Influence of nonadiabatic dynamics on competing photodissociation channel

These experiments measure the time-of-flight and angular distributions of the photofragments from the dissociation of allyl chloride and nitric acid upon excitation at 193 nm. In both nitric acid and cis-allyl chloride, electronically nonadiabatic dynamics markedly influence the branching ratio between energetically accessible reaction channels. Both systems demonstrate that when the adiabatic reaction coordinate requires a dramatic change in the electronic wavefunction, significant branching to another channel is observed.;The experiments on allyl chloride examine the competition between HCl elimination and C-Cl bond fission upon excitation of the nominal $pipisp*$(C=C) transition. In planar geometries, the individual orbital symmetry is not conserved along the excited C-Cl reaction coordinate so that HCl elimination, a ground state process, can compete. The experimental results are analyzed along with supporting ab initio electronic structure calculations to examine the potential influence of nonadiabaticity along the C-Cl fission reaction coordinate for the two conformers of allyl chloride.;The experiments on nitric acid, HONO$sb2,$ elucidate the dynamics of the competing reaction pathways upon excitation of the $pi$(nb,O) $to pisp*$(NO$sb2$) transition at 193 nm. Since the adiabatic OH($sp2Pi$) + NO$sb2$(1$sp2$B$sb2$) reaction coordinate requires the individual orbital symmetries on the NO$sb2$ chromophore to change, significant branching to a second OH + NO$sb2$ channel occurs. Product channels that require both the OH and NO$sb2$ functional groups to switch symmetries along the reaction coordinate are not experimentally observed. The change in the electronic wavefunction required for the molecule to follow one of these pathways is even more difficult than a change in the individual orbital symmetry when both sets of orbitals are localized on the same moiety. Thus, we propose a "restricted adiabatic" correlation diagram to disallow correlations to product channels that involve large electronic changes on both the OH and NO$sb2$ functional groups. This approach, which is useful for identifying viable reaction channels, can be applied to other systems when the relevant molecular orbitals are localized in both the reactants and products.