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Studies of geometrical isomers by photo-ionization mass spectrometry Stewart, William Brien 1968

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STUDIES OF GEOMETRICAL ISOMERS BY PHOTOIONIZATION MASS SPECTROMETRY by WILLIAM BRIEN STEWART B. S c , U n i v e r s i t y of B r i t i s h Columbia, 1966 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN THE DEPARTMENT OF CHEMISTRY We accept t h i s t h e s i s as conforming to the re q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA September, 1968. In 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 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 a d v a n c e d d e g r e e a t t h e 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 a g r e e 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 a n d S t u d y . I f u r t h e r a g r e e 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 b y t h e Head o f my D e p a r t m e n t o r b y 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 c o p y i n g o r p u b l i c a t i o n 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 . D e p a r t m e n t o f CHEMISTRY The 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 V a n c o u v e r 8, C a n a d a D a t e OCTOBER 2 5 , 1 9 6 8 . ABSTRACT A s e r i e s of isomeric t r i c y c l i c [3.2.1.0 ' ] oct-8--y£ d e r i v a t i v e s have been studied by photoionization mass spectrometry and the low re s o l u t i o n mass spectra of the compounds obtained.. The observed differences i n the r e l a t i v e i n t e n s i t i e s i n the spectra of the isomers are explained on the basis of the geometry of the t r i c y c l i c system. The major fragments of the low resolut i o n " s p e c t r a were 'mass measured' to determine t h e i r elemental composition and, from the information obtained, possible fragmentation pathways are postulated. In a d d i t i o n , the i o n i z a t i o n p o t e n t i a l of a l l isomers was determin by electron impact. -11-TABLE OF CONTENTS Page ABSTRACT . i ACKNOWLEDGEMENT v I INTRODUCTION 1 II THEORETICAL 8 A. Photoionization and D i s s o c i a t i v e Ionization 8 B. Ionization Potentials by Electron Impact 9 C. Operation of the Mass Spectrometer 13 III EXPERIMENTAL 16 A. Instrumental 16 B. Samples 22 C. Mass Spectra 25 D. Ionization Potentials 27 IV RESULTS AND DISCUSSION 29 A. Ketones 29 B. Methoxy Compounds 32 C. Hydrocarbons 34 D. Acetates 37 E. Alcohols 43 F. High Resolution 51 G. Ionization Potentials 58 V CONCLUSION 68 VI BIBLIOGRAPHY 70 - i i i -LIST OF TABLES Page I High Resolution Results f or T r i c y c l i c Ketones 52 II High Resolution Results f or Saturated and 53 Unsaturated T r i c y c l i c Methoxy Compounds III High Resolution Results f o r T r i c y c l i c Hydrocarbons 54 IV High Resolution Results f o r T r i c y c l i c Acetates 55 V High Resolution Results f o r T r i c y c l i c Alcohols 56 VI High Resolution Results f or B i c y c l i c Alcohols . 57 - i v -LIST OF FIGURES Page 1. The Franck-Condon P r i n c i p l e 11 2. The M.S. 9 Mass Spectrometer 17 3. Dual Photon and Electron Impact Ion Source f o r M.S. 9 18 4. Photoionization Source 20 5. Compounds Studied 24 6. Mass Spectra of T r i c y c l i c Ketones 30 (a) Helium Light Source (b) Hydrogen Light Source 7. Mass Spectra of T r i c y c l i c Methoxy Compounds 33 (a) Helium Light Source (b) Hydrogen Light Source 8. Mass Spectra of Unsaturated T r i c y c l i c Methoxy Compound 35 (a) Helium Light Source (b) Hydrogen Light Source 9. Mass Spectra of T r i c y c l i c Hydrocarbons 36 (a) Helium Light Source (b) Hydrogen Light Source 10. Mass Spectra of T r i c y c l i c Acetates, Helium Light Source 39 11. Mass Spectra of T r i c y c l i c Acetates, Hydrogen Light Source 40 12. Mass Spectra of T r i c y c l i c Alcohols, Helium Light Source 44 13. Mass Spectra of T r i c y c l i c Alcohols, Hydrogen Light Source 45 14. Mass Spectra of B i c y c l i c Alcohols 50 (a) Helium Light Source (b) Hydrogen Light Source 15. Fragmentation Scheme f o r T r i c y c l i c Ketones 59 16. Fragmentation Scheme f o r Unsaturated T r i c y c l i c Methoxy Compound 60 17. Fragmentation Scheme f o r T r i c y c l i c Hydrocarbons 61 18. Fragmentation Scheme for T r i c y c l i c Acetates 62 63 19. Fragmentation Scheme for T r i c y c l i c Alcohols 20. Fragmentation Scheme for Bicyclo [2.2.1] heptan-7-ol 64 21. Fragmentation Scheme for a n t i - b i c y c l o [2.2.1] hepten-7-ol 65 22. Ionization Potentials f o r Compounds Studied. 67 -V-ACKNOWLEDGEMENT I wish to express my sincere g r a t i t u t e to Dr. C.E. Brion f o r his continual help and guidance throughout the course of t h i s work. I would also l i k e to thank Dr. D.C. Frost, Dr. G.R. Branton, Mr. G.E. Thomas, Mr. L.A.R. Olsen and Mr. J.S. Sandhu f o r t h e i r h e l p f u l discussions and assistance during the preparation of t h i s t h e s i s . Special thanks are due to Dr. R.E. Pincock and Dr. J.S. Haywood-Farmer who generously provided the samples used i n t h i s study. In a d d i t i o n , I would l i k e to thank Mr. G.D. Gunn for his assistance i n the operation of the mass spectrometer. I INTRODUCTION L i t t l e a t tention has been devoted to mass spectrometry with regard to the influence of stereochemistry on the fragmentation of organic molecules. Only two aspects have been demonstrated so far,, namely the influence of the shape of the molecule which may be regarded as a crowding e f f e c t and the influence of the distance between i n t e r a c t i n g groups. For instance, Biemann (1) has discussed the d i f f e r e n c e i n the behaviour of borneol and isoborneol on the basis of s t r a i n r e l i e f experienced by the molecular ion upon fragmentation. Elimination of water takes, place to a greater extent i n the more compact a l c o h o l . Mandelbaum and Ginsberg (2) have shown that i n some cases the fragmentation patterns of two epimers are dramatically d i f f e r e n t , where in a c y c l i c t r a n s i t i o n state a migration of one hydrogen i s possible for only one of the isomers due to geometrical' considerations. Gurst and Djerassi (3) have shown that androstane-2-one gives a very intense M-58 ion correspondin to the elimination of one molecule of acetone, p o s s i b l y through a four membered t r a n s i t i o n state. Deuteration proves such a migration i s impossible i n the 10 a-epimer and t h i s peak i s not observed. In t h i s example the distance f a c t o r i s the e f f e c t which influences the fragmentation.. It has been concluded (4) that i f cases of hydrogen migration which can take place i n one isomer only are ignored, the e f f e c t of stereochemistry on fragmentation i n mass spectrometry i s small, but there appears to be a c o r r e l a t i o n between s t a b i l i t y i n the thermodynamic sense and the a b i l i t y f o r the compound to fragment upon electron impact. The d i f f e r e n c e i n structure between geometrical isomers i s characterized by the r e l a t i v e s p a t i a l p o s i t i o n of some atoms or groups i n the molecule. The proximity of these atoms or groups, i n a p a r t i c u l a r s p a t i a l configuration, can produce non-bonded i n t e r a c t i o n s i n the molecule, and a s t e r i c hindrance often e x i s t s i n one-isomer but not i n the other (5). The study of geometrical isomers by mass spectrometry has been the subject of a number of in v e s t i g a t i o n s (1-14). The majority of these studies have been c a r r i e d out u t i l i z i n g a conventional electron impact i o n i z a t i o n source and only very slight, differences are observed i n the mass spectra of two isomers of the same- substance. The observed dif f e r e n c e s between the mass spectra of the two isomers have been found to depend on the energy of the i o n i z i n g electrons, being generally more pronounced at lower electron energies. Using electron impact mass spectrometry f o r the study of geometrical isomers Na t a l i s (5) has found, (1) f o r the trans isomer, the r e l a t i v e abundance of the molecular ion i s greater, than that of the c i s isomer, e s p e c i a l l y f o r low electron energies. (2) the i o n i z a t i o n p o t e n t i a l i s e s s e n t i a l l y the same f o r both isomers, however, the appearance p o t e n t i a l s of the main fragment ions are lower f o r the c i s isomer. This indicates that less energy i s required to cause fur t h e r decomposition of the ionized c i s molecule. (3) the p r o b a b i l i t y of decomposition of metastable ions i s larger i n the c i s isomer than i n the trans, f o r several processes. Brion and H a l l (18) have studied the c i s and trans isomers of 4-t-butylcyclohexanol using a photoionization source (19, 20) i n a mass spectrometer. They have observed s i g n i f i c a n t differences i n the mass spectra of the two isomers which have been a t t r i b u t e d to s t e r i c f a c t o r s . Previous studies of these compounds, by the same authors, using a conventional electron impact ion source gave r e s u l t s which were inconclusive and could not be reproduced. These r e s u l t s were a t t r i b u t e d to spurious processes caused by the hot filament and the accompanying high temperature of the ion chamber. These high temperatures are one of the major disadvantages of electron impact ion sources, since compounds may undergo s i g n i f i c a n t thermal decomposition. The use of photoionization eliminates many of the problems associated with conventional electron impact ion sources. Poschenrieder and Warneck (21) have shown that the lack of a filament also minimizes outgassing and memory e f f e c t s . Another advantage i s that the photon energy i s p r e c i s e l y determined whereas i n electron impact work there i s not only a Maxwellian thermal spread of electron energies but also space charge and contact p o t e n t i a l s which can modify the nominal energy. This i s e s p e c i a l l y serious at low electron energies and can lead to lack of r e p r o d u c i b i l i t y i n the mass spectra due to the d i f f i c u l t y of obtaining the same absolute electron energy. While i t i s true that fragmentation can be reduced i f an electron impact source i s operated at low energies, the r e s u l t i n g loss i n s e n s i t i v i t y cannot usually be t o l e r a t e d (22). -4-Photoionization spectra are also more amenable to comparison with theories of mass spectra since the energy of the i o n i z i n g r a d i a t i o n i s c l o s e l y defined. Poschenrieder and Warneck have discussed the use of an u l t r a v i o l e t monochromator as a convenient, narrow band energy se l e c t o r . With energy s e l e c t i o n applied i n the mass spectrometric analysis of complex gas mixtures, a s e l e c t i v e i o n i z a t i o n of only a few of the involved components can be achieved (21) i n contrast to the i o n i z a t i o n of a l l the components as i s customary with electron impact sources. Accordingly, the mass spectrum i s s i m p l i f i e d and the o"\erlap and interference r e s u l t i n g from i n d i v i d u a l components can be minimized. The e a r l i e s t d e s c r i p t i o n of the use of a beam of photons to produce i o n i z a t i o n was by Terenin and Popov (23). From the beginning the technique has developed u n t i l i t i s possible to measure the cross-sections f o r photoionization i n gases at low pressures (24) and to measure the i o n i z a t i o n p o t e n t i a l of gases (25). Watanabe and co-workers (25) used a o 1-meter normal .incidence vacuum monochromator with a r e s o l u t i o n of 1 A. However, photoionization sources f or mass spectrometers received r e l a t i v e l y l i t t l e a t tention probably due to the technical d i f f i c u l t i e s involved i n t h e i r operation. Interest i n the technique was revived i n 1956, when Lossing and Tanaka (26) described some preliminary experiments on the performance and c h a r a c t e r i s t i c s of a photoionization source, used i n conjunction with a mass spectrometer. Instead of a monochromator they used the d i r e c t u l t r a v i o l e t l i g h t from a krypton discharge lamp which had been f i t t e d with a lithium f l u o r i d e window. The -5-li t h i u m f l u o r i d e window w i l l only transmit wavelengths greater than o 1050 A ( < 11.8 e.v.) and molecules with i o n i z a t i o n p o t e n t i a l s greater than t h i s therefore cannot be studied. Terenin and Vilessov (27) and Morrison, Hurzeler and Inghram (28, 29) have used a combination of a vacuum monochromator and.a mass spectrometer i n d e t a i l e d studies of the formation of ions by photon impact. The source of l i g h t was a high voltage hydrogen lamp with a lithium f l u o r i d e window to i s o l a t e the r e s i d u a l gases i n the l i g h t source from the i o n i z a t i o n chamber. I f very e f f i c i e n t d i f f e r e n t i a l pumping i s employed i t i s not necessary to i s o l a t e the l i g h t source from the high vacuum of the mass spectrometer with a lithium f l u o r i d e window. A windowless d i f f e r e n t i a l l y pumped system places no r e s t r i c t i o n on the energy of the transmitted photon beam. Weissler, Samson, Ogawa and Cook (30) and also Comes and Lessmann (31) were able to obtain r e s u l t s up to about 30 e.v. using a low pressure r e p e t i t i v e spark source and d i f f e r e n t i a l pumping. A simple windowless system f o r photoelectron spectroscopy has been used by Al-Joboury and Turner (32) and by Frost, McDowell and Vroom (33). Photoionization sources have been exploited i n mass spectrometry by Frost, Mak and McDowell (34), by DibeTer' and Reese (35) and by Berkowitz and Chupka (36). However, these instruments have been p r i m a r i l y designed for the study of threshold i o n i z a t i o n phenomena. In t h i s work grating monochromators are used to vary the wavelength i n a precise manner. The r e s u l t i n g photon fluxes are very low due to the high r e s o l v i n g power and low r e f l e c t i v i t y of the d i f f r a c t i o n grating at the wavelengths used. Such a device i s generally not s u i t a b l e f o r the i o n i z i n g source of an a n a l y t i c a l mass spectrometer f o r which a high i o n i z i n g f l u x at a single and preferably high energy i s usually required. The choice of a l i g h t source f o r vacuum u l t r a v i o l e t r a d i a t i o n depends on the p a r t i c u l a r a p p l i c a t i o n . Continuum sources are generally more desirable for providing information at a l l wavelengths, as f o r example i n absorption studies. Continuous r a d i a t i o n can be produced both by the i n t e r a c t i o n of electrons with atoms or molecules and by the a c c e l e r a t i o n (synchrotron radiation) or deceleration (bremsstrahlung radiation) of free electrons. Line r a d i a t i o n i s often preferred for mass spectrometries studies. It. i s usually produced by e l e c t r o n i c t r a n s i t i o n s between d i f f e r e n t energy le v e l s i n neutral atoms and molecules and i n ions. Roman numerals placed a f t e r the symbols for the elements indi c a t e whether the r a d i a t i o n i s emitted from the neutral atom (I) or from ions of various degrees of i o n i z a t i o n (II, III . . . . ) . Radiation produced by t r a n s i t i o n s between excited states and the ground state of the atom or ion i s c a l l e d resonance r a d i a t i o n . The problem of producing r a d i a t i o n of a given wavelength ( i . e . of a given energy) involves the formation of the appropriate excited atom or ion. In general, the shortest wavelengths (highest energy) are produced from the most highly ionized atoms. A dc glow discharge tends to produce r a d i a t i o n from neutral atoms whereas more energetic discharges such as the spark discharge are necessary to produce highly ionized atoms. Line sources using undispersed r a d i a t i o n and s u i t a b l e for use i n a mass spectrometer have been described by Brion (19) , Omura and Doi (37, 38, 39) and also Beynon and co-workers (40). In these studies -7-o a microwave or dc discharge i s used to produce ei t h e r the 584.3 A o (21.21 e.v.) helium resonance l i n e (He 1) or the 1215.7 A (10.19 e.v.) hydrogen Lyman-alpha l i n e (H I ) . The r e s u l t s of photoelectron spectro-scopy indicate the e s s e n t i a l monochromaticity of these l i g h t sources. The i n t e n s i t y of ions produced, i n a p a r t i c u l a r state, by photons i s u s u a l l y a maximum at threshold and thereafter generally decreases with increasing photon energy (42) while f o r electron impact i o n i z a t i o n the i n t e n s i t y of ions increases as a function of electron energy above threshold (41). Due to the nature of the photoionization threshold law the l i g h t sources used i n t h i s study provide s u f f i c i e n t energy to give s a t i s f a c t o r y mass spectra. . '. . -8- . II THEORETICAL A. Photoionization and D i s s o c i a t i v e Ionization This work i s concerned mainly with the i o n i z i n g c o l l i s i o n s of photons with molecules. A molecule can absorb a photon, of energy hv e x c i t i n g the system from a state of lower energy E" to a state of higher energy E', i . e . hv = E' - E" 1. where h i s Planck's constant and v i s the frequency of the r a d i a t i o n . Photoexcitation i s the process of absorption of r a d i a t i o n by a molecule. It can be represented by xy + hv ->• xy* 2. where xy and xy are ground and excited states of the molecule r e s p e c t i v e l y . Photoionization can occur by the i n t e r a c t i o n of a photon of s u f f i c i e n t l y high energy with the molecule xy. The major processes r e s u l t i n g from the photoionization of molecules may be summarized as follows: 1. Simple Ionization xy + hv ->• xy + + e 3. The minimum photon energy necessary f o r t h i s process i s c a l l e d the adiabatic i o n i z a t i o n p o t e n t i a l of xy. Beyond the energy of the i o n i z a t i o n threshold there i s a region of Continuous absorption f o r each quantum state. -9-2. D i s s o c i a t i v e Ionization xy + hv x y + + e b x + + y 4. The minimum photon energy necessary for t h i s process i s c a l l e d the appearance p o t e n t i a l of x +. The d i s s o c i a t i o n energy or bond strength of the molecule D(x-y) can be calculated from the appearance p o t e n t i a l by the following r e l a t i o n s h i p V ( x + ) = D(x - y) + I(x) + K.E. + E.E. 5. where V ( x + ) i s the appearance p o t e n t i a l of atom x, I(x) i s the i o n i z a t i o n p o t e n t i a l of x, K.E. i s the excess k i n e t i c energy of the process and E.E. i s the e x c i t a t i o n energy which ion or neutral fragment may possess. Most atomic i o n i z a t i o n p o t e n t i a l s are known from o p t i c a l spectroscopy and provided that the K.E. and E.E. are known the d i s s o c i a t i o n energy can be obtained by measurement of the appearance p o t e n t i a l . 3. Ion Pair Formation xy + hv -* xy* I—^  X + + y" 6 . B- IONIZATION POTENTIALS BY ELECTRON IMPACT An extensive study of the i o n i z a t i o n p o t e n t i a l s of a large number of atoms and molecules has been c a r r i e d out by a large number of workers, f o r example see F i e l d and Fr a n k l i n (46). In addition to the -10-technique of electron impact the following methods of measuring i o n i z a t i o n p o t e n t i a l s have been employed': o p t i c a l spectroscopy^ photoelectron spectroscopy, t h e o r e t i c a l and semi-empirical c a l c u l a t i o n s , charge t r a n s f e r and photon impact. The i o n i z a t i o n p o t e n t i a l of an atom or molecule i s t h e o r e t i c a l l y defined (43) as the minimum amount of energy required to completely remove an electron from the neutral species i n i t s ground state. xy + e -> x y + + 2e 7. However, the i o n i z a t i o n p o t e n t i a l s measured may not correspond to t h i s d e f i n i t i o n because the p o s s i b i l i t y exists that the product or products of the i o n i z a t i o n process may be i n a v i b r a t i o n a l l y excited state. Thus the i o n i z a t i o n p o t e n t i a l i s generally defined i n electron impact studies, as the minimum energy of the bombarding electrons at which the formation of 'parent' ions can be detected. The Franck-Condon p r i n c i p l e , f i r s t proposed by Franck (44) and l a t e r formulated mathematically by Condon states that, i n an e l e c t r o n i c t r a n s i t i o n , the nuclear separation and v e l o c i t y of r e l a t i v e motion a l t e r to a n e g l i g i b l e extent i . e . the t r a n s i t i o n takes place so quickly that there i s no change i n the nuclear coordinates. Figure 1 i l l u s t r a t e s the a p p l i c a t i o n of the Franck-Condon p r i n c i p l e t o ' i o n i z a t i o n phenomena for a diatomic molecule. The t r a n s i t i o n s are a l l taken as o r i g i n a t i n g from the zeroth v i b r a t i o n a l l e v e l of the i n i t i a l e l e c t r o n i c state. The maximum p r o b a b i l i t y f o r t r a n s i t i o n occurs i n the center of the Franck-Condon region. internuclear distance (a) internuclear distance (b) -12-Figure 1(a) represents a t r a n s i t i o n between two states f o r which the equilibrium internuclear distances (r ) are s i m i l a r . Here, the Franck-Condon p r i n c i p l e requires that the probable t r a n s i t i o n i s to the lowest v i b r a t i o n a l l e v e l of the upper i o n i c state. Figure 1(b).shows a moderate change i n r e on formation of the ion. From the diagram i t can be seen that the most probable t r a n s i t i o n would occur to the v' = 3 l e v e l of the upper e l e c t r o n i c state i f the Franck-Condon p r i n c i p l e holds. However, there w i l l also be t r a n s i t i o n s to other v i b r a t i o n a l l e v e l s . Two terms are often used to describe the t r a n s i t i o n s occurring. A t r a n s i t i o n to the v' = o l e v e l of the upper e l e c t r o n i c state i s c a l l e d an 'adiabatic' t r a n s i t i o n . This process requires the least amount of energy to reach the appropriate e l e c t r o n i c l e v e l . The ' v e r t i c a l ' t r a n s i t i o n corresponds to the most probable t r a n s i t i o n . It i s represented by the s o l i d l i n e s i n f i g u r e 1. In f i g u r e 1(a) the v e r t i c a l and adiabatic processes are equivalent. The s i t u a t i o n depicted by f i g u r e 1(c) shows a large change i n r . Here the v e r t i c a l t r a n s i t i o n i n t e r s e c t s the i o n i c p o t e n t i a l energy curve at an energy above the d i s s o c i a t i o n l i m i t . This r e s u l t s i n d i s s o c i a t i o n of the ion into charged and neutral fragments. This process may be represented as xy + e ->- x + + y + 2e 8. There i s a f i n i t e p r o b a b i l i t y of t r a n s i t i o n over the e n t i r e width of the Franck-Condon region. The fragments, therefore, have a spread i n k i n e t i c energy. -13-I f a t r a n s i t i o n occurs to a repulsive i o n i c curve as shown i n fig u r e 1(d) d i s s o c i a t i o n also occurs. In the case of polyatomic molecules, p o t e n t i a l energy curves are replaced by multidimensional p o t e n t i a l energy surfaces. D i s s o c i a t i o n occurs f i r s t at the weakest bond. Such a process can be written ABCD •+ e -v A + .+ BCD + '2e 9. An electron c o l l i d i n g with an atom or molecule can lose i t s energy in one of two ways. I f an e l a s t i c c o l l i s i o n takes place, the electron loses part of i t s energy to the target p a r t i c l e i n such a way that the t r a n s l a t i o n a l energy of the two body system i s conserved. A l t e r n a t i v e l y an i n e l a s t i c c o l l i s i o n may occur i n which case a change i n the i n t e r n a l energy, within the atom or molecule takes place. An i n e l a s t i c c o l l i s i o n may r e s u l t i n one or more of e x c i t a t i o n , i o n i z a t i o n and d i s s o c i a t i o n of the molecule. The theory of electron-molecule c o l l i s i o n s has been described elsewhere (46, 47) and a l l that w i l l be said i s that when the energy of the electron i s less than the i o n i z a t i o n p o t e n t i a l of the system studied, i o n i z a t i o n t r a n s i t i o n s cannot occur and the i o n i z a t i o n cross-section i s zero. As the energy of the electron i s increased above the c r i t i c a l voltage, the cross-section f o r the t r a n s i t i o n between the two given l e v e l s increases. C. OPERATION OF THE MASS SPECTROMETER Many reviews have appeared on the subject of the theory of mass spectrometry (43, 48, 49, 50) and only the "mass spectrometry equations" w i l l be presented here. The p o s i t i v e ions of mass m(kg.) formed i n the ion source are accelerated through a p o t e n t i a l V (voltage) and the ions acquire k i n e t i c energy eV. I f the energy increment i s large compared with the i n i t i a l energy, then eV = mv2 10 2 where v (m/sec.) i s the v e l o c i t y of the ion a f t e r a c c e l e r a t i o n and e (coulombs) i s the charge of the ion. If the ion now traverses a magnetic f i e l d , i t w i l l experience a c e n t r i p e t a l force, Hev, which i s normal to the d i r e c t i o n of the magnetic f i e l d and to the d i r e c t i o n of motion. The r e s u l t i s a c i r c u l a r o r b i t , such that, the c e n t r i f u g a l force balances the d e f l e c t i n g force 2 mv = Hev 11 R where H (amperes/meter) i s the strength of the magnetic f i e l d and R (m.).: i s the radius of curvature of the ion beam. Combining equations 10 and 11 v i s eliminated and 2 2 m = R H e 2V -15-Thus the ion i s deflected with a c e r t a i n value of R, according to i t s r a t i o m/e. Hence, magnetic d e f l e c t i o n analyzes according to the momentum of an ion rather than mass alone. Thus f o r a given mass, v a r i a t i o n i n V w i l l cause a v a r i a t i o n i n R and hence a loss i n r e s o l u t i o n . The energy band width of the ion beam can be g r e a t l y reduced by i n s e r t i n g an e l e c t r o s t a t i c analyzer i n the ion beam before i t enters the magnetic f i e l d . Thus, i f the ion, emerging from the ion source, as described by equation 10 i s projected into a r a d i a l e l e c t r o s t a t i c f i e l d E (volts/meter) at r i g h t angles to the boundary, i t w i l l experience a force eE, normal to the d i r e c t i o n of motion. The r e s u l t i s a c i r c u l a r o r b i t of radius r given by: 2 eE = mv r 2 or r = mv 13. eE A r a d i a l e l e c t r i c f i e l d therefore d e f l e c t s ions according to t h e i r k i n e t i c energy and allows ions with a given value of k i n e t i c energy to pass to the magnetic analyzer. Thus instruments, using e l e c t r o s t a t i c and magnetic f i e l d s achieve both v e l o c i t y and d i r e c t i o n focussing and are described as double focussing mass' spectrometers. -16-I I I EXPERIMENTAL A. Instrumental The instrument used for t h i s work i s a General E l e c t r i c - A s s o c i a t e d E l e c t r i c a l Industries M.S. 9 mass spectrometer. It i s a double focussing, magnetic scanning instrument featuring a r e s o l v i n g power of up to 20,000, c o n t r o l l e d by manually v a r i a b l e source and c o l l e c t o r s l i t s . Two peaks of equal height are said to be resolved i f the v a l l e y between them i s 10% of t h e i r height. Resolving power i s then M/AM, where M i s the average mass and AM the mass differ e n c e of the peaks. Figure 2 shov/s the main parts of the instrument which has been f u l l y described elsewhere (51). The convential electron impact ion source, employing a heated filament, has been replaced with a newly designed dual photoionization - electron impact ion source (52). An exploded view of t h i s ion source i s shown i n fi g u r e 3. In previous photoionization ion sources used i n t h i s laboratory (19, 20), i t was necessary to replace the electron trap and filament assemblies i n the conventional ion source by s t a i n l e s s s t e e l plates containing 3 mm. square apertures. The apertures i n the s t a i n l e s s s t e e l plates were necessary f o r passage of the photon beam which traversed the path normally occupied by the electron beam. To change from electron impact to photoionization operation i t was necessary to remove the conventional ion source and replace i t with a modified source. In addition, i t was also necessary to remove the electron beam c o l l i m a t i n g magnets since they Figure 2. The M.S. 9 Mass Spectrometer. Figure 3. D U A L P H O T O N & E L E C T R O N I M P A C T (ON S O U R C E F O R M.S.9. -19-completely blocked the passage of the photon beam. To make the required changes i t was necessary to l e t the i o n i z a t i o n chamber to atmospheric pressure and to remove the source housing. The whole procedure res u l t e d i n a heavy loss of machine time. With the dual photon-electron impact ion source, i t i s no longer necessary to make the above changes. In the new ion source the photon beam traverses the same path as i n the standard source except that the mounting of the electron trap and filament assemblies as well as the electron beam co l l i m a t i n g magnets have been rotated by 45°. This permits simultaneous use of photoionization and electron impact i o n i z a t i o n (53). One source of photons i s a lew pressure microwave discharge i n helium (He I ) , i t was f i r s t used as a f a r u l t r a v i o l e t source by Frost and McDowell (54). This provides a l i n e emission spectrum and i n the f a r u l t r a v i o l e t the majority of the emission has a wavelength o of 584.3 A, corresponding to an energy of 21.21 e.v. This spectral l i n e a r ises from the 2^P l^S resonance t r a n s i t i o n i n helium (55). The a l t e r n a t i v e source of photons was a low pressure microwave discharge i n a hydrogen-helium mixture. This produces Lyman-alpha o r a d i a t i o n (H I ) , 1215.7 A", corresponding to an energy of 10.19 e.v. This spectral l i n e also a r i s e s from a 2^P •> 1*S t r a n s i t i o n . The l i g h t source, shown i n fi g u r e 4, i s mounted on the side of a s p e c i a l l y modified ion source housing so that the l i g h t beam traverses the path formerly occupied by the electron beam i n a 2 4 5 0 M/cs pumps pumps J [ gas inlet Figure 4. PHOTOIONIZATION SOURCE (II) -21-conventional M.S. 9 ion source. A needle valve controls the flow of commercial helium (Canadian Li q u i d A i r Co.) into the quartz tube at a pressure of approximately 1 mm. of Hg. The pressure i n the discharge region i s also s t a b i l i z e d by a c o n s t r i c t i o n at the other end of the quartz tube. The c o n s t r i c t i o n also serves to f a c i l i t a t e d i f f e r e n t i a l pumping of the helium between the l i g h t source and the i o n i z a t i o n chamber. by ' The discharge i s produced i n the cavity^power from a 'Microtron 200' microwave generator (manufactured by El e c t r o Medical Supplies Ltd., England). This unit produces microwave r a d i a t i o n at a frequency of 2450 Mc./sec. with a maximum power output of 200 watts. The s i l v e r plated microwave ca v i t y i s constructed of brass and i s a modified form of that described by Z e l i k o f f , Wychoff, Auschenbrand and Loomis (56). The discharge i s i n i t i a t e d with.a Tesla c o i l . The quartz discharge tube i s cooled with compressed a i r to prevent melting. To prevent excessive helium from entering the mass spectrometer the lower portion of the source consists of a short, earthed brass tube which protrudes through the source housing and has an i n t e r n a l diameter of 2 mm. This c a p i l l a r y transmits a narrow l i g h t beam whilst impeding the flow of helium. A 4-cm. pumping l i n e i s situated close to the mouth of the c a p i l l a r y and helium i s pumped away by a 100 l i t e r s / s e c . o i l d i f f u s i o n pump backed by a 400 l i t e r s / m i n . mechanical pump. With t h i s arrangement and the optimum helium pressure ( that giving the maximum l i g h t i n t e n s i t y ) , the mass spectrometer operating source pressure was about 5.0 x 10 mm. Hg. To obtain optimum s t a b i l i t y i t -22-was- necessary to allow the helium to flow f o r about 1/4 hour, to purge any r e s i d u a l a i r . To produce the Lyman-alpha r a d i a t i o n commercial tank hydrogen (Canadian Liquid A i r Co.) was allowed to flow into the l i g h t source with the helium l i g h t already on. The flow of hydrogen into the system was adjusted using a needle valve u n t i l the maximum ion beam was obtained. With t h i s arrangement the mass spectrometric source pressure was about 1.0 x 10 ^ mm. Hg. A photon f l u x of 9 approximately 3.0 x 10 photons/sec. has been reported by Samson (57) i n a s i m i l a r type of source. B. Samples This t h e s i s deals with the spectra of some isomeric t r i c y c l o [3.2.1.0 2'^] octanes. The numbering of the t r i c y c l o [3.2.1.0 2'^] octane r i n g system i s shown below. a n t i syn The pr e f i x e s endo and exo r e f e r to the stereochemistry of the cyclopropane r i n g r e l a t i v e to the norbornyl r i n g system, whereas syn and a n t i r e f e r to the stereochemistry of the substituents at carbon-8 r e l a t i v e to the side of the molecule containing the cyclopropyl r i n g . -23-This convention i s maintained i n compounds containing both a double bond and a cyclopropyl r i n g . The compounds used i n t h i s study are shown i n f i g u r e 5, they were generously supplied by Drs. J.S. Haywood-Farmer and R.E. Pincock. Abbreviations are used i n the f i g u r e f o r some common func t i o n a l groups. These are: a) acetate, OAc and b) methyl, Me. The preparation of the compounds shown i n f i g u r e 5 has been reported (58). Samples to be run on the mass spectrometer were c o l l e c t e d , i n glass c a p i l l a r y melting point tubes, by g a s - l i q u i d p a r t i t i o n chromatography using a Wilkins-Aerograph Model A-90-P and a Varian Aerograph 90-P chromatograph, with helium as the c a r r i e r gas (flow rate 42 ml./min.) and with thermal conductivity detection. Three columns were used i n the chromatographs, at temperatures between 100°C. and 130°C; a 6 f t . x 1/4 i n . , 1,2,3-tris-(2-cyanoethoxy) propane (20%) on 45/60 mesh chromasorb P (TCEP) column; a . 6 f t . x 1/4 i n . , acid-washed Carbowax 20M (25%) on 60/80 mesh chromasorb W (acid-washed Carbowax) column and a 10 f t . x 3/8 i n . , Carbowax 20M (20%) on 60/80 mesh chromasorb S (Carbowax prep) column. A f t e r c o l l e c t i o n of the sample, the sample tube was sealed to prevent loss or contamination. The p u r i t y and i d e n t i f i c a t i o n of the compounds had previously been determined by nuclear magnetic resonance using Varian Associates Model A-60 60 Mc. and Model HA-100 100 Mc. spectrometers, by u l t r a v i o l e t spectroscopy using an Applied Physics Corporation Cary II spectrophotometer, by i n f r a r e d spectroscopy using a Perkin-Elmer O A c OAc ' A c O exo-anti A c O exo-syn endo-syn ,2,4 [3.2.1.0 ' ] octan-8-yls Acetate exo-anti HO H O exo-syn endo-syn 2,4-- t r i c y c l o [3.2.1.0 ' ] octan-8-ol exo - t r i c y c l o [3.2.1.0 2' 4] octane o o exo 2,4-- t r i c y c l o [3.2.1.0 ' ] octan-8-one M e O ^ O M e M e O v O M e M e O Y O M e exo -8,8-dimethoxytricyclo [3.2.1.0 ' ] octane OH OH - 8,8-dimethoxytricyclo-[3.2.1.0 2' 4] oct-6-ene b i c y c l o [2.2.1]-heptan-7-ol a n t i - b i c y c l o [2.2.1] hepten-7-ol Figure 5. Compounds Studied. -25-Model 137 B Infracord spectrophotometer with sodium chloride optics and a Perkin-Elmer Model 21 spectrophotometer using matched 0.523 mm. sodium ch l o r i d e c e l l s . In addition microanalysis and corrected melting points were obtained for a l l samples. Also no extraneous peaks of mass greater than those associated with the parent were found i n the mass spectra of the samples. C. Mass Spectra Low r e s o l u t i o n mass spectra were obtained f o r a l l samples using both the helium resonance l i n e and the hydrogen Lyman-alpha l i n e l i g h t sources. A l l samples were s u f f i c i e n t l y v o l a t i l e at room temperature so that no heating was required and the samples could be introduced i n t o the mass spectrometer through the d i r e c t i n l e t system. The d i r e c t i n l e t system which has no expansion chamber provided a means of introducing small q u a n t i t i e s of v o l a t i l e samples d i r e c t l y into the mass spectrometer i o n i z a t i o n chamber. Before any sample was introduced i n t o the instrument, the l i g h t source was switched on and the e l e c t r o n i c s allowed to warm up. With the accelerating voltage at 8 Ke.v. the instrument was focussed on nitrogen at m/e 28, so that the maximum ion beam was obtained at-the c o l l e c t o r . Also a background spectrum was always recorded before the sample was introduced. To introduce the sample, one of the sealed melting point c a p i l l a r y tubes, containing 10-20 y£ of sample, was opened and the sample-containing tube placed i n the d i r e c t i n l e t system. A i r and any -26-non-condensible gases were pumped away by means of freeze-thaw cycle s . The amount of sample admitted to the i o n i z a t i o n chamber was adjusted using two bellows valves so that the pressure i n the source was constant and so that there was s u f f i c i e n t sample to obtain a s a t i s f a c t o r y mass spectrum. The t o t a l pressure i n the source was measured by an i o n i z a t i o n gauge, which i s located j u s t above the source d i f f u s i o n pump. When the helium l i g h t source was used the t o t a l pressure i n the source, due to sample and helium, was 2.0-3.0 x 10 ^ mm. Hg., with the Lyman-alpha l i g h t source the t o t a l pressure, due to sample and the hydrogen helium mixture, was 2.5 - 3.5 x 10 ^ mm. Hg. Detailed i n s t r u c t i o n s f o r the mechanics of running the mass spectrum may be found i n the "G.E. - A.E.I. M.S. 9 Mass Spectrometer - Instructions f o r Operation and Maintenance" manual. The mass spectra of a l l compounds were recorded under conditions as i d e n t i c a l as poss i b l e , the band width, recorder paper speed, magnetic scan speed and m u l t i p l i e r settings were the same for a l l spectra. The mass spectrometer used i n t h i s study employs the peak matching technique f o r the accurate measurement of the mass of an ion. Knowing the nominal mass of the ion, the r e l a t i o n s h i p between the mass of that ion and that of an accurately known reference i s determined by comparison of the ion a c c e l e r a t i n g voltages necessary to bring the ion beams on to the c o l l e c t o r s l i t , at a constant magnetic f i e l d . Provided that the masses of the two ions do not d i f f e r by more than 10%, the s t a b i l i t y of the c i r c u i t s and the r e s o l v i n g power of the instrument are s u f f i c i e n t f o r the measurement of mass r a t i o s with a p r e c i s i o n of a few parts i n 10^. -27-In t h i s study, i n addition to the determination of the exact molecular composition of the parent, the major peaks of the low r e s o l t u i o n mass spectra of a l l samples were 'mass measured' to determine the elemental composition of the ions. The reference compound used was heptacosafluorotributylamine. The high r e s o l u t i o n mass measurements were performed using both the helium resonance l i n e and the Lyman-alpha l i n e l i g h t sources with a r e s o l u t i o n of about 10,000. D. Ionization Potentials The method used f o r the determination of the i o n i z a t i o n p o t e n t i a l s i s s i m i l a r to that described by Lossing, Tickner and Bryce (59). They found that the semi-log p l o t s of the i o n i z a t i o n e f f i c i e n c y curves f o r the molecule-ions from a number of substances were e s s e n t i a l l y p a r a l l e l f o r voltages i n the region of the i o n i z a t i o n p o t e n t i a l . Since t h i s was the case, they took the i o n i z a t i o n p o t e n t i a l as the energy at which the ion current was 1% of i t s value when the electron energy was 50 e.v. Values they obtained were reproducible to +_ 0.01 e.v. by t h i s method. Argon was used as the c a l i b r a t i n g gas and was introduced simultaneously with the sample under i n v e s t i g a t i o n . In t h i s study the i o n i z a t i o n p o t e n t i a l was taken as the energy at which the ion current i s 0.1% of i t s value when the electron energy i s 70 e.v.. Argon was also used as the c a l i b r a t i n g gas and was introduced into the mass spectrometer along with the sample. This method while d i f f i c u l t to j u s t i f y on a t h e o r e t i c a l basis provides a convenient method f o r comparing the i o n i z a t i o n -28-p o t e n t i a l s of s i m i l a r compounds. It i s expected that f o r s i m i l a r compounds the same quantum states w i l l be a v a i l a b l e . -29-IV RESULTS AND DISCUSSION The explanations given f o r the differences observed i n the r e l a t i v e abundances of the fragments of the isomers are based on the spectra obtained using the helium l i g h t source, however, the r e s u l t s obtained when the hydrogen Lyman-alpha resonance l i n e i s used as the l i g h t source are e s s e n t i a l l y the same. The differences i n i n t e n s i t i e s of the isomers may be accounted f o r i n most cases i n terms of the geometry of the t r i c y c l i c systems and the bonding o r b i t a l s of the fused cyclopropane r i n g . The mechanisms used to account f o r the decomposition of excited ions formed from the molecules are merely empirical r a t i o n a l i z a t i o n s of gross s t r u c t u r a l e f f e c t s applied to the molecules. A. Ketones The i n t e n s i t y of the parent ion f o r the exo isomer i s more than twice as great as that of the endo isomer (29% of base peak v. 13% of base peak). This may be seen i n figure 6 which shows the spectra obtained f o r the exo and endo ketones for both l i g h t sources. A previous study (60) of the mass spectrum of e x o - t r i c y c l o 2 4 [3.2.1.0 ' ] oct-6-en-8-one u t i l i z i n g a conventional electron impact ion source showed no parent ion. This c l e a r l y demonstrates one of the p r i n c i p a l advantages of photoionization. I f decarbonylation of'the molecular ion occurs with equal p r o b a b i l i t y f o r both isomers i t i s expected that there w i l l be twice as much of the fragment of m/e 94 (C ?H „t) -31-present i n the exo isomer than i n the endo isomer since there i s twice as much exo molecular ion. However, there are nearly equal amounts of the decarbonylated fragment. Thus i t appears that the endo isomer must undergo decarbonylation more e a s i l y than the exo isomer to form a seven membered r i n g system which i s quite stable. Decarbonylation studies 2 4 of the exo and endo-tricyclo [3.2.1.0 ' ]oct-i6-'en-8-one confirm the greater ease of formation of the seven membered r i n g system from the endo isomer (60,61,62). The extra s t a b i l i t y of the seven membered r i n g tends to i n h i b i t further fragmentation. The ease of decarbonylation of the isomers and formation of the seven membered rin g may be explained i n terms of the geometry of the t r i c y c l i c systems and the bonding o r b i t a l s of the fused cyclopropane r i n g . In the endo ketone the o r b i t a l s forming the cyclopropyl 'banana' bond between carbon-2 and carbon-4 are i d e a l l y s i t u a t e d f o r i n t e r a c t i o n and subsequent p i bond formation with the p type o r b i t a l s a v a i l a b l e on fragmentation at carbon-1 and carbon-5. Cyclopropyl p a r t i c i p a t i o n i n the decarbonylation step i s thus predicted and a concerted process involving simultaneous loss of carbon monoxide and breaking of the cyclopropyl bond can occur to form the seven membered rin g system. The geometry of the fused cyclopropyl r i n g i n the exo ketone does not allow overlap of the carbon-2 -32-and carbon-4 bond o r b i t a l s with the a v a i l a b l e p o r b i t a l s and cyclopropyl p a r t i c i p a t i o n i s not expected. Furtrier fragmentation of the exo isomer a f t e r decarbonylation i s expected since the geometry of the cyclopropane r i n g i n the exo isomer nakes i t less l i k e l y that the formation of the stable seven membered r i n g w i l l occur. Thus the differences i n the r e l a t i v e i n t e n s i t i e s of the fragments at m/e 80 (C^Hg*) and at m/e 55 (composed mainly of C^H^+) can be r a t i o n a l i z e d provided that the fragment ions a r i s e from further fragmentation of the decarbonylated fragment. To account f o r the equal amounts of base peak at m/e 79, which has.j the formula C£\ *, i t i s necessary to suppose that ore of the bonds of the cyclopropyl r i n g i s broken and a rearrangement occurs to form a methyl group which i s then l o s t . I f t h i s occurs the exo and endo isomers w i l l have the same configuration and decarbonylation should then occur with equal p r o b a b i l i t y from e i t h e r isomer. Thus i f the formation and loss of the methyl group occurs with about equal p r o b a b i l i t y i n both isomers the amount of fragment at m/e 79 should be nearly equal and t h i s i s observed. B. Methoxy Compounds It can be seen from fi g u r e 7, that within experimental u n c e r t a i n t i e s , there i s no d i s c e r n i b l e d i f f e r e n c e in the spectra of the exo and endo isomers 2 4 of 8,8-dimethoxy [3.2.1.0 ' ] octane. The s t r i k i n g feature of the spectra i s that there i s no peak which i s greater than 10% of the base peak. The base peak, at m/e 101, has the formula C^H^O^ and probably arises by the breaking of the carbon-carbon bonds between carbon atoms 1-8, 6-7 and 4-5. 10.0 -\ M e O s ^ O M e 1 2 0 M e O ^ O M e 1 2 0 MeCX OMe ILL. 1 6 0 1 6 0 1 ~T— 1 2 0 M e O . O M e m/e t 120 JLU_ Figure 7. 1 6 0 • 1 4 0 1 6 0 Mass Spectra of T r i c y c l i c Methoxy Compounds, (a) Helium Light Source (b) Hydrogen Light Source -34-This appears to be the most probable manner of obtaining the fragment. Ad d i t i o n a l evidence : that the postulated fragmentation scheme i s correct may be obtained by the study of the mass spectrum of endo-8, 8-'dimethoxy 2 4 [3.2.1.0 ' ] oct-6-ene. It i s expected that the fragmentation pattern of t h i s compound, which has a double bond between carbon atoms 6 and 7, would be quite d i f f e r e n t than that of the saturated isomer i f the proposed fragmentation i s correct. As may be seen i n fi g u r e 8 t h i s i s the case. The i n t e n s i t y of the fragment at m/e 101 has been considerably reduced and the base peak has been s h i f t e d to m/e 91 which has the formula C^H^+. C, Hydrocarbons From f i g u r e 9 i t can be seen that there i s s l i g h t l y more parent present f o r the exo isomer than for the endo isomer (10% of base peak v. 7% of base peak). This may be due to the i n t e r a c t i o n of the o r b i t a l s of the hydrogen atoms of the cyclopropyl r i n g with those of the hydrogen atoms of carbons 6 and 7 which causes a greater amount of s t e r i c i n t e r a c t i o n i n the endo isomer. Due to t h i s s t e r i c i n t e r a c t i o n s i t would be expected that fragmentation would occur more r e a d i l y i n the endo isomer. The fragment at m/e 93, which i s C^H^+, i s more intense f o r the endo isomer than f o r the exo isomer (31% of base peak v. 23% of base peak). This fragment probably has eit h e r a monocyclic seven membered r i n g structure or a b i c y c l i c norbornene type structure. A seven membered r i n g i s formed more r e a d i l y f o r the endo isomer than f o r the exo isomer because of the overlap of the carbon-2 and carbon-4 bond o r b i t a l s with the p type 1 0 0 -8 0 -6 0 -t 4 0 -in -z. UJ t 20-UJ > UJ 0 V 4 0 (A) MeO. rOMe I • -l I i i , i | r • 1 h i , l 1 1. 1 1 II ,1, 100 -I 5 0 6 0 H 4 0 2 0 J •40 6 0 ( B ) 80 m/e 100 120 M e O Y O M e 140 J_J_I HiUL 160 ill! 6 0 Figure 8. i 8 0 m/e 1 0 0 r 20 140 160 Mass Spectra of Unsaturated T r i c y c l i c Methoxy Compound, (a) Helium Light Source (b) Hydrogen Light Source 1 0 0 -I 8 0 6 0 A ^4 0 ^ 2 0 H u i 0 4 > 4 0 < u cc 1 0 0 -j 8 0-I 6 0 -. 4 0 -2 0 -AO-(A) 6 0 8 0 m/e (A) 6 0 A 1 0 0 8 0 m/e 1 0 0 1 0 0 8 0 -j 6 0 -J 4 0 -| 2 0 4 0 1 0 0 8 0 -: 6 0 -4 0 20 A 0 4 0 (B) 6 0 ( B ) — T — 6 0 Figure 9. Mass Spectra of T r i c y c l i c Hydrocarbons. (a) Helium Light Source (b) Hydrogen Light Source -37-o r b i t a l s at carbon-1 and carbon-5 which occurs f o r the endo isomer and which cannot occur f o r the exo isomer. The b i c y c l i c system can be formed by loss of a methyl group and i f the b i c y c l i c system i s formed furth e r fragmentation w i l l occur more r e a d i l y than for the seven membered r i n g system. Loss of a CH^ group would give, the fragment at m/e 79 (C^H^+) which i s the base peak f o r the endo isomer and the second largest peak f o r the exo isomer. The fragment at m/e 67 i s C<.H^ + and i s more intense f o r the endo isomer than f o r the exo isomer (44% of base peak v. 35% of base peak) The greater i n t e n s i t y of the fragment for the endo isomer i s probably due to a greater s t e r i c i n t e r a c t i o n i n the endo isomer than occurs f o r the exo isomer. The base peak f o r the exo isomer i s the fragment at m/e 66 which corresponds to C^ -H^ * and i s only s l i g h t l y more intense than the endo isomer. D. Acetates While i t i s reasonably easy to compare the spectra of the isomers of the ketones, methoxys and hydrocarbons since only the exo and endo isomers are p o s s i b l e , i t i s more d i f f i c u l t to compare the spectra of the acetates and alcohols since there are four possible isomers; they are exo-anti, endo-anti, exo-syn and endo-syn. Thus the spectra of the alcohols and acetates may be compared on the basis of the o r i e n t a t i o n of the cyclopropyl group when the substituent on the'flagpole' i s f i x e d , -38-that i s , by an exo v. endo comparison, as has been done with the previous compounds or the cyclopropyl group may be held r i g i d and the o r i e n t a t i o n of the substituent on the 'flagpole' varied, that i s a syn v. a n t i comparison. In many cases both methods w i l l be employed i n an attempt to explain s i g n i f i c a n t differences i n the i n t e n s i t i e s of the fragments present. The isomeric acetates have the smallest molecular ion i n t e n s i t i e s of any seri e s of compounds studied. The spectra of the acetates are shown i n figures 10 and 11. The low i n t e n s i t y of the molecular ion i s probably due to the bulky acetate group which causes more s t e r i c i n t e r a c t i o n than smaller substituents and consequently a less stable + molecular ion. The fragment at m/e 124 i s predominantly CgH^O - and corresponds to the loss of ketene (^^O) leaving an alcohol type structure. There i s more of t h i s fragment present i n the spectrum of the endo-anti isomer that i n that of the endo-syn isomer (16% of base peak v. 3% of base peak) because i n the endo-anti isomer the non-bonding o r b i t a l s of the oxygen atom i n t e r a c t with the o r b i t a l s of the hydrogen atoms bonded to carbon atoms 6 and 7 to a greater extent than takes place when the i n t e r a c t i o n occurs with the hydrogen atoms bonded to carbons 2 and 4. This occurs because the hydrogen atoms bonded to carbons 6 and 7, " s t i c k up" out of the plane more than those hydrogen atoms of carbons 2 and 4 do. There i s also more of t h i s fragment present i n the endo-anti spectrum than i s present i n the exo-anti spectrum (16% of base peak v. 6% of base peak) probably due to greater s t e r i c i n t e r a c t i o n of the cyclopropyl r i n g l—J-JJJLL i l L j O A c T j r 1-40 O A c jJLU. r r""' i r A c O 1 4 0 1 4 0 8 0 6 0 4 0 - | 2 0 0 4 0 6 0 iL. 8 0 1 0 0 m/e A c O 1 2 0 1 4 0 1 6 0 Figure 10. Mass Spectra of T r i c y c l i c ' A c e t a t e s , Helium Light Source. -"40-1 0 0 * ' m/e ' • • • Figure 11. Mass Spectra of T r i c y c l i c Acetates, Hydrogen Light Source. hydrogen with the hydrogen atoms of carbons 6 and 7 which are below the plane i n the endo isomer. The fragment at m/e 106 has the formula CgH^Q* which corresponds to the loss of a c e t i c a c i d from the parent, probably by a McLafferty rearrangement (63). As occurs for most fragments the i n t e n s i t y of the endo-anti fragment i s the greatest. The amount of fragment for the endo-anti isomer i s greater than that f o r the endo-syn isomer (75% of base peak v. 14% of base peak) probably because there i s greater s t e r i c i n t e r a c t i o n of the non-bonding o r b i t a l s of the oxygen atom with the o r b i t a l s of the hydrogen atoms which are bonded to carbon atoms 6 and 7 i n the endo-anti isomer. There i s also more of the fragment present for the endo-anti isomer than for the exo-anti isomer (75% of base peak v . 43% of base peak) and t h i s may be due to greater s t e r i c i n t e r a c t i o n of the cyclopropyl group i n the endo isomer. There i s also more of the fragment at m/e 106 present i n the spectrum of the exo-syn isomer than i s present i n that of the endo-syn isomer (30% of base peak v. 14% of base peak) probably due to the closer proximity of the hydrogen o r b i t a l s of the cyclopropyl r i n g to the non-bonding o r b i t a l s of the oxygen atom i n the exo-syn isomer. The r e l a t i v e amount of fragment present f o r the d i f f e r e n t isomers f o r the fragments at m/e 80 and 79, which are CgHg* and C^H^+ r e s p e c t i v e l y , i s very s i m i l a r to that at m/e 106 and the same reasoning as was used f o r the fragment at m/e 106 may be employed to explain the differences i n the spectra. The fragment at m/e 91 has the' formula C._H^+ and probably has a tropylium ion type structure. The i n t e n s i t y of the endo isomers i s -42-greater than that of the corresponding exo isomer because i n the endo isomers the o r b i t a l s forming the cyclopropyl 'banana' bond between carbons 2 and 4 are i d e a l l y s i tuated f o r i n t e r a c t i o n and subsequent p i bond formation with the p o r b i t a l s at carbons 1 and 5 whereas i n the exo isomer they are not. The trends of the i n t e n s i t i e s f o r the various isomers f o r the fragment at m/e 70, which has the formula C^H^O*, are the same as those of the fragment at m/e 124 and s i m i l a r explanations may be used i n an attempt to explain the d i f f e r i n g i n t e n s i t i e s f o r the various isomers. The base peak f o r the exo isomers i s at m/e 78 and corresponds to the fragment C^ -H^ * while the base peak f o r the endo isomers i s at m/e 43 and has the formula C^H^O*. The dif f e r e n c e i n base peak may be accounted f o r on the basis of the formation of a seven membered r i n g f o r the endo isomer which occurs to a lesser extent i n the exo isomer. I f the bond between carbon atoms 1 and 8 or 5 and 8 i s broken, there i s a greater p r o b a b i l i t y of forming a seven membered rin g i n the endo isomers because the cyclopropyl 'banana' bond between carbon.atoms 2 and 4 i s i d e a l l y s i tuated f o r i n t e r a c t i o n and subsequent p i bond formation with the developing p o r b i t a l at carbon 1 or 5 whereas i n the exo isomer the geometry of the fused cyclopropyl r i n g does not allow overlap of the bond o r b i t a l s of carbons 2 and 4 with the developing p o r b i t a l s . Once the formation of the 7 membered r i n g has occurred i t i s u n l i k e l y that the . fragment at m/e 78 w i l l be formed. Thus i t i s expected that the i n t e n s i t y of the fragment w i l l be greater f o r the exo isomer. -43-The fragment at m/e 43 which i s the base peak f o r the endo isomers i s the second most intense peak i n the spectra of the exo isomers. It a r i s e s by the breaking of the carbon-oxygen and bond and retention of the p o s i t i v e charge by the oxygen atom of the acetate group. E. Alcohols Only small differences are observed i n the i n t e n s i t y of the molecular ion i n the spectra of the isomeric alcohols as may be seen from fi g u r e s 12 and 13 and because the absolute i n t e n s i t y i s quite small no attempt is. made to explain these differences. The fragment at m/e 106 has the formula CgH^Q* and corresponds to the loss of water from the molecular ion. Biemann (1) has suggested that i n the process of water elim i n a t i o n from alcohols which occurs i n the mass spectrometer occurs such that f o r epimeric polycyclic. alcohols, such as borneol and isoborneol, the one with the most s t e r i c a l l y hindered hydroxyl function e x h i b i t s the lower i n t e n s i t y molecular ion and the higher i n t e n s i t y M-H^O (parent ion minus water) peak. This e f f e c t i s not observed with the t r i c y c l i c alcohols used i n t h i s study probably due to the geometry of the t r i c y c l i c system which leads to other s t e r i c i n t e r a c t i o n s . The i n t e n s i t y of the molecular ion minus water peak i n the endo-syn spectrum i s greater than that i n the exo-syn spectrum (40% of base peak v. 24% of base peak) and the i n t e n s i t y i n the endo-anti spectrum i s greater than that i n the exo-anti spectrum (31% of base peak v. 20% of base peak). Also there i s s l i g h t l y more of t h i s fragment present i n the spectrum of the exo-syn isomer than i n that of the exo-anti (24% of base peak v. 20% of base peak), s i m i l a r l y the endo-syn isomer has a more intense peak o CD O' Co o-o ro -i o ro O i_ o CD O RELATIVE Co o O o o x I N T E N S I T Y ro O ..O j CD o 3 oo IT O" o ro -O J> CD CD (5 0 o o o 1 I I I o CD o 3 oo -- o~i ft o -o ro o ro O _ L _ O _u_ CD o I _ CO o o o _1 _!_ 4^  O CD O 3 Co ^ o o-o ro -O ro O O i CD O _ J _ Co o I o o -17V-m/e m/e Figure 13. Mass Spectra of T r i c y c l i c Alcohols, Hydrogen Light Source. than the endo-anti isomer. There appears to be a d i r e c t c o r r e l a t i o n between the i n t e n s i t y of the molecular ion minus water fragment and the ease of formation of a seven membered r i n g . The endo isomers form the seven membered r i n g more e a s i l y than the exo isomers because i n the endo isomers the o r b i t a l forming the cyclopropyl 'banana' bond are i d e a l l y situated f o r i n t e r a c t i o n and subsequent p i bond formation whereas t h i s i s not the case with the exo isomers. Also the i n t e n s i t y of the molecular ion minus water fragment i s greater i n the spectrum of the syn isomer than i n the corresponding a n t i isomer due to the fac t that the hydrogen l o s t i n the formation of water i s from the syn side of the molecule, the formation of a seven membered r i n g i s f a c i l i t a t e d . The fragment which occurs at m/e 95 i s a doublet consisting of C^H^ + and C^H^0+. The C^H^ + fragment corresponds to the loss of COH' from the molecular ion and may only a r i s e by a rearrangement process. Because the i n t e n s i t y of the fragment i s greater i n the endo-anti spectrum than i n that of the exo-anti (13% of base peak v. 9% of base peak) and the i n t e n s i t y of the endo-syn i s greater than that of exo-syn (37% of base peak v. 8% of base peak) i t i s expected that a seven membered •ring i s involved i n the fragmentation process. The C^H^0+ fragment arises by the loss of C^H^ from the molecular ion. I f the loss occurs i n two stages, f i r s t the loss of C^H^ and then the loss of a hydrogen r a d i c a l , then the C^H^ l o s t i s probably that of carbons 6 and 7. The p o r b i t a l s of the cyclopropyl group of the endo isomers are i n a position.which . f a c i l i t a t e s the formation of a double bond i n a s i x membered r i n g more e a s i l y than occurs i n the case of the exo isomers. Thus i t i s expected .-47-that the endo isomers w i l l have more intense peaks than the corresponding exo isomer. The s i x membered r i n g a r i s i n g from the endo isomers w i l l probably be i n a chair type configuration while that a r i s i n g from the exo isomer w i l l be i n a boat type configuration. I f the fragment at m/e 77 arises predominantly from the loss of water from the fragment at m/e 95 (C^H^O )then, using s i m i l a r reasoning to that used by Brion and H a l l (18), who account f o r the loss of water from the isomers of t-butylcyclohexanol on the basis of configuration, i t would be expected that the syn isomer would have more intense m/e 77 peaks than the corresponding a n t i isomer and t h i s i s observed. Also i t i s to be expected that the exo-syn isomer i f i t has boat symmetry w i l l exhibit a more intense m/e 77 peak than the endo-syn i f the l a t t e r has chair symmetry. However, the i n t e n s i t y of the endo-syn isomer i s greater than that of the exo-syn isomer (17% of base peak v. 11% of base peak). This behaviour may be accounted f o r on the basis that the fragment at m/e 95 i s the precursor of the fragment at m/e 77. For the fragment at m/e 95 the endo-syn isomer i s more than three times as intense as the exo-syn isomer (25% of base peak v. 8% of base peak), thus i f the i n t e n s i t y of the fragment at m/e 95 was the same for both isomers • i t would be expected that the exo-syn isomer would be more intense than the endo-syn isomer. The fragment at m/e 91 has the formula CyHy + and probably has a tropylium ion type structure. As with previous fragments the i n t e n s i t y of the endo-syn isomer i s greater than that of the exo-syn isomer (61% of base peak v. 22% of base peak) and the i n t e n s i t y of the endo-anti -48-i s greater than that of the exo-anti isomer (22% of base v. 14% of base peak). The more intense peaks occur f o r the endo isomers because the o r b i t a l s forming the cyclopropyl 'banana' bond are i d e a l l y s i tuated f o r i n t e r a c t i o n and subsequent p i bond formation whereas i n the exo isomer they are not. In addition, the i n t e n s i t y of the fragment i s greater f o r the exo-syn isomer than for the exo-anti isomer (22% of base peak v. 14% of base peak) and s i m i l a r l y the i n t e n s i t y of the endo-syn isomer i s greater than that of the endo-anti isomer (61% of base peak v.22% of base peak). The i n t e n s i t y of the peak i n the syn isomer i s greater than i n the corresponding a n t i isomer because elimination of the hydroxyl group occurring from the syn side of the molecule f a c i l i t a t e s the formation of a seven membered r i n g system. The fragment at m/e 80 i s C,H„* and follows the same trend as the O o other fragments i n that the i n t e n s i t y of the endo isomer i s greater than that of the corresponding exo isomer. However, for t h i s fragment the abundance of the a n t i isomer i s greater than that of the corresponding syn isomer which may a r i s e because the elimination of the hydroxyl group occurs by a d i f f e r e n t mechanism than occurs f o r the other fragments. The base peak for the exo-anti, exo-syn and endo-syn isomers and the second largest peak i n the spectrum of the endo-anti isomer i s the fragment which occurs at m/e 78 and which i s C^H^*. The base peak f o r the endo-anti isomer occurs at m/e 70 and has the formula C.H,0*. The 4 6 amount of t h i s fragment present for the endo-anti isomer i s greater than that f o r the exo-anti isomer and the amount present i n the endo-syn spectrum i s greater than that i n the exo-syn spectrum. The greater i n t e n s i t y -49-of the fragment f o r the endo isomers than f o r the corresponding exo isomer indicates the probable formation of a seven membered r i n g as an intermediate. The fragment at m/e 68 corresponds to C^Hg* and can only a r i s e i f a rearrangement process occurs. Since deuterated compounds were not used i n t h i s study the rearrangement that occurs cannot be i d e n t i f i e d . The peak at m/e 57 i s C^H^0+ f o r which the i n t e n s i t y i n a l l isomers i s the same within experimental error. The spectra of b i c y c l o [2.2.1] heptan-7-o£ and a n t i - b i c y c l o [2.2.1] hepten-7-o£ are shown i n f i g u r e 14 and are included f o r comparison with the t r i c y c l i c alcohols. The spectrum of b i c y c l o [2.2.1] heptan-7-o& i s very s i m i l a r to that of the isomeric t r i c y c l i c alcohols i n that many of the same fragments are present i n both cases although t h e i r i n t e n s i t i e s are d i f f e r e n t . The fragments i n common are: the molecular + + ion minus water, those at m/e 70 (C^H^O -), m/e 68 (C^Hg*) and m/e 57 (C^H^0 +). It i s not s u r p r i s i n g that the other major fragments of the t r i c y c l i c alcohols are not present i n the spectrum of the b i c y c l i c compound since seven membered rings were postulated to account f o r these fragments and the seven membered rin g cannot be e a s i l y formed from the b i c y c l i c compound. The base peak for the b i c y c l o [2.2.1] heptan-7-oJl i s the molecular ion minus water fragment at m/e 94. The other major peaks i n the spectrum are at m/e 79 and m/e 81 which are C.H_+ and C,H n + b / b y r e s p e c t i v e l y . The spectrum of a n t i - b i c y c l o [2.2.1] hepten-7-o£ also has fragments at m/e 79 and m/e 81. The peak at m/e 79 i s C^H^+ and i s the base peak f o r the spectrum. The peak at m/e 81 i s a doublet consisting of 100H 6 0 H tr>o oo ^2 0 La O-j-L^ E= 4 0 DC 100-1 8 0 4 6 0 4 0 2 0 4 0 "i l ! — r ( A ) Hii 6 0 i 8 0 m/e 1 1— 1 0 0 ( B ) 6 0 i 8 0 m/e 1 0 0 —1T~ 12b 120 100-8 0 ^ 6 0 4 0 -2 0 . o p -40 1 0 0 8 0 -j ; 6 0 4 0 -2 0 -4 0 6 0 T 8 0 m/e 6 0 Figure 14. Mass Spectra of B i c y c l i c Alcohols. (a) Helium Light Source (b) Hydrogen Light Source 8 0 m/e ( A ) 1 0 0 ( B ) 100 120 120 i-o i T 5 1 -C rH rO + and C,H_+ which are present i n the r a t i o 1:5 when the helium 5 b o y r l i g h t source i s used. In common with the isomeric t r i c y c l i c alcohols there i s an abundant molecular ion minus water peak which occurs at m/e 92 i n the spectrum. . Also i n common with the t r i c y c l i c alcohols i s the fragment at m/e 95 which has the formula C^H^0+. The other major peaks i n the spectrum are those at m/e 82 (Cj.H^.0* and C ^ i o * present i n the r a t i o 4:1 r e s p e c t i v e l y , f o r the helium l i g h t source) and at m/e 67 (C^H^ +). There i s no apparent c o r r e l a t i o n of these fragments with e i t h e r the t r i c y c l i c alcohols or with b i c y c l o [2.2.1] heptan-7-o£. F. High Resolution The high r e s o l u t i o n mass spectrometer r e s u l t s f or the compounds used i n t h i s study are shown i n tables 1-6. In addition to the determination of the molecular composition of the molecular ion, the composition of the major fragments i n the low r e s o l u t i o n mass spectra were also determined. The l i n e s i n the table indicate that no attempt was made to determine the composition of the fragment ion because of i n s u f f i c i e n t i n t e n s i t y . The doublets observed are shown i n the tables together with t h e i r r e l a t i v e 2 4 r a t i o s . For example, i n the spectrum of endo-syn t r i c y c l o [3.2.1.0 ' ] octan-8-o£ a doublet occurs at m/e 95 which i s composed of C^H^ + and C^H^0+. For the helium l i g h t source the C^H^ + and C^H_,0+ fragments are present i n .the r a t i o 3:2 whereas for the hydrogen l i g h t source the ions are present i n the r a t i o 1:1. In a number of instances a knowledge of the elemental composition of a fragment permits the formulation of a probable fragmentation scheme -52-m/e 122 94 C 8 H 1 0 ° C7 H10 o C 8 H 1 0 ° C7 H10 80 79 77. 66 55 C 6 H 8 C 6H ? C 6 H 5 C 5 H 6 C 4 H 7 C 6 H 8 C 6 H 7 C 6 H 5 C 5 H 6 C 4 H 7 C 3H 30 (Trace) C 3H 30 (Trace) TABLE I High Resolution Results f o r T r i c y c l i c Ketones -53-m/e MeO^ OMe MeO^ OMe 168 101 C10 H16°2 C5 H9°2 MeO^ OMe C10 H16°2 C 5 H 9 ° 2 m/e m/e 166 165 151 135 119 C10 H14°2 C10 H13°2 C 9 H 1 1 ° 2 C 9 H 1 1 ° C 8 H 7 ° 101 92 91 59 C 5 H 9 ° ; C 7 H 8 C 7H ? C 2 H 3 ° 2 C 3 H 7 ° He* H* 2: 2 1 TABLE II High Resolution Results f o r Saturated and Unsaturated T r i c y c l i c Methoxy Compounds See Text f o r explanation -54-; m/e 108 93 91 80 79 78 67 66 54 C8 H12 C 7 H 9 C ?H 7 C 6 H 8 C 6 H 7 C 6 H 6 C 5H 7 C 5 H 6 4 6 C8 H12 C 7 H 9 C 7H y C 6 H 8 C 6 H 7 C 6 H 6 C 5 H 7 C 5 H 6 4 6 TABLE III High Resolution Results f o r T r i c y c l i c Hydrocarbons m/e AcO AcO OAc OAc 166 124 C10 H14°2 C 8 H 1 2 ° C 7H g0 2 (Trace) C10 H14°2 C 8 H 1 2 ° C 7H g0 2 (Trace) C10 H14°2 C8 ;Hl2° C 7H g0 2 (Trace) C10 H14°2 C 8 H 1 2 ° C 7H g0 2 (Trace) 106 91 80 79 78 C8 H10 C 7H ? C 6 H 8 C 6 H 7 C 6 H 6 C8 H10 C 7H ? C 6 H 8 C 6 H 7 C 6 H 6 C8 H10 C 7H y C 6 H 8 C 6 H 7 C 6 H 6 r H L8 10 C 6 H 8 C 6 H 7 C 6 H 6 i tn • On 70 He* H 2* 4 6 1 only C 3 H 2 ° 2 Trace He* H 2* 4 6 1 only C 3 H 2 ° 2 Trace He* H 2* 4 6 1 only C 3 H 2 ° 2 Trace He* H 2* 4 6 1 only C 3 H 2 ° 2 Trace 43 C 2 H 3 ° C 2H 30 C 2 H 3 ° C 2 H 3 ° See text f o r explanation. TABLE IV High Resolution Results f o r T r i c y c l i c Acetates m/e H O 124 106 C 8 H 1 2 ° C8 H10 C 8 H 1 2 ° C8 H10 C 8 H 1 2 ° C8 H10 C 8 H 1 2 ° C8 H10 95 He* H 2* C7 H11 3 2 C 6 H 7 ° 2 1 He* H 2* C7 H11 1 1 C 6 H 7 ° 1 1 He* H 2* C7 H11 ~ T ~ 2 C 6H y0 » •• He* H 2* C7 H11 7 7 C 6H y0 3 91 80 79 78 70 68 6 7 57 C ?H 7 C 6 H 8 C 6 H 7 C 6 H 6 C.1L0 4 6 C5 H< C 5 H 7 C 3 H 5 ° C 7H ? C 6 H 8 C 6 H 6 C.H^O 4 6 C 5 H 8 C 3 H 5 ° C 7H y C 6 H 7 C 6 H 6 C 4 H 6 ° C 5 H 8 C 5 H 7 C 3 H 5 ° C 6 H 8 C 6 H 6 4 6 C 5 H 8 C 3 H 5 ° * See text f o r explanation. TABLE V High Resolution Results f o r T r i c y c l i c Alcohols -57-m/e O H m/e O H 112 94 81 79 70 C 7H 1 20 C7 H10 C 6 H 9 C 6 H 7 C.H,0 4 6 110 95 92 91 82 C ?H 1 00 C,H_0 D / C 7 H 8 C 7H ? * * He H^ C 5 H 6 ° 4 1 57 C 5 H 8 C 3 H 5 ° 81 C6 H10 1 1 * * He H 2 C 5H 50 1 Trace 79 78 67 C 6 H 9 C 6 H 7 o 6 C 5 H 7 5 1 TABLE VI High Resolution Results f o r B i c y c l i c Alcohols See Text f o r explanation and probable structure f o r the ion. While a knowledge of the elemental composition of the fragments aids i n the i n t e r p r e t a t i o n of the mass spectra no unequivocal decision can be made as to which atoms o r i g i n a l l y , present i n the molecule are retained by the p a r t i c u l a r fragment. Probable fragmentation schemes together with probable structures f o r the major fragments of the compounds studied are shown i n figures 15-21. Although, only one isomer of a group i s shown, the proposed scheme i s applicable to a l l isomers of the group. For many fragments more than one pathway e x i s t s f o r the formation o f that fragment. Also since i s o t o p i c s u b s t i t u t i o n was not used, i t cannot be stated unequivocally that the proposed pathway a c t u a l l y e x i s t s . G. Ionization P o t e n t i a l s Electron impact i o n i z a t i o n p o t e n t i a l s were obtained f o r a l l compounds used i n t h i s study by use of a method s i m i l a r to that described by Lossing et a l (59), the r e s u l t s obtained are shown i n fig u r e 22. Previous work i n t h i s laboratory, using the same technique, has shown that the r e p r o d u c i b i l i t y i s about +_ 0.1 e.V. The values obtained show that the i o n i z a t i o n p o t e n t i a l of the exo isomer i s greater than that of the corresponding endo isomer i n a l l instances. The release of non-bonded i n t e r a c t i o n energy i n the more crowded endo isomer may be used to account f o r the lower i o n i z a t i o n p o t e n t i a l s of the endo isomer compared to the corresponding exo isomer. C 2 ' H 3 0 ' C 6 H 7 m/e 7 9 \ = - H i> C 5 H 7 m/e 77 O 11 • C 4 H 3 0 -C 4 H 7 + m/e 5 5 - C O ^ C-^H^-•7' '11 m/e 9 4 C 3 H 3 0 m/e 55 m/e 8 0 - C 2 H 4 C 5 H 6 + m/e 6 6 <<^=0+ Figure 15. Fragmentation Scheme for T r i c y c l i c Ketones. M e O v M eC? C 5 H 9 ° 2 m/e 101 K - C 5 H 5 \ - H M e O - O O - 3 H 7 ° + m/e 5 9 - C - H - O CQH/|>| O 2 m/e 151 CH-C 1 0 H 1 3 ° 2 m/e 1 6 5 + C H 2 ° T O M e C 8 H 1 1 V +  C 2 H 3 ° 2 m/e 5 9 0 - " C - 0 M e C H 3 ° r H o +  J v C 9 H 1 1 ° m/e 135 \ ^ C 3 H 6 0 2 C y H 8 t m/e 9 2 - H + OvOMe OMe x x - C H 4 > C 8 H 7 d m/e 119. C 7 H 7 + m/e 91 Figure 16. Fragmentation Scheme f o r Unsaturated T r i c y c l i c Methoxy Compound. C 6 H 8 m/e 8 0 \\ - H , C 6 H 6 + m/e 7 8 1 + C H 3 > C 7 H 9 m/e 9 3 C 5 H 6 + m/e 6 6 C 5 H 7 m/e 6 7 m/e 5 4 ^ v 6 7 m/e 7 9 Figure 17. Fragmentation Scheme f o r T r i c y c l i c Hydrocarbons. C 6 H ? + m/e 7 9 C 6 H 8 + \\ m/e 8 0 m/e 7 8 ' C 4 H 7 ° 2 1i C 4 H 6 ° 2 C 8 H 1 1 ° ' C 2 H s O m/e 4 3 + O r C - C H -jrOA(? + - C 3 H 6 C 7 H g 0 2 t m/e 124 OAc C ? H 4 ° 2 r H + • > 8 M 1 0 m/e 106 \ C 2 H 2 0 C 8 H 1 2 ° - - C 4 H 6 - C H 3 ^ C 7 H 7 V + m/e 124 OhP +m/e 91 C 4 H 6 ° + m/e 7 0 O H f to I Figure 18. Fragmentation Scheme for T r i c y c l i c Acetates. C 5 H 8 + m/e 6 8 C 5 H 4 0 C 5 H 7 C 3 H 5 0 + m/e 5 7 C 4 H e O + m/e 7 0 C 4 H 6 O H H 2 0 C 6 H 8 + m/e 8 0 C 2 H 4 0 ^ C 8 H 1 0 ' m/e 1 0 6 C 2 H 5 C O H C 6 H 7 ° m/e 9 5 O H C 7 H 1 1 + m/e 9 5 C 6 H 6 + m/e 7 8 C 2 H 4 V - C H 3 > C 7 H / m/e 91 Figure 19. Fragmentation Scheme f o r T r i c y c l i c Alcohols, C 3 H 5 0 ' m/e 5 7 A C 4 H 7 C 5 H 8 + m/e 6 8 C 2 H 4 0 O H - H o O C-vH 7 n 1 0 ' m/e 9 4 ~ C 3 H 6 ^ 4 ^ Q O m/e 7 0 X X O H - C K C 6 H g + m/e 81 m/e 7 9 Figure 20. Fragmentation Scheme f o r Bicyclo [2.2.1] heptan-7-ol C 5 H 7 m/e 6 7 - C H , C 6 H y O + m/e 9 5 / O H C 6 H 1 0 + m/e 8 2 A C 2 H s O - C O C 5 H 6 0 + m/e 82 • C 2 H 4 OH — C O H " \k C 6 H 9 + m/e 81 • H 2 0 > C-vH 7 ^ 8 m/e 9 2 ^ T C 2 H 5 C 5 H 5 O m/e 81 / O H O H • C H _ C 6 H 7 + rn/e 7 9 Figure 21. Fragmentation Scheme f o r a n t i - b i c y c l o [2.2.1] hepten-7-ol. -66-It can also be seen from f i g u r e 22 that, f or the cases studied the i o n i z a t i o n p o t e n t i a l of the unsaturated compounds are less than that of the corresponding saturated ones as i s expected (64). The reduction i n i o n i z a t i o n p o t e n t i a l may be a t t r i b u t e d to the d e r e a l i z a t i o n e f f e c t of the p i system which r e s u l t s i n the electrons being less t i g h t l y bound. Also from f i g u r e 22 i t can be seen that the i o n i z a t i o n p o t e n t i a l of the a n t i isomer i s greater than that of the corresponding syn isomer. No reason has been found to account f o r the phenomenon. N a t a l i s (5) has found from thermodynamic data that c i s isomers are generally less stable than the corresponding trans isomer, the energy di f f e r e n c e being a measure of the i n t e r a c t i o n energy a r i s i n g from the s t e r i c conformation. Also since the c i s isomer i s less stable, fragmentation occurs more r e a d i l y . The e f f e c t that the more s.terically hindered (less stable) isomer fragments more r e a d i l y also appears to be present i n the isomers studied. .-68-V CONCLUSION The mass spectra f o r the isomers of a group are q u a l i t a t i v e l y very s i m i l a r . However, s i g n i f i c a n t differences are observed i n the r e l a t i v e abundances of the p r i n c i p a l fragments. The observed differences are more pronounced when the hydrogen l i g h t source i s used than when the helium l i g h t source i s used, however, with the hydrogen l i g h t source fewer differences are observed. The f a c i l i t y of fragmentation appears to be a function of s t e r i c hindrance within the molecule. Thus the endo isomers fragment more r e a d i l y than do the exo ones except f o r the exo-syn acetate i n which a large 'bowsprit-flagpole' i n t e r a c t i o n i s present. This occurs because the exo compounds have a f i x e d boat l i k e conformation i n which s t e r i c i n t e r a c t i o n may occur between the cyclopropyl r i n g and any large group on the syn 'flagpole' p o s i t i o n . For the compounds studied the more crowded ( i . e . the larger the substituent on the 'flagpole' p o s i t i o n ) the isomer, the smaller the r e l a t i v e abundance of the molecular ion. The probable formation of a seven membered r i n g appears to occur more r e a d i l y f o r endo isomers than for the corresponding exo isomer because i n the endo isomers the o r b i t a l s forming the cyclopropyl 'banana' bond are i d e a l l y situated f or i n t e r a c t i o n and subsequent p i bond formation with the p type o r b i t a l s a v a i l a b l e on fragmentation. Thus fragments which probably involve a seven membered rin g as an intermediate are more intense f o r the endo isomer than the corresponding exo isomer. -69-Th e i o n i z a t i o n p o t e n t i a l of the endo isomer i s lower than that of the corresponding exo isomer. The release of non-bonded i n t e r a c t i o n energy i n crowded molecules i s presumably the reason f o r the lower i o n i z a t i o n p o t e n t i a l . Because the mass spectra of isomers are q u a l i t a t i v e l y very s i m i l a r i t appears that the use of photoionization mass spectrometry w i l l only be s l i g h t l y more useful than electron impact mass spectrometry f o r the i d e n t i f i c a t i o n of geometrical isomers. Using photoionization i t w i l l be necessary to have previously determined the mass spectra of a l l isomers of a group before any p a r t i c u l a r isomer can be i d e n t i f i e d . -7(1-VI BIBLIOGRAPHY 1. K. 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