No abstract

Reactive scattering of F atoms with C3H5Br and C6H51 molecules leading to Br and I atom abstraction has been studied at an initial translational energy E -40 kJ mol-' using a supersonic beam of F atoms seeded in He buffer gas. The center-of-mass angular distributions of BrF and IF scattering show peaking in the forward and backward directions, which is consistent with reaction via persistent C3H5-BrF and C6H5-1-F complexes with lifetimes greater than two rotational periods. The sharply peaked angular distribution observed for F + C3HsBr is consistent with a microcanonical description, whereby reactive scattering arises from a product transition state which approximates to a strongly prolate symmetric top. The mildly peaked angular distribution observed for F + C6H51 is consistent with a phase space description whereby unconstrained rotation is established between the nascent molecules in the product transition state. The product translational energy distributions are both consistent with randomization of energy over internal modes of the collision complex. The lifetime of the C3H5BrF collision complex greatly exceeds that of the FCH2--'CH-CH2Br free radical intermediate in the Br atom displacement pathway, showing that these radicals are not coupled via a four-membered-ring transition state. Similarly, the rate of coupling to the 'CH2-CHF-CH2Br radical must be slower than the rate of dissociation of the C3H5BrF radical, and the rate of migration of the F atom to the n system of the very long-lived C6H5IF radical must also be slower than its rate of dissociation.

Reactive scattering of F atoms with CzH3Br molecules leading to both H and Br atom displacement has been studied a t an initial translational energy E -40 kJ mol-' using a supersonic beam of F atoms seeded in He buffer gas. The center-of-mass angular distribution of CzH3F reactive scattering shows a sharp forward peak with a lower backward peak of relative intensity -0.5. The product translational energy distribution peaks a t a low fractionfbk -0.1 of the total available energy with a tail extending up to higher energy. The branching ratio -10 strongly favors Br atom displacement over H atom displacement. The H atom displacement pathway occurs in competition with F atom migration in the FCHZCHBr' radical formed by F atom addition to the C=C double bond. The Br atom displacement occurs in the 'CHZCHBrF radical formed either by F atom migration from FCHZCHBr'or directly from F atom addition to the C 4 d o u b l e bond. In either case, Br atom displacement occurs rapidly compared with the rate of F atom migration over the potential energy barrier in the bridged configuration, resulting overall in a short-lived collision complex mechanism for the formation of C2H3F reaction products. IntroductionThe bromine atom displacement reactions of chlorine atoms with vinyl and allyl bromide molecules were among the first systems to be studied' in crossed-beam reactive scattering experiments. A long-lived collision complex mechanism was observed' for vinyl bromide compared with a short-lived complex for allyl bromide. This difference was attributed to preferred C1 atom addition to the terminal CH2 group of C2H3Br followed by C1 atom migration to the CHBr group and then Br atom displacement, compared with "bond migration" following C1 atom addition to the terminal CH2 group of C3H5Br. The reaction of C1 atoms with C2H3Br molecules has also been investigated293 at higher collision energy using supersonic beams of C1 atoms seeded in inert buffer gas. A short-lived collision complex mechanism was observed,3 but evidence for Cl atom migration remained cir~umstantial.~ Indeed, early studied of F atoms reacting with 1,l-dichloroethene molecules demonstrate that addition to the CH2 group yields H atom displacement while addition to the CCl2 group yields C1 atom displacement. Comparison with the F atom reaction of 1 ,Zdichloroethene molecules where only C1 atom displacement is observed5 suggests that decomposition of the collision complex must be faster than F atom migration between the carbon atoms. Previous measurements2 of F atoms reacting with C2H3Br molecules have beenconfined to the forward scattering of C2H3F reaction products. Hence, a more extensive study of the F + C2H3Br reaction was undertaken in order to investigate the extent of migration in the fluorobromoethyl radical intermediate, in which both Br atom and H atom displacement pathways were observed.

No abstract

. 72,523 (1994).Reactive scattering of F atoms with Br2 molecules has been studied at an initial translational energy E -16 kJ mol-I using a supersonic beam of F atoms seeded in Ne buffer gas. Laboratory angular and velocity distributions of BrF product have been measured. They show sharp peaking in the forward direction with almost isotropic wide angle scattering of relative intensity -0.2. Approximately half the total available energy is disposed into product translation for scattering in the forward and backward directions with slightly lower translational energy for the sideways scattering. The combination of sharp forward scattering and high product translational energy is attributed to early migratory trajectories in large impact parameter collisions whereby the F atom reacts with the more distant Br atom of the Br2 molecule lying in the forward hemisphere. The isotropic wide angle scattering arises from direct trajectories in which the F atom reacts with the nearer Br atom of the Br2 molecule. Migratory dynamics are promoted by charge transfer interaction of the form Br2+F which stabilises the potential energy surface in elongated isosceles triangular FBr2 configurations as well as in slightly bent FBrBr configurations. Comparison is made with the dynamics of the F + C12, I?, and CCI3Br reactions.ZHEN ZHENG ZHU, RICHARD W.P. WHITE, DAVID 5. SMITH et ROGER GRICE. Can. J. Chem. 72,523 (1994).Utilisant un rayon supersonique d'atomes de fluor places dans un gaz tampon de Ne et operant i une Cnergie initiale de translation d'environ 16 kJ mol-', on a Ctudit la diffusion rkactive des atomes de fluor par des molCcules de brome. On a mesurC les distributions des vitesses et des angles du produit BrF; la vitesse prCsente un important maximum dans la direction avant avec un grand angle de dispersion pratiquement isotrope d'intensitC relative d'environ 0,2. Environ la moitit de 1'Cnergie totale disponible est utilisCe pour provoquer une translation du produit dans les directions avant et arrikre alors qu'une quantitt ICgtrement infkrieure d'Cnergie de translation est utilisCe pour une diffusion IatCrale. On attibue la combinaison d'une diffusion importante vers l'avant et d'une tnergie de translation tlevee du produit i d e s trajectoires de migration precoces dans des collisions avec des paramktres d'impact importants alors que l'atome de fluor rCagit avec l'atome de brome de la moltcule de Br2 se trouvant dans l'htmisphtre avant. Le grand angle de dispersion isotrope dtcoule de trajectoires directes dans lesquelles l'atome de fluor rCagit avec l'atome de brome le plus prks dans la molCcule de Br2 Les dynamiques migratoires sont amplifiCes par l'interaction de transfert de charge de la forme B r 2 + F qui stabilise la surface d'tnergie potentielle dans les configurations ClongCes de forme triangle isoctle du FBr2 ainsi que dans les configurations Itgerement dCformCes du FBrBr. On a fait une comparaison des dynamiques des reactions des atomes de fluor avec respectivement le Clz, le I2 et le CC13Br.[Traduit par la re...