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7 Crossed Molecular Beam Studies of Fluorine Chemistry

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J. M . FARRAR and Y. T. L E E Materials and Molecular Research Division, Lawrence Berkeley Laboratory, and Department of Chemistry, University of California, Berkeley, CA 94720

Fluorine atom chemistry has received significant attention from chemical kineticists in recent years. Indeed, the present volume bears testimony to the proliferation of activity in this field. The molecular beam technique, which has come of age during the past several years, has been applied with great success to the chemistry of fluorine atoms and it is this subject which we will address in the present article. The crossed molecular beam technique (1) applied to problems in chemical kinetics involves colliding well-defined beams of atoms and molecules in a vacuum chamber; from the direct measurements of results of two-body collisions, in particular, angular and energy distributions of reaction products, one attempts to visualize in great detail how the reaction occurs. These dynamical features of chemical reactions, the favored orientation of molecules, reaction intermediate lifetimes, and product energy distributions are among the major concerns of a crossed molecular beam experiment. The experimental results which we will describe are primarily those obtained in this laboratory but a few experimental data exist which have been collected elsewhere. Our experimental program in fluorine atom chemistry has been motivated primarily by two facts which have also been important to studies performed by other methods: (1) atomic fluorine abstraction of hydrogen atoms from appropriate molecules has been demonstrated to be an important class of reactions for chemical lasers (2). In particular, the reactions of F + H -->HF + H and F + D -->DF + D have been investigated in great detail by various theoretical and experimental approaches (3-11); the latter reaction provides us with an example from the general class of reactions of fluorine atoms with diatomic molecules. (2) Substitution reactions of fluorine atoms with unsaturated hydrocarbons involving the formation of C-F bonds frequently are observed to proceed through a "complex" which 2

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Root; Fluorine-Containing Free Radicals ACS Symposium Series; American Chemical Society: Washington, DC, 1978.

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l i v e s many r o t a t i o n a l p e r i o d s (12-19). Since the C-F bond i s the strongest s i n g l e bond i n v o l v i n g a carbon atom, these r e a c t i o n s are g e n e r a l l y q u i t e exoergic with cleavage of C-C, C-H, and C-Cl bonds o c c u r r i n g r e a d i l y . Since many o f these r e a c t i o n s do proceed through i n t e r m e d i a t e s , the competition among v a r i o u s modes of bond cleavage i n the l o n g - l i v e d intermediate y i e l d s v a l u a b l e i n f o r m a t i o n on i n t r a m o l e c u l a r energy t r a n s f e r i n the complex p r i o r to decomposition to products. Various s t u d i e s o f r e a c t i o n s o f f l u o r i n e atoms with hydrocarbons, both a l i p h a t i c and aromatic, as a f u n c t i o n of s u b s t i t u e n t placement and i d e n t i t y chain l e n g t h , and i n i t i a l k i n e t i c energy of the r e a c t a n t s thus make the f l u o r i n e atom chemistry observed a very i n t e r e s t i n g probe of unimolecular decay. Many of the f i r s t s t u d i e s of unimolecular decompositions from t h i s l a b o r a t o r y were concerned with the determination of product branching r a t i o s and r e c o i l v e l o c i t y d i s t r i b u t i o n s i n o l e f i n s , aromatics, and h e t e r o c y c l i c s a t a f i x e d c o l l i s i o n energy. C e r t a i n c h a r a c t e r i s t i c f e a t u r e s of these r e a c t i o n s began to emerge i n terms of the nature of the e x i t channel, that i s , whether the c r i t i c a l c o n f i g u r a t i o n corresponded to a p o t e n t i a l energy maximum or merely r e s u l t e d from angular momentum, and the nature of the group emitted. A d d i t i o n a l work on s e l e c t e d systems as a f u n c t i o n of the i n i t i a l c o l l i s i o n energy has l e d to a f u r t h e r understanding o f the r o l e of the p o t e n t i a l s u r f a c e i n unimolecular decompositions. 9

Experimental Method The molecular beam technique, i n general * i s i d e a l l y s u i t e d to the study of r e a c t i o n s of h i g h l y r e a c t i v e s p e c i e s such as f l u o r i n e atoms; s i n c e beam i n t e n s i t i e s and d e t e c t o r s e n s i t i v i t i e s are l i m i t e d even under the most f a v o r a b l e experimental c o n d i t i o n s , only f o r r e a c t i o n s w i t h l a r g e c r o s s s e c t i o n s and f a v o r a b l e kinematic r e l a t i o n s can one hope to measure both energy and angular d i s t r i b u t i o n s of product molecules by c r o s s i n g two w e l l d e f i n e d r e a c t a n t beams at a given c o l l i s i o n energy. Since the i n t e r n a l s t a t e s or the v e l o c i t i e s of the r e a c t a n t s can be s e l e c t e d , the i n i t i a l r e l a t i v e v e l o c i t y v e c t o r can be w e l l - d e f i n e d i n magnitude and d i r e c t i o n and thus the measured q u a n t i t i e s i n a molecular beam experiment, the angular and energy d i s t r i b u t i o n s of s c a t t e r e d products, can be r e l a t e d to the mechanics of s i n g l e collisions. The d i f f e r e n t i a l c r o s s s e c t i o n f o r chemical r e a c t i o n , which i s the q u a n t i t y most e a s i l y r e l a t e d to c l a s s i c a l and quantum mechanical models based on c o l l i s i o n theory and from a knowledge of p o t e n t i a l energy hypersurfaces, i s determined r a t h e r d i r e c t l y i n a beam experiment, although present experimental data y i e l d cross s e c t i o n s averaged to a great extent over i n i t i a l impact parameter and f i n a l i n t e r n a l s t a t e d i s t r i b u t i o n s . This s i t u a t i o n c o n t r a s t s , however, with the much more h i g h l y averaged r a t e constant o f c l a s s i c a l k i n e t i c s which i s an average over the i n i t i a l Boltzmann d i s t r i b u t i o n of the r e a c t i o n c r o s s s e c t i o n at a p a r t i -


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c u l a r r e l a t i v e v e l o c i t y weighted by the r e l a t i v e v e l o c i t y . In p r i n c i p l e the beam technique provides the means f o r studying s t a t e - t o - s t a t e chemical k i n e t i c s on f a v o r a b l e systems, and h i g h l y r e f i n e d experiments employing two crossed beams with s p e c i f i e d i n t e r n a l s t a t e s and r e l a t i v e v e l o c i t i e s are d e f i n i t e l y p o s s i b l e i n the near f u t u r e . In molecular beam experiments, a l l measurements a r e c a r r i e d out i n a l a b o r a t o r y system of c o o r d i n a t e s , but one wishes to i n t e r p r e t data i n terms o f a c o o r d i n a t e system which moves with the center o f mass (cm.) of the c o l l i d i n g system. The manner i n which t h i s t r a n s f o r m a t i o n i s made i s well-understood (20-23), but the Jacobian i n v o l v e d i n the t r a n s f o r m a t i o n d i s t o r t s c e r t a i n p o r t i o n s of the l a b data; i n a d d i t i o n , the l a b - c m . t r a n s f o r mation i s o f t e n not s i n g l e - v a l u e d . In order to circumvent t h i s problem, data f i t t i n g r o u t i n e s have been developed which assume c m . angular and v e l o c i t y d i s t r i b u t i o n s , averaging them and transforming them back to the l a b f o r comparison with the data. When the i n i t i a l v e l o c i t y d i s t r i b u t i o n s are q u i t e narrow, t h i s technique allows one to recover the c m . d i s t r i b u t i o n s r e l i a b l y . A v e l o c i t y v e c t o r diagram shown i n F i g u r e 1 i l l u s t r a t e s the nature of the t r a n s f o r m a t i o n . The v e c t o r C, which p o i n t s i n the d i r e c t i o n of the c m . determined by p a r t i t i o n i n g the i n i t i a l r e l a t i v e v e o c i t y v e c t o r a c c o r d i n g to c o n s e r v a t i o n of l i n e a r momentum, allows us to r e l a t e l a b o r a t o r y v e l o c i t y , γ, and s c a t t e r i n g angle, Θ, to c m . v e l o c i t y , u, and angle, θ, through the v e c t o r a d d i t i o n u + Ç = v . Since we determine y and we d e s i r e u, we must deconvolute C. I f the i n i t i a l c o n d i t i o n s c r e a t e a s u b s t a n t i a l spread i n v e c t o r s Ç, then the s e t of v e c t o r s u which we determine to f i t a measured y are not unique. In f a c t , some experimental data have been i n t e r p r e t e d i n c o r r e c t l y by f a i l i n g to account c o r r e c t l y f o r the spread i n i n i t i a l cond i t i o n s and a l s o f a i l i n g to use the c o r r e c t Jacobian to r e l a t e lab and c m . f l u x e s . We r e f e r the reader to an e a r l i e r review (24) f o r f u r t h e r d i s c u s s i o n of t h i s p o i n t . The development of a crossed molecular beam apparatus capable of the d e t e c t i o n of a r b i t r a r y molecular fragments r e s u l t i n g from chemical r e a c t i o n has been c e n t r a l to the advancement of molecular beam k i n e t i c s . The f i r s t r e l i a b l e and s e n s i t i v e apparatus of t h i s s o r t employing electron-bombardment i o n i z a t i o n of r e a c t i o n products i n an u l t r a h i g h vacuum chamber followed by mass spectrometric d e t e c t i o n became o p e r a t i o n a l i n 1968 (25) and a number of instruments of t h i s type are now employed i n v a r i o u s l a b o r a t o r i e s . This so-called "universal" d e t e c t o r together with the advent of f r e e j e t molecular beams (26) moved molecular beam chemical k i n e t i c s out of the " a l k a l i age", so named because surface i o n i z a t i o n d e t e c t i o n techniques used p r e v i o u s l y l i m i t e d the s p e c i e s to be s t u d i e s to a l k a l i atoms and a l k a l i halides. The general experimental arrangement employed i n a crossed molecular beam apparatus i s shown i n F i g u r e 2. Reagent atomic species formed by thermal d i s s o c i a t i o n i n a heated



Figure 2.

Experimental arrangement for molecular beam studies of fluorine atom chemistry


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oven are c o l l i m a t e d i n t o a beam; separate beam formation, c o l l i m a t i o n , and r e a c t i o n chambers are used to i n s u r e that the beams are w e l l - d e f i n e d and, depending on the nature of t h e i r formation, do not present a l a r g e gas load to the c o l l i s i o n chamber. The d e t a i l s of halogen-atom beam formation are d i s c u s s e d i n greater d e t a i l below. The secondary beam of reactant molecules i s formed i n a s i m i l a r manner and i n t e r s e c t s the atomic halogen beam at 90° i n a c o l l i s i o n chamber maintained at 10"" t o r r by an o i l d i f f u s i o n pump. C o l l i s i o n s then occur at the i n t e r s e c t i o n zone of the beams; f o r measurements of angular d i s t r i b u t i o n s , r e a c t i v e l y s c a t t e r e d products are detected i n the plane d e f i n e d by the beams as a f u n c t i o n of l a b o r a t o r y s c a t t e r i n g angle, Θ, by a r o t a t a b l e mass spectrometer d e t e c t o r . T h i s d e t e c t o r i s comprised of three nested vacuum chambers, each pumped with a separate i o n pump (and cryogenic pumping i n the innermost chamber) to achieve a pressure of l e s s than 1 0 " ^ torr i n the i o n i z a t i o n r e g i o n . An e l e c t r o n bombardment i o n i z e r of approximately 0.1% e f f i c i e n c y produces ions which are then i n j e c t e d i n t o a quadrupole mass f i l t e r . While the pressure i n the i o n i z a t i o n r e g i o n i s ~10"^® t o r r , the p a r t i a l pressure of r e a c t i o n products may be as low as lO""-^ to 10""-^ t o r r when they reach i o n i z a t i o n r e g i o n . At such low s i g n a l l e v e l s , very c a r e f u l work must be done to e x t r a c t meaningful s i g n a l s , i n c l u d i n g mechanical design to e l i m i n a t e sources of background molecules i n the d e t e c t o r , e l a b o r a t e cryogenic pumping, and beam modulation techniques as discussed below. A f t e r the ions pass through the mass f i l t e r , they are a c c e l e r a t e d and c o l l i m a t e d i n t o a beam which i s d i r e c t e d to an aluminum coated cathode h e l d at -30 kV. Several secondary e l e c t r o n s are emitted per i n c i d e n t i o n ; these e l e c t r o n s are a c c e l e r a t e d i n the same 30 kV p o t e n t i a l to a A l coated p l a s t i c s c i n t i l l a t o r which emits s e v e r a l photons per secondary electron. The photon pulses are a m p l i f i e d by a p h o t o m u l t i p l i e r tube and counted u s i n g c o n v e n t i o n a l techniques. As i n d i c a t e d above, beam modulation i s used f o r l o w - l e v e l s i g n a l recovery and to e f f e c t t h i s , the secondary (molecular) beam i s chopped with a t u n i n g f o r k at 150 Hz. A s i g n a l from the chopper i s processed by a d i g i t a l l o g i c c i r c u i t which gates a dual s c a l e r i n syn c h r o n i z a t i o n w i t h the beam modulation. In t h i s c o n f i g u r a t i o n , one s c a l e r counts s i g n a l p l u s background corresponding to the beam open p e r i o d ; the other s c a l e r c o l l e c t s background o n l y . In t h i s manner, r e a c t i v e s c a t t e r i n g s i g n a l s which g e n e r a l l y range from a few counts per second to s e v e r a l hundred per second can be recovered. 7

The t o t a l energy of the system under study c o n s i s t s of the i n i t i a l r e l a t i v e and i n t e r n a l energies p l u s the e x o e r g i c i t y of the r e a c t i o n . T h i s energy can be d i v i d e d among i n t e r n a l degrees of freedom p l u s r e l a t i v e t r a n s l a t i o n a l energy of the products. In our molecular beam experiments, we measure v e l o c i t y d i s t r i b u t i o n s of s c a t t e r e d products to i n f e r i n t e r n a l energy d i s t r i butions by c o n s e r v a t i o n of energy. On the other hand, d e t a i l e d


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s t a t e d i s t r i b u t i o n s f o r some simple diatomic product molecules could be measured by using the molecular beam resonance method (27,28) or l a s e r - i n d u c e d f l u o r e s c e n c e (29,30). V e l o c i t y d i s t r i b u t i o n s of the r e a c t i v e l y s c a t t e r e d products can be measured by time of f l i g h t (TOF) methods i n which the v e l o c i t y of a p a r t i c l e i s determined by measuring the time r e q u i r e d f o r the p a r t i c l e to t r a v e r s e a known d i s t a n c e i n the d e t e c t o r . Although not i n d i c a t e d i n F i g u r e 2, a chopper with very narrow s l i t s chops the r e a c t i v e l y s c a t t e r e d products as they enter the d e t e c t o r at a frequency (1600 Hz, t y p i c a l l y ) which i s high compared to the secondary beam modulation frequency. The burst of molecules then enters the d e t e c t o r at time t and the pulse spreads i n time during t r a n s i t through the d e t e c t o r i n accordance with the v e l o c i t y d i s t r i b u t i o n of molecules i n the p u l s e . A multi-channel s c a l e r then records the d i s t r i b u t i o n of f l i g h t times of the products; s i g n a l averaging i s accomplished by minicomputer c o n t r o l of the TOF u n i t . F l i g h t times of 100-400 ysec through our d e t e c t o r (pathlength 17 cm) are measured q u i t e r e a d i l y using t h i s technique. Having discussed i n general terms how angular and v e l o c i t y d i s t r i b u t i o n s of r e a c t i v e l y s c a t t e r e d products are measured using a " u n i v e r s a l " molecular beam apparatus, we now focus a t t e n t i o n on methods of beam production, i n p a r t i c u l a r , t e c h niques f o r the production of s t a b l e , i n t e n s e beams of f l u o r i n e atoms. Since the bond energy of F 2 i s 36 k c a l mole"*"-*-, thermal d i s s o c i a t i o n of F£ i n a heated n i c k e l oven has been found to be a s u c c e s s f u l method f o r f l u o r i n e atom p r o d u c t i o n . The high r e a c t i v i t y o f f l u o r i n e atoms w i t h v a r i o u s m a t e r i a l s r u l e s out a great number of them f o r oven c o n s t r u c t i o n and we have found that n i c k e l a f f o r d s the best r e a d i l y a v a i l a b l e m a t e r i a l . Thermal d i s s o c i a t i o n of F2 i n a n i c k e l oven at a pressure of ~0.1 - 1.0 t o r r at 750°C can be accomplished with the degree of d i s s o c i a t i o n approaching 50 - 90%. At such low p r e s s u r e s , beam formation i s determined by e f f u s i o n through an o r i f i c e and the v e l o c i t y d i s t r i b u t i o n of the atoms i s the Maxwell-Boltzmann d i s t r i b u t i o n . In the absence of v e l o c i t y s e l e c t i o n , such a broad d i s t r i b u t i o n of i n i t i a l speeds leads to a very broad d i s t r i b u t i o n of i n i t i a l k i n e t i c energies, with the r e l a t i v e v e l o c i t y v e c t o r and the center of mass v e c t o r d i s t r i b u t e d over a l a r g e r e g i o n of v e l o c i t y space. A s l o t t e d d i s c v e l o c i t y s e l e c t o r (31) can be used to reduce t h i s spread i n i n i t i a l c o n d i t i o n s and much of the work discussed i n t h i s paper was performed with such a s e l e c t o r with a f u l l width h a l f maximum (FWHM) v e l o c i t y spread of 20%. Such a small spread i n the i n i t i a l r e l a t i v e v e l o c i t y of the r e a c t a n t s enables the l a b - c m . transformation to be made more uniquely, thus a s s i s t i n g us i n the i n t e r p r e t a t i o n of the data. The v e l o c i t y s e l e c t e d f l u o r i n e atom source was the workhorse f o r much of the work done i n t h i s l a b o r a t o r y , but recent developments i n seeded supersonic beam technology have l e d to much more e f f i c i e n t means of halogen atom beam production. The 0


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expansion of a gas at a few hundred t o r r through a small o r i f i c e i n t o a vacuum has been shown to be an e f f e c t i v e means f o r the production of high i n t e n s i t y , narrow v e l o c i t y spread atomic and molecular beams. By mixing 1 to 2% F2 w i t h a r a r e gas c a r r i e r such as argon or helium and expanding the mixture at 500 t o r r through an o r i f i c e (0.1 mm d i a ) i n a n i c k e l oven heated to 750800°C, f l u o r i n e atom beams were produced which were one to two orders of magnitude more i n t e n s e than v e l o c i t y s e l e c t e d beams and had v e l o c i t y d i s t r i b u t i o n s which were somewhat narrower than s e l e c t e d beams, g e n e r a l l y with a 15% FWHM spread. Such a method of beam p r o d u c t i o n maintains the low p a r t i a l pressure of halogen to make the e q u i l i b r i u m favor atom p r o d u c t i o n w h i l e p r o v i d i n g a high pressure expansion under hydrodynamic flow c o n d i t i o n s l e a d i n g to a narrowing of the v e l o c i t y d i s t r i b u t i o n . Under the hydrodynamic flow c o n d i t i o n s of supersonic expansions, s p e c i e s of d i f f e r e n t masses i n a molecular beam achieve the same t e r m i n a l v e l o c i t y which i s r e l a t e d to the average mass number of the expanding s p e c i e s . T h i s technique can be used to i n c r e a s e the speed of a heavy p a r t i c l e r e l a t i v e to i t s thermal speed i f a small f r a c t i o n of the gas mixture i s the d e s i r e d heavy s p e c i e s and the bulk of the mixture i s a l i g h t c a r r i e r gas. T h i s technique has been used very s u c c e s s f u l l y to produce hyperthermal beams of Xe atoms formed by expansions of mixtures composed of 1% Xe i n 99% H2. By v a r y i n g the d i l u e n t gas from argon to helium, high anergy beams of f l u o r i n e atoms can be produced f o r r e a c t i o n s i n the hyperthermal r e g i o n (1040 k c a l rnole "^). T h i s technique of halogen atom p r o d u c t i o n i s not unique to f l u o r i n e ; i n f a c t , the f i r s t halogen atom beam produced i n t h i s l a b o r a t o r y was a c h l o r i n e atom beam produced by d i s s o c i a t i o n of C I 2 , i n argon b u f f e r gas i n a heated g r a p h i t e oven (32). The technique should be a p p l i c a b l e to a l l of the halogen atoms and a v a s t v a r i e t y of s c a t t e r i n g experiments can be i n i t i a t e d u s i n g such sources. The molecular beams i n our experiments are a l s o formed by supersonic expansion. In a d d i t i o n to enhanced i n t e n s i t y and narrow v e l o c i t y d i s t r i b u t i o n s obtained i n t h i s way, the molecules i n such beams can g e n e r a l l y be considered to be i n t h e i r lowest v i b r a t i o n a l and r o t a t i o n a l s t a t e s . T h i s " f r e e z i n g out" of i n t e r n a l degrees of freedom i s important i n f u r t h e r d e f i n i n g the i n i t i a l experimental c o n d i t i o n s . In understanding the mechanics of c o l l i s i o n s , one can o b t a i n s i m p l i f i c a t i o n s i n i n t e r p r e t a t i o n by knowing that r e a c t a n t molecules are r o t a t i o n a l l y and v i b r a tionally "cold". T h i s point w i l l be d i s c u s s e d f u r t h e r i n l a t e r s e c t i o n s of t h i s paper. -

Examples of Experimental R e s u l t s I . F + Dg DF + D. The r e a c t i o n of atomic f l u o r i n e with molecular hydrogen c o n t a i n i n g molecules has been demonstrated to


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be the c h i e f mechanism r e s p o n s i b l e f o r the HF chemical l a s e r and has r e c e i v e d s i g n i f i c a n t experimental and t h e o r e t i c a l a t t e n t i o n . In a d d i t i o n to chemical l a s e r (2,11,33,34) and i n f r a r e d chemiluminescence (_5,60 measurements on t h i s system which have y i e l d e d values f o r a number of r a t e constant r a t i o s f o r production of s p e c i f i c v i b r a t i o n a l s t a t e s of HF, the molecular beam technique (_3,4) has a l s o provided s i g n i f i c a n t data on t h i s r e a c t i o n . The F + U£ r e a c t i o n to form DF i s exoergic by 31.5 k c a l mole -'-, y i e l d i n g enough energy i n c o n j u n c t i o n with t r a n s l a t i o n to produce DF i n e x c i t e d v i b r a t i o n a l s t a t e s up to v' = 4. The equal gain and zero gain v a r i a t i o n s of the chemical l a s e r technique (2) a l l o w f o r the determination of r a t e constant r a t i o s ky-t/k^.».^ f o r v = 1, 2, 3, and 4 where v i s the DF v i b r a t i o n a l quantum number and i n f r a r e d chemiluminescence measurements allow f o r determinations of t h i s r a t i o f o r v = 2-4. In c o n t r a s t , the crossed molecular beam technique i s not very i d e a l f o r measurements of r e l a t i v e s t a t e populations although, as we s h a l l see, some i n f o r m a t i o n of t h i s type can be determined under f a v o r a b l e conditions. -

f

f

f

The laws of c o n s e r v a t i o n of energy and angular momentum p l a y a very important r o l e i n determining what i n f o r m a t i o n i s contained i n the molecular beam data on t h i s system. A cons i d e r a t i o n of r a t e constant data (35) f o r t h i s system i n c o n j u n c t i o n with the r e c o g n i t i o n t h a t the r a t e constant i s an ensemble average of cross s e c t i o n weighted by r e l a t i v e v e l o c i t y , allows us to estimate the maximum impact parameter c o n t r i b u t i n g to chemical r e a c t i o n . At 300°K. the cross s e c t i o n i s < 1 Â2 S O ο ' ο b i s roughly 1 A. For an impact parameter of 1 A and v i ~ 2 χ cm sec**l ( f o r a c o l l i s i o n energy of 1.7 k c a l m o l e " I ) , L = yvb or approximately 10h i n magnitude. Since at room temper a t u r e , J , the r o t a t i o n a l angular momentum of D£, i s no more than 2h, we can use the c o n s e r v a t i o n r e l a t i o n , ^= ^ + J = L + J , to allow us to conclude that J * i s l i k e l y to be small a l s o (10~12h). Since the e x i t channel reduced mass i s s m a l l , even i f a l l of the i n i t i a l o r b i t a l angular momentum appears as product r o t a t i o n a l e x c i t a t i o n , the r o t a t i o n a l energy of DF w i l l be approximately 3-5 k c a l which i s l e s s than the v i b r a t i o n a l l e v e l spacing of 8 kcal. Thus, by c o n s e r v a t i o n of energy, only d i s c r e t e regions of product t r a n s l a t i o n a l energy space w i l l be populated and " q u a n t i z a t i o n " of the product v e l o c i t y d i s t r i b u t i o n should occur. In a crossed beam experiment with w e l l - d e f i n e d beam v e l o c i t i e s , one thus expects s i g n i f i c a n t s t r u c t u r e i n the product v e l o c i t y spectrum. Under very f a v o r a b l e kinematic circumstances, p a r t i c u l a r l y at low c o l l i s i o n energies where the angle between Y r e l and YD£ i s q u i t e l a r g e , such s t r u c t u r e should a l s o be observable i n the angular d i s t r i b u t i o n s . F i g u r e 3 demonstrates t h i s point c l e a r l y . Two d i s c r e t e peaks are observable at both the c o l l i s i o n energies; the lower panel of t h i s f i g u r e shows C a r t e s i a n p l o t s corresponding to these experiments. The s i g n i f i c a n t f e a t u r e of both of these contour maps i s that the DF m a x

r e

T


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product s c a t t e r s predominantly i n the backward d i r e c t i o n i n the c m . system, corresponding to net r e p u l s i v e encounters; as the c o l l i s i o n energy i s i n c r e a s e d , the sharpness of the backward s c a t t e r i n g diminishes and products are formed s c a t t e r e d through angles s i g n i f i c a n t l y smaller than 180°. Experiments performed at higher c o l l i s i o n energies continue to show the trend toward more forward s c a t t e r i n g , but the predominance of backward s c a t t e r i n g i s s t i l l i n evidence. While a l l of the experimental data show s t r u c t u r e a r i s i n g from i n d i v i d u a l v i b r a t i o n a l s t a t e s of DF, determination of r a t e constant r a t i o s i s very d i f f i c u l t s i n c e one must i n t e g r a t e f l u x over c m . v e l o c i t y to assess these r a t i o s . The l / u ^ Jacobian s i n g u l a r i t y near the c m . i s most pronounced i n c a l c u l a t i n g the k4/k3 r a t i o and the c e n t r o i d d i s t r i b u t i o n , although narrow by contemporary standards, i s broad enough to i n t r o d u c e p r o h i b i t i v e l y l a r g e u n c e r t a i n t i e s i n t o c a l c u l a t i o n s of r e l a t i v e populations. The o b s e r v a t i o n of DF predominatly s c a t t e r e d i n t o the backward hemisphere at the c o l l i s i o n energies s t u d i e d here suggests that the favored o r i e n t a t i o n f o r r e a c t i o n i s c o l l i n e a r . Ab i n i t i o c a l c u l a t i o n s of the p o t e n t i a l energy s u r f a c e f o r t h i s system (10,36) suggest that c o l l i n e a r approach of F + H£ i s the dominating geometry l e a d i n g to chemical r e a c t i o n . Indeed, the backward s c a t t e r e d DF product i s that which one expects from low impact parameter c o l l i s i o n s . One can thus conclude that the beam data are c o n s i s t e n t with a p o t e n t i a l energy s u r f a c e with predominant c o l l i n e a r c h a r a c t e r . For simple systems c o n t a i n i n g l i g h t atoms, the comparison between t h e o r e t i c a l s t u d i e s and experimental r e s u l t s w i l l provide f u r t h e r understanding of r e a c t i o n dynamics from f i r s t p r i n c i p l e i n the very near f u t u r e . II. S u b s t i t u t i o n Reactions of F Atoms with Hydrocarbons.

Unsaturated

A. General Remarks on Unimolecular Decompositions of Chemically A c t i v a t e d R a d i c a l s . A major p o r t i o n of the molecular beam s t u d i e s of f l u o r i n e atom chemistry (12-19,37) has been concerned with a c l a s s of r e a c t i o n s c h a r a c t e r i z e d by the formation of a t r a n s i e n t s p e c i e s from b i m o l e c u l a r a s s o c i a t i o n of the r e a c t a n t s and whose l i f e t i m e i s long compared to i t s r o t a t i o n a l or v i b r a t i o n a l p e r i o d s . The formation of such a long l i v e d complex i m p l i e s that the r e a c t a n t s experience a net a t t r a c t i o n and consequently the p o t e n t i a l energy s u r f a c e f o r the r e a c t i o n possesses a deep w e l l ; of course, the t o t a l energy of the system i s greater than that r e q u i r e d to d i s s o c i a t e the intermediate, e i t h e r to r e a c t a n t s or products, so that i n the absence of a t h i r d body or r e l a t i v e l y improbable photon emission ( r a d i a t i v e l i f e t i m e > 10" 2 s e c ) , the intermediate must decay p r i o r to d e t e c t i o n . Under c e r t a i n f a v o r a b l e c o n d i t i o n s where a l a r g e


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number of i n t e r n a l degrees of freedom p a r t i c i p a t e i n the r e d i s t r i b u t i o n of the e x c i t a t i o n energy, the r e a c t i v e i n t e r mediate could l i v e long enough to t r a v e l to our d e t e c t o r , but none of the systems s t u d i e d to date have e x h i b i t e d t h i s behavior. The decay of a complex formed i n such a bimolecular encounter provides us with an example of the general c l a s s of r e a c t i o n s denoted as unimolecular decompositions (38). Under circumstances i n which a l o n g - l i v e d complex decomposes to products, a v a s t c o l l e c t i o n of questions can be asked regarding the dynamics of the decomposition, i n c l u d i n g the extent to which the i n i t i a l e x c i t a t i o n energy of the intermediate species i s d i s t r i b u t e d throughout the e n t i r e molecule before a s u f f i c i e n t amount l o c a l i z e s i n the bond to be broken as the products form. The most widely accepted model of unimolecular decay, the R i c e Ramsperger-Kassel-Marcus (RRKM) theory (38,39), s t a t e s that a q u a s i e q u i l i b r i u m between the complex and the i n i t i a l c o n f i g u r a t i o n i s maintained through the r e a c t i o n . T h i s assumption i s equivalent to s t a t i n g that energy randomization i n the complex i s r a p i d compared to chemical r e a c t i o n . The technique of chemical a c t i v a t i o n developed by Kistiakowsky (40) and e x t e n s i v e l y employed by Rabinovitch and co-workers (41) has provided k i n e t i c i s t s with a l a r g e body of experimental data, the m a j o r i t y of which lend support to the assumptions of RRKM theory. In t h i s t e c h nique, the complex i s " s y n t h e s i z e d " i n a bulb r e a c t o r by a bimolecular r e a c t i o n and the r a t i o of the c o n c e n t r a t i o n of complexes s t a b i l i z e d by c o l l i s i o n s with background gas molecules to products of decomposition i s measured as a f u n c t i o n of p r e s s u r e . Under these circumstances, the " c l o c k " f o r these experiments becomes the time between c o l l i s i o n s and i f one assumes that the "strong c o l l i s i o n " assumption i s v a l i d , that i s that an a c t i v a t e d molecule (one with enough energy to decompose) can be d e a c t i v a t e d i n a s i n g l e c o l l i s i o n with a b u f f e r gas molecule, one can i n f e r whether or not the product y i e l d s are c h a r a c t e r i s t i c of a s t a t i s t i c a l d i s t r i b u t i o n of energy i n the complex. D i r e c t comparisons with RRKM theory can be made s i n c e t h i s model i s concerned with complex l i f e t i m e s and r a t e s of decomposition, q u a n t i t i e s which these conventional k i n e t i c methods determine directly. Information of a d i f f e r e n t s o r t i s obtained i n a molecular beam experiment, although the means f o r producing the s p e c i e s undergoing unimolecular decomposition i s a l s o chemical a c t i v a t i o n . Whereas the conventional k i n e t i c s t u d i e s y i e l d r e a c t i o n r a t e s for d i r e c t comparison with RRKM l i f e t i m e s , the beam technique y i e l d s product r e c o i l energy d i s t r i b u t i o n which, i n p r i n c i p l e , c o n t a i n i n f o r m a t i o n regarding e x i t channel dynamics s p e c i f i c a l l y ignored i n RRKM. Comparison of experimental r e s u l t s with RRKM theory i s i n d i r e c t , r e q u i r i n g a d d i t i o n a l assumptions whose v a l i d i t y must be determined. F o r t u n a t e l y , however, s t a t i s t i c a l t h e o r i e s of a d i f f e r e n t s o r t e x i s t which base t h e i r p r e d i c t i o n s on asymptotic (and t h e r e f o r e measureable) p r o p e r t i e s of the


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products; these "phase space" t h e o r i e s (42,43) w i l l be d i s c u s s e d i n greater d e t a i l below. Reactions s t u d i e d using the chemical a c t i v a t i o n technique i n beam or bulb experiments y i e l d important i n f o r m a t i o n because the intermediate r a d i c a l s have w e l l - d e f i n e d e x c i t a t i o n energies; unimolecular decompositions i n v o l v i n g the formation of a C-F bond have a d d i t i o n a l appeal because many competing channels f o r decomposition open up and s t u d i e s of branching r a t i o s f o r C-H, C-C, or C-Cl bond cleavage provide important chemical i n f o r m a t i o n . In a s t a t i s t i c a l model, the r a t e f o r a p a r t i c u l a r channel i s p r o p o r t i o n a l to the d e n s i t y of s t a t e s at the c r i t i c a l c o n f i g u r a t i o n ; consequently, one expects that more exoergic r e a c t i o n s should proceed with g r e a t e r r a t e s s i n c e more s t a t e s are a c c e s s i b l e under such circumstances. In the d i s c u s s i o n of s p e c i f i c experimental systems which f o l l o w s , the bulk of our remarks w i l l be l i m i t e d to a d i s c u s s i o n of product r e c o i l v e l o c i t y d i s t r i b u t i o n s and the i n f o r m a t i o n which such s t u d i e s provide f o r us. An understanding of the e n e r g e t i c s of unimolecular decomposition and t h e i r r e l a t i o n to product v e l o c i t y d i s t r i b u t i o n s can be f a c i l i t a t e d by r e f e r r i n g to F i g u r e 4. The e x c i t a t i o n energy of the complex, Ε*, i s determined by the i n i t i a l t r a n s l a t i o n a l and i n t e r n a l energy of the r e a c t a n t s p l u s the w e l l depth of the complex. Unimolecular decomposition to products can occur when energy E flows i n t o the r e a c t i o n coordinate such that the b a r r i e r V i s surmounted. T h i s b a r r i e r can i n c l u d e c e n t r i f u g a l energy p l u s p o t e n t i a l energy terms ; whenever terms of the l a t t e r type are present, the c r i t i c a l c o n f i g u r a t i o n i s " t i g h t " , that i s , i n c i p i e n t product r o t a t i o n s are not f r e e , but correspond to bending v i b r a t i o n s (44). T h i s p o t e n t i a l energy b a r r i e r i m p l i e s that the r e v e r s e a s s o c i a t i o n r e a c t i o n has a nonzero a c t i v a t i o n energy ; the nature of t h i s b a r r i e r and i t s d i s p o s a l as product t r a n s l a t i o n and i n t e r n a l energy i s a v i t a l f a c t o r i n i n t e r p r e t i n g measured product r e c o i l distributions. The energy i n excess of the c r i t i c a l c o n f i g u r a t i o n b a r r i e r , E-p-V , where Εχ = Etrans + E i n t - Δ ϋ ° i s the t o t a l a v a i l a b l e energy, i s that q u a n t i t y of energy which can be d i s t r i b u t e d among the v a r i o u s product o s c i l l a t o r s p l u s the r e l a t i v e t r a n s l a t i o n a l energy of the s e p a r a t i n g fragments. I f some energy at the c r i t i c a l c o n f i g u r a t i o n i s apportioned to the coordinate which becomes product t r a n s l a t i o n , then ε*^* should be a lower bound to the t r a n s l a t i o n a l energy of the products. The manner i n which the e x i t channel b a r r i e r r e l e a s e s i t s energy V among product degrees of freedom i s a dynamical c o n s i d e r a t i o n not addressed by c o n v e n t i o n a l RRKM theory, although recent e f f o r t s to i n c l u d e these dynamics i n a " t i g h t " t r a n s i t i o n s t a t e theory have been made (45). In the case where the c r i t i c a l c o n f i g u r a t i o n i s t i g h t , the product r e c o i l d i s t r i b u t i o n s cannot be p r e d i c t e d by RRKM theory and any assumptions which one makes to connect the energy d i s t r i b u t i o n at the c r i t i c a l c o n f i g u r a t i o n with that of r e a c t i o n products must be assessed c a r e f u l l y . Certain Q

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202

F + D — - DF + D 2

1.5 ~ 1.0

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:

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ί '"r"""! ι ι 1

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0° 30° LAB SCATTERING ANGLE,θ

60°

90°

60°

90°

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Figure 3. Upper panel: DF angular distributions at 0.80 kcal/mol and 1.68 kcal/mol collision energy. Lower panel: Cartesian flux contour maps generated from these data.

20 Η ω-

~Γ

f

I

Figure 4. Schematic reaction coordinate for the system F + C H relating various ener gies of complex and critical configuration along the reaction coordinate. g

4


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t r a j e c t o r y c a l c u l a t i o n s (46) formulated to probe energy r e l e a s e from such a r e p u l s i v e f o r c e i n the e x i t channel suggest that when a l i g h t atom i s emitted from a complex, such as during C-H bond cleavage, most o f the e x i t channel b a r r i e r V appears i n product t r a n s l a t i o n a l energy. While such an energy t r a n s f e r mechanism i s v a l u a b l e i n i n t e r p r e t i n g our experimental data, we f e e l that one should avoid u s i n g RRKM theory without c o n s i d e r i n g the e x i t b a r r i e r to p r e d i c t product r e c o i l energy d i s t r i b u t i o n s . Other models have been formulated which p r e d i c t product r e c o i l energy d i s t r i b u t i o n s , most notably the phase space theory of L i g h t and co-workers (42,43). This model possesses the d e s i r a b l e a t t r i b u t e that c r o s s s e c t i o n s are computed by e v a l u a t i n g the volume i n phase space which the products can sample which i s a c c e s s i b l e from a s t a t i s t i c a l complex whose conserved quantum numbers a r e given by the t o t a l energy, the t o t a l angular momentum and i t s p r o j e c t i o n on a s p a c e - f i x e d a x i s . Thus the theory i s parameterized i n terms of the observable p r o p e r t i e s of product molecules; however, establishment o f the l i m i t s on the formation and decomposition o f a s t r o n g - c o u p l i n g complex f r e q u e n t l y n e c e s s i t a t e s c o n s i d e r a t i o n s of intermediate geometry, p a r t i c u l a r l y when the r o t a t i o n a l energy o f the complex must become product angular momentum under the d i c t a t e s o f an e x i t channel b a r r i e r . A number o f v a r i a t i o n s on the phase space theory have been proposed and the l i t e r a t u r e can be consulted f o r d e t a i l s (16). A comparison of RRKM and phase space theory i n d i c a t e s that the degree and philosophy f o r parameterizing these models i s quite d i f f e r e n t . In the former case, the only constant of the motion i s the t o t a l energy o f the system. Phase space theory i s appealing because two more constants o f the motion corresponding to angular momentum v a r i a b l e s have been considered i n the derivation. The recent v e r s i o n s of t r a n s i t i o n s t a t e theory which consider e x p l i c i t l y angular momentum i n l i m i t i n g cases where the t o t a l angular momentum becomes e i t h e r product r o t a t i o n a l or o r b i t a l angular momentum y i e l d r e s u l t s i n accord w i t h phase space theory ( 4 5 ) . The v a r i o u s models f o r product r e c o i l d i s t r i b u t i o n s can be i l l u s t r a t e d most f r u i t f u l l y by c o n s i d e r a t i o n of s e v e r a l systems to which we now t u r n . a

B. F + C2H4. A system which has r e c e i v e d s u b s t a n t i a l a t t e n t i o n i n molecular beam experiments i n t h i s l a b o r a t o r y and which provides us with an opportunity to d i s c u s s many of the important f e a t u r e s of unimolecular decay i s the F + C2H4 r e a c t i o n (12,19). The r e a c t i o n to form C2H3F p l u s a hydrogen atom i s exoergic by approximately 14 k c a l mole"! and the s t a b i l i t y of the intermediate r a d i c a l , C2H4F, i s estimated to be -50 k c a l mole"-'- with respect to r e a c t a n t s . A molecular beam experiment performed as i n d i c a t e d i n the previous s e c t i o n , then, r e v e a l s the c h a r a c t e r i s t i c formation o f the C2H3F product near the d i r e c t i o n of the center of mass o f the system, as shown i n Figure 5. V e l o c i t y a n a l y s i s o f the r e a c t i o n products confirms


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the symmetry of the product emission about 0 - Tr/2 as expected f o r a complex which l i v e s many r o t a t i o n a l p e r i o d s . I n i t i a l angular and v e l o c i t y d i s t r i b u t i o n measurements on t h i s system y i e l d e d v a l u a b l e i n f o r m a t i o n on the c m . angular and energy d i s t r i b u t i o n s with the f o l l o w i n g notable p o i n t s : the c m . angular d i s t r i b u t i o n measured at the r e l a t i v e c o l l i s i o n energy of 1.98 k c a l mole"-' was symmetric about π/2 as expected, but the d i s t r i b u t i o n was peaked a t π/2 i n the c m . system. T h i s peaking was explained by c o n s i d e r i n g the angular momentum d i s p o s a l i n the chemical r e a c t i o n : w h i l e the explanation has appeared i n d e t a i l i n previous p u b l i c a t i o n s (18), we repeat the s a l i e n t f e a t u r e s here. R e c a l l i n g that the t o t a l angular momentum must be conserved i n a c o l l i s i o n , we note that s i n c e the C2H4 molecules a r e produced i n a nozzle expansion and a r e t h e r e f o r e r o t a t i o n a l l y " c o l d " , the i n i t i a l o r b i t a l angular momentum then becomes the t o t a l angular momentum o f the system and w i l l be perpendicular to the i n i t i a l r e l a t i v e v e l o c i t y v e c t o r . During the l i f e t i m e of the complex, the t o t a l angular momentum i s the r o t a t i o n a l angular momentum of the complex, which r o t a t e s i n the plane defined by the heavy atom C-C-F framework. As the C-H bond extends and breaks along the reformed p o r b i t a l on the sp^ h y b r i d i z e d carbon atom, the emitted hydrogen atom i s emitted along the r o t a t i o n a l angular momentum v e c t o r . T h i s v e c t o r becomes the f i n a l product r o t a t i o n a l angular momentum s i n c e the emitted hydrogen atom cannot remove a s i g n i f i c a n t amount of o r b i t a l angular momentum. We thus see that the products r e c o i l along J which i s p e r p e n d i c u l a r to V , thus producing the observed sideways peaking. A second notable f e a t u r e of the observed c m . d i s t r i b u t i o n s i s found i n the c m . energy distribution: the shape o f the d i s t r i b u t i o n i s q u i t e wide and y i e l d s an average product k i n e t i c energy which i s ~50% of the t o t a l a v a i l a b l e energy, f a r i n excess o f s t a t i s t i c a l p r e d i c t i o n s . The i n t e r p r e t a t i o n of t h i s r e s u l t at a s i n g l e c o l l i s i o n energy i s complicated by the f a c t that hydrogen a d d i t i o n to a halogenated o l e f i n on the end o f the molecule where the halogen r e s i d e s pro ceeds with an a c t i v a t i o n of energy of ~3 k c a l mole" ( 4 7 ) , there f o r e suggesting that the e x i t channel f o r the C H F decomposition to C2H3F + Η has a b a r r i e r . The manner i n which energy i s r e l e a s e d i n descending t h i s b a r r i e r then destroys the energy d i s t r i b u t i o n at the c r i t i c a l c o n f i g u r a t i o n which RRKM theory p r e d i c t s . A contour map o f c m . product f l u x as a f u n c t i o n of p o l a r coordinates (u,Θ) i s shown i n F i g u r e 6. C# m

-

z

1

1

2

4

In order to provide f u r t h e r i n s i g h t i n t o the p e r t u r b a t i o n of the c r i t i c a l c o n f i g u r a t i o n energy d i s t r i b u t i o n by the e x i t b a r r i e r , a d d i t i o n a l experiments on t h i s system a t higher c o l l i s i o n energies have been undertaken. The seeded beam technique can be used to vary the c o l l i s i o n energy from 2.2 k c a l mo l e " to 12.1 k c a l mole" , thereby i n c r e a s i n g the t o t a l energy a v a i l a b l e to the system from 16 to 28 k c a l mole" . The determination of the i n i t i a l c o l l i s i o n energy dependence of the unimolecular r e a c t i o n dynamics can i n p r i n c i p l e , provide 1

1

1


7.

FARRAR

A N D L E E


205

F+C H — C H F + H 2

§,

4

2

3

EXPERIMENT -12-OSCILLATOR P H A S E - S P A C E MODEL

-·—5-0SCILLAT0R P H A S E - S P A C E MODEL

30°

40°

LAB Figure 5.

50°

60°

Λ\ \R

70°

80°

SCATTERING ANGLE

Experimental angular distribution for from F + C ll E = 1.98 kcal/mol 2

h

CHF 2

3

rel

Figure 6. Center of mass contour map showing equal intensity points for formation of C H F from F + C H 2

3

2

Jf


206


RADICALS

two kinds of i n f o r m a t i o n . Since the energy which can be d i s t r i b u t e d amoung the v a r i o u s modes of the c r i t i c a l c o n f i g u r a t i o n i s the t o t a l a v a i l a b l e energy i n excess of the b a r r i e r , i n c r e a s i n g t h i s t o t a l energy should minimize the nonrandom c o n t r i b u t i o n of V to P ( E ) as Εχ i n c r e a s e s . Thus, i f a s t a t i s t i c a l model i s c o r r e c t and r e q u i r e s c o r r e c t i o n s a r i s i n g from the s p e c i f i c dynamics a s s o c i a t e d with V , then as the c o l l i s i o n energy i s increased, P ( E ) should approach s t a t i s t i c a l behavior more c l o s e l y . This i s the information which we seek i n the C2H4F system. In p r i n c i p l e , information of a d i f f e r e n t s o r t can be obtained from the i n i t i a l c o l l i s i o n energy dependence of g(6), the c m . angular d i s t r i b u t i o n f u n c t i o n . When the b i n d i n g energy E of the chemically a c t i v a t e d intermediate becomes comparable to the c o l l i s i o n energy, the complex l i f e t i m e should approach one r o t a t i o n a l p e r i o d and the g(6) f u n c t i o n should l o s e i t s symmetry about π/2. By then estimating the r o t a t i o n a l p e r i o d of the complex, one can then i n f e r the complex lifetime. I f one then knows a c c u r a t e l y the s t a b i l i t y , E , of the complex, model c a l c u l a t i o n s to t e s t s t a t i s t i c a l t h e o r i e s of unimolecular decay can be made. T h i s experimental handle on the decay of a l o n g - l i v e d complex y i e l d s l i f e t i m e s which can be compared d i r e c t l y with q u a n t i t i e s to which RRKM theory addresses i t s e l f d i r e c t l y . This d e s i r a b l e s t a t e of a f f a i r s i s g e n e r a l l y not achieved i n beam experiments on chemically a c t i v a t e d r a d i c a l s ; the l a r g e s t a b i l i t y of the intermediate complex (~30-60 k c a l mole~l) compared to a v a i l a b l e c o l l i s i o n energies i n our experiments does not allow us to use the r o t a t i o n a l p e r i o d as a " c l o c k " . The C2H^F system, however, does allow us to use the f i r s t method f o r s e p a r a t i n g the r o l e of the e x i t channel b a r r i e r i n the measured r e c o i l energy d i s t r i b u t i o n s . The experimental data obtained over the range of c o l l i s i o n energies 2.2 to 12.1 k c a l mole" are shown i n Figure 7. Over t h i s range of c o l l i s i o n energies, the r e a c t i o n appears to proceed through a l o n g - l i v e d complex, as evidenced by angular d i s t r i b u t i o n s symmetric about π/2 i n the center of mass coordinate system. The sideways peaking noted i n the e a r l i e r work i s observed at a l l energies here, suggesting that the f r a c t i o n a l p a r t i t i t i o n i n g of the t o t a l angular momentum i n t o r o t a t i o n a l and o r b i t a l angular momentum remains constant. The s i g n i f i c a n t c o n c l u s i o n from these experimental data, however, i s that the r e c o i l energy d i s t r i b u t i o n s maintain a r a t h e r constant shape as a f u n c t i o n of the i n i t i a l k i n e t i c energy, channeling ~50% of the t o t a l energy i n t o t r a n s l a t i o n of the products. A comparison of the measured average k i n e t i c energy of the products with c a l c u l a t i o n s performed with various s t a t i s t i c a l models i s q u i t e i n s t r u c t i v e . We have performed phase space c a l c u l a t i o n s of the r e c o i l energy d i s t r i b u t i o n s i n c l u d i n g the phase space volume a s s o c i a t e d w i t h a l l of the product v i b r a t i o n a l modes and with a s e l e c t e d subset of product v i b r a t i o n a l modes; the former c a l c u l a t i o n , denoted " 1 2 - o s c i l l a t o r " f

a

a

f

0

Q

1


7.

FARRAR AND L E E


207

model y i e l d s average k i n e t i c energy r e l e a s e s which place a smaller f r a c t i o n of the t o t a l energy i n t o t r a n s l a t i o n as the t o t a l energy i n c r e a s e s . The l a t t e r phase space c a l c u l a t i o n i n which the phase space volume a s s o c i a t e d with 5 product o s c i l l a t o r s i n c l u d i n g only those modes expected to be a c t i v e based on gross geometry changes during the r e a c t i o n y i e l d s r e s u l t s i n c l o s e r accord with experiment, but s t i l l somewhat discordant. This model, which h e u r i s t i c a l l y i n c o r p o r a t e s p a r t i a l a c t i v a t i o n of product o s c i l l a t o r s , p l a c e s a l a r g e r f r a c t i o n of the t o t a l excess energy i n t r a n s l a t i o n s i n c e fewer o s c i l l a t o r s share the energy, but again, the f r a c t i o n of the t o t a l energy appearing i n t r a n s l a t i o n decreases at higher energies, i n contrast to the 50% value observed at a l l energies. A t h i r d model, r e c e n t l y proposed by Marcus (45) accounts f o r the nature of the " t i g h t " c r i t i c a l configuration by assuming that the quantum numbers f o r the bending v i b r a t i o n s i n the t r a n s i t i o n s t a t e which become product r o t a t i o n s are " a d i a b a t i c on the average". T h i s model then channels a d d i t i o n a l energy i n t o t r a n s l a t i o n s i n c e the energy of a bending v i b r a t i o n a l quantum i s greater than f o r a r o t a t i o n a l quantum, the d e f i c i t accounted f o r by t r a n s l a t i o n ; c a l c u l a t i o n s with t h i s model y i e l d r e s u l t s q u i t e s i m i l a r to the 1 2 - o s c i l l a t o r phase space c a l c u l a t i o n . The s t a t i s t i c a l models, then, y i e l d values of the average k i n e t i c energy which are n e i t h e r q u a l i t a t i v e l y or q u a n t i t a t i v e l y c o r r e c t . In a l l cases, the models place too l i t t l e t r a n s l a t i o n a l energy i n the products and the f r a c t i o n of the t o t a l energy appearing i n t r a n s l a t i o n decreases w i t h i n c r e a s i n g c o l l i s i o n energy. The 5 - o s c i l l a t o r phase space model y i e l d s r e s u l t s i n c l o s e r accord with experiment, but t h i s model c e r t a i n l y cannot be c o r r e c t because a c a l c u l a t i o n of the complex l i f e t i m e performed u s i n g the d e n s i t y of s t a t e s of these v i b r a t i o n s y i e l d s l i f e t i m e s which are smaller than one r o t a t i o n a l period at a l l c o l l i s i o n e n e r g i e s . Since the experimentally observed angular d i s t r i b u t i o n s p l a c e a lower bound on the l i f e t i m e of the complex of s e v e r a l r o t a t i o n a l p e r i o d s , we can be sure that the C2H3F r e c o i l energy d i s t r i butions cannot be explained by simple r e d u c t i o n of the number of o s c i l l a t o r s p a r t i c i p a t i n g i n the decomposition. Rowland (48) has studied the C2H4F system by measuring the pressure dependence of the s t a b i l i z a t i o n - d e c o m p o s i t i o n ratio for C2H4F produced i n the hot atom r e a c t i o n of + C2H4; h i s r e s u l t s suggest that the l i f e t i m e of the complex i s on the order of 10~^ sec, a r e s u l t which can only be explained e a s i l y by i n c l u d i n g a l l of the complex's v i b r a t i o n s i n a l i f e t i m e c a l c u l a t i o n . An e x p l a n a t i o n of the l i f e t i m e r e s u l t s and the r e c o i l d i s t r i b u t i o n s must c e r t a i n l y r e q u i r e an e x p l i c i t treatment of the dynamics of i n t r a m o l e c u l a r energy t r a n s f e r connecting the complex w i t h the c r i t i c a l c o n f i g u r a t i o n . The normal modes a s s o c i a t e d w i t h formation of the C2H4F complex are q u i t e d i f f e r e n t from those required to form the products


208


RADICALS

(49); slow energy m i g r a t i o n between those two subsets of o s c i l l a t o r s might e x p l a i n the long observed l i f e t i m e while the p a r t i c i p a t i o n of a reduced s e t of v i b r a t i o n a l modes i n the decomposition may account f o r the d i s p r o p o r t i o n a t e l y l a r g e f r a c t i o n of the t o t a l energy i n product t r a n s l a t i o n . While the mechanical e x p l a n a t i o n f o r the l a r g e average k i n e t i c energy r e l e a s e i n t h i s system has not y e t been developed, the study of t h i s r e a c t i o n as a f u n c t i o n of i n i t i a l c o l l i s i o n energy suggests s t r o n g l y that the e x i t channel b a r r i e r does not account f o r the n o n s t a t i s t i c a l d i s t r i b u t i o n of the excess energy. C. The E f f e c t of P o t e n t i a l Energy B a r r i e r i n the E x i t Channel. We have i n d i c a t e d i n the previous s e c t i o n that e x i t channel i n t e r a c t i o n s can p l a y a s i g n i f i c a n t r o l e i n determining the t r a n s l a t i o n a l energy d i s t r i b u t i o n of product molecules. In the decomposition of C2H4F, we have demonstrated that to a l a r g e extent, the e f f e c t of the b a r r i e r can be minimized by studying the energy dependence of the product r e c o i l d i s t r i b u t i o n . A number of important p o i n t s can be made regarding the systematics of the e x i t channel b a r r i e r by c o n s i d e r i n g the decompositions of s u b s t i t u t e d fluorobenzenes. Table I and F i g u r e 8 show the e n e r g e t i c s a s s o c i a t e d with a number of these r e a c t i o n s and Figure 9 p o r t r a y s some of the r e c o i l d i s t r i b u t i o n r e s u l t s graphically. Table I E n e r g e t i c s and Average Product T r a n s l a t i o n a l Energi es, F l u o r i n e + S u b s t i t u t e d Benzenes Reactants

Exoergicity k c a l mole

Products

Barrier k c a l mole"

f

1

k c a l mole"-1

6 6

C.KLF + Η 6 5

13

~4

7.7

F + toluene

C H F + Η

13

~4

6.8

22

-7.5

9.3

F + m-xylene

C,H_F + CH Ό 3 j CgH F + Η

13

-4

7.8

C H F + CH

22

-7.5

C.H.C1F + Η 6 4

13

~4

8.2

C.H F + C l O 3

29

-1

3.4

F

+

C

H

7

?

0

9

?

F + C.H C1 6 5 C

7

C

3

11.0

A number of c o n c l u s i o n s can be reached from an examination of these data. We f i n d that average product t r a n s l a t i o n a l energy does not c o r r e l a t e w e l l with r e a c t i o n e x o e r g i c i t y . In


FARRAR A N D L E E

0°

Crossed

Molecular

Beam

20° 40° 60° 80°

Studies

0

5

10

6

1.0 0.5 0 1.0

Ε

:

4.3?

ο

ural/muie

0.5

1 χ ο " F - f*V14

kco'/moie

0.5 Ho kcal/rtiole

2

0.5

1

00°

Figure 7.

20° 40° 60° 80° LA3 SCATTERING ANGLE,θ

5 10 15 20 VELOCITY (10 cm/sec) 4

Experimental angular and TOF distribution data for F C H -» C H F at 4 collision energies 2

k

2

—

Figure 8.

3

0

+

X (X = C I , H , C H ) 3

Schematic reaction coordinate for the reactions ofF-\- substituted benzenes


+

210

FLUORINE-CONTAINING

FREE

RADICALS

the case of C l emission, t h i s very exoergic channel r e s u l t s i n only 3 . 4 k c a l mole" i n t r a n s l a t i o n whereas the l e s s exoergic Η-atom emission r e a c t i o n s r e s u l t i n much higher t r a n s l a t i o n a l energy r e l e a s e . Higher s t i l l are the t r a n s l a t i o n a l energy r e l e a s e s a s s o c i a t e d with CH3 emission which are accompanied by very l a r g e e x i t channel b a r r i e r . One i s immediately l e d to the c o n c l u s i o n that the energy released i n descending the b a r r i e r i s channeled p r i m a r i l y i n t o product t r a n s l a t i o n ; very high t r a n s l a t i o n a l energy r e l e a s e i s found i n CH3 emission where a l a r g e b a r r i e r i s expected, whereas C l emission proceeds with very low t r a n s l a t i o n a l e x c i t a t i o n i n accordance with a very small e x i t channel b a r r i e r . Hydrogen atom emission provides an intermediate case i n which a 4 k c a l b a r r i e r r e l e a s e s energy i n t o t r a n s l a t i o n with l y i n g between values found f o r CH3 or C l emission. One can t h e r e f o r e conclude that p r e d i c t i o n s of product energy d i s t r i b u t i o n s from the decompositon of l o n g - l i v e d complexes must f i r s t consider the height of the e x i t channel b a r r i e r . Most r e a c t i o n s under c o n s i d e r a t i o n here have e x o e r g i c i t i e s of 20 k c a l or l e s s ; i f the t o t a l excess energy of the complex i s d i s t r i b u t e d randomly, then only -1-2 k c a l can be expected i n the r e a c t i o n coordinate i f more than 10 v i b r a t i o n a l degrees of freedom share the energy. Consequently, a b a r r i e r of height 5 to 10 k c a l could channel much more energy than the s t a t i s t i c a l amount i n t o product t r a n s l a t i o n . For d i f f e r e n t r e a c t i o n s with comparable e x o e r g i c i t i e s , one thus expects that the higher the e x i t channel b a r r i e r , the " c o l d e r " the products. Angular momentum c o n s e r v a t i o n p l a y s an important r o l e i n many of these unimolecular decompositions, p a r t i c u l a r l y at higher c o l l i s i o n energies. In the case of decompositions i n v o l v i n g emission of a hydrogen atom from the c o l l i s i o n complex, the l i g h t atom cannot remove s i g n i f i c a n t o r b i t a l angular momentum and consequently i n i t i a l o r b i t a l angular momentum become product r o t a t i o n a l e x c i t a t i o n . Consequently angular momentum c o n s e r v a t i o n does not p l a c e k i n e t i c energy r e s t r i c t i o n s on the product energy p a r t i t i o n i n g and one expects t h i s kinematic freedom to hold at higher c o l l i s i o n energies. In the case of heavy p a r t i c l e emission, however, the con s e r v a t i o n laws can play a very important r o l e . E a r l i e r i n t h i s s e c t i o n we have discussed the conversion of e x i t channel p o t e n t i a l energy i n t o t r a n s l a t i o n ; one should a l s o note, however, that when t h i s energy i s released along a l i n e which does not pass through the center of mass of the c o l l i s i o n complex, s u b s t a n t i a l product r o t a t i o n a l e x c i t a t i o n can occur, p a r t i c u l a r l y i f the moments of i n e r t i a are l a r g e . Under these circumstances product o r b i t a l angular momentum might be sub s t a n t i a l l y decreased thereby removing r e s t r i c t i o n s on the r e c o i l energy d i s t r i b u t i o n . Conversely, i f s i g n i f i c a n t o r b i t a l angular momentum i s c a r r i e d away by the products, reduced masses 1

f


7.

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211

and i n t e r a c t i o n p o t e n t i a l ranges might r e q u i r e that a l a r g e f r a c t i o n of the r o t a t i o n a l energy of the complex be c a r r i e d away as product t r a n s l a t i o n such that r e c o i l v e l o c i t i e s might be l a r g e r than one i n i t i a l l y expects from simple s t a t i s t i c a l factors. In p a r t i c u l a r , the "hot atom p r e p a r a t i v e c o n d i t i o n s " d i s c u s s e d by Bunker (50) r e g a r d i n g unimolecular decomposition of species formed from t r a n s l a t i o n a l l y "hot" r e a c t a n t s may p l a y a very important r o l e i n decomposition l i f e t i m e s and r e c o i l energy d i s t r i b u t i o n s . I f an a c t i v a t e d molecule i s formed from high angular momentum r e a c t a n t s , i t s decomposition may be s i g n i f i c a n t l y governed by c o n s e r v a t i o n laws; e s p e c i a l l y when the t o t a l angular momentum i s placed i n t o f i n a l o r b i t a l angular momentum, we expect t r a n s l a t i o n a l l y "hot" products to be formed. T h i s s i t u a t i o n should be p a r t i c u l a r l y marked i n cases where a s l i g h t a c t i v a t i o n energy f o r the r e a c t i o n causes the t o t a l cross s e c t i o n to i n c r e a s e with c o l l i s i o n energy. Under such circumstances, the i n i t i a l o r b i t a l angular momentum i n c r e a s e s d r a m a t i c a l l y w i t h energy and i t s conversion i n t o product o r b i t a l angular momentum r e q u i r e s the mean v e l o c i t y with which the products r e t r e a t from one another to i n c r e a s e d r a m a t i c a l l y . The net e f f e c t of the c o n s e r v a t i o n laws then i s to f o r c e the average f r a c t i o n of the t o t a l energy i n t r a n s l a t i o n to i n c r e a s e w i t h c o l l i s i o n energy, i n o p p o s i t i o n to the u s u a l s t a t i s t i c a l r e s u l t . This p o i n t i s shown more c l e a r l y i n the F + CH3I r e a c t i o n d i s c u s s e d i n the s e c t i o n immediately f o l l o w i n g . III.

Other Examples.

A. F + CH3I -> IF + CH3. T h i s chemical r e a c t i o n provides us w i t h an example or an a b s t r a c t i o n r e a c t i o n from an a l i p h a t i c compound ; three channels are open i n t h i s system as i n d i c a t e d here : F + CH I -> IF

+ CH

3

-> HF

3

+ CH I 2

-> CH F + 1 3

ΔΗ = -11

kcal

ΔΗ = -35

kcal

ΔΗ = -53

kcal

The l a s t r e a c t i o n , w h i l e h i g h l y e x o e r g i c , has been found by T a l ' r o s e (51) to be of n e g l i g i b l e importance i n t h i s system; the second r e a c t i o n i s known to produce v i b r a t i o n a l l y e x c i t e d HF y i e l d i n g l a s e r a c t i o n , but we have not studied t h i s channel. T a l r o s e has a l s o s t u d i e d the r e a c t i o n to produce IF u s i n g mass spectrometric probing of f l u o r i n e flames and has measured a r a t e constant k = 2 χ 1 0 ~ ^ cm^ sec~^. We have s t u d i e d the IF p r o d u c t i o n r e a c t i o n (37) at two c o l l i s i o n e n e r g i e s , 2.6 and 14.1 k c a l mole -*- produced by Ar and He seeded F atom beams ; the l a b o r a t o r y angular d i s t r i b u t i o n f o r 1

-


212

FLUORINE-CONTAINING

FREE

RADICALS

IF a t the lower c o l l i s i o n energy i n F i g u r e 10 shows some symmetry about the c e n t r o i d , suggesting that the r e a c t i o n proceeds through a l o n g - l i v e d complex. The time o f f l i g h t data i n c o n j u n c t i o n w i t h the l a b o r a t o r y angular d i s t r i b u t i o n data indeed c o n f i r m that the c m . angular d i s t r i b u t i o n i s symmetric about θ = π/2 as expected from decomposition of a complex which l i v e s many r o t a t i o n a l p e r i o d s . The angular d i s t r i b u t i o n i n the center o f mass system i s o f the form (1 + a c o s 6 ) , symmetric about π/2 but a n i s o t r o p i c , thereby p r o v i d i n g i n f o r m a t i o n about angular momentum p a r t i t i o n i n g i n the decomposition o f the complex. In p a r t i c u l a r , the s t r o n g forward-backward peaking a t t h i s c o l l i s i o n energy i n d i c a t e s that the complex i s q u i t e p r o l a t e , which i s not s u r p r i s i n g s i n c e CH3I i s a h i g h l y p r o l a t e (cigar-shaped) symmetric t o p , and that the complex r o t a t i o n a l angular momentum becomes product o r b i t a l angular momentum. T h i s s i t u a t i o n occurs, as pointed out by M i l l e r , Safron, and Herschbach (52), because the c r e a t i o n o f products with low r o t a t i o n a l e x c i t a t i o n i n c o n j u n c t i o n w i t h the azimuthal symmetry of ]L about V causes the product V v e c t o r s t o map out a sphere and the i n t e n s i t y of products " p i l e s up" a t the p o l e s . T h i s s i t u a t i o n i s r e l a t i v e l y common i n a l k a l i a t o m - a l k a l i h a l i d e exchange r e a c t i o n s i n which long range f o r c e s created by d i p o l e - p o l a r i z a b l e atom i n t e r a c t i o n s make L and L l a r g e ; the F + CH3I case, however, i s unique i n that the f o r c e s are r e l a t i v e l y short range. The d i s p o s a l of angular momentum i n t h i s case i s governed by the h i g h l y p r o l a t e nature of C H 3 I F . 2

f

T

T

f

The r e c o i l d i s t r i b u t i o n f o r t h i s system i s r a t h e r i n t e r e s t i n g ; we f i n d that the products a r e formed w i t h -30% of the t o t a l energy i n t r a n s l a t i o n . T h i s r e s u l t i s q u i t e c l o s e to s t a t i s t i c a l p r e d i c t i o n s and, i n f a c t , the phase space volume a s s o c i a t e d w i t h the high frequency Η-atom motions i n the CH3 fragment i s so small that the experimental data do not a l l o w one to conclude the extent to which v i b r a t i o n a l energy i s shared among the modes of the c o l l i s i o n complex. At higher c o l l i s i o n energy the angular d i s t r i b u t i o n i n Figure 11 shows q u i t e d r a m a t i c a l l y the forward-backward peaking mentioned p r e v i o u s l y . Now, however, the peaks are not o f equal i n t e n s i t y i n the c m . system; t h i s r e s u l t would be expected i f the c o l l i s i o n complex l i v e d only a f r a c t i o n of a r o t a t i o n a l p e r i o d . One can convolute the random l i f e t i m e d i s t r i b u t i o n expected f o r the decomposition w i t h the c l a s s i c a l angular momentum form f a c t o r s to r e l a t e the r a t i o of the forward-backward i n t e n s i t i e s to the r o t a t i o n a l p e r i o d o f the complex. This " o s c u l a t i n g " model (53) p r e d i c t s that a forward-backward r a t i o of ~1.5 corresponds to a complex l i f e t i m e of h a l f a r o t a t i o n a l p e r i o d . Based on estimates o f the t o t a l r e a c t i o n c r o s s s e c t i o n , we can compute the maximum o r b i t a l angular momentum quantum number l e a d i n g to r e a c t i o n and compute a r o t a t i o n a l p e r i o d of 8 χ 1 0 " sec. A comparison of t h i s l i f e t i m e with RRKM theory 1 2


7.

FARRAR A N D L E E

Crossed Molecular

Beam

213

Studies

CM. RECOIL ENERGY,Ε (kcal/mole) Figure 9.

ι

Product recoil energy distributions for F + substi tuted benzenes

1

1

1

F + CH I — I F 3

+ CM

1—~ 3

i

r

Ε = 2 . 6 0 kcal/mole

LAB SCATTERING ANGLE, 0 Figure 10. IF product angular distribution from the reaction F + CH I at E = 2.6 kcal/mol S

rei


214


i n d i c a t e s that the "observed" value l i e s between p r e d i c t i o n s based upon f u l l a c t i v a t i o n of the CH3 v i b r a t i o n s and a c t i v a t i o n of the \>2 mode only. However, t h i s v a r i a t i o n i s l e s s than an order of magnitude and one must thus conclude that t h i s system does not allow us to make a s u f f i c i e n t l y s e n s i t i v e t e s t of the energy randomization hypothesis i n RRKM theory. Phase space c a l c u l a t i o n s of the product r e c o i l energy d i s t r i b u t i o n s are shown i n Figure 12 and y i e l d much the same c o n c l u s i o n regarding the r o l e of high frequency v i b r a t i o n s i n determining these q u a n t i t i e s . At the higher energy, however, one does observe the e f f e c t of o r b i t a l angular momentum i n that the conservation law places a higher f r a c t i o n of the t o t a l energy i n t r a n s l a t i o n than at 2.6 k c a l ; examination of our data i n d i c a t e s that the r e a c t i v e cross s e c t i o n increases with energy and places more energy i n t r a n s l a t i o n , with ~55% of the t o t a l energy i n t r a n s l a t i o n . Contour maps i n the c m . system f o r IF production are shown i n Figure 13. The observation that the CH3IF "molecule" must be s t a b l e with respect to the r e a c t a n t s F + CH3I i s somewhat s u r p r i s i n g initially. That the complex l i v e s s e v e r a l r o t a t i o n a l periods at a c o l l i s i o n energy of 2.6 k c a l mole" t e l l s us that i t i s bound by at l e a s t 20 k c a l mole" . In a separate experiment (54), we have observed the production of CH3IF i n the endoergic bimolecular r e a c t i o n F + CH3I -> F + C H 3 I F . The e n e r g e t i c s of t h i s process and the corresponding r e a c t i o n of atomic f l u o r i n e are shown i n F i g u r e 14. The energy s c a l e which l o c a t e s the s t a b i l i t y of CH3IF with respect to both r e a c t i o n s has been determined by i n t e g r a t i n g the d i f f e r e n t i a l cross s e c t i o n f o r CH3IF production as a f u n c t i o n of i n i t i a l k i n e t i c energy. Such measurements i n d i c a t e that the t h r e s h o l d f o r CH3IF formation i s ~11 k c a l , thus suggesting that the r a d i c a l i s s t a b l e with respect to F + CH3I by at l e a s t 25 k c a l . Knowledge of the "well depth" of a " s t i c k y " c o l l i s i o n of t h i s kind i s g e n e r a l l y u n a v a i l a b l e and t h i s technique of endoergic bimolecular s y n t h e s i s appears to be of g e n e r a l i t y . In the f i n a l s e c t i o n of t h i s paper, we d i s c u s s f u r t h e r development of t h i s method f o r observing h i t h e r t o unknown f r e e r a d i c a l s p e c i e s . 1

1

2

B. A New S y n t h e t i c Method f o r F l u o r i n e - Containing R a d i c a l s . The CH3IF r a d i c a l produced i n the endoergic r e a c t i o n of F + CH3I i s only one example of t r a n s i e n t species produced i n r e a c t i o n s of F2 with other molecules. More recent work on s i m i l a r systems i n v o l v i n g t r i h a l o g e n s I I F and C 1 I F and the pseudo-trihalogen HIF has been performed (55,56), y i e l d i n g information on the endoergic r e a c t i o n s 2

F

2

+ I

F

2

+ ICI

2

+ IIF

+ F

C1IF + F


FARRAR A N D L E E

Crossed

F + CH3I -

Molecular

Beam

Studies

E = HIF + F.

The IIF r a d i c a l i s of some importance i n understanding the macroscopic r e a c t i o n mechanism i n the F2 + I2 system, p a r t i c u l a r l y i n view of p r e d i c t e d s t a b i l i t i e s of t r i h a l o g e n molecules from molecular o r b i t a l c a l c u l a t i o n s (54), termolec u l a r recombination work (58) and matrix i s o l a t i o n (59). One must r e a l i z e that the r e a c t i o n F

2

+ XI + XIF + F (X = H, C l , I)

can only y i e l d observable XIF product i f the decomposition of XIF i n t o IF and X i s endoergic. By measuring thresholds f o r the above r e a c t i o n s f o r X = H, C l , I, we f i n d values of 11, 6, and 4 k c a l r e s p e c t i v e l y and, f u r t h e r , the predominant products formed are XIF and F. The d i s s o c i a t i o n of XIF i n t o IF + X at higher c o l l i s i o n energies becomes important f i r s t f o r F2 + I2 at 7 k c a l , and then f o r F2 + ICI; the d i s s o c i a t i o n of HIF i s an i n a c c e s s i b l e channel f o r the c o l l i s i o n energy range of these experiments. In the F2 + ICI system, no product mass peaks at m/e 56 (C1 F) or m/e (C1 F) were observed and no m/e 20 (HF) peak was observed f o r the F2 + HI r e a c t i o n . These mass s p e c t r a l r e s u l t s i n d i c a t e that the t r i h a l o g e n and pseudo-trihalogen species formed i n v o l v e bonding schemes such as C1IF and HIF with the more e l e c t r o p o s i t i v e atom i n the center of the molecule as p r e d i c t e d by Walsh's r u l e s (60,61). Figure 15 shows a contour map f o r the F2 + ICI system which i s t y p i c a l of a l l the data. Very forward peaked angular d i s t r i butions f o r the products suggest that a b s t r a c t i o n of an F atom from F2 by XI proceeds through a somewhat bent geometry, i . e . , F-F-I or F-I-X angle l e s s than 180° i n the system F-F-I-X. Aside from the unique chemical i d e n t i t y of these t r i h a l o g e n s , the s t a b i l i t y of the I2F molecule i s of importance i n e l u c i d a t i n g some features of the k i n e t i c s of the F2/I2 system (62). F i r s t of a l l , i t i s important to note that the gas phase generation of F atoms i n F2 and I2 mixtures only r e q u i r e s 4 k c a l of r e l a t i v e k i n e t i c energy between F2 and 12 through the F2 + I2 "** l 2 r e a c t i o n . A recent study of t h i s system i n d i c a t e s that e l e c t r o n i c a l l y e x c i t e d IF i s formed as evidenced by chemiluminescence observations. The four center exchange r e a c t i o n F2 + I2 2IF i s exoergic by 60 k c a l , more than enough to produce e l e c t r o n i c a l l y e x c i t e d IF, but our crossed molecular beam experiments have shown that at low energies c o l l i s i o n s between F2 + I2 produce I2F + F or IF + I + F, but not 2IF. I f I F were not s t a b l e , only the F + I2 and I + F2 r e a c t i o n s would be exoergic and then only by 30 k c a l , too l i t t l e to account f o r the chemiluminescence. However, the r e a c t i o n F + I2F -* 2IF i s exoergic by 64 k c a l and, i n view of the beam measurements of I2F s t a b i l i t y and the ease of production, could account f o r the observed chemiluminescence 37

35

F +

2


F

218


(a)

F+CH I 3

HOI F + CH3

-20-

D (F ) 0

2

CH3IF + F

-30 -40

^

F - F +CH3I

Figure 14. Schematic energy level showing the relationship between the formation of CH IF as a transient species via the F + CHJ path or as a stable radical via the F -fCH I pathway 3

2

S


7. FARRAR AND LEE


in F2 and I2 mixtures. Abstract A summary of work in fluorine chemistry as determined from crossed molecular beam studies of reaction dynamics is presented. Special emphasis is given to studies of unimolecular decay of long-lived complexes formed from bimolecular association of fluorine atoms with unsaturated hydrocarbons. The experimental results are discussed in terms of statistical models for energy randomization in the complex as well as the role of the exit channel potential energy barrier in determining the product translational energy distribution. Experimental results on recent studies of reactions of F2 with a variety of species are also presented, with special attention to the utility of the method of endoergic bimolecular reactions in "synthesizing" unstable radicals such as trihalogens and pseudotrihalogens. Results are presented for F2 + I2 -> I F + F, F +HI->HIF + F, F + CH3I->CH3IF, and F + ICl -> FICl + F. 2

2

2

2

Literature Cited 1.

Lawley, K. P. ed, "Molecular Scattering", Adv. Chem. Phys. 30, Wiley, New York, 1975. 2. Molina, M. J., and Pimentel, G. C., IEEE J. Quantum Electron. (1973), QE9, 64. 3. Schafer, T. P., Siska, P. Ε., Parson, J. M., Tully, F. P., Wong, Y. C., and Lee, Y. T., J. Chem. Phys. (1970), 53, 3385. 4. Schafer, T. P., Ph.D. Dissertation, Univ. of Chicago (1972). 5. Polanyi, J. C., and Woodall, Κ. B., J. Chem. Phys. (1972), 57, 1574. 6. Jonathan, Ν., Melliar-Smith, C. Μ., and Slater, D. Η., Mol. Phys. (1971), 20, 93. 7. Wilkins, R. L . , J. Chem. Phys. (1972), 57, 912. 8. Muckerman, J. T., J. Chem. Phys. (1971), 54, 1155. 9. Schatz, G. C., Bowman, J. Μ., and Kuppermann, Α., J. Chem. Phys. (1973), 58, 4023. 10. Bender, C. F., O'Neil, S. V., Pearson, P. Κ., and Schafer, H. F., J. Chem. Phys. (1972), 56, 4626. 11. Berry, M. J., J. Chem. Phys. (1973), 59, 6229. 12. Parson, J. Μ., and Lee, Y. T., J. Chem. Phys. (1972), 56, 4658. 13. Parson, J. M., Shobatake, Κ., Lee, Y. T., and Rice, S. Α., J. Chem. Phys. (1973), 59, 1402. 14. Shobatake, Κ., Parson, J. Μ., Lee, Y. T., and Rice, S. Α., J. Chem. Phys., (1973), 59, 1416. 15. Shobatake, Κ., Parson, J. M., Lee, Y. T., and Rice, S. Α., J. Chem. Phys. (1973), 59, 1427.


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43. Lin, J., and Light, J. C., J. Chem. Phys. (1966), 45, 2545. 44. Wieder, G. Μ., and Marcus, R. Α., J. Chem. Phys. (1966), 37, 1835. 45. Marcus, R. Α., J. Chem. Phys. (1975), 62, 1372. 46. Shobatake, K., Rice, S. Α., and Lee, Y. T., J. Chem. Phys. (1973), 59, 2483. 47. Jones, W. E., Macknight, S. D., Teng, L., Chem. Rev. (1973), 73, 407. 48. Williams, R. L., and Rowland, F. S., J. Chem. Phys. (1972), 76, 3509. 49. Light, J. C., private communication. 50. Bunker, D. L., J. Chem. Phys. (1972), 57, 332. 51. Leipunskii, I.O.,Morozov, I. I., and Tal'rose, V. L., Dokl. Akad. Nauk. SSSR (1971), 198, 1367. 52. Miller, W. B., Safron, S. Α., and Herschbach, D. R., Faraday Discuss. Chem. Soc. (1967), 44, 108. 53. McDonald, J. D., Ph.D. Dissertation, Harvard Univ. (1971). 54. Farrar, J. Μ., and Lee, Y. T., J. Am. Chem. Soc. (1974), 96, 7570. 55. Valentini, J. J., Coggiola, M. J., and Lee, Y. T., Faraday Discuss. Chem. Soc. (1976), to be published. 56. Valentini, J. J., Coggiola, M. J., and Lee, Y. T., J. Am. Chem. Soc. (1976), 98, 853. 57. Peyerimhoff, S. D., and Buenker, R. J., J. Chem. Phys. (1968), 49, 2473. 58. Bunker, D. L., and Davidson, Ν., J. Am. Chem. Soc. (1958), 80, 5090. 59. Nelson, L. Υ., and Pimentel, G. C., J. Chem. Phys. (1967), 47, 3671. 60. Walsh, A. D., J. Chem. Soc. (1953), 2266. 61. Walsh, A. D., J. Chem. Soc. (1953), 2288. 62. Birks, J. W., Gabelnick, S. D., and Johnston, H. S., J. Mol. Spectr. (1975), 57, 23.


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