COMPETING C-BR AND C-C BOND FISSION FOLLOWING (1)[N(O),PI-ASTERISK(C=O)] EXCITATION IN BROMOACETONE - CONFORMATION DEPENDENCE OF NONADIABATICITY AT A CONICAL INTERSECTION

These experiments investigate the competition between C-C and C-Br bon d fission in bromoacetone excited in the (1)[n(O),pi()(C=O)] absorpti on, 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 wi th 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 o f 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 fragment s show dissociation is not occurring via internal conversion to the gr ound electronic state and allow us to identify these channels in the c losely related systems of bromoacetyl- and bromopropionyl chloride. In the second part of the work we focus on the marked conformation depen dence to the branching between C-C fission and C-Br fission. Photofrag ment angular distribution measurements show that C-Br fission occurs p rimarily 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 barr iers, we investigate the possible mechanisms for the conformation depe ndence of the nonadiabatic recrossing with low-level ab initio electro nic structure calculations on the C-Br reaction coordinate and qualita tive 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 dissociati on samples regions of the phase space near the conical intersection al ong 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 confi rms that the branching ratio changes with the relative conformer popul ations in accord with this model.