UBC Theses and Dissertations

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UBC Theses and Dissertations

Circular dichroism of nitrate esters Barton, Richard Edgar 1971

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CIRCULAR DICHROISM o f NITRA.TE ESTERS by RICHARD EDGAR BARTON B . S c . f U n i v e r s i t y o f B r i t i s h Columbia; , 1966 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n t h e Depar tment o f CHEMISTRY We a c c e p t t h i s t h e s i s a.s c o n f o r m i n g t o t he r e a u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA J u n e , 1971 i v I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l l m e n t o f t h e r e q u i r e m e n t s f o r an advanced degree a t t he U n i v e r s i t y o f B r i t i s h C o l u m b i a , I agree t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t he Head o f my D e p a r t -ment o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t copy-i n g o r p u b l i c a t i o n s o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . The U n i v e r s i t y o f B r i t i s h Co lumbia Vancouver 8, B.C. A B S T R A C T The c i r c u l a r d i c h r o i s m o f o p t i c a l l y a c t i v e a l k y l c y c l o -h e x y l - , b i c y c l o \Z,2.1J h e p t y l - , and o x o l a n y l n i t r a t e s was m e a s u r e d . A c h i r a l i t y r u l e i s p r o p o s e d w h i c h c o r r e l a t e s t h e s t e r e o c h e m i s t r y o f t h e n i t r a t e e s t e r s w i t h t h e s i g n o f t h e i r 2 3 0 nm CD b a n d s : when t h e p l a n a r n i t r a t o chromophore i s o r i e n t e d i n r i g h t - h a n d e d r e c t a n g u l a r c a r t e s i a n c o o r d i n a t e s w i t h t h e a l k o x y l oxygen a t t h e o r i g i n , t h e C - 0 bond on t h e z a x i s , and t h e n i t r o m o i e t y i n t h e p o s i t i v e yz p l a n e , t h e s i g n o f t h e 2 3 0 nm c i r c u l a r d i c h r o i s m band w i l l be p o s i t i v e when t h e p e r t u r b i n g atoms l i e i n t h e p o s i t i v e x - d i r e c t i o n . The r u l e can be e f f e c t i v e l y a p p l i e d o n l y when a f a v o u r e d c o n f o r m a t i o n o f t h e n i t r a t e e s t e r can be d e t e r m i n e d . Of f o r t y - e i g h t n i t r a t e e s t e r s e x a m i n e d , t h e r u l e c o r r e c t l y p r e d i c t e d t h e band s i g n o f t w e n t y - e i g h t ; t h e c o n f o r m a t i o n s o f e i g h t e e n and t h e band s i g n s o f two r e m a i n u n c e r t a i n . No d i r e c t c o n t r a d i c t i o n s were o b s e r v e d . M e n t h y l and c a r v o m e n t h y l n i t r a t e s gave " d o u b l e - h u m p e d " bands i n t h e 2 5 0 - 3 3 0 nm s p e c t r a l r e g i o n and e v i d e n c e o f c o n -f o r m a t i o n a l e q u i l i b r i a . I n a s e r i e s o f seven a l k y l b i c y c l o -[ 2 . 2 . l ] h e p t y l n i t r a t e s , a p o s i t i v e 2 3 0 nm band was o b s e r v e d f o r compounds c o n t a i n i n g a 2 - e n d o n i t r a t o g r o u p w h i l e a n e g a t i v e band was o b s e r v e d f o r compounds w i t h a 2 - e x o n i -t r a t o g r o u p . The i n f l u e n c e o f t e m p e r a t u r e and s o l v e n t on t h e m o l e c u l a r e l l i p t i c i t y o f t h e 2 7 0 and 2 3 0 nm CD bands o f n i t r a t e e s t e r s i i i was examined . The 270 nm band i s t h e more s e n s i t i v e t o e n -v i r o n m e n t a l e f f e c t s , and i n some cases i t s s i g n may be r e -v e r s e d by c h a n g i n g t h e s o l v e n t o r t e m p e r a t u r e . T h i s i s a t -t r i b u t e d t o r o t a t i o n o f t h e n i t r a t o g roup abou t t he 0-C bond , a l t h o u g h n o n - c o n f o r m a t i o n a l e f f e c t s may a l s o be i m p o r t a n t . S t u d i e s on m e n t h y l and f e n c h y l n i t r a t e s showed no a p p r e c i a b l e i n f l u e n c e o f s o l v e n t on t h e e n e r g y o f t h e 270 nm e l e c t r o n i c . t r a n s i t i o n . V TABLE OF CONTENTS Page A b s t r a c t <>. . . .o i i L i s t o f T a b l e s 0........0 „ v i L i s t o f F i g u r e s . . . . . . < > <> o . . v i i i Acknowledgments 0 0 . . . . . . x i i INTRODUCTION . o . . . . 1 C h i r a l i t y R u l e s < > . . . . 0 0 0 0 6 E n v i r o n m e n t a l E f f e c t s Ik, CD o f N i t r a t e E s t e r s 35 Chromophores R e l a t e d t o R0NCr2 43 C o n f o r m a t i o n o f t h e N i t r a t o G r o u p . . . 55 T r a n s i t i o n s I n N i t r a t e E s t e r s . . o . 0 . . 65 Bond ing Of The N i t r a t o G r o u p . 0 . o . . . o 73 RESULTS AND DISCUSSION c 80 U l t r a v i o l e t S p e c t r a Of N i t r a t e E s t e r s 80 C h i r a l i t y Ru le For N i t r a t e E s t e r s . . . 84 A l k y l c y c l o h e x y l N i t r a t e s . • 0 . . . o 9k N i t r a t e s Of A l k y l b i c y c l o [ 2 , 2 , f l -h e p t a n o l s o o I l k N i t r a t e s W i t h Oxolane R i n g s . l4l S t e r o i d a l N i t r a t e s . 155 A c y c l i c N i t r a t e s o . . • o • . • • 0 • • o . • . • . o • 160 Summary. • • • . < > • . o • . • • • 165 EXPERIMENTAL . . . . o . . o . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 Source o f C h i r a l C a . r b i n o l s . . . o . . o . . . 168 S y n t h e s i s o f N i t r a t e E s t e r s . . . . . . . . . 168 UV S p e c t r a . . . . . . 0 0 . . . . . 173 CD Measurements 173 Source o f I n s t r u m e n t E r r o r . . . . . . . . . . 174 Low Tempera tu re C D . M e a s u r m e n t s . . . • • o 176 v i LIST OF TABLES Page I Some E n v i r o n m e n t a l E f f e c t s Seen i n CD S p e c t r a . • • 17 I I C o n f i g u r a t i o n a.nd t h e S igns o f t h e C o t t o n E f f e c t Of N i t r a t e s of. G l u c o s i d e s o o . . 36 I I I CD o f S t e r o i d a l N i t r a t e s i n E t h a n o l . 40 IV Chromophores C o n t a i n i n g N i t r o g e n - O x y g e n . . . . . . . . • 44 V UV Spect ra , o f N i t r a t e E s t e r s 66 V I P r o b a b l e E l e c t r o n i c T r a n s i t i o n s R e s p o n s i b l e For The CD Bands Of N i t r a t e E s t e r s . . . 72 V I I L e n g t h o f N i t r o g e n Bonds 0 76 • V I I I UV Spect ra , o f N i t r a t e E s t e r s , c . . . 81 IX CD o f N i t r a t e s W i t h One Cyc lohexane R i n g . o o . . . o . 96 X A l k y l c y c l o h e x y l N i t r a t e s i CD Band S i g n s and P r e d i c t e d C h i r a l i t y <, 0 . . . . . . . . » .... « ... 98 X I AG Va lues o f V a r i o u s S u b s t i t u e n t s . . . . . . . . . • 100 X I I V a r i a b l e Tempera tu re S tudy o f 1R»3R14S-Menthyl N i t r a t e ( I ) . 0 0 . o . . . . . . . . 0 0 106 X I I I C i r c u l a r D i c h r o i s m o f 1R»3R»4S-Menthyl N i t r a t e S o l v e n t E f f e c t s . . . . . ° o 0 . . . . o . . 0 . . . . . . . . 109 XIV CD Of N i t r a t e s Of A l k y l b i c y c l o [ 2.2 . l ] h e p t a n o l s . oll6 XV Band S i g n s and C h i r a l i t y R u l e P r e d i c t i o n s For N i t r a t e s o f A l k y l b i c y c l o [2.2 .1] h e p t a n o l s . . . o • o . 0 I I 8 XVI LTCD o f B o r n y l , I s o b o r n y l and F e n c h y l N i t r a t e . ..123 X V I I E f f e c t o f S o l v e n t s on t h e CD o f A l k y l b i c y c l o -[2.2ol] h e p t y l N i t r a t e s . . . . . . . 0 . 0 . 0 131 X V I I I LTCD o f Ca .mpha .ne-2,3 -d io l -d in i t ra tes ( M e O H ) . o . . . 1 3 7 XIX CD o f L - T h r e i t a . n D i n i t r a t e and o f D- and L-<x-N i t r a t o - ^ ' / ^ - d i m e t h y l - l f - b u t y r o l a c t o n e . o o o o . . . . . . . 1 4 3 XX CD o f N i t r a t e s W i t h Oxolane R ings 0 . 145 v i i L IST OF TABLES (CONTD) Page XXI CD o f L - T h r e i t a n D i n i t r a t e ( 3 0 ° C ) o . . 1^9 X X I I CD o f S t e r o i d a l N i t r a t e s , . o » ' . . . » 156 X X I I I N i t r a t e s o f o ( - H y d r o x y A c i d s . . . . l 6 l XXIV 230 nm S i g n P r e d i c t i o n s by...Planar N i t r a t o R u l e . o o . . o . . . . . « « « o o « « . . o a . . * . . « o . . . . * . o o o o l 6 5 XXV Sources o f C h i r a l A l c o h o l s . . . . . . o • « , . . . . . . . . 169 XXVI E l e m e n t a l A n a l y s i s o f N i t r a t e E s t e r s . . 0 . 0 o o 172 v i i i LIST OF FIGURES Pa.ge 1 . R e l a t i o n s h i p Between t h e U l t r a v i o l e t A b s o r p t i o n , ORD and CD 3 2 . Three C l a s s e s o f Chromophores 0 6 3 « Spheres o f P o s s i b l e D i s s y m m e t r y . • o 7 4 . The O c t a n t R u l e . . . . . . . . . . . . . . . . 11 5. C 2 v C h a r a c t e r T a b l e and t h e A 2 P s e u d o s c a l a r R e p r e s e n t a t i o n . . • 12 6 . E n v i r o n m e n t a l CD E f f e c t s 0 . . o . . . < > . . . 1 6 7 . CD o f ( - ) Menthone i n D e c a l i n a t + 2 5 * C , -74*C a.nd + l62*C (A) and i n V a r i o u s S o l v e n t s a t 2 5 °C ( B ) . . . c . o . . . . . o . . a o o o s . o o a . . . . . o o . . o . . . o « o 21 8 . G e n e r a t i o n o f a Double-Humped CD C u r v e . . . . o o . c • 2 2 9 . ORD and CD o f 2 , 3 , 4 , 6 - T e t r a - 0 - a . c e t y l - l - o -N i t r o - o i - D - g a l a c t r o p y r a n o s e (A) and M e t h y l 4 i 6 - 0 -B e n z y l i d e n e - Q ( - D - a l t r o p y r a . n o s i d e 3 - N i t r a . t e ( B ) . o 3 7 1 0 . F ree R o t a t i o n i n t h e N i t r i t o , N i t r o and N i t r a t o Chromophores . 43 1 1 . CD o f ?* -Ch lo ro -7>S-Ni t ro -5o ( -Cho les ta .ne a t +22- C a.nd -188°C ( 5 0 ) . . 48 1 2 . N i t r o S e c t o r R u l e . . o . . . 49 1 3 . A p p l i c a t i o n o f t h e N i t r o S e c t o r Ru le • • 5 0 1 4 . A p p l i c a t i o n o f R u l e R e l a t i n g C o n f i g u r a t i o n o f C - n i t r o A l c o h o l s t o t h e S i g n o f t h e 3 1 0 nm CD band ( 6 5 ) . . . . . . < > . . . • * . . . . . • • • • o . • . . . . . . . . . . . . . o 5 2 1 5 c S e c t o r Ru le f o r t h e N i t r o s o a m i n e Chromophore (upper s e c t o r s ) ( 6 4 ) . 0 . . . . 0 5 3 1 6 . C o n f i g u r a t i o n o f S t e r o i d a l A c e t a t e s . 5 5 17• Proposed S t r u c t u r e s f o r N i t r a t e E s t e r s . . . . . . . . . 5 7 i x Pa.ge 1 8 . Bond R o t a t i o n s i n N i t r a t e E s t e r s 57 1 9 . M e t h y l N i t r a t e Energy S u r f a c e ( 8 9 ) ° . . . . . . . . . . . o . 19 2 0 . Some T r a n s i t i o n s o f MeON02 . . . . . o . 68 2 1 . UV Spect rum o f M e t h y l N i t r a t e R e s o l v e d i n t o i t s Component Bands ( 8 9 ) . . . . . . . o . . . . . » . . o o . o . . o o 68 22 Energy L e v e l s and T r a n s i t i o n s o f t h e N i t r o Chromophore and N i t r a . t e I o n . • o 0 o . . . . . o o . . • 69 2 3 . E t h y l N i t r a t e as Viewed f o r C h a r g e - T r a n s f e r Bands <>.. 71 2 4 . Nonbonding and Tr o r b i t a l s o f t h e N i t r a t o G r O U p . . o . . . . . . . . . s . . . . . . . * . . . o . . « e . e . . . e . . o . e . . . 7^ 2 5 . M o l e c u l a r O r b i t a l D e n s i t y C o n t o u r Maps f o r M e t h y l N i t r a t e , a c c o r d i n g to Csizma.dia. e_t a . l . ( 8 9 ) . . * ° . 0 . 0 0 . a o o . o . o o . e « o . . . . « . o . . . o o o « . . o . e . . . 75 260 UV Spec t rum o f M e n t h y l N i t r a t e ( M e O H ) 0 o 0 0 » . o . . o . 82 2 7 . Noda l P lanes o f t h e TT and Nonbonding O r b i t a l s o f t h e N i t r a t o G r o u p • • . • < > • • • . . . . . . . o . o . . o e . . . o . . 84 2 8 . A n t i b o n d i n g O r b i t a l s (A) and Noda l S u r f a c e Of C — O N O 2 ( B ) o o . . o . o . e o . o o o o . . . o . . o o e . Q . . . o . * . . a . 86 2 9 . C o o r d i n a t e s and D i p o l e Moment o f t h e N i t r a t o Group . . . . . . . o o . . . . . . . o . . 0 . 0 . 0 . 0 0 . . o . . . . . . . 86 3 0 . P l a n a r Ru le ( f r o m Cs p o i n t group) f o r t h e N i t r a t o G r o u p . . o o . o . . . . . . 87 3 1 . P o s s i b l e P o s i t i o n s o f t h e n and I T * Noda l S u r f a c e s o f t h e N i t r a t o G r o u p . o . o . . . . . . . o . . o o . . . 89 3 2 . V i e w i n g t h e N i t r a t © Group A long t h e 0-C b o n d . . o . 90 3 3 . A p p l i c a t i o n o f t h e N i t r a t o C h i r a l i t y Ru le t o I S s 2 R — B o r n y l N i t r a . t e « . o o o . » . o . . o e o » o . » 9 B » . . o o . . . 91 3 4 . P r o j e c t i o n s o f t h e N i t r a t o G r o u p . . . . . . . . . . . . . . . . 92 3 5 . CD o f C y c l o h e x y l N i t r a t e s ( i n CH-^CN). 0 0 0 . . . . . . . . 99 360 Reso lved Bands ( n o t t o s c a l e ) o f t h e 270 nm T r a n s i t i o n o f M e n t h y l and C a r v o m e n t h y l N1 ~tr*3."t 6 S • e o c o o o o o o o » o f i o o * o e « o o » o o o o o o o o o o o o » o o o o 99 X Page 37. CD (band I ) o f I s o c a r v o m e n t h y l N i t r a t e 0 . . . 0 0 102 3 8 . LTCD o f M e n t h y l N i t r a t e ( I ) i n M e t h a n o l . . . . . . . . . . . 106 39. V a r i a t i o n o f t h e 270 nm Band o f M e n t h y l N i -t r a t e ( I ) w i t h T e m p e r a t u r e . . . . . • o 108 4 0 . M e n t h y l N i t r a t e ( I ) J R e l a t i o n o f £ ? r y n t o [e] 2 7 0 . . . . . . . • • • • • • • • • •1 0 8 4 1 . P l o t o f D i e l e c t r i c C o n s t a n t (A) and Z Va lue (B) A g a i n s t M 2 7 0 o f M e n t h y l N i t r a t e ( I ) 113 4 2 . CD o f I s o b o r n y l , B o r n y l , P e n c h y l N i t r a t e s 117 4 3 . CD o f Camphane-2 ,3 -d i o l - d i n i t r a t e s (CH^CN) 120 4 4 . C h i r a l i t y Ru le P r o j e c t i o n s o f N i t r a t o Groups i n X I I and XIV 121 4 5 . LTCD o f L S i 2 R - B o m y l N i t r a t e (MeOH) 126 i 4 6 . Tempera tu re V a r i a t i o n (Band I ) o f I s o b o r n y l and B o r n y l N i t r a t e s (MeOH). o . . . 0 • . • • o . 127 4 7 . LTCD o f I S i 2 R - B o r n y l N i t r a t e ( I X ) . . . 129 4 8 . CD o f l R » 2 R - I s o b o r n y l N i t r a t e ( S o l v e n t S t u d y ) . . 4 9 . CD o f B o r n y l ( I X ) and F e n c h y l (X) N i t r a t e s . . . . . . . 133 50o LTCD o f Camphane-2 ,3 -d i o l - d i n i t r a t e s ( M e O H ) . . . . . . 139 51. LTCD o f C a . m p h a . n e - 2 , 3 - d i o l - d ' i n i t r a . t e s ( M e O H ) . . . . . . 140 52. CD o f L - T h r e i t a . n D i n i t r a t e and L - C k - N i t r a t o -0, ^ - d i m e t h y l - * - b u t y r © l a c t o n e . • • o . . . . o . . . . . 1 4 4 53• P r o j e c t i o n s f o r I s o i o d i d e and Isomann ide Mono-N i t r a t e s and I s o s o r b i d e D i n i t r a t e (X IX) 146 54. P r o j e c t i o n s o f N i t r a t o Groups i n L - T h r e i t a . n • D i n i t r a t e . . . o o o . . . . . . . . . e . . . . . . . . . . . . . . o . . o . . . . o . 147 55«> CD o f L - T h r e i t a n D i n i t r a t e . . . . . o . 150 56. LTCD o f L - T h r e i t a n D i n i t r a t e . 151 1 x i Page 57. CD o f L - T h r e i t a n D i n i t - r a t e w i t h P o s s i b l e Band R e s o l u t i o n (MeOH a t 3 G * C ) . . o 0 . . 152 58. LTCD o f D - « - N i t r a t o - ^ , ^ - d i m e t h y l - y - . b u t y r o -l a c t o n e . . . . . . . . 0 o . . . . o e . o . . . . 153 59. CD o f N i t r a t e s o f o ( - H y d r o x y A c i d s ( C H - C N ) . . . . . . . 162 x i i ACKNOWLEDGMENTS To ray men to r and g u i d e , P r o f e s s o r L . D . Hayward , who i n t r o d u c e d me t o t h e f i e l d s o f c i r c u l a r d i c h r o i s m and n i t r a t e e s t e r s , and t o whom I am d e e p l y i n d e b t e d f o r h i s g u i d a n c e and encouragement , t o P r o f e s s o r J . T r o t t e r f o r v a l u a b l e d i s c u s s i o n s on t h e x - r a y a n a l y s i s o f n i t r a t e e s t e r s , t o D r . R.No T o t t y ' s p r e s e n c e i n t h e l a b o r a t o r y and h i s w i l l i n g n e s s t o o f f e r v a l u a b l e h e l p , t o P r o f e s s o r s S. S c h r o e t e r and S. A n g y a l f o r g i f t s o f c h i r a l e a r b i n o l s , t o my w i f e , Peggy, who spen t h o u r s o f t i m e d e c i p h e r i n g my h a n d w r i t i n g i n t o n e a t t y p e s c r i p t . x i i i 1 . I N T R O D U C T I O N 2 INTRODUCTION A most c h a r a c t e r i s t i c p r o p e r t y o f a l a r g e number and v a r i e t y o f n a t u r a l l y - o c c u r r i n g o r g a n i c compounds i s t h e i r o p t i c a l r o t a t o r y power , t h a t i s , t h e i r i n t e r a c t i o n w i t h p o l a r i z e d l i g h t . To e x h i b i t o p t i c a l a c t i v i t y , t h e m o l e -c u l e s o f t h e compound must be c h i r a l . Lo rd K e l v i n ( l ) d e f i n e d c h i r a l i t y o v e r s e v e n t y y e a r s a g o , when he s a i d , " I c a l l any g e o m e t r i c a l f i g u r e o r g r o u p o f p o i n t s c h i r a l and say i t has c h i r a l i t y i f i t s image i n a p l a n e m i r r o r i d e a l l y r e a l i z e d , cannot be b r o u g h t t o c o -i n c i d e w i t h i t s e l f . " Thus a c h i r a l m o l e c u l e i s one t h a t i s n o n s u p e r i m p o s a b l e w i t h i t s m i r r o r image by t r a n s l a t i o n s and r o t a t i o n s o n l y ( 2 a ) . To be c h i r a l , i t must l a c k t h e symmetry e l e m e n t s Sn, t h a t i s , have no p l a n e , c e n t e r , o r a l t e r n a t i n g a x i s o f symmetry . The most d i r e c t measure o f m o l e c u l a r c h i r a l i t y i s t h e p r e f e r e n t i a l a b s o r p t i o n o f t h e r i g h t o r l e f t c i r c u l a r l y p o l a r i z e d l i g h t known as c i r c u l a r d i c h r o i s m (CD) . The CD o f t h e sample i s e x p r e s s e d as t h e d i f f e r e n c e between t h e m o l a r a b s o r p t i o n c o e f f i c i e n t s f o r t h e two fo rms o f c i r -c u l a r l y p o l a r i z e d l i g h t . A p l o t o f t h e d i f f e r e n t i a l a b s o r p t i o n , At, a g a i n s t w a v e l e n g t h i s t h e CD s p e c t r u m o f t h e c h i r a l compound ( F i g u r e 1 ) . The s i g n o f At depends on t h e c h i r a l sense o f t h e m o l e c u l e s , i . e . on t h e i r a b s o l u t e c o n f i g u r a t i o n . 3 i F i g u r e 1 . R e l a t i o n s h i p between t h e u l t r a v i o l e t a b s o r p t i o n , ORD and CD. The d i f f e r e n t i a l d i s p e r s i o n o f r i g h t - and l e f t - h a n d e d c i r c u l a r l y p o l a r i z e d l i g h t , known as o p t i c a l r o t a t o r y d i s -p e r s i o n (ORD), may be measured w i t h a s p e c t r o p o l a r i m e t e r and l i n e a r l y p o l a r i z e d l i g h t i n s p e c t r a l r e g i o n s where t h e c h i r a l sample i s t r a n s p a r e n t . The two phenomena, CD and ORD, a r e r e l a t e d by t h e K r o n i g - K r a m e r s t r a n s f o r m . The phenomena o f ORD and CD t o g e t h e r a r e known as t h e C o t t o n  e f f e c t f r o m t h e i r d i s c o v e r e r , C o t t o n (I895) ( 2 b ) . The r e l a t i o n s h i p o f t h e e l e c t r o n i c a b s o r p t i o n , ORD and o f a c h i r a l t r a n s i t i o n i s i l l u s t r a t e d i n F i g u r e 1 . Note t h a t t h e CD band i s c o n f i n e d t o t h e a r e a u n d e r t h e u l t r a -4 . v i o l e t a b s o r p t i o n b a n d , w h i l e t h e ORD cu rve e x t e n d s ( t h e o -r e t i c a l l y ) o v e r t h e e n t i r e s p e c t r u m . C i r c u l a r d i c h r o i s m d a t a a r e a l s o e x p r e s s e d as m o l e -c u l a r e l l i p t i c i t y , [e] , w h i c h i s r e l a t e d t o t h e d i f f e r e n t i a l a b s o r p t i o n b y t h e e x p r e s s i o n The g e n e r a l f i e l d o f c i r c u l a r d i c h r o i s m and o p t i c a l r o t a t o r y d i s p e r s i o n has* been r e v i e w e d by s e v e r a l a u t h o r s ( 3 - 7 ) . The w a v e l e n g t h w e i g h t e d a r e a u n d e r t h e CD band i s p r o -p o r t i o n a l t o t h e r o t a t i o n a l s t r e n g t h , R, w h i c h i s a measure o f t h e d i s s y m m e t r y o f t h e chromophore and o f t h e i n t e n s i t y o f t h e d i c h r o i s m . Each t r a n s i t i o n o f t h e chromophore w i l l have i t s own r o t a t i o n f o r compounds w i t h o v e r l a p p i n g CD bands . The r o t a t i o n a l s t r e n g t h , R, i s a s i g n e d q u a n t i t y and s e r v e s as a measure o f t h e ch romophore*s i n t e r a c t i o n w i t h i t s d i s s y m m e t r i c e n -v i r o n m e n t . T h i s q u a n t i t y i s a l s o i m p o r t a n t because o f i t s t h e o r e t i c a l s i g n i f i c a n c e . Th rough t h e r o t a t i o n a l s t r e n g t h , t h e t h e o r y o f o p t i c a l a c t i v i t y can be r e l a t e d t o an e x -p e r i m e n t a l q u a n t i t y . The r o t a t i o n a l s t r e n g t h o f t h e t r a n s i t i o n , R k , i s [9 ] = 3300 A t A a l s t r e n g t h and t h e r e f o r e t h e c a l c u l a t i o n can be d i f f i c u l t 'o ( c . g . s . u n i t s ) where c i s t h e v e l o c i t y o f l i g h t , h i s P l a n c k * s c o n s t a n t , i s t h e number o f o p t i c a l l y a c t i v e m o l e c u l e s p e r cm 3, and 6K i s t h e p a r t i a l e l l i p t i c i t y f o r t h e k ^h t r a n s i t i o n . I t i s a l s o i m p o r t a n t t o n o t e t h a t t h e e x p e r i m e n t a l l y o b t a i n e d r o t a t i o n a l s t r e n g t h w i l l v a r y w i t h t e m p e r a t u r e and s o l v e n t . I n s p i t e o f t h e m e d i c i n a l and t e c h n i c a l i m p o r t a n c e o f n i t r a t e e s t e r s ( R 0 N 0 2 ) , knowledge o f t h e i r p h y s i c a l p r o -p e r t i e s and m o l e c u l a r s t r u c t u r e i s r e l a t i v e l y l i m i t e d . The n i t r a t o g r o u p ( - 0 N 0 2 ) i s an e a s i l y a c c e s s i b l e ch romophore , w h i l e t h e h y d r o x y l g r o u p f r o m w h i c h i t i s u s u a l l y d e r i v e d a b s o r b s a t t o o low a w a v e l e n g t h t o be reached by t h e u s u a l s p e c t r o p h o t o m e t e r o r p o l a r i m e t e r . S ince t h e h y d r o x y l g roup can be r e a d i l y c o n v e r t e d i n t o t h e n i t r a t o g r o u p , t h e c i r -c u l a r d i c h r o i s m o f t h e n i t r a t e e s t e r s o f t h e w ide range o f n a t u r a l l y o c c u r r i n g a l c o h o l s may be s t u d i e d . F o r t h e s e r e a s o n s i t was d e s i r a b l e t o d e v i s e a c h i r a l i t y r u l e t h a t w o u l d c o r r e l a t e t h e s t e r e o c h e m i s t r y o f a n i t r a t e e s t e r t o t h e s i g n s o f i t s c i r c u l a r d i c h r o i s m bands . 6 C h i r a l i t y R u l e s C i r c u l a r d i c h r o i s m i s o f u n i q u e v a l u e f o r t h e d e t e r -m i n a t i o n o f a b s o l u t e c o n f i g u r a t i o n s i n c e i t i s a p r o p e r t y t h a t depends d i r e c t l y on t h e c h i r a l i t y o f t h e m o l e c u l e . When c h i r o p t i c a l p r o p e r t i e s a r e c o n s i d e r e d , chromo-p h o r e s a r e d i v i d e d i n t o t h r e e c l a s s e s ( F i g u r e 2 ) . F i g u r e 2 . T h r e e c l a s s e s o f c h r o m o p h o r e s . The d i s s y m m e t r i c chromophore i s c h i r a l b y i t s e l f and w i l l e x h i b i t c i r c u l a r d i c h r o i s m ; t h e c o u p l e d - o s c i l l a t o r r e q u i r e s two chromophores w h i c h become o p t i c a l l y a c t i v e by t h e i r m u t u a l i n t e r a c t i o n ; t h e s y m m e t r i c a l chromophore i s i n a c t i v e b y i t s e l f , b u t becomes o p t i c a l l y a c t i v e by a d i s s y m m e t r i c p e r t u r b a t i o n o f i t s m o l e c u l a r o r b i t a l s b y t h e c h i r a l e n -v i r o n m e n t i n w h i c h i t i s s i t u a t e d . The n i t r a t o g r o u p b e -l o n g s t o t h i s t h i r d t y p e o f ch romophore . The n i t r a t o g r o u p i s s y m m e t r i c a l and i t s t r a n s i t i o n s w i l l o n l y be o p t i c a l l y a c t i v e when i t i s p l a c e d i n a d i s s y m m e t r i c e n v i r o n m e n t . S n a t z k e ( 8 a ) c o n s i d e r e d t h e m o l e c u l e as d i v i d e d i n t o s p h e r e s a ) D i s s y m m e t r i c chromophore b ) C o u p l e d - O s c i l l a t o r s c ) s y m m e t r i c T r o g e r ' s base chromophore 3 - m e t h y l -c y c l o h e x a n o n e 7 o f i n f l u e n c e and p o s t u l a t e d t h a t t h e sphere n e a r e s t t h e s y m m e t r i c chromophore d e t e r m i n e s t h e s i g n o f t h e CD band t o t h e g r e a t e s t e x t e n t ( F i g u r e 3 ) . 1 s t sphere F i g u r e 3» Spheres o f p o s s i b l e d i s s y m m e t r y . I n e f f e c t , t h e p e r t u r b a t i o n o f a chromophore w i l l be a f u n c t i o n o f t h e d i s t a n c e between i t and t h e p e r t u r b i n g atoms ( 8 b ) . Thus t h e r o t a t i o n a l s t r e n g t h i s p r o p o r t i o n a l t o » 2 7 i x yj7 7 J X D; where t h e 7's a r e t h e d i r e c -t i o n a l c o s i n e s o f t h e l i n e j o i n i n g t h e oxygen atoms o f t h e c a r b o n y l g r o u p w i t h t h e p e r t u r b i n g a t o m , and D i s a f u n c t i o n o f t h e d i s t a n c e between t h e p e r t u r b i n g atom and t h e chromo-p h o r e . Thus t h e i n h e r e n t l y s y m m e t r i c chromophore , such as t h e c a r b o n y l Or n i t r a t o , whose t r a n s i t i o n s a re r e a d i l y a c c e s s i b l e , can be used as a s t e r e o c h e m i c a l p robe when p l a c e d i n a d i s s y m m e t r i c e n v i r o n m e n t . I t may be used t o deduce t h e a b s o l u t e c o n f i g u r a t i o n o f t h e compound. A s c e r t a i n i n g t h e a b s o l u t e c o n f i g u r a t i o n o f a c h i r a l compound c o n t a i n i n g a s y m m e t r i c chromophore r e q u i r e s t h e compar i son o f i t s CD s p e c t r u m w i t h t h e s p e c t r a o f r e f e r e n c e 8. compounds o f known a b s o l u t e c o n f i g u r a t i o n . S ince t h e CD s p e c t r u m o f a c o n f o r m a t i o n a l l y l a b i l e compound i s t h e p o p -u l a t i o n - w e i g h t e d ave rage o f t h e s p e c t r a o f i t s i n d i v i d u a l c o n f o r m e r s ( v i d e i n f r a ) , t o d e t e r m i n e t h e a b s o l u t e c o n f i g -u r a t i o n , t h e c o n f o r m a t i o n a l e q u i l i b r i a o f t h e compound must be known. On t h e o t h e r hand , when t h e a b s o l u t e c o n f i g u r -a t i o n o f a compound i s known, CD measurements can g i v e u s e -f u l i n f o r m a t i o n a b o u t t h e c o n f o r m a t i o n a l e q u i l i b r i a . The a b s o l u t e c o n f i g u r a t i o n s o f t h e n i t r a t e e s t e r s s e l e c t e d f o r t h e p r e s e n t s t u d y a r e known, and i t was hoped t h a t t h e s e w o u l d f o r m a s e t o f r e f e r e n c e s t a n d a r d s on w h i c h t o base a r u l e r e l a t i n g t h e s i g n s o f t h e CD bands t o c o n f i g u r a t i o n . C h i r a l i t y r u l e s a r e based on t h e i m p l i c i t a s s u m p t i o n o f an i n t e r a c t i o n be tween a chromophore and i t s m o l e c u l a r e n v i r o n m e n t . C o n s e q u e n t l y , a c a r e f u l e v a l u a t i o n o f t h e mechanism o f o p t i c a l a c t i v i t y must p recede t h e d e s i g n o f a c h i r a l i t y r u l e ( 13) . Over t h e y e a r s , t h r e e complementary mechanisms have been p r o p o s e d ( l4)» 1. O n e - e l e c t r o n mechanism 2 . K i r k w o o d c o u p l i n g mechanism 3 . M a g n e t i c - e l e c t r i c c o u p l i n g mechanism The a m p l i t u d e o f t h e CD band depends on t h e magn i tude o f t h e e l e c t r i c and m a g n e t i c t r a n s i t i o n moments and t h e c o -s i n e o f t h e a n g l e be tween them. I n t h e o n e - e l e c t r o n mechanism, t h e m a g n e t i c and e l e c t r i c t r a n s i t i o n s o c c u r i n t h e same chromophore and t h e r e m a i n d e r o f t h e m o l e c u l e a c t s as a p e r t u r b i n g f i e l d w h i c h p a r t i a l l y 9 b r e a k s down t h e symmetry o f t h e chromophore . Thus t h e mag-n e t i c and e l e c t r i c t r a n s i t i o n s a r e " m i x e d " . I n t h e K i r k w o o d c o u p l e d o s c i l l a t o r m o d e l , t h e m o l e c u l e r e q u i r e s a t l e a s t two c h r o m o p h o r i c g r o u p s d i s s y m m e t r i c a l l y o r i e n t e d i n s p a c e . The chromophores have e l e c t r i c t r a n s -i t i o n s and t h e s e e l e c t r o n i c t r a n s i t i o n s a r e c o u p l e d by t h e i r d i p o l a r f i e l d s t o p roduce a m a g n e t i c moment. The e x c i t o n t h e o r y i s a s p e c i a l case o f t h i s mechanism. The m a g n e t i c - e l e c t r i c c o u p l i n g mechanism a l s o i n v o l v e s two ch romophores . One g r o u p has an e l e c t r i c t r a n s i t i o n w h i l e t h e o t h e r has a m a g n e t i c t r a n s i t i o n . These t r a n s i t i o n s a r e c o u p l e d i n t h e m o l e c u l e t o g i v e r o t a t o r y s t r e n g t h t o b o t h t r a n s i t i o n s . A m o l e c u l e w i t h o n l y one i m p o r t a n t chromophore s h o u l d be a d e q u a t e l y t r e a t e d by t h e o n e - e l e c t r o n t h e o r y . S c h e l l m a n (13) s t a t e s t h a t i f t h e t r a n s i t i o n u n d e r c o n s i d e r a t i o n i s weak and e s s e n t i a l l y m a g n e t i c , i t i s u s u a l l y a l l o w a b l e t o i g n o r e t h e d i p o l e - d i p o l e c o u p l i n g t h e o r y e n t i r e l y and t r e a t t h e C o t t o n e f f e c t by t h e o n e - e l e c t r o n mechanism. A s t r o n g a b s o r p t i o n band i s u s u a l l y a s s o c i a t e d w i t h an e l e c t r i c a l l y a l l o w e d t r a n s i t i o n w h i l e o n l y a s h o u l d e r appears f o r a m a g n e t i c a l l y a l l o w e d one. The l o w e s t e n e r g y bands o f many o r g a n i c m o l e c u l e s c o n t a i n i n g h e t e r o atoms a r e e s s e n -t i a l l y p -p t r a n s i t i o n s w i t h l a r g e m a g n e t i c moments ( n — > n * t r a n s i t i o n s ) and can be c o n s i d e r e d i n t h e f ramework o f t h e o n e - e l e c t r o n t h e o r y . 10* I f t h e C o t t o n e f f e c t a r i s e s f r o m a band w i t h a l a r g e e l e c t r i c d i p o l e and no m a g n e t i c d i p o l e ( fT - * r f f o r example) and i f t h e r e a r e no m a g n e t i c t r a n s i t i o n s i n t h e n e a r - u l t r a -v i o l e t , i t i s a f a i r l y s a f e a s s u m p t i o n t h a t t h e r o t a t o r y s t r e n g t h i s d e v e l o p e d l a r g e l y f r o m t h e d i p o l e - d i p o l e coup-l i n g mechanism (13a). Thus t h e C o t t o n e f f e c t s o f s t r o n g a b s o r p t i o n bands a r e t r e a t e d by t h e c o u p l e d - d i p o l e t h e o r y , a l t h o u g h t h e p o s s i b i l i t y o f o n e - e l e c t r o n c o n t r i b u t i o n s t o such bands i s u s u a l l y p r e s e n t . A c h i r a l i t y r u l e can be d e v e l o p e d b y e x a m i n i n g t h e t r a n s i t i o n i n v o l v e d , and t h e n d i v i d i n g t h e space a round t h e chromophore i n t o s i g n e d r e g i o n s t h a t c o n t r i b u t e t o t h e o p -t i c a l a c t i v i t y . I n t h e case o f t h e c a r b o n y l ch romophore , t h e symmetry and n o d a l p l a n e s o f t h e o r b i t a l s o f t h e t r a n s -i t i o n a r e t h e s u r f a c e s t h a t d i v i d e up space a round t h e chromophore i n t o e i g h t r e g i o n s . Thus , an o c t a n t r u l e i s d e v i s e d (3). The i n f l u e n c e o f an atom on t h e d i s s y m m e t r y o f t h e c a r b o n y l g r o u p i s c h a r a c t e r i z e d by a s i g n a c c o r d i n g t o t h e o c t a n t i n w h i c h i t i s l o c a t e d ( F i g u r e 4 ) . There h a s , however , been an i n c r e a s i n g number o f r e p o r t s o f a n t i o c t a n t b e h a v i o u r (13b). 11 Back O c t a n t s + F r o n t O c t a n t s + A. B. C. F i g u r e 4 . The O c t a n t Rules A. n v r r ' t r a n s i t i o n . B. O c t a n t p r o j e c t i o n . C. S i g n o f t h e o c t a n t r e g i o n s . Symmetry r u l e s have been d e v e l o p e d f o r a s u r p r i s i n g l y l a r g e number o f c h r o m o p h o r e s , and have been r e v i e w e d b y a number o f a u t h o r s ( 9 - 1 2 ) . S c h e l l m a n , on t h e o t h e r h a n d , has used a d i f f e r e n t a p p r o a c h t o g e n e r a t e c h i r a l i t y r u l e s (13a, 1 5 ) . F o r t h e t r a n s i t i o n s , a g r o u p t h e o r y a p p r o a c h i s a p p l i e d w i t h i n t h e o n e - e l e c t r o n mechan ism. S c h e l l m a n ' s method c o n c e r n s t h e a p p l i c a t i o n o f g r o u p t h e o r y f o r g e n e r a t i n g t h e s i g n d e t e r -m i n i n g r e g i o n s a s s o c i a t e d w i t h p e r t u r b e r s o f i n h e r e n t l y s y m m e t r i c c h r o m o p h o r e s . The e q u a t i o n f o r t h e r o t a t o r y s t r e n g t h i s g i v e n b y - Vqp R a p = 1 2 . Reps r o t a t o r y s t r e n g h M = e l e c t r i c d i p o l e v e c t o r m = m a g n e t i c d i p o l e v e c t o r a , P= s t a t e s a and P va,p= p e r t u r b a t i o n e n e r g y between t h e two e x c i t e d s t a t e s w h i c h p r o d u c e s c o u p l i n g The f o r m o f V w h i c h i s r e q u i r e d t o i n d u c e c h i r a l i t y i n t h e chromophore depends on t h e symmetry o f t h e c h r o m o p h o r i c g r o u p i t s e l f . I n p a r t i c u l a r , Sche l lman shows t h a t a p a r t o f V must b e l o n g t o t h e p s e u d o s c a l a r r e p r e s e n t a t i o n ( t h e r o t a t o r y s t r e n g t h i s a p s e u d o s c a l a r w h i c h changes s i g n on r e f l e c t i o n and i n v e r s i o n i n t h e o r i g i n ) . The c a r b o n y l ch romophore , f o r examp le , b e l o n g s t o t h e C 2 V p o i n t g r o u p . C o n s u l t i n g t h e C 2 V c h a r a c t e r t a b l e r e v e a l s t h a t t h e A 2 r e p r e s e n t a t i o n i s t h e p s e u d o s c a l a r p a r t o f t h e p o t e n t i a l ( F i g u r e 5 ) J c 2 V E C 2 o1 o " Vlxi.) VtyjL) A, 1 1 1 1 A 2 1 1 - 1 - 1 B, 1 - 1 1 - 1 B 2 1 - 1 - 1 1 t + + A 2 r e p r e s e n t a t i o n F i g u r e 5« C 2 V c h a r a c t e r t a b l e and t h e A 2 p s e u d o s c a l a r r e p r e s e n t a t i o n . 13 . Thus a q u a d r a n t r u l e i s p r e d i c t e d f o r t h e c a r b o n y l g r o u p by t h i s a p p r o a c h . I t s h o u l d be n o t e d , however , t h a t S c h e l l m a n * s method can s p e c i f y o n l y a minimum number o f s p a t i a l r e g i o n s ( l 6 ) . There may be a d d i t i o n a l n o d a l s u r -f a c e s , n o t d e t e r m i n e d by symmetry , w h i c h can l e a d t o f u r -t h e r s u b d i v i s i o n s . The t h i r d a p p r o a c h t o a c h i r a l i t y r u l e i s t h e p u r e l y e m p i r i c a l one. A l a r g e number o f compounds o f known a b -s o l u t e c o n f i g u r a t i o n a r e examined and t h e i r c i r c u l a r d i c h r o -i sm s p e c t r u m r e c o r d e d . A r u l e may t h e n be d e v e l o p e d by l o o k i n g f o r c o r r e l a t i o n s between c o n f i g u r a t i o n and CD band s i g n . Whatever t h e r u l e , t h e f i n a l t e s t i s i t s a b i l i t y t o c o r r e c t l y p r e d i c t t h e a b s o l u t e c o n f i g u r a t i o n f r o m t h e s i g n o f t h e C o t t o n e f f e c t , o r v i c e v e r s a . These t h r e e app roaches can be c o n s i d e r e d i n t h e deve lopment o f a c h i r a l i t y r u l e f o r n i t r a t e e s t e r s . 14 E n v i r o n m e n t a l E f f e c t s I n t h e a p p l i c a t i o n o f c h i r a l i t y r u l e s and i n t h e i r deve lopment , t e m p e r a t u r e , s o l v e n t , and c o n f o r m a t i o n f a c -t o r s a r e i n v o l v e d . These may be c o n s i d e r e d as i n t r a - and i n t e r m o l e c u l a r e n v i r o n m e n t a l e f f e c t s . The i n t r a m o l e c u l a r e f f e c t i s c o n f o r m a t i o n a l , w h i l e t h e i n t e r m o l e c u l a r e f f e c t i s due t o s o l v e n t — s o l u t e i n t e r -a c t i o n . Tempera tu re can a f f e c t b o t h o f t h e s e e n v i r o n m e n t a l e f f e c t s ; i t can a f f e c t t h e c o n f o r m a t i o n o f t h e ch romophore , and a l s o t h e i n t e r a c t i o n o f t h e chromophore w i t h s o l v e n t . When t h e t e m p e r a t u r e a n d / o r s o l v e n t i s v a r i e d , f i v e changes i n t h e CD s p e c t r u m may be observed ( F i g u r e 6 ) . 1 . s i g n r e v e r s a l . 2 . doubled-humped peaks c h a n g i n g i n i n t e n s i t i e s , p e r h a p s even m e r g i n g i n t o one peak ( 2 8 ) . 3 . marked changes i n t h e i n t e n s i t y o f CD bands . 4 . i n d u c e d c i r c u l a r d i c h r o i s m . 5« f r e q u e n c y s h i f t s o f CD b a n d s . I n t h i s s t u d y , t h e cause and s i g n i f i c a n c e o f f r e q u e n c y s h i f t s w i l l n o t be c o n s i d e r e d i n d e t a i l . The f i r s t f o u r e f f e c t s have been a s c r i b e d t o : 1 . E q u i l i b r i u m between an u n a s s o c i a t e d and an a s s o c i a t e d f o r m ( s e l f - a s s o c i a t i o n ) . 2 . C o n f o r m a t i o n a l changes i n t h e m o l e c u l e - an e q u i l i b r i u m between two o r more c o n f o r m e r s . 3 . Assymmet r i c s o l v a t i o n - v a r i a t i o n s i n t h e s o l -15 v e n t cage . 4 . E q u i l i b r i u m between a s o l v a t e d and u n s o l v -a t e d f o r m . 5. A c o m b i n a t i o n o f c o n f o r m a t i o n a l and s o l v a t i o n e f f e c t s . These e f f e c t s a r e n o t j u s t c o n f i n e d t o one chromophore , as can be seen f r o m T a b l e I . These e f f e c t s a r e s e e n , f o r example , i n k e t o n e s , x a n t h a t e s ( 4 9 ) , o t - h y d r o x y a c i d s ( 3 9 ) , e s t e r s , b e n z y l s u l p h o x i d e s ( 2 1 ) , azoxy compounds ( 2 0 ) , n i t r i t e s ( 4 9 ) , n i t r o s t e r o i d s ( 5 0 ) , and o(-amino e s t e r s ( 1 8 ) . 16 C. INTENSITY CHANGE. D. INDUCED CD. FIGURE 6. ENVIRONMENTAL CD EFFECTS . 17. Table I Some Environmental E f f e c t s Seen i n CD Spectra Compound E f f e c t Postulated Cause Ref trans-2-chloro-5-methyl-cyclohexanone double-humped peak conformational 28 menthone double-humped peak conformational 28 2-oxo-1-p-menthanol double-humped peak conformational 48 3/5-acetoxyhexanordammar-20-one i n t e n s i t y change with AT conformational -r o t a t i o n a l isom-erism 22 isofenchone double-humped peak asymmetric s o l -vation 25 epiisofenchone double-humped peak asymmetric s o l -vation 25 5-hydroxy-epiisofenchone double-humped peak asymmetric s o l -vation 2 5 camphenylone double-humped peak asymmetric s o l -vation ' 25 K-acetoxy-camphor double-humped peak asymmetric s o l -vation 25 oC-hydroxy-epiisof enchone double-humped peak asymmetric s o l -vation 25 2a-methyl-adamantanone sign reversal solvent e f f e c t 19 l a c t i c acid double-humped peak conformati onal ( r o t a t i o n a l isomerism) 39 42 camphor i n t e n s i t y change with AT asymmetric s o l -vation 43 L-malic acid double-humped peak conformational ( r o t a t i o n a l isomerism) 39 1 8 . T a b l e I c o n t d . Some E n v i r o n m e n t a l E f f e c t s Seen I n CD S p e c t r a Compound E f f e c t P o s t u l a t e d Cause i ief. d i m e t h y l - L ( - ) - m a l a t e doub le-humped peak c o n f o r m a t i o n a l 39 e t h y l - L ( + ) - l a c t a t e doub le-humped peak c o n f o r m a t i o n a l 4o ( * -pu legone o x i d e and /3-pu legone o x i d e doub le-humped peak and s i g n r e v e r s a l c o n f o r m a t i o n a l and s o l v a t i o n e q u i l i b r i a 17 s(-amino e s t e r s doub le-humped peak c o n f o r m a t i o n a l 18 LL-BH8?2o( and e l a i o m y c i n ( a z o x y chromophore C-g=N-C) doub le-humped peak s o l v a t i o n e q u i l i b r i u m 20 b e n z y l m e t h y l s u l p h o x i d e s i g n r e v e r s a l c o n f o r m a t i o n a l ( r o t a t i o n a l i s o m e r i s m ) 2 1 < < - c h l o r o - b e n z y l m e t h y l s u l p h o x i d e s i g n r e v e r s a l i n 220 nm t r a n s i t i o n c o n f o r m a t i o n a l e q u i l i b r i u m a n d / o r a s y m m e t r i c s o l v a t i o n o f a r o m a t i c chromo-phore 21 E t i a n i c a c i d s and e s t e r s i n t e n s i t y change w i t h temp. c o n f o r m a t i o n a l 34 c i s - ( S ) - 4 - m e t h y l — 2 - h e x e n e and t r a n s - ( S ) - 4 - m e t h v l - 2 -hexene double-humped peak c o n f o r m a t i o n a l 35 N - d i t h i o c a r b e t h o x y - L -a s p a r t i c a c i d s i g n r e v e r s a l w i t h s o l v e n t c o n f o r m a t i o n a l ( r o t a t i o n a b o u t C-0 bond) 49 c h o l e s t a n - 3 £ - o l m e t h y l x a n t h a t e s i g n r e v e r s a l w i t h AT c o n f o r m a t i o n a l ( r o t a m e r e q u i l -i b r i u m ) 49 19'• T a b l e I c o n t d . Some E n v i r o n m e n t a l E f f e c t s Seen I n CD S p e c t r a Compound E f f e c t P o s t u l a t e d Cause Ref N - p h t h a l o y l - 2 0 r t - a m i n o - 5 t f -p r e g n a n - 3 £ - o l shape change w i t h s i g n r e -v e r s a l o f one peak w i t h AT c o n f o r m a t i o n a l o r s o l v a t i o n ' : e q u i l i b r i a 49 c h o l e s t a n - X - o l n i t r i t e s i g n r e v e r s a l w i t h A T r o t a t i o n a l i s o -mer ism 4-9 7<*-chloro-70 - n i t r o - 5 < r t -c h o l e s t a n e s i g n r e v e r s a l w i t h AT r o t a t i o n a l i s o -mer ism 50 2 0 ; The p r o b l e m i n many cases i s t o s e p a r a t e p u r e c o n f o r -m a t i o n a l f r o m p u r e s o l v a t i o n e f f e c t s . By c o n f o r m a t i o n a l e f -f e c t , i t i s meant t h a t t h e changes i n t h e CD s p e c t r u m r e s u l t f r o m changes i n e i t h e r t h e c o n f o r m a t i o n o f t h e who le m o l e c u l e ( e g . m e n t h o n e ) , o r changes i n t h e c o n f o r m a t i o n o f j u s t t h e ch romophore . The r o t a t i o n o f t h e n i t r o g r o u p i n n i t r o s t e r -o i d s i s an example o f t h i s l a t t e r t y p e o f c o n f o r m a t i o n a l e f -f e c t . Even t h o u g h a change o f s o l v e n t may a l t e r t h e e q u i l i b -r i u m between two c o n f o r m e r s i n s o l u t i o n , i t i s s t i l l c l a s s i -f i e d as a c o n f o r m a t i o n a l e f f e c t - an i n t r a m o l e c u l a r e n v i r o n -m e n t a l e f f e c t . A CD s o l v e n t e f f e c t , on t h e o t h e r h a n d , i s d e f i n e d i n t h i s t h e s i s as a v a r i a t i o n i n t h e CD s p e c t r u m due t o t h e s o l v e n t a l o n e , a p a r t f r o m any c o n f o r m a t i o n a l change. I n t h i s c a s e , t h e s o l v e n t m o l e c u l e s a f f e c t t h e m o l e c u l a r o r -b i t a l s o f t h e chromophore so as t o change t h e i r d i s s y m m e t r i c p e r t u r b a t i o n , o r t o change t h e e n e r g y r e q u i r e m e n t s o f t h e t r a n s i t i o n . Pure s o l v e n t e f f e c t s a r e most e a s i l y d e t e c t e d i n r i g i d compounds. A who le s e r i e s o f e f f e c t s have been r e p o r t e d f o r t h e r i g i d b i c y c l o [ 2 , 2 , l ] hep tanones ( 2 5 ) ( F i g u r e 6 - B ) . I n t h e case o f 2 a - m e t h y l - a d a m a n t a n o n e , no c o n f o r m a t i o n a l f reedom i s c o n c e i v e d , y e t on c h a n g i n g f r o m d i o x a n e t o i s o -o c t a n e , t h e comp le te CD s p e c t r u m i s r e v e r s e d , f i n e s t r u c t u r e and a l l ( F i g u r e 6 - A ) . T h i s can t h e r e f o r e be a t t r i b u t e d t o a p u r e s o l v e n t e f f e c t - an i n t e r m o l e c u l a r e n v i r o n m e n t a l e f f e c t . 21. In compounds which have conformational mobility, any solvent e f f e c t w i l l be superimposed upon the conformational e f f e c t . These conformational e f f e c t s occur because the CD band i s very s e n s i t i v e to population v a r i a t i o n . The CD spectrum i s a population-weighted average of the r o t a t i o n a l contributions of the separate species i n sol u t i o n . One of the most commonly encountered types of spectra f o r conformationally f l e x i b l e molecules i s the double-humped curve exemplified by the CD spectrum of (-)-menthone shown i n Figure 7» Figure 7« CD of (-)-menthone i n decalin at +25*C, - 7 4 eC and +l62*C (A) and i n various solvents at 25*C (B). Djerassi et a l . (28) showed that a complex CD curve with two oppositely signed extrema could a r i s e whenever two Cotton e f f e c t s of s i m i l a r amplitudes, but opposite sign, are 22. super imposed w i t h t h e i r i n d i v i d u a l maxima s e p a r a t e d by 1 t o 20 nm. I n t h e k e t o n e s s t u d i e d , t h e two e x t r e m a were s e p -a r a t e d by a b o u t 30 nm. I n o t h e r w o r d s , t h e r e were a t l e a s t two d i f f e r e n t s p e c i e s i n s o l u t i o n , and each s p e c i e s abso rbed a t a s l i g h t l y d i f f e r e n t w a v e l e n g t h w i t h CD peaks o f oppo-s i t e s i g n ( F i g u r e 8 ) . The r e s u l t a n t CD c u r v e i s a d o u b l e -humped one . . F i g u r e 8 . G e n e r a t i o n o f a doub le-humped CD c u r v e . F o r a s e p a r a t i o n (A,-A^of a b o u t 1 nm f o r peaks o f s i m i l a r a m p l i t u d e , t h e r e s u l t i n g a p p a r e n t r o t a t i o n a l s t r e n g t h s a r e o n l y a b o u t 1/25 o f t h o s e o f t h e c o n t r i b u t i n g s p e c i e s . I f t h e r e i s an e q u i l i b r i u m between two c o n f o r m e r s , t h e r o t a t i o n a l s t r e n g t h a t t e m p e r a t u r e T i s g i v e n by R T = ( R A - R b ) ( l + e x p ( - A G ° / N k T ) ) " 1 + R F E where R A and R ^ a r e t h e r o t a t i o n a l s t r e n g t h s o f t h e i n d i v i -d u a l c o n f o r m e r s , A G * i s t h e f r e e e n e r g y d i f f e r e n c e between t h e 2 3 . c o n f o r m e r s , k i s B o l t z m a n ' s c o n s t a n t , and N i s A v o g a d r o ' s number . By p l o t t i n g R T a g a i n s t l + e x p ( - a G ° / N k T ) - 1 f o r d i f -f e r e n t v a l u e s o f AG*, a f a m i l y o f c u r v e s w i l l be o b t a i n e d , o n l y one o f w h i c h w i l l be a s t r a i g h t l i n e , c o r r e s p o n d i n g t o t h e c o r r e c t AG°. The i n t e r c e p t w i l l be R^. T h e r e f o r e , by t h i s g r a p h i c a l m e t h o d , AG0, R & and can be o b t a i n e d and t h e e q u i l i b r i u m c o n s t a n t can be e a s i l y c a l c u l a t e d . I f an e q u i l i b r i u m i s e s t a b l i s h e d be tween two c o n f o r m e r s i n s o l u t i o n , t h i s e q u i l i b r i u m i s e x p e c t e d t o be a f f e c t e d ; by b o t h s o l v e n t and t e m p e r a t u r e . So i t i s i n t h e case o f ( - ) -menthone , and many o t h e r compounds. ( - ) - m e n t h o n e ( d i e q u a t o r i a l ) ( - ) - m e n t h o n e ( d i a x i a l ) F i g u r e 7 shows t h e changes i n t h e CD c u r v e o f ( - ) - m e n t h o n e as s o l v e n t i s changed, t h i s p r o b a b l y r e f l e c t i n g t h e change i n t h e c o n f o r m e r p o p u l a t i o n s . F i g u r e 7 a l s o shows t h e e f f e c t o f t e m p e r a t u r e change. I t wou ld be e x p e c t e d t h a t t h e e q u i -l i b r i u m w o u l d be s h i f t e d t o t h e s i d e o f t h e more s t a b l e c o n f o r m e r as t h e t e m p e r a t u r e i s l o w e r e d . I n EPA s o l v e n t a t - 1 9 2 * C , ( - ) - m e n t h o n e has an i n t e n s e p o s i t i v e CD c u r v e ; no n e g a t i v e hump was d e t e c t e d , i n d i c a t i n g t h e more s t a b l e c o n f o r m e r had been f r o z e n o u t . I n n o n p o l a r s o l v e n t s , a x i a l 2 4 . o r b o a t l i k e f o r m s somet imes p r e d o m i n a t e ( 2 9 ) . T r a n s - 2 -c h l o r o - 5 - n i e t h y l c y c l o h e x a n o n e i s a n o t h e r example o f t h i s t y p e o f e q u i l i b r i u m ( 2 7 ) . ( + ) CD i n p o l a r s o l v e n t s ( - ) CD i n n o n p o l a r s o l v e n t s I n p o l a r s o l v e n t s , t h e d i e q u a t o r i a l c o n f o r m e r p r e d o m i n a t e s , p r o d u c i n g a p o s i t i v e CD b a n d , w h i l e t h e d i a x i a l c o n f o r m e r p r e d o m i n a t e s i n a n o n p o l a r medium, p r o d u c i n g a n e g a t i v e CD b a n d . The a c t u a l i n t e r a c t i o n mechanism c o u l d i n v o l v e a d i -p o l a r i n t e r a c t i o n o f t h e s o l u t e m o l e c u l e w i t h t h e f i e l d i n -duced by t h e s o l v e n t m o l e c u l e s , w h i c h c o u l d change t h e e q u i -l i b r i u m o f t h e c o n f o r m e r s , and t h e r e b y change t h e CD spec -t r u m . S i m i l a r double-humped CD c u r v e s a r e s e e n , f o r example , i n «A-amino e s t e r s ( 1 8 ) , l a c t i c and m a l i c a c i d ( 3 9 ) , and e t h y l - L ( + ) - l a c t a t e . S ( + ) - l a c t i c a c i d has bands a t 210 and 240 nm w h i c h have been a t t r i b u t e d t o t h e r o t a m e r s A and B r e s p e c t i v e l y ( 2 3 ) . 2 5 . 0 . O H H 0 V 0 V ' x / H O — C — H H 0 - — P — H 6H- 6* more s t a b l e S ( + ) - L a c t i c A c i d Double-humped peaks a r e a l s o seen i n c o n f o r m a t i o n a l l y  r i g i d m o l e c u l e s and a r e a s c r i b e d t o s o l v a t i o n e q u i l i b r i a ( F i g u r e 6 B ) . Some a u t h o r s have e x p l a i n e d t h i s phenomenon as an e q u i l i b r i u m between s o l v a t e d and u n s o l v a t e d s p e c i e s ( 2 4 - 2 7 ) . s o l u t e + s o l v e n t ^ ,T [so l u t e - s o l v e n t ] The s h o r t - w a v e l e n g t h band has been a s s i g n e d t o t h e s o l v a t e d s p e c i e s w h i l e t h e l o n g - w a v e l e n g t h band has been a t t r i b u t e d t o t h e u n s o l v a t e d s p e c i e s . As t h e t e m p e r a t u r e o f t h e sys tem i s l o w e r e d , t h e band due t o s o l v a t e d s p e c i e s t h e n s h o u l d grow a t t h e expense o f t h e band a s s o c i a t e d w i t h t h e u n s o l v a t e d s p e c i e s . W i t h t h e more p o l a r s o l v e n t s (where t h e r e w o u l d be g r e a t e r a s s o c i a t i o n o f s o l u t e and s o l v e n t ) one wou ld e x p e c t t h e s o l v a t e d band t o be l a r g e r . Thus b o t h c o n f o r m a t i o n a l and s o l v a t i o n e q u i l i b r i a can 2 6 . p roduce a doub le-humped CD s p e c t r u m . U n l e s s t h e m o l e c u l e has a r i g i d c o n f o r m a t i o n , i t i s v e r y d i f f i c u l t t o d i s t i n g u i s h between s o l v a t i o n and c o n f o r m a t i o n a l e q u i l i b r i a . One c a n -n o t a l w a y s a rgue t h a t c o n f o r m a t i o n a l e f f e c t s w i l l be much l a r g e r t h a n s o l v a t i o n e f f e c t s . The a m p l i t u d e s o f t h e s p e c t r a o f ( - ) - m e n t h o n e and e p i i s o f e n c h o n e a r e s i m i l a r , y e t t h e s p e c t r u m o f ( - ) - m e n t h o n e has been e x p l a i n e d v i a a c o n f o r -m a t i o n a l e q u i l i b r i u m w h i l e t h a t o f e p i i s o f e n c h o n e was a t t r i b u t e d t o a s o l v a t i o n e q u i l i b r i u m . However, D j e r a s s i e t a l . ( 2 8 ) a l s o s t a t e t h a t s o l v e n t e f f e c t s a r e o p e r a t i v e w i t h ( - ) - m e n t h o n e a t l o w e r t e m p e r a t u r e s . There a r e a number o f p o s s i b l e ways t o d i s t i n g u i s h between t h e two e f f e c t s . One wou ld p o s t u l a t e c o n f o r m a t i o n a l e q u i l i b r i a i f t h e low t e m p e r a t u r e CD measurements were i n c o n s i s t e n t w i t h t h e s o l v a t i o n h y p o t h e s i s t h a t t h e s h o r t e r - w a v e l e n g t h band s h o u l d i n c r e a s e a t t h e expense o f t h e l o n g e r - w a v e l e n g t h band as t h e t e m p e r a t u r e i s l o w e r e d . An e x a m i n a t i o n o f F i g u r e 7 w i l l show t h a t t h i s was t h e r e a s o n a c o n f o r m a t i o n a l e q u i l i b -r i u m was p o s t u l a t e d f o r ( - ) - m e n t h o n e . A n o t h e r p o s s i b l e method f o r d i s t i n g u i s h i n g between t h e two e q u i l i b r i a i s t o examine t h e CD s p e c t r u m o f t h e v a p o u r . I f doub le-humped peaks a r e seen i n t h e v a p o u r , t h e n t h e s o l v e n t i s n o t t h e s o l e cause and c o n f o r m a t i o n a l e q u i l i b r i a must a l s o be i n v o l v e d . I t i s a l s o p o s s i b l e f o r b o t h c o n -f o r m a t i o n a l and s o l v a t i o n e q u i l i b r i a t o be o p e r a t i v e . Two e q u i l i b r i a can be d e t e c t e d t h r o u g h v a r i a b l e t e m p e r a t u r e 2 7 . s t u d i e s . I f t h e CD band passes t h r o u g h a maximum and t h e n b e g i n s t o d e c r e a s e a g a i n , t h i s i s e v i d e n c e f o r b o t h c o n -f o r m a t i o n a l and s o l v e n t e f f e c t s . D j e r a s s i e t a l . have a l s o shown t h a t t h e degree o f s o l -v a t i o n does n o t a l w a y s r e m a i n c o n s t a n t o v e r t h e l o w - t e m p -e r a t u r e r a n g e ( 2 8 ) . 5 ^ - C h o l e s t a n - 3 - o n e seems t o show i n -c r e a s e d s o l v a t i o n a t low t e m p e r a t u r e . F o r a t h o r o u g h i n v e s t i g a t i o n , one s h o u l d n o t e n t i r e l y r u l e o u t t h e p o s s i b i l i t y o f s e l f - a s s o c i a t i o n ( 1 7 ) . By m o l e -c u l a r w e i g h t d e t e r m i n a t i o n s and by an a n a l y s i s o f CD, ORD and UV s p e c t r a o v e r t h e same t e m p e r a t u r e r a n g e , one c o u l d p o s s i b i l y d e c i d e i f s e l f - a s s o c i a t i o n i s an i m p o r t a n t f a c t o r . S e l f - a s s o c i a t i o n s h o u l d become a more d o m i n a n t f a c t o r a t h i g h c o n c e n t r a t i o n s . The second t y p e o f e n v i r o n m e n t a l e f f e c t i s a c o m p l e t e r e v e r s a l o f t h e CD b a n d . T h i s o c c u r s , f o r examp le , i n some k e t o n e s ( 1 9 ) , i n b e n z y l m e t h y l s u l p h o x i d e and i n o c - c h l o r o b e n z y l m e t h y l s u l p h o x i d e ( 2 1 ) . The r i g i d 2 a - m e t h y l - a d a m a n -t a n o n e-4- ( I S ) has a r e l a t i v e l y weak p o s i t i v e CD i n d i o x a n e ( a n t i o c t a n t b e h a v i o u r ) . I t s most i n t e n s e peak i s AC+0.093 (303nm). On g o i n g t o t h e n o n p o l a r s o l v e n t , i s o o c t a n e , t h e c o m p l e t e CD s p e c t r u m (340-235nm) i s r e v e r s e d , s t i l l s h o w i n g v i b r a t i o n a l s t r u c t u r e . A g a i n , t h e CD s p e c t r u m was r e l a t i v e l y weak (most i n t e n s e b a n d , AG - 0 . 0 5 2 (300nm).). However , t h i s b e h a v i o u r was n o t seen i n t h e o t h e r adamantanones t h a t S n a t z k e and h i s c o - w o r k e r s (19) s t u d i e d . S i n c e t h e adaman-2 8 . tanone has a r i g i d s t r u c t u r e , t h e e f f e c t must be due t o t h e s o l v e n t a l o n e . A second example o f a comp le te r e v e r s a l o f t h e CD spec t r u m i s b e n z y l m e t h y l s u l p h o x i d e , i n w h i c h a r o m a t i c t r a n s -i t i o n s and t h e n-TT* t r a n s i t i o n o f t h e s u l p h o x i d e were o b -s e r v e d . V i b r a t i o n a l s t r u c t u r e was a l s o seen i n one o f t h e a r o m a t i c t r a n s i t i o n s ( 2 5 0 - 2 7 0 n m ) . S i n c e f r e e r o t a t i o n e x -i s t s a b o u t t h e C-C and C-S bonds , t h e r e v e r s a l o f t h e s p e c -O II C H 2 - £ j S — C H 3 t r u m i n g o i n g f r o m e t h a n o l t o i s o o c t a n e was a t t r i b u t e d t o t h e e x i s t e n c e o f d i f f e r e n t p r o p o r t i o n s o f r o t a m e r s i n s o l -u t i o n . The a r o m a t i c 2 5 0 - 2 7 0 nm t r a n s i t i o n was n o t r e v e r s e d f o r b e n z y l t - b u t y l s u l p h o x i d e i n w h i c h t h e r e wou ld be more s t e r i c h i n d r a n c e t o i n t e r n a l r o t a t i o n . T h e r e f o r e , b o t h s o l v e n t and c o n f o r m a t i o n a l e f f e c t s can g i v e r i s e t o t h e doub le-humped CD s p e c t r u m and t h e comple te r e v e r s a l o f a CD s p e c t r u m . The most r e c e n t l y i n v e s t i g a t e d s o l v e n t e f f e c t i s t h a t o f i n d u c e d c i r c u l a r d i c h r o i s m where an i n a c t i v e s u b -s t a n c e , S, ( e i t h e r a s y m m e t r i c a l m o l e c u l e o r a r a c e m i c m i x t u r e ) i s d i s s o l v e d i n a c h i r a l s o l v e n t , C, and t h e t r a n s -i t i o n o f t h e s u b s t r a t e becomes o p t i c a l l y a c t i v e ( 3 6 - 3 8 ) . I n o t h e r w o r d s , c h i r a l i t y has been i n d u c e d by t h e s o l v e n t , 2 9 . C, i n t h e s u b s t r a t e , S. I t was conc luded t h a t t h i s e f f e c t does n o t r e q u i r e s p e c i f i c b o n d i n g be tween C and S ( 3 8 ) . T h e o r i e s o f S o l v e n t E f f e c t s W i t h t h e c u r r e n t i n t e r e s t i n i n d u c e d c i r c u l a r d i c h r o i s m , where t h e CD s p e c t r u m o f an a c h i r a l s u b s t r a t e i s r e c o r d e d i n a c h i r a l s o l v e n t , more a t t e n t i o n has been d e v o t e d t o an e x a m i n a t i o n o f t h e p o s s i b l e mechanisms o f s o l u t e - s o l v e n t i n t e r a c t i o n s and how s o l v e n t s a f f e c t CD s p e c t r a . The t h e o r y o f an e q u i l i b r i u m between a s o l v a t e d and u n s o l v a t e d s p e c i e s i s one o f t h e more common e x p l a n a t i o n s . Some such e q u i l i b r i a have been n o t e d , such as t h e one r e -p o r t e d by R i t c h i e and P r a t t (30) f o r complex f o r m a t i o n o f n i t r i l e s and k e t o n e s w i t h d i m e t h y l s u l p h o x i d e . They a l s o s t a t e t h a t t h e b u l k o f t h e s o l v e n t need n o t a p -p r o a c h c l o s e l y t o t h e b u l k o f t h e s o l u t e s p e c i e s . Such complex f o r m a t i o n m i g h t have i m p o r t a n t e f f e c t s on t h e c o n -f o r m a t i o n s o f t h e s o l u t e . I n a d d i t i o n , t h e complex f o r -m a t i o n m i g h t be e x p e c t e d t o be q u i t e s e n s i t i v e t o s t e r i c e f f e c t s . T h i s c o u l d cause a marked e f f e c t i n t h e CD spec -I s 1 complex 30. t r u m . Complex f o r m a t i o n w i t h camphor has even been r e p o r t e d K l y n e , K i r k and W a l l i s ( 2 9 ) have p r o p o s e d two h y p o -t h e s e s f o r s o l v e n t e f f e c t s ( n o t e : t h e s e a r e c a l l e d h y p o -t h e s e s , n o t t h e o r i e s ) . The f i r s t h y p o t h e s i s i s t h a t o f d i s s y m m e t r i c s o l v a t i o n . I n t h i s h y p o t h e s i s , t h e e f f e c t i s due t o t h e t i m e - a v e r a g e d a r r a n g e m e n t o f s o l v e n t m o l e c u l e s a r o u n d t h e s o l u t e m o l e c u l e . The s o l v e n t cage t h e n p e r -t u r b s t h e s o l u t e ch romophore . F o r n o n - a s s o c i a t i n g m o l e -c u l e s ( n o n p o l a r ) , t h e p e r t u r b a t i o n o f t h e chromophore i s o n l y due t o t h e s o l u t e m o l e c u l e i t s e l f , w h i l e f o r p o l a r a s s o c i a t i n g m o l e c u l e s , t h e p e r t u r b a t i o n i s a l s o due t o t h e d i s s y m m e t r y o f s o l v e n t cage a r o u n d t h e s o l u t e m o l e c u l e . F o r a c h i r a l m o l e c u l e , t h e r e w i l l u s u a l l y be a p r e f e r e n c e o f a s s o c i a t i o n f r o m one s i d e o f t h e m o l e c u l e . The r e s u l t s h o u l d b e , as K l y n e e t a l . ( 2 9 ) s t a t e , " t h e m a g n i t u d e o f t h e s o l v e n t e f f e c t s h o u l d be g r e a t e s t f o r t h o s e compounds where one s i d e o f t h e c a r b o n y l g r o u p i s much more h i n d e r e d b y s u b s t i t u e n t s t h a n t h e o t h e r ; t h e d i s s y m m e t r y w o u l d t h e n be l a r g e r . " Thus t h e r e s h o u l d be a g r e a t e r s o l v e n t e f f e c t f o r f e n c h o n e t h a n f o r n o r b o r n a n o n e . f o r t h e n o n p o l a r s o l v e n t s CCl^ and c y c l o h e x a n e ( 3 1 ) . s i d e o f c a r b o n y l more h i n d e r e d i n fenchone by g e m i n a l m e t h y l g r o u p s . 31. However , i n t h e compounds s t u d i e d , t h e y f o u n d no t r e n d o f t h i s n a t u r e . I n t h e i r second h y p o t h e s i s , " s o l v e n t - d e p e n d e n t g r o u p i n c r e m e n t s " , t h e s o l v e n t e f f e c t r e s u l t s f r o m t h e p e r t u r -b a t i o n o f t h e chromophore by t h e s o l u t e m o l e c u l e i t s e l f w h i c h i s , i n t u r n , p e r t u r b e d by t h e s o l v e n t . Thus t h i s p r o -posed i n t e r a c t i o n i s be tween t h e chromophore and v a r i o u s s o l v e n t - i n f l u e n c e d d i s s y m m e t r i c f e a t u r e s o f t h e c a r b o n y l compound. T h i s c o u l d be the dominan t f a c t o r when t h e c a r b o n y l g roup i s i n an u n h i n d e r e d e n v i r o n m e n t . However, t h e r e was no d e f i n i t i o n o f a s p e c i f i c s o l u t e - s o l v e n t i n t e r -a c t i o n p a t h . These a r e j u s t " h y p o t h e s e s " , and s t i l l r e -q u i r e a d d i t i o n a l s t u d y . I f t h e s o l v e n t m o l e c u l e approached t h e s o l u t e , i t c o u l d d i s s y m m e t r i c a l l y p e r t u r b t h e chromophore . The mech-an ism o f p e r t u r b a t i o n c o u l d i n v o l v e t h e f i e l d i n d u c e d by t h e s o l v e n t w h i c h d i s t o r t s t h e s o l u t e m o l e c u l e , and t h i s d i s t o r t i o n wou ld be p r o p o r t i o n e d t o t h e f i e l d s t r e n g t h (44). Of c o u r s e , any d i s t o r t i o n o f t h e chromophore o r s o l u t e m o l e c u l e w i l l e f f e c t i t ' s o p t i c a l a c t i v i t y . S ince t h e r e i s a g r e a t v a r i a t i o n o f CD w i t h s o l v e n t i n r i g i d m o l e c u l e s , V e l l u z e t a l . ( 4 4 ) have s u g g e s t e d t h a t t h e s o l v e n t e f f e c t r e s u l t s f r o m a d i r e c t v i c i n a l a c t i o n o f t h e s o l v e n t on t h e ch romophore . A c t u a l l y , d i p o l e f o r c e s a r e t h o u g h t t o be t h e p e r t u r b i n g f i e l d w i t h i n t h e f ramework o f t h e o n e - e l e c t r o n t h e o r y o f o p t i c a l a c t i v i t y . 32. I f t h e n a t u r e o f s o l u t e - s o l v e n t i n t e r a c t i o n were m a i n l y e l e c t r o s t a t i c , t h e n one w o u l d e x p e c t some c o r r e l a t i o n w i t h t h e d i e l e c t r i c c o n s t a n t s and Z v a l u e s o f s o l v e n t s . I f t h e m o l e c u l e i s c o n s i d e r e d a r i g i d s p h e r e , t h e n i t can be shown t h a t t h e r o t a t i o n a l s t r e n g t h s h o u l d v a r y l i n e a r i l y w i t h "a 9 K - 1 d K + 2 where M 0 i s t h e d i p o l e moment o f t h e s o l u t e , K i s t h e d i e l e c -t r i c c o n s t a n t and d i s t h e mean d i s t a n c e between the c h i r a l s o l u t e m o l e c u l e and t h e s o l v e n t m o l e c u l e s i n a d i l u t e s o l -u t i o n ( 4 4 ) . I f t h e i n t e r a c t i o n i s e l e c t r o s t a t i c , t h e r e s h o u l d be a l i n e a r r e l a t i o n between Afi and K - l / K + 2 . There does seem t o be such a r e l a t i o n s h i p f o r many s t e r o i d a l k e -t o n e s ( 4 4 ) . T h i s c o r r e l a t i o n i s n o t g e n e r a l , however . Wel lman e t a l . ( 2 8 ) , were n o t a b l e t o c o r r e l a t e Ag o f 5oC-c h o l e s t a n - 3 - o n e and 5 « - a n d r o s t a n - l 6 - o n e w i t h d i e l e c t r i c c o n s t a n t o r s o l v e n t p o l a r i t y (Z v a l u e ) . On t h e o t h e r h a n d , Rassa t and Coulombeau (25,45) d i d c o r r e l a t e t h e n e g a t i v e CD min ima o f i s o f e n c h o n e w i t h t h e p o l a r i t y o f t h e s o l v e n t (Z v a l u e ) . One i s n o t , a t t h i s p o i n t , concerned w i t h f r e q u e n c y s h i f t s o f t h e n - rr* t r a n s i t i o n s i n t h e UV a b s o r p t i o n s p e c t r a w h i c h have been c o r r e l a t e d w i t h d i e l e c t r i c c o n s t a n t s . Here 33 t h e more p o l a r s o l v e n t s r e s u l t i n s t a b i l i z a t i o n o f t h e g round s t a t e due t o e l e c t r o s t a t i c i n t e r a c t i o n s be tween s o l u t e and s o l v e n t m o l e c u l e s and r e s u l t i n a b l u e s h i f t . I n t h e CD o f i s o f e n c h o n e , t h e r e were no a p p r e c i a b l e f r e q u e n c y s h i f t s , b u t o n l y t h e c h a n g i n g o f i n t e n s i t i e s o f t h e doub le-humped peaks* t h e s e i n t e n s i t y changes were c o r r e l a t e d t o Kosower*s Z v a l u e s . Wel lman e t a l . ( 2 8 ) , f o u n d o n l y a r o u g h c o r -r e l a t i o n between t h e r o t a t i o n a l s t r e n g t h o f ( - ) - m e n t h o n e and Z v a l u e s . Weigang ( 4 6 ) a l s o s t u d i e d t h e s o l v e n t f i e l d c o r r e c t i o n s f o r r o t a t o r y s t r e n g t h and f o u n d t h a t t h e r e a r e nonze ro c o r -r e c t i o n s even f o r r a n d o m l y o r i e n t e d s o l u t e and s o l v e n t m o l e -c u l e s . The s o l v e n t f i e l d c o r r e c t i o n f a c t o r n o t o n l y i n -v o l v e d t h e L o r e n t z f i e l d c o r r e c t i o n w h i c h i s a f u n c t i o n o f t h e r e f r a c t i v e i n d e x o f t h e s o l v e n t , b u t a l s o c o n t a i n e d a f u n c t i o n o f t h e i n d u c t i v e and d i p o l a r s o l u t e - s o l v e n t i n t e r -a c t i o n s w h i c h i n v o l v e d a t e r m i n c l u d i n g t h e d i e l e c t r i c c o n -s t a n t o f t h e s o l v e n t . F o r a r a n d o m l y o r i e n t e d s o l u t e - s o l -v e n t s y s t e m , t h e e f f e c t o f t h e s o l v e n t f i e l d due t o t h e s t a t i c d i p o l e moment o f t h e s o l v e n t i s t o p r e d i c t a dec rease i n o p t i c a l a c t i v i t y . Because o f t h e c o m p l e x i t y o f t h e p r o -b l e m , Weigang c o u l d n o t p l a c e t h e s o l v e n t f i e l d c o r r e c t i o n on a p u r e l y q u a n t i t a t i v e f o o t h o l d . H i s c o n s i d e r a t i o n s i n -v o l v e d a p e r t u r b a t i o n by s o l v e n t m o l e c u l e s w h i c h were r a n -domly o r i e n t e d . From t h e p r e v i o u s c o n s i d e r a t i o n s p r e s e n t e d c o n c e r n i n g d i s s y m m e t r i c o r i e n t a t i o n o f s o l v e n t m o l e c u l e s a b o u t a c h i r a l m o l e c u l e , i t may v e r y w e l l be t h a t t h e s o l -3*. v e n t m o l e c u l e s a r e n o t r a n d o m l y o r i e n t e d . N o n e t h e l e s s , t h e s o l v e n t f i e l d c o r r e c t i o n w i l l i n v o l v e b o t h t h e d i e l e c t r i c c o n s t a n t and t h e r e f r a c t i v e i n d e x o f t h e s o l v e n t . T h e r e f o r e , i t can be seen why c o r r e l a t i o n s o f t h e CD band i n t e n s i t y w i t h a d i e l e c t r i c c o n s t a n t o r r e f r a c t i v e i n d e x f u n c t i o n a r e u s u a l l y c o n s i d e r e d , even t h o u g h such c o r r e l a t i o n s may n o t a l w a y s be f o u n d . Weigang has p r o p o s e d an a l t e r n a t i v e p a t h i n w h i c h t h e s o l v e n t c o u l d a f f e c t t h e CD s p e c t r u m . T h i s i n v o l v e s t h e deg ree o f e l e c t r o n i c - v i b r a t i o n a l c o u p l i n g ( 3 2 ) . V i b r o n i c components o f t h e m a g n e t i c moment can make i m p o r t a n t c o n -t r i b u t i o n s t o o p t i c a l a c t i v i t y , and v i b r a t i o n a l f r e q u e n c i e s a r e known t o be s e n s i t i v e t o t h e n a t u r e o f t h e s o l v e n t . S t i g t e r and S c h e l l m a n ( 3 3 ) v o i c e a p o s s i b l e s o l v e n t e f f e c t r e l a t e d t o t h e p a c k i n g and o r i e n t a t i o n s t r a i n i n t h e s o l v e n t e f f e c t s o f i s o t r o p i c a b s o r p t i o n bands . T h i s mech-a n i s m c o u l d p o s s i b l y i n v o l v e a d i r e c t p e r t u r b a t i o n o f t h e w a v e - f u n c t i o n s o f t h e c h r o m o p h o r e . A g a i n , c l o s e c o n t a c t be tween s o l u t e and s o l v e n t m o l e c u l e s w o u l d be p o s t u l a t e d . U n t i l t h e p r e s e n t , no s y s t e m a t i c s t u d y o f t h e e n v i r o n -m e n t a l e f f e c t s on t h e CD s p e c t r u m o f n i t r a t e e s t e r s has been made. 3 5 CD Of N i t r a t e E s t e r s Up u n t i l 1 9 7 1 , t h e r e has been no p a p e r p u b l i s h e d c o r -r e l a t i n g t h e c o n f i g u r a t i o n o f t h e n i t r a t e e s t e r s w i t h t h e s i g n s o f t h e i r CD s p e c t r a . There has appeared no r u l e a n a l o g o u s t o t h e " o c t a n t r u l e " f o r k e t o n e s . The f i r s t s t u d y o f t h e o p t i c a l r o t a t o r y d i s p e r s i o n o f n i t r a t e e s t e r s was r e p o r t e d i n 1 9 6 5 by T s u z u k i et a l . ( 5 1 ) . T h e i r f i r s t p a p e r was r e s t r i c t e d t o t h e ORD o f mono and d i -n i t r a t e s o f g l u c o p y r a n o s e . Even f r o m t h e f i r s t p a p e r one c o u l d see how l i m i t e d ORD was i n e x a m i n i n g t h e n i t r a t o chromophore . They were a b l e t o p e n e t r a t e o n l y t o 2 6 5 nm. These suga r n i t r a t e s had s t r o n g background r o t a t i o n , and i n most c a s e s , s h o u l d e r s o r i n f l e c t i o n s were t h e o n l y e f -f e c t seen a round 3 0 0 nm. L a t e r t h e y made a c o r r e l a t i o n between t h e c o n f i g u r a t i o n and t h e s i g n o f t h e C o t t o n e f f e c t (due t o t h e n i t r a t o g r o u p ) o f hexose and g l u c o s e n i t r a t e s ( 5 2 , 5 3 ) . They f o u n d , i r r e s p e c t i v e o f t h e p o s i t i o n o f t h e n i t r a t o g roup on t h e r i n g , © ( - g l y c o s i d e s w i t h a C-0N0 2 chromophore o f an O C c o n f i g u r a t i o n ; showed n e g a t i v e C o t t o n e f f e c t s , w h i l e t h o s e w i t h a C-0N0 2 g r o u p o f a ^ c o n f i g u r a t i o n e x h i b i t p o s i t i v e C o t t o n e f f e c t s . The s i g n s o f t h e C o t t o n e f f e c t s o f ^ - g l y c o s i d e s were r e v e r s e d . T h e i r c o n c l u s i o n s a r e summarized i n T a b l e I I . 3&. T a b l e I I C o n f i g u r a t i o n And The S i g n s Of The C o t t o n E f f e c t Of N i t r a t e s Of G l u c o s i d e s C o n f i g u r a t i o n o f C-ONO2 C i C o n f i g u r a t i o n S i g n Of Co t ton , E f f e c t oL CX -anomer (-) c< 0 -anomer (+) fi ck -anomer (+) /S -anomer (-) C x = * C 1 = ft 01 = o< C1 = ft C 2 = 0N02 =«< c 2 = 0N02 = o< C-j = 0N02 =/3 C^ = 0N02 =^ C o t t o n e f f e c t C o t t o n E f f e c t :•. C o t t o n e f f e c t C o t t o n e f f e c t ( - ) ( + ) ( + ) ( - ) 3 7 . Because o f t h e s t r o n g backg round r o t a t i o n o f t h e com-pounds , t h e s i g n o f t h e C o t t o n e f f e c t was a s c e r t a i n e d by-a p p l y i n g t h e Drude e q u a t i o n . I t was a l s o i n 1 9 6 6 t h a t T s u z u k i ( 5 5 ) r e p o r t e d t h e f i r s t CD o f n i t r a t e e s t e r s . From F i g u r e 9 i t can be seen how much more u s e f u l CD i s t h a n ORD i n d e t e r m i n i n g t h e s i g n o f t h e C o t t o n e f f e c t o f n i t r a t e e s t e r s . T h a t i s why CD was chosen o v e r ORD f o r t h e p r e s e n t s t u d y . F i g u r e 9 . ORD and CD o f 2 , 3 , 4 , 6 - t e t r a - 0 - a c e t y l - l - 0 - n i t r o - c t -D - g a l a c t o p y r a n o s e (A) and m e t h y l 4 i 6 - 0 - b e n z y l i -d e n e - o t - D - a l t r o p y r a n o s i d e 3 - n i t r a t e ( B ) . T s u z u k i e t a l . ( 5 4 ) d e v o t e d a second p a p e r t o t h e CD o f hexose n i t r a t e s i n w h i c h t h e y c o n f i r m t h e i r c o r r e l a t i o n 3 8 between t h e ORD and t h e s t e r e o c h e m i s t r y o f t h e n i t r a t e s . They a l s o r e p o r t t h a t t h e CD maximum ( 2 6 0 - 2 7 0 nm r e g i o n ) i s r e d - s h i f t e d by s t e r i c h i n d r a n c e . I n a d d i t i o n , t h e c o n -t r i b u t i o n t o t h e e l l i p t i c i t y i n p o l y n i t r a t e s con fo rms t o t h e o r d e r C 2 > C^ > C^ > C , j . T h e i r s t u d y w o u l d have been much b e t t e r , i f t h e y c o u l d have p e n e t r a t e d t o a t l e a s t 2 2 0 nm i n t h e CD. They n e v e r d i d see t h e 2 3 0 nm band o f t h e n i t r a t e ch romophore . A second c l a s s o f n i t r a t e s t h e y s t u d i e d was t h e ec -hydroxy -c a r b o x y l i c a c i d s ( 5 3 ) . However, t h e y o n l y p r e s e n t e d l i m i t e d ORD d a t a . E i g h t n i t r a t e s were examined , and n i t r a t e s o f c l - D - h y d r o x y - c a r b o x y l i c a c i d s (R c o n f i g u r a t i o n ) showed p o s i -t i v e C o t t o n e f f e c t s , w h i l e t h o s e o f o c - L - h y d r o x y - c a r b o x y l i c a c i d s ( S - c o n f i g u r a t i o n ) showed n e g a t i v e C o t t o n e f f e c t s . A g a i n , t h e y o n l y p e n e t r a t e d t o a b o u t 2 6 5 - 2 7 0 nm w i t h ORD, and i n some cases t h e r e was u n c e r t a i n t y i n d e t e r m i n i n g t h e s i g n o f t h e C o t t o n e f f e c t due t o t h e n i t r a t o g r o u p . The c i r c u l a r d i c h r o i s m o f t h r e e c ( - h y d r o x y - c a r b o x y l i c a c i d s have been i n c l u d e d i n t h i s p r e s e n t s t u d y . U n t i l 1 9 6 6 t h e r e was no r e p o r t o f CD bands o t h e r t h a n t h e 2 7 0 nm one r e c o r d e d by T s u z u k i and c o - w o r k e r s ( 5 * 0 . I t was i n 1 9 6 7 t h a t a commun ica t i on appeared by Hayward and C laesson ( 5 6 ) r e p o r t i n g t h e d i s c o v e r y o f t h e 2 3 0 nm band o f n i t r a t e e s t e r s . Thus t h i s " h i d d e n " t r a n s i t i o n had been 3 9 . d i s c o v e r e d and t h i s showed t h e v a l u e o f CD i n r e v e a l i n g t r a n s i t i o n s n o t seen i n t h e u l t r a v i o l e t s p e c t r a because o f b e i n g o b s c u r e d by more i n t e n s e ones . I n t h e i r c o m m u n i c a t i o n , Hayward and C l a e s s o n d i s c u s s e d t h e CD s p e c t r a o f e i g h t n i -t r a t e e s t e r s - mono and d i n i t r a t e s o f i s o s o r b i d e , i s o i o d i d e and isomann ide , and t h a t o f 3 j 3 - c h o l e s t e r y l n i t r a t e . I t was f o u n d t h a t t h e s i g n o f t h e 2 6 5 nm band was i n d e p e n d e n t o f t h e c o n f i g u r a t i o n o f t h e o ( - c a r b o n a t o m , b u t t h e 2 3 0 nm band was p o s i t i v e f o r t h e compounds w i t h e n d p _ - ( R ) - n i t r a t o g r o u p s and n e g a t i v e f o r t h o s e w i t h e x o - ( S ) - n i t r a t o g r o u p s . D u r i n g t h e y e a r 1 9 6 9 , w h i l e t h e p r e s e n t s t u d y was i n p r o g r e s s , a t h i r d band o f t h e n i t r a t o chromophore was r e -p o r t e d by a g r o u p o f German w o r k e r s - S n a t z k e , L a u r e n t and W i e c h e r t ( 5 7 ) . T h i s was a n o t h e r band w h i c h was n o t seen i n UV s p e c t r a , and i s l o c a t e d a t a b o u t 2 1 0 nm. The band i s p r o b a b l y n o t due t o e x c i t o n s p l i t t i n g s i n c e t h e r o t a t i o n a l s t r e n g t h s o f t h e 2 1 0 and 2 3 0 nm bands a r e d i f f e r e n t . S n a t z k e e t a l . r e p o r t e d CD d a t a f o r a s e r i e s o f s t e r o i d a l n i t r a t e s , and c o n c l u d e d t h a t t h e c o m b i n a t i o n o f t h e C o t t o n e f f e c t s a r e c h a r a c t e r i s t i c o f t h e l o c a t i o n o f t h e n i t r a t o g r o u p , as w e l l as t h e c o n f i g u r a t i o n o f t h e g r o u p . The s i g n s o f t h e CD bands o f some t y p e s o f s t e r o i d a l n i t r a t e s a r e p r e s e n t e d i n T a b l e I I I . TABLE I I I CD Of S t e r o d i a l N i t r a t e s I n E t h a n o l ( N i t r a t o bands o n l y ) ONO2 P o s i t i o n S t e r o i d C o n f i g u r a t i o n S i g n s o f CD Bands I I I I I I 3£> ( e q u a t o r i a l ) A/B t r a n s + -3 d ( a x i a l ) A/B t r a n s (weak] + 3 * A/B c i s (5 /8-and-r o s t a n ) (weak) - + 5 * A/B t r a n s - + + HP A./B c i s - + 17/8 A/B t r a n s + + -19 A/B t r a n s + + 19 A/B c i s + — N o t e i adap ted f r o m d a t a o f (57) Band I - 3r270nm Band I I - »230nm Band I I I - «210nm 4 1 However, o n l y f o u r t e e n s t e r o i d a l n i t r a t e s were s t u d i e d , w h i c h i s r e a l l y t o o few t o p e r m i t c o n c l u s i o n s r e l a t i n g a c h a r a c t e r i s t i c CD s p e c t r u m t o t h e p a r t i c u l a r c o n f i g u r a t i o n and p o s i t i o n o f t h e n i t r a t o chromophore . I n most cases i t was p o s s i b l e t o d e t e c t t h e CD bands i n t h e p resence o f k e t o g r o u p s . No d e t a i l e d s o l v e n t s t u d y o f n i t r a t e e s t e r s has y e t appeared i n t h e c h e m i c a l l i t e r a t u r e . Snatzke e t a l . measured a few s t e r o i d a l n i t r a t e s i n b o t h e t h a n o l and i s o -o c t a n e . I n most cases b a t h o c h r o m i c s h i f t s were seen f o r a l l t h r e e bands i n g o i n g t o t h e n o n p o l a r s o l v e n t . No v i b r a t i o n a l f i n e s t r u c t u r e was d e t e c t e d i n any o f t h e t h r e e bands o f n i t r a t e s t h a t a r e seen i n n i t r i t e e s t e r s . The g r e a t e s t changes i n i n t e n s i t y were seen i n band I I ( 2 3 0 n m ) . For some compounds, t h e i n t e n s i t y o f band I I i n c r e a s e d , w h i l e f o r o t h e r n i t r a t e s , i t d e c r e a s e d . I n some cases t h i s i n -t e n s i t y change o f band I I amounted t o 300%, The i n t e n s i t y change was no d o u b t due t o changes i n t h e c o n f o r m a t i o n a l e q u i l i b r i u m o f t h e n i t r a t o g r o u p . T h i s f a c e t o f t h e s o l -v e n t e f f e c t s on t h e CD o f n i t r a t e e s t e r s w i l l be d i s c u s s e d l a t e r . Because o f the c o n f o r m a t i o n a l m o b i l i t y o f t h e n i -t r a t o g r o u p . Snatzke and h i s c o - w o r k e r s were n o t a b l e t o p r e s e n t a s e c t o r r u l e r e l a t i n g t h e c o n f i g u r a t i o n o f t h e n i t r a t o g roup t o t h e s i g n s o f t h e CD b a n d s . These a u t h o r s d i d n o t s t u d y t h e low t e m p e r a t u r e CD o f n i t r a t e e s t e r s ( 5 7 ) . 42. In 1970, there appeared an a d d i t i o n a l reference to the CD of n i t r a t e esters. Snatzke and Eckhardt (58) were studying the CD of c h i r a l adamantanones and the e f f e c t of various substituents on t h e i r CD. This was not a study directed at n i t r a t e esters. Three keto-nitrates were re-ported. I t i s i n t e r e s t i n g to note that a n i t r a t o group i n an equatorial p o s i t i o n obeys the octant rule f o r ketones, whereas an a x i a l substituent shows antioctant behaviour. This has been shown, however, only f o r n i t r a t e s positioned on a P carbon atom to the carbonyl group and not f o r other p o s i t i o n s . U n t i l the present, there has been no rule presented r e l a t i n g the configuration of the n i t r a t o group to the s i g n of i t s CD band; there has been no detailed systematic study of the e f f e c t of solvent on t h e i r CD spectra, and there has been no variable temperature study. 4 a-Nitryloxy-adaman tanone-(2)-(lS). 4-Nitryloxy-adam-antandione-(2,6) -(1R). 4 3 . Chromophores R e l a t e d t o R0N0 2 U s e f u l i n f o r m a t i o n can a l s o he o b t a i n e d by s t u d y i n g t h e c i r c u l a r d i c h r o i s m o f o t h e r n i t r o g e n - c o n t a i n i n g chromophores r e l a t e d t o t h e n i t r a t o g r o u p . Some o f t h e s e chromophores a r e p r e s e n t e d i n Tab le I V . 1 . N i t r i t o Group The n i t r i t o and n i t r o chromophores a r e p r o b a b l y t h e two n i t r o g e n - o x y g e n chromophores most c l o s e l y r e l a t e d t o t h e n i t r a t o g r o u p . A l l t h r e e chromophores have f r e e r o t a t i o n abou t a C-0 o r C-N b o n d . ( F i g u r e 1 0 ) . F i g u r e 10 . F ree r o t a t i o n i n t h e n i t r i t o , n i t r o and n i t r a t o D j e r a s s i and h i s c o - w o r k e r s have r e p o r t e d b o t h CD and ORD d a t a f o r a number o f n i t r i t e e s t e r s , ( 4 0 , 5 9 , 6 0 ) . T h i s chromophore a b s o r b s a b o u t 320-400 nm and shows v i b r a t i o n a l s t r u c t u r e . Because o f i t s f r e e r o t a t i o n , D j e r a s s i ( 4 0 ) used t h e v a r i a b l e t e m p e r a t u r e t e c h n i q u e i n s t u d y i n g t h e CD and ORD o f n i t r i t e e s t e r s . G e n e r a l l y , i n s t e r o i d a l n i -t r i t e s where t h e f r e e r o t a t i o n was i m p a i r e d , o n l y a l i m -0 ii chromophores . 44. T a b l e IV Chromophores C o n t a i n i n g N i t r o g e n - O x y g e n S t r u c t u r e Name A b s o r p t i o n (nm) Notes R e f . R0N0 2 n i t r a t o 270,230,210 f r e e r o t a t i o n abou t 0-0 bond 51-57 RONO n i t r i t o 320-440 v i b r a t i o n a l s t r u c -t u r e ; f r e e r o t a -t i o n ; no g e n e r a l r u l e p roposed 4 0 , 5 9 , 6 0 RN0 2 n i t r o 270-290,330 f r e e r o t a t i o n ; s e c t o r r u l e 6 0 , 2 6 , 6 5 , 5 0 , 6 4 , 6 3 R-NO C - n i t r o s o 500-700 monomers u n s t a b l e ; m u l t i p l e p e a k s ; f r e e r o t a t i o n 67 R f \ ) N - n i t r o s o -amine ( N - n i t r o s c 370 s e c t o r r u l e 6 4 % +.0" n i t r o n e 270 ,240 ,210 270 ,240 bands s t r -o n g l y s o l v e n t s h i f t e d 6 8 R«P ~ > I - 0 ' n i t r o x i d e 450,320 camphenyl t - b u t y l n i t r o x i d e r a d i c a l ; b a t h o c h r o m i c s h i f t i n n o n p o l a r s o l -v e n t s 6 9 •^C=N'NC ? 2 n i t r i m i n o • 4 5 i t e d change was seen on l o w e r i n g t h e t e m p e r a t u r e . However, i n s t e r o i d a l n i t r i t e s where t h e r e i s l i t t l e i n h i b i t i o n t o f r e e r o t a t i o n , t h e r e was observed a l a r g e i n c r e a s e i n t h e m a g n i t u d e o f t h e r o t a t i o n a l s t r e n g t h as t e m p e r a t u r e was l o w e r e d ( 40 ) . F o r example , i n t h e case o f cholestan-3<< -o l n i t r i t e , a comp le te i n v e r s i o n o f t h e CD s i g n a l was seen as t h e t e m p e r a t u r e was l owered f r o m -74 'C t o -192* C. T h i s was , o f c o u r s e , a t t r i b u t e d t o r o t a t i o n a l i s o m e r i s m . The l a r g e s t changes i n t h e CD s p e c t r u m o f s t e r o i d a l n i t r i t e s o c c u r r e d between -74° and -192*C. T h i s i n d i c a t e s t h a t t h e e n e r g y b a r r i e r t o f r e e r o t a t i o n a b o u t t h e C-0 s i n g l e bond i s s m a l l . One w o u l d e x p e c t a s l i g h t l y h i g h e r e n e r g y b a r r i e r t o f r e e r o t a t i o n i n n i t r a t e (RONO2) e s t e r s . Even t h o u g h t h e r e a r e r o t a t i o n a l i s o m e r s p r e s e n t , i t i s i m p o r -t a n t t o n o t e t h a t D j e r a s s i f o u n d t h a t t h e most s t a b l e c o n -f o r m a t i o n was u s u a l l y a l r e a d y p r e d o m i n a n t a t room temp-e r a t u r e . As i n t h e case o f n i t r a t e e s t e r s , no g e n e r a l r u l e has appeared r e l a t i n g t h e c o n f i g u r a t i o n o f t h e n i t r i t e e s t e r (RONO) t o t h e s i g n o f t h e CD a b s o r p t i o n . D j e r a s s i s u g g e s t s t h a t t h e v a l u e o f m e a s u r i n g t h e ORD/CD o f n i t r i t e e s t e r s i s t o use t h e c h a r a c t e r i s t i c n a t u r e o f t h e C o t t o n e f f e c t f o r d i f f e r e n t i a t i o n between e p i m e r i c p a i r s o f a l c o h o l s (59 ) . Because o f t h e complex n a t u r e o f t h e CD band o f n i t r i t e e s t e r s , i t i s n o t a l w a y s p o s s i b l e t o c h a r a c t e r i z e t h e who le 46 o f t h e band by a s i n g l e s i g n . V e l l u z , Legrand and G r o s j e a n ( 6 1 ) c o n f e r t h e s i g n o f t h e most p r e p o n d e r a n t peak i n t h e CD s p e c t r u m . They were a l s o a b l e t o d i s t i n g u i s h between t h e r e l a t i v e c o n f i g u r a t i o n s o f e p i m e r s . When t h e c o n f i g u r a t i o n o f t h e c e n t e r b e a r i n g t h e n i t r i t o chromophore was i n v e r t e d , t h e n i n v e r s i o n o f t h e s i g n o f t h e CD o c c u r r e d . However, a g a i n , t h e y were n o t a b l e t o c o n s t r u c t a g e n e r a l r u l e r e -l a t i n g t h e a b s o l u t e c o n f i g u r a t i o n t o t h e s i g n o f t h e c i r -c u l a r d i c h r o i s m . For s t e r o i d s where t h e n i t r i t o g roup i s a t t a c h e d t o a c h a i n ( 2 0 « < - n i t r i t o , f o r example) as w e l l as f o r n i t r i t o g r o u p s a t t a c h e d t o 6-membered r i n g s (jfi - n i t r i t o , f o r example) i t seemed t h a t an S - c o n f i g u r a t i o n o f t h a t c e n t e r gave r i s e t o a p o s i t i v e b a n d , w h i l e an R - c o n f i g u r a t i o n gave r i s e t o a n e g a t i v e band ( 6 l ) . However, t h i s r u l e i s n o t g e n e r a l . Legrand r e p o r t s t h a t t h e o p p o s i t e i s t r u e f o r n i t r i t e s o f 1 7 - h y d r o x y s t e r o i d s ( i e . on f i v e - m e m b e r e d r i n g s ) . I n a d d i t i o n , s t r u c t u r a l changes i n t h e m o l e c u l e even remote f r o m t h e n i t r i t o g r o u p can r e v e r s e t h e s i g n o f i t s CD ( 6 1 - 6 2 ) . 4 7 . Hayward and T o t t y (66) r e p o r t e d CD d a t a f o r n i n e n i t r i t e e s t e r s . Of t h e n i n e , f o u r f i t t e d t h e t e n t a t i v e r u l e s o f V e l l u z e t : a l . ( 6 1 ) , one ( ( - ) - R - 2 - o c t y l n i t r i t e ) c o n t r a d i c t e d t h e s e r u l e s * w h i l e t h e o t h e r f o u r were am-b i g u o u s . The s i m i l a r i t i e s between t h e n i t r i t o and t h e n i t r a t o g roup p rompted Snatzke (57) t o examine t h e CD d a t a f o r b o t h , b u t he f o u n d no c o r r e l a t i o n between t h e s i g n s o f t h e i r CD bands . 2. N i t r o Chromophore A second chromophore s i m i l a r t o t h a t o f t h e n i t r a t o i s t h e n i t r o g r o u p . As w i t h t h e n i t r i t o g r o u p , t h e r e w i l l be an e q u i l i b r i u m between c o n f o r m e r s w h i c h w i l l be t e m p e r a t u r e d e p e n d e n t . I n t h e low t e m p e r a t u r e s t u d y o f n i t r o s t e r o i d s , i t was f o u n d t h a t t h e change i n r o t a t i o n a l s t r e n g t h i n g o i n g t o l o w e r t e m p e r a t u r e s was s m a l l f o r compounds where t h e r e was s t e r i c h i n d r a n c e o f t h e n i t r o g r o u p ( 2 6 ) . I n compounds where t h e r e was o n l y a w e a k l y h i n d e r e d r o t a t i o n o f t h e n i t r o g r o u p , t h e r e was f o u n d a g r e a t change i n r o t a t i o n -a l s t r e n g t h as t h e t e m p e r a t u r e was l o w e r e d . F o r n i t r o s t e r -o i d s , t h e s t r o n g e s t changes i n t h e CD o c c u r r e d between - 5 0 ' C and - 1 7 0 ' C where a h i g h p o p u l a t i o n o f t h e most s t a b l e c o n f o r m e r i s o b t a i n e d . 7 o \ - C h l o r o - 7 / S - n i t r o - 5 o C - c h o l e s t a n e was t h e o n l y n i t r o s t e r o i d w h i c h showed i n v e r s i o n o f s i g n on 48. c o o l i n g ( F i g u r e 1 1 ) . 22 C 350 -0.5- - a t 22*C a t -188«C F i g u r e 1 1 . CD o f 7 ° < - c h l o r o - 7 ^ - n i t r o - 5 < < - c h o l e s t a n e a t +22*C T h i s i n v e r s i o n i s a p p a r e n t l y caused by t h e s t e r i c i n -t e r a c t i o n o f t h e 1 5 - m e t h y l e n e c a r b o n and t h e g e m i n a l c h l o r -i n e w i t h t h e a x i a l n i t r o g r o u p (26). There i s a t e n d e n c y f o r t h e g e m i n a l c h l o r i n e t o t u r n t h e n i t r o g roup away f r o m what w o u l d n o r m a l l y be i t s most f a v o u r e d c o n f o r m a t i o n . There have been no g e n e r a l r u l e s r e l a t i n g c o n f i g u r a -t i o n t o t h e s i g n s o f t h e CD bands f o r n i t r a t e and n i t r i t e e s t e r s , b u t t h e r e has been one p roposed f o r t h e n i t r o g r o u p (63,64). The o r i g i n a l r u l e was an o c t a n t one s i m i l a r t o t h e one p r o p o s e d f o r t h e c a r b o n y l chromophore (63). Sna tzke made r e f i n e m e n t s , c o n s i d e r i n g a l l t h e n o d a l p l a n e s o f t h e o r b i t a l s i n v o l v e d i n t h e n - T T * t r a n s i t i o n , and now t h e r u l e has e v o l v e d i n t o a s e c t o r r u l e (64). and - 1 8 8 ' C ( 5 0 ) . 49. n V TT* A . O R B I T A L S A N D N O D A L P L A N E S O F n—^TT* . B. S E C T O R R U L E F O R N I T R O C H R O M O P H O R E — U P P E R S E C T O R S . C . N O D A L A N D S Y M M E T R Y P L A N E S O F RNG>. FIGURE 12. NITRO SECTOR RULE. F i g u r e 1 2 shows t h e s e c t o r s f o r t h e N i t r o R u l e , w h i l e F i g -u r e 1 3 i l l u s t r a t e s t h e a p p l i c a t i o n . (+T CD e x p e r i m e n t ( + ) CD e x p e r i m e n t F i g u r e 1 3 » A p p l i c a t i o n o f t h e N i t r o S e c t o r R u l e . Fo r t h e s e n i t r o s t e r o i d s , t h e most f a v o u r e d c o n f o r m e r was s e l e c t e d by e x a m i n i n g m o l e c u l a r m o d e l s . T h i s i s t h e t y p e o f r u l e t h a t i s d e s i r e d f o r b o t h t h e n i t r a t o and n i t r i t o ch romophores . A n o t h e r , s i m p l e r s e t o f r u l e s , has been p roposed r e -l a t i n g t h e c o n f i g u r a t i o n o f c a r b o h y d r a t e C - n i t r o a l c o h o l s t o t h e s i g n o f t h e CD a b s o r p t i o n (65). 51. These t h r e e r u l e s s t a t e : 1 . C a r b o h y d r a t e C - n i t r o a l c o h o l s i n w h i c h t h e asym-m e t r i c ca rbon atom a d j a c e n t t o t h e n i t r o m e t h y l g roup b e l o n g s t o t h e S - s e r i e s e x h i b i t p o s i t i v e C o t t o n e f f e c t s and p o s i t i v e CD maxima ( o r s h o u l d e r s ) , whereas t h e C - n i t r o a l c o h o l s i n w h i c h t h e a d j a -c e n t ca rbon atom has t h e R - c o n f i g u r a t i o n show n e g a t i v e C o t t o n e f f e c t s and n e g a t i v e CD maxima ( o r s h o u l d e r s ) . 2 . The s i g n o f t h e C o t t o n e f f e c t o r t h e e l l i p t i c i t y i s n o t a f f e c t e d b y s u b s t i t u t i o n a t C-2 , w h e t h e r by a h y d r o x y l o r an a c e t a m i d o g r o u p . 3 . The a b s o l u t e c o n f i g u r a t i o n s o f o t h e r a s y m m e t r i c c e n t e r s do n o t c o n t r i b u t e t o t h e s i g n o f t h e C o t t o n e f f e c t . These r u l e s do n o t r e q u i r e t h e e x a m i n a t i o n o f m o l e -c u l a r models t o d e t e r m i n e t h e p r e f e r r e d c o n f o r m a t i o n o f t h e n i t r o g r o u p . They a r e v e r y easy t o a p p l y t o t h e c a r b o -h y d r a t e C - n i t r o a l c o h o l s ( F i g u r e 1 4 ) . 5 2 . NO? I I H-C-OH I HO-GH I HCOH I HCOH CH 20H NO, I 2 CHo I H-C-NHAc I HO-CH I HCOH I HCOH CH 20H NO, CH, AcHN-C-H ) - A H HOHCOH I HCOH CH 20H S - c o n f i g u r a t i o n a t S - c o n f i g u r a t i o n a t R - c o n f i g u r a t i o n a t C-2 C-2 C-2 ( + ) - C D p r e d i c t e d (+ ) -CD p r e d i c t e d ( - ) - C D p r e d i c t e d ( + ) - C D e x p e r i m e n t a l (+ ) -CD e x p e r i m e n t a l ( - ) - C D e x p e r i m e n t a l 1 - d e o x y - l - n i t r o - D - 2 - a c e t a m i d o - l , 2 -g l u c i t o l d i d e o x y - l - n i t r o -D - g l u c i t o l 2 - a c e t a m i d o - l , 2 -d i d e o x y - l - n i t r o - D -m a n n i t o l F i g u r e 14 . A p p l i c a t i o n o f R u l e R e l a t i n g C o n f i g u r a t i o n o f C - n i t r o a l c o h o l s t o t h e s i g n o f t h e 310 nm CD band ( 6 5 ) . No d e t a i l e d s t u d y has been made o f t h e e f f e c t o f s o l -v e n t on t h e CD s p e c t r a o f n i t r o compounds. No w a v e l e n g t h s h i f t was o b s e r v e d i n c h a n g i n g t h e s o l v e n t f r o m m e t h a n o l t o d i o x a n e t o i s o o c t a n e ( 6 0 ) . I t i s p o s s i b l e , however , t h a t t h e s o l v e n t c o u l d a f f e c t t h e d i s s y m m e t r y o f t h e t r a n s -i t i o n w h i c h wou ld be m i r r o r e d i n t h e c h a n g i n g i n t e n s i t y o f t h e CD bands . Snatzke ( 2 6 ) s t a t e s "To d i f f e r e n t i a t e w i t h c e r t a i n t y be tween s o l v e n t e f f e c t s and t h o s e caused by c o n -f o r m a t i o n a l m o b i l i t y many more measurements need s t i l l t o be made. " 5 3 . T h i s s t a t e m e n t sums up t h e e x t e n t o f t h e knowledge a b o u t s o l v e n t e f f e c t s o f n i t r o compounds. Two o t h e r n i t r o g e n - o x y g e n chromophores a r e t h e C - n i t r o s o (R-NO) and t h e N - n i t r o s o ( R - ^ N - N O ) . The C - n i t r o s o has had v e r y l i t t l e s t u d y . The most r e c e n t p a p e r appeared i n 1 9 7 0 , b u t t h i s j u s t p r e s e n t e d p r e l i m i n a r y r e s u l t s ( 6 7 ) . The chromophore has m u l t i p l e bands i n t h e r e g i o n o f 5 0 0 t o 7 0 0 nm. The monomers a r e u n s t a b l e , and as w i t h t h e o t h e r n i -t r o g e n - o x y g e n chromophores s t u d i e d so f a r , i t has f r e e r o -t a t i o n . The o n l y c o n c l u s i o n was t h e " n i t r o s o a b s o r p t i o n shows shape and s i g n i n d i v i d u a l l y c h a r a c t e r i s t i c o f t h e p o s i t i o n o f t h e chromophore on t h e s t e r o i d n u c l e u s " ( 6 7 ) . The CD o f t h e N - n i t r o s o g r o u p , on t h e o t h e r h a n d , has had a f a r more t h o r o u g h i n v e s t i g a t i o n ( 6 4 ) . A s e c t o r r u l e has been p roposed c o r r e l a t i n g t h e c o n f i g u r a t i o n o f N - n i t r o s o -amines t o t h e i r n-TT* C o t t o n e f f e c t ? * ( F i g u r e 1 5 ) . F i g u r e 1 5 . S e c t o r Ru le f o r t h e N i t r o s o a m i n e chromophore ( u p p e r s e c t o r s ) (64), These a r e t h e n t h e most f a m i l i a r n i t r o g e n - o x y g e n chromo-p h o r e s and t h e ones t h a t a r e most c l o s e l y r e l a t e d t o t h e n i t r a t o g r o u p . G e n e r a l l y , t h e y a l l have f r e e r o t a t i o n . 5 5 . C o n f o r m a t i o n o f t h e N i t r a t o Group I f a c h i r a l i t y r u l e i s t o "be d e r i v e d r e l a t i n g t h e a b s o l u t e c o n f i g u r a t i o n o f n i t r a t e e s t e r s t o t h e s i g n o f t h e i r C o t t o n e f f e c t , i n f o r m a t i o n a b o u t t h e c o n f o r m a t i o n o f t h e n i t r a t o g r o u p s h o u l d be known. The c a r b o x y l s e c t o r r u l e as a p p l i e d t o s t e r o i d a l a c e t a t e s and c a r b o x y l i c a c i d s i s a good a n a l o g y . When t h i s chromophore a p p e a r s as a l a c t o n e , t h e n t h e r e i s u s u a l l y no p r o b l e m o f c o n f o r m a t i o n s i n c e i t c a n n o t r o t a t e f r e e l y . The c a r b o x y l s e c t o r r u l e was f i r s t d e v e l o p e d f o r l a c t o n e s ( 7 0 ) , and i t was t h e n l a t e r a p p l i e d t o s t e r o i d a l a c e t a t e s ( 7 1 ) and c a r b o x y l i c a c i d s ( 7 2 ) . I n s t e r o i d a l a c e t a t e s , because o f t h e p o s s i b l e f r e e r o t a t i o n o f t h e ch romophore , i t was f i r s t n e c e s s a r y t o ' a n a l y z e t h e i r c o n f o r m a t i o n . P r i n c i p a l l y , t h e c o n f o r -m a t i o n was deduced f r o m x - r a y a n a l y s i s ( 7 3 ) » and t h e n a p p l i e d t o t h e m o l e c u l e i n s o l u t i o n ( F i g u r e 1 6 ) . ! A X I A L E Q U A T O R I A L F i g u r e 1 6 . C o n f o r m a t i o n o f S t e r o i d a l A c e t a t e s . 5 6 . There i s a l w a y s a d a n g e r i n a p p l y i n g c o n f o r m a t i o n s i n t h e c r y s t a l t o c o n f o r m a t i o n s i n s o l u t i o n . However , M a t h i e s o n ( 7 3 ) s u g g e s t e d t h a t t h e c o n f o r m a t i o n o f s t e r o i d a l a c e t a t e s i n t h e s o l i d s t a t e w i l l a l s o p r o b a b l y p r e d o m i n a t e i n t h e i s o l a t e d m o l e c u l e . The c o n f o r m a t i o n s o f t h e e s t e r g r o u p as shown i n F i g u r e 1 6 c o r r e s p o n d t o t h e f a v o u r e d t r a n s c o n f o r m a t i o n d i s c u s s e d b y Oki and N a k a n i s h i ( 7 ^ ) . I n t h e case o f n i t r a t e e s t e r s , t h e r e a r e no a v a i l a b l e compounds i n w h i c h t h e chromophore i s h e l d r i g i d l y i n a c y c l i c s t r u c t u r e . What we know a b o u t t h e n a t u r e o f t h e n i t r a t o chromophore and i t s c o n f o r m a t i o n come f r o m s p e c -t r o s c o p i c measurements . The f i r s t q u e s t i o n t o be answered was w h e t h e r t h e atoms o f t h e n i t r a t o g r o u p , a l o n g w i t h t h e oc-carbon a t o m , a l l l i e i n a p l a n e ( F i g u r e 1 7 ) . I n a n : e l e c t r o n d i f f r a c t i o n s t u d y o f m e t h y l n i t r a t e , d a t i n g back t o 1 9 3 7 , P a u l i n g and Brockway ( 7 5 ) f a v o u r e d a c o n f i g u r a t i o n o f a p l a n a r n i t r a t o g r o u p w i t h t h e m e t h y l g r o u p o u t o f t h i s p l a n e . They d i d n o t , h o w e v e r , c o m p l e t e l y e l i m i n a t e t h e p o s s i b i l i t y o f a c o m p l e t e l y p l a n a r s t r u c t u r e ( F i g u r e 1 7 ) . F ree r o t a t i o n a b o u t t h e N-0 bond o f t h e n i t r a t o g r o u p was f a v o u r e d i n a l a t e r s t u d y ( 7 6 ) . A more r e c e n t m ic rowave s t u d y was made b y D i x o n and W i l s o n ( 7 7 ) , where t h e y f o u n d t h a t a l l t h e f i v e heavy atoms o f m e t h y l n i t r a t e were c o m p l e t e l y p l a n a r . C a l c u l a t i o n s showed t h a t t h e N 0 ? g r o u p was n o t even t w i s t e d P L A N A R N I T R A T O - W I T H CARBON MOIETY OUT O F CARBON IN P L A N E . P L A N E O F N I T R A T O BROUP. FIGURE 17. PROPOSED STRUCTURES FOR NITRATE ESTERS. FIGURE 18. BOND ROTATIONS IN NITRATE E S T E R S . 58. as l i t t l e as one degree f r o m t h e p l a n e . T r o t t e r and h i s c o - w o r k e r s s i m i l a r l y f o u n d r e l a t i v e l y p l a n a r n i t r a t o g r o u p s i n x - r a y s t u d i e s o f 2 - 0 - ( p - b r o m o -b e n z e n e s u l p h o n y l ) - l i 4 , 3 : 6 - d i a n h y d r o - D - g l u c i t o l - 5 - n i t r a t e , i n c i s - 1 , 2 - a c e n a p h t h e n e d i o l d i n i t r a t e , and i n c i s - b e n z o -c y c l o b u t e n e - l , 2 - d i o l d i n i t r a t e ( 7 8 - 8 0 ) . C a l c u l a t i o n s show t h a t t h e c a r b o n atom o f t h e C-ONOg g r o u p i s o n l y a b o u t 0 . 1 A o u t o f t h e p l a n e o f t h e ONOg a t o m s . I t seems t h a t t h e C-ONOg atoms want t o be p l a n a r , and n o t j u s t as a r e -s u l t o f c r y s t a l p a c k i n g s i n c e p l a n a r n i t r a t o g r o u p s a r e seen f o r d i f f e r e n t c h e m i c a l and c r y s t a l e n v i r o n m e n t s . I t i s more l o g i c a l t o c o n c l u d e t h a t t h e s l i g h t d i s t o r t i o n f r o m p l a n a r i t y i n c i s - b e n z o c y c l o b u t e n e - l , 2 - d i o l d i n i t r a t e i s a r e s u l t o f c r y s t a l p a c k i n g . I t i s r e a s o n a b l e t o assume t h e n , w i t h o u t f u r t h e r e v i d e n c e t o t h e c o n t r a r y , t h a t t h e C-OMOg g r o u p w i l l be p l a n a r i n s o l u t i o n . I n a d d i t i o n , t h e s t r u c t u r e o f n i t r i c a c i d was shown t o be p l a n a r ( 8 1 ) . However , t h i s p l a n a r s t r u c t u r e i s n o t t h e c o n f o r m a t i o n w i t h t h e e n e r g y minimum w h i c h i s e a s i l y seen by e x a m i n i n g m o l e c u l a r m o d e l s . F o r m e t h y l n i t r a t e , t h e r e i s a c l o s e r a p p r o a c h o f t h e m e t h y l h y d r o g e n s t o t h e oxygen o f t h e n i t r o m o i e t y . C s i z m a d i a e t a l . ( 8 9 ) , have shown t h a t t h e s t r u c -t u r e w i t h t h e <X-carbon atom l y i n g i n a p l a n e p e r p e n d i c u l a r t o t h e n i t r a t o g r o u p l i e s a t 4 . 9 K c a l . / m o l e be low t h a t o f t h e s t r u c t u r e w i t h t h e « - c a r b o n i n t h e same p l a n e as t h e 59* n i t r a t o g r o u p . T h e r e f o r e , t h e r e must be o t h e r f a c t o r s , such as b o n d i n g p e r h a p s , w h i c h wou ld f a v o u r such a s t r u c -t u r e s i n c e s t e r i c c o n s i d e r a t i o n s f a v o u r t h e p e r p e n d i c u l a r s t r u c t u r e . But i n l i g h t o f t h e r e c e n t x - r a y a n a l y s i s and D i x o n ' s ( 7 7 ) mic rowave s t u d y , i t was d e c i d e d t o use a p l a n a r n i t r a t o g r o u p i n t h i s s t e r e o c h e m i c a l i n v e s t i g a t i o n . S i n c e t h e n i t r a t o g r o u p i s c o n s i d e r e d t o be p l a n a r , o n l y r o t a t i o n a b o u t t h e C-0 and C-C bonds need now be c o n s i d e r e d ( F i g u r e 1 8 ) . The e f f e c t o f r o t a t i o n a l i s o m e r i s m a b o u t C-C bonds i n n i t r a t e e s t e r s has been obse rved v i a i n f r a r e d s p e c -t r o s c o p y . I n a s e r i e s o f n i t r a t e e s t e r s , where X was a l a r g e s u b s t i t u e n t , and t h u s t h e r e wou ld be s t e r i c h i n d r a n c e t o r o t a t i o n , t h e 1630 c m " 1 and 1280 cm"" 1 bands were s p l i t (82-86). T h i s s p l i t t i n g was a t t r i b u t e d t o r o t a t i o n a l i s o m e r s - t h e t r a n s i s o m e r and t h e gauche . ONOg ©BOg T R A N S G A U C H E 60. The t r a n s i s o m e r s have l o w e r TP as NOg and "Vs NO^ t h a n t h e gauche i s o m e r s . However, o n l y a s i n g l e band ( n o t s p l i t ) i s seen when t h e s t e r i c h i n d r a n c e i s n o t g r e a t o r i f t h e r e i s o n l y one p r o b a b l e c o n f o r m a t i o n , such as i n t h e case o f b e n z y l n i t r a t e . U r b a n s k i and W i t a n o w s k i (82,83) b a s i c a l l y s t u d i e d s i m p l e a l i p h a t i c n i t r a t e s and d i n i t r a t e s , b u t i t was a l s o r e p o r t e d t h a t t h e r e was a b a r e l y d e t e c t a b l e s p l i t -t i n g f o r c y c l o h e x y l n i t r a t e w h i c h was a t t r i b u t e d t o s t e r i c d i f f e r e n c e s between a x i a l and e q u a t o r i a l p o s i t i o n s o f t h e 0N0 2 g r o u p . I n a l l t h e cases where r o t a t i o n a l i s o m e r i s m a b o u t a C-C bond i s p o s s i b l e , t h e r e i s a l s o t h e p o s s i b i l i t y o f r o t a t i o n a b o u t t h e C-0 b o n d . The r o t a t i o n a b o u t t h e C-0 bond i s t h e t y p e o f r o t a t i o n a l i s o m e r i s m t h a t w i l l be i m p -o r t a n t i n t h e n i t r a t e e s t e r s o f t h e p r e s e n t s t u d y . D i p o l e moment and s p e c t r o s c o p i c s t u d i e s f a v o u r such f r e e r o t a t i o n i n p e n t a e r y t h r i t o l t e t r a n i t r a t e m o l e c u l e s i n s o l u t i o n (83). On t h e o t h e r h a n d , i n t h e mic rowave s t u d y o f m e t h y l n i -t r a t e , D i x o n and W i l s o n (77) showed t h a t t h e hyd rogen atoms o f t h e m e t h y l g r o u p a r e s t a g g e r e d w i t h r e s p e c t t o t h e n e a r e s t oxygen o f t h e n i t r a t o g r o u p . They e x p l a i n e d , " t h a t t h e r e i s a s t e e p r e p u l s i v e i n t e r a c t i o n , s t e r i c h i n d r a n c e , between t h e m e t h y l hyd rogens and t h e n e a r b y o x y g e n . " By " s t e e p " b a r r i e r , t h e y mean s t e e p i n compar i son t o o t h e r 6 1 . CH<j-0- b a r r i e r s . The b a r r i e r t o r o t a t i o n o f t h e m e t h y l g r o u p i n m e t h y l n i t r a t e was s e t a t 2 . 3 K c a l / m o l e + C s i z m a d i a e t a l . ( 8 9 ) , have c a l c u l a t e d a p o t e n t i a l e n e r g y s u r f a c e f o r a number o f n i t r a t e e s t e r s . By a n a l y z i n g t h e i r r e s u l t s , i t can be seen t h a t t h e b a r r i e r t o m e t h y l g r o u p r o t a t i o n i n m e t h y l n i t r a t e i s a b o u t 1 5 - 1 6 K c a l / m o l e ( F i g u r e 1 9 ) . I n F i g u r e 19 t h e c o n t o u r l i n e s have been s k e t c h e d i n a t 5 K c a l / m o l e ( 8 9 ) . F o r m e t h y l n i t r a t e , one w o u l d f a v o u r t h e v a l u e f r o m mic rowave s p e c t r a r a t h e r t h a n t h e t h e o r e t i c a l one . There w i l l be g r e a t e r s t e r i c h i n d r a n c e t o r o t a t i o n i n t h e more complex n i t r a t e e s t e r s . I n some cases o f d i n i t r a t e s and t h e n i t r a t e s c o n t a i n -i n g o x o l a n e r i n g s , t h e r e i s a second i n t e r a c t i o n t h a t has a b e a r i n g on r o t a t i o n i s o m e r i s m a b o u t t h e C - 0 b o n d s . There seems t o be an i n t e r a c t i o n be tween t h e r e l a t i v e l y p o s i t i v e a l k o x y l oxygen and t h e r e l a t i v e l y n e g a t i v e oxygen o f t h e n i t r o g r o u p . The m o l e c u l a r o r b i t a l c a l c u l a t i o n s o f Zhanov ( 8 6 ) show a p o s i t i v e e l e c t r o n i c charge on t h e oxygen o f an o x o l a n e r i n g . I n 2 - 0 - ( p - b r o m o b e n z e n s u l p h o n y l ) - l , 4 : 3 , 6 - d i a n h y d r o - D - g l u c i t o l - 5 - n i t r a t e , T r o t t e r e t a l . ( 7 8 ) f o u n d o n l y 2 . 9 A between a n i t r o g r o u p oxygen and t h e a l k o x y l oxygen i n t h e o t h e r f i v e - m e m b e r r i n g . The sum o f van d e r -W a a l ' s r a d i i f o r oxygen atoms i s 2 . 8 A . The 0 . . . . 0 c o n t a c t ( 2 . 9 A ) i s o n l y a l i t t l e g r e a t e r t h a n t h e v a n d e r W a a l ' s d i s t a n c e , i n d i c a t i n g a t t r a c t i v e f o r c e s between t h e r e l a t i v e l y n e g a t i v e n i t r o oxygen and t h e r e l a t i v e l y p o s i t i v e a l k o x y l o x y g e n . 6 3 . ONO. 2 . 9 4 A s e p a r a t i o n be tween oxygen atoms R-0 • A * H \ V 2 . 8 6 A s e p a r a t i o n be tween oxygen ' a toms T h i s same i n t e r a c t i o n was a l s o d e t e c t e d i n an i n f r a -r e d s t u d y ( 8 ? ) , where d i f f e r e n t f r e q u e n c i e s f o r t h e V s ( N 0 2 ) band appeared f o r t h e n i t r a t e e s t e r s o f t h e . l , 4 : 3 » 6 - d i a n h y -d r o h e x i t o l s . The T?s(N0 2 ) f r e q u e n c y o f endo and exo n i -t r a t o g r o u p s v a r i e d w i t h t h e d i h e d r a l a n g l e ( • ) be tween t h e a d j a c e n t C - 0 b o n d s . The d i h e d r a l angle,<|>, i s t h e a n g l e b e -t w e e n t h e C - 0 bond o f t h e n i t r a t o g r o u p and t h e C - 0 bond i n t h e a d j a c e n t r i n g . <fr endo = 0 * + 3 0 * " p s ( N 0 2 ) = 1282 c m "1 <j) exo = 1 2 0 ° + 3 0 - p s ( N 0 2 ) = 1 2 7 4 c m "1 The s h i f t t o h i g h e r f r e q u e n c i e s o f t h e endo i s o m e r i s a g a i n a t t r i b u t e d t o t h e c l o s e a p p r o a c h and i n t e r a c t i o n o f t h e r i n g 6 4 . oxygen w i t h t h a t o f t h e n i t r a t o g r o u p . T h i s same t y p e o f i n t e r m o l e c u l a r a t t r a c t i o n has a l s o been r e p o r t e d t o be p r e s e n t i n c i s - l , 2 - a c e n a p h t h e n e d i o l d i n i t r a t e ( ? 9 ) . B o t h n i t r a t o g r o u p s a r e i n c l i n e d t o t h e p l a n e o f t h e c a r b o n a toms , and b o t h i n c l i n e d i n t h e same d i r e c t i o n . The c l o s e s t i n t e r m o l e c u l a r n o n b o n d i n g d i s t a n c e i s 2 . 9 l A between an o x y -gen atom o f one n i t r o g r o u p and t h e n e i g h b o u r i n g a l k o x y l oxygen ( ? 8 ) . A g a i n , t h e f r e q u e n c y l ) s ( N 0 2 ) o f t h e c i s and t r a n s n i t r o g r o u p s was r e l a t e d t o t h e d i h e d r a l a n g l e b e -tween n i t r a t o g r o u p s ( 8 8 ) . These c o n s i d e r a t i o n s w i l l be u s e f u l i n a n a l y z i n g t h e c o n f o r m a t i o n o f t h e n i t r a t o g r o u p s i n d i n i t r a t e s and i n n i t r a t e s c o n t a i n i n g o x o l a n e r i n g s . 65-T r a n s i t i o n s I n N i t r a t e E s t e r s A c h i r a l i t y r u l e i s u s u a l l y d e v e l o p e d by e x a m i n i n g t h e t r a n s i t i o n s and t h e o r b i t a l s o f the ch romophore . I n t h e case o f n i t r a t e e s t e r s , t h e r e i s a l a c k o f d e f i n i t e i n f o r m a t i o n a b o u t t h e n a t u r e o f t h e t r a n s i t i o n , t h e b o n d i n g and t h e o r b i t a l s o f t h e chromophore . F o u r s p e c t r a l bands have now been observed i n t h e s p e c t r a o f n i t r a t e e s t e r s , and. CD has been i n v a l u a b l e i n d e t e c t i n g two o f t h e s e . The f i r s t s p e c t r a l band was seen i n t h e UV s p e c t r a o f n i t r a t e e s t e r s and i s c h a r a c t e r i z e d as an i n f l e c t i o n o r s h o u l d e r a round 2 7 0 nm. The second band was f i r s t d e t e c t e d i n a c i r c u l a r d i c h r o i s m s t u d y o f t h e n i -t r a t o g roup and i s l o c a t e d n e a r 2 3 0 nm ( 5 6 ) . The t h i r d s p e c t r a l b a n d , a l s o f i r s t d e t e c t e d t h r o u g h CD s p e c t r o s c o p y , i s l o c a t e d n e a r 2 1 0 nm ( 5 7 ) . The f o u r t h and l a s t t r a n s -i t i o n d e t e c t e d was f o u n d i n t h e UV s p e c t r a o f e t h y l n i t r a t e , w h i c h had a maximum a t 1 9 3 nm ( £ = 5 8 0 0 ) ( 9 0 ) . The 2 3 0 nm and 2 1 0 nm t r a n s i t i o n s were n o t seen i n t h e u l t r a v i o l e t s p e c t r a ; t h e s e t r a n s i t i o n s a r e r e l a t i v e l y weak and a r e h i d -den u n d e r t h e i n t e n s e 1 9 3 nm t r a n s i t i o n . Tab le V l i s t s UV d a t a o f some n i t r a t e e s t e r s . Ungnade and S m i l e y ( 9 1 ) r e p o r t t h a t l a r g e r m o l e c u l e s o f s e c o n d a r y a l k y l n i t r a t e had h i g h e r £ v a l u e s t h a n l o w e r m o l e -c u l a r w e i g h t n i t r a t e s . A d d i t i o n a l l y , Cs i zmad ia r e p o r t e d 6 6 . Table V UV Spectra of Nitrate Esters Nitrate Solvent i Ref. E t h y l n i t r a t e pet. ether 270 17 99 E t h y l n i t r a t e n-heptane 193 5800 90 1-Octyl n i t r a t e EtOH 270 15 91 Cyclohexylmethyl n i - EtOH 270 16,20 91 trate Cyclopentylmethyl EtOH 270 16 91 n i t r a t e 2-Butyl n i t r a t e EtOH 270 17 91 4-Heptyl n i t r a t e EtOH 270 28 91 2 - 0 c t y l n i t r a t e EtOH 270 37 91 Cyclohexyl n i t r a t e EtOH 270 22 91 5-Methyl -2- o c t y l EtOH 270 33 91 n i t r a t e Isoamyl n i t r a t e MeOH 270 19 100 6 7 . t h a t , i n t h e s e r i e s CH^ONOg t o EtONOg t o i - p r O N 0 2 t h e low i n t e n s i t y band was s h i f t e d t o l o w e r e n e r g y ( 8 9 ) . A l t h o u g h f o u r s p e c t r a l bands o f t h e n i t r a t o g r o u p have been d e t e c t e d , t h e r e i s u n c e r t a i n t y c o n c e r n i n g t h e i r n a t u r e . I t s h o u l d a l s o be n o t e d t h a t an a p p a r e n t l y s i m p l e a b s o r p t i o n c u r v e can have a complex band p a t t e r n ( 8 9 ) . Csiz raad ia e t a l . c a l c u l a t e d t h e t h e o r e t i c a l s p e c t r a o f e i g h t n i t r a t e e s t e r s , and m e t h y l n i t r a t e , f o r example had f i v e r e s o l v e d bands up t o 5 e v . ( t h r e e n-»Tf t r a n s i t i o n s , oneTT"-»TT* t r a n s i t i o n , and o n e t f - ^ t f * ) . The re i s g e n e r a l agreement t h a t t h e 270 nm band i s a n + T T * t r a n s i t i o n . Eremenko et a l . (92) c a l l t h e 265 nm band o f n i t r i c a c i d a t r a n s i t i o n o f an e l e c t r o n f r o m t h e n o n b o n d i n g o r b i t a l o f oxygen t o an a n t i b o n d i n g m o l e c u l a r o r b i t a l . Lao (93) s i m i l a r l y i d e n t i f i e s t h i s as a n-flT* t r a n s i t i o n . C s i z m a d i a ' s (89) t h e o r e t i c a l s t u d y i n d i c a t e s n-»TY*" t r a n s i t i o n s a t 274 and 249 nm. F i g u r e 20 shows some o f t h e p o s s i b l e t r a n s i t i o n s i n m e t h y l n i t r a t e . The d a t a f o r t h e m o l e c u l a r o r b i t a l e n e r g i e s were t a k e n f r o m T a b l e s I V , V and V I I o f C s i z m a d i a and h i s c o - w o r k e r s ( 8 9 ) . F i g u r e 21 shows t h e UV s p e c t r a o f m e t h y l n i t r a t e r e s o l v e d i n t o i t s Gauss ian component b a n d s . The n-vr? band i s s i m i l a r t o t h a t o f t h e n i t r o chromophore (270 n m ) . F i g u r e 22 d e p i c t s t h e e n e r g y l e v e l s o f t h e n i t r o g r o u p and n i t r a t e i o n f o r c o m p a r i s o n . 68. +•2 A ev 1-3 3n 2n m IT FIGURE 20. SOME TRANSITIONS OF MeONO-,. FIGURE 21. UV SPECTRUM OF METHYL NITRATE RESOLVED INTO ITS COMPONENT BANDS (8 9) 69. A . T R A N S I T I O N S IN R N 0 2 ( 9 4 ) . B. T R A N S I T I O N S IN T H E 2 N I T R A T E J O N ( 9 3 , 9 S ) FIGURE 2 2. ENERGY LEVELS OF THE NSTRO 7 0 . Less i s known a b o u t t h e 230 nm t r a n s i t i o n ; t h e r e i s n o t even an i n f l e c t i o n i n t h e UV; t h e band i s c o m p l e t e l y h i d d e n u n d e r t h e more i n t e n s e t r a n s i t i o n s . The i n t e n s i t y o f t h e UV enve lope a t 230 nm v a r i e d f r o m £=100 t o 200 f o r t h e s e r i e s m e t h y l t o n - b u t y l n i t r a t e ( 8 9 ) . T h i s w i l l n o t be t h e t r u e i n t e n s i t y o f t h e t r a n s i t i o n because o f o v e r l a p o f t h e much more i n t e n s e b a n d s . The low i n t e n s i t y , however , w o u l d t e n d t o r u l e o u t a tr-^Tf4 t r a n s i t i o n . The t h e o r e t i c a l s p e c t r u m o f m e t h y l n i t r a t e shows a n-*Tr* t r a n s i t i o n a t 230 nm (£=28) ( 8 9 ) . Thus Cs i zmad ia e t a l . r e f e r r e d t o t h e 230 nm CD band o f n i t r a t e e s t e r s as r e s u l t i n g f r o m an n-*T? t r a n s -i t i o n . There has been c o n s i d e r a b l e d i s c u s s i o n o f t h e h i g h e r e n e r g y bands o f n i t r a t e e s t e r s o r n i t r a t e i o n . The 200 nm t r a n s i t i o n o f t h e n i t r a t e i o n has been a t t r i b u t e d t o a i r—Tr * t r a n s i t i o n (£=9,700 o r 12,500) ( 9 5 , 9 6 ) , as has t h e 210 nm ir-»T? t r a n s i t i o n o f n i t r i c a c i d and o r g a n i c n i t r o com-pounds ( 9 2 , 9 7 , 9 8 ) . Cs i zmad ia and h i s c o - w o r k e r s e x p e c t a 1 7 — T? band n e a r 207 nm f o r t h e s e r i e s o f m e t h y l , e t h y l , n - b u t y l , i - p r o p y l , n - b u t y l and t - b u t y l n i t r a t e s (£=1300 t o 2500) ( 8 9 ) . They a l s o e x p e c t more low i n t e n s i t y bands i n t h i s r e g i o n , some o f w h i c h w i l l be a s s o c i a t e d w i t h a c h a r g e -t r a n s f e r f r o m t h e a l k y l g r o u p t o t h e TT* o r b i t a l s o f t h e n i t r a t o g r o u p . Kaya e t a l . ( 9 0 ) , i n v e s t i g a t e d t h e p o s s i b i l i t y o f c h a r g e - t r a n s f e r bands i n t h e s p e c t r a o f n i t r a t e e s t e r s ( 9 0 ) . The e t h y l n i t r a t e m o l e c u l e was d i v i d e d i n t o two p a r t s - t h e e t h o x y l g r o u p as e l e c t r o n d o n o r and t h e n i t r o g r o u p as an e l e c t r o n a c c e p t o r ( F i g u r e 2 3 ) . They c a l c u l a t e d t h e TT e l e c t r o n e n e r g y l e v e l s o f e t h y l n i t r a t e , and e x p e c t e d t h e f i r s t Tt—*-TT* t r a n s i t i o n t o appear a t 208 nm. A second s t r o n g hand was e x p e c t e d t o o c c u r n e a r 200 nm. The f i r s t TT-*-tT*band, a c c o r d i n g t o K a y a , may be r e -ga rded as an i n t r a m o l e c u l a r c h a r g e - t r a n s f e r caused by t h e i n t e r a c t i o n between t h e e l e c t r o n d o n a t i n g C 2 H ^ d - g r o u p and t h e e l e c t r o n a c c e p t i n g - N 0 2 . The t r a n s i t i o n s p r o b a b l y r e s p o n s i b l e f o r t h e t h r e e CD bands a r e summarized i n T a b l e V I . E L E C T R O N DONOR •NT E L E C T R O N A C C E P T O R F i g u r e 23 t E t h y l N i t r a t e as v i e w e d f o r c h a r g e - t r a n s f e r bands . 7 2 . T a b l e V I P r o b a b l e E l e c t r o n i c T r a n s i t i o n s R e s p o n s i b l e F o r The CD Bands Of N i t r a t e E s t e r s Band (nm) T r a n s i t i o n 270 230 210 .3190-200 n-TT* n-TT* f T — f T * ( c h a r g e t r a n s f e r ) rr-rf 73-Bond ing Of The N i t r a t o Group There a r e t h r e e resonance s t r u c t u r e s o f t h e n i t r a t o g r o u p t o c o n s i d e r ! There has been some q u e s t i o n abou t t h e i m p o r t a n c e o f s t r u c t u r e I I I . D i x o n (77) showed t h a t a. p l a n a r n i t r a t o g r o u p s u p p o r t s a r o l e f o r s t r u c t u r e I I I . Thus one wou ld need t o c o n s i d e r t h e T Y b o n d i n g o f t h e a l k o x y l o x y g e n . On t h e o t h e r h a n d , M u r r e l l t e l l s u s , " t h e o r g a n i c n i t r a t e s R 0 - N 0 2 o . . can be c o n s i d e r e d as a l k o x y l s u b s t i t u t e d n i t r o compounds as f a r as t h e i r s p e c t r o s c o p i c p r o p e r t i e s a r e c o n -c e r n e d " (101). I n o t h e r w o r d s , he says t o f o r g e t abou t s t r u c t u r e I I I . However , one w o u l d ha.ve t o c o n s i d e r t h e o r b i t a l s o f t h e a l k o x y l oxygen i f one i s t o a c c e p t t h e c h a r g e - t r a n s f e r band ( a l k o x y l g r o u p e l e c t r o n d o n a t i n g ; n i -t r o g r o u p e l e c t r o n a c c e p t i n g ) . There i s c o n s i d e r a b l e u n -c e r t a i n t y c o n c e r n i n g t h e o r b i t a l s i n v o l v e d . Sometimes t h e o r b i t a l s a r e drawn as i f t h e n i t r a t o g r o u p were j u s t an a l k o x y l s u b s t i t u t e d n i t r o compound (102,103), w h i l e o t h e r t i m e s one f i n d s s t a t e m e n t s such as " t h e MO l o c a l i z e d on t h e n i t r a t o I I I I I I 74. g r o u p c o u l d c l e a r l y be i d e n t i f i e d w i t h t h e MO o f t h e n i t r a t e i o n " (89) ( F i g u r e 24 and 2 5 ) . F i g u r e 2 4 . Nonbond ing and T T o r b i t a l s o f t h e n i t r a t o g r o u p . I n F i g u r e 24A, t h e TT e l e c t r o n s a r e shown as b e i n g d e l o c a l i z e d o v e r t h e n i t r o g r o u p , w h i l e F i g u r e 24B d e p i c t s t h e d e r e a l i z a t i o n o v e r t h e e n t i r e ONOg g r o u p . I f t he d e r e a l i z a t i o n were o v e r t h e e n t i r e ONOg g r o u p , t h e n i t w o u l d e x p l a i n t h e p l a n a r chromophore and a l l o f i t s t r a n s i t i o n s . B u t , i f t h i s were t h e c a s e , t h e n one wou ld e x p e c t a s h o r t e r RO-N bond d i s t a n c e . The bond l e n g t h s o f v a r i o u s N=0 and '0-N bonds appear i n T a b l e V I I . As can be seen i n t h i s T a b l e , t h e l e n g t h o f t h e N=0 bond o f t h e n i t r o p o r t i o n o f t h e n i t r a t o g r o u p has t h e u s u a l n i t r o g e n - o x y g e n d o u b l e bond l e n g t h . However, t h e l e n g t h o f t h e RO-NC bond i s t h a t o f a n i t r o g e n - o x y g e n s i n g l e bond . T h e r e f o r e , i t i s 7 5 . 1 i I I 3n (n..Q',)| n—i—r FIGURE 25. MOLECULAR ORBITAL DENSITY •S CSIZMADIA et al. (89). 7 6 . T a b l e V I I L e n g t h o f N i t r o g e n Bonds M o l e c u l e N-OR ( A ) N=0 ( A ) Method R e f . RONOo Compounds m e t h y l n i t r a t e ( C H 3 0 N 0 2 ) 1 . 3 6 1 . 2 6 e l e c t r o n d i f f r a c t i o n 7 5 . 1 0 4 n i t r i c a c i d (H0N0 2 ) 1 . 4 0 5 1 . 2 0 6 mi crowave 8 1 c i s - 1 , 2 - a c e n a p h t h -e n e d i o l d i n i t r a t e 1 .41 1 . 1 9 X - r a y 7 9 p e n t a e r y t h r i t o l t e t r a n i t r a t e C ( C H 2 0 N 0 2 ) ^ 1 .404 1 . 2 2 0 5 , 1 . 2 0 3 X - r a y 108 2 - 0 - ( p - b r o m o b e n z e n e -s u l p h o n y l ) - l , 4 : 3 , 6 -d i anhyd r o - D - g l u -c i t o l 5 - n i t r a t e 1 . 5 3 1 . 1 6 X - r a y 78 c i s - b e n z o c y c l o -b u t e n e - 1 , 2 - d i o l d i n i t r a t e 1 . 3 8 7 , 1 .411 1 . 1 9 8 , 1.190 X - r a y 80 n i t r a t e i o n ( 0 N 0 2 " ) 1.218 1 0 5 N i t r i t e E s t e r s m e t h y l n i t r i t e ( C H 3 - 0 - N = 0 ) 1.37 1.22 104 77-T a b l e V I I c o n t d . L e n g t h s o f N i t r o g e n Bonds M o l e c u l e N-OR (A) N=0 1 Method R e f . O t h e r N-0 Compounds N 2 0 1.197 106 n i t r o m e t h a n e 1.22 104 ( C H 3 N 0 2 ) N0 2C1 1.202 106 N0 2 1.19 3^ 107 ( C H 3 ) 3 ^ 0 1.388 107 ( C H ^ N O - H C I 1 .425 107 H 2N-CHtN0H 1 . 4 1 106 78' c o n c l u d e d t h a t , d e s p i t e C s i z m a d i a ' s (89) c a l c u l a t i o n s , t h e RO-N "bond canno t have much d o u b l e bond c h a r a c t e r and t h e b o n d i n g i n t h i s , p o r t i o n o f t h e n i t r a t o g r o u p w i l l n o t be l i k e t h a t o f t h e n i t r a t e i o n . 1 79 R E S U L T S a n d D I S C U S S I O N 80 . A. U l t r a v i o l e t S p e c t r a o f N i t r a t e E s t e r s U l t r a v i o l e t a b s o r p t i o n s p e c t r a l d a t a f o r a number o f n i t r a t e e s t e r s a r e p r e s e n t e d i n T a b l e V I I I . The i n t e n s i t i e s were c a l c u l a t e d f o r t h e w a v e l e n g t h s 270, 23^, 210 and 200 nm, b u t t h e s e w a v e l e n g t h s do n o t c o r r e s p o n d t o any UV m a x i -mum, as can be seen f r o m F i g u r e 26. The c u r v e s i n F i g u r e 26 a r e t h e i n s t r u m e n t t r a c e s and a r e t y p i c a l c u r v e s f o r t h e n i t r a t e e s t e r s s t u d i e d . G e n e r a l l y , a c e t o n i t r i l e was s e l e c t e d as t h e p o l a r s o l -v e n t w h i l e c y c l o h e x a n e r e p r e s e n t e d a n o n p o l a r one . 1R:3R*4S-M e n t h y l n i t r a t e was examined i n s i x s o l v e n t s o f v a r y i n g p o l a r i t y . I t was g e n e r a l l y f o u n d t h a t t h e a b s o r p t i o n was g r e a t e r i n a c e t o n i t r i l e t h a n i n cyc lohexane a t 270 and 230 nm, w h e r e a s , t h e i n t e n s i t y a t 210 and 200 nm was g r e a t e r i n c y c l o h e x a n e . T h i s i n d i c a t e s t h e p r e s e n c e o f d i f f e r e n t t y p e s o f t r a n s i t i o n s . Because o f t h e v a r i a t i o n s o f e l e c -t r o n i c d e n s i t y i n d i f f e r e n t e l e c t r o n i c s t a t e s , s o l v e n t i n t e r -a c t i o n a f f e c t s d i f f e r e n t l y t h e v a r i o u s e l e c t r o n i c s t a t e s o f t h e m o l e c u l e . N o r m a l l y , t h e g round s t a t e o f t h e n-rjr* t r a n s i t i o n i s s t a b i l i z e d by t h e e l e c t r o s t a t i c i n t e r a c t i o n s between s o l u t e and p o l a r s o l v e n t m o l e c u l e s . M e n t h y l n i t r a t e had an a b n o r m a l l y l a r g e £270 v a l u e i n h e x a f l u o r o i s o p r o p a n o l . T h i s can o n l y be a t t r i b u t e d t o t h e i n t e r a c t i o n caused by t h e e l e c t r o n - w i t h d r a w i n g t r i f l u o r o m e t h y l T a b l e V I I I UV S p e c t r a Of N i t r a t e E s t e r s N i t r a t e S o l v e n t 270 nm 230 nn 210 nm 200 nm menthyl n i t r a t e HFIP CHC1, MeOH CHoCN TMP C 6 H 1 2 44 31 28 30 30 25 1400 830 9^0 1000 650 4200 3900 3900 3800 3800 5000 4800 4900 4600 5300 p - m e n t h - l ( 7 ) - t r a n s -e n e - 2 - o l - r i i t r a t e CH^ CN 38 860 3900. 7700 c i s - 3 - m e t h y l c y c l o -h e x y l n i t r a t e CH^CN 28 336 t r a n s - 3 - m e t h y l c v -c l o h e x y l n i t r a t e CH^ CN 35 b o r n y l n i t r a t e CH^CN C 6*12 3^ 26 1140 800 3800 3900 4400 4100 i s o b o r n y l n i t r a t e CHoCN C 6 % 2 37 30 1100 830 3800 3900 4500 5100 e ( - f e n c h y l n i t r a t e CHc.CN  C 6*12 60 37 1600 1200 3200 3200 4100 4400 c a m p h a n e - 2 - e x o , 3 -e x o - d i o l - d i n i t r a t e CHoCN W l Z 65 58 1700 1300 5900 6700 7600 9700 camphane - 2 -endo , 3 -e n d o - d i o l - d i n i t r a t e CHoCN C6^12 52 47 1400 1200 5100 6000 6700 8700 c a m p h a n e - 2 - e n d o , 3 -e x o - d i o l - d i n i t r a t e CHoCN C 6*12 59 50 i 8 6 0 1260 7200 7400 9500 9900 c t - n i t r a t o - / , ^ - d i m e t h -y l - ^ - b u t y r o l a c t o n e CH3CN C 6 % 2 25 25 670 642 2100 2200 3400 3600 t h r e i t a n d i n i t r a t e CHoCN C 6 H 1 2 28 28 700 550 4700 5200 7700 8900 82. 210 + FIGURE 26. UV SPECTRUM OF MENTHYL NITRATE (MeOH). 83 . g r o u p s on t h e s o l v e n t m o l e c u l e . The l v a l u e a t 200 nm was n o r m a l i n t h i s s o l v e n t . I n t h e s e r i e s b o r n y l , i s o b o r n y l , and f e n c h y l n i t r a t e s , a l l w i t h t h e same e m p i r i c a l f o r m u l a , t h e i n t e n s i t y a t 270 nm (CH^CN s o l v e n t ) i n c r e a s e d as t h e s t e r i c h i n d r a n c e o f t h e n i t r a t o g r o u p i n c r e a s e d . S i m i l a r l y , c a m p h a n e - 2 - e x o , 3 -e x o - d i o l - d i n i t r a t e , i n w h i c h t h e n i t r a t o g r o u p s a r e c i s , had t h e h i g h e s t £ v a l u e a t 270 nm among t h e t h r e e camphane-2 , 3 - d i o l - d i n i t r a t e s s t u d i e d . 8 4 . B. C h i r a l i t y Ru le Fo r N i t r a t e E s t e r s Assuming a p l a n a r n i t r a t o g r o u p , and w i t h a t t e n t i o n f o -cused on the. . n i t r o p o r t i o n o f t h e n i t r a t o ch romophore , t h e no d a l and symmetry p l a n e s o f t h e chromophore were d e t e r m i n e d . The n o d a l p l a n e o f t h e TT o r b i t a l s w i l l be t h e p l a n e c o n -t a i n i n g t h e n i t r a t o g r o u p . T h i s i s a l s o t h e o n l y m o l e c u l a r symmetry p l a n e o f t h e p l a n a r chromophore ( F i g u r e 2 7 ) . N o d a l . p l a n e s o f t h e n o n b o n d i n g o r b i t a l s o f t h e F i g u r e 27. N o d a l p l a n e s o f theTT and n o n b o n d i n g o r b i t a l s o f the n i t r a t o g r o u p . 85. The n o n b o n d i n g o r b i t a l s o f t h e t e r m i n a l oxygens a r e a l s o i n t h i s p l a n e . The n o d a l p l a n e s o f t h e s e n o n b o n d i n g o r b i t a l s o f t h e t e r m i n a l oxygens a r e p l a n e s p e r p e n d i c u l a r t o t h e p l a n e o f t h e n i t r a t o g r o u p and l i e a l o n g t h e N-0 b o n d s . There w i l l a l s o be n o n b o n d i n g e l e c t r o n s a s s o c i a t e d w i t h t h e a l k o x y l o x y g e n . One o f t h e n o d a l p l a n e s a s s o c i a t e d w i t h t h e s e o r b i t a l s w i l l p r o b a b l y be c o i n c i d e n t a l w i t h t h e m o l -e c u l a r p l a n e o f t h e ch romophore . The n o d a l s u r f a c e s o f t h e a n t i b o n d i n g T T * o r b i t a l s a r e more a m b i g u o u s , p r i n c i p a l l y because o f t h e u n c e r t a i n t y o f t h e b o n d i n g i n t h e n i t r a t o g r o u p . Sometimes t h e a n t i b o n d i n g s u r f a c e s a r e a p p r o x i m a t e d by p l a n e s , b u t t h e y a c t u a l l y a r e c u r v e d s u r f a c e s ( F i g u r e 28 ) ( 6 3 , 6 4 ) . A p l a n a r g r o u p w i l l a l w a y s have a t l e a s t one p l a n e o f symmet ry . A p l a n a r n i t r a t o g r o u p has o n l y t h e symmetry p l a n e , and t h e chromophore t h e n b e l o n g s t o t h e C s p o i n t g r o u p , w i t h symmetry e l e m e n t s E and g". I n F i g u r e 29 t h e c h a r a c t e r t a b l e o f t h e C s p o i n t g r o u p i s a d a p t e d t o t h e c o o r d i n a t e sys tem o f 86. FIGURE 28 . ANTIBONDING ORBITALS (A) AND NODAL SURFACE OF e-ON0 2(B). E v . j . A' 1 1 » , Z , R * A" 1 ~ 1 Rii xy.xz C 8 C H A R A C T E R T A B L E . FIGURE 29. COORDINATES AND DIPOLE MOMENT 87. t h e n i t r a t o g r o u p . The y and z axes a r e i n t h e symmetry p l a n e , and do n o t change s i g n on r e f l e c t i o n i n t h e p l a n e . The x a x i s , on t h e o t h e r h a n d , does change s i g n on r e f l e c t i o n t h r o u g h t h e p l a n e , as do t h e s i g n s o f t h e 1* b o n d i n g and t h e TT* a n t i b o n d i n g o r b i t a l s . The d i p o l e moment o f t h e g r o u p w i l l a l s o be i n t h i s symmetry p l a n e . F o r t h e n i t r a t o g r o u p , t h e p s e u d o s c a l a r p a r t o f t h e p o t e n t i a l i s t h e A" r e p r e s e n t a t i o n . U s i n g S c h e l l m a n ' s (15) a p p r o a c h t h e n , t h e r e s h o u l d e x i s t a p l a n a r c h i r a l i t y r u l e f o r t h e n i t r a t o g r o u p , w i t h t h e p l a n e b e i n g t h a t o f t h e n o d a l p l a n e o f t h e T T o r b i t a l s ( F i g u r e 3 0 ) . y F i g u r e 3 0 . P l a n a r r u l e ( f r o m Cs p o i n t g r o u p ) f o r t h e n i t r a t o g r o u p . 88. However, t h i s a p p r o a c h f r o m g roup t h e o r y can o n l y s p e c i f y t h e minimum number o f s p a t i a l r e g i o n s ( 1 6 ) . I n a d d i t i o n t o t h e m o l e c u l a r symmetry p l a n e , t h e r e may be o t h e r n o d a l s u r f a c e s n o t d e t e r m i n e d by symmetry w h i c h can l e a d t o f u r -t h e r s u b d i v i s i o n s . F o r t h e n i t r a t o g r o u p , t h e s e a d d i t i o n a l p l a n e s c o u l d o n l y be t h e n o d a l s u r f a c e s o f t h e n and T T o r b i t a l s ( F i g u r e 3 1 ) . The n o d a l p l a n e B ( F i g u r e 31) o f t h e oxygen n o n b o n d i n g o r b i t a l s i s n o t e x p e c t e d t o be i m p o r t a n t s i n c e i t se ldom passes t h r o u g h t h e p e r t u r b i n g m o l e c u l e . S i m i l a r l y , t h e TT* a n t i b o n d i n g o r b i t a l n o d a l • s u r f a c e o n l y o c c a s i o n a l l y passes t h r o u g h t h e m o l e c u l a r bonds o f t h e compound. I t i s o n l y t h e n o d a l p l a n e A ( F i g u r e 31) o f t h e oxygen n o n b o n d i n g o r b i t a l w h i c h c o u l d a p p r e c i a b l y a f f e c t t h e n a t u r e o f t h e c h i r a l i t y r u l e , s i n c e t h i s p l a n e does pass t h r o u g h t h e p e r -t u r b i n g p a r t o f t h e m o l e c u l e . The d i t h i o c a r b a m a t e chromophore i s s i m i l a r i n s t r u c t u r e t o t h e n i t r a t o g r o u p . I c I N D I T H I O C A R B A M A T E N I T R A T O FIGURE 31. POSSIBLE POSITIONS OF THE n AND TT* SURFACES OF THE NITRATO GROUP. NODAL The n i t r a t o g r o u p has o n l y one symmetry p l a n e , w h i l e t h e d i t h i o c a r b a m a t e has two - t h e m o l e c u l a r p l a n e , and a second p e r p e n d i c u l a r p l a n e l y i n g a l o n g t h e N-C b o n d . The CD s p e c t r a o f d i t h i o c a r b a m a t e s have been i n t e r p r e t e d by a q u a d r a n t r u l e (109). A n a l o g o u s l y , t h e CD o f t h e n i t r a t o g r o u p s h o u l d t h e n be d e s c r i b e d by a p l a n a r r u l e . I n t h e a p p l i c a t i o n o f t h i s " p l a n a r r u l e " t o n i t r a t e e s t e r s , t h e g roup i s v i e w e d down t h e 0-C bond ( F i g u r e 3 2 ) . + (P) 0 \ VIEW F i g u r e 32 . V i e w i n g t h e n i t r a t o g r o u p a l o n g t h e 0-C b o n d . I f t h e m a j o r i t y o f t h e p e r t u r b i n g atoms a r e t o t h e r i g h t o f 91 t h e p l a n e , t h e m o l e c u l e i s s a i d t o have p o s i t i v e ( + ) c h i r a l i t y ; i f t h e m a j o r i t y a r e t o t h e l e f t o f t h e p l a n e , t h e compound i s s a i d t o have n e g a t i v e ( - ) c h i r a l i t y . I n a p p l y i n g t h e r u l e , i t must be r e a l i z e d t h a t f r e e r o -t a t i o n a b o u t t h e C-0 bond i s p o s s i b l e and t h e most s t e r i c a l l y f a v o u r e d c o n f o r m a t i o n i s s e l e c t e d by e x a m i n i n g t h e m o d e l s . T h i s t e c h n i q u e has a l s o been a p p l i e d t o n i t r o s t e r o i d s ( 6 3 ) . The s t e r i c a l l y f a v o u r e d c o n f o r m e r w i l l most l i k e l y be t h e p r e d o m i n a n t i s o m e r even a t room t e m p e r a t u r e . I n some c a s e s , however , t h e "mos t s t e r i c a l l y f a v o u r e d c o n f o r m a t i o n " c o u l d n o t be d e f i n i t e l y s e l e c t e d , so a number o f a l t e r n a t i v e p o s s i b i l i t i e s were c o n s i d e r e d . F i g u r e 33 i l l u s t r a t e s t h e a p p l i c a t i o n o f t h e r u l e t o l S s 2 R - b o r n y l n i t r a t e . N I T R A T E ( - ) C H I R A L I T Y F i g u r e 33• A p p l i c a t i o n o f t h e N i t r a t o C h i r a l i t y r u l e t o l S i 2 R - b o r n y l n i t r a t e . S ince b o t h t h e 270 and 230 nm t r a n s i t i o n s a r e t h o u g h t t o b e r r - * i / , c o r r e l a t i o n was a t t e m p t e d w i t h t h e s i g n s o f b o t h o f t h e s e b a n d s . E m p i r i c a l l y , a c o r r e l a t i o n was o n l y o b t a i n e d w i t h t h e s i g n o f t h e 230 nm b a n d . I n a d d i t i o n t o t h e p r o j e c t i o n o f t h e most s t e r i c a l l y p r o b a b l e c o n f o r m a t i o n , f o u r o t h e r p o s s i b l e c o r r e l a t i o n s were examined* 1. W i t h t h e c o n f i g u r a t i o n o f t h e c h i r a l c e n t e r (R o r S) t o w h i c h t h e 0N0 2 g r o u p i s bonded . 2 . W i t h - 0 N 0 2 c i s t o t h e g e m i n a l h y d r o g e n ( F i g u r e 3^A) 3 . W i t h - 0 N 0 2 t r a n s t o t h e g e m i n a l h y d r o g e n 4 . W i t h t h e p r o j e c t i o n v i e w e d down t h e b i s e c t r i x o f t h e O-N-0 g r o u p ( F i g u r e 34C) (D n - 0 N 0 2 i s c i s t o t h e g e m i n a l h y d r o g e n . Bo - 0 N 0 2 i s t r a n s t o t h e g e m i n a l h y d r o g e n . C. -0N0 2 g r o u p i s v i ewed t h r o u g h t h e 0N0 m o i e t y . F i g u r e 34. P r o j e c t i o n s o f t h e n i t r a t o g r o u p . 9 3 . There was no c o r r e l a t i o n f o u n d between t h e s i g n s o f any o f t h e t h r e e d i c h r o i c t r a n s i t i o n s o f t h e n i t r a t o g r o u p and t h o s e p r e d i c t e d f r o m t h e t h r e e p r o j e c t i o n s o f F i g u r e 34. The c i s and t r a n s p r o j e c t i o n s were n o t e x p e c t e d t o be f a v o u r e d f r o m s t e r i c c o n s i d e r a t i o n s . A d d i t i o n a l l y , no c o r r e l a t i o n o f CD s i g n was seen w i t h t h e c o n f i g u r a t i o n o f t h e c h i r a l c e n t e r t o w h i c h the n i t r a t o g roup i s a t t a c h e d . 9k C. A l k y l c y c l o h e x y l N i t r a t e s Seven a l k y l s u b s t i t u t e d c h i r a l c y c l o h e x y l n i t r a t e s were s t u d i e d ( I - V I I , T a b l e I X ) and r e p r e s e n t a t i v e s p e c t r a a r e shown i n F i g u r e 35* As seen f r o m T a b l e X, t h e r e i s no c o r r e l a t i o n o f t h e c o n f i g u r a t i o n o f t h e c h i r a l c e n t e r s w i t h CD band s i g n . There i s , however , f a i r l y good c o r -r e l a t i o n be tween t h e s i g n o f t h e 230 nm band (band I I ) and t h e s i g n o f t h e c h i r a l i t y p r e d i c t e d f r o m t h e p l a n a r n i t r a t o r u l e . I n t h e m e n t h y l , c a r v o m e n t h y l , and c i s - 3 - m e t h y l c y c l o -h e x y l n i t r a t e s , t h e m o l e c u l e s were c o n s i d e r e d i n t h e c h a i r f o r m , w i t h a l l s u b s t i t u e n t s e q u a t o r i a l l y o r i e n t e d . F e l t k a m p and F r a n k l i n (110) made an NMR s t u d y o f t h e c o n f o r m a t i o n s o f m e n t h o l and c a r v o m e n t h o l i n w h i c h t h e y d e t e r m i n e d t h a t t h e p o p u l a t i o n o f t h e e q u a t o r i a l c o n f o r m e r was c l o s e t o 100$. W i t h f e w e r g r o u p s on t h e r i n g , t h e r e wou ld be more o f t h e a x i a l i s o m e r p r e s e n t . I t has been r e p o r t e d t h a t c y c l o h e x y l n i t r a t e has 73$ e q u a t o r i a l n i t r a t o g roup a t +25*C ( i l l ) . B o t h m e n t h y l and c a r v o m e n t h y l n i t r a t e s have n e g a t i v e peaks n e a r 320 nm w h i c h do n o t c o r r e s p o n d t o any known t r a n s -i t i o n o f n i t r a t e e s t e r s ( F i g u r e 3 5 ) . These s m a l l peaks a r e s e p a r a t e d by a b o u t 50 nm f r o m t h e t r a n s i t i o n a t 270 nm and a r e n o t seen i n t h e CD s p e c t r a o f t h e r i g i d a l k y l b i c y c l o -[2.2.1] h e p t y l n i t r a t e s . They may a r i s e f r o m t h e s m a l l p r o p o r t i o n o f a x i a l c o n f o r m e r s i n s o l u t i o n . F o r m e n t h y l n i t r a t e t h i s 320 nm peak has a v a l u e 0.8$ o f t h a t o f t h e 96 T a b l e IX CD Of N i t r a t e s W i t h One Cyc lohexane R i n g N i t r a t e S o l v e n t X i n nm. »([©]) I m e n t h y l n i t r a t e CH^CN c y c l o h e x a n e 3 2 0(-io ) , 3 i M o ) , 2 7 1 (+1230)" 250(0) ,232(-3260>224 (0) , 215(+7300) 3 1 9 ( - 1 8 ) , 3 1 2 ( 0 ) , 2 7 K + 9 1 0 ) , 2 5 0 ( 0 ) , 2 3 2 ( - 3 2 3 0 ) , 2 2 0 ( 0 ) , 2l4(+5800) I I c a r v o m e n t h y l n i t r a t e CH3CN 3 l 8 ( - 0 ) , 3 1 2 ( 0 ) , 2 7 0 ( + 6 3 0 ) , 2 4 8 ( 0 ) , 2 3 5 ( - 8 7 0 ) , 2 2 7 ( 0 ) , 2l6(+5000) I I I i s o c a r v o m e n -t h y l n i t r a t e CH3CN c y c l o h e x a n e 285(+6o),267(+49),229(+870) 2 9 2 ( + 3 0 ) , 2 6 9 ( 0 ) , 2 6 4 ( - 6 ) , 259(0),226(+1100) IV p - m e n t h - l ( 7 ) -e n e - c i s - 2 - o l -n i t r a t e CH^CN 276(+920 ) ,247(+250 ) ,219 (+9200) V p - m e n t h - l ( 7 ) -e n e - t r a n s - 2 -o l - n i t r a t e CH CN 3 cyc lohexane 2 8 3 ( - 6 1 0 ) , 2 6 2 ( 0 ) , 2 3 7 ( + 6 4 0 0 ) , 2 2 7 ( 0 ) , 2 1 8 ( - 1 2 , 0 0 0 ) 277 ( - 1 0 9 0 ^ 5 4 ( 0 ) , 233 (+6340), 224 ( 0 ) , 2 1 5 ( - 9 3 0 0 ) V I c i s - 3 - m e t h y l -c y c l o h e x y l n i -t r a t e * CH3CN c y c l o h e x a n e n - h e p t a n e 268(+65),255(+55)»240(+90) i n c . 2 9 8 ( - 4 ) , 2 8 0 ( 0 ) , 2 6 5 ( + 1 0 ) i n c . 268(+32),260(+31) ,228(+200) V I I t r a n s - 3 - m e -t h y l c y c l o -h e x y l n i t r a t e * CH3CN c y c l o h e x a n e 2 6 8 ( + 3 3 ) , 2 4 5 ( 0 ) , 2 3 5 ( - 7 0 ) i n c . 2 9 2 ( - 8 ) , 2 6 3 ( - 2 ) , 2 4 0 ( - 3 5 ) i n c . * b y R.N. T o t t y i n c . - and i n c r e a s i n g 9 7 . 8" • n i 3 •2--- 4 -• 6- • - 8 - • - 1 0 " -12 X A MENTHYL NITRATE f-MENTH-IC7)-ENE- 'R AM-2-OL-NITRATE ip^OL- NITRATE •CARVOMENTHYL NITRATE NITRATE 98. T a b l e X A l k y l c y c l o h e x y l N i t r a t e s CD Band S i g n s and P r e d i c t e d C h i r a l i t y Com-NITRATE C o n f i g -u r a t i o n CD Band S i g n P r e d i c t e d S i g n pounc # I I I I l l I m e n t h y l n i t r a t e 1 R : . 3 R I 4 S + . + I I c a r v o m e n t h y l n i t r a t e 1R:2R:4R + - + -I I I i s o c a r v o m e n t h y l , 1 S S 2 S J 4 R + + + IV p - m e n t h - 1 ( 7 ) - e n e -c i s - 2 - o l - n i t r a t e 2 R I 4 R + +? + V p - m e n t h - l ( 7 ) - e n e -t r a n s - 2 - o l - n i t r a t e 2S:4R - + - + V I c i s - 3 - m e t h y l c y -c l o h e x y l n i t r a t e 1S«3R +? + + V I I t r a n s - 3 - m e t h y 1 - I S os +? + 1 c y c l o h e x y l n i t r a t e -f 9 9 . 270 nm p e a k . I t i s s i m i l a r t o t h e doub le-humped s p e c t r u m r e c o r d e d f o r ( - ) - m e n t h o n e ( 2 8 ) . F i g u r e 36 i l l u s t r a t e s how t h e doub le-humped peak f o r m e n t h y l and c a r v o m e n t h y l n i t r a t e s may be g e n e r a t e d . F i g u r e j6. Reso lved bands ( n o t t o s c a l e ) o f t h e 270 nm t r a n s i t i o n o f m e n t h y l and c a r v o m e n t h y l n i t r a t e s . ( + ) - c i s - 3 - M e t h y l c y c l o h e x y l n i t r a t e i s e x p e c t e d t o have a l a r g e r p r o p o r t i o n o f t h e a x i a l c o n f o r m e r . No double-humped peak was d e t e c t e d i n i t s s p e c t r u m , b u t i t was v e r y s e n s i t i v e t o s o l v e n t . I n c y c l o h e x a n e , i t has a n e g a t i v e band a t 298 nm ( [e] -4) w h i l e i n a c e t o n i t r i l e , n - h e p t a n e and m e t h a n o l , a c o m p l e t e l y p o s i t i v e s p e c t r u m was o b t a i n e d . T h i s s e n s i t i v i t y t o s o l v e n t no d o u b t r e f l e c t s v a r i a t i o n s i n t h e e q u i l i b r i u m 100 . 5 between a x i a l and e q u a t o r i a l f o r m s . The e q u a t o r i a l c o n f o r m e r no d o u b t has a p o s i t i v e 270 nm band w h i l e t h e a x i a l has a n e g a t i v e one . Even t h o u g h t h e p r o p o s e d c h i r a l i t y r u l e i s n o t a p p l i e d t o t h e 270 nm t r a n s i t i o n , t h e r u l e does p r e d i c t o p p o s i t e s i g n s f o r t h e e q u a t o r i a l and a x i a l i s o m e r s o f c i s -3 - m e t h y l c y c l o h e x y l n i t r a t e . The c o r r e c t s i g n o f t h e 230 nm band i s a l s o p r e d i c t e d f o r i s o c a r v o m e n t h y l n i t r a t e . I n t h i s n i t r a t e , a l l sub-s t i t u e n t s canno t be i n t h e e q u a t o r i a l p o s i t i o n . A f t e r c o n -s i d e r i n g t h e AG v a l u e s o f v a r i o u s s u b s t i t u e n t s ( T a b l e X I ) , i s o c a r v o m e n t h y l n i t r a t e was drawn w i t h an a x i a l i s o p r o p y l g r o u p . T a b l e X I AG V a l u e s Of V a r i o u s S u b s t i t u e n t s (112) C 6 H n X a x i a T * " c 6 H n X e q a t o r i a l S u b s t i t u e n t X , r A G -GHo -GH<CH-) 0 - 0 N 0 p J c -OH 1.70 2.15 0.59 0„ 52 The a s s u m p t i o n i s made t h a t t h e AG v a l u e o f t h e m o l e c u l e i s t h e sum o f t h e A G v a l u e s o f t h e i n d i v i d u a l s u b s t i t u e n t s . 101. T h i s i s t h e same a s s u m p t i o n made by Felkamp and F r a n k l i n (110) who f o u n d e x p e r i m e n t a l l y t h a t i s o c a r v o m e n t h o l e x i s t s 70$ w i t h an e q u a t o r i a l OH. I t i s e x p e c t e d t h e n t h a t i s o -c a r v o m e n t h y l n i t r a t e wou ld e x i s t w i t h a s l i g h t l y g r e a t e r t h a n 70$ e q u a t o r i a l n i t r a t o g r o u p . The 270 nm CD band o f i s o c a r v o m e n t h y l n i t r a t e ( I I I ) i s much weaker t h a n t h o s e o f m e n t h y l o r c a r v o m e n t h y l n i -t r a t e . I n a c e t o n i t r i l e s o l v e n t , i s o c a r v o m e n t h y l n i t r a t e does n o t e x h i b i t t h e s m a l l peak a t 320 nm. However, b e s i d e s h a v i n g a much weaker 270 nm b a n d , i t i s a l s o s o l v e n t s e n s i t i v e ( F i g u r e 3 7 ) . The s m a l l n e g a t i v e peak i n c y c l o -hexane i n d i c a t e s t h e p r e s e n c e o f a n e g a t i v e band w h i c h i s p r o b a b l y due t o t h e l e s s s t a b l e c o n f o r m e r . P R E D I C T E D T O B E L E S S F A V O U R E D The CD s p e c t r u m o f i s o c a r v o m e n t h y l f r o m 250 t o 350 nm 102. 103 can be l o o k e d upon as one s i m i l a r t o t h a t o f m e n t h y l and c a r v o m e n t h y l n i t r a t e s w i t h t h e s m a l l peak on t h e s h o r t -w a v e l e n g t h s i d e . The low i n t e n s i t y o f t h e 270 nm b a n d , w h i c h seems t o be the most s e n s i t i v e t o c o n f o r m a t i o n , i s p r o b a b l y due t o t h e g r e a t e r p r o p o r t i o n o f o t h e r l e s s s t a b l e c o n f o r m e r s w i t h o p p o s i t e l y s i g n e d p e a k s . By " l e s s s t a b l e " , i t i s t h e thermodynamic s t a b i l i t y t h a t i s r e f e r r e d t o . Because o f t h e l a r g e r v a l u e o f -AG f o r a m e t h y l t h a n a n i t r a t o g r o u p , t r a n s - 3 - m e t h y l c y c l o h e x y l n i t r a t e ( V I I ) was drawn i n t h e p r o j e c t i o n w i t h an a x i a l 0N0 2 and an e q u a t o r i a l CH^. Even s o , a d e c i s i o n on t h e most s t e r i c a l l y f a v o u r a b l e n i t r a t o c o n f o r m a t i o n c o u l d n o t be made, and t h u s t h e s i g n s f r o m two p r o j e c t i o n s appear i n T a b l e X. t r a n s - 3 - M e t h y I c y c l o h e x y l n i t r a t e i s even more s e n -s i t i v e t o s o l v e n t t h a t t h e c i s i s o m e r , w h i c h i s t o be e x -p e c t e d . The t r a n s i s o m e r has weaker CD bands t h a n t h e c i s and has s i g n r e v e r s a l o f t h e 270 nm band on g o i n g f r o m a c e -t o n i t r i l e t o c y c l o h e x a n e . A g a i n , ' t h i s can e a s i l y be e x -p l a i n e d as a c o n f o r m a t i o n a l e f f e c t . I t i s more d i f f i c u l t t o s e l e c t a f a v o u r e d c o n f o r m a t i o n f o r p - m e n t h - l ( 7 ) - e n e - t r a n s - 2 - o l - n i t r a t e ( V ) . 104 C o n f o r m a t i o n A i s f a v o u r e d o v e r B s i n c e t h e n i t r a t o g r o u p i n B w o u l d be v e r y c l o s e t o t h e Tr* o r b i t a l s o f t h e C=CH 2 g r o u p . The NMR s p e c t r u m i n d i c a t e s t h e n i t r a t o g r o u p t o be a x i a l ; t h e g e m i n a l p r o t o n resonance^ w h i c h a p p e a r s as a d i s t o r t e d t r i p l e t , has a band w i d t h o f o n l y 10 c p s . P r o -j e c t i o n A p r e d i c t s a p o s i t i v e b a n d , and a p o s i t i v e 230 nm CD band was o b s e r v e d . The 230 nm band o f p - m e n t h - l ( 7 ) - e n e - c i s - 2 - o l - n i t r a t e ( I V ) i s c o m p l e t e l y h i d d e n u n d e r t h e t r a n s i t i o n o f band I I I . W i t h l ow t e m p e r a t u r e and s o l v e n t s t u d i e s i t may be p o s s i b l e t o c o r r e c t l y a s c e r t a i n t h e s i g n o f t h i s b a n d , a l t h o u g h i t a p p e a r s t o be p o s i t i v e . These two n i t r a t e s ( I V and V) a l s o show marked b a t h o -c h r o m i c s h i f t s ( a b o u t 5 nm) i n g o i n g f r o m c y c l o h e x a n e t o a c e t o n i t r i l e s o l v e n t . A n o t h e r u n u s u a l p r o p e r t y o f t h i s p a i r , when compared t o t h e o t h e r n i t r a t e e s t e r s i n t h i s s e r i e s , i s t h e v e r y s t r o n g i n t e n s i t y o f band I I I (218 n m ) . N i -t r a t e V has a m o l e c u l a r e l l i p t i c i t y o f 2 1 g - 1 2 , 0 0 0 i n 105. a c e t o n i t r i l e . T h i s c o u l d be due t o e l e c t r o s t a t i c i n t e r -a c t i o n s between t h e t r a n s i t i o n s o f o l e f i n i c and n i t r a t o ch romophores ; t h e i n c r e a s e d i n t e n s i t y c o u l d r e s u l t f r o m t r a n s i t i o n moment c o u p l i n g . I t i s t h o u g h t t h a t t r a n s i t i o n moment c o u p l i n g a l s o a c c o u n t s f o r t h e i n c r e a s e d i n t e n s i t y o f t h e CD bands o f /3 - u n s a t u r a t e d k e t o n e s ( 1 1 3 ) . ! R i 3 R t 4 S - M e n t h y l N i t r a t e V a r i a b l e Tempera tu re S t u d y A v a r i a b l e t e m p e r a t u r e s t u d y was c a r r i e d o u t on m e n t h y l n i t r a t e ( I ) i n m e t h a n o l o v e r t h e t e m p e r a t u r e range +30*C t o 90*C ( T a b l e X I I , F i g u r e 3 8 ) . As t h e t e m p e r a t u r e i s l o w e r e d , t h e 320 nm band d e c r e a s e s , w h i l e a t t h e same t i m e , t h e 270 nm band i n c r e a s e s . T h i s i n -d i c a t e s t h a t e q u i l i b r a t i o n i s o c c u r r i n g and what one i s s e e i n g i s t h e i n c r e a s i n g p o p u l a t i o n o f one f o r m a t t h e expense o f a n o t h e r . S ince i t i s t h e s h o r t w a v e l e n g t h peak t h a t i n c r e a s e s w i t h l o w e r i n g o f t e m p e r a t u r e , t h i s b e h a v i o u r i s c o n s i s t e n t w i t h b o t h a s o l v a t i o n and a c o n f o r m a t i o n a l e q u i l i b r i u m . I n t h e case o f ( - ) - m e n t h o n e , i t was t h e l o n g w a v e l e n g t h peak t h a t grew as t e m p e r a t u r e was l o w e r e d ( 2 8 ) . The o p p o s i t e i s t r u e i n t h e case o f m e n t h y l n i t r a t e . The o n l y t i m e doub le-humped peaks were observed i n t h e CD s p e c t r a o f n i t r a t e e s t e r s was when a c o n f o r m a t i o n a l e q u i l i b r i u m was p o s s i b l e , as i n t h e case o f m e n t h y l n i t r a t e , o r where t h e r e was t h e p o s s i b i l i t y o f r o t a m e r s i n s o l u t i o n , as w i t h D-cfr-ni-t r a t o p r o p a n o i c a c i d . T h e r e f o r e , t h e e q u i l i b r i u m seen i n t h e 106, T a b l e X I I V a r i a b l e Tempera tu re S t u d y o f l R j 3 R « 4 S - M e n t h y l N i t r a t e ( I ) Temper- 320 nm Band Band I Band I I e r a t u r e C O > ( n m ) [ e ] >• (nm) [ 6 ] X (nm 30 20 321 322 - 8 . 8 271 271-2 +1080 +1120 232 232 -3100 -3100 -55 270-2 +1350 -90 324-5 - 2 . 2 271-2 + 1480 233 -3520 * a t 15°C A. BAND X (270 nm) . B. 320 nm BAND FIGURE 38. LTCD OF MENTHYL NITRATE (1) IN METHANOL. 10?. CD s p e c t r u m o f m e n t h y l n i t r a t e i s t h o u g h t t o be a c o n f o r -m a t i o n a l one . As t h e t e m p e r a t u r e i s l o w e r e d , t h e p o p u l a t i o n o f t h e e q u a t o r i a l c o n f o r m e r i n c r e a s e s ( p o s i t i v e 270 nm band) w h i l e t h e p o p u l a t i o n o f t h e l e s s s t a b l e c o n f o r m e r d e c r e a s e s ( n e g a -t i v e 230 nm b a n d ) . However, a change i n r o t a m e r p o p u l a t i o n as t e m p e r a t u r e i s changed i s s t i l l a v e r y p r o b a b l e a l t e r n a t i v e e x p l a n a t i o n f o r t h e doub le -humped peak o f m e n t h y l n i t r a t e . Over t h e t e m p e r a t u r e range s t u d i e d , t h e m o l e c u l a r e l -l i p t i c i t y o f t h e 270 nm band i n c r e a s e s l i n e a r l y w i t h t e m p -e r a t u r e ( F i g u r e 3 9 ) • T h i s same t y p e o f b e h a v i o u r was seen f o r c a r v o m e n t h y l n i t r a t e ( I I ) , a l t h o u g h t h e measurements were n o t made q u a n t i t a t i v e l y . Band I I I a l s o shows an i n c r e a s e i n m o l e c u l a r e l l i p t i c i t y as t h e t e m p e r a t u r e i s l o w e r e d , b u t n o t n e a r l y as g r e a t as t h e i n c r e a s e i n t h e 270 nm band (band I ) . 1R; 3R14S-IV!enthyl N i t r a t e S o l v e n t S t u d y The CD o f m e n t h y l n i t r a t e ( I ) was a l s o examined i n f i f t e e n s o l v e n t s , as t h e n e a t l i q u i d and i n t h e v a p o u r s t a t e ( T a b l e X I I I ) . S o l v e n t s i n c l u d e d p o l a r and n o n p o l a r t y p e s , h a l o g e n a t e d s o l v e n t s , and s o l v e n t s w i t h a w ide v a r i e t y 108. 15-r FIGURE 39. T E M P E R A T U R E CcV VARIATION OF THE 270 nm. BAND OF MENTHYL NITRATE (I) WITH TEMPERATURE. an*© 270 FIGURE 40. MENTHYL NITRATE RELATION OF e^o TO zei 270 ' 109. T a b l e X I I I C i r c u l a r D i c h r o i s m o f 1R :3R»4S -Menthy l N i t r a t e : S o l v e n t E f f e c t s Band 320 Band I Band I I S O L V E N T [el [e] H e x a f l u o r o i s o p r o p a n p l 323 - 6 . 4 271 + 1590 234 -2880 D i m e t h y l s u l p h o x i d e 324 - 9 . 3 271 +1410 T r i f l u o r a c e t i c a c i d 272 +1350 C h l o r o f o r m 321 - 1 2 . 0 270 +1300 D i m e t h y l f o r a m i d e 323 - 1 0 . 1 271 +1260 A c e t o n i t r i l e 320 - 1 0 . 271 +1230 232 -3230 A c e t i c a c i d 321 - 1 0 . 5 271 +1190 T r i m e t h y l phospha te 322 - 1 0 . 4 273 +1150 233 -3080 I s o p r o p a n o l 322 -12 .3 271 +1100 234 -3150 M e t h a n o l 321 -11.7 271 +1080 232 -3100 T e t r a h y d r o f u r a n 322 - 1 4 . 4 272 +1080 235 -2820 c i s - 1 , 2 - D i m e t h y l - 1 -c a r b o m e t h o x y c y c l o -p ropane 323 -12 .3 271 +1030 n-Hexane 321 269 +1000 c i s - P i n a n e 324 -16 .7 272 +980 234 -3030 Cyclohexane 319 - 1 8 271 +910 232 -3230 NEAT LIQUID 320 -12.6 271 +1130 VAPOUR 269 ( + ) * 233 ( - ) * * c o n c e n t r a t i o n ( p r e s s u r e ) o f sample unknown. 110. o f f u n c t i o n a l g r o u p s . I t i s n o t p o s s i b l e t o t e l l f r o m t h e u l t r a v i o l e t s p e c t r a o f n i t r a t e e s t e r s w h e t h e r t h e r e i s any w a v e l e n g t h s h i f t , b u t i t i s q u i t e e v i d e n t by e x a m i n i n g CD bands I and I I t h a t t h e r e i s no such s h i f t , w i t h i n t h e p o s s i b l e e x p e r i m e n t a l e r r o r o f +2 nm. S ince t h e r e i s no dependence o f w a v e l e n g t h w i t h s o l v e n t , i t i n d i c a t e s t h a t t h e r e i s no e l e c t r o s t a t i c i n t e r a c t i o n o f t h e s o l v e n t so as t o a l t e r t h e e n e r g y o f t h e t r a n s i t i o n . There i s no e v i d e n c e o f h y d r o g e n b o n d i n g i n h e x a f l u o r o -i s o p r o p a n o l s o l v e n t w h i c h h y d r o g e n bonds v e r y s t r o n g l y w i t h k e t o n e s . The i n f o r m a t i o n f r o m t h e s o l v e n t s t u d y a l s o i n d i c a t e s t h e p r e s e n c e o f an e q u i l i b r i u m . G e n e r a l l y t h e 320 nm peak i s most i n t e n s e i n t h e n o n p o l a r h y d r o c a r b o n s o l v e n t s and • i t s i n t e n s i t y dec reases as t h e p o l a r i t y o f t h e s o l v e n t i n -c r e a s e s . The o p p o s i t e i s t r u e o f t h e 270 nm b a n d , w h i c h i s most i n t e n s e i n t h e h i g h l y p o l a r s o l v e n t s and weakes t i n t h e n o n p o l a r h y d r o c a r b o n s o l v e n t s . T h i s can be i n t e r p r e t e d as an i n c r e a s e d amount o f t h e a x i a l f o r m i n n o n p o l a r s o l v e n t s . S i m i l a r r e s u l t s were o b t a i n e d f o r ( - ) - m e n t h o n e and t r a n s - 2 -c h l o r o - 5 - m e t h y l c y c l o h e x a n o n e ( 1 1 4 ) . There i s a l s o a r o u g h c o r r e l a t i o n o f t h e m o l e c u l a r e l l i p t i c i t y o f t h e 270 nm band w i t h t h e i n t e n s i t y o f t h e UV a b s o r p t i o n a t t h e same w a v e l e n g t h ( F i g u r e 4 0 ) . I t must be remembered t h a t t h e UV i n t e n s i t y i s n o t due t o a s i n g l e t r a n s -i t i o n , b u t i s a l s o t h e r e s u l t o f o t h e r o v e r l a p p i n g bands . I l l The v a l u e s i n h e x a f l u o r o i s o p r o p a n o l do n o t f a l l a l o n g t h e c u r v e . ^ A d d i t i o n a l l y , t h e m o l e c u l a r e l l i p t i c i t y i s v e r y i n -t e n s e i n t h i s f l u o r i n a t e d s o l v e n t . I t has a l s o been n o t e d by o t h e r a u t h o r s (29) t h a t t h e g r e a t e s t s o l v e n t e f f e c t s i n k e t o n e s o c c u r r e d w i t h f l u o r i n a t e d s o l v e n t s . T h i s i n -d i c a t e s t h e n t h a t h e x a f l u o r o i s o p r o p a n o l may a f f e c t n o t o n l y t h e c o n f o r m e r p o p u l a t i o n s , b u t a l s o t h e e l e c t r o n i c t r a n s -i t i o n . However, t h e r e was no w a v e l e n g t h s h i f t o f e i t h e r band I o r I I , b u t o n l y i n c r e a s e d i n t e n s i t y o f band I . I t may be p o s s i b l e t h a t t h e e l e c t r o n - w i t h d r a w i n g f l u o r i n e atoms w o u l d a f f e c t t h e e l e c t r o n d e n s i t y o f t h e n i t r a t o g r o u p . The m o l e c u l a r e l l i p t i c i t y was a l s o examined i n r e l a t i o n t o v a r i o u s s o l v e n t p a r a m e t e r s . There was no c o r r e l a t i o n o f t h e m o l e c u l a r e l l i p t i c i t y o f t h e 270 nm band w i t h r e -f r a c t i v e i n d e x , p o l a r i t y , o r d i p o l e moment o f t h e s o l v e n t . There was no c o r r e l a t i o n w i t h t h e r e l a t i o n K - l / K + 2 . There i s , p e r h a p s , a r o u g h c o r r e l a t i o n w i t h d i e l e c t r i c c o n s t a n t and Z v a l u e ( F i g u r e 4 l ) . Summary Of t h e seven a l k y l c y c l o h e x y l n i t r a t e s , f i v e have 230 nm bands t h a t can be c o r r e l a t e d w i t h t h e s i g n p r e d i c t e d by a p l a n a r n i t r a t o r u l e ; f o r t h e two o t h e r s , band s i g n a s s i g n m e n t o r c h i r a l i t y p r e d i c t i o n was u n c e r t a i n . No a p p r e c i -a b l e s o l v e n t s h i f t o f bands I and I I was d e t e c t e d i n a w ide v a r i e t y o f s o l v e n t s . Low t e m p e r a t u r e and s o l v e n t s t u d i e s 112. i n d i c a t e t h e p resence o f a c o n f o r m a t i o n a l e q u i l i b r i u m i n m e n t h y l , and c a r v o m e n t h y l n i t r a t e s , a l t h o u g h t h e p o s s i b i l i t y o f a s o l v a t i o n e q u i l i b r i u m canno t be c o m p l e t e l y e l i m i n a t e d . 1 1 3 . i i 4 ; D. N i t r a t e s Of Alkylbicyclor2,2 , i " l H e p t a n o l s The CD s p e c t r a o f seven n i t r a t e s o f a l k y l b i c y c l o[2.2. l j -h e p t a n o l s ( V I I I - X I V , T a b l e X I V ) were r e c o r d e d . A l l seven have t h e s i g n s o f t h e i r 230 nm CD band (band I I ) p r e d i c t e d c o r r e c t l y by t h e p l a n a r n i t r a t o c h i r a l i t y r u l e ( T a b l e X V ) . T h i s s e r i e s i s n o t s u b j e c t t o a s k e l e t a l e q u i l i b r i u m . Any c o n f o r m a t i o n a l changes may be a t t r i b u t e d s o l e l y t o r o -t a t i o n o f t h e n i t r a t o g r o u p on t h e C-0 b o n d . There was no d i f f i c u l t y i n s e l e c t i n g t h e s t e r i c a l l y f a v o u r e d c o n f o r m a t i o n f o r b o r n y l ( I X ) and i s o b o r n y l ( V I I I ) n i t r a t e s . I s o b o r n y l n i t r a t e does n o t have d i s t i n c t bands I I and I I I as b o r n y l n i t r a t e d o e s , b u t i t seems t h a t b o t h a r e n e g a t i v e and o v e r l a p ( F i g u r e 42) . Low t e m p e r a t u r e CD (LTCD) measurements c o n f i r m t h a t t h e 230 nm band i s n e g a t i v e . The c h i r a l i t y r u l e p r e d i c t s a n e g a t i v e 230 nm CD band f o r i s o b o r n y l n i t r a t e . I t was more d i f f i c u l t t o s e l e c t t h e s t e r i c a l l y f a v o u r e d c o n f o r m a t i o n f o r D-Q(- fenchy l n i t r a t e ( X ) . There were two f a v o u r e d c o n f o r m a t i o n s - one p r e d i c t i n g a n e g a t i v e c h i r a l i t y and t h e o t h e r c o n f o r m a t i o n a p o s i t i v e c h i r a l i t y . U s i n g P e t e r s o n m o l e c u l a r models and m e a s u r i n g t h e d i s t a n c e s b e -tween a toms , i t was f o u n d t h a t t h e c o n f o r m a t i o n t h a t p r e -d i c t s a p o s i t i v e 230 nm CD band (band I I ) i s s l i g h t l y f a -v o u r e d s t e r i c a l l y . E x p e r i m e n t a l l y , a p o s i t i v e CD band was o b s e r v e d . 116 T a b l e XIV CD Of N i t r a t e s Of A l k y l b i c y c l o [ 2 . 2 . 1 . ] h e p t a n o l s # N i t r a t e S o l v e n t > - (nm) , [© j V I I I 1 Ri 2 R - I s o b o r n y l N i t r a t e CH CN 3 c y c l o h e x a n e 2 7 0 ( + 1 3 0 ) , 2 5 2 (0 ) ,2 2 3 -4 ( - 2 2 0 0 ) 2 6 5 ( - 9 0),245 ( - 2 0 ) m i n . , 2 2 0 ( - 1 8 0 0 ) I X I Si2R-B o r n y l N i t r a t e CH3CN c y c l o h e x a n e 2 7 0 ( - 2 6 0 ) , 2 3 2 ( - 3 3 0 0 ) , 2 2 1 (0) ,212 (+2900) 270(-245) ,231 ( -3400) X D-cC-Fenchyl N i t r a t e CH^CN c y c l o h e x a n e 2 7 7 ( - 7 4 0 ) , 2 6 0 ( 0 ) , 2 3 2 ( + 1 1 , 2 0 0 ) 2 2 2 ( 0 ) , 2 1 2 ( - 1 2 , 7 0 0 ) 277(-475),262(0),233 ( + 1 0 , 5 0 0 0 ) , 2 2 2 ( 0 ) , 2 1 0 ( 9 8 0 0 ) X I Camphane-2-exo, 3 - e x o - d i o l d i -n i t r a t e CH CN cyc lohexane 2 7 3 ( - 5 9 0 ) , 2 2 2 ( 0 ) , 2 1 0 ( + 9 2 0 ) 266(-425) X I I Camphane-2-ejco, 3 - e n d o - d i o l d i -n i t r a t e CH^CN 2 8 l ( + 1 0 1 ) , 2 6 7 ( 0 ) , 2 2 8 - 9 ( - 5 1 0 0 ) , 2 2 1 ( 0 ) , 2 1 l ( + 7 2 0 0 ) X I I I Camphane-2-endo, 3 - ^ n d o - d i o l d i -n i t r a t e CH-jCN c y c l o h e x a n e 2 7 5 ( + 4 0 0 ) , 2 2 9 ( + 4 l 0 0 ) , 2 l 8 ( 0 ) , 2 0 6 ( - 2 0 0 0 ? ) 2 9 0 ( + 3 4 ) , 2 7 0 ( 0 ) , 2 2 8 ( + 3 6 0 0 ) XIV Camphane-2-endo 3 - e x o - d i o l d i -n i t r a t e CH3CN cyc lohexane 2 7 0 ( + 2 0 0 ) * 2 3 0 ( + 9 0 0 0 ) , 2 1 7 ( 0 ) , 209(-9000) 2 8 5 ( - 1 4 8 ) , 2 6 6 ( 0 ) , 2 2 9 ( + 7 1 0 0 ) m i n , - minimum * n o t a maximum 1 1 7 . 118 . T a b l e XV Band S i g n s And C h i r a l i t y Ru le P r e d i c t i o n s F o r N i t r a t e s Of A l k y l b i c y c l o [ 2 . 2 . l ] h e p t a n o l s # N i t r a t e C o n f i g - Band S i g n P r e d i c -u r a t i o n I I I I I I t e d S i g n V I I I I s o b o r n y l N i t r a t e 1 R I 2 R . + ( - ) ( - ) -IX B o r n y l N i t r a t e 1S:2R ± a - + -X D - d - F e n c h y l N i t r a t e 1 R J 2 R - + - + X I C a m p h a n e - 2 - e x o , 3 - e x o -d i o l - d i n i t r a t e 1 R I 2 S I 3 R - + -X I I C a m p h a n e - 2 - e x o , 3 - e n d o -d i o l d i n i t r a t e 1 R : 2 S « 3 S + - + ( + ) X I I I C a m p h a n e - 2 - e n d o , 3 - e n d o -d i o l d i n i t r a t e 1 R J 2 R S 3 S + + - + XIV C a m p h a n e - 2 - e n d o , 3 - e x o -d i o l d i n i t r a t e 1R J 2R:3R - b + + + ( - ) a . ( + ) a t low t e m p e r a t u r e . ( - ) a t room t e m p e r a t u r e . b. ( + ) i n C H 3 C N , ( - ) i n CgH 119 . The f o u r c a m p h a n e - 2 , 3 - d i o l d i n i t r a t e s ( X I - X I V ) a l s o have t h e i r band s i g n s (band I I ) p r e d i c t e d by t h e c h i r a l i t y r u l e . There was no d i f f i c u l t y i n s e l e c t i n g t h e f a v o u r e d c o n f o r m a t i o n o f t h e 2 - e x o , 3 - e x o and 2 - e n d o , 3 - e n d o i s o m e r s ( X I and X I I I ) . The n i t r a t o g r o u p s a r e c i s and w i l l be i n -c l i n e d i n t h e same d i r e c t i o n . B e s i d e s b e i n g t h e s t e r i c a l l y f a v o u r e d c o n f o r m a t i o n s , t h e s e a l s o a l l o w f o r c l o s e c o n t a c t o f t h e n i t r o oxygen o f t h e C-2 n i t r a t o g r o u p w i t h t h e a l k o x y l oxygen o f t h e C-3 n i t r a t o (pages 61 -64 o f t h i s t h e s i s ) . C a m p h a n e - 2 - e x o , 3 - e x o - d i o l d i n i t r a t e has a v e r y weak n e g a t i v e CD a b s o r p t i o n a t 230 nm ( F i g u r e 4 3 , T a b l e X I V ) . The m o l e c u l a r e l l i p t i c i t y does n o t become p o s i t i v e u n t i l 222 nm i s r e a c h e d , and t h i s t h e r e f o r e i n d i c a t e s t h e p resence o f a weak n e g a t i v e 230 nm band . B o t h n i t r a t o g roups g i v e p r o j e c t i o n s w h i c h p r e d i c t a n e g a t i v e s i g n f o r band I I . There was no d i f f i c u l t y i n s e l e c t i n g t h e s t e r i c a l l y f a v o u r e d c o n f o r m a t i o n o f t h e t r a n s d i n i t r a t e s ( 2 - e n d o , 3 - e x o ; 2 - e x o , 3 - e n d o ) . However, t h e 2 and 3 n i t r a t o g r o u p s , when v i e w e d f o r t h e r u l e a p p l i c a t i o n , p r e d i c t o p p o s i t e c h i r a l i t y ( F i g u r e 4 4 ) . I f t h e numbers o f atoms i n t h e p o s i t i v e and n e g a t i v e s e c t o r s a r e a l g e b r a i c a l l y summed t h e n t h e s i g n o f t h e t o t a l g i v e s t h e c o r r e c t p r e d i c t i o n o f CD s i g n . I n a d d i t i o n , t h e second n i t r a t o g r o u p a l w a y s f a l l s i n t h e s e c t i o n w i t h t h e s i g n o f t h e 230 nm band . S i n c e t h e n i t r a t o g r o u p s a r e 0C t o each o t h e r , t h e y a r e e x p e c t e d t o be t h e p r i m e sou rce o f c h r o m o p h o r i c p e r t u r b a t i o n . A second t y p e o f c o n f o r m a t i o n w h i c h a l l o w e d even a c l o s e r 1 2 0 . FIGURE 43. CD OF CAMPHANE- 2,3-DIOL DINITRATES (CH-CN). 121 + + + + C) 9 O 6 A. 3 -endo 2 -exo B. 2 -endo 3 -exo T o t a l atomss ( + ) 9 ( - ) 1 5 E x p e r i m e n t a l l y ( - ) Band I I T o t a l a t o m s : ( + ) 14 ( - ) 9 E x p e r i m e n t a l l y ( + ) Band I I A. 2 - e x o , 3 - e n d o B. 2 - e n d o , 3 - e x o F i g u r e 44 . C h i r a l i t y Rule P r o j e c t i o n s o f n i t r a t o g r o u p s i n i n t e r a c t i o n o f a l k o x y l oxygens w i t h n i t r o oxygen v/as examined. However, t h e s e were n o t t h e most s t e r i c a l l y f a v o u r e d c o n -f o r m a t i o n s . I n t h e s e c o n f o r m a t i o n s , b o t h n i t r a t o g r o u p s p r e d i c t t h e same s i g n , and t h a t s i g n was t h e s i g n o f t h e 230 nm CD band . CD S p e c t r a o f B o r n y l , I s o b o r n y l and F e n c h y l N i t r a t e s I n t h i s s e r i e s o f t h r e e m o n o n i t r a t e s ( V I I I - X ) , t h e CD s p e c t r u m o f D - o ( - f e n c h y l n i t r a t e (X) i s o u t s t a n d i n g . Of t h e t h r e e , i t has t h e most i n t e n s e CD bands ; i t has t h e most i n -t e n s e 230 nm band o f any monon i t ra te so f a r r e p o r t e d ( [©]232+ 1 1 , 2 0 0 K T h i s no doub t r e f l e c t s t h e s t e r i c h i n d r a n c e o f X I I and X I V . 122 t h e g e m i n a l d i m e t h y l g r o u p a d j a c e n t t o t h e n i t r a t o g roup w h i c h w o u l d r e s t r i c t C-0 bond r o t a t i o n . Tempera tu re E f f e c t s The 270 nm band o f b o r n y l , i s o b o r n y l and f e n c h y l n i -t r a t e s i s t e m p e r a t u r e d e p e n d e n t . The 270 nm band o f b o r n y l and i s o b o r n y l n i t r a t e s r e v e r s e s i g n on c o o l i n g , whereas t h i s band o f f e n c h y l n i t r a t e i n c r e a s e s as t h e t e m p e r a t u r e i s l o w e r e d ( T a b l e X V I ) . I t i s d i f f i c u l t t o s e p a r a t e c o n f o r m a t i o n a l f r o m s o l v e n t e f f e c t s . There a r e t h r e e p o s s i b l e e x p l a n a t i o n s o f t h e s e s i g n r e v e r s a l s as t e m p e r a t u r e i s l o w e r e d t 1 ) S ince r o t a t i o n a b o u t t h e C-0 bond i s p o s s i b l e , a l o w e r i n g o f t e m p e r a t u r e w o u l d a l t e r t h e p r o p o r t i o n o f r o t -amers . As t h e t e m p e r a t u r e i s l o w e r e d , t h e p o p u l a t i o n o f t h e most t h e r m o d y n a m i c a l l y s t a b l e r o t a m e r wou ld i n c r e a s e . 2) As t h e t e m p e r a t u r e i s l o w e r e d , t h e r e c o u l d be i n -c reased s o l v e n t i n t e r a c t i o n , w h i c h wou ld a l t e r t h e c o n f o r -m a t i o n o f t h e n i t r a t o g r o u p . 3) The e f f e c t c o u l d be due t o s o l v a t i o n a l o n e ; a c o n -f o r m a t i o n a l mechanism need n o t be i n v o l v e d . T h i s wou ld be s i m i l a r t o t h e s o l v e n t e f f e c t s i n b i c y c l o ( 2 . 2 . l j hep tanones ( 2 5 ) . D-oc-Fenchyl n i t r a t e i s t h e l e a s t a f f e c t e d by t e m p e r a t u r e and s o l v e n t , w h i l e t e m p e r a t u r e v a r i a t i o n has t h e g r e a t e s t 123. T a b l e XVI LTCD Of B o r n y l , I s o b o r n y l And F e n c h y l N i t r a t e S o l v e n t Temp-e r a t u r e Cc ) > ( n m ) Band I [e] " M m * a t Band I I nm, m CH-jCN MeOH EPA CHC1. n - p e n t a n e MeOH n - p e n t a n e ! S t 2 R - B o r n y l N i t r a t e +30 0 - 4 0 +30 +20 -20 -35 -50 -65 -75 -90 ~-100 -10 -95 -145 30 0 -6o o -125 2 7 0 i 275 286 2 7 0 i 2 7 0 i 2 7 0 i 280 270 271 270 268 269 2 7 2 i 270 269 270i 2 7 0 i 2701 2 7 0 i 288 -26O -230 - 8 0 -250 - 2 4 0 - 1 4 0 -55 0 +101 +160 +290 +430 -200 +80 +330 -430 -408 -380 -210 - 4 0 261 260 265 265 263 258 1 R » 2 R - I s o b o r n y l N i t r a t e +30 265i +20 2 7 0 i -15 270 -30 269 -4o 268 -60 268 -75 268 -90 268 -95 268 +20 2 7 3 i -115 269 - 7 5 -70 0 +31 256 +80 252 +170 248 +260 248 +370 247 +440 246 - 2 2 +130 232 233 -3300 •3300 230 230 -6600 -7000 124 . T a b l e X V I c o n t d . LTCD Of B o r n y l , I s o b o r n y l And F e n c h y l N i t r a t e S o l v e n t Temp-e r a t u r e Cc ) Band I X (nm! Band TTT > (nm] [e] B t f 0 % (nm) OS Me OH n - p e n t a n e +5 -95 0 -125 D-*-Fer i c h y l N r ; r a t e 233 233 +11000 +15000 277 276 276 276 -660 -960 -675 -730 261 . 260 260 260 i = i n f l e c t i o n . 125. e f f e c t on t h e I n t e n s i t y o f t h e b o r n y l n i t r a t e bands . T h i s t r e n d a l s o r e f l e c t s t h e i n c r e a s e d s t e r i c h i n d r a n c e t o n i -t r a t o g r o u p r o t a t i o n f r o m b o r n y l t o i s o b o r n y l t o f e n c h y l n i t r a t e . B o r n y l n i t r a t e i s endo , and does n o t have t h e h i n d r a n c e o f t h e g e m - d i m e t h y l b r i d g e ; i s o b o r n y l n i t r a t e i s h i n d e r e d by t h e 7 - m e t h y l g r o u p ; D - « ( - f e n c h y l n i t r a t e has t h e g e m - d i m e t h y l g r o u p ot t o t h e n i t r a t o g r o u p w h i c h s e v e r e l y r e s t r i c t s r o t a t i o n . T h e r e f o r e , t h e o b s e r v e d e f f e c t can be e x p l a i n e d v i a a c o n f o r m a t i o n a l mechanism. The low t e m p e r a t u r e CD (LTCD) o f b o r n y l n i t r a t e i s shown i n F i g u r e 4 5 . The r e l a t i v e s e n s i t i v i t y o f b o r n y l and i s o b o r n y l n i -t r a t e s t o t e m p e r a t u r e can be seen i n F i g u r e 4 6 , B o r n y l n i -t r a t e f l i p s s i g n a t - 5 0 *C, w h i l e i s o b o r n y l n i t r a t e becomes p o s i t i v e a t -10*C. T h i s c o u l d i n d i c a t e t h e p r e s e n c e o f a g r e a t e r p r o p o r t i o n o f l e s s s t a b l e r o t a m e r s a t room t e m p e r a -t u r e f o r b o r n y l n i t r a t e . A t -95*C t h e m o l e c u l a r e l l i p t i c i t i e s o f b o r n y l and i s o b o r n y l n i t r a t e s a r e s i m i l a r . The m o l e c u l a r e l l i p t i c i t y i s i n c r e a s i n g r a p i d l y a t - 9 0 * , w h i c h wou ld i n -d i c a t e t h e b a r r i e r t o r o t a t i o n i s b e i n g approached and an i n c r e a s i n g number o f m o l e c u l e s a r e a d o p t i n g t h e p r e f e r r e d c o n f o r m a t i o n o f t h e n i t r a t o g r o u p . V a r y i n g t h e c o n c e n t r a t i o n o f i s o b o r n y l n i t r a t e f r o m 0.036 m o l a r t o 0 .1 m o l a r gave e s s e n t i a l l y t h e same c u r v e 128. FIGURE 4 5 . LTCD OF B-2R BORNYL NITRATE (MeOH). 127. FIGURE 46. TEMPERATURE VARIATION (BANDI) OF ISOBORNYL AND BORNYL NITRATES (MeOH) 128 . ( F i g u r e 4 6 ) when t e m p e r a t u r e was p l o t t e d a g a i n s t t h e m o l e -c u l a r e l l i p t i c i t y o f band I . S o l v e n t does have an a p p r e c i a b l e e f f e c t on t h e LTCD o f n i t r a t e s V I I I - X ( T a b l e X V I , F i g u r e 4 ? ) . I n n - p e n t a n e , t h e 270 nm band was s t i l l n e g a t i v e a t - 1 15 s C f o r b o r n y l n i t r a t e , w h i l e t h e s i g n o f t h i s band had a l r e a d y f l i p p e d f o r i s o -b o r n y l n i t r a t e by t h e t i m e t h i s t e m p e r a t u r e had been r e a c h e d . There was l i t t l e v a r i a t i o n i n c h l o r o f o r m , w h i l e t h e g r e a t e s t e f f e c t was seen i n m e t h a n o l . Pure c o n f o r m a t i o n a l and p u r e s o l v a t i o n e f f e c t s canno t be s e p a r a t e d . I t seems, t h o u g h , t h a t t h e c o n f o r m a t i o n a l e f f e c t w i l l p r e d o m i n a t e . The low t e m p e r a t u r e s o l v e n t e f f e c t c o u l d a r i s e f r o m s o l v e n t a f f e c t i n g t h e c o n f o r m a t i o n o f t h e chromophore . There were no " d o u b l e - h u m p e d " peaks d e t e c t e d i n any o f t h e s e n i t r a t e s , n o r were any d e t e c t e d i n t h e LTCD o f camphor and f e n c h o n e , t h e p a r e n t k e t o n e s , a l t h o u g h t h e r e was v a r i a t i o n i n i n t e n s i t y . There was no a p p r e c i a b l e w a v e l e n g t h s h i f t i n b o r n y l n i -t r a t e (MeOH) as t h e t e m p e r a t u r e was l o w e r e d f r o m 0° t o -95*Ci s i m i l a r l y , t h e bands o f f e n c h y l and i s o b o r n y l n i t r a t e s d i d n o t a p p r e c i a b l y s h i f t as t e m p e r a t u r e was l o w e r e d . I f d i s s y m m e t r i c s o l v a t i o n were t h e p r i n c i p a l cause o f CD v a r i a t i o n , one wou ld e x p e c t t h e e f f e c t t o be g r e a t e s t f o r D -«* - fenchy l n i t r a t e . S ince a p p r o a c h t o one s i d e o f t h e n i -t r a t o g r o u p i s b l o c k e d by t h e g e m - d i m e t h y l g r o u p , s o l v e n t 34 130. m o l e c u l e s w o u l d be a b l e t o p e r t u r b o n l y one s i d e o f t h e chromophore . The r e s u l t s do n o t s u p p o r t t h i s h y p o t h e s i s . I n b o t h t h e c y c l o h e x y l n i t r a t e s and t h e n i t r a t e s o f t h e b i c y c l o [ 2 . 2 . l ] h e p t a n o l s , t h e r e was no case where t h e 230 nm band changed s i g n . The 270 nm band i s t h e most s e n s i t i v e t o t e m p e r a t u r e a n d / o r s o l v e n t . However, t h e r e was an a p -p r e c i a b l e i n c r e a s e i n i n t e n s i t y o f t h e 230 nm bands o f b o t h b o r n y l and f e n c h y l n i t r a t e s ; b o r n y l n i t r a t e had a g r e a t e r p e r c e n t a g e i n c r e a s e t h a n f e n c h y l n i t r a t e , a g a i n r e f l e c t i n g t h e g r e a t e r s t e r i c h i n d r a n c e t o r o t a t i o n i n t h e f e n c h y l n i t r a t e . Room Tempera tu re CD S t u d i e s . The same t r e n d t h a t was seen i n t h e v a r i a b l e t e m p e r a t u r e e x p e r i m e n t was a l s o seen i n t h e room t e m p e r a t u r e s o l v e n t s t u d y ( T a b l e X V I I ) . B o t h b o r n y l and f e n c h y l n i t r a t e s showed no s i g n r e v e r s a l as t h e n a t u r e o f t h e s o l v e n t was v a r i e d ( F i g u r e 48). The 270 nm band o f i s o b o r n y l n i t r a t e was e x -amined i n e l e v e n s o l v e n t s . Of t h e e l e v e n , t h e m o l e c u l a r e l l i p t i c i t y was p o s i t i v e i n f o u r o f t hem, and n e g a t i v e i n seven ( F i g u r e 4-9). There was no s i g n r e v e r s a l o f t h e 230 nm band ; i n f a c t , s o l v e n t had v e r y l i t t l e e f f e c t on t h e i n -t e n s i t y o f band I I . I n D-oC-fenchyl n i t r a t e , no w a v e l e n g t h s h i f t was d e t e c t e d i n e i t h e r bands I o r I I , c o n f i r m i n g t h a t t h e r e i s no e l e c t r o -s t a t i c i n t e r a c t i o n o f s o l v e n t so as t o a l t e r t h e e n e r g y r e -131. Table XVII E f f e c t Of Solvents On the CD Of Alk y l b i c y c l o £ 2 . 2 . 1 J h e p t y l Nitrates Solvent l R i 2 S-Isoborn-v l Nitrate lR»2S-Borny] Nitrate D-ot-Fenchyl Nitrate X (nm) x(nm] X(nm) e Trimethyl phos-phate 270 +170 274 232 -230 -3000 A c e t o n i t r i l e 270 223 +130 - 2 2 0 0 270 233 -260 -3300 277 232 - 7 4 0 +11 ,200 Dimethylsul-phoxide 2 7 0 a +120 274 -370 Dimethylforamide 2 7 5 a +100 2 7 4 a - 2 7 0 Hexane 260 - 3 6 Acetic acid 2 6 5 a - 7 4 270 - 2 5 8 Methanol 265 -75 270 233 -250 -3260 277 233 - 6 6 0 b . + 1 1 , 0 0 0 ° Cyclohexene 260 - 9 0 Cyclohexane 265 220 - 9 0 -1800 270 231 -2^5 - 3 4 o o 277 233 - 4 7 5 +10,500 Chloroform 266 -260 270 234 -440 - 3 1 8 0 277 234 - 6 9 0 +11,100 Hexafluoroiso-propanol 270 234 -570 -3300 277 233 -1050 +12,500 Carbon t e t r a -chloride 270 -320 NEAT 270 -130 270 - 2 9 0 277 -560 'n-pentane 270 - 2 2 276 - 5 7 5 ° a1 not a maximum, but end absorption, b t at +5*C. c« at 0*C. 132 . 1 3 3. FIGURE 49. CD OF BORNYL OX) AND FENCHYL - O NITRATES. 134. q u i r e m e n t s o f t h e e l e c t r o n i c t r a n s i t i o n . There i s no c o r -r e l a t i o n o f t h e band I i n t e n s i t y o f f e n c h y l n i t r a t e w i t h t h e n o r m a l s o l v e n t p a r a m e t e r s , n o r w i t h t h e s i z e o f t h e s o l v e n t m o l e c u l e . A t low t e m p e r a t u r e s , b o t h b o r n y l n i t r a t e ( I X ) and i s o -b o r n y l n i t r a t e ( V I I I ) have n e g a t i v e bands (band I ) . I f t h e r e s u l t s were due t o c o n f o r m a t i o n a l m o b i l i t y , one w o u l d e x -p e c t more o f t h e l e s s s t a b l e r o t a m e r s o f b o r n y l n i t r a t e t h a n t h e l e s s s t a b l e r o t a m e r s o f i s o b o r n y l n i t r a t e t o e x i s t a t room t e m p e r a t u r e . I t c o u l d b e , t h e n , t h a t t h e s o l v e n t c a n -n o t a f f e c t t h e c o n f o r m a t i o n o f enough o f t h e r o t a m e r s o f b o r n y l n i t r a t e so as t o r e v e r s e t h e s i g n o f i t s 2?0 nm CD b a n d . On t h e o t h e r h a n d , s o l v e n t w o u l d have t o e x e r t an i n f l u e n c e on a s m a l l e r number o f i s o b o r n y l n i t r a t e m o l e c u l e s so as t o r e v e r s e i t s band s i g n . F e n c h y l n i t r a t e has t h e g e m - d i m e t h y l g r o u p ot t o t h e n i t r a t o g r o u p and t h e r e f o r e w o u l d be l e s s s u s c e p t i b l e t o c o n f o r m a t i o n a l i n f l u e n c e f r o m t h e s o l v e n t . I t i s s t i l l u n -c l e a r , however , wha t t h e e x a c t n a t u r e o f t h e s o l v e n t i n t e r -a c t i o n i s . A d d i t i o n a l l y , p u r e s o l v a t i o n e f f e c t s s t i l l c a n -n o t be e l i m i n a t e d as a cause o f t h i s phenomenon. C a m p h a n e - 2 , 3 - d i o l d i n i t r a t e s The f o u r c a m p h a n e - 2 , 3 - d i o l d i n i t r a t e s each have c h a r a c t e r i s t i c CD s p e c t r u m w h i c h can be used f o r i d e n t i f i -c a t i o n ( F i g u r e 4 3 ) . A d d i t i o n a l l y , a l l t h e compounds w i t h 2 -endo n i t r a t o g r o u p s ( I X , X , X I I I , X I V ) , and r e l a t e d t o t h e 135. c o n f i g u r a t i o n o f ( + ) - c a m p h o r , have p o s i t i v e 230 nm CD bands . The l S i 2 R - b o r n y l n i t r a t e examined i s d e r i v e d f r o m ( - ) - c a m p h o r . I t s e n a n t i o m e r , d e r i v e d f r o m ( + ) - c a m p h o r , w i l l have a CD s p e c t r u m w h i c h w i l l be t h e m i r r o r image o f t h e CD s p e c t r u m o f l S i 2 R - b o r n y l n i t r a t e . The t h r e e compounds w i t h 2 -exo n i t r a t o g r o u p s ( V I I I , X I , X I I ) a l l have n e g a t i v e 230 nm CD bands . A r e l a t i v e l y s t r o n g 210 nm CD band (band I I I ) was d e -t e c t e d f o r t h e 2 - e x o , 3 - e n d o and 2 - e n d o , 3 - e x o i s o m e r s . How-e v e r , t h e 2 - e x o , 3 - e x o and 2 - e n d o , 3 - e n d o i s o m e r s have r e l a -t i v e l y weak 210 nm CD bands . The 210 nm band i s b e l i e v e d t o be a T r - ^ T r * t r a n s i t i o n , and t h e s e t r a n s i t i o n s a r e e l e c -t r o n i c a l l y a l l o w e d and a r e u s u a l l y s t r o n g , i n c o n t r a s t w i t h t h e f o r b i d d e n n-*Tr* t r a n s i t i o n s . Thus one u s u a l l y i n v o k e s t h e K i r k w o o d c o u p l i n g mechanism f o r s t r o n g e l e c t r o n i c a l l y a l l o w e d t r a n s i t i o n s ( 15) , i n w h i c h two chromophores a r e c o u p l e d e l e c t r o n i c a l l y . One f e a t u r e o f t h i s model i s t h a t when two chromophores a r e c o p l a n a r , no r o t a t i o n r e s u l t s ( 1 4 ) . When t h e r e a r e two i d e n t i c a l chromophores c l o s e l y s i t u a t e d i n a m o l e c u l e , t h e e l e c t r o n i c t r a n s i t i o n o f t h e one chromophore a p p e a r s t o be a m a g n e t i c t r a n s i t i o n t o t h e second chromophore , and v i c e v e r s a . S ince t h e CD a m p l i t u d e i s p r o p o r t i o n a l t o t h e p r o d u c t o f t h e e l e c t r o n i c t r a n s i t i o n moment and t h e m a g n e t i c t r a n s i t i o n moment and t h e c o s i n e o f 136. t h e a n g l e be tween them, i t i s a symmetry r e q u i r e m e n t t h a t t h e r e w i l l be no r o t a t i o n f o r c o p l a n a r ch romophores . The two n i t r a t o g r o u p s a r e e x p e c t e d t o be n e a r l y c o p l a n a r f o r t h e 2 - e x o , 3 - e x o and 2 - e n d o , 3 - e n d o - d i o l d i n i t r a t e s , and t h e r e -f o r e a weak CD band wou ld be e x p e c t e d . The n i t r a t o g r o u p s i n t h e 2 - e n d o , 3 - e x o and 2 - e x o , 3 - e n d o i s o m e r s a r e e x p e c t e d t o be r e l a t i v e l y p e r p e n d i c u l a r , and t h e r e s u l t o f t h e e l e c -t r o n i c c o u p l i n g s o f t h e two n i t r a t o g r o u p s s h o u l d be e v i d e n t i n t h e CD. Bo th o f t h e s e i s o m e r s have s t r o n g 210 nm b a n d s . Because o f o t h e r o v e r l a p p i n g b a n d s , t h e s p l i t t i n g ( e x c i t o n ) c o u l d n o t be d e t e c t e d . The 270 nm band i s t h e one t h a t i s most s e n s i t i v e t o e n v i r o n m e n t a l e f f e c t s . I n t h e two s o l v e n t s , a c e t o n i t r i l e and c y c l o h e x a n e ( C ^ H ^ g ) , t h e r e was s i g n r e v e r s a l i n band I f o r t h e 2 - e n d o , 3 - e x o i s o m e r . The 2 - j § n d o , 3 - e n d o - d i o l d i -n i t r a t e had a v e r y weak p o s i t i v e band I i n c y c l o h e x a n e , and t h e r e i s a l s o t h e p o s s i b i l i t y o f t h e e x i s t e n c e o f a weak  n e g a t i v e band n e a r 270 nm ( T a b l e X I V ) . There was no s i g n r e v e r s a l f o r t h e 2 - e x o , 3 - e x o i s o m e r . Of t h e f o u r i s o m e r s , t h e 2 - e x o , 3 - e x o has. t h e g r e a t e s t , s t e r i c h i n d r a n c e t o r o -t a t i o n because o f t h e 7 - m e t h y l g r o u p . Over t h e t e m p e r a t u r e range +30* C to- -9.0* C, t h e bands o f t h e 2 - e n d o , 3 - e n d o and 2 - e x o , 3 - e n d o i s o m e r s changed s i g n s ( T a b l e X V I I I , F i g u r e s 50, 51 ) • There were i n t e n s i t y changes i n t h e 270 nm bands o f t h e o t h e r two i s o m e r s , b u t no s i g n r e v e r s a l was n o t e d . The i n t e n s i t y change i n band I o f t h e 137* T a b l e X V I I I LTCD Of Camphane — 2 , 3 - D i o l D i n i t r a t e s (MeOH) I somer Tempera tu re ( *C) X i n n m , ( [ej ) 2 - e x o , 3 - e x o ( X I ) +20 • " -90 270 ( -640) 272 ( -600) 2 - e x o , 3 - e n d o ( X I I ) +10 -36 -90 280 ( + 1 9 0 ) , 2 6 2 ( 0 ) , 2 2 9 ( -4800) 2 8 2 ( + 8 4 ) , 2 6 4 ( 0 ) 2 8 0 ( - 6 8 ) a , 2 2 9 ( - 4 6 0 0 ) 2 - e n d o , 3 - e n d o ( X I I I ) +15 -60 -90 2 7 0 - 5 ( + l 4 0 ) a , 2 3 0 ( + 4 0 0 0 ) 270(-190) 2 6 5 ( - 4 8 0 ) , 2 3 0 ( + 7 4 0 0 ) 2 - e n d o , 3 - e x o ( X I V ) +10 -5 -90 2 8 0 ( - 9 5 ) , 2 6 6 ( 0 ) 230(+7600) 2 7 2 ( - 1 2 0 ) , 2 6 1 ( 0 ) , 2 3 0 ( + 8 l 0 0 ) a« n o t a maximum. 138. 2-6X0,3-6X0 i s o m e r was v e r y s m a l l o v e r t h e range o f +30*C t o - 9 0 * C, p e r h a p s a g a i n r e f l e c t i n g t h e s t e r i c h i n d r a n c e t o r o -t a t i o n o f t h e n i t r a t o g r o u p s . I n c o n t r a s t w i t h band I , t h e r e was no s i g n r e v e r s a l w i t h t e m p e r a t u r e o f band I I ( T a b l e X V I I I ) , n o r g r e a t i n -t e n s i t y change i n t h e 2 - e x o , 3 - e n d o and 2 - e n d o , 3 - e x o d i -n i t r a t e s . I n summary, t h e n i t r a t o p l a n a r r u l e has been s u c c e s s -f u l l y a p p l i e d t o t h e 230 nm band o f t h e c a m p h a n e - 2 , 3 - d i o l d i n i t r a t e s . E n v i r o n m e n t a l e f f e c t s have been a n a l y z e d and can p a r t i a l l y be e x p l a i n e d by t h e c o n f o r m a t i o n a l m o b i l i t y o f t h e n i t r a t o g r o u p s . 139. 140'. FIGURE 5). LTCD OF CAMPHANE-2,3- DIOL DINITRATES (MeOH). E N i t r a t e s W i t h Oxolane R i n g s The n i t r a t o p l a n a r c h i r a l i t y r u l e was a p p l i e d t o e l e v e n n i t r a t e s c o n t a i n i n g o x o l a n e r i n g s . The CD o f compounds XV t o X X I I have been r e p o r t e d p r e v i o u s l y ( 5 6 ) ; t h e CD o f com-pounds X X I I I t o XXV a r e b e i n g r e p o r t e d here f o r t h e f i r s t t i m e ( T a b l e X I X , F i g u r e 5 2 ) . Since t h e r i n g s i n t h i s s e r i e s a r e n o t b r i d g e d , t h e r e w i l l be a g r e a t e r degree o f f l e x i b i l i t y t h a n i n t h e a l k y l -b i c y c l o [2.2.3T) h e p t y l n i t r a t e s . S e l e c t i o n o f t h e c o n f o r -m a t i o n o f t h e n i t r a t o g r o u p i n t h i s s e r i e s o f n i t r a t e s was a i d e d by t h e x - r a y s t u d y o f a d e r i v a t i v e o f i s o s o r b i d e mono-n i t r a t e ( X V I ) i n w h i c h t h e r e was f o u n d an i n t r a m o l e c u l a r i n t e r a c t i o n o f t h e n i t r o oxygen o f t h e n i t r a t o g roup w i t h t h e oxygen o f t h e o t h e r r i n g ( 7 8 ) . T h e r e f o r e , i n t h i s s e r i e s o f n i t r a t e s , t h e n i t r a t o g r o u p s were p l a c e d i n c o n f o r m a t i o n s where such an i n t e r a c t i o n w o u l d be p o s s i b l e . F o r n i t r a t e s X V - X V I I I , XX, and X X I , t h e c h i r a l i t y r u l e p r e d i c t e d t h e same s i g n as t h e s i g n o f t h e 230 nm CD band ( T a b l e X X ) . I n t h e s e compounds t h e r e was no a m b i g u i t y o f CD s i g n n o r o f s i g n p r e d i c t e d by t h e r u l e , s i n c e i n f o r m a t i o n b e a r i n g on n i t r a t o c o n f o r m a t i o n was known. For i s o i o d i d e d i n i t r a t e ( X I X ) and i somann ide d i n i t r a t e ( X X I ) , b o t h n i t r a t o g r o u p s gave t h e same s i g n p r e d i c t i o n . I n i s o s o r b i d e d i n i t r a t e ( X I X ) , on the. o t h e r hand, t h e two n i t r a t o g r o u p s gave o p p o s i t e s i g n p r e d i c t i o n s ( F i g u r e 5 3 ) . F o r t h e 2 -exo p r o j e c t i o n , t h e 143 T a b l e XIX CD Of L - T h r e i t a n D i n i t r a t e And Of D-and L - ° \ - N i t r a t o - r i , ^ - d i m e t h y l -, t f - b u t y r o l a c t o n e N i t r a t e S o l v e n t "X i n nm, ( [0j ) X X I I I L - T h r e i t a n d i n i t r a t e CH^CN C y c l o -hexane 2 5 9 ( - 5 2 0 ) , 2 5 0 ( - 4 9 0 ) m i n , 2 4 0 ( - 5 7 0 ) 2 3 l ( 0 ) , 210(+l6 ,000) 2 6 3 C - 6 9 0 ) , 2 3 9 ( 0 ) , 210(+l6 ,000) XXIV D - « t - n i t r a t o - £ ,& -d i m e t h y l -Tf-b u t y r o l a c t o n e CH^CN C y c l o -hexane 270 (+600 )*,2 32 (+6500) 2 2 2 ( 0 ) , 2 0 8 ( - 1 7 , 0 0 0 ) 270(+480),230(+3500), 2 2 1 (o ) , 2 1 0 ( - 6 2 0 0 ) XXV L - « . - n i t r a t o - ^ ,$ -d i m e t h y l - t f -b u t y t o l a c t o n e CHjCN C y c l o -hexane 2 7 0 ( - 6 0 0 ) * , 2 3 2 ( - 6 5 0 0 ) 2 2 2 ( 0 ) , 2 0 8 ( + 1 7 , 0 0 0 ) 2 7 0 ( - 5 0 0 ) * , 2 3 2 ( - 3 5 0 0 ) , 221(0),210(+7000) * ! n o t a maximum. 145. T a b l e XX CD Of N i t r a t e s W i t h Oxolane R i n g s NITRATE Con-f i g u r -a t i o n CD Band S i g n • P r e -d i c t e d S i g n I I I I I I XV I s o s o r b i d e m o n o n i t r a t e (2-exo) 2 S : 5 R + - -XVI I s o s o r b i d e m o n o n i t r a t e ( 5 - e n d o ) 2 S : 5R + + + X V I I I s o i o d i d e m o n o n i t r a t e ( 2 - e x o ) 2Ss5S + - -X V I I I I somann ide m o n o n i t r a t e ( 2 - e n d o ) 2 R : 5 R + + + XIX I s o s o r b i d e d i n i t r a t e 2 S s 5 R + + + XX I s o i o d i d e d i n i t r a t e 2Ss5S + - -XXI Isomann ide d i n i t r a t e 2 R : 5 R + + + X X I I 1,2»5»6 - D i - 0 - i s o p r o p y -l i d e n e - c x - D - g l u c o f u r a -n o s e - 3 - n i t r a t e 3S + — + X X I I I L - T h r e i t a n D i n i t r a t e 2S»3S - + + + XXIV D - ^ - N i t r a t o - ^ - d i m e t h y l -" t f - b u t y r o l a c t o n e R + + - + XXV L - « - N i t r a t o - / S ,$ - d i m e t h y l tf-butyrolactone S - - + -\ H o X o J A 8) /0~C f — / - - - ^ h o * 5 -endo 2 - e x o I s o i o d i d e Isomann ide I s o s o r b i d e d i n i t r a t e m o n o n i t r a t e ( X V I I ) m o n o n i t r a t e ( X V I I I ) ( X I X ) F i g u r e 5 3 • P r o j e c t i o n s f o r i s o i o d i d e and i somann ide mono-n i t r a t e s and i s o s o r b i d e d i n i t r a t e ( X I X ) . 5 - e n d o n i t r a t o g r o u p i s i n t h e p l a n e , w h i l e i n t h e p r o j e c t i o n o f t h e 5 - e n d o n i t r a t o , t h e 2 - e x o g r o u p i s w h o l l y w i t h i n t h e p o s i t i v e s e c t o r . I s o s o r b i d e d i n i t r a t e has a p o s i t i v e 2 3 0 nm CD b a n d . One f a v o u r e d c o n f o r m a t i o n c o u l d n o t be s e l e c t e d f o r X X I I . The 5 , 6 - i s o p r o p y l i d e n e g r o u p can r o t a t e a b o u t t he C-C bond and t h e n i t r a t o g roup c o u l d i n t e r a c t w i t h oxygens i n e i t h e r o f t h e i s o p r o p y l i d e n e g r o u p s . For L - t h r e i t a n d i n i t r a t e , t h e r e was u n c e r t a i n t y i n b o t h t h e s i g n o f band I I and t h e s i g n o f t h e c h i r a l i t y p r e d i c t i o n . 14? ; I f t h e n i t r a t o g r o u p s a r e p l a c e d i n a c o n f o r m a t i o n w h i c h a l l o w s c l o s e i n t e r a c t i o n w i t h t h e r i n g o x y g e n , t h e n a p o s i -t i v e c h i r a l i t y i s p r e d i c t e d by b o t h n i t r a t o g roups ( F i g -u r e 54 A ) . A. g r e a t e s t i n t e r a c t i o n B. b o t h ONOg g r o u p s i n c l i n e d w i t h r i n g oxygen . i n t h e same d i r e c t i o n . F i g u r e 54. P r o j e c t i o n s o f n i t r a t o g r o u p s i n L - T h r e i t a n d i n i t r a t e . However, i f s t e r i c f a c t o r s a r e c o n s i d e r e d and t h e n i t r a t o g r o u p s a r e b o t h i n c l i n e d i n t h e same d i r e c t i o n , t h e n a n e g a t i v e c h i r a l i t y i s p r e d i c t e d ( F i g u r e 54 B ) . 1 148 There was no d i s t i n c t 230 nm CD band o f L - t h r e i t a n d i -n i t r a t e d e t e c t e d . I n p o l a r s o l v e n t s , a n e g a t i v e peak i s seen n e a r 238 nm, and t h e n t h e m o l e c u l a r e l l i p t i c i t y d e c r e a s e s s h a r p l y t o become p o s i t i v e a t a b o u t 230 nm ( T a b l e X X I , F i g -u r e 5 5 ) • I n t h e n o n p o l a r s o l v e n t s t h e r e i s no s i g n o f band I I a t a l l . I t i s b e l i e v e d t h a t t h i s band i s h i d d e n u n d e r t h e v e r y s t r o n g p o s i t i v e 210 nm CD band ( [@j 2 1 Q + 16 ,000, CH^CN). The d i f f e r e n c e s i n t h e CD s p e c t r a i n p o l a r and n o n p o l a r s o l -v e n t s can be i n t e r p r e t e d as an e q u i l i b r i u m between c o n f o r m e r s . The n i t r a t o g r o u p s have f r e e r o t a t i o n a n d , i n a d d i t i o n , 6 x -o l a n e r i n g s have a degree o f f l e x i b i l i t y t h e m s e l v e s and a r e known t o be n o n p l a n a r . The low t e m p e r a t u r e CD s p e c t r a o f L - t h r e i t a n d i n i t r a t e i n EPA and i n m e t h a n o l a l s o add a d d i t i o n a l i n f o r m a t i o n . There i s no s i g n o f a 230 nm band i n EPA. As t h e t e m p e r a t u r e i s l o w e r e d , t h e i n t e n s i t y o f band I d e c r e a s e s , u n t i l a t - 6 0 e C , t h e c u r v e c r o s s e s t h e b a s e l i n e a t 236 nm ( F i g u r e 5 6 ) . How-e v e r , by -105*C, t h e c r o s s o v e r p o i n t has moved t o 247+3 nm. I n o t h e r w o r d s , t h e m o l e c u l a r e l l i p t i c i t y n e a r 230 nm has become more p o s i t i v e . Between - 1 1 5 ° C and - l 4 5 ' C , t h e m o l e -c u l a r e l l i p t i c i t y o f band I b e g i n s t o i n c r e a s e a g a i n . T h i s i n d i c a t e s t h e p resence o f more t h a n one e q u i l i b r i u m . S i m i -l a r l y , t h e i n t e n s i t y o f band I i n m e t h a n o l d e c r e a s e s as t h e t e m p e r a t u r e i s l o w e r e d ( F i g u r e 5 6 ) . There i s no d o u b t , how-149, T a b l e XXI CD Of L - T h r e i t a n D i n i t r a t e (30»C) S O L V E N T Band I Band I I £3 =° TTT 1 >- (nm M >>• (nm) M n m , M n m ; "9] 1 Cyc lohexane 263 -690 _ 241 210 +16,000 n - h e p t a n e 264 -602 - - 241 n - p e n t a n e 263 -580 - - 242 T H F i c y c l o h e x a n e ( 1 4 . 8 6 ) 263 -738 - - 237 CH^CN 259 -520 240 - 5 7 0 231 210 +16,000 MeOH 258 -500 237 - 6 4 0 228 208 +12,000 2 , 2 , 2 - T r i f l u o r o -e t h a n o l 261 -490 239 -390 232 THF 263 - 7 4 0 - 238 E t h e r - P e n t a n e -E t h a n o l (EPA) (5 '5«2) 260 -534 - - 236 > 1 5 0 . FIGURE 55. CD OF L-THREITAN DINITRATE. 151 . 152. e v e r , t h a t a t room t e m p e r a t u r e , i n p o l a r s o l v e n t s , L - t h r e i t a n d i n i t r a t e has a n e g a t i v e 230 nm CD band ( F i g u r e 5 7 ) . F i g u r e 57. CD o f L - T h r e i t a n d i n i t r a t e w i t h p o s s i b l e band r e s o l u t i o n (MeOH a t +30 -C) . S ince t h e i n t e n s i t y o f t h e 210 nm band i s s i m i l a r i n b o t h p o l a r and n o n p o l a r s o l v e n t s , one canno t a rgue t h a t t h e absence o f t h e hump a t 238 nm i n t h e n o n p o l a r s o l v e n t s i s due t o g r e a t e r o v e r l a p o f t h e s t r o n g p o s i t i v e 210 nm b a n d . I t a p p e a r s , i n t h e l i g h t o f t h e LTCD measurements , t h a t t h e 230 nm band has f l i p p e d s i g n a t low t e m p e r a t u r e , and t h a t i n n o n p o l a r s o l v e n t s a t room t e m p e r a t u r e , i t i s e i t h e r much weaker o r has a l s o f l i p p e d s i g n . The p o s s i b l e c o n f o r m a t i o n s o f t h e n i t r a t o g r o u p s p r e s e n t e d i n F i g u r e 5kk and 5^B" p r e -d i c t o p p o s i t e s i g n s f o r t h e 230 nm band . 153. (MeOH) • 5 154. The c h i r a l i t y r u l e f o r t h e n i t r a t o g roup p r e d i c t s a p o s i t i v e hand I I f o r D - ^ - n i t r a t o - ^ , A - d i m e t h y l - l f - b u t y -r o l a c t o n e ( X X I V ) ; t h e m o l e c u l a r e l l i p t i c i t y i s p o s i t i v e ( j © J 2 3 3 +6500). D ( - ) - P a n t o l a c t o n e , t h e p a r e n t compound, has a s t r o n g n e g a t i v e CD band (^22j - 8200 (CH^CN); a l s o ^ 2 1 9 -1370 i n HC1 (116)). The CD maximum o f band I I o c c u r s a t t h e e x p e c t e d f r e q u e n c y f o r a n i t r a t e e s t e r . Microwave s t u d i e s have shown t h a t t h e r i n g atoms i n V-b u t y r o l a c t o n e a r e n o n p l a n a r and i n d i c a t e t h e e x i s t e n c e o f " t w o e q u i v a l e n t n o n p l a n a r c o n f i g u r a t i o n s s e p a r a t e d by a m o d e r a t e l y h i g h b a r r i e r t o t h e p l a n a r s t r u c t u r e " ( 1 1 5 ) . T h i s f l e x i b i l i t y o f t h e r i n g no d o u b t a c c o u n t s f o r t h e l a r g e s o l v e n t e f f e c t i n t h e CD s p e c t r a o f XXIV and XXV ( T a b l e X I X ) . T h i s e f f e c t i s most p ronounced i n t h e 210 nm band , a r e g i o n where t h e r e i s l e s s l a c t o n e a b s o r p t i o n . A d d i t i o n a l l y , t h e r e i s a l m o s t a 100$ i n c r e a s e i n t h e m o l e -c u l a r e l l i p t i c i t y o f band I I as t h e t e m p e r a t u r e i s l o w e r e d f r o m +5*C t o -90*C ( F i g u r e 5 8 ) . On t h e o t h e r h a n d , band I d e c r e a s e s i n i n t e n s i t y . I n summary, o f t h e e l e v e n n i t r a t e s w i t h o x o l a n e r i n g s , e i g h t had c o r r e c t s i g n s p r e d i c t e d by t h e c h i r a l i t y r u l e . Fo r t h e o t h e r t h r e e , t h e CD band s i g n s o r t h e c h i r a l i t y r u l e p r e d i c t i o n s were amb iguous . 155. F S t e r o i d a l N i t r a t e s The CD o f t w e n t y - t h r e e s t e r o i d a l n i t r a t e e s t e r s was e x -amined . T w e n t y - t w o o f t h e s e were r e p o r t e d by Sna tzke e t a l . (57)» w h i l e compound X X X I I was r e p o r t e d by Hayward and C laesson ( 5 6 ) . The f i r s t seven n i t r a t e s a r e a l l 3 - n i t r a t o s t e r o i d s . F o r t h e s e v e n , two c o n f o r m a t i o n s o f t h e n i t r a t o g r o u p seemed e q u a l l y f a v o u r e d , and t h e s e gave o p p o s i t e c h i r a l i t y p r e d i c t i o n s . N i t r a t e s X X X I I I and XXXIV a r e 5 - n i t r a t o s t e r o i d s . The n i t r a t o g r o u p i s more s t e r i c a l l y h i n d e r e d i n t h e s e s t e r o i d s and ope c o n f o r m a t i o n was f a v o u r e d . The s i g n o f t h e p r o j e c t i o n o f t h e s e two n i t r a t e s c o r r e s p o n d s t o t h e s i g n o f band I I . There were e i g h t 1 7 - n i t r a t o s t e r o i d s c o n s i d e r e d (XXXV t o X L I I ) . F o r n i t r a t e s XXXV t o XXXIX, one s t e r i c a l l y f a v -oured c o n f o r m a t i o n o f t h e n i t r a t o g r o u p was s e l e c t e d , and when t h e p l a n a r n i t r a t o r u l e was a p p l i e d , t h e s i g n s o f t h e i r p r o j e c t i o n s were t h e same as t h e 230 nm band s i g n s . A s i n g l e n i t r a t o g r o u p c o n f o r m a t i o n c o u l d n o t be s e l e c t e d f o r compounds XL t o X L I I , b u t a p o s i t i v e c h i r a l i t y was s l i g h t l y f a v o u r e d . I n t h e s e t h r e e n i t r a t e s , t h e r e were p r o b l e m s w i t h t h e c o n -f o r m a t i o n o f t h e 17/J g r o u p , t h e c o n f o r m a t i o n o f t h e l?o{ n i -t r a t o g r o u p , and t h e f l e x i b i l i t y o f t h e f i v e - m e m b e r e d D r i n g . The CD s p e c t r a o f t h r e e 1 1 - n i t r a t o s t e r o i d s ( X L I I I t o XLV) were examined . There was u n c e r t a i n t y i n t h e s i g n o f t h e p r o j e c t i o n o f X L I I I . T h i s was due t o t h e n e a r l y e q u a l 156. T a b l e X X I I CD Of S t e r o i d a l N i t r a t e s Comp-ound # N I T R A T E N i t r a t o 3roup C o n f i g -u r a t i o n CD Band S i g n s S i g n From I I I I I I P r o j -e c t i o n 3 - n i t r a t o s t e r o i d s XXVI 3 d - n i t r a t o - 5 ^ - c h o l -e s t a n e 3R - + + X X V I I 3 < r t - n i t r a t o - l ? A - a c e -t o x y - 5 * - a n d r o s t a n e 3R - + + X X V I I I 3 <<-n i t ra to -5 fX -and -r o s t a n - 1 7 - o n e 3R + + XXIX 3 c X - n i t r a t o - 1 7 £ - a c e -t o x y - 5 ^ - a n d r o s t a n e 3R - - + + XXX 3 / # - n i t r a t o - 5 c t - c h o l -e s t a n e 3S + - + XXXI 3 ^ - n i t r a t o - 1 7 4 - a c e t o x y -%• a n d r o s t a n e •3S + + X X X I I 3 / - c h o l e s t e r y l n i t r a t e 5 - n i t r a t o s t e r o i d s 3S + + + X X X I I I 5 - n i t r a t o - 3 / 3 - a c e t o x y -5 « \ - c h o l e s t a n - 6 - o n e 5R +a - b + XXXIV 5 - n i t r a t o - 3 ^ » 6 ^ - d i a c e -t o x y - 5<< - cho l e s t a n e 1 7 - n i t r a t o s t e r o i d s 5S +c + + XXXV 1 7 / S - n i t r a t o - 4 - a n d r o s -t e n e - 3 - o n e 17R + + - + XXXVI 170 - n i t r a t o - 17<<-eth-i n y l - 4 - e s t r e n e - 3 - o n e 17S + - + XXXVII 17^ - n i t r a t o - 5 « ( - a n d r o s -t a n - 3 ^ - o l 17R + + - + 157. T a b l e X X I I ( c o n t i n u e d ) CD Of S t e r o i d a l N i t r a t e s Comp-ound # N I T R A T E N i t r a t o Group C o n f i g -u r a t i o n CD Band S i g n s i v1 S i g n f r o m P r o j -e c t i o n ! I I I I I I X X X V I I I 17/* - n i t r a t o - 5 ° ( - a n d -r o s t a n - 3 - o n e 17R + - + XXXIX V7B - n i t r a t o - 3 0 - a c e -t o x y - 5 ° ( - a n d r o s t a n e 17R + + - • + XL 1 7 - n i t r a t o - 1 9 - n o r -4 - p r e g n e n - 3 » 2 0 - d i o n e 17R +d + f + X L I 1 7 - n i t r a t o - 4 - p r e g -n e n - 3 » 2 0 - d i o n e 17R + + + X L I I 1 7 - n i t r a t o - 2 1 - a c e -t o x y - 4 - p r e g n e n - 3 » 2 0 -d i o n e 1 1 - n i t r a t o s t e r o i d s 17R + + + + X L I I I 1 1 0 - n i t r a t o - 3 < < » 1 7 * -d i a c e t o x y - 5 ^ - a n d r o s -t a n e U S — + — +? XLIV 6 r t - f l u o r o - l L y - n i t r a t o -2 1 - a c e t o x y - l 6 « < - m e t h y l -l , 4 - p r e g n a d i e n e - 3 » 2 0 - d i o n e U S +g +g - g + XLV 6o(-f l u o r o - l l t x - n i t r a t o - 2 1 - a c e t o x y - l 6 o ( - m e -t h y l - l , 4 - p r e g n a d i e n e -3 , 2 0 - d i o n e 1 9 - n i t r a t o s t e r o i d s 11R - g +g XLVI 1 9 - n i t r a t o - 3 r ? - a c e t o x y -5<<-cholestane - + + 158. T a b l e X X I I ( c o n t i n u e d ) CD Of S t e r o i d a l N i t r a t e s Comp-ound N I T R A T E N i t r a t o Group C o n f i g -u r a t i o n CD Band S i g n s • S i g n From P r o j -e c t i o n I I I I I I 1 9 - n i t r a t o s t e r o i d s ( c o n t i n u e d ) X L V I I 1 9 - n i t r a t o - 3 » 4 - a c e -t o x y - 5 ^ - c h o l e s t a n e - + X L V I I I 1 9 - n i t r a t o - 5 4 - c h o l -e s t a n - 3 - o n e a., mm — a . max. a t 247 nm. b. a t 221 nm. c . i n f l e c t i o n a t 244 nm. d . a t 287 nm. f . a t 240 nm g . u n c e r t a i n i f due t o n i t r a t o a b s o r p -t i o n . 159. d i s t r i b u t i o n o f atoms t o t h e r i g h t and l e f t o f t h e n i t r a t o g r o u p . One f a v o u r e d c o n f o r m a t i o n was s e l e c t e d f o r XLIV and XLV. However, t h e s i g n o f t h e n i t r a t o band was i n q u e s t i o n s i n c e t h e p a r e n t a l c o h o l o f XLIV a l s o has a b s o r p t i o n bands o f t h e same s i g n i n t h i s s p e c t r a l r e g i o n . Three 1 9 - n i t r a t o s t e r o i d s were p r e p a r e d by Sna tzke et a l . b u t a g a i n no f a v o u r e d c o n f o r m a t i o n o f t h e n i t r a t o g roup c o u l d be a s s i g n e d because o f f r e e r o t a t i o n a b o u t t h e C-C bond o f t h e 19yS - g r o u p . The CD bands o f t h e s e 1 9 - n i t r a t o s t e r o i d s a r e v e r y weak. 160. G. A c y c l i c N i t r a t e s The CD o f t h r e e n i t r a t e s o f c t - h y d r o x y a c i d s were b r i e f -l y examined ( T a b l e X X I I I , F i g u r e 5 9 ) . C00H H-C-ONO, CH, XLIX C00CHoCH 2 3 C00CH_CH 2 3 0 2 N 0 - C - H 0 2 N 0 - C - H CHgCOOCHgCH^ H-C-ONO, C00CHoCH 2 3 L I T h e i r comp le te CD s p e c t r a down t o 200 nm were measured . T s u z u k i e t _ a l . ( 5 5 ) , had p r e v i o u s l y measured t h e ORD o f compounds L and L I down t o 280 nm. B o t h XLIX and L showed t h e e x i s t e n c e o f r o t a m e r s . The CD bands o f D - K - n i t r a t o - p r o p a n o i c a c i d were weak, and b o t h t h i s n i t r a t e and the n i t r a t e s y n t h e s i z e d f r o m d i e t h y l - L -m a l a t e showed weak l o n g w a v e l e n g t h peaks a t 292 nm and 310 nm r e s p e c t i v e l y . T h i s b e h a v i o u r i s s i m i l a r t o t h e r e s u l t s f o u n d f o r l a c t i c and m a l i c a c i d s ( 3 9 - 4 1 , 1 1 7 ) . These peaks no d o u b t r e s u l t f r o m d i f f e r e n t r o t a m e r s i n s o l u t i o n . F o r ( - ) - l a c t i c a c i d , t h e most s t a b l e r o t a m e r i s e x -p e c t e d t o be (39) OH H ( / c; R H 1 6 1 . T a b l e X X I I I N i t r a t e s Of «M-Hydroxy A c i d s # N I T R A T E S o l v e n t >. i n nm, ( [©] ) XLIX D - ^ - N i t r a t o - p r o p a n o i c a c i d CH3CN 292 ( + 2 2 ) , 281 ( 0 ) , 249 ( -217)242 ( 0 ) , 2 3 3 ( + 3 6 0 ) 228 ( 0 ) , 2 1 9 ( - 2 3 0 0 ) L D i e t h y l - L - m a l a t e n i t r a t e CH^CN 3 1 0 ( - 9 . 6 ) , 3 0 0 ( 0 ) , 2 6 7 (+170),252(0),232 ( - 2 1 0 0 ) , 2 2 2 ( 0 ) , 2 0 9 (+4000) L I D i e t h y l - L - t a r t r a t e d i n i t r a t e CH^CN 2 6 2 ( + 4 4 0 ) i , 2 4 2 ( + 6 6 0 ) , 2 3 4 ( 0 ) , 2 1 0 ( - 1 1 , 0 0 0 ) i = i n f l e c t i o n 162. FIGURE 59. CD OF NITRATES OF *-HYDROXY ACIDS (CH,CN). 163. and t h i s i s t h e r o t a m e r b e l i e v e d t o be r e s p o n s i b l e f o r t h e n e g a t i v e 210 nm band i n t h e CD s p e c t r u m o f D ( - ) - l a c t i c a c i d . I n t h e n i t r a t e o f D ( - ) - l a c t i c a c i d , t h e r e i s a l s o t h e p o s s i b i l -i t y o f r o t a t i o n a b o u t t h e C-0 bond o f t h e n i t r a t o g r o u p . I f t h e same c o n f o r m a t i o n i s s e l e c t e d f o r t h e n i t r a t e e s t e r , t h e n t h e c o n f o r m a t i o n wou ld b e , <h ONOo \ I C C- C H , HO^ and i f t h e s t e r i c a l l y f a v o u r e d c o n f o r m a t i o n o f t h e n i t r a t o  g r o u p were s e l e c t e d , t h e n t h e c h i r a l i t y r u l e wou ld p r e d i c t a p o s i t i v e 230 nm CD band , w h i c h i s what i s o b t a i n e d . D-<*-NITRATO-PROPANOIC ACID The s e l e c t i o n o f p r e f e r r e d c o n f o r m a t i o n s o f n i t r a t e s L and L I was d i f f i c u l t because o f t h e l o n g e r f l e x i b l e c h a i n . 164. The CD s p e c t r u m o f t h e d i n i t r a t e o f d i e t h y l - L - t a r t r a t e ( L I ) d i d n o t show a l o n g w a v e l e n g t h CD b a n d . S i m i l a r l y , t h e CD s p e c t r u m o f t a r t a r i c a c i d o n l y has a s i n g l e band a t a b o u t 200 nm ( 3 9 ) . The 209 nm band o f L cannot be w h o l l y a t t r i b u t e d t o t h e n i t r a t e s i n c e t h e p a r e n t d i e t h y l - L - m a l a t e a l s o has a p o s i t i v e band a t t h i s w a v e l e n g t h ( 3 9 ) . There was no e v i d e n c e f r o m t h e CD s p e c t r a o f n i t r a t e s XLIX t o L I t o s u p p o r t t h e c o n c l u s i o n s o f T s u z u k i and c o w o r k e r s (55) t h a t n i t r a t e s o f cA-D-hydroxy a c i d s have p o s i t i v e C o t t o n e f f e c t s w h i l e t h o s e d e r i v e d f r o m « t - L - h y d r o x y a c i d s have n e g a t i v e C o t t o n e f f e c t s . I n h i s ORD s t u d y , he o n l y saw t h e ORD o f many o v e r l a p p i n g CD, bands . The o n l y s i m i l a r i t y i n t h e CD s p e c t r a o f n i t r a t e s L and L I , b o t h d e r i v e d f r o m o i - L - h y d r o x y a c i d s , was t h e i r p o s -i t i v e 270 nm CD band . The n i t r a t e f r o m D ( - ) - l a c t i c a c i d had a n e g a t i v e 270 nm b a n d . I n g e n e r a l , i t was d i f f i c u l t t o a p p l y t h e n i t r a t o c h i r a l i t y r u l e t o t h e s e t h r e e n i t r a t e s because o f t h e f l e x i b i l i t y o f t h e g r o u p s . 165. H. Summary F o r t h e p l a n a r n i t r a t o c h i r a l i t y r u l e t o be a p p l i e d s u c c e s s f u l l y , t h e n i t r a t o g roup must be i n an e n v i r o n m e n t w h i c h r e s t r i c t s , t o some d e g r e e , i t s f r e e r o t a t i o n , o r a l l o w s , f o r s p e c i f i c i n t r a m o l e c u l a r i n t e r a c t i o n s . I f t h i s i s * t h e s i t u a t i o n , t h e n a p r e f e r r e d c o n f o r m a t i o n o f t h e n i t r a t o may be s e l e c t e d . The r u l e was a p p l i e d t o f o r t y - e i g h t n i t r a t e e s t e r s i n f o u r d i f f e r e n t c h e m i c a l s e r i e s ( T a b l e X X I V ) . Of t h e f o r t y -e i g h t , t w e n t y - e i g h t gave t h e c o r r e c t s i g n o f t h e 230 nm b a n d . There were t w e n t y n i t r a t e s where t h e p r e d i c t i o n c o u l d n o t be made. T a b l e XXIV 230 nm S i g n P r e d i c t i o n s b y P l a n a r N i t r a t o Rule N i t r a t e S e r i e s C o r r e c t S i g n P r e -d i c t e d I n c o r r e c t S i g n P r e -d i c t e d S i g n P r e d i c t i o n U n c e r t a i n m o n o c y c l i c s ix -membered r i n g 6 _ 1 b i c y c l o [ 2 . 2 . 1 ] h e p t a n o l 5 - 2 w i t h o x o l a n e r i n g s 8 3 s t e r o i d , 9 - 14 2 8 ' - 20 166 In most cases t h i s r e s u l t e d from the i n a b i l i t y to s e l e c t a p r e f e r r e d conformation of the n i t r a t o group. There were a few cases where the s i g n of the 230 nm CD band was u n c e r t a i n . There was no d i r e c t c o n t r a d i c t i o n to the r u l e among the f o r t y - e i g h t n i t r a t e e s t e r s . Thus, the r u l e can be s u c c e s s -f u l l y a p p l i e d to n i t r a t e s i n which a p r e f e r r e d n i t r a t o con-f o r m a t i o n i s d e f i n e d . The CD s p e c t r a o f n i t r a t e e s t e r s a l s o i n d i c a t e d the presence of a s t r u c t u r a l e q u i l i b r i u m . In some cases t h i s appeared to be a c o n f o r m a t i o n a l e q u i l i b r i u m of the whole molecule (eg. menthyl n i t r a t e ) , w h i l e i n o t h e r cases the changes i n the CD s p e c t r a appeared to r e s u l t from r o t a t i o n of the n i t r a t o group i t s e l f ( b o r n y l and i s o b o r n y l n i t r a t e s , f o r example). 167. E X P E R I M E N T A L 168 E x p e r i m e n t a l A. Source o f C h i r a l A l c o h o l s ( - ) - I s o D o r n e o l was o b t a i n e d by r e d u c t i o n o f ( + ) - c a m p h o r w i t h LiAlH^j, ( 1 1 8 ) a t -6o*C t o i n c r e a s e t h e p r o p o r t i o n o f t h e exp i s o m e r ( T a b l e XXV). A f t e r t h r e e s u c c e s s i v e t r e a t -ments w i t h p h t h a l i c a n h y d r i d e (119,120) a c o n s t a n t o p t i c a l r o t a t i o n was reached ( J«J 2 2 - 3 3 . 6 , c = 9 . 8 EtOH; l i t . [oC] 1 9 -33«5»c=8 .0 EtOH (123)). G . L . C . ( A e r o g r a p h A-700; 20$ c a r b o -wax 20 M) showed no ( + ) - b o r n e o l c o n t a m i n a n t (<1$). The s o u r c e s o f t h e o t h e r o p t i c a l l y a c t i v e a l c o h o l s a r e shown i n T a b l e XXV. B. S y n t h e s i s o f N i t r a t e E s t e r s A l l t h e n i t r a t e e s t e r s were s y n t h e s i z e d f r o m t h e c o r -r e s p o n d i n g c h i r a l a l c o h o l s . Anhydrous n i t r i c a c i d was p r e -p a r e d f r o m f u m i n g n i t r i c a c i d ( 1 2 4 ) . I n a t y p i c a l e x p e r i -men t , 100 mgm (O.OOO65 m o l e s ) o f t h e c h i r a l a l c o h o l was d i s s o l v e d i n a m i x t u r e o f 0.2 m l o f a c e t i c a c i d a n d . 0 . 2 m l o f a c e t i c a n h y d r i d e . The s o l u t i o n was t h e n c o o l e d i n an i c e - w a t e r b a t h . The n i t r a t i n g s o l u t i o n was p r e p a r e d by a d d i n g 0.125 m l (0.0035 mo les ) o f anhyd rous n i t r i c a c i d t o a c o o l e d s o l u t i o n (0*C) o f 0.25 m l o f a c e t i c a n h y d r i d e and 0.125 m l o f a c e t i c a c i d . T h i s n i t r a t i n g s o l u t i o n was s l o w l y added d r o p w i s e t o t h e cold s o l u t i o n o f a l c o h o l , a n d , a f t e r a d d i t i o n was c o m p l e t e d , t he r e a c t i o n m i x t u r e was s t o r e d i n a r e f r i g e r a t o r (+5°C) f o r two h o u r s . The r e -169; T a b l e XXV Sources o f C h i r a l A l c o h o l s C a r b i n o l Source ( - ) - l R » 2 R - i s o b o r n e o l L iAlHj j , r e d u c t i o n o f ( + ) - c a m p h o r a t - 6 0 » C (118); s e p a r a t i o n f r o m ( + ) - b o r n e o l (119,120) ( - ) - l S » 2 R - b o r n e o l 5% KOH-MeOH h y d r o l y s i s o f 1 - b o r n y l a c e t a t e ( A l d r i c h ) D - c ( - f e n c h y l a l c o h o l K o c h - L i g h t L a b o r a t o r i e s ( - ) - m e n t h o l Matheson , Coleman and B e l l c a r v o m e n t h o l h y d r o l y s i s o f i t s 3 , 5 - d i n i t r o b e n z o a t e , a g i f t o f S . S c h r o e t e r (121) i so carvomentho1 h y d r o l y s i s o f i t s 3 » 5 - d i n i t r o b e n z o a t e , a g i f t o f S. S c h r o e t e r (121) p - m e n t h - l ( 7 ) - e n e -t r a n s - 2 - o l h y d r o l y s i s o f i t s p - n i t r o b e n z o a t e , a g i f t o f S. S c h r o e t e r (121) p - m e n t h - 1 ( 7 ) - e n e -c i s - 2 - o l h y d r o l y s i s o f i t s 3 , 5 - d i n i t r o b e n z o a t e , a g i f t o f S. S c h r o e t e r (121) c a m p h a n e - 2 , 3 - d i o l s a g i f t o f S . J . A n g y a l (122) 3 - m e t h y l c y c l o -h e x a n o l s p r e p a r e d i n t h i s l a b o r a t o r y by R.N. T o t t y T j - t h r e i t a n p r e p a r e d i n t h i s l a b o r a t o r y by D. Dong • D- and L - p a n t o -l a c t o n e A b b o t t L a b o r a t o r i e s D ( - ) - l a c t i c a c i d M i l e s L a b o r a t o r i e s D i e t h y l - L - t a r t r a t e Eastman-Kodak 170. a c t i o n m i x t u r e was t h e n p o u r e d on 10 gm o f c rushed i c e . The e x c e s s a c i d s were n e u t r a l i z e d w i t h kofo sodium h y d r o x i d e (pH m e t e r ) , w h i c h was f o l l o w e d by e x t r a c t i o n o f t h e p r o d u c t w i t h m e t h y l e n e c h l o r i d e . A f t e r s u c c e s s i v e w a s h i n g s w i t h w a t e r and $fo sod ium b i c a r b o n a t e s o l u t i o n , t h e CHgClg s o l -u t i o n was d r i e d o v e r D r i e r i t e , and t h e s o l v e n t was removed . G e n e r a l l y t h e n i t r a t e s were p u r i f i e d by d i s t i l l a t i o n ("v0.5 t o r r ) . D-c<-Fenchyl n i t r a t e and i s o b o r n y l n i t r a t e were a l s o p u r i f i e d by co lumn c h r o m a t o g r a p h y on s i l i c a g e l (CHCLj d e v e l o p e r ) . P u r i f i c a t i o n was c o n t i n u e d u n t i l T . L . C . ( s i l i c a g e l ? C ^ H 1 2 , CH^CN ( 2 0 i l ) d e v e l o p e r ) i n d i c a t e d no i m p u r i t i e s . l R i 3 R « 4 S - M e n t h y l n i t r a t e was a l s o p r e p a r e d v i a i t s c h l o r o f o r m a t e . The p r o c e d u r e o f Boshan (125) p r o v i d e d a g u i d e f o r t h i s s y n t h e s i s . The C-H-N a n a l y s e s o f t h e n i t r a t e e s t e r s 171 a r e c o l l e c t e d i n T a b l e XXVI . N.M.R. s p e c t r a ( V a r i a n A-60 and T - 6 0 , CCl^ s o l v e n t ) were r e c o r d e d and t h e i r a n a l y s i s showed t h a t c h e m i c a l s h i f t s and c o u p l i n g c o n s t a n t s ag reed w i t h t h e s t r u c t u r a l a s s i g n -m e n t s . I n f r a - r e d s p e c t r a were r e c o r d e d on P e r k i n - E l m e r models 21 and 137 s p e c t r o m e t e r s as s o l u t i o n s p e c t r a (CCl^ s o l v e n t ) , l i q u i d f i l m s , and p o t a s s i u m bromide w a f e r s . The s t r e t c h i n g f r e q u e n c i e s o f t h e n i t r a t o bands were c o n s i s t e n t w i t h p u b l i s h e d IR s p e c t r a l d a t a f o r n i t r a t e e s t e r s (82-85, 8 7 , 8 8 ) . A t t e m p t e d s y n t h e s i s o f ( - ) - t r a n s - c a r v e o l ( L I I ) , ( - ) - c i s -c a r v e o l ( L I I I ) , ( + ) - t r a n s - c a r v o t a n a c e t o l ( L I V ) , and 1 - i s o -p u l e g o l (LV) were u n s u c c e s s f u l . Upon a d d i t i o n o f t h e 172. T a b l e XXVI E l e m e n t a l A n a l y s i s Of N i t r a t e E s t e r s N i t r a t e C a l c u l a t e d Found % C % H % N % C % H % N l R t 2 R - i s o b o r n y l n i t r a t e 60.28 8.60 7.03 60.15 8.69 7.08 l S»2R - b o r n y l n i t r a t e 60.28 8o60 7.03 60,12 8.56 7.10 camphane-2 -endo,3 - e x o - d i o l - d i n i t r a t e 46.15 6.20 10.77 46.25 6.18 io . 9 6 camp hane - 2 -endo ,3 - e n d o - d i o l - d i n i t r a t e 46.15 6.20 10.77 46.20 6.12 10.90 camphane - 2 -exp , 3 -exo - d i o l - d i r i i t r a t e 46.15 6.20 10.77 46.31 6.25 IO . 6 5 D-c^-f e n c h y l n i t r a t e 60.28 8,60 7.03 60.31 8.44 7.02 1R«3R«4S-menthyl n i t r a t e 59.67 9.52 6.96 59.39 9.72 6.92 i s o c a r v o m e n t h y l n i t r a t e 59.67 9.52 6.96 59.99 9 .70 6.91 p - m e n t h - 1 ( 7 ) - e n e -2 - o l - n i t r a t e 60.28 8.60 7.03 60.15 8.51 7.10 D - « ( - n i t r a t o - 4 4-di-m e t h y l - c f - bu t y r o l a c -t o n e 41.14 5.18 8.00 41.28 5.16 8.07 173. c o l o u r on s t a n d i n g . The i s o l a t e d p r o d u c t s were v e r y v i s c o u s and r e d - b r o w n i n d i c a t i n g t h e p r e s e n c e o f n i t r o g e n d i o x i d e . I t i s e v i d e n t t h a t p o l y m e r i z a t i o n had o c c u r r e d . I t has been r e p o r t e d t h a t c ( , o i - d i m e t h y l a l l y l n i t r a t e e x h i b i t s a - s t r o n g t e n d e n c y t o p o l y m e r i z e ( 126) . N i t r a t i o n o f c a r b i n o l L v a t -25* C was s i m i l a r l y u n s u c c e s s f u l . N i t r a t i o n o f c e d r o l ( L V I ) a t -50»C by a AcgO-HOAc-HNO^ m i x t u r e , o r by n i t r i c a c i d i n CHgClg a t -60 "C y i e l d e d c e d r e n e . S i m i l a r l y , an a t t e m p t e d p r e p a r a t i o n v i a t h e c h l o r o f o r m a t e y i e l d e d o n l y c e d r e n e . These r e s u l t s r e f l e c t t h e l a b i l e n a t u r e o f a t e r t i a r y c a r b i n o l . d -3 -Bromocamphor r e s i s t e d n i t r a t i o n by d i s s o l v i n g t h e b romo-ke tone i n 100$ n i t r i c a c i d , and a l s o by r e p l a c e m e n t o f b romine by n i t r a t e i o n (AgNO^ i n CH^CN). C. UV S p e c t r a UV s p e c t r a were r u n on a Cary 15 s p e c t r o p h o t o m e t e r i n 10 mm and 1 mm q u a r t z c e l l s . R e p e t i t i v e measurements were made o v e r t h e span o f a y e a r t o check t h e i r r e p r o d u c i b i l i t y . C o n c e n t r a t i o n s were s e l e c t e d t o m a i n t a i n a s l i t w i d t h be low 1 mm and an absorbance be low 1 .0 . D. CD Measurements C i r c u l a r d i c h r o i s m s p e c t r a were measured on JASCO ORD/ UV/CD-5 ( J - 5 ) and J-20 s p e c t r o p h o t o m e t e r s . The m a j o r i t y o f t h e s p e c t r a were measured on t h e J -5 w i t h a s e n s i t i v i t y o f 174. 5x10 d i f f e r e n t i a l abso rbance u n i t s p e r m i l l i m e t e r o f c h a r t p a p e r . The e n t i r e o p t i c a l p a t h was u n d e r c o n s t a n t n i t r o g e n f l u s h i n g . The i n s t r u m e n t was o p e r a t e d w i t h a programmed v a r i a b l e s l i t - w i d t h . The i n s t r u m e n t was c a l i b r a t e d each day b e f o r e measu re -ments were p e r f o r m e d ; 5 -< * - ch loes tan - 3 -one (A^+1.13 a t 295 nm (127) ) i n 1,4-dfoxane was chosen as a s t a n d a r d . Measurements were made i n 10 mm, 1 mm and 0,1 mm q u a r t z c e l l s , and were r e p e a t e d a t i n t e r v a l s o v e r 3 y e a r s on f r e s h l y p r e p a r e d s o l u t i o n s . To g u a r d a g a i n s t b a s e l i n e s h i f t s , s o l v e n t b a s e l i n e s were , r e c o r d e d b e f o r e and a f t e r sample r u n s . A l l s p e c t r a were scanned t w i c e and each compound was measured a t two o r more c o n c e n t r a t i o n s . I n one e x p e r i m e n t t h e 230 nm band o f 1S:2R-b o r n y l n i t r a t e was examined e i g h t t i m e s (CH^CM s o l v e n t ) and t h e m o l e c u l a r e l l i p t i c i t y v a r i e d o n l y + 5$ f r o m t h e a v e r a g e v a l u e . E. S o u r c e s o f I n s t r u m e n t E r r o r I f s t r a y l i g h t i s a s i g n i f i c a n t sou rce o f e r r o r , one w o u l d e x p e c t d e v i a t i o n s f r o m B e e r ' s l a w . L i n e a r i t y o f a b s o r b a n c e v e r s u s c o n c e n t r a t i o n v e r i f i e d t h a t s t r a y l i g h t was n o t s i g n i f i c a n t i n t h e J-5 o v e r t h e s p e c t r a l range 210-300 nm. The o p t i c a l r o t a t i o n o f t h e sample o r o f t h e c e l l s does 175 n o t i n f l u e n c e t h e d i f f e r e n t i a l a b s o r p t i o n measurements , s i n c e t h e a p p a r a t u s i s based on t h e measurement o f l u m i n o u s f l u x e s ( 1 2 8 ) . S i m i l a r l y , b i r e f r i n g e n c e s o f t h e e x i t window o f t h e sample c e l l and o f t h e e n t r a n c e f a c e o f t h e p h o t o c e l l have no e f f e c t on t h e measurement ( 1 2 8 ) . However , b i r e f r i n g e n c e s e n c o u n t e r e d be tween t h e p o l a r i z e r and t h e sample c o u l d a f f e c t t h e measurement , b u t t h e s e e f f e c t s a r e e x p e c t e d t o be s m a l l ( 1 2 9 ) . The CD s p e c t r u m o f b o t h e n a n t i o m e r s o f e { - n i t r a t o -4 » f * -d i m e t h y l - t f - b u t y r o l a c t o n e (XXIV and XXV) were measured and no l a r g e d i s c r e p a n c i e s i n t h e i r m o l e c u l a r e l l i p t i c i t i e s were n o t e d . The a b s o l u t e p r e c i s i o n o f t h e measurements were l i m i t e d by t h e ' b a c k g r o u n d n o i s e o f t h e p h o t o m u l t i p l i e r ( s h o t n o i s e ) . The JASCO i n s t r u m e n t s a l s o r e c o r d a t r a c e o f t h e p h o t o -m u l t i p l i e r v o l t a g e . I t was f o u n d t h a t t h e measurements were u n r e l i a b l e when t h e p h o t o m u l t i p l i e r v o l t a g e i n c r e a s e d above 800 v o l t s . When t h i s o c c u r s , t h e n o i s e i n c r e a s e s and t h e i n s t r u m e n t t e n d s t o have a p o s i t i v e b i a s . F o r t h i s r e a s o n , a l l measurements were made v / i t h t h e p h o t o m u l t i p l i e r v o l t a g e b e l o w 800 v o l t s by d i l u t i n g t h e sample o r b y g o i n g t o a s h o r t e r p a t h l e n g t h . T h i s , t h e n , d e c r e a s e d t h e i n s t r u m e n t n o i s e . I n s t r u m e n t a r t i f a c t s a r e a l s o known t o o c c u r when s h a r p a b s o r p t i o n peaks a r e scanned ( a r o m a t i c t r a n s i t i o n s , f o r e x a m p l e ) . The n i t r a t e e s t e r s do n o t s u f f e r f r o m t h i s p h e n -omenon, as seen i n t h e UV s p e c t r a o f m e n t h y l n i t r a t e 176 ( F i g u r e 2 6 ) . No a r t i f a c t s were d e t e c t e d i n any o f t h e CD s p e c t r a . The s l i t was programmed so t h a t i t opened t o 2 mm. a t 2^5 nm. T h i s was n e c e s s a r y because o f t h e i n c r e a s e d i s o -t r o p i c a b s o r p t i o n o f t h e sample be low 24o nm. w h i c h caused t h e p h o t o m u l t i p l i e r v o l t a g e t o i n c r e a s e . A consequence o f t h e i n c r e a s e d s l i t o p e n i n g i s t h e i n c r e a s e d s p e c t r a l b a n d -w i d t h and t h e d e c r e a s e i n r e s o l u t i o n . However* t h i s i s c o u n t e r e d by t h e i n c r e a s e d d i s p e r s i o n o f t h e p r i s m i n t h i s s p e c t r a l r e g i o n . The s p e c t r a l b a n d w i d t h a t 220 nm f o r a s l i t - o p e n i n g o f 2 mm w i l l be a b o u t 2 nm. F . Low Tempera tu re CD Measurements Low t e m p e r a t u r e measurements were made i n 10 mm and 1.0mm c e l l s i n a q u a r t z dewar c o o l e d w i t h t h e b o i l - o f f o f l i q u i d n i t r o g e n ( 3 8 ) . 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