The Chemistry of Combustion Processes - American Chemical Society

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 30, 2017 | http://pubs.acs.org Publication Date: April 16, 1983 | doi: 10.1021/bk-1983-0249.ch004

4 Reactivities and Structures of Some Hydrocarbon Ions and Their Relationship to Soot Formation JOHN R. EYLER Department of Chemistry, University of Florida, Gainesville, FL 32611 +

The reactions of three ions, C H3 , C H +, and C H +, which have been sampled from fuel-rich and sooting flames, have been studied with a variety of flame neutrals using an ion cyclotron resonance (icr) mass spectrometer. Three different mass spectrometric techniques have been used to differentiate isomeric forms of the ions whose reaction rate coefficients were measured. These include icr reactivity differ ences, reactive collisions in a triple quadrupole mass spectrometer, and collision-induced dissociaton reac tions in a reversed-geometry, double-focusing mass spectrometer. As a complement to experimental work, theoretical calculations have been carried out to predict the visible and ultraviolet absorption spectra of several isomeric forms of C H3 and C H5 , as well as the relative stability of a number of isomeric forms of the latter ion. The structure determination and the ion/molecule reactivity studies both provide results consistent with and supportive of recently proposed ionic mechanisms for soot nucleation. 3

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The e x i s t e n c e o f i o n s i n f l a m e s has been known f o r many y e a r s , w i t h L a n g m u i r p r o b e s (1) and mass s p e c t r o m e t r i c s a m p l i n g (2) u s e d i n a number o f t h e e a r l y i n v e s t i g a t i o n s . H o w e v e r , i o n number d e n s i t i e s were f o u n d t o be much h i g h e r t h a n c o u l d be e x p l a i n e d by e q u i l i b r i u m thermal i o n i z a t i o n at the temperatures occuring i n flames. C a l c o t e (3) f i r s t s u g g e s t e d t h e c h e m i i o n i z a t i o n mechanism CH* + 0 + HCO + e " +

(1)

w h i c h i s now g e n e r a l l y a c c e p t e d as t h e most i m p o r t a n t i o n f o r m a t i o n mechanism i n many f l a m e s . I n r e c e n t y e a r s a number o f g r o u p s ( 4 - 7 ) have used i m p r o v e d mass s p e c t r o m e t r i c s a m p l i n g t e c h n i q u e s t o 0097-6156/84/0249-0049506.00/0 © 1984 American Chemical Society Sloane; The Chemistry of Combustion Processes ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


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CHEMISTRY OF COMBUSTION PROCESSES

o b t a i n d e t a i l e d p r o f i l e s o f i o n s i n b o t h f u e l - l e a n and - r i c h f l a m e s , and t o e x p l a i n t h e i o n c h e m i s t r y i n v o l v e d i n t h e f o r m a t i o n and l o s s o f t h e o b s e r v e d i o n s ( 8 ) . S i n c e most c o m b u s t i o n pro c e s s e s have been shown t o p r o c e e d v i a n e u t r a l and f r e e - r a d i c a l r e a c t i o n s , the importance of i o n s i n combustion ( a s opposed t o t h e i r e x i s t e n c e ) has n o t been e s t a b l i s h e d . One a r e a o f c o m b u s t i o n c h e m i s t r y where i o n s and i o n / m o l e c u l e r e a c t i o n s may p l a y a c r i t i c a l r o l e , and w h i c h has been e m p h a s i z e d i n o u r s t u d i e s t o d a t e , i s t h a t of soot n u c l e a t i o n i n f u e l - r i c h flames. A r e c e n t r e v i e w by C a l c o t e (9) d i s c u s s e s many o f t h e p r o p o s e d mechanisms o f s o o t n u c l e a t i o n , b o t h n e u t r a l and i o n i c , and p r e s e n t s t h e c a s e f o r an i o n / m o l e c u l e scheme, b e g i n n i n g w i t h CoH^ and sequentially a d d i n g p r i m a r i l y a c e t y l e n e and p o l y a c e t y l e n e molecules i n rapid condensation and c o n d e n s a t i o n - e l i m i n a t i o n r e a c t i o n s which l e a d t o p o l y c y c l i c a r o m a t i c hydrocarbon i o n s of m/z 500 - 1000 amu. This p a r t i c u l a r ion/molecule soot n u c l e a t i o n m o d e l has been e l u c i d a t e d f u r t h e r i n an a r t i c l e by C a l c o t e and O l s o n ( 10) where a s e r i e s o f i o n / m o l e c u l e r e a c t i o n s were combined w i t h a c e t y l e n e o x i d a t i o n r e a c t i o n s and a c o m p u t a t i o n a l model developed. T h i s model gave i o n p r o f i l e s r e a s o n a b l y s i m i l a r t o those a c t u a l l y observed i n s o o t i n g f l a m e s , and a l s o p r e d i c t e d c o n c e n t r a t i o n s o f i o n s s u f f i c i e n t l y h i g h t h a t t h e y m i g h t be c o n s i d e r e d as s o o t n u c l e a t i o n s i t e s . A s p e c t s o f an i o n i c mechanism f o r s o o t n u c l e a t i o n a r e d i s c u s s e d i n more d e t a i l i n a n o t h e r a r t i c l e i n t h i s v o l u m e , so no more w i l l be s a i d a t t h i s p o i n t about t h e g e n e r a l f e a t u r e s o f t h e scheme. I t s h o u l d be n o t e d , h o w e v e r , t h a t o b j e c t i o n s t o i t have been r a i s e d r e c e n t l y ( 1 1 ) . Our work i n t h i s a r e a has been prompted by a number o f weak n e s s e s o f t h e i o n / m o l e c u l e s o o t n u c l e a t i o n scheme o f C a l c o t e and O l s o n ( 10) ( n o t e d by t h e a u t h o r s i n t h e i r a r t i c l e ) . V e r y few e x p e r i m e n t a l s t u d i e s have been c a r r i e d out t o d e t e r m i n e t h e r a t e c o e f f i c i e n t s f o r r e a c t i o n s of s m a l l hydrocarbon ions w i t h v a r i o u s flame n e u t r a l s . N o t i n g t h i s l a c k o f d a t a , C a l c o t e and O l s o n s e t a l l r a t e c o e f f i c i e n t s i n t h e i r model t o t h e ( n o t u n r e a s o n a b l e ) v a l u e o f 2 χ 10~ cm / s . As w i l l be s e e n l a t e r i n t h i s a r t i c l e , a l a r g e number o f i s o m e r i c s t r u c t u r e s a r e p o s s i b l e f o r e v e n s m a l l h y d r o c a r b o n i o n s , and l i t t l e , i f a n y , r e l i a b l e t h e r m o c h e m i c a l d a t a e x i s t s f o r any o f them. C a l c o t e and O l s o n , i n t h e i r m o d e l , c h o s e what seemed t o be t h e most r e a s o n a b l e s t r u c t u r e f o r many o f t h e i o n s i n v o l v e d , and e s t i m a t e d h e a t s o f f o r m a t i o n i n t h e many c a s e s where none c o u l d be found i n t h e l i t e r a t u r e . In an a t t e m p t t o p r o v i d e more a c c u r a t e d a t a on i o n / m o l e c u l e r e a c t i o n s , i o n s t r u c t u r e s , and i o n t h e r m o c h e m i s t r y w h i c h may be r e l e v a n t t o s o o t f o r m a t i o n , we have i n i t i a t e d a p r o g r a m t o o b t a i n such i n f o r m a t i o n i n c o n t r o l l e d l a b o r a t o r y s t u d i e s . Ion/molecule r e a c t i o n r a t e c o e f f i c i e n t s f o r t h e r e a c t i o n s o f m a j o r (and i n i t i a l l y low m o l e c u l a r w e i g h t ) i o n i c s p e c i e s found i n f u e l - r i c h and s o o t i n g f l a m e s w i t h a v a r i e t y o f f l a m e n e u t r a l s have been and a r e c o n t i n u i n g t o be d e t e r m i n e d . We have used s e v e r a l mass s p e c t r o m e t r i c t e c h n i q u e s , i n c l u d i n g i o n c y c l o t r o n resonance ( i c r ) mass

Sloane; The Chemistry of Combustion Processes ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

4.

Hydrocarbon

EYLER

Ions and Soot

Formation

51

spectrometry, triple quadrupole tandem mass s p e c t r o m e t r y , and c o l l i s i o n - i n d u c e d d i s s o c i a t i o n i n a r e v e r s e d - g e o m e t r y ZAB-2F mass s p e c t r o m e t e r t o probe t h e s t r u c t u r e s o f s m a l l hydrocarbon ions. And we have complemented o u r e x p e r i m e n t a l s t u d i e s w i t h t h e o r e t i c a l c a l c u l a t i o n s o f t h e e n e r g i e s and s p e c t r a o f c e r t a i n i o n s whose s t r u c t u r e s a r e n o t w e l l e s t a b l i s h e d . The r e m a i n d e r o f t h i s a r t i c l e w i l l d i s c u s s o u r work on t h r e e i o n i c systems w h i c h a r e s e e n i n e a r l y stages of t h e ion/molecule chemistry i n f u e l r i c h flames: CU* ,C H and C H . +


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Experimental The m a j o r i t y o f t h e s t u d i e s t o be r e p o r t e d h e r e have been c a r r i e d out u s i n g i o n c y c l o t r o n r e s o n a n c e mass s p e c t r o m e t r y . This v a r i a t i o n o f mass s p e c t r o m e t r y and i t s many a p p l i c a t i o n s have been d i s c u s s e d i n d e t a i l i n s e v e r a l r e v i e w a r t i c l e s ( 1 2 - 1 4 ) and a t l e a s t one book ( 1 5 ) . B r i e f l y , i t u s e s c o m b i n a t i o n s o f s t a t i c e l e c t r i c and m a g n e t i c f i e l d s t o t r a p gaseous i o n s a t l o w p r e s s u r e s for t i m e p e r i o d s o f up t o s e v e r a l s e c o n d s i n d u r a t i o n . During t h e s e l o n g t r a p p i n g p e r i o d s , t h e i o n s c a n be s u b j e c t e d t o e l e c t r o magnetic r a d i a t i o n , o f t e n from tunable l a s e r s , t o study their spectroscopic properties, or their r e a c t i v i t y with various neutral compounds p r e s e n t i n t h e t r a p p i n g r e g i o n c a n be f o l l o w e d . With t h e p u l s e d i c r method ( 16) d e t e r m i n a t i o n o f i o n / m o l e c u l e r e a c t i o n r a t e c o e f f i c i e n t s f o r q u a s i - t h e r m a l i o n s a t temperatures from c a . 300 Κ t o s e v e r a l h u n d r e d Κ h i g h e r c a n be made i n a s t r a i g h t f o r w a r d manner ( 1 7 - 1 9 ) . The s t r u c t u r e s o f i s o m e r i c i o n s c a n a l s o be probed w i t h t h i s t e c h n i q u e s i n c e i o n s o f d i f f e r e n t s t r u c t u r e o f t e n e x h i b i t d i f f e r e n t r e a c t i v i t i e s toward selected neutrals. This a p p r o a c h was f i r s t employed by G r o s s and c o - w o r k e r s ( 20) i n a s t u d y o f C^H, i s o m e r s , and h a s been u t i l i z e d more r e c e n t l y by A u s l o o s and L i a s ( 2 1 - 2 2 ) f o r C H and C ^ H ^ i o n s . F o r one s t u d y i n v o l v i n g t h e d i f f e r e n t i a t o n o f C ^ H ^ i s o m e r s , a F i n n i g a n t r i p l e q u a d r u p o l e mass s p e c t r o m e t e r was e m p l o y e d . This t y p e o f mass s p e c t r o m e t r i c i n s t r u m e n t a t i o n h a s been d e s c r i b e d i n s e v e r a l p u b l i c a t o n s ( 2 3 - 2 4 ) and h a s been a p p l i e d t o a number o f p r o b l e m s ( 2 5 - 2 6 ) i n a n a l y t i c a l c h e m i s t r y . The f i r s t q u a d r u p o l e i s used t o s e l e c t an i o n o f i n t e r e s t , t h e s e c o n d , i n a n r f - o n l y mode, i s used as a t r a p i n w h i c h t h e i o n u n d e r g o e s ( i n o u r case r e a c t i v e ) c o l l i s i o n s w i t h a s e l e c t e d n e u t r a l g a s , and t h e t h i r d q u a d r u p o l e i s used t o a n a l y z e t h e r e s u l t s o f t h o s e c o l l i s i o n s . Work on C,H^ s t r u c t u r e d i f f e r e n t i a t i o n was c a r r i e d o u t on a VG A n a l y tical Instruments ZAB-2F r e v e r s e - g e o m e t r y d o u b l e - f o c u s i n g mass spectrometer (27) a t the Naval Research Laboratory i n Washington, DC. U s i n g t h e MIKES-CID ( 2 8 ) a p p r o a c h , a p a r t i c u l a r m/z i o n i s passed through t h e magnetic s e c t o r o f t h e s p e c t r o m e t e r and, w i t h k i n e t i c e n e r g y u s u a l l y i n t h e 4-8 keV r a n g e , c o l l i d e s w i t h a g a s i n t h e second f i e l d f r e e r e g i o n . The p r o d u c t s o f t h i s c o l l i s i o n a r e e n e r g y a n a l y z e d by an e l e c t r o s t a t i c a n a l y z e r , w i t h t h i s i n f o r mation then related t o t h e mass o f t h e c o l l i s i o n products. +

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T h e o r e t i c a l s t u d i e s on i o n s t r u c t u r e s and s p e c t r a were c a r r i e d out u s i n g i n i t i a l l y t h e Amdahl 450-V7 and more r e c e n t l y t h e IBM 3081D computer of t h e N o r t h e a s t R e g i o n a l D a t a C e n t e r a t t h e U n i v e r s i t y o f F l o r i d a , and a D i g i t a l Equipment C o r p . VAX 11/780 m i n i c o m p u t e r i n the Quantum T h e o r y P r o j e c t a t UF. Results CoHo^. 3 3 m a j o r i o n sampled f r o m a w i d e range of f u e l T i c h a n d s o o t i n g f l a m e s , and has been t a k e n as t h e s t a r t i n g p o i n t f o r t h e i o n i c s o o t f o r m a t i o n scheme p r o p o s e d by C a l c o t e and O l s o n ( a l t h o u g h t h e r e i s s t i l some u n c e r t a i n t y as t o i t s e x a c t f o r m a t i o n mechanism i n c e r t a i n f l a m e s ) ( 1 0 ) . Two i s o m e r i c s t r u c t u r e s a r e important i n d i s c u s s i n g i t s r o l e i n flame systems. The f i r s t i s t h e c y c l o p r o p e n y l i u m i s o m e r , I , w h i c h has been most o f t e n formed


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and s t u d i e d i n mass s p e c t r o m e t r i c and t h e o r e t i c a l work t o d a t e . T h i s i s g e n e r a l l y r e c o g n i z e d as the most s t a b l e C^H^ isomer, w i t h a t h e o r e t i c a l l y c a l c u l a t e d h e a t of f o r m a t i o n of 253 k c a l / m o l (29), w h i c h i s i n q u i t e good agreement w i t h t h e 256 ± 2 k c a l / m o l d e t e r mined ( 3 0 ) by e x p e r i m e n t . C^H^"*" i o n s p o s s e s s i n g t h i s s t r u c t u r e can be formed f o r s t u d y i n a mass s p e c t r o m e t e r by e l e c t r o n i m p a c t on a number of p r e c u r s o r s , i n c l u d i n g aliène (C^H^) and t h e v a r i o u s p r o p a r g y l h a l i d e s (C^H^X). A second and p o t e n t i a l l y more i m p o r tant C^H^ s t r u c t u r e i s t h a t of t h e l i n e a r p r o p a r g y l i u m i o n , I I .

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4.

EYLER

Hydrocarbon

Ions and Soot

53

Formation

The h e a t o f f o r m a t i o n o f t h i s i o n has been c a l c u l a t e d ( 2 9 ) t o be 31-34 k c a l / m o l h i g h e r t h a n t h a t o f t h e c y c l o p r o p e n y l i u m i o n , a g a i n i n f a i r agreement w i t h t h e 25 k c a l / m o l d i f f e r e n c e f o u n d ( 3 0 ) experimentally. F a r l e s s t h e o r e t i c a l and e x p e r i m e n t a l a t t e n t i o n has been g i v e n t h i s f o r m o f t h e C^H^"" i o n , a l t h o u g h t h e r e was a t l e a s t one s u g g e s t i o n ( 3 1 ) t h a t i t m i g h t be i m p o r t a n t i n f l a m e s . Mass s p e c t r o m e t r i c s t u d y o f t h e p r o p a r g y l i u m i s o m e r h a s become more s t r a i g h t f o r w a r d w i t h t h e r e p o r t ( 3 2 ) by A u s l o o s and L i a s t h a t s i g n i f i c a n t f r a c t i o n s o f t h e i s o m e r c a n be p r o d u c e d by c h a r g e t r a n s f e r reactions of small ions ( A r , X e , C 0 , N e , etc.) w i t h p r o p a r g y l c h l o r i d e and b r o m i d e . Work i n o u r l a b o r a t o r i e s h a s shown t h a t even h i g h e r p r o p o r t i o n s o f I I r e l a t i v e t o I c a n be obtained by e i t h e r e l e c t r o n i m p a c t on o r c h a r g e exchange w i t h p r o p a r g y l i o d i d e ( s y n t h e s i z e d by a h a l i d e exchange r e a c t i o n w i t h propargyl bromide). O t h e r i s o m e r i c C^H^* s t r u c t u r e s have been c a l c u l a t e d ( 2 9 ) t o be 1 eV o r h i g h e r i n e n e r g y t h a n I I , and have not been s e r i o u s l y s t u d i e d o r d i s c u s s e d i n c o n n e c t i o n w i t h f l a m e mechanisms. 1


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Because of i t s s t a b i l i t y , t h e c y c l o p r o p e n y l i u m i o n , I , has been t h o u g h t t o be r e l a t i v e l y u n r e a c t i v e t o w a r d s i m p l e h y d r o c a r b o n fuels. T h i s has been c o n f i r m e d i n a s t u d y ( 3 3 ) by w o r k e r s f r o m t h e N a t i o n a l B u r e a u o f S t a n d a r d s , who found t h a t I was u n r e a c t i v e with ethylene, aliène, and more i m p o r t a n t l y , a c e t y l e n e and diacetylene. T h i s i s o m e r i c f o r m o f t h e i o n d i d show moderate r e a c t i v i t y t o w a r d some h y d r o c a r b o n s s u c h as i s o - C ^ H g , t r a n s - 2 p e n t a n e , and 1 , 3 - c y c l o - C ^ H g . I o n s w i t h t h e s t r u c t u r e I I were q u i t e r e a c t i v e w i t h many o r t h e 26 n e u t r a l s p e c i e s s t u d i e d . I n p a r t i c u l a r , r e a c t i o n o f I I w i t h a c e t y l e n e p r o d u c e d C5H3 and C^Hc i o n p o p u l a t i o n s w i t h r e a c t i v e and u n r e a c t i v e components. The r e a c t i v e i s o m e r s o f t h e s e i o n s combined w i t h a c e t y l e n e t o f o r m CyH i o n s , which probably possessed a s t a b l e , c y c l i c s t r u c t u r e . I t t h u s a p p e a r s t h a t i f an i o n i c s o o t f o r m a t i o n mechanism s i m i l a r t o t h a t p r o p o s e d by C a l c o t e ( 9 ) i s important i n combustion systems, the l i n e a r , propargylium i o n , II, i s the important reactive precursor. A s u g g e s t i o n h a s been made ( 3 4 ) t h a t C^H^"" d o e s n o t r e a c t s e q u e n t i a l l y w i t h a c e t y l e n e t o form s m a l l p o l y c y c l i c i o n s , but r a t h e r d i r e c t l y w i t h aromatic n e u t r a l s (benzene, t o l u e n e , methyln a p h t h a l e n e s , i n d e n e ) t o f o r m t h e i n i t i a l p o l y c y c l i c i o n s i n one step. W h i l e t h e s e n e u t r a l s have o n l y been seen as m i n o r compon e n t s i n f l a m e s , t h e i r r e a c t i o n s w i t h C ^ H ^ may n o n e t h e l e s s be important s i n c e t h e y would e l i m i n a t e 3-5 s e q u e n t i a l acetylene r e a c t i o n s , each w i t h i t s p o s s i b i l i t y f o r b r a n c h i n g i n t o u n r e a c t i v e as w e l l as r e a c t i v e s p e c i e s . We have used ( 3 5 ) e l e c t r o n i m p a c t on p r o p a r g y l i o d i d e t o p r o d u c e b o t h c y c l o p r o p e n y l i u m and p r o p a r g y l i u m i o n s i n o u r i c r mass s p e c t r o m e t e r and t h e n s t u d i e d t h e i r s u b s e quent ion/molecule reactions with small aromatic neutral molecules. The r e s u l t s f o r b o t h i o n i c s t r u c t u r e s a r e g i v e n i n Table I , along w i t h the expected r a t e c o e f f i c i e n t c a l c u l a t e d a s s u m i n g a n i o n - i n d u c e d d i p o l e model ( 3 6 ) . One sees t h a t as w i t h x

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+




54

other hydrocarbon s p e c i e s , the propargylium i o n r e a c t s quite r a p i d l y w i t h a l l of the n e u t r a l s , w h i l e the c y c l o p r o p e n y l i u m form r e a c t s slower or not at a l l . These r e s u l t s i n d i c a t e t h a t d i r e c t r e a c t i o n o f ^βΗβ* w i t h c y c l i c n e u t r a l s may i n d e e d be an i m p o r t a n t c h a n n e l i n an i o n i c s o o t f o r m a t i o n mechanism, s i n c e t h e r a t e c o e f f i c i e n t s are quite high. However, i m p o r t a n c e o f t h i s c h a n n e l relative t o s e q u e n t i a l a c e t y l e n e a d d i t i o n r e a c t i o n s cannot be a s s e s s e d u n t i l more o f t h e r a t e c o e f f i c i e n t s , i o n i c s t r u c t u r e s , and h e a t s of f o r m a t i o n have been d e t e r m i n e d f o r t h e r e a c t i o n s i n t h e l a t t e r scheme.

Table I .

R a t e C o e f f i c i e n t s f o r Some R e a c t i o n s o f C^H-j +

+ Reactant Neutral

Reactant Neutral

Cyclic

C H 3

Acetylene

3

(C2H2)

-» P r o d u c t s Linear

Langevin

N.R.

12

11

Benzene

(C^H^)

N.R.

15

16

Toluene

(CyHg)

0.17

15-20

16

Naphthalene

(C

H )

0.17

7.0

13

N.R

5.7

2-Methylnaphthalene ( n i o ^

0.21

1.6

Indene

4.4

18-20

— — —

1 Q

8

1-Methylnaphthalene

(C^H^) c

(CgHg)

H

A l l r a t e c o e f f i c i e n t s i n cm /s χ 10 . N.R. = no r e a c t i o n o r r e a c t i o n l e s s t h a n c e l l — = not c a l c u l a t e d .

loss.

The r e a c t i v i t y o f t h e p r o p a r g y l i u m f g r m and t h e n o n r e a c t i v i t y o f t h e c y c l o p r o p e n y l i u m f o r m o f C^H^ toward a c e t y l e n e l e d t o d e v e l o p m e n t o f a new mass s p e c t r o m e t r i c t e c h n i q u e f o r t h e d i f f e r e n t i a t i o n of s t r u c t u r a l isomers. I n c o l l a b o r a t i o n (37) w i t h D r s . Y o s t and F e t t e r o l f i n o u r d e p a r t m e n t we have used r e a c t i v e c o l l i s i o n s o f low k i n e t i c e n e r g y i o n s i n t h e c e n t e r q u a d r u p o l e of a t r i p l e q u a d r u p o l e mass s p e c t r o m e t e r t o d i f f e r e n t i a t e between t h e two i s o m e r i c forms of C^H/**. C^H^ i o n s were formed by e l e c t r o n i m p a c t on p r o p a r g y l c h l o r i d e , b r o m i d e , and i o d i d e , m a s s - s e l e c t e d i n t h e f i r s t q u a d r u p o l e , t h e n p a s s e d i n t o t h e second q u a d r u p o l e w i t h e n e r g i e s o f 2-20 eV where t h e y r e a c t e d w i t h a c e t y l e n e ^ Ions r e s u l t i n g f r o m t h e s e r e a c t i o n s , as w e l l as u n r e a c t e d C^H^ ions, w e r e t h e n m a s s - a n a l y z e d i n t h e t h i r d q u a d r u p o l e . The i n t e n s i t y o f C Η , formed by t h e r e a c t i o n o f t h e p r o p a r g y l i u m form o f C Η +

+

+


4.

EYLER

Hydrocarbon

Ions and Soot

Formation

with acetylene, increased s l i g h t l y r e l a t i v e to unreacted C3H3 when propargyl bromide instead of c h l o r i d e was used as precursor of ; a marked increase was seen when propargyl iodide was used. These r e s u l t s are consistent with i c r ion r e a c t i v i t y data that show e l e c t r o n impact upon propargyl iodide produces predomi nantly the l i n e a r , propargylium C ^ H ^ i o n , while use of the bromide or c h l o r i d e produces almost e x c l u s i v e l y the c y c l o p r o p e n y l ium i o n . Since quadrupole mass spectrometers have been employed i n some flame sampling s t u d i e s , our d i f f e r e n t i a t i o n of ion s t r u c t u r e s with a t r i p l e quadrupole instrument suggests the p o s s i b i l i t y of a c t u a l l y determining the s t r u c t u r e s of selected ions sampled mass s p e c t r o m e t r i c a l l y from flames. Such s t r u c t u r e determination could serve to confirm, or f u r t h e r e l u c i d a t e , an i o n i c soot n u c l e a t i o n pathway. The number d e n s i t i e s determined for C^H^"*" ions i n s e v e r a l experimentally studied flames are s u f f i c i e n t l y high that use of l a s e r induced fluorescence ( l i f ) might be considered as a r e l a t i v e l y n o n - i n t r u s i v e probe for mapping the i n t e n s i t y d i s t r i b u t i o n of these ions i n the flame. However, some knowledge of the ener gies of the various excited states of the ions i s necessary before a serious attempt at using l i f can be made. The excited s t a t e energetics depend, of course, on the s t r u c t u r e of the C ^ H ^ isomer under c o n s i d e r a t i o n . No experimental determination of the excited states of e i t h e r form of the ion considered here has been c a r r i e d out. One rather complete t h e o r e t i c a l c a l c u l a t i o n has been p e r formed (38) on the cyclopropenylium i o n , with the SCF LCAO-MO CI r e s u l t s i n d i c a t i n g that the f i r s t excited s i n g l e t of the i o n l i e s some 8.5 eV above the ground s t a t e . The e x c i t a t i o n wavelength f o r lif studies on t h i s s t r u c t u r e would thus be ca. 146 nm, shorter than that c u r r e n t l y a v a i l a b l e from commercial l a s e r s . However^ one might expect that the l i n e a r , propargylium form of the C3H3 ion would possess l o w e r - l y i n g excited s t a t e s . No c a l c u l a t i o n of the excited state energetics of t h i s s t r u c t u r a l isomer has been performed. In c o l l a b o r a t i o n with D r s . Zerner and Edwards of the U n i v e r s i t y of F l o r i d a Quantum Theory P r o j e c t , we are c u r r e n t l y i n v e s t i g a t i n g t h i s problem. A spectroscopic INDO program (39) has been used to obtain e x c i t a t i o n energies for the propargylium structure, and to confirm the e a r l i e r (38) results for the cyclopropenylium i o n . Several low l y i n g excited s t a t e s , i n the range 3-5 eV above the ground s t a t e , have been p r e d i c t e d , but a l l are symmetry forbidden to f i r s t o r d e r , and the forbiddeness i s not broken by v i b r o n i c c o u p l i n g . A second, more d e t a i l e d set of c a l c u l a t i o n s , i n c o l l a b o r a t i o n with Drs. Sabin and Oddershede, using a p o l a r i z a t i o n propagator method (40) has produced r e s u l t s somewhat more promising than reported above. A transition r e q u i r i n g 5.36 ev energy (corresponding to a photon wavelength of 231 nm) with an o s c i l l a t o r strength of 0.14 has been p r e d i c t e d . This t r a n s i t i o n might be excited by l a s e r r a d i a t i o n from a frequency doubled dye l a s e r mixed with the fundamental output of a Nd:YAG pump l a s e r . Thus l i f d e t e c t i o n of l i n e a r , propargylium +


55



56

C3H3 i o n s i n f l a m e s i s a p o s s i b i l i t y , a l t h o u g h i n t e r f e r e n c e s f r o m other (possibly aromatic) s p e c i e s e x c i t e d by l i g h t i n this w a v e l e n g t h r e g i o n would have t o be e l i m i n a t e d .

1

^ H ^ . The C^H^" " i o n , a l t h o u g h n o t a m a j o r i o n i n a c e t y l e n e Tlames, has been s e e n i n c e r t a i n o t h e r s t u d i e s ( 4 1 ) , i n c l u d i n g t h o s e p r o b i n g benzene f l a m e s . We a l s o o b s e r v e d t h i s i o n i n one o f our e a r l y i n v e s t i g a t i o n s ( 4 2 ) o f s e q u e n t i a l i o n / m o l e c u l e r e a c t i o n s i n a c e t y l e n e , where i t was p r o d u c e d by t h e r e a c t i o n o f C ^ H ^ i o n s w i t h t h e p a r e n t n e u t r a l compound. Similar to the i c r result d i s c u s s e d above f o r C ^ H ^ , t h e C ^ H ^ i o n was p r o d u c e d as a m i x t u r e o f isomers^, one r e a c t i v e toward a c e t y l e n e , and one u n r e a c t i v e . As w i t h CUH^ , t h e two most l i k e l y i s o m e r i c s t r u c t u r e s a r e a c y c l i c one I I I , t h e p h e n y l i u m i o n , and an a c y c l i c i o n IV.


+

+

H III

IV

An ab i n i t i o ( 4 3 ) and a MINDO/3 ( 4 4 ) c a l c u l a t i o n have g i v e n somewhat d i f f e r i n g v a l u e s f o r t h e heat o f f o r m a t i o n o f t h e p h e n y l i u m i o n ( 2 8 0 v s . 245 k c a l / m o l , a l t h o u g h see r e f . 44 f o r a d i s c u s s i o n which resolves t h i s d i f f i c u l t y ) . E x p e r i m e n t a l AH v a l u e s a r e i n t h e r a n g e 266-270 k c a l / m o l ( 4 5 , 4 6 ) . S i m i l a r l y , t h e open c h a i n i s o m e r IV has been c a l c u l a t e d t o have a heat o f f o r m a t i o n 17 and 16 k c a l / m o l h i g h e r t h a n t h e p h e n y l i u m by t h e ab i n i t i o and MINDO/3 calculations, respectively. The MINDO/3 r e s u l t s g i v e s u b s t a n t i a l l y h i g h e r heats of f o r m a t i o n f o r o t h e r i s o m e r i c forms. We used b o t h i c r r e a c t i v i t y s t u d i e s and c o l l i s i o n - i n d u c e d d i s s o c i a t i o n ( c i d ) i n a ZAB-2F d o u b l e - f o c u s i n g , reversed-geometry mass s p e c t r o m e t e r t o s t u d y t h e s t r u c t u r e s o f C ^ H c i s o m e r s ( 4 7 ) . B o t h l i n e s o f i n v e s t i g a t i o n made u s e o f t h e e a r l i e r r e p o r t ( 4 8 ) t h a t C ^ H ^ i o n s p o s s e s s i n g t h e p h e n y l i u m s t r u c t u r e c o u l d be formed by p r o t o n t r a n s f e r t o h a l o b e n z e n e s , f o l l o w e d by t h e l o s s o f H X . I o n s s o formed i n t h e i c r mass s p e c t r o m e t e r were found t o be unreactive toward acetylene, indicating that i n sequential i o n / m o l e c u l e r e a c t i o n s i n a c e t y l e n e , t h e p h e n y l i u m C^-Hc i s o m e r i s f

+

+


4.

Hydrocarbon

EYLER

Ions and Soot

57

Formation

unreactive and the a c y c l i c isomer IV i s the r e a c t i v e species* This conclusion was f u r t h e r borne out by an extensive s e r i e s of cid experiments c a r r i e d out i n c o l l a b o r a t i o n with Dr. J . E . Campana at the Naval Research Laboratory. ^6 5 *formed i n a v a r i e t y of ways: d i r e c t e l e c t r o n impact on a number of n e u t r a l precursors; proton-assisted dehydrohalogenation reactions like those shown (48) to produce phenylium i o n s ; and s e q u e n t i a l ace tylene ion/molecule r e a c t i o n s . The l a t t e r two formation schemes were c a r r i e d out i n the mass spectrometer's high-pressure chemical i o n i z a t i o n source. Mass selected C ^ H ^ ions were i n v e s t i g a t e d using both charge s t r i p p i n g (49) reactions and fragmentation i n t o C^H^ and C^Hg i o n s , where the t h e o r e t i c a l c a l c u l a t i o n s (44) predicted that the a c y c l i c isomer should produce r e l a t i v e l y more of the l a t t e r i o n . Both classes of high energy processes v e r i f i e d that the r e a c t i v e isomer produced by s e q u e n t i a l ion/molecule reactions i n acetylene has the a c y c l i c s t r u c t u r e IV and the unreactive isomer possesses the phenylium s t r u c t u r e . Thus s i m i l a r behavior ( a c y c l i c r e a c t i v e , c y c l i c unreactive) has been found for both C H and C H ions. H

+

o n s

w

e

r

e


+

+

3

+

3

6

5

C^Hç^. Reaction of the propargylium form of CoH~"*" with acetylene "Has" been shown ( 33) to produce both C ^ H ^ ana C ^ H . In flame sampling experiments the l a t t e r ion i s u s u a l l y more abundant (34) (although under c e r t a i n c o n d i t i o n s , the r e l a t i v e abundances of the two product ions are reversed (50)). Our work to date has concen trated on C ^ H ^ because i t can be formed i n r e l a t i v e l y high abun dance i n our i c r mass spectrometer from a number of n e u t r a l p r e cursors. There i s s u b s t a n t i a l confusion, however, as to the r e l a t i v e s t a b i l i t y of the s e v e r a l p o s s i b l e isomeric s t r u c t u r e s of this ion. Most t h e o r e t i c a l studies (51) have concentrated on the c y c l i c structure, V, +

+

3

+

H

V while some experimental work was i n t e r p r e t e d (52) i n terms of both a c y c l i c and an a c y c l i c isomer. However, a number of other s t r u c tures , as seen i n Figure 1, can be e n v i s i o n e d , and a few of even the more u n l i k e l y looking of these have been the subject of t h e o r e t i c a l c a l c u l a t i o n s (53). We have formed C H ions from four n e u t r a l precursors and studied t h e i r r e a c t i o n s with various flame molecules. In a d d i t i o n , because of the lack of a consistent 5

5





4.

EYLER

Hydrocarbon

Ions and Soot

Formation

59

theoretical treatment of the energetics of the many p o s s i b l e isomeric forms of C c H ^ , we have c a r r i e d out MINDO/3 c a l c u l a t i o n s on many of them, and more d e t a i l e d ab i n i t i o c a l c u l a t i o n s on three of them. Two d i f f e r e n t primary i o n i z a t i o n schemes were employed to form C ^ H ^ i o n s . The f i r s t was conventional e l e c t r o n impact on cyclopentadiene, d i c y c l o p e n t a d i e n e , l-penten-3-yne, and norbornadiene. The second involved charge t r a n s f e r reactions i n the i c r analyzer c e l l from CO to norbornadiene (54). Results (55) of these r e a c t i v i t y studies are given i n Tables II - V. As can be seen, d i f f e r e n t n e u t r a l precursors produce isomers of d i f f e r i n g r e a c t i v i t y , ranging from quite slow ( c a . 3 χ 10 cc/s i n Table IV) to very high ( c a . 5 χ 1 0 ~ i n Table I V ) . Because of uncer t a i n t y i n the i d e n t i t y of the various C c H ^ s t r u c t u r a l isomers, we have l a b e l l e d them simply A-D i n the Tables. The r e a c t i o n c o e f f i cients determined f o r isomer A are at the lower range of those which can be conveniently measured by the i c r technique, and may represent non-reactive ion loss from the i c r analyzer c e l l . +


+

10

+

Table I I .

Rate C o e f f i c i e n t s for Some Reactions of C c H Formed from Cyclopentadiene C H 5

+ 5

Ions

q

+ Reactant Neutral — * Products Ion Structure

Reactant Neutral

A

Β

Acetylene

Aliène

0.20 0.45 0.32 0.16 0.62 0.27

(C~H,)

Benzene Toluene Naphthalene 1-Methylnaphthalene 2- Methylnaphthalene Indene All

rate c o e f f i c i e n t s

C

0.54 0.56 0.22 0.18 0.21 0.39 i n cm /s

χ 10

.


D

60


Table I I I .

Rate C o e f f i c i e n t s f o r Some Reactions of CjH^ Ions Formed from Dicyclopentadiene + Reactant Neutral —«• Products Ion S t r u c t u r e


Reactant N e u t r a l

A

B

C

D

Acetylene Ethylene Propane 1,3-Butadiene Aliène

0.71 0.49 0.70 0.72 0.38

Benzene Toluene Naphthalene 1-Methylnaphthalene 2-Methylnaphthalene Indene

0.93 1.10 0.31 0.33 0.39 0.46

All

rate c o e f f i c i e n t s

Table IV.

i n cm /s χ 10

·

Rate C o e f f i c i e n t s for Some Reactions of C H from l-penten-3-yne 5

C H 5

+ 5

5

Formed

+ Reactant Neutral —•+ Products Ion Structure

Reactant Neutral

A

Β

C

D

Acetylene Ethylene Propane Methane 1,3-Butadiene Aliène

0.070 0.050 0.070 0.030 0.16 0.020

0.40 2.9 1.10 0.60 2.5 2.3

Benzene Toluene Naphthalene 1-Methylnaphthalene 2-Methylnaphthalene Indene

0.40 0.30 0.030 N.R. 0.023 0.090

3.4 4.0 1.0 0.70 3.0 8.0

A l l rate c o e f f i c i e n t s i n cm /s χ 10 N.R. = no r e a c t i o n or r e a c t i o n less than c e l l

loss.


4.

Hydrocarbon

EYLER

T a b l e V·

Ions and Soot

61

Formation

R a t e C o e f f i c i e n t s f o r Some R e a c t i o n s o f C H Formed f r o m N o r b o r n a d i e n e 5

C H 5

+ 5

+ 5

Ions

+ Reactant N e u t r a l — + Products Ion S t r u c t u r e


Reactant N e u t r a l

A

B

C

D

Acetylene Ethylene Propane Methane 1,3-Butadiene Aliène

0.20 — 0.23 — 0.63 —

0.53

Benzene Toluene Naphthalene 1- M e t h y l n a p h t h a l e n e 2-M e t h y l n a p h t h a l e n e Indene

0.51 —

4.4

0.27 0.20

2.4 8.6

All

2.7 2.2

3 10 r a t e c o e f f i c i e n t s i n cm /s χ 10 .

Isomers Β and C, whose r a t e c o e f f i c i e n t s d i f f e r by l e s s t h a n a f a c t o r o f two i n most c a s e s , may a c t u a l l y be t h e same i s o m e r formed w i t h d i f f e r i n g d e g r e e s o f i n t e r n a l e x c i t a t i o n . However, t h e s u b s t a n t i a l r e a c t i v i t y d i f f e r e n c e s between A and Β o r C, and Β o r C and D, l e a d us t o t h e c o n c l u s i o n t h a t a t l e a s t t h r e e , and p o s s i b l y f o u r , d i f f e r e n t i s o m e r i c s t r u c t u r e s a r e formed f r o m t h e s e precursors. To a i d i n i s o m e r i c i d e n t i f i c a t i o n i n t h e C^H^ r e a c t i o n s , we u n d e r t o o k a s e r i e s o f MINDO/3 c a l c u l a t i o n s t o d e t e r m i n e t h e h e a t s o f f o r m a t i o n o f t h e s t r u c t u r e s shown i n F i g u r e 1. The MINDO/3 method h a s been d e m o n s t r a t e d t o g i v e q u i t e r e a s o n a b l e r e s u l t s f o r g e o m e t r i e s and h e a t s o f f o r m a t i o n o f b o t h n e u t r a l and p o s i t i v e l y charged hydrocarbon s p e c i e s ( 5 6 ) . We used a g r a d i e n t g e o m e t r y o p t i m i z a t i o n r o u t i n e w i t h a m o d i f i e d MINDO/3 p r o g r a m on o u r campus Amdahl 450-V7 c o m p u t e r . The r e s u l t s ( 5 5 ) a r e shown i n T a b l e V I , where t h e l o w e s t e n e r g y i s o m e r i s p r e d i c t e d t o be t h e v i n y l c y c l o propenylium i o n , w i t h s e v e r a l a c y c l i c isomers next h i g h e s t i n e n e r g y , and some o f t h e c y c l i c s p e c i e s h i g h e r s t i l l . MINDO/3 i s known t o have s e v e r a l d e f i c i e n c i e s , i n c l u d i n g u n d e r e s t i m a t i o n o f s m a l l r i n g s t r a i n and p r e d i c t i o n o f t h e s t a b i l i t y o f t r i p l e bonds ca. 5 k c a l / m o l t o o l o w . S i n c e t h e lowest heat o f f o r m a t i o n found i n o u r c a l c u l a t i o n s was f o r a compound w i t h a three-membered r i n g , and s i n c e t h e a c y c l i c i s o m e r s c o n t a i n t r i p l e b o n d s , t h e MINDO/3 +



62

+

Table VI.

MINDO/3 R e s u l t s f o r 0 Η,- I s o m e r s ς


Structure* m 1, k, j, i, h c, f, e, d, b, a,

AH

h

g Dc , triplet cis-"linear" trans-"linear" "linear" vinylcyclopropenylium h

Figure 1 for structural

(kcal/mol) 288 270 268 266 257 256 255 253 252 247 242 238

pyramid D^ , singlet dimethyleneeyclopropenylium methylenecyclobutenylium

*See t e x t and

f

details.

r e s u l t s are suspect. We have t h u s c h o s e n t h r e e i s o m e r i c f o r m s , a , b and c f r o m F i g u r e 1 and c a r r i e d out ab i n i t i o c a l c u l a t i o n s u s i n g a 4-31G b a s i s s e t and a m o d i f i e d G a u s s i a n 80 ( 57) package on our Quantum T h e o r y P r o j e c t VAX 11/780 computer. These more d e t a i l e d c a l c u l a t i o n s c o n f i r m t h e o r d e r of s t a b i l i t y f o r the t h r e e i s o m e r s f o u n d u s i n g MINDO/3. We a r e c u r r e n t l y u s i n g a f o u r t h o r d e r manybody p e r t u r b a t i o n t h e o r y package ( 5 8 ) w h i c h i n c l u d e s single, d o u b l e and q u a d r u p l e e x c i t a t i o n s t o see i f the r e l a t i v e s t a b i l i ties of the three isomeric forms chosen shift further. P r e l i m i n a r y r e s u l t s show t h a t i s o m e r s a and b have e s s e n t i a l l y t h e same h e a t of f o r m a t i o n , w h i l e t h e ΔΗ^ of c r e m a i n s c a . 15 k c a l / m o l higher. The r e l a t i v e s t a b i l i t y o r d e r p r e d i c t e d by our MINDO/3 c a l c u l a t i o n s has l e d to a t e n t a t i v e assignment of isomers, a l t h o u g h t h i s may have t o be m o d i f i e d when f i n a l r e s u l t s of the more d e t a i l e d quantum m e c h a n i c a l c a l c u l a t i o n s d i s c u s s e d above a r e available. We a s s i g n the v i n y l c y c l o p r o p e n y l i u m s t r u c t u r e (a) c a l c u l a t e d t o be l o w e s t i n e n e r g y , t o the u n r e a c t i v e i s o m e r A. The m o d e r a t e l y r e a c t i v e i s o m e r s Β and C, w h i c h may be t h e same i s o m e r w i t h d i f f e r e n t amounts of i n t e r n a l e n e r g y , we a s s i g n t o one o r more of t h e a c y c l i c forms s i m i l a r t o b i n F i g u r e 1. The h i g h l y r e a c t i v e i s o m e r , D, we a s s i g n the n e x t h i g h e r e n e r g y c y c l i c s t r u c t u r e , c, f r o m F i g u r e 1. R e a c t i v i t y d i f f e r e n c e s between the s p e c i e s A and Β o r C, and Β o r C and D a r e so g r e a t t h a t we b e l i e v e t h e y a r e t h e r e s u l t of the i o n s p o s s e s s i n g different i s o m e r i c s t r u c t u r e s , and not j u s t c o n t a i n i n g d i f f e r i n g amounts o f i n t e r n a l energy. I f t h i s s t r u c t u r a l assignment proves c o r r e c t , i t p r o v i d e s a d i f f e r e n t c a s e f r o m t h a t w h i c h h e l d f o r C^H^"" and 1



4.

EYLER

Hydrocarbon

Ions and Soot

63

Formation

C^Hc . Now the c y c l i c isomer i s less s t a b l e , and more r e a c t i v e , with the a c y c l i c less r e a c t i v e , and a d i f f e r e n t , mixed c y c l i c and l i n e a r s t r u c t u r e (the v i n y l c y c l o p r o p e n y l i u m ion) i s the l e a s t reactive species. A synthesis of both experimental and t h e o r e t i c a l results generated by e a r l i e r workers and our group s t i l l seems i n s u f f i cient to determine c o n c l u s i v e l y which isomeric s t r u c t u r e should be assigned to the various r e a c t i v e species which have been observed experimentally. The technique of p h o t o d i s s o c i a t i o n of gaseous ions trapped i n an i c r mass spectrometer has been used s u c c e s s fully for s t r u c t u r a l i d e n t i f i c a t i o n i n a number of previous studies ( 59), and we hope to apply i t to the present case. However, i t i s necessary to have some idea of the approximate spectra of the various C ^ H ^ isomers which might be expected, i n order to see i f they are s u f f i c i e n t l y d i f f e r e n t that s t r u c t u r a l d i f f e r e n t i a t i o n might be accomplished. A l s o , some idea of the p o s i t i o n s of various absorption bands i s necessary i n order to know which l i g h t sources (and most often which wavelengths of tunable l a s e r s ) must be used for the p h o t o d i s s o c i a t i o n . In c o l l a b o r a t i o n with Drs. Zerner and Edwards of the UF Quantum Theory P r o j e c t , we have t h e o r e t i c a l l y predicted the spectra of four C5H5*" isomers, using the INDO s p e c t r a l p r e d i c t i o n program (39). Results are shown i n Figure 2 for four of the p o s s i b l e isomeric s t r u c t u r e s given i n Figure 1. One sees that at l e a s t one i s predicted to have a moderately intense absorption band i n the near u l t r a v i o l e t r e g i o n , which should be a c c e s s i b l e by e i t h e r excimer-pumped or Nd:YAG-pumped dye l a s e r s (but unfortunately not by our e x i s t i n g flashlamp-pumped dye l a s e r ) . With an improved laser c a p a b i l i t y (or perhaps using m u l t i p l e photon i r laserinduced d i s s o c i a t i o n (60) with our cw CO2 l a s e r ) , we should be able to assign more completely the C^H^ isomeric s t r u c t u r e s . +

+

Conclusion Studies of the s t r u c t u r e and r e a c t i v i t i e s of the three ions d i s cussed i n t h i s a r t i c l e have continued to bear out the p o s s i b i l i t y of an i o n i c mechanism for soot formation. The r e a c t i o n rate c o e f f i c i e n t s are c e r t a i n l y rapid and i n general l a r g e r than those used by Olson and Calcote (10) i n t h e i r model, at l e a s t for c e r t a i n isomeric forms of the ions and at the temperatures of ca. 325 Κ used i n our work. However, u n t i l more s o p h i s t i c a t e d flamesampling mass spectrometers are employed, the exact isomeric form of the many ions seen i n flames cannot be known, and thus cannot be r e l a t e d d i r e c t l y to our work. As i s to be expected, d e t a i l s of the r e a c t i v i t y or n o n r e a c t i v i t y of various isomers vary as one moves from one i o n i c species to another. Thus i n some cases (C^H^ ) a c y c l i c forms are less r e a c t i v e , while the c y c l i c isomers react r a p i d l y , while i n others (C^H^" ", C^H/ ") the opposite i s the case. Those channels where the c y c l i c Ions are l e s s r e a c t i v e suggest an opportunity f o r the formation of c y c l i c neutrals i n 1

1



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The Chemistry of Combustion Processes - American Chemical Society

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