UBC Theses and Dissertations

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UBC Theses and Dissertations

FOURIER transform ion cyclotron resonance spectroscopy Melka, Joe David 1978

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FOURIER TRANSFORM ION CYCLOTRON RESONANCE SPECTROSCOPY by ' JOE DAVID MELKA B.S., Michigan T e c h n o l o g i c a l U n i v e r s i t y , 1976. A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN THE FACULTY OF GRADUATE STUDIES THE DEPARTMENT OF CHEMISTRY We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA December, 1978. (cj Joe David Melka, 1978 In presenting th i s thes is in pa r t i a l fu l f i lment of the requirements for an advanced degree at the Univers i ty of B r i t i s h Columbia, I ag ree that the L ibrary shal l make it f ree ly ava i lab le for reference and s tudy . I further agree that permission for extensive copying of t h i s thesis for scho lar ly purposes may be granted by the Head of my Department or by his representat ives. It is understood that c o p y i n g o r p u b l i c a t i o n o f th i s thes i s fo r f i nanc ia l gain sha l l not be allowed without my written permission. n . r CHEMISTRY Department o f The Univers i ty of B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 DEC. 13, 1978 ABSTRACT Chapter I I of. t h i s t h e s i s d e s c r i b e s the o p e r a t i o n of a F o u r i e r Transform Ion C y c l o t r o n Resonance Spectrometer. Included i s a d e t a i l e d d i s c u s s i o n of software which was developed to run the spectrometer and analyze the data. An automatic data a c q u i s i t i o n system f o r k i n e t i c experiments i s a l s o d e s c r i b e d . Chapter I I I contains a short d i s c u s s i o n of d i s c r e t e versus continuous methods of data a c q u i s i t i o n and' problems with the FT-ICR method which u t i l i z e s d i s c r e t e sampling. Two t e c h -niques are d i s c u s s e d which have been found to s o l v e problems a s s o c i a t e d with a d i g i t a l system, a c q u i s i t i o n by "mixing" and z e r o - f i l l i n g . A l s o , mass c a l i b r a t i o n s done on the FT-ICR spectrometer are presented. The r e a c t i o n s of some negative ions with e s t e r s i n the gas phase have been s t u d i e d by FT-ICR and are d i s c u s s e d i n 18 Chapter IV. The r e a c t i o n s of 0 l a b e l l e d methoxide with methyl t r i f l u o r o a c e t a t e , methyl benzoate, dimethyl carbonate, d i e t h y l carbonate and d i p r o p y l carbonate have been s t u d i e d . 18 I t was found that there i s i n c o r p o r a t i o n of 0 i n product ions of the form RCOO showing that methoxide a t t a c k s e s t e r s i n the gas phase p a r t i a l l y at c a r b o n y l carbon. Product d i s t r i -b u tions are found to be c o n s i s t e n t with two competing * -mechanisms, B 2 and S 2. Thus i t was found that CH 0 r e a c t s w i t h C^HVCOOCH- 92% by a B._2 type mechanism and 8% by 6 5 3 AC 18 an S 2 type mechanism. The amount of 0 i n c o r p o r a t i o n i n - l i i -p r o d u c t i o n s was seen. to. decrease w i t h the p r e s e n c e o f 3 hydrogens, due t o t h e presence o f an e l i m i n a t i o n c h a n n e l . The r e a c t i o n o f e t h o x i d e w i t h the ab.ove named e s t e r s was s t u d i e d and the p r o d u c t s are a l s o i n t e r p r e t e d i n terms o f t h r e e competing mechanisms, B^n2., S M 2 and e l i m i n a t i o n . - i v -TABLE OF CONTENTS CHAPTER ONE I n t r o d u c t i o n 1 CHAPTER TWO The FT-ICR Spectrometer 6 A. I n t r o d u c t i o n to ICR Theory 6 B. The PT-ICR Spectrometer 9 i . Overview 9 i i . Vacuum System 9 i i i . ICR c e l l and s i g n a l route 13 i v . Computer, P u l s e r , Disk and Sy n t h e s i z e r 15 v. E l e c t r o n E j e c t i o n 18 C. Software 18 i . FTZB and the Pulse Sequence 18 i i . Double Resonance 23 i i i . K i n e t i c Data A c q u i s i t i o n by FTZB 24 i v . Peak P i c k i n g 26 v. Rate Determination 27 References 30 CHAPTER THREE D i g i t a l Techniques and T h e i r A p p l i c a t i o n to PT-ICR • 31 A. Problems With D i s c r e t e A n a l y s i s 31 i . L i m i t e d Memory S i z e , 31 i i . D i s c r e t e v--. Continuous 32 • - V -B. S o l u t i o n s to. the D i s c r e t e P o i n t s Problem 37 i . Curve F i t t i n g 37 i i . S p e c t r a l Segment E x t r a c t i o n o r Mixing .. 38 i i i . Z e r o - F i l l i n g 40 C. Accuracy Obtained by. Z e r o - F i l l i n g 41 i . I n t e n s i t i e s 4 l i i . F requencies 50 D. Leakage 56 E. S e l e c t i v e Z e r o - F i l l i n g 57 F. Mass C a l i b r a t i o n 62 References 66 CHAPTER FOUR The Gas Phase Reactions of Carbonates 67 A. A B r i e f Review of Previous E s t e r S t u d i e s .... 67 B. Experimental 69 i . I nstrumental 69 i i . Chemicals - Commercial 71 i i i . Chemicals - S y n t h e s i s 71 C. R e s u l t s 73 D. D i s c u s s i o n of R e s u l t s 82 References 90 - v i -LIST OP FIGURES F i g u r e Page 1 General ICR pulse sequence- , , . , 8 2 Schematic diagram of the FT-ICR vacuum system 10 3 A sample Ba r a t r o n pressure v. Ion gauge pressure p l o t f o r CH^ 12 4 Schematic diagram of the FT-ICR spectrometer 14 5 Receiver c o n f i g u r a t i o n f o r d i r e c t a c q u i s i t i o n 16 6 Receiver c o n f i g u r a t i o n f o r s p e c t r a l segment e x t r a c t i o n or mixing 16 7 Pulse sequence f o r FTZB 20 8 Time p l o t f o r the r e a c t i o n C H 4 + + CH^ — C H 5 + + -CH 29 9 Continuous and d i s c r e t e magnitude mode peaks 33 10 Continuous and d i s c r e t e a b s o r p t i o n mode s p e c t r a l peaks 35 11 CH^O /HNO spectrum obtained u s i n g the mixing technique 39 ' 12 R e l a t i v e e r r o r due to n o n i n f i n i t e z e r o -f i l l i n g f o r an a b s o r p t i o n mode l i n e s h a p e . 46 13 R e l a t i v e e r r o r due to n o n i n f i n i t e zero-f i l l i n g f o r a magnitude mode li n e s h a p e .. 48 14 P l o t of mass v. AM f o r v a r i o u s values of DW 51 15 P l o t of mass v. AM f o r v a r i o u s values of n where n = number of z e r o - f i l l i n g s .. 54 16 "Wide band spectrum of t r i s ( p e r f l u o r o -h e p t y l ) - s - t r i a z i n e 58 17 Hanning window f u n c t i o n a p p l i e d to t r a n s i e n t of f i g u r e 16 59. - V l ' l -18. High mass p o r t i o n of f i g u r e 16 obtained u s i n g the s e l e c t i v e , f i l l i n g technique of Pajer and Arr.itage 6 l 19 U n f i l l e d high mass p o r t i o n of f i g u r e 16 .. 61 2 0 B u f f e r gas curves f o r the dimethyl carbon-ate, system 7 4 21 Time p l o t f o r CD^O- + (n-C 3H 70) 2CO —>- products 80 22 Time p l o t f o r * -CH 3 0 + ( n - C ^ O ^ C O — • products 8 l 23 Products of CH * 0 ~ + (CD^O) CO at low r e a c t i o n time r 1 86 - v i i i -LIST OF TABLES Table Page 1 Output pulses from 293-A and t h e i r uses 21 2 C a l i b r a t i o n data f o r the FT-ICR spectrometer 63 3 Summary of r e a c t i o n s s t u d i e d 75 4 C o n t r i b u t i o n of B A Q 2 and S N2 to s e l e c t e d gas phase r e a c t i o n s 79 - i x -ACKNOWLEDGEMENTS I would l i k e to acknowledge' the support and. encourage-ment of P r o f e s s o r M e l v i n Comlsarow d u r i n g my car e e r at U.B.C.. A l s o , thanks must be extended to the t e c h n i c a l ' s t a f f at U.B.C. who provided much needed advice and s e r v i c e . At t h i s time i t gives me great p l e a s u r e to c i t e the e f f o r t s of my c o l l e a g u e s and good f r i e n d s , Dr. Gerald P a r i s o d and V a l e r i o G r a s s i . Without t h e i r c o - o p e r a t i o n and a s s i s t a n c e , t h i s t h e s i s would not have been p o s s i b l e . Thanks must a l s o be pro v i d e d to the f o l l o w i n g people f o r v a r i o u s s e r v i c e s performed: Barry Hames ( p r o o f r e a d i n g ) , Howard Morton (G.L.C. a n a l y s i s ) , Gord Rickards (advice on s y n t h e s i s ) and Randy Miku l a (worthy b a s k e t b a l l opponent). Last but not l e a s t , I would l i k e to thank my parents f o r s u p p o r t i n g my e d u c a t i o n a l goals and o f f e r i n g needed encouragement. -X-"I don't get no r e s p e c t . " Rodney D a n g e r f i e l d , famous comedian. - 1 -I. INTRODUCTION Reactions between ions and n e u t r a l molecules i n the gas phase have been a c t i v e l y s t u d i e d by many experimental techniques d u r i n g the past two decades. V a r i o u s types of i n s t r u m e n t a t i o n which have been developed to perform these s t u d i e s i n c l u d e tandem-mass spectrometers, d r i f t tubes, high pressure mass spectrometers and i o n c y c l o t r o n resonance 1 2 (ICR) spectrometers. 5 The s t u d i e s of ion-molecule r e a c t i o n s can be d i v i d e d i n t o two broad areas, thermodynamics and k i n e t i c s . Thermochemical p r o p e r t i e s that have been s t u d i e d i n c l u d e proton a f f i n i t i e s , e l e c t r o n a f f i n i t i e s , a c i d i t i e s , b a s i c i t i e s , i o n i c heats of formation and Lewis acid-Lewis base i n t e r a c t i o n s . These r e s u l t s have been obtained by measurements i n the absence of a s o l v e n t so that the i n t r i n s i c p r o p e r t i e s of a molecule are r e v e a l e d . Gas phase r e s u l t s can be compared to s o l u t i o n r e s u l t s to pr o v i d e i n t e r e s t i n g c o n t r a s t s and com-p a r i s o n s . E x e m p l i f y i n g the d i f f e r e n c e s between s o l u t i o n and gas phase r e a c t i o n s i s the f o l l o w i n g gas phase b a s i c i t y order which was d i s c o v e r e d by Brauman and B l a i r Me.O" > EtO~ > t-BuO~ This i s i n c o n t r a s t to the w e l l known s o l u t i o n order t-BuO~ > EtO~ > MeO~ The d i f f e r e n c e has been e x p l a i n e d by p o l a r i z a b i l i t y e f f e c t s which determine the gas phase b a s i c i t y order as compared to i n s o l u t i o n where so l v e n t e f f e c t s dominate. By s t u d y i n g - 2 -f a m i l i e s of compounds i n the gas phase and comparing to s o l u t i o n r e s u l t s , much can be l e a r n e d about s o l v a t i o n e f f e c t s which leads to a b e t t e r understanding of chemical r e a c t i o n s . On the k i n e t i c f r o n t , t h e o r i e s have been advanced to d e s c r i b e f a c t o r s which i n f l u e n c e c o l l i s i o n r a t e s and r e a c t i o n r a t e s . Notable developments i n the past decade i n c l u d e the average d i p o l e o r i e n t a t i o n (ADO) theory which i s used to d e s c r i b e the c o l l i s i o n frequency between an i o n and a p o l a r 4 molecule. And, the theory of unimolecular decomposition has s u c c e s s f u l l y been used to d e s c r i b e the fragmentation to products of a r e a c t i o n i n t e r m e d i a t e f o r some S N 2 r e a c t i o n s 5 i n the gas phase. The sum of thermochemical and k i n e t i c i n f o r m a t i o n obtained from gas phase s t u d i e s can then l e a d to a b e t t e r understanding of the f o r c e s which c o n t r o l a chemical r e a c t i o n . ICR i s a powerful t o o l f o r determining a b s o l u t e and r e l a t i v e r a t e c o n s t a n t s , making e q u i l i b r i u m measurements to e s t a b l i s h a c i d i t y and b a s i c i t y s c a l e s , p r o b i n g r e a c t i o n mech-anisms, determining e l e c t r o n a f f i n i t i e s by photodetachment experiments and e l u c i d a t i n g complex r e a c t i o n schemes by 2 double resonance techniques. T h i s t h e s i s d e s c r i b e s work which a p p l i e s the technique of F o u r i e r t r a n s f o r m i o n c y c l o t r o n resonance (FT-ICR) spectroscopy to the study of n e g a t i v e ions with e s t e r s i n the gas phase. FT-ICR i s a new method f o r s t u d y i n g gas phase r e a c t i o n s which has r e c e n t l y been developed and has been shown to have advantages i n r e s o l u t i o n , speed, mass range and s e n s i t i v i t y over c o n v e n t i o n a l ICR methods. - 3 -Chapter I I of t h i s t h e s i s d e s c r i b e s the FT-ICR spec-trometer used f o r t h i s work and how i t operates. Included i s a d e t a i l e d d i s c u s s i o n of software which was developed to run the spectrometer and analyze the data. An automatic data a c q u i s i t i o n system f o r k i n e t i c experiments i s a l s o d e s c r i b e d . Chapter I I I c o n t a i n s a short d i s c u s s i o n of d i s c r e t e versus continuous methods of data a c q u i s i t i o n and problems with the FT-ICR method which u t i l i z e s d i s c r e t e sampling. V a r i o u s techniques are presented f o r s o l v i n g problems a s s o c i a t e d with a d i g i t a l system. Examples are presented which show t y p i c a l experimental problems i n FT-ICR and how they are s o l v e d . A l s o , the problem of mass c a l i b r a t i o n i s d i s c u s s e d and the r e s u l t s of mass c a l i b r a t i o n s done on an FT-ICR spectrometer are compared f a v o r a b l y w i t h c a l i b r a t i o n s done on a c o n v e n t i o n a l instrument. Chapter IV d i s c u s s e s the r e a c t i o n s of some ne g a t i v e ions w i t h e s t e r s i n the gas phase as s t u d i e d by FT-ICR. The r e a c t i o n s of methoxide and ethoxide with methyl benzoate, m e t h y l t r i f l u o r o a c e t a t e and three a l k y l carbonates (methyl, e t h y l and n-propyl) have been i n v e s t i g a t e d . E x t e n s i v e l a b e l l i n g schemes have been used to probe m e c h a n i s t i c path-ways. I t i s shown that f o r the e s t e r s used i n t h i s study, numerous pathways e x i s t that l e a d to products. Rates have been determined u s i n g the data a c q u i s i t i o n system presented i n chapter I I and compared to ADO theory to o b t a i n r e a c t i o n e f f i c i e n c i e s . V a r i o u s m e c h a n i s t i c p o s s i b i l i t i e s are d i s -cussed and the r e a c t i o n s of carbonates are compared to the -the r e a c t i o n s of other e s t e r s with negative ions i n the gas phase. -5-REPERENCES 1. P. Ausloos (Ed.), " I n t e r a c t i o n s Between Ions and Molecules", Plenum Press, New York, 1975. 2. T. A. Lehman and M. M. Bursey, "Ion C y c l o t r o n Resonance Spectrometry", John Wiley and Sons, New York, 1976. 3- J . I. Brauman and L. K. B l a i r , J . Am. Chem. Soc. 90, 6561(1968). 4. Reference 1, pages 163-183-5. W. N. Olmstead and J . I. Brauman, J . Am. Chem. Soc. £9., 4219(1977). 6. M. Comisarow i n "Advances i n Mass Spectrometry", N. R. Daly (Ed.), Heyden and Son L t d . , London 1978. -6-I I . THE FT-ICR SPECTROMETER A. I n t r o d u c t i o n to ICR Theory Ion C y c l o t r o n Resonance Spectroscopy i s a form of mass spectroscopy t h a t i s p a r t i c u l a r l y s u i t e d f o r the study of ion-molecule r e a c t i o n s . Numerous review a r t i c l e s d e s c r i b e i n d e t a i l the theory of ICR and the.types of experiments that 1-3 can be done by t h i s method. Consequently, only a b r i e f i n t r o d u c t i o n to the technique w i l l be given here. A t y p i c a l ICR experiment i s conducted by forming ions In an i o n t r a p , c a l l e d a c e l l , which i s enclosed i n a vacuum chamber c o n t a i n i n g a p p r o p r i a t e sample gases at p r e s s u r e s of _q _4 10 " to 10 T o r r . A f t e r a s u i t a b l e delay time which ranges from m i l l i s e c o n d s to seconds, the ions i n the c e l l are de-t e c t e d . By v a r y i n g the time between formation and d e t e c t i o n of the i o n s , the r e a c t i o n s o f ions with a n e u t r a l gas can be f o l l o w e d . Trapping v o l t a g e s are'.used'-to keep- ions of a c e r t a i n s i g n i n the c e l l . R e v e r sing the p o l a r i t y o f the t r a p p i n g v o l t a g e e f f e c t i v e l y removes the ions from the c e l l , a process commonly r e f e r r e d to as a quench p u l s e . A g e n e r a l pulse sequence i s g i v e n i n f i g u r e 1 which i l l u s t r a t e s the p u l s e d ICR technique; formation of Ions, r e a c t i o n time, d e t e c t i o n and quench. I t i s w e l l known t h a t an i o n of charge q and mass m i n a homogeneous magnetic f i e l d of s t r e n g t h B w i l l be c o n s t r a i n e d to a c i r c u l a r path that i s p e r p e n d i c u l a r to the magnetic f i e l d and w i l l o r b i t at a c y c l o t r o n frequency to where -7-io=qB/m ( § - 1 ) A mass spectrum i s obtained by determining the f r e q u e n c i e s of the sample ions and then c o n v e r t i n g to masses by equation 2 - 1 . Conventional ICR spectrometers d e t e c t ions i n the f o l l o w -i n g manner. A r a d i o f r e q u e n c y e l e c t r i c f i e l d i s used to e x c i t e the motion of the sample ions by connecting the output of the RF o s c i l l a t o r to the top and bottom p l a t e s of the ICR c e l l (see f i g u r e 4 ) . I f the frequency of an i o n i s equal to the frequency of the a p p l i e d RF f i e l d , the i o n w i l l absorb energy from t h a t f i e l d , a process c a l l e d i o n c y c l o t r o n resonance. A marginal o s c i l l a t o r c i r c u i t p r o v i d e s the RF f i e l d as w e l l as measuring i o n i n t e n s i t i e s by r e l a t i n g power a b s o r b t i o n from the c i r c u i t to the number of i o n s which are i n resonance. By v a r y i n g the magnetic f i e l d , the sample ions are equated'with the f i x e d frequency of the marginal o s c i l l a t o r c i r c u i t and detected. A p l o t of power a b s o r b t i o n of the c i r c u i t versus magnetic f i e l d s t r e n g t h leads to a mass spectrum. This method of d e t e c t i o n , although proved to be very r e l i a b l e , has s e v e r a l drawbacks. Since only one frequency can be d e t e c t e d at a given time, i t can take up to 2 0 minutes to r e c o r d a mass spectrum of m/e = 1 5 to 2 0 0 . A l s o , r e s o l u t i o n i s poor. An a l t e r n a t e method of d e t e c t i o n i n v o l v e s e x c i t i n g the c y c l o t r o n motion of a l l the ions i n the c e l l and d e t e c t i n g the v o l t a g e Induced i n the top and bottom p l a t e s of the ICR 4 c e l l a f t e r the RF f i e l d i s removed. The s i g n a l w i l l o s c i l l a t e with time due to the c y c l o i d a l motion of the i o n s . In t h i s G r i d R e a c t I O N / P e r i o d Pulse. I CO I Pu\s* T I N A E F i g u r e 1 General ICR pulse sequence. - 9 -method, e x c i t a t i o n and d e t e c t i o n are temporally d i s t i n c t . The time domain s i g n a l can be F o u r i e r transformed to give the corresponding frequency domain spectrum. A l a r g e s a v i n g i n time i s r e a l i z e d as a l l the f r e q u e n c i e s are de t e c t e d i n the same time i t takes to detec t one frequency by the c o n v e n t i o n a l method. S e n s i t i v i t y i s enhanced through the use of s i g n a l averaging and the u s e f u l mass range has been extended to at l e a s t 1200. B. The FT-ICR Spectrometer i . Overview The FT-ICR spectrometer used i n t h i s work was b u i l t i n the chemistry department at the U n i v e r s i t y of B r i t i s h Columbia. The remainder of t h i s chapter w i l l be devoted to d e s c r i b i n g the instrument and software t h a t runs the spectrometer and processes the data i n d e t a i l . The main p a r t s of the FT-ICR spectrometer are the vacuum system, c e l l , magnet and computer with i t s associated.programmable p u l s e r and RF generator. i i . Vacuum System A diagram of the vacuum system i s given i n f i g u r e 2. The vacuum c o n t a i n e r i s made of s t a i n l e s s s t e e l and i s designed such that the end c o n t a i n i n g the ICR c e l l w i l l f i t between the poles of a 24 kGauss magnet. The magnet i s on a t r a c k , making i t moveable, thus f a c i l i t a t i n g r e p a i r s to the vacuum system and bake-out. The main vacuum system i s pumped by a 225 1/s Veeco Maglon pump which can be valved o f f by a bellows v a l v e . Base pressures of the system a f t e r o v e r n i g h t bake-out are t y p i c a l l y 2-4 x 1 0 - 1 0 T o r r as measured with a V a r i a n S H O R T P r V T H IIs/LET N T O T o r t PurAP LlCUj\0 Nx TR»\p H DtFF U S l O i s / r-J P U I S A P 5 " = & & $ $ SPMAPLE INLET J = t s— -> TO OlFF PUfAP I C R C E L L i M O I TO ROVXGHIMG PUN\P F i g u r e 2 Schematic diagram of the FT-ICR vacuum system. -11-Model 971-1008 i o n i z a t i o n gauge. The i o n i z a t i o n gauge can -5 -4 be c a l i b r a t e d f o r v a r i o u s gases i n the range 10 to 10 T o r r by a Baratron capacitance manometer. I t i s assumed that the Barat r o n g i v e s the tr u e pressure i n t h i s pressure range. However, ICR experiments are u s u a l l y conducted at lower p r e s -sures n e c c e s s i t a t i n g c a l i b r a t i o n of the i o n gauge a g a i n s t the Baratron at these higher p r e s s u r e s . F i g u r e 3 shows a c a l -i b r a t i o n curve f o r CH^. A c a l i b r a t i o n f a c t o r P-D • . / P T can be determined from the slope of the Baratron Ion Gauge ^ l i n e . The pressure i s then equal to the i o n gauge r e a d i n g m u l t i p l i e d by the c a l i b r a t i o n f a c t o r which i s assumed to be constant over a wide pressure range. Sample i n t r o d u c t i o n can be accomplished by two methods, l e a k i n g vapors i n t o the vacuum system or by u s i n g a short path i n l e t f o r n o n v o l a t i l e l i q u i d s and s o l i d s . Four i n l e t s , equipped with Sl4 male adapters welded to Cajon f i t t i n g s are a v a i l a b l e f o r sample i n t r o d u c t i o n . Sample gases are leaked Into the main vacuum system by a d j u s t a b l e leak v a l v e s . The i n l e t system i s pumped by an 11 1/s d i f f u s i o n pump which i s separated from the sample c o n t a i n e r s by a l i q u i d n i t r o g e n t r a p . A mechanical pump i s used as a forepump. I n v o l a t i l e samples can be i n t r o d u c e d by p l a c i n g them i n a g l a s s tube which has been se a l e d to a Cajon f i t t i n g . A simple on/off v a l v e separates t h i s short path i n l e t from the.main vacuum chamber. Dynamic pressure r e g u l a t i o n i s accomplished by mani p u l a t i o n of the bellows v a l v e . The short path i n l e t can a l s o be pumped by the d i f f u s i o n pump. Of T O •z o <c 00 ui D 1/7 uO UJ Q_ 4 i 3 J 2 1 5 7 1 P R E S S U R E I O N G f V J G E X J O 4 T O K R , Figure 3 A sample Baratron pressure v. Ion gauge pressure plot for -13-The e n t i r e vacuum system, except the par t c o n t a i n i n g the c e l l , c a l l e d the can, and the i o n pump i s housed i n a l a r g e box which Is l i n e d with i n s u l a t i o n £nd equipped with heaters f o r bake out purposes. The i o n pump can be heated by fou r t u b u l a r heaters which extend i n t o the i o n pump body and a removeable can heater was b u i l t to heat the c e l l area. Bake out temperatures reach 180°C i n the can area and 120 °C i n the main system. A f t e r the main vacuum system has been opened to the atmosphere, i t can be pumped down by the d i f f u s i o n pump to a pressure of about 10 ^ T o r r . At t h i s p o i n t , the i o n pump can be s t a r t e d and the d i f f u s i o n pump va l v e d o f f . I t was found that with no l i q u i d n i t r o g e n t r a p , backstreaming of d i f f u s i o n pump o i l vapors i n t o the main system d u r i n g pump down was a problem, i d e n t i f i e d by burp i n g , i . e . , sudden pressure f l u c t u a t i o n s i n the i o n pump at lower p r e s s u r e s . A d d i t i o n of the t r a p s o l v e d t h i s problem. i i i . ICR C e l l and S i g n a l Route A cubic trapped i o n c e l l s i m i l a r to the o r i g i n a l Mclver design i s used. The c e l l i s i l l u s t r a t e d i n f i g u r e 4 . Trapping v o l t a g e s are a p p l i e d to p l a t e s T to t r a p ions i n the magnetic f i e l d . I t i s a l s o p o s s i b l e to apply s u i t a b l e v o l t a g e s to p l a t e s N, S, E, W f o r t u n i n g purposes. A g r i d of copper mesh i s p l a c e d between the t r a p p i n g p l a t e and the rhenium f i l a m e n t . By b i a s i n g the g r i d p o t e n t i a l , e l e c t r o n s can be pulsed i n t o the c e l l at the a p p r o p r i a t e time and t h e i r c u r r e n t measured by a c o l l e c t o r on the opposite t r a p p i n g p l a t e . RF e x c i t a t i o n i s a p p l i e d to p l a t e s E and W. The s i g n a l N,S - receiver plates E . W - liunsmiiler plates T - trapping plates B - moanetic field 6 - grid C - electron collector P - preamplifier A - transmitter amplifier CUBIC TRAPPED-ION CELL FREQUENCY SYNTHESIZER T T Y DISC C R T A D C C P U I/O R A M MEMORY C O M P U T E R C O N T R O L UNIT P L O T T E R i P U L S E GEN. F i g u r e 4 Schematic diagram of the FT-ICR spectrometer. induced i n p l a t e s N and S by the e x c i t e d i o n motion i s a m p l i f i e d by a 60 dB preamp, su b j e c t e d to a high pass f i l t e r and a m p l i f i e d again by 20 dB. The s i g n a l can now be fed d i r e c t l y i n t o the computer's a n a l o g - d i g i t a l c o n v e r t e r (ADC) and subjected to F o u r i e r t r a n s f o r m a t i o n . T h i s process i s known as " d i r e c t a c q u i s i t i o n " and i s i l l u s t r a t e d i n f i g u r e 5. A l t e r n a t e l y , the s i g n a l can be mixed with a r e f e r e n c e f r e -quency, passed through a low pass f i l t e r to e x t r a c t a c e r t a i n band of f r e q u e n c i e s and then d i g i t i z e d . T h i s i s known as "mixing" and i s i l l u s t r a t e d i n f i g u r e 6. i v . Computer, P u l s e r , Disk and S y n t h e s i z e r The spectrometer i s c o n t r o l l e d by a N i c o l e t 1180 m i n i -computer which has 24k memory and uses 20 b i t words. E i t h e r assembly language or BASIC can be used f o r programming. The computer i s equipped with a f a s t ADC which operates i n v a r -ious manners depending on the d i g i t i z a t i o n r a t e . For s i g n a l s up to 333 kHz, the ADC has 12 b i t r e s o l u t i o n and the d i g -i t i z e d s i g n a l i s fed d i r e c t l y i n t o the randon access memory (RAM). For s i g n a l s from 333 kHz to 2.5 MHz, the ADC s t i l l has 12 b i t r e s o l u t i o n but the d i g i t i z e d s i g n a l can't be fed d i r e c t l y i n t o the RAM at t h i s high r a t e so the s i g n a l i s fed i n t o a b u f f e r memory, or s h i f t r e g i s t e r memory, and then at a slower r a t e f e d i n t o the RAM. For s i g n a l s from 2.5 to 5 MHz, 9 b i t r e s o l u t i o n i s obtained and the b u f f e r mode i s used. Programs and s p e c t r a can be s t o r e d on a magnetic d i s k which has room f o r 1,143,296 words of 20 b i t s each or about 60 s p e c t r a of 16k words each. -16-RECEIVER CONFIGURATION FOR DIRECT ACQUISITION PREAMP FILTER ADC S R MEMORY RAM F i g u r e 5 Receiver c o n f i g u r a t i o n f o r d i r e c t a c q u i s i t i o n RECEIVER CONFIGURATION FOR SPECTRAL SEGMENT EXTRACTION PREAMP FILTER MIXER I SYNTHESIZER FILTER F i g u r e 6 Receiver c o n f i g u r a t i o n f o r s p e c t r a l segment e x t r a c t i o n or mixing. A N i c o l e t 2 9 3 - A programmable p u l s e r which has 16 memories and 7 output l i n e s i s used to send pulses to a p p r o p r i a t e d e v i c e s i n the spectrometer thereby c o n t r o l l i n g the experiment. The 2 9 3 - A i s under c o n t r o l of the computer so that p u l s e lengths can be e a s i l y a l t e r e d as w e l l as the pu l s e sequence changed to s u i t the experiment. Pulse lengths range from 3 2 ns to 2147 s. Pulses from the 2 9 3 - A can be used to t u r n devices on and o f f . A l s o , an i n t e r r u p t p u l s e can be sent to the computer which w i l l then determine what the value of the 2 9 3 - A program counter i s . D i f f e r e n t program counter numbers are a s s o c i a t e d with d i f f e r e n t subroutines that w i l l be executed by the computer when i t r e c e i v e s an i n t e r r u p t p u l s e . T h i s i s how the f r e -quency s y n t h e s i z e r outputs at the c o r r e c t time d u r i n g the pulse sequence a double resonance e j e c t i o n frequency, an e x c i t a t i o n frequency, band and a mixing frequency. A frequency sweep e x c i t a t i o n i s used to e x c i t e the c y c l o t r o n motion of the i o n s . A d e s i r e d frequency band i s covered i n steps of . 8 7 5 ys, each step being a c e r t a i n frequency jump. Thus to e x c i t e a band of 200 kHz u s i n g a step s i z e of 200 Hz w i l l take . 8 7 5 ms. Since the frequency s y n t h e s i z e r generates up to three d i f f e r e n t outputs on the same l i n e d u r i n g each p u l s e sequence, s w i t c h i n g c i r c u i t r y was b u i l t to route the output to the 7 a p p r o p r i a t e p l a c e . ' The s w i t c h i n g c i r c u i t r y i s a c t i v a t e d by pulses from the 2 9 3 - A . v.; E l e c t r o n E j e c t i o n When d e a l i n g w i t h negative i o n s , i t i s common t o t r a p s t r a y e l e c t r o n s i n the c e l l . I f n egative ions can be formed by the d i s s o c i a t i v e attachment of thermal e l e c t r o n s to n e u t r a l molecules, i . e . , CH^ONO + e~ >> CH^O" + NO", the t o t a l i o n c o n c e n t r a t i o n w i l l not be constant as a f u n c t i o n of time i f e l e c t r o n s are trapped i n the c e l l . The frequency of an e l e c t r o n i s much too high f o r c o n v e n t i o n a l double resonance techniques; that i s , a p p l i c a t i o n of an RP p u l s e to p l a t e s E and W at the frequency of an i o n t o be e j e c t e d . Beauchamp has shown t h a t an i o n i n an e l e c t r i c ''(trapping) f i e l d w i l l have a c e r t a i n o s c i l l a t o r y frequency i n the t r a p -• r . . . . . g p i n g . f i e l d . T h i s f a c t can be used to e j e c t e l e c t r o n s by a p p l y i n g an RP pulse to the t r a p p i n g p l a t e s . A Wavetek s i g n a l generator capable of g e n e r a t i n g s i g n a l s up to 30 MHz was used to e j e c t e l e c t r o n s . A simple delay c i r c u i t was used to send t h i s pulse to the c e l l a c e r t a i n time a f t e r the beam pu l s e had been completed to e j e c t any s t r a y e l e c t r o n s . T h i s technique y i e l d s t o t a l i o n i n t e n s i t i e s t h a t are constant with time i f the emission c u r r e n t i s high. T o t a l i o n i n t e n s i -t i e s that are constant with time can a l s o be obtained i f a low emmision c u r r e n t and no e l e c t r o n e j e c t i o n are used. C.. • Software i . FTZB and the Pulse Sequence PT-ICR experiments are under c o n t r o l of a program c a l l e d FTZB. T h i s program was w r i t t e n in' assembly language and i s approximately 8 k words i n l e n g t h l e a v i n g 16k,of - 1 9 -memory f o r data a c q u i s i t i o n and d i s p l a y purposes. FTZB i s an extremely powerful t o o l f o r running double resonance experiments and c o l l e c t i n g data f o r k i n e t i c experiments. T h i s program w i l l be e x p l a i n e d i n d e t a i l as a thorough knowledge of i t s workings manifests I t s e l f i n a r i g o r o u s understanding of how an FT-ICR experiment i s performed. The p u l s e sequence f o r FTZB i s g i v e n i n f i g u r e 7 where PC r e f e r s to the program counter number In the 2 9 3 - A which 9 s t a r t s at zero. The p u l s e sequence i n f i g u r e 7 i s e s s e n t i a l l y the same as the p u l s e sequence i n f i g u r e 1 w i t h the e x c e p t i o n t h a t d e t e c t i o n i s performed i n a d i f f e r e n t manner and p u l s e s f o r double resonance e j e c t i o n experiments have been added. Va r i o u s parameters c r i t i c a l t o the experiment such as s t a r t i n g frequency of the e x c i t a t i o n sweep, width of the e x c i t a t i o n sweep, d i g i t i z a t i o n r a t e , r e a c t i o n time, mixing frequency and beam l e n g t h are e i t h e r typed i n t o the computer or set u s i n g c o n t r o l knobs on the f r o n t of the computer. The p u l s e sequence i s i n i t i a t e d by a subroutine c a l l e d GO. The computer w i l l l o a d the 2 9 3 - A programmable p u l s e r w i t h t i m i n g words which t e l l the 2 9 3 - A how many pu l s e s are to be out-p u t t e d on each of i t s seven output l i n e s d u r i n g the pulse sequence and the d u r a t i o n of each p u l s e . Table 1 . l i s t s the output p u l s e s and t h e i r uses. The p u l s e sequence i s given i n f i g u r e 7 -CL •z o "2 (VI Cu Cf) (Vl Qu - 2 0 -CL Q_ UJ 2 1 -Table 1 Output Pulses Prom 2 9 3 - A and T h e i r Uses Pulse Number Use PO Quench P2 I n t e r r u p t P3 T r a n s m i t t e r s On P 4 R e c e i v e r On P5 Beam P6 or PCRES Reset PC. The f i r s t output o f the 2 9 3 - A i s sent back t o the computer, a process c a l l e d an i n t e r r u p t . R e c e i v i n g an i n t e r r u p t t e l l s the computer t o search i t s l i s t o f subroutines to f i n d the a p p r o p r i a t e one to execute when the PC i s at a c e r t a i n number. When the PC i s 1'(after a pu l s e i s i n i t i a t e d , the PC i s incremented by one so t h a t even though the zero p u l s e i s s t i l l on, the PC has a value o f 1 ) , the computer executes a subroutine that loads the Rockland frequency s y n t h e s i z e r w i t h the double resonance frequency i f a double resonance experiment i s b e i n g done, otherwise a frequency o f zero i s loaded. The l o a d i n g process takes . 3 ms. A f t e r . 3 ms, the output on l i n e P 2 goes from a high to a low value s i g n a l l i n g the Rockland frequency s y n t h e s i z e r to begin output of the double resonance frequency which i s sent to the c e l l . Simultaneously, the beam p u l s e ( P 5 ) which b i a s e s the g r i d to allow e l e c t r o n s i n t o the c e l l , forming i o n s , i s turned on. At t h i s p o i n t , ions are being formed i n the c e l l and ions of a c e r t a i n frequency are being e j e c t e d i f a double resonance experiment i s being done. A f t e r the beam p u l s e , t h e r e i s a delay o f l e n g t h DL - 2 2 -which means l e n g t h of double resonance pulse a f t e r beam. No output from t h e ' 2 9 3 - A i s used here, i t j u s t a c t s as a timer. A f t e r time DL, an i n t e r r u p t p u l s e (P2) i s again sent to the computer which executes a subroutine which loads the Rockland c o n t r o l l e r w i t h frequency sweep e x c i t a t i o n parameters. This i n t e r r u p t p u l s e e f f e c t i v e l y ends the double resonance e j e c t i o n p e r i o d as new parameters are loaded i n t o the Rockland. Pulse P2 i s kept on u n t i l time RT from the end of beam has passed. Loading the Rockland c o n t r o l l e r only takes a f r a c t i o n of a m i l l i s e c o n d but i n some experiments i t i s d e s i r a b l e to have a longer time between the end of double resonance e j e c t i o n and d e t e c t i o n . Frequency sweep e x c i t a t i o n begins when P2 i s turned o f f and P3 i s turned on. P3 a c t i v a t e s s w i t c h i n g c i r c u i t r y t h a t sends the frequency sweep e x c i t a t i o n t o p l a t e s E and W. So f a r , ions have been formed, some e j e c t e d , allowed to r e a c t f o r time RT and t h e i r c y c l o t r o n motion e x c i t e d by frequency sweep e x c i t a t i o n . When the e x c i t a t i o n i s complete, an i n t e r r u p t (P2) i s again sent to the computer from the 2 9 3 - A t e l l i n g the com-puter to loa d the mixing frequency MX, i n t o the Rockland I f a c q u i s i t i o n i s to be done v i a the mix mode as i n f i g u r e 6 . When P2 goes down, the Rockland begins to output the mixing frequency and simultaneously pulse P4 i s sent to the computer's ADC i n s t r u c t i n g i t t o begin d i g i t i z a t i o n of the ICR s i g n a l . The ICR s i g n a l has been taken from p l a t e s N and S, a m p l i f i e d , f i l t e r e d and sent to the ADC d i r e c t l y or mixed with a r e f e r e n c e frequency to e x t r a c t a band of f r e -quencies and s h i f t them to lower v a l u e s , thereby a l l o w i n g f o r slower d i g i t i z a t i o n r a t e s . .Output from the Rockland always" s t a r t s at the same phase v a l u e , t h e r e f o r e the d i g -i t i z e d ICR s i g n a l can be c o h e r e n t l y added from scan to scan to i n c r e a s e s e n s i t i v i t y . A c q u i s i t i o n time i s d i c t a t e d by d i g i t i z a t i o n r a t e and the number of data p o i n t s to be c o l l e c t e d . When a c q u i s i t i o n i s complete, PO (quench) r e v e r s e s the s i g n of the t r a p p i n g f i e l d , - t h e r e b y e j e c t i n g any ions remaining i n the c e l l , a process c a l l e d quench. Another i n t e r r u p t (P2) i s sent to the computer a f t e r the quench p u l s e , t e l l i n g the computer to l o a d a frequency of zero i n t o the Rockland, ending the mixing frequency output. The l a s t p u l s e , P6, i s sent to the computer i n s t r u c t i n g i t to set PC to zero. The p u l s e sequence i s repeated'over and over u n t i l the d e s i r e d number of scans, NS, has been completed. A f t e r each scan, the d i g i t i z e d ICR s i g n a l i s added to the values i n memory:. ;'.This Is known as s i g n a l averaging. I f n o i s e i n the system i s random, s i g n a l to n o i s e enhancement i s (NS) 2 where NS i s the number of scans. The d i g i t i z e d t r a n s i e n t . ICR s i g n a l now r e s i d e s i n memory where i t i s amenable to F o u r i e r t r a n s f o r m a t i o n , storage on d i s k or other treatment. i i . Double Resonance Ft-IGR double resonance experiments have r e c e n t l y been d i s c u s s e d elsewhere so only a c u r s o r y mention w i l l be made 10 31 here. ' ~ FTZB was w r i t t e n such that an i o n can be e j e c t e d f o r any p e r i o d of time up to e x c i t a t i o n , a l l o w i n g . 3 ms f o r l o a d i n g of the e x c i t a t i o n parameters i n t o the Rockland c o n t r o l l e r . Hence FTZB i s very v e r s a t i l e and as such i t i s a powerful t o o l f o r e l u c i d a t i n g Ion molecule r e a c t i o n path-ways v i a double resonance experiments. i i i . K i n e t i c Data A c q u i s i t i o n By FTZB One of the u s e f u l f e a t u r e s of ICR i s the a b i l i t y to determine r a t e constants f o r i o n molecule r e a c t i o n s . FTZB was w r i t t e n f o r the a c q u i s i t i o n of s p e c t r a at v a r i o u s r e a c t i o n times and the storage of s a i d s p e c t r a on d i s k f o r a n a l y s i s to determine r a t e constants. Since the d e n s i t y of ions i s orders of magnitude s m a l l e r than the n e u t r a l gas d e n s i t y at ICR p r e s s u r e s , i t i s assumed that the disappearance of an i o n i s a pseudo f i r s t order p r o c e s s , A " + B c " + D ( 2 - 2 ) The c o n c e n t r a t i o n of A Is simply [*1 - U"]t-O e" k , t < 2 " 3 > where k' i s the r a t e of disappearance of A and t i s time. k' i s converted to an absolute r a t e constant by d i v i d i n g k' by the n e u t r a l gas d e n s i t y . V a r i o u s options were w r i t t e n i n t o FTZB f o r the k i n e t i c experiments. The f i r s t r e a c t i o n time i s typed i n t o the com-puter along w i t h the r e a c t i o n time increment, t h a t i s how much the r e a c t i o n time changes between each spectrum. A l s o , the number of f i l e s ( s p e c t r a ) to a c q u i r e and the name of the f i r s t f i l e Is entered. A double resonance o p t i o n allows the l e n g t h of the double resonance p u l s e (DL) to be incremented between s p e c t r a by the r e a c t i o n time increment or kept constant. Thus, continuous e j e c t i o n of an i o n i s p o s s i b l e even though the r e a c t i o n time i s changing.. The f i l e names are i n the format FILE01.EXT where FILE and EXT are chosen to be d e s c r i p t i v e of the experiment and 01 i s the number of the f i r s t f i l e . F i l e numbers can range from 01 to 99• The f i r s t ICR t r a n s i e n t i s c o l l e c t e d at the I n i t i a l r e a c t i o n time and a u t o m a t i c a l l y s t o r e d on d i s k with the name FILE01.EXT. The r e a c t i o n time i s then incremented, a l s o DL i f d e s i r e d , and a second ICR t r a n s i e n t s i g n a l c o l l e c t e d and st o r e d on d i s k with the name FILE02.EXT. This process con-t i n u e s u n t i l the d e s i r e d number of f i l e s i s s t o r e d on d i s k . Three o p t i o n s , chosen at the s t a r t of the experiment, are a v a i l a b l e f o r treatment of the t r a n s i e n t s on the d i s k . 1) Leave the t r a n s i e n t s on d i s k as i s f o r f u t u r e a n a l y s i s . 2) A u t o m a t i c a l l y c a l l the t r a n s i e n t s o f f the d i s k and F o u r i e r t r a n s f o r m them, then s t o r e the transformed s p e c t r a back on the d i s k with the f i l e name FILEXY.EXT where XY i s a running number that begins with a value that i s one more than the number of the l a s t a c q u i r e d ICR t r a n s i e n t . 3 ) Do the same as opt i o n 2 except that the F o u r i e r t r a n s f o r m a t i o n i s over a longer data set than the o r i g i n a l d ata s e t . T h i s i s known as z e r o - f i l l i n g and i s d i s c u s s e d i n more d e t a i l i n Chapter 31. The maximum t r a n s f o r m a t i o n by FTZB i s 16K p o i n t s . No i n t e r -v e n t i o n by the operator i s needed a f t e r the i n i t i a l parameters and o p t i o n s have been s e l e c t e d and the s t a r t command, JO, has been given. Thus a c q u i s i t i o n of k i n e t i c data i s -26-automatic and f a s t , iv.. Peak P i c k i n g ICR frequency domain s p e c t r a c o n t a i n peaks which must be i d e n t i f i e d by frequency (or mass) and i n t e n s i t y . A program c a l l e d CUBS was w r i t t e n i n BASIC to do j u s t t h i s . B r i e f l y , CUBS operates i n the f o l l o w i n g manner. The operator types i n the f i l e name, s t a r t i n g frequency of the f i l e , s p e c t r a l width and number of p o i n t s i n the f i l e . The program t h e n . c a l l s the f i l e o f f the d i s k and c a l c u l a t e s the mean nois e f o r the spectrum and f i n d s the l a r g e s t peak simply by l o o k i n g f o r the maximum p o i n t i n the spectrum. Both the mean nois e and the maximum are p r i n t e d out. The operator then chooses a value f o r the t h r e s h o l d of a peak and types i t i n t o the computer. U s u a l l y the t h r e s h o l d i s at l e a s t f i v e times '; the value of the mean n o i s e . CUBS then searches f o r peaks u s i n g an a l g o r i t h m p u b l i s h e d by Cooper and a f t e r the search i s completed, outputs the frequency of each peak, i n t e n s i t y . 12 and r e l a t i v e i n t e n s i t y . R e l a t i v e i n t e n s i t y i s determined by comparing each peak t o the l a r g e s t peak which i s assign e d a value of 100. F o u r i e r t r a n s f o r m a t i o n of an ICR time domain s i g n a l w i l l r e s u l t i n a frequency domain spectrum. That i s , the spectrum i s l i n e a r i n frequency not mass. CUBS has an o p t i o n f o r c o n v e r t i n g f r e q u e n c i e s i n t o masses and f o r p l o t t i n g a spectrum l i n e a r l y i n mass. The accuracy o f c o n v e r s i o n of fr e q u e n c i e s i n t o masses i s d i s c u s s e d in' Chapter 3- I t w i l l be seen that mass c a l i b r a t i o n s done by FT-ICR are b e t t e r than mass c a l i b r a t i o n s done by c o n v e n t i o n a l ICR. v.. Rate Determinations Peak p i c k i n g of s p e c t r a with many p o i n t s i s very time consuming. T h e r e f o r e , a second program c a l l e d PIK was w r i t t e n i n BASIC to get the peak i n t e n s i t i e s i n many s p e c t r a but the f r e q u e n c i e s of the peaks must be known bef o r e u s i n g t h i s program. PIK simply c a l l s a spectrum o f f the d i s k , goes to the f i r s t frequency that was entered by the operator and p r i n t s out the i n t e n s i t y at that frequency. I t then goes to the next frequency t h a t was input and p r i n t s out the i n t e n s i t y at that frequency, e t c . . Output f o r each f i l e i s frequency, i n t e n s i t y and r e l a t i v e i n t e n s i t y . PIK does t h i s f o r the d e s i r e d number of f i l e s . A l s o i n c l u d e d i n PIK i s an o p t i o n f o r k i n e t i c a n a l y s i s . A frequency i s s e l e c t e d , pressure of the n e u t r a l gas entered and the c a l i b r a t i o n c o e f f i c i e n t f o r the i o n i z a t i o n gauge i s a l s o typed i n t o the computer. PIK then determines the i n t e n s i t y of the peak at that frequency from each spectrum that was recorded at d i f f e r e n t r e a c t i o n times by FTZB. From a l e a s t squares a n a l y s i s , a slope of the decay of t h a t i o n with time i s determined along with a c o r r e l a t i o n c o e f f i c i e n t The slope i s converted to an absolute r a t e constant by d i v i d i n g by the number d e n s i t y of n e u t r a l molecules. Rates are p r i n t e d out i n u n i t s of cm /molecule-sec. Using the combination of FTZB and PIK, r a t e determinations are f a i r l y f a s t . The standard r e a c t i o n f o r r a t e determinations i n the f i e l d of i o n molecule r e a c t i o n s i s - 2 8 -C H 4 + + CH^ — T C H 5 + + .CH 3 (2-4) An average r a t e of 1.11 x 10"^ cm^/molecule-sec has been obtained f o r t h i s r e a c t i o n u s i n g v a r i o u s experimental 13 techniques. J F i g u r e 8 shows some CH^ data which was obtained by u s i n g FTZB and PIK. Using the c a l i b r a t i o n from f i g u r e 3 , the r a t e of decay of C H ^ was found to be 1.11 + .02 x 10~ 9 3 cm /molecule-secj i n good agreement with the l i t e r a t u r e value. o-, eO to A z U J a a a a CHi i i 1 1 1 1 1 1 1 1 ZO 6 0 100 n-o ISO T I M E IM NAtLU SECONDS Figure 8 .Time plot for the reaction CH^ "*" + CH^ •—> CE^+ + • CH - 3 0 -REPERENCES AND NOTES 1 . T. A. Lehman and M. M. Bursey, "Ion C y c l o t r o n Resonance Spectrometry", John Wiley and Sons, New York, 1 9 7 6 . 2 . J . I. Brauman and L. K. B l a i r , Ion C y c l o t r o n Resonance Spectroscopy, i n P. C. Nachod and.J. J . Zuckerman (E d s . ) , Determination of Organic S t r u c t u r e s by P h y s i c a l Methods, V o l . 5 , Academic, New York, 1 9 7 6 . 3 . J . L. Beauchamp, Ion C y c l o t r o n Resonance Spectroscopy, Ann. Rev. Phys. Chem.."22, 5 2 7 ( 1 9 7 D -4 . M. B. Comisarow i n "Advances i n Mass Spectrometry", N. R. D a l y ' ( E d . ) , Heyden and Son L t d . , London, 1 9 7 8 and r e f e r e n c e s t h e r e i n . 5 . R. T. Mclver J r . , Rev. S c i . Instrum. 4 1 , 5 5 5 ( 1 9 7 0 ) . 6 . N i c o l e t Instrument C o r p o r a t i o n , Madison, Wisconsin. 7 . The s w i t c h i n g c i r c u i t r y was b u i l t by Dr. Ge'rald P a r i s o d . 8 . J . L. Beauchamp and J . T. Armstrong, Rev. S c i . Instrum. 4 0 , 1 2 3 ( 1 9 6 9 ) . 9 . The pulse sequence was developed with the a i d of V a l e r i o Grass!. 1 0 . V a l e r i o G r a s s i , M.Sc. T h e s i s , i n p r e p a r a t i o n . 1 1 . M = B. Comisarow, V. G r a s s i and G. P a r i s o d , Chem. Phys. L e t t . 5 7 , 4 1 3 ( 1 9 7 8 ) . 1 2 . James W. Cooper, "The Minicomputer i n the Laboratory", John Wiley and Sons, 1 9 7 7 , pp. 2 4 8 - 2 5 5 -1 3 . W. T. Huntress, J . B. Laudenslager and R. F. P i n i z z o t t o , I n t . J . Mass Spectrom. Ion Phys., 1 3 , 3 3 1 ( 1 9 7 4 ) . - 3 1 -I I I . DIGITAL TECHNIQUES AND THEIR APPLICATIONS TO FT-ICR A. An I n t r o d u c t i o n to Problems With D i s c r e t e A n a l y s i s 1, L i m i t e d memory s i z e FT-ICR i s a r e l a t i v e l y new method f o r o b t a i n i n g ICR s p e c t r a . Fundamental d i f f e r e n c e s between FT-ICR and con-v e n t i o n a l ICR l e a d to d i f f e r e n t techniques f o r data a c q u i -s i t i o n and data r e d u c t i o n . I t i s the purpose of t h i s chapter to d i s c u s s these techniques as a p p l i e d to FT-ICR. The g e n e r a t i o n of t r a n s i e n t ICR s i g n a l s has been d i s -cussed i n Chapter 2 . The a c q u i s i t i o n of the t r a n s i e n t s i g n a l s which e x i s t i n FT-ICR must s a t i s f y the Nyquist Sampling Theorem which s t a t e s t h a t the s i g n a l must be sampled at a r a t e which exceeds twice the band width of the s i g n a l . For an FT-ICR spectrometer o p e r a t i n g at 2 0 kGauss, the c y c l o t r o n f r e q u e n c i e s extend from about 1 MHz to 3 0 kHz f o r a mass range of m/e = 3 0 to 1 0 0 0 . A c q u i s i t i o n of t h i s broad band s i g n a l r e q u i r e s a sampling r a t e of at l e a s t 2 MHz, much too f a s t f o r d i r e c t a c q u i s i t i o n onto a magnetic d i s k . Hence, the number of data p o i n t s that can be a c q u i r e d i s t h e r e f o r e r e s t r i c t e d by the random access memory s i z e of the computer. R e s o l u t i o n i n FT-ICR has been shown to be i n v e r s e l y p r o p o r t i o n a l to mass and dependent upon the a p p l i e d magnetic 1 f i e l d as w e l l as the computer memory a v a i l a b l e . T h e r e f o r e , a c q u i s i t i o n of a broad band s i g n a l w i t h i n the framework of l i m i t e d memory s i z e can l e a d to problems due to the l a c k of p o i n t s to a c c u r a t e l y d e s c r i b e s p e c t r a l l i n e s h a p e s . Accurate f r e q u e n c i e s and i n t e n s i t i e s are mandatory f o r the study of ion-molecule r e a c t i o n s , i i . D i s c r e t e v. Continuous In: experimental p r a c t i c e , the' continuous time domain response ( i . e . , the t r a n s i e n t ICR s i g n a l ) i s not a n a l y t i c a l l y transformed to produce a continuous frequency spectrum. Rather, the time domain response i s sampled at a f i n i t e number of p a r t i c u l a r p o i n t s i n time to produce a d i s c r e t e time domain response. The d i s c r e t e time domain response i s then n u m e r i c a l l y transformed to produce a d i s c r e t e frequency 2 spectrum. The d i s c r e t e frequency spectrum i s d e f i n e d at M s p e c i f i c f r e q u e n c i e s g i v e n by, m/T Hz, m = 0 , 1 , 2 • ' M ( 3 - D where T i s the a c q u i s i t i o n time of the time domain s i g n a l . C l e a r l y , the spacing In the d i s c r e t e frequency spectrum i s 1/T Hz. Unless the frequency to be determined happens to be e x a c t l y one of the p a r t i c u l a r f r e q u e n c i e s given by equation 3 - l 3 the i n t e n s i t i e s i n the d i s c r e t e frequency spectrum w i l l not correspond to the peak maximum i n the continuous frequency spectrum. F i g u r e s 9 and 1 0 show two continuous l i n e s h a p e s , A and B, where the maximum of A i s e x a c t l y on one of the d i s c r e t e f r e q u e n c i e s of a d i s c r e t e frequency spectrum and the maximum of B i s half-way between the two d i s c r e t e f r e q u e n c i e s . S t r a i g h t l i n e connection of the amplitudes i n the d i s c r e t e frequency spectrum leads to the d i s c r e t e l i n e s h a p e s A* and B*. -33-F i g u r e 9 . Continuous and d i s c r e t e magnitude mode s p e c t r a l peaks. Curves A and B are continuous s p e c t r a l peaks of i d e n t i c a l l i n e s h a p e and i n t e n s i t y which were c a l c u l a t e d from equation 3-5 with a value of T/T = 1.0. The maximum of Curve A f a l l s e x a c t l y on one of the f r e q u e n c i e s of the d i s c r e t e frequency spectrum l a b e l l e d n = 0. The maximum of Curve B f a l l s e x a c t l y h a l f way between two p o i n t s of the d i s c r e t e frequency spectrum l a b e l l e d n = 0. Curves A* and B* are d i s c r e t e frequency s p e c t r a l l i n e s h a p e s formed by s t r a i g h t l i n e connection of the p o i n t s i n the d i s c r e t e spectrum l a b e l l e d n = 0. The d i s c r e t e frequency spectrum l a b e l l e d n = 1 has twice the r e s o l u t i o n of the n = 0 d i s c r e t e spectrum. The open c i r c l e s are the values of Curve A i n the n = 1 d i s c r e t e spectrum. A A N=Of N= 1 t Magnitude B B' i t t t t t t t t t t t t t t r t t t g u r e 9 --35-Flgure. 10,. Continuous and d i s c r e t e a b s o r p t i o n mode s p e c t r a l peaks. Curves A and B are continuous s p e c t r a l peaks of i d e n t i c a l l i n e s h a p e and i n t e n s i t y which were c a l c u l a t e d from equation 3-4 with a value of T/T = 1.0. The maximum of Curve A f a l l s e x a c t l y on one of the f r e q u e n c i e s of the d i s c r e t e frequency spectrum l a b e l l e d n = 0. The maximum of Curve B f a l l s e x a c t l y h a l f way between two p o i n t s of the d i s c r e t e frequency spectrum l a b e l l e d n = 0. Curves A' and B' are d i s c r e t e frequency s p e c t r a l l i n e s h a p e s formed by s t r a i g h t l i n e connection of the p o i n t s i n the d i s c r e t e spectrum l a b e l l e d n = 0. The d i s c r e t e frequency spectrum l a b e l l e d n = 1 has twice the r e s o l u t i o n of the n = 0 d i s c r e t e spectrum. The open c i r c l e s are the values of Curve A i n the n = 1 d i s c r e t e spectrum. Absorption B B' N = 0 t N = H • i CA I t < t t t • • t i t t t t t t Figure 10. - 3 7 -I t i s obvious that A' p r o v i d e s an exact estimate of the amplitude and frequency of A. A' a l s o p r o v i d e s a reasonable approximation to the l i n e w i d t h of A. However, the amplitude, the frequency and the l i n e s h a p e of B are p o o r l y estimated by B' .. The problem i l l u s t r a t e d i n f i g s . 9 3 1 0 has been r e c o g n i z e d i n the l i t e r a t u r e . Two g e n e r a l s o l u t i o n s are a v a i l a b l e , curve f i t t i n g and u s i n g more p o i n t s . B.. S o l u t i o n s To The D i s c r e t e P o i n t s Problem i . Curve F i t t i n g For cases i n which the a n a l y t i c a l l i n e s h a p e i s known i n advance with only the l o c a t i o n of the peak maximum and the i n t e n s i t y of the peak maximum being unknown, curve f i t t i n g techniques may be used to f i t the few p o i n t s i n the v i c i n i t y of the peak maximum to the a n a l y t i c a l l i n e s h a p e . In t h i s way, the peak maximum and frequency are obtained. FT-ICR l i n e s h a p e s are complex as shown by 3 - 4 and 3 - 5 . The l i n e s h a p e i s determined by the value of T/x where T i s a c q u i s i t i o n time and T i s the r e l a x a t i o n time of the t r a n s i e n t ICR s i g n a l . x i s not the same f o r a l l peaks i n a spectrum as x i s r e l a t e d to the mass of the ions i n q u e s t i o n . T h e r e f o r e , a s l i g h t l y d i f f e r e n t l i n e s h a p e needs to b e ^ f i t to each peak. T h i s i s a c h a l l e n g i n g problem that was not gone i n t o deeply because p r e l i m i n a r y c a l c u l a t i o n s showed that f i t t i n g experimen-3 t a l p o i n t s to FT-ICR l i n e s h a p e s g i v e s poor r e s u l t s and other techniques are a v a i l a b l e that are of proven q u a l i t y . - 3 8 -i i , S p e c t r a l Segment E x t r a c t i o n or Mixing S p e c t r a l segment e x t r a c t i o n techniques have been used p r e v i o u s l y t o demonstrate the r e s o l u t i o n a c h i e v a b l e i n FT-ICR. In t h i s procedure, the time domain FT-ICR s i g n a l i s m u l t i -p l i e d by a r e f e r e n c e s i g n a l ( i . e . mixing s i g n a l ) to produce output s i g n a l s at the sum and d i f f e r e n c e f r e q u e n c i e s between the two input s i g n a l s . T h i s output s i g n a l Is then passed through a low-pass f i l t e r which e x t r a c t s j u s t the d i f f e r e n c e frequency s i g n a l . The net e f f e c t of t h i s process i s to e x t r a c t a band of ICR f r e q u e n c i e s which are s h i f t e d down i n frequency. Slower d i g i t i z a t i o n r a t e s are now p o s s i b l e due to the lower f r e q u e n c i e s being sampled. As r e s o l u t i o n i s p r o p o r t i o n a l to o b s e r v a t i o n time, extremely high r e s o l u t i o n i s p o s s i b l e by u s i n g a r e f e r e n c e frequency very c l o s e to the frequency of a p a r t i c u l a r i o n . F i g u r e 1 1 shows the HNCT/CH^O doublet formed from the d i s s o c i a t i v e attachment of thermal - 9 e l e c t r o n s t o CH^ONO at a p r e s s u r e of % 1 x 1 0 T o r r . The c y c l o t r o n f r e q u e n c i e s of these ions i n a 2 0 kGauss f i e l d are 9 9 0 , 5 ^ 3 Hz and 9 9 0 , 1 4 1 ' H z r e s p e c t i v e l y . By mixing the time-domain ICR s i g n a l from these ions with a frequency of 9 8 9 kHz and d i g i t i z i n g at 1 6 kHz , r e s o l u t i o n of (M/AM^50%) 5 0 0 , 0 0 0 i s obtained. Mixing i s the method of choice f o r o b s e r v i n g ions that have s i m i l a r c y c l o t r o n f r e q u e n c i e s as extremely high r e s o l u t i o n i s p o s s i b l e . However, to o b t a i n a spectrum cover-i n g a wide mass range, r e p e a t i n g t h i s technique many times i s neccessary, a process that i s not used i n p r a c t i c e . - 3 9 -CH30" m/e = 31.01839 ^ = 5 0 0 , 0 0 0 — NOH" m/e = 31.00581 i i i 1 1 31.000 31.005 31.010 31.015 31.020 Figure 11 CH^O /HNO doublet obtained using the "mixing" technique. - 4 0 -i i i . Z e r o - f i l l i n g Another technique that has been found to be u s e f u l i n 5 - 9 many a p p l i c a t i o n s i s z e r o - f i l l i n g . In t h i s method, the time domain data t a b l e i s extended by adding zeroes to the end of the sampled t r a n s i e n t s i g n a l p r i o r to F o u r i e r t r a n s -formation. U s u a l l y zeroes are added u n t i l the data t a b l e l e n g t h has been extended by a f a c t o r of 2 n where n = 1 , 2 • • • , but t h i s procedure i s v a l i d f o r any p o s i t i v e value of n. In t h i s way, the e f f e c t i v e " z e r o - f i l l e d a c q u i s i t i o n time", T , i s now longer and the spacing i n the d i s c r e t e frequency spectrum i s now reduced to 1/T from 1/T. T T x 2 n ( 3 - 2 ) In the l i m i t , n — • °°, the numerical F o u r i e r t r a n s f o r m becomes I d e n t i c a l to the a n a l y t i c a l F o u r i e r t r a n s f o r m and the d i s c r e t e frequency spectrum becomes i d e n t i c a l to the continuous frequency spectrum. A f u r t h e r advantage of t h i s i n t e r p o l a t i o n method i s that the f i r s t set of added zeroes i n c r e a s e s the s i g n a l to noise r a t i o of the f i n a l a b s o r p t i o n mode spectrum. F u r t h e r z e r o - f i l l i n g s w i l l only i n t e r p o l a t e to the c o r r e c t l i n e s h a p e . I t i s the c o r r e c t l i n e s h a p e that i s d e s i r e d i n FT-ICR ex-periments so the p o i n t of how much z e r o - f i l l i n g i s needed w i l l be pursued i n some d e t a i l . CV Accuracy Obtained by Z e r o - f i l l i n g i . I n t e n s i t i e s While the e f f e c t of z e r o - f i l l i n g has been known f o r q u i t e some time, we are unaware of q u a n t i t a t i v e c r i t e r i a f o r determining the number of z e r o - f i l l i n g s which i s r e q u i r e d f o r a p a r t i c u l a r accuracy. T h i s c r i t e r i u m i s r e q u i r e d f o r FT-ICR experiments because accurate i n t e n s i t i e s and f r e -quencies are c r u c i a l to the success of an experiment. Determination of the e r r o r r e s u l t i n g from f i n i t e z e r o -f i l l i n g of F o u r i e r t r a n s f o r m f a r a d a i c admittance data has o r e c e n t l y been examined by Smith. However, no g u i d e l i n e s were given f o r ge n e r a l a p p l i c a t i o n s of t h i s technique. Also. H o r l i c k has shown the e r r o r i n peak maximum measurements as a f u n c t i o n of the number of p o i n t s above the half-maximum 7 f o r v a r i o u s l i n e s h a p e s . We w i l l d e r i v e here expressions f o r the accuracy of peak maximum de t e r m i n a t i o n as a f u n c t i o n of T/T. That i s , the only parameter r e q u i r e d i s how much the time domain s i g n a l has decayed d u r i n g the o b s e r v a t i o n time. Consider a continuous time domain s i g n a l of the form F ( t ) = exp(-T/x)coscot 0 < t < T ( 3 - 3 ) The continuous a b s o r p t i o n mode frequency spectrum ( i . e . the a n a l y t i c a l F o u r i e r transform) of 3 - 3 i s " 1 " 1 A ( ' A w ) = . _ r — - | l + e x p ( - T / T ) ( ( A w ) T S i n ( ( A w ) T ) - c o s ( ( A w ) T ) ) ) l + ( A o ) ) V \ I ( 3 - 4 ) The continuous magnitude mode frequency spectrum of 3 - 2 i s ^ 1 -42-C(Au>.) =/ T. \^/l-2e.xp(-T/T)cos( (Ao))T)+exp (-2T/T )\ ( 3 - 5 ) t l + ( A . . ) 2 T 2 J 1 I In equations 3 - 3 to 3 - 5 , T i s the r e l a x a t i o n time of the s i g n a l , AOJ i s the frequency d i s t a n c e from the maximum (Ao)=o)-w where w' i s the frequency at the peak maximum) and T i s the time over which the s i g n a l was observed. Equations 3 - 3 to 3 - 5 are very g e n e r a l equations and are a p p l i c a b l e to many forms of spectroscopy and i n p a r t i c u l a r to ICR and NMR. Now, when 3 - 3 i s sampled at a s e r i e s of d i s c r e t e , times, the d i s c r e t e frequency spectrum r e s u l t i n g from F o u r i e r t r a n s f o r m a t i o n w i l l e x i s t only at the p a r t i c u l a r f r e q u e n c i e s given by 3 - 1 - Thus, the only allowed values f o r the d i s t a n c e between the p o i n t s i n the d i s c r e t e frequency spectrum are given by Af = 1/T (Hz) ( 3 - 6 ) i f no z e r o - f i l l i n g i s done p r i o r to F o u r i e r t r a n s f o r m a t i o n , and by Af = 1/T = (Hz) n = 0, 1, 2 ' * * ( 3 - 7 ) z 2 " T i f the time domain s i g n a l i s z e r o - f i l l e d n times p r i o r to F o u r i e r t r a n s f o r m a t i o n . For a continuous s p e c t r a l peak which happens to f a l l e x a c t l y on one of the d i s c r e t e f r e q u e n c i e s of the d i s c r e t e frequency spectrum, as curve A i n f i g u r e s 9 and 10, the i n t e n -s i t y of the l i n e s h a p e w i l l be d e f i n e d only at the d i s c r e t e frequency values g i v e n by - 4 3 -A w = 2TTN N = ± 0, 1, 2 ( 3 - 8 ) 2 N T where n i s the number of z e r o - f i l l i n g s . For a value of N = +1, the i n t e n s i t y of the peak at the f i r s t d i s c r e t e frequency above the peak maximum w i l l be obtained. S u b s t i t u t i n g equation 3 - 8 with a value of N •= +1 i n t o 3 - 4 g i v e s A(Aoo) = T ^ l + Y _ y 1 ^ l + e " Y ( Y s i n ( Y ) - c o s ( Y ) ^ ( 3 - 9 ) X where X = T/T ( 3 - 1 0 ) and Y = 2JT ( 3 - H ) 2 n Equation 3 - 9 g i v e s the d i s c r e t e i n t e n s i t y value f o r the f i r s t d i s c r e t e frequency above the peak maximum ( i . e . N = +1) f o r an a b s o r p t i o n mode li n e s h a p e as a f u n c t i o n of X, the r a t i o of the a c q u i s i t i o n time T, to the r e l a x a t i o n time T, and n, the number of z e r o - f i l l i n g s . Equation 3-12 i s the correspond-i n g e x p r e s s i o n f o r a d i s c r e t e magnitude l i n e s h a p e . C(AUJ) = T ( l + ( Y / X ) 2 ) ~ ^ ( l - 2 e x p ( - X ) c o s ( Y ) + e x p ( - 2 X ) ) ( 3 - 1 2 ) Examination of f i g u r e 9 leads to a systematic procedure f o r d etermining the maximum amplitude e r r o r due to the f i n i t e frequency r e s o l u t i o n of a d i s c r e t e frequency spectrum. Curve A i n f i g u r e 9 i s a continuous mode li n e s h a p e c a l c u l a t e d from 3 - 3 f o r T/T = 1.0. Curve B i n f i g u r e 9 i s a continuous l i n e -shape which i s i d e n t i c a l , to Curve A but i s at a d i f f e r e n t - 4 4 -frequency. I f peak A was obtained by sampling a time domain s i g n a l and F o u r i e r t r a n s f o r m a t i o n , and i f the maximum of Curve A was e x a c t l y on one of the p o i n t s i n the d i s c r e t e f r e -quency spectrum, the d i s c r e t e magnitude mode l i n e s h a p e obtained by connecting the p o i n t s i n the d i s c r e t e frequency spectrum would be A 1. I f the sampling c o n d i t i o n s were appro-p r i a t e f o r Curve A, then the maximum of Curve B would be i n c o r r e c t l y i n d i c a t e d by the d i s c r e t e magnitude l i n e s h a p e , Curve B*. F i g u r e 10 i s the same as f i g u r e 9 except that a b s o r p t i o n mode lin e s h a p e i s used. I f the t r a n s i e n t which leads to Curve A was z e r o - f i l l e d once (n=l) p r i o r to F o u r i e r t r a n s f o r m a t i o n , the p o i n t s i n -d i c a t e d as open c i r c l e s i n f i g u r e 9 (and 1 0 ) would be obtained f o r s p e c t r a l peak A. Since the maximum f o r the continuous peak B f a l l s e x a c t l y half-way between two of the d i s c r e t e f r e q u e n c i e s of the non z e r o - f i l l e d (n= 0 ) d i s c r e t e frequency spectrum, measurement of s p e c t r a l peak B' w i l l l e a d to the worst p o s s i b l e estimate f o r the frequency and the amplitude of Curve B. I f peak B was l e s s than half-way between two f r e -quencies of the n=0 d i s c r e t e spectrum, B* would have an amplitude c l o s e r to that of Curve B. Comparison of Curve B with the open c i r c l e s of Curve A leads to the f o l l o w i n g c o n c l u s i o n : The minimum amplitude f o r a peak which f a l l s between two p o i n t s of a non z e r o - f i l l e d spectrum w i l l be equal to the amplitude of the f i r s t (N=l) p o i n t away from the maximum of the " z e r o - f i l l e d once" (n=l) spectrum of a peak - 4 5 -whose maximum f a l l s e x a c t l y on one of the. f r e q u e n c i e s of the non z e r o - f i l l e d (n=0). spectrum. The preceeding s t a t e -ments may be g e n e r a l i z e d t o : (Maximum r e l a t i v e e r r o r \ / obtained a f t e r n J =[1.0 - equation 3-12 (n=n+l) \ z e r o - f i l l i n g s / V , . _ .„ . . I > equation 3-12 (n=°°) / and Maximum r e l a t i v e e r r o r obtained a f t e r n ] =[1.0 - equation 3-9 z e r o - f i l l i n g s (3-13) ;n=n+l) \ ( n = o o ) J N equation 3-9 ( 3 - 1 4 ) Equation 3-13 g i v e s the maximum r e l a t i v e e r r o r f o r a mag-nitude mode li n e s h a p e as a f u n c t i o n of n and T/T. Equation 3 - 1 4 i s the corresponding a b s o r p t i o n mode ex p r e s s i o n . Equations 3 - 1 3 5 l 4 depend upon the r a t i o T/T but are Independent of the absolute values of T and T. F i g u r e 12 shows the percentage amplitude e r r o r r e s u l t i n g from f i n i t e z e r o - f i l l i n g f o r the a b s o r p t i o n mode l i n e s h a p e . As expected, the e r r o r ' i s r a p i d l y reduced by extended zero-f i l l i n g p r i o r to F o u r i e r t r a n s f o r m a t i o n . As the time domain s i g n a l i s r e l a x e d d u r i n g the a c q u i s i t i o n time ( i n c r e a s i n g T / T ) , the magnitude of the amplitude e r r o r f o r f i x e d n a l s o becomes l e s s . F i g u r e 13 shows the percentage amplitude e r r o r f o r the magnitude mode l i n e s h a p e . The dependence upon n and T/T i s the same as i n the case f o r the a b s o r p t i o n mode. However, the amplitude e r r o r f o r the magnitude mode i s l e s s f o r any -46-F i g u r e 1 2 , R e l a t i v e e r r o r due to n o n i n f i n i t e z e r o - f i l l i n g f o r an ab s o r p t i o n mode l i n e s h a p e . Each curve gives the maximum percentage e r r o r f o r a p a r t i c u l a r r a t i o of T/T as a f u n c t i o n of the number of z e r o - f i l l i n g s . The e r r o r at i n t e g r a l values of n I s the most important but the curves are v a l i d f o r n o n i n t e g r a l values of n a l s o . The curves were c a l c u l a t e d from equation 3 - 1 4 . Note that the e r r o r s c a l e i s d i f f e r e n t than that of f i g u r e 1 3 • Absorption mode Number of zero fillings Figure 12. - 4 8 -F i g u r e 1 3 , R e l a t i v e e r r o r due to n o n i n f i n i t e z e r o - f i l l i n g f o r a magnitude mode li n e s h a p e . Each curve giv e s the maximum percentage e r r o r f o r a p a r t i c u l a r r a t i o of T/T as a f u n c t i o n of the number of z e r o - f i l l i n g s . The e r r o r at i n t e g r a l values of n i s the most important but the curves are v a l i d f o r n o n i n t e g r a l values of n a l s o . The curves were c a l c u l a t e d from equation 3 - 1 3 - Note that the e r r o r s c a l e i s d i f f e r e n t than that of f i g u r e 1 2 . Magnitude mode 0 1 2 3 4 5 0 2 3 4 5 0 1 2 3 4 5 0 1 2 3 4 5 Number of zero fillings I Figure 1 3 -g i v e n values of n and T/T than f o r the a b s o r p t i o n mode. This i s because the magnitude l i n e s h a p e i s broader than the a b s o r p t i o n l i n e s h a p e . One gen e r a l c o n c l u s i o n which f o l l o w s d i r e c t l y from f i g u r e s 12 and 1 3 i s that three z e r o - f i l l i n g s are enough to decrease the amplitude e r r o r to a few per cent f o r undamped t r a n s i e n t s . For more damped t r a n s i e n t s , l e s s z e r o - f i l l i n g i s neccessary. i i . F requencies A c q u i s i t i o n time,T, i s r e l a t e d to the dwellAtime,DW, or the time that the d i g i t i z e r spends on each p o i n t and the number of p o i n t s t o be a c q u i r e d , #pts., by the formula T = DW x #pts. ( 3 - 1 5 ) where DW Is i n s e c / p o i n t . S u b s t i t u t i n g 3 - 1 5 i n t o 3 - 6 y i e l d s Af = 1 ( 3 - 1 6 ) DW x #pts The number of r e a l p o i n t s i n the frequency domain i s #pts/2 because the F o u r i e r t r a n s f o r m a l g o r i t h m used produces both r e a l and imaginary c o e f f i c i e n t s even i f a l l the data i s 10 r e a l . Equation 3 - 1 5 now becomes Af = 1 ( 3 - 1 7 ) 2 x DW x #pts i n p r a c t i c e . Af/2 i s t h e r e f o r e the maximum e r r o r i n the frequency d e t e r m i n a t i o n of a peak. F i g u r e 14 shows a p l o t of mass v. AM f o r v a r i o u s d w e l l times•where mass i s i n amu and AM i s the maximum e r r o r i n mass de t e r m i n a t i o n as found from -51-F i g u r e 14. P l o t of mass v. AM f o r v a r i o u s values of DW, The curves were c a l c u l a t e d from' equation 3-18 where i t i s assumed that B = 20 kGauss and #pts = 16k. DW i s i n u s e c / p b i n t . AM = 1 . 0 3 7 x l o " 7 (m 2/qB)Af ( 3 - 1 8 ) where B'ls i n kGauss, q i s In m u l t i p l e s of elementary charge 1 1 and m i s mass i n AMU. F i g u r e 14 was obtained assuming d i r e c t a c q u i s i t i o n of s i g n a l , i . e . no mixing. I t i s seen that when DW i s low (high sampling r a t e ) , the e r r o r i n mass de t e r m i n a t i o n at the high mass end of the s c a l e becomes ex c e s s i v e . By u s i n g higher values of DW (lower sampling r a t e ) , AM can be reduced, but only at the expense of mass range. F o r t u n e a t e l y , z e r o - f i l l i n g can be used i n such a way that allows a broad band spectrum to be recorded i n one experiment that g i v e s a c c e p t a b l e values f o r AM. Combining 3 - 7 and 3 - 1 5 y i e l d s Af = 1 ( 3 - 1 9 ) 2 x DW x #pts x 2 n where n i s a p o s i t i v e i n t e g e r equal to the number of times the o r i g i n a l data set was extended by a f a c t o r of two .with zeroes p r i o r to F o u r i e r t r a n s f o r m a t i o n . F i g u r e 1 5 shows the e f f e c t of z e r o - f i l l i n g on.AM f o r the case where DW i s h e l d constant. By comparing f i g u r e s 14 and 1 5 , i t i s apparent that z e r o - f i l l i n g and i n c r e a s i n g DW have the same e f f e c t on AM with the important d i f f e r e n c e that with z e r o - f i l l i n g the mass range can be h e l d constant at any value. I t must be emphasized that while z e r o - f i l l i n g can improve AM, the accuracy of mass d e t e r m i n a t i o n , i t can not improve r e s o l u t i o n . R e s o l u t i o n enhancement can only be achieved by o b s e r v i n g the t r a n s i e n t f o r a longer time. A - 5 4 -F i g u r e 15. P l o t of mass. v. AM for. various, values of n where n = number of z e r o - f i l l i n g s . The curves were c a l c u l a t e d from equation 3-18 where i t i s assumed that B = 20 kGauss, #pts = 16k and DW i s constant at .5 u s e c / p o i n t . program w r i t t e n at N i c o l e t Instrument C o r p o r a t i o n c a l l e d DNMR allows us to do d i s k based F o u r i e r transforms of 512 k words. Th i s allows us to extend a 16k data set by a f a c t o r of 2 or z e r o - f i l l i n g 5 times, D. Leakage One problem that i s encountered with z e r o - f i l l i n g i s the appearance of s i d e bands on peaks when a t r a n s i e n t t hat 13 has not decayed 100% i s extended by zeroes-. The appearance of these si d e bands i s commonly r e f e r r e d to as leakage i n the spectrum. These s i d e lobes are not caused by z e r o - f i l l i n g , they e x i s t due to t r u n c a t i o n of the t r a n s i e n t . However, s i n c e the F o u r i e r t r a n s f o r m process produces both r e a l and imaginary p a r t s , the p o i n t s that d e f i n e the s i d e lobes seem to be m i s s i n g u n l e s s one power-of-two of z e r o - f i l l i n g i s used to r e cover them. T h i s Is r e a d i l y seen i n f i g u r e 9 where the open c i r c l e s r e p r e s e n t p o i n t s that are obtained a f t e r z e r o - f i l l i n g once. I t i s seen that with the a d d i t i o n of these p o i n t s , s i d e lobes have appeared t h a t were not v i s i b l e i n the u n f i l l e d spectrum. I t i s neccessary to reduce leakage i n FT-ICR experiments as small peaks i n the v i c i n i t y of l a r g e r peaks can e a s i l y be l o s t i n the leakage. V a r i o u s window f u n c t i o n s such as Hanning, e x p o n e n t i a l and t r a p e z o i d a l are a v a i l a b l e i n software pack-ages to reduce leakage. A p p l i c a t i o n of a Hanning window f u n c t i o n p r i o r to e x t e n s i o n of the data set by zeroes has been found to reduce leakage i n FT-ICR experiments to accept-able l e v e l s . 1 ^ There i s some l i n e broadening with t h i s window -57-f u n c t i o n but the leaka g e i s removed and t h e peak I n t e n s i t i e s have the same r e l a t i v e v a l u e s . F i g u r e 16 shows a broad band spectrum o f t r i s ( p e r f l u o r o -h e p t y l ) - s - t r i a z i n e o b t a i n e d by e x t e n s i o n o f the o r i g i n a l 16-k d a t a s e t t o 128k. w i t h zeroes p r i o r t o F o u r i e r t r a n s f o r m a t i o n . The base peak at m / e = 8 6 6 has been expanded t o show t h e e f f e c t o f le a k a g e more c l e a r l y . A l l peaks i n f i g u r e 16 have bothersome s i d e l o b e s but they a re not r e a d i l y apparent at low masses because f i g u r e 16 i s p l o t t e d on a mass scale,:\not a f r e q u e n c y s c a l e . Hence, f r e q u e n c i e s a re compressed a t the low mass end o f t h e spectrum and t h e l e a k a g e i s not seen t h e r e u n l e s s the s c a l e i s expanded. The peak a t m / e = 6 9 i s a l s o expanded i n f i g u r e 16 t o show t h a t t h e e f f e c t o f l e a k a g e i s i n h e r e n t throughout t h e spectrum. F i g u r e 1 7 was o b t a i n e d i n e x a c t l y the same manner as f i g u r e 16 except f o r t h e a p p l i -c a t i o n o f a Hanning window f u n c t i o n p r i o r t o z e r o - f i l l i n g . A g a i n , t h e peak at m / e = 8 6 6 I s expanded but the l e a k a g e i s removed by t h e window f u n c t i o n . E.' S e l e c t i v e Z e r o - f i l l i n g H igh o r d e r s o f z e r o - f i l l i n g can reduce AM t o an a c c e p t -a b l e minimum but l o n g d i s k based F o u r i e r t r a n s f o r m s a r e v e r y time consuming. F o r example, a 16k t r a n s f o r m t a k e s 14 seconds, 64k t a k e s 4 minutes w h i l e 512k t a k e s over 50 m i n u t e s ! S i n c e the problem o f mass d e t e r m i n a t i o n o c c u r s a t the h i g h mass end o f t h e s c a l e , i t would be advantageous t o be a b l e t o z e r o - f i l l o n l y t h i s p o r t i o n o f t h e spectrum f o r a c c u r a t e mass d e t e r m i n a t i o n . F o l l o w i n g t h e method o f P a j e r and 866 > 6 9 —P4 Jilj. L u 2 . © o — I — 4 o o — i — 600 1 — i l d . 1 u i 00 1 "TT 800 / O O O I z o o Figure 16 Wide band spectrum of t r i s ( p e r f l u o r o h e p t y l ) - s - t r i a z i n e . O r i g i n a l 16k data set expanded to 128k before Fourier transformation. © 6 6 i v o I JL A. 2op 400 — 1 — 600 3oo l o o o 1200 Figure 1 7 Hanning window function applied to transient of figure 1 6 . Armitage i t i s p o s s i b l e to. s e l e c t i v e l y z e r o - f i l l u s i n g the f o l l o w i n g procedure: c o l l e c t d a t a and F o u r i e r t r a n s f o r m as u s u a l to o b t a i n a frequency domain spectrum, save only the d e s i r e d high mass segment (or any other segment f o r that matter) with i t s a s s o c i a t e d imaginary p a r t , F o u r i e r t r a n s f o r m t h i s back to the time domain then z e r o - f i l l to the d e s i r e d amount before f i n a l l y F o u r i e r t r a n s f o r m i n g back to the f r e -quency d o m a i n ^ F i g u r e 18 shows how t h i s method was used to s e l e c t i v e l y f i l l the high mass p o r t i o n of f i g u r e 16. The o r i g i n a l 16k t r a n s i e n t was F o u r i e r transformed to y i e l d a frequency domain spectrum. Then, lk r e a l and Ik imaginary p o i n t s corresponding to the high mass end of the spectrum were s e l e c t e d and transformed back to the time domain to y i e l d a 2k t r a n s i e n t . T h i s t r a n s i e n t was z e r o - f i l l e d to 16k and. f i n a l l y F o u r i e r transformed to give f i g u r e 18. Thus, the segmentrcorresponding to mass ^ l - ^ o w a s s e l e c t i v e l y zero-f i l l e d . F i g u r e 19 i s the u n f i l l e d spectrum shown f o r comparison. Note the d i f f e r e n c e In l i n e s h a p e s between the two examples and how the peak p o s i t i o n i s more r e a d i l y apparent i n f i g u r e 18. T h i s i s a powerful technique as i t allows f o r a broad band spectrum to be c o l l e c t e d and analyzed segmentally or wholly, but u n l i k e the mixing technique only one experiment needs to be done. Of course r e s o l u t i o n i s not as high as i t can be by u s i n g the mixing technique. [ISO — I <2oo 1175 Figure 1 8 High mass portion of figure 1 6 obtained using the selective f i l l i n g technique of Armitage and Pajer. F. Mass C a l i b r a t i o n As a F o u r i e r t r a n s f o r m a t i o n of a time domain ICR s i g n a l produces a frequency domain spectrum, an accurate method must be a v a i l a b l e f o r c o n v e r t i n g f r e q u e n c i e s to masses. Beauchamp and Armstrong have d e r i v e d the f o l l o w i n g e x p r e s s i o n f o r the frequency of an i o n i n a c r o s s e d e l e c t r i c and magnetic field"'"^ co2 = qH 2 - 4qV (3-20) mc ,2 md In equation 3-20, co i s frequency, V i s the t r a p p i n g v o l t a g e , d i s the d i s t a n c e between the top and bottom p l a t e s of the ICR c e l l , q i s the charge on the i o n , H i s magnetic f i e l d , c i s the speed of l i g h t and m i s mass o f the i o n . Equation 3-20 can be s o l v e d f o r m/q and a f t e r i n t r o d u c t i o n of the parameters 2 2 A=(H/c) and B=4V/d , the f o l l o w i n g e x p r e s s i o n due to 17 Mclver i s obtained m = -B + ( B 2 + 4Aa) 2)^ (3-21) Once A and B are known, a l l that i s r e q u i r e d f o r the mass de t e r m i n a t i o n of an i o n i s the frequency of t h a t i o n . A and B are determined by u s i n g two known mass values and t h e i r experimental f r e q u e n c i e s to s o l v e equation 3-21. Table 2 g i v e s the major ions seen i n f i g u r e 16 and e x p e r i m e n t a l l y determined f r e q u e n c i e s . A l s o i n c l u d e d are the c a l c u l a t e d and a c t u a l masses and d e v i a t i o n s f o r each i o n . V a r i o u s combinations of masses have been used f o r the f i t t i n g Table 2 C a l i b r a t i o n Data f o r the FT-ICR Spectrometer ION EXACT m/q FREQUENCY Hz : CALCULATED3-• : m/q ERROR CALCULATED m/q b ERROR CALCULATED0 ERROR m/q CF + 6 8 . 9 9 5 2 454148 6 8 . 9 9 5 2 0. 000 6 8 . 9 9 5 2 0. 000 " 6 8 . 9 9 5 2 1. 9 7 8 8 E - 5 118.9920 2 6 3 1 8 3 1 1 8 . 9 8 9 -0. 003 118.992 0. 000 1 1 8 . 9 9 1 2. 3 9 3 2 E - 4 130.9920 23.9037 130.991 0. 000 130 . 9 9 5 0. 003 130.994 -2. 0 5 9 9 E - 3 C 3 F 7 + 1 6 8 . 9 8 8 8 1 8 5 2 1 9 1 6 8 . 9 7 9 -0. 010 168.986 -0. 003 1 6 8 . 9 8 4 4 . 0 5 6 4 E - 3 C 8 F l 4 N + 3 7 5 - 9 8 0 7 8 3 0 5 3 . 8 375-950 -0. 030 3 7 5 . 9 9 8 0. 017 3 7 5 - 9 8 4 - 3 - 8 0 6 I E - 3 C 1 0 F 1 7 N 2 " 4 7 0 .9790 6 6 2 3 0 . 7 470. 930.. -0. 049 471.009 0. 030 470.986 - 6 . 9742E- 3 C 1 6 F 2 9 N 2 770 . 9 5 9 8 4 0 3 2 1 . 4 770.856 -0. 104 7 7 1 . 0 8 3 0. 123 7 7 1 .020 - 6 . 0 1 2 9 E - 2 C 1 8 P 3 2 N 3 .865 . 9 5 8 2 3 5 8 6 5 . 7 8 6 5 . 6 6 8 -0. 2 9 0 8 6 5 . 9 5 8 0. 000 8 6 5 . 8 7 7 0. 0 8 0 8 5 C 2 4 P 4 4 N 3 1165.9390 26542.7 M165-64 -0. 299 1166 . 18 0. 241 1166.03 -0. 09371 C24 P45 N 5 1184.9374 2 6 1 0 0 . 2 1 1 8 5 . 1 3 0. 193 1 1 8 5 . 6 9 0. 753 1 1 8 5 .53 0. 39940 a-m/q= eq. 3-21. 6 8 . 9 9 5 2 and 130.9920 used to f i t eq. 3 -c A l l p o i n t s f i t to eq. 3-21 to f i n d •21. best k m/q=68. values of 9 9 5 2 and A and B 8 6 5 . 9 5 8 2 used to f i t -64-of equation 3-21 with good success, Mass. c a l i b r a t i o n by FT-ICR i s seen to be q u i t e good when compared to p r e v i o u s ICR c a l i b r a t i o n s . For example, Mclver measured the mass of C 3 5 C 1 2 3 7 C 1 + (m/q=ll8.903) as 118.887 f o r an e r r o r of .016 amu. We have measured the mass of a s i m i l a r i o n C^F^"1" (m/q=ll8.992) as II8.989 or an e r r o r of .003 amu. A l s o , the mass range has been extended to at l e a s t 1200 whereas pr e v i o u s ICR experiments have been l i m i t e d to l e s s than m/q=400 with poor r e s o l u t i o n . Good accuracy i s obtained by t h i s mass c a l i b r a t i o n method but i t i s not p o s s i b l e to i d e n t i f y d i f f e r e n t i s o b a r i c ions i n a broad band spectrum s o l e l y by mass because: 1) An i n o r d i n a t e number of p o i n t s must be transformed to determine f r e q u e n c i e s to a high degree.of accuracy. 2) The equation of motion of ions i n a cubic trapped i o n c e l l i s not known so the use of equation 3-20 i s q u e s t i o n a b l e , although Table 2 shows i t to be reasonably a c c u r a t e . 3) The e f f e c t t h a t ions have on each others' f r e q u e n c i e s due to induced magnetic f i e l d s and space charges can not be pre -d i c t e d . 4) V a r i a t i o n s of the a p p l i e d magnetic f i e l d d u r i n g experiments and f i e l d inhomogeheities can cause l o s s of r e s o l u t i o n . Methods have been presented f o r o b t a i n i n g FT-ICR s p e c t r a . I t has been shown that there i s a p l a y - o f f between r e s o l u t i o n , mass range and computational time r e q u i r e d when performing FT-ICR experiments. In p r a c t i c e , t h i s p l a y - o f f i s not - 6 5 -s e r i o u s as asset of s a t i s f a c t o r y a c q u i s i t i o n parameters can u s u a l l y be found that allow the experiment t o be performed i n an e f f i c i e n t manner. A l s o , data a c q u i s i t i o n and pro-c e s s i n g are under computer c o n t r o l a l l o w i n g f o r l a r g e amounts of data to be a c q u i r e d and analyzed f a i r l y q u i c k l y . i - 6 6 -REFERENCES AND NOTES 1. M. B.. Comisarow and A. G. M a r s h a l l , J , Chem, Ph y s i c s 64, 1 1 0 ( 1 9 7 6 ) . 2. E. 0 . Brigham, "The Fast F o u r i e r Transform", P r e n t i c e -H a l l , Inc., Englewodd C l i f f s , New J e r s e y , 1 9 7 4 , pp. 132-146. 3 . V a l e r i o G r a s s i , p r i v a t e communication, 4. M. B. Comisarow, •:.G'. P a r i s o d and V. G r a s s i , proceedings, of 2 6 t h ASMS meeting, St. L o u i s , MO., 1978, paper RD5. 5- E. B a r t h o l d i and R. R. E r n s t , J . Mag. Resonance 1 1 , 9 ( 1 9 7 3 ) • 6. P. R. G r i f f i t h s , ! Appl. Spectrosc. 2 9 , ;L11 (197 5 ) . 7. G. H o r l i c k and W. K. Yuen, Anal. Chem. 48 , 1 6 4 3 ( 1 9 7 6 ) . 8. R. J . 0 ' H a l l o r a n and D. E. Smith, Anal. Chem. 5 0 , 1391(1978) . 9. M. Forker and J . D. Rogers, Nucl. Instrum. and Methods 9 6 , 4 5 3 ( 1 9 7 1 ) . 10. J . W. Cooper, "The Minicomputer i n the Laboratory", Wiley, New York, 1977, pp. 2 6 1 - 2 9 4 . 11. M. B. Comisarow and A. G. M a r s h a l l , unpublished r e s u l t s . 12. N i c o l e t Instrument C o r p o r a t i o n , 5225 Verona Rd., Madison, Wisconsin. 13- r e f e r e n c e 2 , pp. 140-146. 14. The window f u n c t i o n s used are pro v i d e d i n Lab - 1 1 8 0 General S i g n a l Averaging Package, N i c o l e t Instrument Corp., 5225 Verona Rd., Madison, Wisconsin. 15. R. T. Paj e r and I. M. Armitage, J . Mag. Resonance 2 1 , 4 8 5 ( 1 9 7 6 ) . 16. J . L. Beauchamp and J . T. Armstrong, Rev. S c i . Instrum. 40, 1 2 3 ( 1 9 6 9 ) . 17. E. B. Ledford and R. T. Mclver, I n t . J . Mass Spectrom. Ion Phys. 2 2 , 3 9 9 ( 1 9 7 6 ) . IV. THE GAS PHASE REACTIONS OF CARBONATES A. A B r i e f Review of Previous E s t e r S t u d i e s The a t t a c k of a n u c l e o p h i l i c reagent at a c a r b o n y l center i s a common f e a t u r e of many organic r e a c t i o n s and thus has been a c t i v e l y s t u d i e d by many investigators."'" The mechanistic d e t a i l s of many s o l u t i o n processes such as e s t e r h y d r o l y s i s are w e l l understood. However, re c e n t gas phase work has shown that i t i s s t i l l not known how the i n t r i n s i c p r o p e r t i e s of the reagents a f f e c t the outcome of the r e a c t i o n . Attempts to improve the knowledge of chemical r e a c t i o n s have concentrated on two areas: t h e o r e t i c a l approaches and r e -c r e a t i n g s o l u t i o n processes i n the gas phase where the r o l e of the s o l v e n t i s e l i m i n a t e d . T h i s chapter i s concerned with the l a t t e r approach as s t u d i e d by FT-ICR. 18 A c l a s s i c s o l u t i o n experiment u t i l i z i n g 0 l a b e l l i n g showed that the a l k a l i n e s a p o n i f i c a t i o n of amyl.acetate 2 proceeds by breakage of the acyl-oxygen bond. Overwhelming evidence by Bender showed that the r e a c t i o n i n v o l v e s the i n i t i a l formation of a t e t r a h e d r a l i n t e r m e d i a t e from the a d d i t i o n of OH to the c a r b o n y l group. His evidence was based upon the a l k a l i n e h y d r o l y s i s of ethyl.benzoate- which 18 was l a b e l l e d with 0 i n the c a r b o n y l oxygen. The r e a c t i o n 16 was stopped before completion and a n a l y s i s showed some 0 i n the c a r b o n y l oxygen, evidence f o r the equivalence of oxygens i n an i n t e r m e d i a t e . T h i s i s commonly c a l l e d a BA^,2 process or simply a t e t r a h e d r a l mechanism. The corresponding mechanism, whereby the a l k y l - o x y g e n bond i s broken i s l e s s - 6 8 -common. Th i s i s an S.T2 or B. T2 process and i s r a r e being N AL ^ • known mainly f o r methyl and e t h y l e s t e r s r e a c t i n g with powerful 4 n u c l e o p h i l e s . V a r i o u s r e s e a r c h groups have s t u d i e d the r e a c t i o n s of negative ions with e s t e r s i n the gas phase and have found that s e v e r a l mechanisms become competitive i n the absence of 5 12 8 a s o l v e n t . ~ Notable has been the work of Rive r o s who s t u d i e d the r e a c t i o n s of F with a l k y l a c e t a t e s and p ropionates and concluded: (1) S N2 processes are important i n the gas. phase r e a c t i o n s of some e s t e r s and (2) e s t e r s c o n t a i n i n g 6 hydrogens l e a d to d i f f e r e n t products than methyl e s t e r s due to the a d d i t i o n ' o f an e l i m i n a t i o n type r e a c t i o n F + CH_C00CoHc. 3 2 5 -~ 0 — C H 0 FUC* N F — H * ^ ' ] CH 3C00 + HF + C 2H^ (4-1) * _ Very r e c e n t l y Riveros r e p o r t e d the r e a c t i o n s of H 0 with v a r i o u s methyl and e t h y l e s t e r s i n an e f f o r t to determine q the importance of 8 ^ 2 , S^ .2 and e l i m i n a t i o n p r o c e s s e s . 18 Measurement of the 0 content i n product ions was used to e s t a b l i s h the importance of these three processes. Riveros concluded that the r e a c t i o n of H 0 with CF^COOCH^ goes 2k% by a B 2 process and 76% by an S 2 process while the r e a c t i o n i~i KJ INI H*0~ + C HVC00CH o goes 92% B ? and 8% S „ 2 . 6 5 3 AG N E a r l i e r work by Comisarow showed that the f o l l o w i n g 11 gas phase r e a c t i o n s occur -69-CD 30~ + C 6H 5COOCH 3 CgH^COO" + CD 3OCH 3 (4-2) CD30~+ CF 3COOCH 3 >- CF 3COO~ + CD^CH^ (4-3) In l i g h t of 4-2 and 4-3, an i n v e s t i g a t i o n of the r e a c t i o n s of CH 30~ and C2H,-0_ with e s t e r s i n the gas phase by FT-ICR was c a r r i e d out. Reactions 4-2 and 4-3 were r e i n v e s t i g a t e d u s i n g CH"3 0 to determine i f both B A Q 2 and S N2 processes were r e s p o n s i b l e f o r the formation of the c a r b o x y l a t e anions. Other e s t e r s that were i n c l u d e d i n t h i s study are the a l k y l carbonates (R0)„CO where R i s CH_, CD_, C„H,_ or n-C_H„. The 2 3 3 2 b 3 1 a l k y l carbonates are an i d e a l system to study because 1) the l a c k of an a c i d i c proton i n the p o s i t i o n a to the carb o n y l group excludes proton t r a n s f e r to the n u c l e o p h i l e and 2) the symmetry of the carbonates are h e l p f u l i n i d e n t i -f y i n g s t r u c t u r e - e n e r g y r e l a t i o n s h i p s . B. Experimental i . I nstrumental A l l work was done on a home b u i l t FT-ICR spectrometer which has been d e s c r i b e d i n Chapter 2. Ion e j e c t i o n t e c h -niques were used to co n f i r m r e a c t i o n pathways and r a t e constants were measured as d e s c r i b e d i n Chapter 2. Pressure measurements were made with a V a r i a n Model 971-1008 i o n i z a t i o n gauge which was c a l i b r a t e d f o r each gas i n the pre s s u r e range 10 J to 10 T o r r pressure a g a i n s t an MKS Ba r a t r o n c a p a c i t a n c e manometer. For compounds that have low vapor p r e s s u r e s and are hard to pump, accurate c a l i b r a t i o n s w i t h the Bara t r o n were not p o s s i b l e F o r t h i s reason, the method of Otvos and Stevenson was used f o r the c a l i b r a t i o n of C^H,_COOCH_ and D 5 3 13 (n-C^H^O^CO. B r i e f l y , t h i s method involves c a l c u l a t i n g the i o n i z a t i o n c r o s s - s e c t i o n f o r a molecule by summing the c r o s s - s e c t i o n s of the atoms which comprise the molecule. The c a l c u l a t e d c r o s s - s e c t i o n i s then used to determine a c a l -i b r a t i o n f a c t o r . For compounds where a c a l i b r a t i o n f a c t o r . could be measured a c c u r a t e l y as w e l l as c a l c u l a t e d , agree-ment between the two methods was very good. A r a t e constant of 1.11 Z .02 x 10 cm /molecule-sec f o r the r e a c t i o n + + CH^ + CH^ —>> CH[- + "CH^ was obtained comparing very f a v o r --9 ably with an average l i t e r a t u r e value of 1.11 x 10 cm-ymolecule-sec which has been obtained by numerous techniques. Alkoxide ions i n these experiments were generated by 15 e l e c t r o n impact upon the a p p r o p r i a t e a l k y l n i t r i t e . Thus CD^ONO gi v e s CD^O and DN0~ i n a 3:1 r a t i o when impacted with .5 - 1. 0 eV e l e c t r o n s . CD CT>20NO give s CD^CD^", DN0~ and CD^CDO- under the same c o n d i t i o n s . The e s t e r s used i n these experiments do not d i s p l a y a negative i o n ICR s i g n a l . In a t y p i c a l experiment, n i t r i t e i s leaked i n t o the - 7 system to a pressure of ^  1 x 10 T o r r . An e s t e r i s then leaked i n t o the system to achieve a t o t a l p r essure of about - 7 5 x 10 T o r r . These pressures are r e l a t i v e l y low even by ICR standards because secondary r e a c t i o n s become bothersome at h i g h e r p r e s s u r e s . I t was d e s i r e d to study the r a t i o of r e a c t i o n products at low con v e r s i o n percentages i n order to deduce mechanistic pathways. Secondary r e a c t i o n s were found to change product r a t i o s with time, hence both low pr e s s u r e s and low r e a c t i o n times were needed i n these s t u d i e s . E l e c t r o n e j e c t i o n experiments were performed and were found to have no n o t i c e a b l e e f f e c t i f the f i l a m e n t c u r r e n t was kept low. i i . Chemicals - Commercial Whenever p o s s i b l e , chemicals were obtained from commercial sources. C e r t a i n chemicals were s y n t h e s i z e d by l i t e r a t u r e methods. Dimethyl carbonate ( A l d r i c h ) , d i e t h y l carbonate (Eastman), methyl t r i f l u o r o a c e t a t e ( P i e r c e ) and methyl benzoate ( A l d r i c h ) were used with no f u r t h e r p u r i f i -c a t i o n . i i i : : Chemicals - Synt h e s i s Methyl n i t r i t e s were s y n t h e s i z e d u s i n g the a p p r o p r i a t e l y l a b e l l e d methyl a l c o h o l . CH^ONO, CD^ONO and CH^ 0 N 0 were sy n t h e s i z e d . In a t y p i c a l p r e p a r a t i o n , 1 . 3 5 g ••( • 042'moles) of MeOH and 2 . 2 g ( . 0 2 2moles) of H" 2S0^ i n 5 ml H^O were slowly d r i p p e d i n t o a 25 ml, 3 neck f l a s k which contained 3 - 5 g ( . 0 5 1 moles) of NaONO i n 18 ml of H 20 at 0 °C. The f l a s k was o u t f i t t e d with a magnetic s t i r r e r , the a d d i t i o n f u n n e l , N 2 i n l e t and an o u t l e t tube which l e a d to a small c o l d t r a p which was immersed i n an acetone/dry i c e s l u s h (-78 °C ). A pale yellow l i q u i d was c o l l e c t e d i n the c o l d t r a p , d i s t i l l e d on a vacuum l i n e to another t r a p c o n t a i n i n g KOH and then f i n a l l y d i s t i l l e d i n t o : a 3 0 0 ml g l a s s bulb. The f i n a l product i s a c o l o r l e s s gas. The g l a s s bulb was s t o r e d i n the dark to avoid photochemical decomposition. E t h y l n i t r i t e , was prepared i n a s i m i l a r manner s t a r t i n g from CH CH2OH or CD^CDgOD -(..99% D, S t o h l e r I s o t o p e s ) . 18 17 Methanol- 0 was prepared by the method of Sawyer i n which 15 ml t r i - n - b u t y l orthoformate was hydr o l y z e d by 1 .0 g 18 H^O (Norsk Hydro, 99% enriched) under a c i d c o n d i t i o n s to 18 give b u t y l formate-carbonyl- 0 . Reduction of the formate 18 e s t e r with l i t h i u m aluminum hydride gave 1 .55 g methanol- 0 18 (80% e n r i c h e d ) , 85% o v e r a l l y i e l d based on s t a r t i n g E^O CD^OD (99% D) was purchased' from S t o h l e r Isotopes. Deuterated dimethyl carbonate was prepared by the method 18 of Renaud and L e i t c h . 5 g ( -051 moles) of D^CBr ( S t o h l e r Isotopes, 99.5% D) and 7 .15 g ( . 0 2 6 moles) of s i l v e r carbonate were p l a c e d i n a t h i c k g l a s s tube. The tube was evacuated on a vacuum l i n e and sea l e d with a t o r c h . I t was p l a c e d i n an oven at 55 °C. and o c c a s i o n a l l y shaken. A f t e r 15 days, the tube was opened and the contents d i s t i l l e d y i e l d i n g 1.1 g (47% y i e l d ) of dg-dimethyl carbonate. D i - n - p r o p y l carbonate was prepared by the t r a n s e s t e r -i f i c a t i o n of dimethyl carbonate. Hence, 40 ml ( . 5 3 moles) of n-propanol, .4 g KOH and 10 ml ( . 1 2 3 moles) of dimethyl carbonate were p l a c e d i n a 100 ml f l a s k and methanol was co n t i n u o u s l y d i s t i l l e d o f f . The organic m a t e r i a l was washed twice with H 20, d r i e d over Mg SO^ and f i l t e r e d . The crude product was d i s t i l l e d y i e l d i n g d i - n - p r o p y l carbonate. C. R e s u l t s Carboxylate anion i s the s o l e product of the r e a c t i o n of methoxide and ethoxide with the methyl and e t h y l e s t e r s used i n t h i s study. Proton a b s t r a c t i o n by a l k o x i d e was not observed. Table 3 g i v e s the products, per cent of each product at low convers i o n r a t i o s , r a t e constants and r e a c t i o n e f f i c i e n c i e s f o r each r e a c t i o n done i n t h i s study. For a l l r e a c t i o n s , the n e u t r a l products are assumed. Product r a t i o s were determined at low con v e r s i o n r a t i o s , t y p i c a l l y l e s s than 10%, to minimize secondary r e a c t i o n s which l e a d to product r a t i o s t h a t change with time. F i g u r e 20 shows the r a t i o o f CD OCOOVCHgOCOO" with time as formed by the r e a c t i o n CH 30~ + (CD 30) 2CO — > - CH 30C00~ + CD 3OCD 3 (4-4a) — > • CD 30C00" + CD 3OCH 3 (4-4b) The product r a t i o c l e a r l y changes from % 2:1 at low r e a c t i o n times to over 4:1 at longer r e a c t i o n times. R e a c t i o n 4-4 i s 20 estimated to be exothermic by about 35 kcal/mole. The products of r e a c t i o n 4-4, c a r b o x y l a t e anion and n e u t r a l , must t h e r e f o r e c o n t a i n 35 kcal/mole of excess energy between them. Reaction 4-5 i s thermoneutral and i s t h e r e f o r e expected to be very slow or non e x i s t a n t . However, i f CH 30C00 i s CH 30C00~ + (CD 30) 2CO >- CD 30C00" + CD 3OCOOCH 3 (4-5) e x c i t e d , r e a c t i o n 4-5 may have an a p p r e c i a b l e r a t e constant. B u f f e r gas experiments were performed to determine i f RATIO OF CD30C0"/CH30ro v. TIME AS A FUNCTION OF CH, PRESSURE 4.0 _ 4 3.5 o •H 3.0 - p K - p o 3 2.5 o 2 . 0 J • : PRESSURE (CD 30) 2C0 = 1.0 X 10" 6 o : PRESSURE CH 4 = 7 X 10" x : PRESSURE CH 4 = 2 X 10""5 I 1.5 25 50 — n — TIME IN MSECS —r— 100 12T ~150 Figure 20 Buffer gas curves for the dimethyl carbonate system. Table 3 Summary of Reactions Studied # ANION i:. O H "o" NEUTRAL CF 3COOCH 3 2 CH 0 0 C^HrCOOCH_ 3 6 5 3 PRODUCTS AND ,a CP3COO + CH 3 OCH3 CF 3CO*0~ + CH 3OCH 3 C rH cCOO~ + CH *OCH0 6 5 3 3 C.HVCO 0~ + CH o0CH o 6 5 3 3 92$ 52% iri/q K u  e x P 113 8 . 3 115 121 1 6 . 9 123 K /K.^n AH exp ADO rxn 357 732 - 6 0 d -54 11 3 CH 3 0 (CD 3 0) 2CO CH3OCOO + CD 3OCD 3 CD OCOCf + CD 3OCH 3 30$ 70$ 75 78 5.1 378 4 CD 3 0 (CH 3 0) 2CO 5 C H 3 * ° (CH 3 0) 2CO 6 C H 3 * 0 (CD 3 0) 2CO 7 CD 3CD 2 0 (CH 3 0) 2CO CD3OCOO + CH 3OCH 3 CH3OCOO~ + CD 3OCH 3 * CH3OCOO + CH 3 OCH 3 CH30*COO" + CH 3OCH 3 CH3*OCOO~ + CD 3OCD 3 CD3OCOO. + CD 3 OCH 3 CD3OCO*0~ + CD 3OCH 3 CH o0C00~ + C oD t-0CH o 3 2 t> 3 and/or C ^ + DOCH, C^OCOO" + CH o0CH Q 2|p j j 12% 51% 49$ 52$ 78 75 75 77 77 78 80 75 8 .5 697 i l 16$ Table 3 Continued 8 CD 0" (C oH c0)_C0 C oH_0C00~ + CD o0C oH_ 78% 3 2 5 2 2 5 3 2 5 and/or CD OH + C ^ CD_0C00~ + C oH c0C oH r 22% 3 2 5 2 5 and/or C 2H OH + C ^ 9 C H 3 * ° ~ ( C 2 H 5 0 ) 2 C 0 CH3*OCOO" + n e u t r a l s 24% C 2H 5OCO 0 + n e u t r a l s 7% CgH 0C00~ + n e u t r a l s 69% 10 CD 3CD 20~ ( C 2 H 5 0 ) 2 C 0 C ^ O C O O - + n e u t r a l s 75% C 2D 5OCOO~ + n e u t r a l s 25% 11 CD 30" (n-C 3H 70) 2CO CH 2=CHCH 20~ + n e u t r a l s 2% CH 3CH 2CH 20" + " 44% CD 30C00~ + " 14% C ^ O C O O - + " 40% 89 4 .12 .23 78 77 91 89 89 4 . 9 .27 1 94 I 57 5 . 4 5 .26 59 78 103 Table 3 Continued 12 CH 3 0 '3 7 2 13 14 CH 3 0 CH 3 0 CHo0C,-H._ 3 6 5 3 2 6 5 CH 2 = CHCH"20 + n e u t r a l s 2$ 57 CH 3CH 2CH 20~ CH3*OCOO~ + tt 47$ 59 + tt 16$ 77 C H OCOO" + t! 30$ 103 C 3H 7OCO*0~ + t! 5$ 105 C.H^O" + CH o 5 3*OCH 3 100$ 93 2. 75 .132 - 3 0d CvH_0~ + CH 6 5 * 3 OCH 2 CH 3 100$ 93 12. 1 • 555 - 3 0 e * and/or CH OH + C 2 H4 -14 a $ r e f e r s to r e l a t i v e amounts of each product at low conversion r a t i o s . b U n i t s — 10 3 f o r Rexp are 10 cm /molecule-sec. Reactions 3-6 have the same r a t e constant. L i k e -wise 8,9 and 11,12 are the same. c Kexp/K A D Q Is the r e a c t i o n e f f i c i e n c y . K A D Q was c a l c u l a t e d by the method of M. T. Bowers and T. Su, Theory of Ion P o l a r Molecule C o l l i s i o n s i n " I n t e r a c t i o n s Between Ions and Molecules", P. Ausloos, Ed., Plenum Press, New York, 1975. d. W. N. Olmstead and J . I. Brauman, J . Amer. Chem. Soc. 99, 4219(1977). e C a l -c u l a t e d from heats of formation f o r Q 2H OCH , CH^OH and C ^ from D. R. S t u l l , E. F. C. Sinke, "The Chemical Thermodynamics of Organic Compounds", Wiley, Westrum, J r . and G. New York, I969. EA f o r CH^O' and CgH^O from K. J . Reed, PhD t h e s i s , Stanford U n i v e r s i t y , 1975. an e x c i t e d carboxylate. anion i s r e s p o n s i b l e f o r the product r a t i o of r e a c t i o n 4-4 changing with time. Prom f i g u r e 2 0 , i t i s seen that as the pressure of CH^ b u f f e r gas i s r a i s e d , the product r a t i o approaches a constant value. T h i s shows that the c a r b o x y l a t e anions are becoming t h e r m a l i z e d and hence u n r e a c t i v e by c o l l i s i o n s with CH^ which i s i n a l a r g e r number d e n s i t y than the n e u t r a l e s t e r . Product r a t i o s f o r other carbonate r e a c t i o n s i n Table 3 were a l s o seen to change with time, presumably due to the same e f f e c t . Experiments with CH^ 0 were conducted i n order to * _ e l u c i d a t e r e a c t i o n pathways. CH^ 0 was formed by the attachment of thermal e l e c t r o n s to CH^ 0N0 which was l a b e l l e d 1 ft — — 80% with 0 . HNO and CR^O , both m/e=31, are a l s o formed by t h i s process. HNO was found to r e a c t slowly with e s t e r s to y i e l d c a r b o x y l a t e anions. Continuous e j e c t i o n of CH^O and HNO was u t i l i z e d as d e s c r i b e d i n Chapter 2 to o b t a i n product r a t i o s as a f u n c t i o n of time f o r the methoxide and ethoxide r e a c t i o n s . In t h i s way, methoxide was e f f e c t i v e l y 18 l a b e l l e d 100% with 0 and no c o r r e c t i o n s f o r u n l a b e l l e d methoxide .were neccessary. The r e a c t i o n of CH^ 0 with methyl t r i f l u o r o a c e t a t e and methyl benzoate leads to i n c o r p o r a t i o n of 0 i n the product i o n s . F o l l o w i n g the arguements of Riv e r o s who s t u d i e d the r e a c t i o n of H 0 with these two e s t e r s , the amount of 0 i n the product ions leads to an estimate of the importance of v a r i o u s mechanisms which c o n t r i b u t e to the f o r m a t i o n of these i o n s . ^ i t w i l l be assumed that r e a c t i o n s 1 and 2 i n Table 3 proceed, by two mechanisms: 1) a simple S^2 process where the methoxide a t t a c k s the a l k y l group, t h a t i s a t t a c k i s at the methyl group and 2) a t t a c k of methoxide i s at the c a r b o n y l carbon to form a t e t r a h e d r a l i n t e r m e d i a t e which can rearrange to give products; t h a t i s , a B A C ; 2 type process. I f the assumption i s made that the methoxy groups i n the t e t -r a h e d r a l i n t e r m e d i a t e are e q u i v a l e n t , the r e l a t i v e importance of S^2 and BA (-,2 type processes can be estimated from the 0 * -content i n the products. Table 4 giv e s the CH^ 0 r e s u l t s * _ along with the H 0 r e s u l t s of Riveros f o r comparison. I t i s seen that the r e a c t i o n s of CH^ 0~ and H 0 with methyl t r i f l u o r o a c e t a t e and methyl benzoate are s i m i l a r . Table 4 C o n t r i b u t i o n of B n r i 2 and SA T2 to AC N S e l e c t e d Gas Phase Reactions Reactahts BAC^ S N 2 H*0~ + CP 3COOCH 3 24% 76% CH *0~ + CF oC00CH o 16% 84% 3 3 3 H 0~ + CgH 5COOCH 3 92% 8% CH3*0~+ C 6H 5COOCH 3 96% 4% The r e a c t i o n of methoxide and ethoxide with methyl and e t h y l carbonate'-leads to the e x c l u s i v e formation of carbox-y l a t e anions. However, the r e a c t i o n of methoxide with n-propyl carbonate y i e l d s propoxide as w e l l as c a r b o x y l a t e s . F i g u r e s 21 and 22 give the temporal i o n i n t e n s i t i e s f o r r e a c t i o n s H a n d 12 of Table 3 showing that propoxide i s an i n i t i a l product which r e a c t s f u r t h e r with n-propyl carbonate. An i o n at m/e = 57 i s a l s o formed i n small amounts which appears to be u n r e a c t i v e with n-propyl carbonate. - 8 1 -Pigure 22 Time plot; for .CH'3 .0" + (n-^H 0)2C0' products. I t has been r e p o r t e d that the r e a c t i o n s of e t h y l e s t e r s with negative ions have a higher r e a c t i o n e f f i c i e n c y than methyl e s t e r s due to the presence of 3 hydrogens which open 7 8 ^ — up an e l i m i n a t i o n channel. ' Since experiments with CH^ 0 * _ and H o show that both S N2 or e l i m i n a t i o n and BA(_,2 type processes l e a d to the formation of c a r b o x y l a t e anions, i t i s d e s i r a b l e to know how much 3 hydrogens can speed up product formation. C o n t r o l experiments with a n i s o l e and phenetole were performed to determine the i n f l u e n c e of 3 hydrogens on r e a c t i o n e f f i c i e n c y . These compounds are expected to form products only by S^2 and/or e l i m i n a t i o n processes. The r e a c t i o n of CH^ o with a n i s o l e and phenetole leads only to phenoxide. No * 0 i s seen i n the product ions showing that as expected, only S^2 and/or e l i m i n a t i o n r e a c t i o n s are o c c u r r i n g . From Table 3, i t i s seen that phenetole r e a c t s with methoxide approximately f o u r times as f a s t as a n i s o l e . Thus, the presence of 3 hydrogens can i n c r e a s e the r e a c t i o n e f f i c i e n c y due to e l i m i n a t i o n by up to a f a c t o r of fou r i n some cases. D.' D i s c u s s i o n of R e s u l t s The r e s u l t s i n Table 3 show that methyl e s t e r s r e a c t predominantly by a B A C 2 type process as judged by the amount of 0 i n c o r p o r a t i o n i n the product i o n s . E t h y l e s t e r s have l e s s 0 showing the i n c r e a s e d importance of e l i m i n a t i o n . A B 2 type process i s i n accord with t h e o r e t i c a l c a l c u l a t i o n s by Tomasi who showed that t e t r a h e d r a l i n t e r m e d i a t e s of 21 ca r b o n y l systems are expected to l i e on an energy minima. For the r e a c t i o n , H 0 ~ +.H2NCHO >- NH 3 + H C 0 0 ~ ( 4 - 6 ) the i n t e r m e d i a t e from a d d i t i o n of HO to the c a r b o n y l group was c a l c u l a t e d to be s t a b l e compared to reagents by 1 0 4 kcal/mole. A l s o , t e t r a h e d r a l i n t e r m e d i a t e s have been de t e c t e d i n the gas phase by ICR l e n d i n g credence to the s u g g e s t i o n that r e a c t i o n s 1 - 1 2 i n Table 3 proceed at l e a s t p a r t i a l l y v i a a B 2 or t e t r a h e d r a l process. As mentioned p r e v i o u s l y , simple c a l c u l a t i o n s show the r e l a t i v e importance of B A ^ 2 and S N 2 type processes f o r CF COOCH„ and C . H , - C 0 0 C H o . However, the v a l i d i t y of the 3 3 t> \) $ assumption concerning an energy randomized t e t r a h e d r a l i n t e r -mediate can not be a s c e r t a i n e d at t h i s j u n c t u r e i n time f o r these two e s t e r s . The r e a c t i o n of methoxide w i t h dimethyl carbonate should y i e l d i n f o r m a t i o n concerning t h i s assumption as a symmetrical i n t e r m e d i a t e may be formed. Reactions 3 - 6 of Table 3": u t i l i z e v a r i o u s l a b e l s to study the dimethyl carbonate/methoxide system. V a r i o u s i n -t e r n a l c o n s i s t e n c i e s are noted. Thus, r e a c t i o n s 3 3 4 and 6 which u t i l i z e deuterium l a b e l l i n g each show that t h e r e i s a 7 0 : 3 0 r a t i o of H/D or D/H i n the product i o n s . Reactions 5 1 8 1 8 and 6 use o l a b e l l i n g and each g i v e s the same amount of 0 i n the product i o n s , about 50%. These i n t e r n a l c o n s i s t e n c i e s are important i n that they preclude the i n f l u e n c e of i s o t o p e e f f e c t s and show that the measurements are not' i n f l u e n c e d by experimental techniques or i n s t r u m e n t a l a r t i f a c t s , problems that must be r e c o n c i l e d when d e a l i n g with new i n s t r u m e n t a t i o n or methods. Rea c t i o n 6 i s p a r t i c u l a r l y i n f o r m a t i v e concerning the mechanism of r e a c t i o n . Scheme I o u t l i n e s the products of r e a c t i o n 6 that would be expected f o r an S^2 process only while Scheme I I g i v e s the products f o r a b A Q 2 type process which proceeds through an energy randomized t e t r a h e d r a l i n t e r m e d i a t e . Scheme I CH 3 0 + CD3OCOOCD3 [ c H q*0*« »CD q- ••OCOOCD3] { CD (m/e=78) 3 CR"3 OCD 3 + 3OCOO 1 0 0 % Scheme I I * _ CH 3 0 + CD3OCOOCD3 rGH3"o|oCD3 J i CH 3 0CD 3 +:CD30C00 (m/e=78) 3 3 % * _ CH 3OCD 3 + CD 30C0 0 3 3 % ( m / e = 8 0 ) CD 3OCD 3 + CH 3 0C00 3 3 % (m/e=77) Fi g u r e 2 3 i s a spectrum o f the products of r e a c t i o n 6 taken at low r e a c t i o n time, No product a t m/e = 7 5 , corresponding to CH^OCOO", was seen showing t h a t t h e r e i s no scrambling of methyl groups o c c u r r i n g i n the i n t e r m e d i a t e of Scheme I I . Scheme I produces mass 78 only while Scheme I I produces 7 7 , 7 8 .and 80 i n equal amounts. Thus, i f r e a c t i o n 6 proceeds only v i a processes d e p i c t e d i n Schemes I and I I , products at m/e = 77 and 80 should appear i n equal amounts. However, i n s p e c t i o n of the r e s u l t s shows t h i s not to be the case, m/e = 77 Is formed i n 28$ y i e l d while m/e = 80 i s formed i n only 20$ y i e l d . I m p l i c a t i o n s of these r e s u l t s are: 1) the formation of a t r u e t e t r a h e d r a l i n t e r m e d i a t e i s not neccessary f o r r e a c t i o n to occur, or 2) another mechanism i s i n e f f e c t . No other s u i t a b l e mechanism can be g i v e n t h a t w i l l f i t the products observed. However, r e a c t i o n s of e s t e r s which proceed from n o n e q u i l a b r a t e d p o s i t i o n s have been r e p o r t e d by R i v e r o s l e n d i n g credence to the former i m p l i c a t i o n . ^ The r e a c t i o n of CH^ o with e t h y l and n-propyl carbonates i s c h a r a c t e r i z e d by the d i m i n i s h i n g abundance of 0 i n the 18 product i o n s . T o t a l 0 i n c o r p o r a t i o n i n the products i s seen to be 31$ and 21$ f o r the e t h y l and n - p r o p y l carbonates r e s p e c t i v e l y . However, the r e a c t i o n e f f i c i e n c y i s not seen to i n c r e a s e over the methyl carbonate system showing that a t t a c k of the n u c l e o p h i l e at the c a r b o n y l carbon i s s t i l l important. An e l i m i n a t i o n channel i s now open with the presence of 3. hydrogens. E l i m i n a t i o n could c o n c e i v a b l y occur by d i r e c t a t t a c k of the n u c l e o p h i l e on the a l k y l group or -86-through an in t e r m e d i a t e such as, Scheme I I I CD 30 + (C 2H 50) 2CO 9' CH_CH 0COCHo~ 0 H . CD„ i CH^CR^OCOO + CD3OH + Presumably, s i m i l a r processes can occur i n the p r o p y l carbonate system. Formation of an a l k o x i d e i s f i r s t seen i n the r e a c t i o n of methoxide with n-propyl carbonate. Propoxide c o u l d be formed as f o l l o w s Scheme IV CD 30 + (n-C 3H 70) 2CO OJ C-H7-0-C-0-CoH,7 i ( | 3 7 OCD 3 1 C 3H ?0 + CD 3OCOOC 3H 7 Carboxylates could be formed by the rearrangement of the in t e r m e d i a t e i n Scheme IV or by a process s i m i l a r to that i n Scheme I I I , a l s o by d i r e c t S N 2 a t t a c k . The i d e n t i t y of the product at m/e = 57 has been assigned to CH 2=CHCH 20~ although t h i s has not been s u b s t a n t i a t e d by experiment. Deuterium l a b e l l i n g of the p r o p y l group could help to i d e n t i f y t h i s i o n . The r e a c t i o n of ethoxide with methyl and e t h y l carbonate leads to c a r b o x y l a t e anions. These r e a c t i o n s appear t o be very s i m i l a r to the methoxide r e a c t i o n s and presumably no new mechanisms need to be i n t r o d u c e d . The process by which an in t e r m e d i a t e such as that d e p i c t e d i n Scheme I I rearranges to products i s open to s p e c u l a t i o n . The simplest e x p l a n a t i o n i n v o l v e s a f o u r - c e n t e r mechanism such as Scheme V CH *0 + (CD 30) 2CO CD n-0-C-0-CD o 3 s i H 3 0-CH o 3 0 ii *_ CD 0C0 + CH OCD 3 Four center mechanisms have a l s o been p o s t u l a t e d f o r 23 the gas phase r e a c t i o n s of methoxide with f l u o r o a l k a n e s , 0...CH 3 T CH 30 + C F 2 C F 2 F L F ^ - X F J OCFCF 2 + CH 3F ( 4 - 7 ) Beauchamp has s t u d i e d the r e a c t i o n s of v a r i o u s a l k o x i d e s with f l u o r o a l k a n e s and a r r i v e d at c o n c l u s i o n s s i m i l a r to those presented here; d i r e c t e l i m i n a t i o n of F or OR anions does not occur. Rather, rearrangement occurs to e l i m i n a t e an e n e r g e t i c a l l y f a v o r a b l e n e u t r a l such as HF, CH 3F e t c . . For e s t e r s used i n t h i s study, (except (n-PrO) 2CO ), the same co n c l u s i o n s hold. ROR' i s e l i m i n a t e d from an i n t e r m e d i a t e r a t h e r than R0~, * _ Riveros has s t u d i e d the r e a c t i o n of H 0 with some of the e s t e r s used i n t h i s study and a l s o a r r i v e d at the same c o n c l u s i o n that d i r e c t e l i m i n a t i o n of RO - i s not a favored g pathway f o r some e s t e r s . T h i s does become important f o r a l k y l p i v a l a t e s however, ^ » ^ r C H 0" + (CH,)_CC00CD q CD 0 + (CH KCCOOCH 5 _ 5 5 5 3 5 3? *(CH o) oCC00 + CD o0CH o (4-8) * \ 5 V ^ 3 3 3 3 where K D 0/K 0 „ = 1.5-B A C 2 S N 2 T h i s work has shown that the gas phase r e a c t i o n s of negative ions with e s t e r s c o n t a i n s very r i c h chemistry. F u r t h e r s t u d i e s should i n c l u d e n u c l e o p h i l e s c o n t a i n i n g S and N to see i f new r e a c t i o n paths are opened. A l s o , vary-i n g the a l k y l group on the e s t e r w i l l l e a d to a b e t t e r under-sta n d i n g of the f o r c e s that determine how much of the r e a c t i o n goes by a p a r t i c u l a r mechanism. A r e a c t i o n that has beenBstudied only b r i e f l y that o f f e r s a glimpse of the p o t e n t i a l f o r study and i n t e r p r e t a t i o n i n t h i s area of gas phase chemistry i s DN0~ + (R0) 2C0 —>• R0C00CH 2~ + ???• R0C00 - + ??? (4-9) where R i s CH , CH 3CH 2 or n - C ^ . - 9 0 -REFERENCES AND NOTES 1 . "The Chemistry of the Carbonyl Group", S. P a t a i , Ed., I n t e r s c i e n c e , New"York,. 1 9 6 6 . 2 . P o l a n y i and Szabo, Trans, Faraday S o c 3 0 , 5 0 8 ( 1 9 3 4 ) . 3 . M . L. Bender, J . Am. Chem. S o c . , ' £ 3 , 1 6 2 6 ( 1 9 5 1 ) . 4 . J . McMurry i n "Organic R e a c t i o n s , V o l . 2 4 " , W. G. Dauben, Ed., New York, 1 9 7 6 . 5 . L. K. B l a i r , P. C. I s o l a n i and J . M. R i v e r o s , J . Amer. Chem. Soc. 9_5, 1 0 5 7 ( 1 9 7 3 ) . 6 . P. W. Tiedman and J . M. R i v e r o s , J . Am. Chem. Soc. £ 6 , 1 8 5 ( 1 9 7 4 ) . 7 . J . F. G. F a i g l e , P. C. I s o l a n i and J . M. R i v e r o s , J . Amer. Chem. Soc. 9 8 , 2 0 4 9 ( 1 9 7 6 ) . 8 . S. M. Jose and J . M. R i v e r o s , Nouveau.J."". Ohim' 1; . . 1 1 3 ( 1 9 7 7 ) . 9 . K. Takashima and J . M . R i v e r o s , J . Amer. Chem. Soc. 1 0 0 , 6 1 2 8 ( 1 9 7 8 ) . 1 0 . 0 . I. As u b i o j o , L.K. B l a i r and J . I. Brauman, J . Amer. Chem. Soc. 9 7 , 6 6 8 5 ( 1 9 7 5 ) . 1 1 . M. B. Comisarow, Can. J . Chem. 55 . , 1 7 1 ( 1 9 7 7 ) . 1 2 . W. N. Olmstead and J . I. Brauman, Jv Amer. Chem. Soc. 9 9 , 4 2 1 9 ( 1 9 7 7 ) . 1 3 . J . W. Otvos and D. P. Stevenson, J . Am. Chem. Soc. 7 8 , 5 4 6 ( 1 9 5 6 ) . 1 4 . W. T. Huntress, J . B. Laudenslager and R. F. P i n i z z o t t o , Int . J . Mass Spectrom. Ion Phys. 1 3 , 3 3 1 ( 1 9 7 4 ) . 15. K . Jager and A. H e i n g l e i n , Z. N a t u r f o r s c h . A., 2 2 , 7 0 0 ( 1 9 6 7 ) . 1 6 . D. F. Hunt, G. C. S t a f f o r d , F. W. Crow and J . W. R u s s e l , Anal. Chem. 4_8, 2 0 9 8 ( 1 9 7 6 ) . 17. C. B. Sawyer, J . Org. Chem.- 3 7 , 4 2 2 5 ( 1 9 7 2 ) . 1 8 . R.Renaud and L. C. L e l t c h , Can. J, Chem. 3 4 , 1 8 1 ( 1 9 5 6 ) . 1 9 . L. S. Bondar, P. P. Rodionov., V. I. P a u s k i l , V. A. Masten and R. A. Okunev, Izv. Akad. Nauk SSSR, Serthim 1972, 3 0 8 . Chem. A b s t r a c t s 77-19122, -91-20.. AHrxn was determined from a Born c y c l e . E,A. f o r C H 3 O i s 36.65 kcal/mole. from K, J , Reed, Ph.D. T h e s i s S t a n f o r d U n i v e r s i t y , 1975, p. 273- E.A.. f o r C H 3 O C O C " was estimated to be 79« 5- kcal/mole. Bond energies used were D(CH"3C00 CH~3) = 88 kcal/mole and D(H3C OCH-3) = 80 kcal/mole from J . A. Kerr, Chem. Rev. 66, 465(1966). 21. G. Alagona, E. Scrocco and J . Tomasi, J . Amer. Chem. Soc. 97, 6976(1975). 22. J . H. Bowie and B. D. W i l l i a m s , Aust. J . Chem. 27,- 1923 (1974). 23. S. A. S u l l i v a n and J . L. Beauchamp, J . Amer. Chem. Soc. 99, 5017(1977). 

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