DISTANCE DEPENDENCE OF NONADIABATICITY IN THE BRANCHING BETWEEN C-BR AND C-CL BOND FISSION FOLLOWING 1[N(O),PI-ASTERISK(C=O)] EXCITATION INBROMOPROPIONYL CHLORIDE

These experiments on bromopropionyl chloride investigate a system in w hich the barrier to C-Br fission on the lowest 1A'' potential energy s urface is formed from a weakly avoided electronic configuration crossi ng, so that nonadiabatic recrossing of the barrier to C-Br fission dra matically reduces the branching to C-Br fission. The results, when com pared with earlier branching ratio measurements on bromoacetyl chlorid e, show that the additional intervening CH2 spacer in bromopropionyl c hloride reduces the splitting between the adiabatic potential energy s urfaces at the barrier to C-Br fission, further suppressing C-Br fissi on by over an order of magnitude. The experiment measures the photofra gment velocity and angular distributions from the 248 nm photodissocia tion of Br (CH2)2COCl, determining the branching ratio between the com peting primary C-Br and C-Cl fission pathways and detecting a minor C- C bond fission pathway. While the primary C-Cl:C-Br fission branching ratio is 1:2, the distribution of relative kinetic energies impar-ted to the C-Br fission fragments show that essentially no C-Br fission re sults from promoting the molecule to the lowest 1A'' potential energy surface via the 1[n(O),pi(C-O)] transition; C-Br fission only results from an overlapping electronic transition. The results differ markedl y from the predictions of statistical transition state theories which rely on the Born-Oppenheimer approximation. While such models predict that, given comparable preexponential factors, the reaction pathway wi th the lowest energetic barrier on the 1A'' surface, C-Br fission, sho uld dominate, the experimental measurements show C-Cl bond fission dom inates by a ratio of C-Cl:C-Br=1.0: <0.05 upon excitation of the 1[n(O ),pi(C=O)] transition. We compare this result to earlier work on brom oacetyl chloride, which evidences a less dramatic reduction in the C-B r fission pathway (C-Cl:C-Br = 1.0:0.4) upon excitation of the same tr ansition. We discuss a model in which increasing the distance between the C-Br and C=O chromophores decreases the electronic configuration i nteraction matrix elements which mix and split the 1n(O)pi(C=O) and n p(Br)sigma(C-Br) configurations at the barrier to C-Br bond fission i n bromopropionyl chloride. The smaller splitting between the adiabats at the barrier to C-Br fission increases the probability of nonadiabat ic recrossing of the barrier, nearly completely suppressing C-Br bond fission in bromopropionyl chloride. Preliminary ah initio calculations of the adiabatic barrier heights and the electronic configuration int eraction matrix elements which split the adiabats at the barrier to C- Br and C-Cl fission in both bromopropionyl chloride and bromoacetyl ch loride support the interpretation of the experimental results. We end by identifying a class of reactions, those allowed by overall electron ic symmetry but Woodward-Hoffmann forbidden, in which nonadiabatic rec rossing of the reaction barrier should markedly reduce the rate consta nt, both for ground state and excited state surfaces.