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

These experiments on bromopropionyl chloride investigate a system in which the barrier to C-Br fission on the lowest 1A\u27\u27 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 impar-ted to the C-Br fission fragments show that essentially no C-Br fission results from promoting the molecule to the lowest 1A\u27\u27 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 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\u27\u27 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: \u3c0.05 upon excitation of the 1[n(O),pi*(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)pi*(C=O) and np(Br)sigma*(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 ah 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

Competing C–Br and C–C Bond Fission Following 1[n(O),π∗(C=O)] Excitation in Bromoacetone: Conformation Dependence of Nonadiabaticity at a Conical Intersection

These experiments investigate the competition between C-C and C-Br bond fission in bromoacetone excited in the (1)[n(O),pi(*)(C=O)] absorption, elucidating the role of molecular conformation in influencing the probability of adiabatically traversing the conical intersection along the C-C fission reaction coordinate. In the first part of the paper, measurement of the photofragment velocity and angular distributions with a crossed laser-molecular beam time-of-flight technique identifies the primary photofragmentation channels at 308 nm. The time-of-flight spectra evidence two dissociation channels, C-Br fission and fission of one of the two C-C bonds, BrH2C-COCH3. The distribution of relative kinetic energies imparted to the C-Br fission and C-C fission fragments show dissociation is not occurring via internal conversion to the ground electronic state and allow us to identify these channels in the closely related systems of bromoacetyl- and bromopropionyl chloride. In the second part of the work we focus on the marked conformation dependence to the branching between C-C fission and C-Br fission. Photofragment angular distribution measurements show that C-Br fission occurs primarily from the minor, anti, conformer, giving a beta of 0.8, so C-C fission must dominate the competition in the gauche conformer. Noting that the dynamics of these two bond fission pathways are expected to be strongly influenced by nonadiabatic recrossing of the reaction barriers, we investigate the possible mechanisms for the conformation dependence of the nonadiabatic recrossing with low-level ab initio electronic structure calculations on the C-Br reaction coordinate and qualitative consideration of the conical intersection along the C-C reaction coordinate. The resulting model proposes that C-C bond fission,cannot compete with C-Br fission in the anti conformer because the dissociation samples regions of the phase space near the conical intersection along the CC fission reaction coordinate, where nonadiabaticity inhibits C-C fission, while from the gauche conformer C-C fission can proceed more adiabatically and dominate C-Br fission. A final experiment confirms that the branching ratio changes with the relative conformer populations in accord with this model