Product Branching in the Photodissociation of Chloroacetaldehyde and the Incorporation of Angular Momentum in Statistical Predictions of Product Branching

Abstract

These studies combine velocity map imaging and crossed laser-molecular beam scattering to study the primary photodissociation channels of chloroacetaldehyde and the unimolecular dissociation channels of the vinoxy radical, CH2CHO. In our velocity map imaging study, we investigate the unimolecular dissociation of vinoxy radicals prepared with high internal energy imparted by photodissociation of chloroacetaldehyde, CH2ClCHO, at 157 nm. We measured the speed distribution of the recoiling chlorine atoms, Cl(2P3/2) and Cl(2P1/2), and derived from this the resulting distribution of kinetic energy imparted to the Cl + vinoxy fragments upon dissociation. The recoil translational energy distributions, P(ET)s, derived for the C−Cl bond fission presented in this work suggest the vinoxy radicals are formed in the A and B state. We also took ion images at m/z = 15 and m/z = 42 to characterize the branching between the unimolecular dissociation channels of the vinoxy radical to H + ketene and methyl + CO products. In our scattering experiments, we further characterized the primary photodissociation pathways of chloroacetaldehyde and found evidence of HCl photoelimination and C−C bond fission in addition to C−Cl bond fission. This is the first direct evidence of the C-C bond fission channel in chloroacetaldehyde and we found that it significantly competes with the C−Cl bond fission channel. The branching between these channels suggests the presence of interesting excited state dynamics in chloroacetaldehyde following excitation. We also found evidence of secondary dissociation of the vibrationally excited products formed from these primary photodissociation channels. While we detected methyl and ketene products from the unimolecular dissociation of vinoxy in both studies, we were unable to determine the branching ratio between these product channels. However, we observed that the production of ketene is favored over the production of methyl for the high internal energy vinoxy radicals produced at 157 nm; prior work which formed lower internal energy vinoxy radicals found the CH3 + CO product channel was dominant. In addition to the experimental studies, we developed a model for the branching between unimolecular dissociation channels that takes into account how the change in rotational energy en route to the products affects the vibrational energy available to surmount the barriers to the channels. The model predicts the portion of the C-Cl bond fission P(ET) that produces dissociative vinoxy radicals, then predicts the branching ratio between the H + ketene and CH3 + CO product channels at each ET. The model uses RRKM rate constants at the correct sums and densities of vibrational states while accounting for angular momentum conservation. We find that the predicted portion of the P(ET) that produces vinoxy that dissociates to H + ketene products best fits the experimental portion of the ketene speed distribution from the velocity map imaging study (that we derive by taking advantage of conservation of momentum) if we use a barrier height for the H + ketene channel that is 4.0 ± 0.5 kcal/mol higher than the isomerization barrier en route to CH3 + CO products. Using the G4 computed isomerization barrier of 40.6 kcal/mol, this gives an experimentally determined barrier to the H + ketene channel of 44.6 kcal/mol. From these calculations, we also predict the branching ratio between the H + ketene and methyl + CO channels to be approx. 2.1:1.