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An infrared study of D₂O and H₂O inert matrices Shurvell, Herbert F. 1962

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AN INFRARED STUDY OP D 20 AND H 20 IN INERT MATRICES by HERBERT P. SHTJRVELL B.Sc. EXETER, 1 9 5 9 . A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department of CHEMISTRY We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA A p r i l 1 9 6 2 . In presenting this thesis in p a r t i a l fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make i t freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department The University of British Columbia, Vancouver 8, Canada. Date ftPrtlL ( i i ) A B S T R A C T Infrared spectra of and D2O trapped i n s o l i d argon, krypton and nitrogen have been obtained at IL°K. In addition, spectra have been recorded of D2O i n carbon tetrachloride and ammonia matrices at 77°K, and of a d i l u t e solution of D2O i n carbon tetrachloride at 25°C. D i l u t i o n studies involving changing the matrix to water r a t i o from $0 to 500 have been carried out with DpO i n argon at i|_0K. Changes i n the spectra of D2O ana H2O i n argon, krypton and nitrogen matrices during warm-up have been observed. The d i l u t i o n and warm-up studies have made i t possible to assign certain peaks to monomer, low polymer and higher polymers. The complex spectra of H2O and D2O i n argon, krypton and nitrogen at l i q u i d helium ..temperatures are discussed, and previous simple explanations involving molecular association, r o t a t i o n and mul-t i p l e trapping sites advanced by other workers are shown to be inadequate. .Consideration i s given to the isotope e f f e c t and interactions with the matrix i n an attempt to account for the observed spectra. ( i ) ACKNOWLEDGMENT I would l i k e to express my gratitude to Dr. K. B. Harvey for h i s help and-encouragement during the course of thi s ?<?ork. Thanks are also due to Mr. R. Muehlchen for h i s help with the apparatus and making the l i q u i d helium; also to Mr. P. Horn who ran mass spectra on the materials used. ( i i i ) CONTENTS Page Acknowledgment i Abstract i i L i s t of Tables V L i s t of Figures v i Chapter 1 Introduction 1 1-1 The Problem 1 1-2 Summary of Previous Work 1 1-3 An Outline of the Present Work 2 Chapter 2 Experimental 6 2-1 Techniques Employed 6 2-2 The Low Temperature C e l l 6 2 - 3 Materials 8 2-k The Spectrometer 8 Chapter 3 Results 10 3-1 D 2 0 i n Various Matrices @ ifOK 10 3 - 2 D i l u t i o n Studies on D 2 0 i n Argon 10 3 - 3 Warm-up Studies 13 3-k D 2 0 i n Various Matrices @ 77°K 13 3 - 5 D 2 0 i n Carbon tetrachloride 114-3-6 H 2 0 i n Various Matrices @ i|°K 1k Chapter Ij. Discussion 33 k-1 Assignments to Polymeric Species 33 (iv) CONTENTS Page Li-2 Isotope E f f e c t s i\2 I4-—3 Multiple Trapping Sites lilx JL|_— JLj_ Rotation of Monomer 5 2 LL-5 Interactions with the Matrix 5 6 IL-6 Conclusions 5 9 J4.-7 Suggestions for Further Work 5 9 Appendix I Properties and Characteristics of Matrices 6 0 Appendix II Symmetry Properties, Selection Rules and Energy Levels of HgO and 6 3 Bibliography 6 6 (v) LIST OF TABLES• Page TABLE 1 Van T h i e l e t a l - H 2 0 i n N i t r o g e n a t 20°K„ 3 2 P r e v i o u s S p e c t r a o f HgO and i n Rare Gas M a t r i c e s II 3 -A Summary o f I n f r a r e d S t u d i e s o f H 2 0 and D2O by the M a t r i x I s o l a t i o n Technique 5 if. S p e c t r a o f D2O i n V a r i o u s M a t r i c e s a t 11 5 D i l u t i o n S t u d i e s - D2O i n Argon 12 6 H2O i n Argon, K r y p t o n and N i t r o g e n a t L\PK 16 7 Assignments t o H2O Polymers 38 8 Assignments to D2O Polymers ill 9 Peaks A s s i g n e d t o Monomer i n S p e c t r a of D2O and H2O k$ 10 Diameters o f L a t t i c e S i t e s l\h 11 S t r o n g e s t Monomer Peaks i n S p e c t r a o f D 2 0 and H 2 0 lp3 12 D2O and H2O a t IL°K - A l l o w e d T r a n s i t i o n s and C a l c u l a t e d I n t e n s i t i e s . 53 13 C a l c u l a t e d F r e q u e n c i e s f o r the ArOD^ m o l e c u l e s $8 lit P h y s i c a l P r o p e r t i e s o f M a t r i c e s 61 15 Energy L e v e l s o f H2O and D 2 0 65 (vi) LIST OF FIGURES To Follow Page F i g . 1(a) The Low Temperature C e l l (h) The Apparatus - Vacuum System and Low Temperature C e l l SPECTRA: Figs. 2-k D i l u t i o n Studies - D 2 0 i n Argon at 4°K 17 F i g s . 5 -7 DpO i n Argon, Krypton and Nitrogen at l+°K 21 Figs. 8-10 HpO i n Argon, Krypton and Nitrogen tt k°K 25 F i g . 11 A Warm-up Study - D 2 0 i n Krypton 29 F i g . 12(a) D 2 0 i n C C l ^ Matrix at 77°K 31 (b) D 2 0 i n CCl^ Solution @ 2S°G F i g . 13(a) The Water Molecule ip9 (b) The Normal Vibrations of the Water .Molecule F i g . l k Structures of S o l i d Matrices F i g . 15 The Normal Vibrations of an XYZ 2 .Molecule $6 1. CHAPTER 1. INTRODUCTION 1-1 The Problem. Infrared spectroscopy of water i n gas, l i q u i d , and s o l i d phases, has been the subject of extensive research by many workers. The very complex gas phase spectra of both H£0 and D£0 have been completely analysed (1) - (k). The gross features of the l i q u i d phase spectrum are simple and e a s i l y interpreted (5) - ( 8 ) , and that of the s o l i d phase, although exhibiting several i n t e r e s t i n g features, has been explained (9) (10) ( 1 1 ) . More recent work ( 1 2 ) . - . ( 1 5 ) has suggested that water molecules trapped i n s o l i d i n e r t matrices at low temperatures possess unexpected properties which manifest themselves i n complex i n f r a r e d spectra. The problem i s then to supplement exis t i n g data, by obtaining spectra for regions hitherto unexamined by the matrix i s o l a t i o n technique, to confirm and improve previous work by repeating under higher resolution, and to f i n d a s a t i s f a c t o r y explanation f o r the observed spectra. 1-2 Summary of Previous Work. Van Thiel,Becker & Pimentel (12) studied H 2 0 i n a nitrogen matrix at 20°K. By varying the matrix to water r a t i o from 10 to 1 0 0 0 , they assigned frequencies to water monomers, dimers, and higher polymers. (See Table 1 ) . Catalano and M i l l i g a n (lk) have recorded spectra of H 2 0 and D26 i n argon, krypton and xenon i n the temperature range 20 - k.2°K. These workers observed several cl o s e l y spaced 2 bands In the bending r e g i o n (see Table I I ) , and i n t e r p r e t e d t h e i r r e s u l t s on the b a s i s t h a t and D 2 0 r o t a t e i n s o l i d argon, k r y p t o n and xenon. G l a s s e l (13) has s t u d i e d the spectrum of ortho and para H 2 0 i n argon and xenon m a t r i c e s a t 20°K, and concluded t h a t the spectrum c l o s e l y approximates the gas spectrum of a mixture of 1$% ortho and p a r a H 2 0 at 20°K. O g i l v i e (15) r e p e a t e d the work with H 2 0 i n argon a t _|i°K, and suggested r o t a t i o n i n m u l t i p l e t r a p p i n g s i t e s as a p o s s i b l e e x p l a n a t i o n of h i s r e s u l t s . 1-3 An O u t l i n e of the Present Work. The most s i g n i f i c a n t r e s u l t s d e s c r i b e d i n t h i s t h e s i s are the s p e c t r a o b t a i n e d a t lt°K f o r D 2 0 i n argon, krypton and n i t r o g e n , and f o r H 2 0 i n krypton. The way i n which these r e s u l t s supplement e x i s t i n g data i s shown i n Table I I I . Previo u s workers used double beam p r i s m instruments to r e c o r d t h e i r s p e c t r a ; the spectrometer used i n t h i s work was a s i n g l e beam g r a t i n g instrument capable of v e r y h i g h r e s o -l u t i o n . I t was decided, t h e r e f o r e , to rep e a t the p r e v i o u s work i n H 2 0 i n n i t r o g e n and krypton. In d i s c u s s i n g the complexity of the observed s p e c t r a , the f o l l o w i n g p o i n t s need to be c o n s i d e r e d : (a) polymeric s p e c i e s , (b) m u l t i p l e t r a p p i n g s i t e s , (c) r o t a t i o n of the water monomer, (d) i n t e r a c t i o n s w i t h the m a t r i x . 3 TABLE 1. The Work of Van Th i e l et a l (12) Observed Infrared Suectrum of H£0 i n Nitrogen at 20°K. Band Probable Band Width Frequency S h i f t cm"' Species y% cm-' -«-(y0-v ) cm-' 1600 monomer 5 l 6 l 5 _ -1620 dimer _ _ 1633 polymer 35 — 3355 polymers 365 320 3222 tetramer (?) 80 kSk 3318 - 358 3355 trimer (?) 32 311 3510 - 166 351|-6 dimer 23 130 3691 27 -15 3627 monomer 20 _ 3725 25 Uo - 3676cm" i s the average of the monomer peaks (3627 and 3725 cm-' ) TABLE I I . Previous Spectra of H2O and D 2 0 i n Rare Gfas Matrices. (a) Bending Region ( V 2 ) frequencies i n om' Catalano & M i l l i g a n (lk) Ogilvie ( 1 5 ) D 2 0 H 2 0 H 2 0 M/R = 800 ::-M/R . 500 M/R - 300 i n Argon i n Argon i n Krypton i n Xenon i n Argon l i f t 1557 1163 1572 1572 1560 157k 1178 1593 1590 1585 1593 1190 1602 1195 1608 1605 1602 1608 1203 1610 1213 1622 1620 1615 l 6 2 k 1220 1638 1630 1628 1 6 3 ? 1260 1655 164.O 161+2 1663 -::-M/R means matrix to water r a t i o . (b) Stretching Region ( V , & 1?3 ) of H20---;:- frequencies i n cm"' . Catalano & M i l l i g a n ( l k ) Glasel (13) Ogilvie (15) M/R unknown M/R = 6 0 0 M/R = 300 i n Argon i n Krypton i n Argon i n Xenon i n Argon 3327 3376 3396 3 k l 6 3508 3521 3510 3539 357k 3636 3582 3 6 3 3 . 5 3 6 5 9 - 2 0 3613 3651 3689 3686 3695 3708 3730*20 3 7 2 k 3737 3725 3750*20 37k8 3758 3 7 5 7 . 5 3772120 3768 3777 -::--::-There have been no previous matrix i s o l a t i o n studies on D 2 0 i n t h i s region. 5 TABLE I I I . A Summary of the Infrared Studies of H2O & D 20 by the Matrix I s o l a t i o n Technique Including the Present Work. The Instruments Used by the Various Workers are Given. Matrix H 20 D 20 Bending region Stretching region Bending region Stretching region Nitroger Van T h i e l et a l (12) (Perkin Elmer 21) THIS WORK (Perkin Elmer 112 grating) Argon Catalano & Mi l l i g a n (Ik) (P.E. 21 & 112 prism) Glasel (13) (Beckmann D.K.I) Catalano & M i l l i g a n THIS WORK Krypton Catalano & M i l l i g a n THIS WORK THIS WORK Xenon Catalano & Mi l l i g a n Glasel -* In the present work H 20 i n nitrogen has been repeated. Ogilvie (15) has repeated the work with HoO i n argon, using the Perkin Elmer 112 Grating Instrument. 6. CHAPTER 2. EXPERIMENTAL  2-1 Techniques Employed; Water molecules were trapped i n i n e r t m a t r i c e s a t low temperatures u s i n g the m a t r i x i s o l a t i o n method d e s c r i b e d by Becker and Pimentel (l6), W h i t t l e , Dows and Pimentel (17) and Bass and B r o i d a (18). The technique c o n s i s t s of the d i s p e r s a l of a c h e m i c a l l y a c t i v e substance - i n t h i s case D2O or H2O -i n a l a r g e excess o f an i n e r t s o l i d m a t r i x at a temperature low enough to prevent d i f f u s i o n of the a c t i v e m olecules. I t may be noted t h a t water i s c o n s i d e r e d a c t i v e s i n c e i t " r e a c t s " to form dimers or polymers by hydrogen bonding. Mixtures of water vapour with a m a t r i x gas were pr e p a r e d s e v e r a l days b e f o r e an experiment; m a t r i x to water r a t i o s o f between 200 and 300-to-l were used. Thorough mixing of the gases was achieved by means of a c o n v e c t i o n c u r r e n t produced by h e a t i n g the lower p a r t of the storage b u l b . During a run the mixture was passed through a d e p o s i t i o n tube i n t o the low temperature c e l l where the gas stream was allowed to impinge on a caesium i o d i d e p l a t e c o o l e d by l i q u i d h e l i u m to k. 2°K (or l i q u i d n i t r o g e n to 77°K.) A needle v a l v e was used to c o n t r o l gas-flow and the r e l a t i v e r a t e of flow was i n d i c a t e d by the p r e s s u r e r e a d i n g of a thermocouple gauge. (See f i g . l b ) 2-2 The Low Temperature C e l l : The c e l l used f o r t h i s work i s of the Duerig-Mador (19) type, and i s shown i n f i g . l a . I t c o n s i s t s e s s e n t i a l l y of a c e n t r a l l i q u i d h e l i u m c o n t a i n e r surrounded by a r a d i a t i o n s h i e l d , and an outer v e s s e l equipped with o p t i c a l windows of fig la. THE LOW TEMPERATURE CELL l iqu id _ nitrogen radiat ion^ shield hole to admit infrared beam Cs I windo w . l i qu id he l i um to pumping system WWA/ A/vWA Cs I deposi t ion window hole to admit gas from deposi t ion tube deposition tube 7 . caesium i o d i d e . The c e l l i s connected to a vacuum system of the c o n v e n t i o n a l type. The l i q u i d h e lium c o n t a i n e r i s made of copper and i s suspended by the neck to minimize i n f l o w of heat by conduction, w i t h subsequent l o s s of l i q u i d helium. The r a d i a t i o n s h i e l d ( a l s o copper) can be f i l l e d w i t h a l i q u i d r e f r i g e r a n t such as l i q u i d n i t r o g e n . Set i n a copper b l o c k below the l i q u i d h e l ium c o n t a i n e r i s a caesium i o d i d e window on which the d e p o s i t of D^O/matrix forms. The c o l d j u n c t i o n of a g o l d - s i l v e r / g o l d - c o b a l t thermocouple i s a t t a c h e d to the copper b l o c k . The E.M.P. from the thermocouple i s r e c o r d e d as a t r a c e on the same c h a r t paper as the spectrum thus g i v i n g a r e c o r d of the temperature a t which the spectrum was observed. T h i s i s p a r t i c u l a r l y u s e f u l f o r warm-up s t u d i e s . The l i q u i d h e l i u m c o n t a i n e r may be turned through 90° so that the caesium i o d i d e p l a t e can f a c e e i t h e r the d e p o s i t i o n tube or the windows of the outer v e s s e l . The procedure f o r o p e r a t i n g the low temperature c e l l i n a t y p i c a l run i s as f o l l o w s : -F i r s t , the r a d i a t i o n s h i e l d i s f i l l e d w ith l i q u i d n i t r o g e n , then l i q u i d h e l i u m i s t r a n s f e r r e d s l o w l y to c o o l the c o n t a i n e r to k . 2°K. T h i s u s u a l l y r e q u i r e s 1% l i t r e s and takes about 30 minutes. A more r a p i d t r a n s f e r of l i q u i d h e l ium to minimize l o s s by e v a p o r a t i o n i s then c a r r i e d out u n t i l the c o n t a i n e r i s f i l l e d (1 l i t r e ) . The gas mixture i s d e p o s i t e d u n t i l a f i l m of condensed m a t e r i a l s u i t a b l e f o r i n f r a r e d study i s produced. T h i s may take s e v e r a l hours, 8 . depending on the rate of deposition and i t i s frequently necessary to make a second transfer of l i q u i d helium and a further deposit i n order to obtain s a t i s f a c t o r y spectra. When deposition i s complete the caesium iodide plate i s turned through 90° a*id the appropriate regions of the spectrum are scanned. 2 - 3 Materials: Regular grade argon and p r e p u r i f i e d nitrogen were obtained from Matheson Co., Inc., and neon and krypton from A i r Reduction Co. Mass spectroscopic analyses of these gases indicated an upper l i m i t of impurity f o r the argon and nitrogen of 5-10 parts per m i l l i o n , and 5 0 - 6 0 parts per m i l l i o n for the neon and krypton. D 2 0 of 9 9 . 8 $ p u r i t y and double d i s t i l l e d H 2 0 were treated by repeated freezing and pumping on the s o l i d to remove traces of non-condensable gas before the mixtures were prepared. Dried reagent grade carbon tetrachloride was used as a matrix, and for the work on D 2 0 i n GGl^ solution. 2-lx. The Spectrometer: The Perkin Elmer 112G spectrometer i s a high resolution, single beam, double pass instrument. The main features are: a 6o° Potassium bromide fore-prism, which acts as a f i l t e r to eliminate the energy of unwanted orders, and a 75 l i n e s per millimeter echelette grating, blazed for maximum i n t e n s i t y at 12 p ( 8 5 0 c m - ') i n the f i r s t order. 'Two detectors are available: a thermocouple, and a lead sulphide c e l l which has a s e n s i t i v i t y more than twenty times as great as that of the thermocouple but can only be used above 3500 cm-'. The instrument was 9. c a l i b r a t e d u s i n g t h e a c c u r a t e l y k n o w n l i n e s o f t h e v i b r a t i o n -r o t a t i o n s p e c t r a o f H 2 O , H C l , CO2 , e t c . ( 2 0 ) . S i n c e t h e o p t i c a l p a t h l e n g t h o f t h e 1 1 2 G i n s t r u m e n t i s a b o u t 5 m e t r e s , w a t e r i n t h e a t m o s p h e r e s h o w s s t r o n g a b s o r p t i o n a n d t h e r e f o r e , w h e n t h e s p e c t r u m t o b e o b s e r v e d i s i n t h e r a n g e o f 6jA o r 3/^ , a s i n t h e c a s e o f t h e w o r k w i t h H 2 O , i t i s d e s i r a b l e t o r e d u c e t h e a t m o s p h e r i c w a t e r a n d t h i s i s a c h i e v e d b y p a s s i n g a c u r r e n t o f d r y n i t r o g e n g a s t h r o u g h t h e i n s t r u m e n t f o r a b o u t a n h o u r b e f o r e t h e s p e c t r u m i s r e c o r d e d . 1 0 . CHAPTER 3 . RESULTS 3 - 1 D?0 In Various Matrices at k°K. Mixtures of D 2 0 i n nitrogen, argon and krypton were prepared and deposited at %°K. The recorded spectra of D2O i n these matrices are shown i n figures 5 - 7 * and the observed frequencies are l i s t e d i n Table IV. Other materials used as matrices at l i q u i d helium temperature were oxygen and carbon tetrachloride. Unfortunately, severe scattering of the incident r a d i a t i o n by the deposit prohibited the recording of spectra of D 2 0 i n these matrices:. 3 - 2 D i l u t i o n Studies on D 2 0 i n Argon. The r e s u l t s l i s t e d i n Table IV were obtained using matrix to water r a t i o s (M/R) of 200 f o r argon and krypton and 2k0 for nitrogen. As an a i d i n assigning the observed frequencies to monomer and polymeric species, d i l u t i o n studies were carr i e d out. M/R ra t i o s of 5 0 , 1 0 0 , 300 and 500 were used, and variations of r e l a t i v e i n t e n s i t i e s of peaks were noted. The r e s u l t s of these studies are given i n Table V. a n<a figures 2 - k . To ensure uniformity of conditions, each mixture was prepared with the same t o t a l pressure, and the mixtures were deposited at a constant needle valve opening, giving 200 microns on the low pressure side of the valve. For 11. TABLE IV. Spectra of D40 i n various matrices at %°K, frequencies i n cm-' Argon Krypton Nitrogen B 210 M/R = 210 M/R « 2ip3 2ii.35 2578 m 2576 s 2589 w 2586 s 2598 m-s 2594 vw 2591 m 2599 s 2609) 261L) vs 2610) _ 26li). m* 2615) 26ll|.) 26ll|. w 2622 w 2625 m 2639 w» 2635) s 2632) _ 2650 w 2637) m 2635) 2655 s 2705 s 2723 w 2727 vs 2725 m 2737 m 27I4.5 vs 2740 s 2738.5 s 2758 m 2751 vw 2757 s 2770 m 2765 m 2765 vs 2782 vs 2775 s 2793 s 2787 m 28o5 vw 2800 vw 1154 w 1159 vw I I 6 4 w II63 w 1170 vw 1175 m 1172: w 1177 s 1176 m 1179 vs 1186 m 1182 s II89 s 1188 m 1195 vs 1191 s 1193 m 1203 w 1200 m 1203 w-* 1212 vw LEGEND v - very, s - strong, w - weak, m - medium, observed during warm-up. 12. Peak Intensity Changes i n D i l u t i o n Studies with D 2 0 i n Argon at l\9K. 50 100 300 13 7 5 3 10 5 14 — _ 2 4 27 25 24 -3 -4 21 12 2 26 11 _ 20 -_ 7 12 18 7 -15 8 7 9 19 20 1 3 8 _ 4 ,9 3 15 48 1 6 21 2 2 3 _ 2 11 1 2 2 11 _ 1 -3 9 9 6 13 17 7 - -2 16 6 9 16 9 13 48 3 7 1 2 5oo 32 8 10 78 18 8 8 108 S f 18 4 12 3 20 30 25 28 76 18 6 13 comparable spectra, the amount of D 2 0 i n the deposit should be constant, hence d i f f e r e n t deposition times were necess-ary ranging from 1 hour for the 50 to 1 mixture up to 54-hour s for the 500 to 1 mixture. The i n t e n s i t i e s i n Table V have been corrected so that they r e f e r to a constant amount of D 2 0 . 3 - 3 Warm-up Studies. The deposits of D 2 0 i n various matrices have been observed during the warming period a f t e r the l i q u i d helium re f r i g e r a n t had evaporated, ^he changes i n peak i n t e n s i -t i e s and widths a s s i s t i n c e r t a i n cases i n the assignment of frequencies to monomer and polymeric D 2 0 . A t y p i c a l warm-up study i s shown i n Pig. 1 1 , where the matrix was krypton. Other warm-up studies have been carried out with D 2 0 i n nitrogen and argon with similar r e s u l t s . 3-J4- DpO i n Various Matrices at 77°K. Three matrices have been employed using l i q u i d nitrogen as ref r i g e r a n t ; carbon tetrachloride, chlorine and ammonia. Each of these materials has a melting point considerably above 77°K, and should therefore be r i g i d enough for use as a matrix at this temperature. However, the observed spectra indicate that D 2 0 monomer i s not is o l a t e d . This may be attr i b u t e d to either the large size of the matrix molecule, permitting d i f f u s i o n of D 2 0 molecules, or the f a c t that an absolute low temperature, rather than a r e l a t i v e one i s necessary to maintain good i s o l a t i o n of i l l -r eactive species. The spectrum of DgO i n carbon tetrachloride at 77°K i n the region 2550 - 2750 cm-' i s shown i n Pig. 1 2 a . The two broad bands correspond to peaks due to polymeric species previously found at k°K. 3 - 5 DgO i n L i q u i d Carbon Tetrachloride. A saturated solution.of D2O i n pure dry carbon tetrachloride was prepared. This solution contains approximately 0 . 1 $ D 2 0 ( 2 1 ) , and corresponds to a "matrix" to water r a t i o of 1 0 0 0 . The solution was introduced into a 10 cm c e l l with NaCl windows and the spectrum i n the region 2600 - 2800 cm"' was recorded (Pig. 1 2 b ) . The s h i f t s of the y, and bands from the gas phase frequencies were 28 and 35 cm"' respectively. The small peak at 2695 cm-' i s the 0D stretching frequency of HOD, formed by hydro/gen exchange between D 2 0 and the trace of H 2 0 present i n the "dry" carbon tetrachloride. Support f o r this assignment was found by repeated d i l u t i o n of the solu t i o n with "dry" CGI4.. The 2695 cm" ' peak increased i n i n t e n s i t y while the peaks at 26I4.3 and 2753 cm- ' decreased. 3 - 6 H ? 0 i n Various Matrices at k°K. The recording of accurate spectra of H 2 0 i n nitrogen and krypton matrices was very d i f f i c u l t using a single i5< beam spectrometer. The reason i s that atmospheric water vapour absorbs very strongly i n the same region as the trapped H2O. Thus, the spectrum must be obtained by subtraction of a "blank" or background water vapour spectrum. Figures 8 - 1 0 show the spectra obtained f o r H£0 i n nitrogen and krypton. The re s u l t s obtained by Ogilvie (15) from H^ O i n argon are included. I t i s f e l t that the spectra obtained may be incomplete, and certain weak peaks may have been missed. The frequencies found are l i s t e d i n Table VI. 1 \ 1 6 . TABLE VI Observed Frequencies (cm-!) fo r H 2 O i n Argon, Krypton and Nitrogen Matrices @ k°K. Argon Krypton Nitrogen M/R = 3 0 0 M/R = 3 8 0 M/R = 2k0 3376 w 3396 vw 3 k l 6 vw 3 k l 8 w 3 k k 0 vw 3kk3 w 3q.66 vw 3510 m 3508 w 3509 w 3526 9 « 3560 m 3 5 k k w 357k vs 3570 s 3 6 3 3 . 5 w 3 5 3 k m 3699 s 3687 s 3686 vs 370b vs 3700 vs 3725 m 3713 m 3725 vs 3 7 5 7 . 5 m 37k6 vs 3777 m 1593 vs 1591 m 1602 m 1600 w 1598 vs 1608 s 1606 s l 6 l 0 s l 6 2 k vs 1625 S-:H:- 1630 m 1638 m I 6 6 3 w -^Observed during warm-up. •---"-Broad band observed i n another run with M/R • 210 NOTE: The s p e c t r a f o r HpO i n argon were o b t a i n e d by O g i l v i e (15) 17. Figures 2, 3 and k D 2 0 i n Argon at k°K, d i l u t i o n studies. The following spectra were traced d i r e c t l y from the recorder charts. Because of space l i m i t a t i o n s only part of the V 2 and >j bands are shown. fig 2, D 2 0 1 ® . in ARGON at 4°l< dilution s tud ies V, region 2635 c m ' 2614 cm- ' 1195 cm- ' 1177 c m " 20. f ig 4 D 2 0 in A R G O N at 4°K di lut ion s t u d i e s y 3 region 2781-5 c m - ' 2745 cm" 21. Figures 5>, 6 and 7. D 20 i n Argon, Krypton and Nitrogen at ij.°K. The following nine spectra were obtained using matrix to water r a t i o s of 210 f o r Argon, 210 for Krypton and 2lp0 for Nitrogen. Intensities are expressed as % absorption i . e . % 1~J« To 22.. 60-•z o — to-t-al CC tro\ O (0 CD W-l 10] fig 5c 0 2 0 ^ H t at 4 ° K Dt region A 2 ^ 0 "i rrr 1 r-Z60 0 2650 1700' 23. 2k-fig 7 INFRARED SPECTRA IN ARGON 50 -O 40-\~ o. 30 on o in 2 0 cn < 10-50 -z: o 4 0-i — c_ 3 a a:o w CO 2 0 < 10-90-8 a 7 0 6 0 50. o I-- 40 a: o 3 0 CD < 2 0 10 of D „ 0 at 4°K 2>< REGION 2 7 0 0 IN KRYPTON 2 7 5 0 2 8 0 0 cm-2 7 0 0 Cc) IN NITROGEN 2 7 5 0 2 8 0 0 c m 2 7 0 0 2 7 5 0 2 8 0 0 cm-1 2 5 . Figures 8 , 9 and 1 0 . H2O i n Argon, Krypton and Nitrogen at J4.0K. The following spectra were obtained using M/R r a t i o s of: 300 for argon, 380 for krypton and 2k0 f o r nitrogen. The r e s u l t s for argon were obtained by Ogilvie (15) and are included here for comparison. ! ~T'. Tj " T : :' i;::: .. . | . . . . "i • • .i • i : •' i : % 1 1 ; T r- 1 " • 1 r T R A : ' : ! ! : : '' .!::1 |... ,. . . . . : . f i g 8 F R A F SEDLLS P £ C Qii.jH-0 ::! :: i a t i ... h : , R E __]i]_ii :. . j . 1 .;..;.!. ; :. •: I IN ,01 !::: • i ; • i ! i . | . . . : 1 . . . . • i . : i M . i : : ! : . : . !' : : : I ' ; .. i . . • •:: i. ' :.::!!; i j ..:: •» on. •; i • ":"I""™-.-I ::' I::: : j :: j : ; .: i 1 : . : :: . 1 . ; . ! . . •ir;;:!;: . ....... t : ; i:, . .Li L: . 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I . . . | . , :: ;|; ' ::: l ; i ... j . . . . •'. j •'• • • :..: j":: :...! :•' :.:'!•::. ,...}—.. . .J . . :::: |: •:: ' . . M . . . . | ' ;i | •:]•::• ';1: i — IT. . . . 1 . :.. | •: , ,::: r:; . : : : | . . : : •:! •' ::.: \ | i : i -«-j"; -• ..., 1... I !:: ... '. ' ' ' J : : l T T - T ~ -::ifi:!| i:::r.:: ::. !:,•" . . , .; ;. •.. ::: ;i i . ::: I •.: • • , . .Tj . , , . • 1' :: •1 i i • • -1 •:.:!: :: ';•;!.:: : •r: r ........ ~7:| ~T : : r r i~ ;~|;-::: : :;;i'Jl! l.-.i: hiii::;: - '::. i : : : : j i • • • • • i" 1 i I j • l 1 :: i • j;:_:;: :::•!:;:: ::.•!:::; ;'.:!::' /j : • 11 :.; 11. •,. - r r rh t r ^:~r rrr: :::: i •:: • • • i • •' :!:3' t 0 0 ;::; i — i ; ; ; ::.: I • 1 3! ( ) 0 •.:.::!•.:. • \ : - - • • . . . . • : : : : j : 00 c nh -. . . i . i . : :.:. i. .; '.... .. 1 . : . CC) IN N I T R O G E . . j . i :: *:- ~r. .:::)::: - — •:: • •; i ; ' ; ; „ ^ ",.':\::. V ' • . i •;: • , ,1 1 jl . . . '. •. j.::: '; •: i ' . ' . .:::]'::, • i :'; j :: ; . I , , 1 . ! • • [ | • - • i • . :: " |.::: :::, : : »40 , :'. i, :•!:•;; ..... ; . . : j . !.. Jl__.lL- J;-!-!•:; ~ r . . . . . . :. !: r-t-r- . . . . . . . . . ' . . . ! . . - . • i ~:-| •:l • I •• I ' :: i::.. • i I..:: i - • • :::•!•: :I-:T;-I~ |: ::!::.•: •-' >><: . . j : . . .,..... .:.:.. L : •! ....... '•\"t- .:.:.! : j. . ,. I:--.: : ± ..: •: ..... | ':: r*:| :: _20 _|.: : •::| ::. ii.'h: !' i : T;-.: b i.;.... : ! • : • : '' •' |: : ::' I.: , • •! r \ ... j.',' ....... :'.'.•[ -.:.M " •:. i • ' ' ' h-1 .: |. .. f: -• : ! •' .. . .:: -j : : * l Q y : i i • •i ! i ' ' . ' :.:.[.::.:.:. : / - ....... :.:!;.. k; . ; J ; ,. / : : \ : . . 1 . :;! li:..-•'-'••!-:-: : •!:•• 400 ::. •: • j..:. i i . 1 \". I i 3!C >0 J—i-—L-L • • ! • : j. : . i ' : ;' . ::|.3 50 0 • •' • l : > ' • c m T.': ....... .p. 27. NER4RE0 28. 29. Figure 11. A Warm-up Study - D 20 i n Krypton. The temperatures were estimated from thermocouple readings during the warming period after the l i q u i d helium r e f r i g e r a n t had evaporated. 30. fig II. A W A R M - U P S T U D Y of D 2 0 in KRYPTON from 4°to 60°K y>. R E G I O N V 2 REGION 2 600 2650 2700 2750 cm-i 31. Figure 12a shows the spectrum of D 2 0 i n a C G I 4 . matrix at 77°K. The two broad bands are assigned to and y 3 of D 2 0 . Figure 12b was obtained from a solution of C C I 4 . saturated with D 2 0 at 25°C. The two strong bands are due to X and >$ of D 2 0 , the weaker peak i s assigned to the OD stretch of HDO. 32. 3 3 , CHAPTER k DISCUSSION Assignments to Polymeric Species. Complex spectra have been observed for the regions of the three fundamentals of H 20 and D 20 i n argon, krypton and nitrogen matrices. A preliminary step i n the quantitative discussion of these spectra i s to assign certain peaks to polymeric species, for example to dimer, other low polymers and higher polymers. Experimental Evidence for Polymers;-Certain i n f r a r e d absorption peaks have been assigned to polymeric species on the following experimental evidence. (i) Peak width at h a l f height ( Peaks have been observed with widths ranging from 2 to 50 cm"' i n the spectra of water i n various matrices. I t i s reasonable to assign the very broad bands to high polymer, since the bands i n ice due to H20 and D20 polymer are several hundred wave numbers i n width at h a l f height. The very sharp peaks on the other hand approximate i n width to the gas phase peaks, and are probably due to i s o l a t e d water monomers. For peaks intermediate i n width, assignment i s usually made to low polymers. It i s not possible to make unequivocal assignments to dimers on peak width evidence alone. ( i i ) Behaviour during warm-up:-Changes i n peak i n t e n s i t i e s and widths have 3 4 been observed d u r i n g the warming p e r i o d a f t e r the l i q u i d h e l i u m r e f r i g e r a n t has evaporated. C e r t a i n peaks decrease r e g u l a r l y as the temperature r i s e s , others i n c r e a s e a t f i r s t then decrease, and a t h i r d type i n c r e a s e and broaden as warm-up p r o g r e s s e s . Pimentel (l6) found t h a t a s o l i d m a t r i x softens a t temperatures c o n s i d e r a b l y below i t s m e l t i n g p o i n t . T h i s would allow d i f f u s i o n of water molecules through the c r y s t a l l a t t i c e . I f two water molecules should come together then d i m e r i z a t i o n w i l l occur. Observed peaks that i n c r e a s e i n i n t e n s i t y d u r i n g the e a r l y p a r t of the warm-up and then decrease as the warm-up pr o g r e s s e s can u s u a l l y be a s s i g n e d to dimer or low polymer. As the warming proceeds, the water molecules become more a c t i v e and movement becomes l e s s r e s t r i c t e d by the m a t r i x ; consequently, p o l y m e r i z a t i o n can occur, and the broader peaks t h a t i n c r e a s e i n width and i n t e n s i t y r e g u l a r l y a t the expense of sharper peaks are u s u a l l y a s s i g n e d to polymer. An example of t h i s behaviour i s shown i n F i g u r e 11, where i t i s seen that the peak a t 2625cm""' i n c r e a s e s i n i n t e n s i t y as the d e p o s i t warms from ij. to 20°K. In l a t e r stages of warm-up when the m a t r i x i s b e g i n n i n g to sublime away from the window, onl y broad polymer bands remain i n the spectrum. ( i i i ) I n t e n s i t y changes i n d i l u t i o n s t u d i e s : -D i l u t i o n s t u d i e s w i t h D20 i n argon have shown s e v e r a l i n t e r e s t i n g e f f e c t s i n the i n f r a r e d spectrum. 35. Going from M/R = 5 0 to 5 0 0 , c e r t a i n peaks increase i n r e l a t i v e i n t e n s i t y , while others decrease. When the M/R r a t i o i s increased i t i s reasonable to expect that the proportion of monomer i s o l a t e d w i l l increase, and sharp peaks which show a continuous increase are thus assigned to monomer. On the other hand, broad peaks which decrease i n i n t e n s i t y on d i l u t i o n , must be due to polymers. Figures 2-1). i l l u s -trate the changes i n the spectra observed as the argon to D 2 0 r a t i o was changed from 5 0 to 5 0 0 . An in t e r e s t i n g feature of the spectra obtained from d i l u t i o n studies i s that c e r t a i n peaks at f i r s t increase i n i n t e n s i t y as M/R i s increased and subsequently decrease at the higher M/R r a t i o s . To make use of this behaviour i n the assignment of such peaks, the species present at various M/R r a t i o s are considered. At an M/R of 5 0 the predominant species present i n the deposit are high polymers and monomers, low polymers being less l i k e l y because of the p r o b a b i l i t y of further polymerization. At M/R of 5 0 0 the majority of the water w i l l be present as i s o l a t e d monomers. So i t i s reasonable to expect that a peak that increases at f i r s t and then decreases as M/R i s changed from 5 0 to 5 0 0 i s due to dimer or low polymer. The 2 6 3 7 cnr'peak i n the V, band of D 2 0 In argon c l e a r l y exhibits this behaviour (see Figure 2 ) . (iv) Frequency s h i f t s i n condensed states:-It i s well known that when water i s condensed from the gas phase to the s o l i d state, there are s h i f t s i n 36. the observed frequencies. The stretching modes s h i f t to lower wave numbers, while the bending mode s h i f t s to a lesser degree to higher frequencies. The magnitude of the s h i f t depends on the degree of polymerization. For example, the V3 band of D 2 0 ice i s s h i f t e d by some 300 cm""' from the gas phase frequency, while lower polymers produce a frequency s h i f t of the order of 50 to 100 cm-'. In the spectra recorded f o r D 2 0 i n a carbon tetrachloride matrix at 77°K, two f a i r l y broad bands were observed. The frequency s h i f t of the i-band i s -75 cm-' from the gas phase. The second peak at 2 6 l 0 cm-' i s assigned to V| with a frequency s h i f t of -70 cm-' . On the basis of the r e l a t i v e l y "small frequ-ency s h i f t , these bands are assigned to low polymers. In the spectra of ice (high polymer) the V, and V, bands overlap and cannot be resolved. Assignments to polymers i n spectra of HpO: On 'the basis of an extensive d i l u t i o n study, Van T h i e l , Becker and Pimentel (12) assigned a l l the observed peaks i n the spectrum of H 2 0 i n nitrogen at 20°K to monomer, dimer and higher polymers (see Table 1 ) . In the present work with H 2 0 i n nitrogen at i|°K, using a M/R r a t i o of 2J4.O, a spectrum very similar to that of Van T h i e l et a l was observed, and s i m i l a r assignments were made based on peak width, frequency s h i f t s and behaviour during warm-up. For H 2 0 i n argon and krypton the spectra are more complex and more than one peak 37 i s assigned to monomer i n the region of the V% and fundamentals. This phenomenon has been observed by Catalano and M i l l i g a n (li}.) who reported complex spectra for the band, and G-lasel (13) who reported s i m i l a r results for 24 . Table VII summarizes the assignments to polymers f o r the spectra of H 2 0 i n argon, krypton and nitrogen. I t i s observed that even at d i l u t i o n s as high as M/R = 5 0 0 , appreciable concentrations of low polymers are found. An explanation f o r this i s that during deposition, the poor thermal conductivity of the window, and s o l i d matrix material, allows each new layer of deposit to remain at a temperature considerably above liPK f o r a f i n i t e time, thus d i f f u s i o n of water molecules i s possible. The temperature difference between the copper block and the newly forming surface of the "'deposit would be expected to Increase as the thickness of the deposit increases. For the high M/R r a t i o s a thick deposit i s necessary, taking several hours at 200 microns pressure to form, so I t i s not surprising that absorption peaks due to polymeric species are found. Further evidence for poor thermal contact between the copper block and the deposit i s the appearance of the deposit on the caesium iodide window, varying from thick at the edges to very t h i n at the centre. This indicates that the centre of the window during deposition, and during recording of the infrared spectra, i s at a temperature considerably above that of the edges, so that the mixture i n i t i a l l y deposited at the centre sublimes and recondenses on the colder parts 38. TABLE VII. Assignments to H?0 Polymers. Matrix Frequency y\ cm"' cm' N 1630 20 Ar l 6 2 k 10 163S 10 Kr 1625 25 N 3 l | l 8 45 31443 3509 30 3691 20 Ar 3376 -3396 -3 k l 6 -3510 20 3699 15 Kr • m 3508 20 3526 1x0 3560 10 3687 20 Evidence for assignment to polymers. Broad band. Frequency s h i f t and broadness of peak. Broad band. Frequency s h i f t and broadness of bands. Broadness of peak. Weak peaks s h i f t e d by lf?0 cm"' from gas phase. Broad peak. Broadness of peak. Frequency s h i f t . Broadness Appeared during warm-up.' Broadness of peak. Increased during warm-up. N.B. A l l other peaks l i s t e d i n Table VI are assigned to monomer. 39. of the window. The s i t u a t i o n could be improved by depositing more slowly thus reducing the warming e f f e c t produced by the stream of "hot" gases impinging on the window. However, present supplies of l i q u i d helium are i n s u f f i c i e n t for the long deposition times that would be needed. Another possible way to improve thermal contact between the • deposit and the r e f r i g e r a n t i s to use a caesium iodide plate mounted on a s i l v e r gauze. The gauze would cause l i t t l e reduction of transmitted i n f r a r e d radiation,' but would conduct heat away from the centre of the plate. Assignments to polymers i n spectra of.DpO: Table VIII shows the peaks assigned to D2O polymers, using s i m i l a r arguments to those employed i n the assign-ments for H 2 O . The doublet peaks at 2635 and 2637 cm"' i n argon and 2632 and 2635 cm"' i n krypton would have been assigned to monomer on the evidence of sharpness and behaviour during warm-up. However, d i l u t i o n studies i n argon have shown that both peaks of the doublet increase to a maximum at M/R = 100 and become weak at M/R = 500, thus they are assigned to dimers. The shoulder at 26l5 cm-' i n argon and 261I4. cm"' i n krypton i s assigned s i m i l a r l y . To make similar assignments for D 2 0 i n nitrogen the peak at 26l7 crn" and the doublet at 2650 and 2655 cm-' were chosen. 4-0. However, there i s no d i l u t i o n evidence to support these assignments, and the sharpness o f the peaks i n q u e s t i o n together with t h e i r behaviour on warm-up makes the a s s i g n -ment to dimer q u e s t i o n a b l e . i n . TABLE V I I I . Assignments to DpO Polymers. Matrix Frequency cm"' Evidence 1 cm" N 1203 20 Appeared during warm-up. Ar 1186 10 Broadness. Kr 1182 10 Ar Kr 1202 1200 6 5 Frequency s h i f t . Ar Kr 1212 1210 8 10 Frequency s h i f t . Ar 2578 6 Kr 2576 10 Broadness Ar - 2589 8 Frequency s h i f t s Kr 2586 10 Warm-up evidence Ar 2594 5 D i l u t i o n studies Kr 2591 8 -N 2598 30 Appeared during warm-up. Ar 2 6 l 5 Sh D i l u t i o n studies. Kr 26:14 Sh N 2617 3 NONE Ar 2635 h D i l u t i o n studies. 2637 5 Kr 2632 2635 s i -N 2650 2655 2 5 NONE Ar 2723 k D i l u t i o n studies. 2737 10 Increased during warm-up. Kr 2727 30 Very broad. N 2725 20 Increased during warm-up. Ar Kr 2805 2800 8 12 Broadness. NOTE Sh means shoulder \\2 k-2 Isotope E f f e c t s . Three i s o t o p i c species i n addition to D^ O could give r i s e to in f r a r e d absorption i n the regions under examination. They are H20, D 2 o ' 8 and D 2 o ' 7 . Mass spectroscopic analysis indicates that the concentration of D 2o'*and D20 1 7 i s very low and need not be considered further. However, a spectrum of D20 ice obtained on the Perkin Elmer 1^ 21 double beam grating instrument revealed the presence of a trace of H20 impurity. To investigate the e f f e c t on the spectra of D20, of HDO produced by hydrogen exchange between D20 and H20 impurity, two runs were c a r r i e d out i n an argon matrix using known percentages of HDO. For the f i r s t run a solution of $0% H20 i n D20 was prepared. This gives %0% HDO and 2%fo of H20 and D20. For the second run, a solution containing 38$ HDO, $6% H20 and 6% D20 was prepared. The M/R r a t i o s were l\\$ f o r the f i r s t run and 110 for the second. The mixtures were deposited f o r three hours at 200 microns pressure. In the f i r s t run peaks due to D20 monomer were small while i n the second run D20 monomer peaks were barely perceptable. New peaks were found at 2693 cm'' (medium) and 2679, 270l|., 2712 cm'1 (very weak) a l l assignable to HDO. Peaks previously assigned to low polymers were unchanged i n frequency, for example, those at 26l5 and 2637 cm-' , but much stronger than peaks k-3 due to D z0 monomer. This i s reasonable since any polymeric species containing an 0-D bond w i l l absorb i n the stretching region. Thus, the M/R r a t i o with respect to polymers approaches the t o t a l matrix to water r a t i o , and since t h i s was low i n the two runs, considerable polymerization would be expected. It has been shown then that the presence of a small amount of H^ O impurity w i l l have l i t t l e or no e f f e c t on the spectra of D2O. In addition, support for the assignment of peaks at 2 6 l 5 and 2637 cm""1 to dimer has been obtained. 4 4 -k-3 Multiple Trapping S i t e s . Having assigned several broad bands and c e r t a i n other peaks to polymeric species, there remain several sharp peaks which are assigned to monomer. These are l i s t e d i n Table IX where i t i s seen that the frequency differences, between peaks observed i n argon and krypton matrices, are small. The water molecule has three fundamental v i b r a t i o n frequencies (see figure 1 3 b ) , and cannot give r i s e to more than one normal v i b r a t i o n a l absorption i n the region of each fundamental. However, i f the p o s s i b i l i t y of multiple trapping sites of d i f f e r e n t size and symmetry i s considered, then more than one peak i n each region i s possible. The c r y s t a l structure of the rare gases i s cubic close packed (22) (figure 1 4 b ) . S o l i d nitrogen exists i n two modifications ( 2 3 ) , the oc form, stable below 35°K i s cubic (figure llpa.). No information i s available f o r the structure of carbon tetrachloride at very low temperatures, but i n the temperature range from 225°K to the melting point (250°K) it'has been reported to be face centred cubic (2I4,). Sub s.ti trot i o n a l and i n t e r s t i t i a l s i t e s are available f o r the trapped species. In the former a water molecule replaces a matrix atom or molecule, while i n the l a t t e r , the molecule i s accommodated i n an octahedral or tetra-hedral hole. Sizes of the various holes i n the rare gases, « - nitrogen and carbon tetrachloride have been calculated and are given i n Table X. 4*. TABLE I X . Peaks A s s i g n e d t o Monomer i n the S p e c t r a o f D 20 and H 2 0 . D p O i [ f r e q u e n c i e s i n cm"' ) A r K r (Ar - K r ) A V 1154 w 1156 1159 vw w 3 1164 w 1163 « 1 1170 vw 1175 ffi 1172 3 1177 s 1176 m 1 1179 vs 1189 s 1188 m 1 1195 vs 1191 3 4 1193 m 2614 vs 2610 s 4 2599 s 2745 vs 2740 s 5 2705 s 2758 m 2751 vw 7 2738 s 2770 in 2765 m 5 2757 s 2783 vs 2775 s 8 2765 vs 2793 s 2787 m 6 3 peaks a t 2 6 l 7 , 2 6 5 0 , 2655 cm"' were a s s i g n e d t o polymers w i t h o u t e v i d e n c e , these peaks may be due t o monomer. 2 . H?0 ( f r e q u e n c i e s i n cm"' ) Ar K r A r - K r 1574 ra 1593 s 1591 m 2 1602 m 1600 w 2 1609 vs 1606 s 3 1598 vs 3574 vs 3570 s 4 363i+ s 3708 vs 3700 vs 8 3725 vs 3724 m 3713 m 11 3757 m 3746 vs 11 3777 m 46. TABLE X. Diameters Of L a t t i c e Sites (in A) Matrix Substitutional Octahedral Tetrahedral Ne 3 . 0 7 1 . 2 8 0 . 6 8 Ar 3 . 7 5 1 . 5 6 0 . 8 6 Kr 4. .02 1 . 6 6 0 . 9 0 Xe 4.31 1 .78 O .96 4.1 ( 3 . 0 ) 1 .7 ( 1 . 2 ) O .94 (O .69) CGI4. 5 . 9 2.4 1 . 4 -::-The maximum diameter for nitrogen i s given f i r s t . Figures i n brackets are calculated from the minimum diameter of the molecule. kl The structure of the water molecule i s shown i n figure 13a, where the atoms are drawn as spheres with r a d i i equal to t h e i r van der Waals r a d i i /values obtained from Pauling (25>)_7 The diameter of the water molecule i s approximately 3.6 A, and by inspection of Table X i t i s evident that only the s u b s t i t u t i o n a l s i t e s can accommodate a ;water molecule, without contact of the van der Waals spheres. The covalent r a d i i of oxygen and hydrogen are considerably less than the van der "Waals r a d i i (2f?) and the diameter of the water molecule calculated on this basis would be 1.9 A, which i s approximately the same size as an octahedral s i t e . Thus, i t appears that s u b s t i t u t i o n a l s i t e s might allow free r o t a t i o n of the water molecule except nitrogen which, owing to i t s c y l i n d r i c a l shape might allow r e s t r i c t e d r o t a t i o n . Rotation w i l l be discussed i n the next section of this chapter. In the observed spectra there i s one noticeably large peak i n the region of each fundamental. These are given i n Table XI. If one postulates that some water monomers are trapped i n s i t e s which prevent r o t a t i o n e n t i r e l y , then the "forbidden" Oo-Oo t r a n s i t i o n becomes allowed. Such a s i t e might be a distorted octahedral s i t e . The frequency s h i f t s i n the various matrices are also given i n Table XI. I t i s noticeable that the spectra obtained using a nitrogen TABLE X I . S t r o n g e s t Monomer Peaks i n the y, S p e c t r a Of D 2C 1 and H20 ( cm-1 ). M a t r i x D20 S h i f t from H20 S h i f t from gas phase gas phase Argon 2614 -57 3574 -83 K r y p t o n 2610 -61 3570 - 8 7 N i t r o g e n 2655 -16 3634 -23 Gas phased 2671 - 3657 -Argon 1195 17 1609 14 K r y p t o n 1191 13 1606 11 N i t r o g e n 1179 1 1598 3 Gas phases- 1178 1595 Argon 2745 -k-3 3708 -48 K r y p t o n 274O -48 3700 -56 N i t r o g e n 2765 -23 3725 -31 Gas phase 2788 3756 :-The f r e q u e n c i e s quoted f o r the gas phase a re the " f o r b i d d e n " Oo-Oo t r a n s i t i o n s . 4 9 . m a t r i x are q u i t e d i f f e r e n t from those r e c o r d e d f o r the r a r e gas m a t r i c e s . T h i s should be s i g n i f i c a n t but with the p r e s e n t evidence i t i s only p o s s i b l e to account f o r such d i f f e r e n c e s by c o r r e l a t i n g with the obvious d i s s i m i l a r i t y of the c y l i n -d r i c a l n i t r o g e n molecule and the s p h e r i c a l r a r e gas atoms. I t i s a l s o worth n o t i n g t h a t the frequency s h i f t s from the gas phase to m a t r i x i s of the same order as t h a t f o r low polymers. The other monomer peaks may be produced by molecules r o t a t i n g i n s u b s t i t u t i o n a l s i t e s . T h i s w i l l be d i s c u s s e d i n the next s e c t i o n . Another p o s s i b i l i t y i s t h a t the water molecules are trapped i n s e v e r a l other s i t e s , such as s i t e s i n which two mat r i x atoms or molecules are r e p l a c e d by a water molecule, or su r f a c e s i t e s produced by d e f e c t s i n the c r y s t a l l a t t i c e of the s o l i d m a t r i x . However, s i n c e n o t h i n g i s known about these s i t e s , i t i s d i f f i c u l t to d i s c u s s the e f f e c t on the spectrum at t h i s time. 5 0 . f ig 13 a The water molecule, the atoms are shown as spheres with r a d i i equal to t h e i r van der Waals r a d i i . fig 13 b The normal v i b r a t i o n s of the water molecule* 51-f ig K a A projection down a cubic A packing drawing of the close axis of the low temperature packed cubic structure of s o l i d form of s o l i d nitrogen. N 2 viewed along a cube axi s . fig U_b. The cubic close packed structure of s o l i d i n e rt gases, i l l u s t r a t i n e an octahedral s i t e . 52 4-4 R o t a t i o n of Monomer. The water molecule i s an asymmetric top s i n c e i t has three d i f f e r e n t p r i n c i p a l moments of i n e r t i a ( I / , ^ 1$ ^ I c ). The allowed gas phase r o t a t i o n a l t r a n s i t i o n s t h a t o r i g i n a t e from the two lowest l e v e l s , are o b t a i n e d u s i n g the energy l e v e l s and s e l e c t i o n r u l e s from Appendix I I , and the c a l -c u l a t e d f r e q u e n c i e s f o r D 20 and H 20 are g i v e n i n Table X I I . I t has been shown (25) t h a t a t ij.°K o n l y the lowest l e v e l w i t h r e s p e c t to each s p e c i e s i s occupied. That i s the Oo l e v e l f o r p a r a molecules and the 1_, l e v e l f o r ortho molecules. I n t e n s i t i e s of r o t a t i o n a l t r a n s i t i o n s have been c a l c u l a t e d (25) and are i n c l u d e d i n Table X I I . I t i s assumed t h a t the normal h i g h temperature ortho para e q u i l i b r i u m r a t i o i s maintained i n the m a t r i x . I f true thermal e q u i l i b r i u m obtains, only one t r a n s i t i o n , namely t h a t o r i g i n a t i n g from the Oo l e v e l , would be allowed a t 4°K. I t may be seen then t h a t i n the r e g i o n of each funda-mental three peaks ( f o u r f o r the y3 r e g i o n ; of H 2 0 ) should be observed i f f r e e r o t a t i o n of the monomer occurs i n the m a t r i x a t i|_°K. However, the experimental s p e c t r a do not support a simple i n t e r p r e t a t i o n based on r o t a t i o n . The main areas of disagreement are: ( i ) The number of observed peaks. In c e r t a i n i n s t a n c e s there are too few peaks observed compared wi t h the p r e d i c t e d spectrum. F o r example, i n the V, r e g i o n of both H 20 and D 20 i n argon and krypton, only 53 TABLE X I I . D p 0 and H-O at [|°K Allowed T r a n s i t i o n s and C a l c u l a t e d I n t e n s i t i e s . Type A Band 3^ D 2 0 cm"1 I n t e n s i t y cm ' H 2 0 I n t e n s i t y i Tr J / R (0) 1_, 2800 20 3779 10 R(D 2_2 2811 20 3801 60 R(D 2 Z l - i 28I4.8 0 . 0 3 3863 k P(D 0 o . 2776 10 3732 30 y, Type B Bands i J r n J r D 2 0 „ , cm c m " I n t e n s i t y R (0) l o 0 o 2692 . 1200 20 Q(D 1 , l _ i 2701 . 1209 15 R(D 2 . , 1 , 2682 1190 15 I II R(o) i c o,, Q ( l ) 1 . 1-. R(D 2^, 1_, H 2 0 cm"' cm"' I n t e n s i t y 3657 1595 10 3675 1617 h$ 3711 1653 kS one peak i s a s s i g n e d to monomer. S i m i l a r l y , f o r both D2O and H£0 i n n i t r o g e n there i s o n l y one monomer peak i n the r e g i o n of the yx fundamental. On the other hand, many more peaks than p r e d i c t e d are observed i n the Vz r e g i o n of H2O and D 20 i n argon, the V3 r e g i o n o f D 2 0 i n argon and k r y p t o n and f o r the yz r e g i o n of D 2 0 i n krypton, ( i i ) Frequency S h i f t s . In comparing gas phase s p e c t r a w i t h those obtained from the m a t r i x i s o l a t i o n s t u d i e s , many anomalous frequency s h i f t s are observed. T h i s i s i l l u s t r a t e d by the f o l l o w i n g example. In the y 3 band of D2O i n n i t r o g e n (see f i g u r e 7c) f o u r sharp peaks are a s s i g n e d to monomer. The peak a t 2705 cm-' i s f u r t h e r a s s i g n e d to the Oo - Oo t r a n s i t i o n o f monomer trapped i n s i t e s t h a t prevent r o t a t i o n , (by the arguments of the pre v i o u s s e c t i o n ) . The three remaining peaks at 2 7 3 8 , 2757 and 2765 cnr' may be compared with p r e d i c t e d f r e q u e n c i e s at 277&, 2800 and 2811 cm -', and the gas-matrix s h i f t are seen to be 3 8 , l±3 and k6 cm"' r e s p e c t i v e l y . These s h i f t s are not constant, which means t h a t the s e p a r a t i o n of peaks i n the observed spectrum i s not the same as i n the gas phase. Furthermore, the magni-tude of the s h i f t s r e p r e s e n t s a p e r t u r b a t i o n of the water molecule t h a t might be expected.to prevent r o t a t i o n e n t i r e l y . I t i s noteworthy t h a t i n a l l cases the gas-matrix s h i f t i s to lower wave numbers. T h i s may be compared with the s h i f t s due to p o l y m e r i z a t i o n , where f o r the s t r e t c h i n g r e g i o n s the s h i f t i s to lower wave numbers, while f o r the bending r e g i o n the s h i f t i s to h i g h e r f r e q u e n c i e s . 5 5 . ( i i i ) I n t e n s i t i e s . A better f i t of observed frequencies with the pre-dicted can be found i n cert a i n cases. For example, i n the V 3 region of D 2 0 i n argon, aft e r assigning the strong peaks at 27i)-5 cm"1 to non-rotating monomer, there remain three strong peaks at 2758, 2783 and 2793 cm"'. Subtracting these from the predicted frequencies, s h i f t s of l 8 , 17 and 18 cm"1 are obtained. The agreement i s seen to be very good and the s h i f t i n t h i s case i s not excessive; however, the i n t e n s i t i e s of the observed peaks do not correspond with the calculated values. The calculated r e l a t i v e i n t e n s i t i e s are i n the r a t i o 1 : 2 : 2 , while the observed i n t e n s i t y r a t i o i s approximately 2 : 7 : 3 . An analagous s i t u a t i o n i s found for >>3 of i n krypton, and Vx of DgO i n argon and krypton. To obtain a go od f i t with the predicted spectrum i n the above cases, three peaks only were assigned to ro t a t i n g monomer. There s t i l l remain several unassigned frequencies i n ; t h e observed spectrum. To account for these i t could be postulated that v i o l a t i o n s of the s e l e c t i o n rules occur i n the matrix. For example, the weak peaks at 2770 and 2805 cm"' i n the spectrum of D 2 0 i n argon, could be the forbidden 10 - 1_, and 2 0 - 1_( t r a n s i t i o n s , calculated for the gas phase at 2796 and 2825 cm"' respectively. However, no peaks corres-ponding to the other forbidden transitions were observed, and there i s no j u s t i f i c a t i o n for making these two tr a n s i t i o n s i n p a r t i c u l a r allowed. 56. k-5 Interactions with the Matrix. The large frequency s h i f t s which increase with the p o l a r i z a b i l i t y of the matrix, are experimental observations that indicate a strong i n t e r a c t i o n between the matrix and the trapped water molecules. For example, i n the V 3 region of DgO i n argon, krypton and nitrogen, the frequency s h i f t s are 20, 25 and kO cm- '• respectively. Various matrix i n t e r a c t i o n models may be postulated, the simplest of which involves a dipole induced dipole i n t e r -action between a matrix atom and the oxygen atom of the water molecule. This would produce an XYZ^ molecule which would have six normal v i b r a t i o n a l modes (figure l5)« f i g 15. Normal vibrations of an XZZZ molecule. O 5 7 . Comparing with figure 13b , i t i s seen that V, y ? and > V correspond closely with V, >*2 and >*3 of the water molecule, while yv and V6 are new frequencies associated with the 0-M "bond" (M being a matrix atom). Using the equations given by Herzberg ( 2 7 ) , frequencies of the normal modes were calculated for the ArOH 2, and ArOD2 molecules. The force constants used i n these calculations were 7 . 8 md/A for the 0-H stretch, and 0.6°, md/A for the H-O-H bend (from Wilson Decius and Cross ( 2 8 ) . ) Values of 0 . 7 8 and O .069 md/A were assumed for,the O-Ar and H-O-Ar force constants i n a preliminary c a l c u l a t i o n ( i . e . 10$ of the corresponding force constants of the water molecule). In subsequent calculations, the force constants were varied as indicated i n Table XIII where the calculated frequencies are given. It i s evident that this simple model does not give frequencies that can be used to account for the observed spectra. However, i t does show that the three fundamentals corresponding to >>, Vt and V, of water, are only s l i g h t l y d i f f e r e n t from the parent molecule. A more complicated model could be used, for example a tetrahedral 0 H 2Ar z mole-cule which would have eight i n f r a r e d active fundamentals. 58. TABLE XIII. Calculated Frequencies f o r Ar0H z and ArOD^. Force Constants Used Mode Frequencies i n cm - 1 Md/A ArOH 2 ArOD_. 0-H 7.8 (100$) 3719 2687 H-O-H O.69 (100$) 331 316 O-Ar 0.78 (10$) 1595 1178 H-O-Ar O.069 (10$) 38k5 2867 356 260 0-H 7.02 (90$) v, 3523 2541 H-O-H O.69 (100$) v t 330 311 1608 1195 O-Ar 0.78 (10$) 3570 2615 H-O-Ar O.069 (10$) 364- 270 0-H 7 M (95$) V, 3619 261k 229 217 H-O-H O.69 (100$) 1608 1195 O-Ar 0.39 (5$;) 3668 2686 H-O-Ar O.034. (5$) 257 191 59-4-6 C o n c l u s i o n s . Many peaks have been s a t i s f a c t o r i l y a s s i g n e d to p o l y -meric s p e c i e s , and assignment of other f r e q u e n c i e s has been d i s c u s s e d . A combination of m u l t i p l e t r a p p i n g s i t e s w i t h allowed and f o r b i d d e n r o t a t i o n a l t r a n s i t i o n s , can account f o r the peaks assi g n e d to monomer i n c e r t a i n cases, f o r example, i n the r e g i o n of D 2 0 i n argon and k r y p t o n . However, t h i s treatment i s i n c o n s i s t a n t i n t h a t i t does not apply to the r e g i o n s of a l l fundamentals of H 2 0 and D 2 0 i n the v a r i o u s m a t r i c e s . A second e x p l a n a t i o n has been put forward i n v o l v i n g a m a t r i x i n t e r a c t i o n model. However, a simple treatment does not g i v e u s e f u l r e s u l t s and more complicated models would i n v o l v e an e x c e s s i v e amount of c a l c u l a t i o n . The p r e s e n t work has shown then, that simple explan-a t i o n s such as those put forward by G l a s e l (13) and Catalano and M i l l i g a n (llj.) can not account f o r the complex s p e c t r a of H 2 0 and D 2 0 trapped i n i n e r t m a t r i c e s at low temperatures. 4-7 Suggestions f o r F u r t h e r Work. A study i n v o l v i n g diatomic molecules might p r o v i d e a key to the c o n d i t i o n s t h a t p r e v a i l when smal l molecules are trapped i n i n e r t m a t r i c e s . P o s s i b l y c o n t r a s t i n g e f f e c t s would be shown by a molecule w i t h a s m a l l d i p o l e moment, e.g. CO , and a molecule w i t h a l a r g e moment, e.g. HCl. In e i t h e r case simpler m a t r i x i n t e r a c t i o n models than those necessary f o r t r i a t o m i c molecules c o u l d be t r e a t e d . 6o. APPENDIX I Properties and Characteristics of Matrices, Some phys i c a l properties of the materials used as matrices i n this work are given i n Table XIV. Becker and Pimentel ( l 6 ) have discussed the desirable character-i s t i c s of a matrix. The matrix must be inert with respect to the species under examination, s u f f i c i e n t l y r i g i d to prevent d i f f u s i o n of the trapped molecules, and trans-parent i n the spectral region of in t e r e s t . Becker and Pimentel noted that small molecules diffuse r a p i d l y and cannot be trapped f o r period of many minutes i f the temperature of the matrix i s allowed to r i s e to the range O.Jj, - 0 .6 of the melting point of the matrix. In the present work i t has been observed that an absolute low temperature i s necessary. These authors also observed that the vapour pressure of the matrix must be below 10" mm Hg at the temperature of the refr i g e r a n t , giving a r e s t r i c t i o n on the upper l i m i t to the temperature range i n which a matrix can be used, e.g. fo r xenon about ?0°K, for argon about 35°K and fo r nitrogen about 30OK. A property which may l i m i t or even preclude the use of a material as a matrix i s l i g h t scattering. For this reason a glassy rather than a c r y s t a l l i n e matrix i s desirable. Catalano and M i l l i g a n (ll].) noted that they were unable to record the spectrum of H^ O i n xenon i n the stretching region because of excessive scattering. In the present work 6 i . TABLE XIV Physical Properties of Matrix Materials. Property Ne Ar Kr Xe N 2 CCl^ oc Melting Point °K - 2 4 8 . 7 - 1 8 9 . 2 - 1 5 6 . 6 - 112 - 2 0 9 . 9 - 2 2 . 8 2 4 . 4 83 .9 116 . 6 l 6 l 6 3 . 3 2 5 0 . 5 °C B o i l i n g Point °K - 2 4 5 . 9 - 1 8 5 . 7 - 1 5 2 . 9 -107 - 1 9 5 . 8 76 .8 27 .3 87 .5 1 2 0 . 3 1 6 6 . 2 7 7 . 4 3 5 0 . 1 Crystal Structure CCP CCP CCP CCP Cubic-::- FCC C e l l Edge a 0 (A) 4 . 5 2 5 . 4 3 5 . 71 6 .25 5 . 6 4 8 . 3 4 Molecular dia-meter i n c r y s t a l (A) 3.07 3 . 7 5 4 . 0 2 4 . 3 1 4 . 1 3 . 0 5 .9 Heat of Fusion cal/mole 80 268 34O 550 170 640 Heat of Vapour-i z a t i o n cal/mole 410 1600 2240 3100 1333 7146 * S o l i d i s cubic below 35°K. The N z molecule i s c y l i n d r i c a l ; the approximate dimensions are 4 . 1 A length and 3 . 0 A diameter. Estimated from the value of a , . 62 at k°K i t was observed that i n t e n s i t y loss due to scattering was n e g l i g i b l e with nitrogen, noticeable with argon, considerable with krypton and very serious with carbon tetrachloride, i n d i c a t i n g that scattering increases with increasing size of matrix atoms or molecules, i n agreement with theory (29). 63 APPENDIX I I . Symmetry Properties, Selection Rules and Energy Levels f o r H2O and D2O. Symmetry Properties of Rotational Levels. Each energy l e v e l of an asymmetric top has one of the symmetries -f--/- -f-— — - f . The f i r s t sign refers to behaviour with respect to a ro t a t i o n by l 8 o ° about the axis of largest moment of i n e r t i a . The second sign refers to a similar rotation about the axis of smallest moment of \ i n e r t i a . The signs are obtained as follows: (i) For the f i r s t sign, the highest value of T f o r any J i s -f~ , the next two lower are — , the next two -f- etc. ( i i ) For the second sign, the lowest y f o r any J i s -f~ , the next two higher a r e — , the next two -f- etc. Hence, the symmetries of the r o t a t i o n a l l e v e l s for J = 0, 1 and 2, are: Oo •+ + 1- i " lo - -1, 2- 2 + + 2-, + -2 o - -2 , - + 6k. Selection Rules. The s e l e c t i o n r u l e s for J are: J = 0, ±1 J s 0 J = 0 The symmetry se l e c t i o n rules are: (i) For A type bands ( i . e . the dipole moment change during v i b r a t i o n i s along the axis of le a s t moment of i n e r t i a ) y, i s of this type. ( i i ) For B type bands ( i . e . the dipole moment change during v i b r a t i o n i s along the axis of i n t e r -mediate moment of in e r t i a ) V, and ^  are of this type. (-/--+-*-?> ) and (-/--<->—+) Energy Levels. The r o t a t i o n a l energy l e v e l s are given i n Table XV. To obtain the frequencies of vib r a t i o n - r o t a t i o n trans-i t i o n s , the lower state energy l e v e l J T n i s s u b t r a c t e d from the upper state energy l e v e l J • and 65. TABLE XV. Energy Levels of H2O and D2O. 1. D 20 (cm"' ) J r 000 100 010 001 0 0.0 2671.7 1178.5 2788.1 l - i 12.1 2683.7 1190.6 2800.1 l o 20.2 2691.7 1199.9 2807.6 l l 22.6 2694.O 1202.5 2810.1 35.8 2707.0 1214.4 2823.6 2-/ q.2.0 2713.1 1221.5 2829.3 2o 49.3 2720.3 1229.2 2836.7 2 , 73.6 2744.3 1256.9 2859-4 2 t 74.1 2744.8 1257.4 2859-9 reference 4 4 3 4 2. H20 (cm-' ) 0 0.0 3657.1 1594.8 3755.8 1-/ 23.8 3680.4 1618.6 3779.4 l o 37.1 3693.3 1635.1 3791.5 1, 42.4 3698.4 1640.6 3796.8 2-2 70.1 3725.9 1665.0 3824.9 2-i 79.5 3734.9 1677.1 3833.4 2o 95.2 3750.6 1693.7 3849.1 2 , 134.9 3788.6 1742.4 3885.6 2 i 136.2 3789.8 1743.6 3887.1 referenc e 2 1 2 1 6 6 . BIBLIOGRAPHY. 1. Benedict and P l y l e r , N.B.S. J . of Research, k 6 , 2k6 (1951) 2 . Dalby and Nielsen, J . Chem. 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