Distance dependence of nonadiabaticity in the branching between C–Br and C–Cl bond fission following 1[n(O),π*(C=O)] excitation in bromopropionyl chloride
Authors: Kash P. W. , Waschewsky G. C. G. , Butler L. J. , Francl M. M. 
Published: July 2002

These experiments on bromopropionyl chloride investigate a system in which the barrier to C–Br fission on the lowest 1A″ potential energy surface is formed from a weakly avoided electronic configuration crossing, so that nonadiabatic recrossing of the barrier to C–Br fission dramatically reduces the branching to C–Br fission. The results, when compared with earlier branching ratio measurements on bromoacetyl chloride, show that the additional intervening CH2 spacer in bromopropionyl chloride reduces the splitting between the adiabatic potential energy surfaces at the barrier to C–Br fission, further suppressing C–Br fission by over an order of magnitude. The experiment measures the photofragment velocity and angular distributions from the 248 nm photodissociation of Br(CH2)2COCl, determining the branching ratio between the competing 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 imparted to the C–Br fission fragments show that essentially no C–Br fission results from promoting the molecule to the lowest 1A″ potential energy surface via the 1[n(O),π*(C=O)] transition; C–Br fission only results from an overlapping electronic transition. The results differ markedly 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 with the lowest energetic barrier on the 1A″ surface, C–Br fission, should dominate, the experimental measurements show C–Cl bond fission dominates by a ratio of C–Cl:C–Br=1.0:&lt;0.05 upon excitation of the 1[n(O),π*(C=O)] transition. We compare this result to earlier work on bromoacetyl chloride, which evidences a less dramatic reduction in the C–Br fission pathway (C–Cl:C–Br=1.0:0.4) upon excitation of the same transition. We discuss a model in which increasing the distance between the C–Br and C=O chromophores decreases the electronic configuration interaction matrix elements which mix and split the 1n(O)π*(C=O) and np(Br)σ*(C–Br) configurations at the barrier to C–Br bond fission in bromopropionyl chloride. The smaller splitting between the adiabats at the barrier to C–Br fission increases the probability of nonadiabatic recrossing of the barrier, nearly completely suppressing C–Br bond fission in bromopropionyl chloride. Preliminary ab initio calculations of the adiabatic barrier heights and the electronic configuration interaction matrix elements which split the adiabats at the barrier to C–Br and C–Cl fission in both bromopropionyl chloride and bromoacetyl chloride support the interpretation of the experimental results. We end by identifying a class of reactions, those allowed by overall electronic symmetry but Woodward–Hoffmann forbidden, in which nonadiabatic recrossing of the reaction barrier should markedly reduce the rate constant, both for ground state and excited state surfaces.