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

Abstract

These experiments investigate the competition between C–C and C–Br bond fission in bromoacetone excited in the 1[n(O),π*(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 β 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 C–C 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.