Concerted stereodynamics of chemical changes: The branching selectivity in the photodissociation of asymmetric‐top molecules, formic acid, and the cold reactivity of the H + H₂ exchange reaction

Abstract

This review article presents our recent examples of the branching selectivity in the photodissociation of asymmetric‐top molecules, halothane CF3CHBrCl and isohaloethane CF2BrCHFCl. The former gave the unexpected branching ratio of ([Cl] + [Cl*])/([Br] + [Br*])≈2 for the Br, Br* and Cl, Cl* atom fragmentations, meaning that the strongly bounded Cl‐C bond gave twice favorable fragmentation, while the later isohaloethane gave almost the same value for all‐atom fragmentations. We interpret this result due to the curve crossing of electronically excited states and the non‐adiabatic interaction on the excited states. The bimodal vibrational distribution of the product CO fragment in the formic acid photodissociation at 193 nm evidenced a roaming signature by using the μs time‐resolved Fourier‐transform infrared emission spectra for the first time. We find the characteristic propensity rule in the time‐dependent interaction potential to judge reactivity in the H + H2 exchange reaction and the roaming‐type of trajectory at temperature 3 K, by use of the impact‐parameter dependent quasi‐classical trajectory simulation, based on the present results, we conclude that reaction dynamics proceeds not only by the prerequisite of energy conservation but also by the timing of the time‐dependent interaction potential which is very sensitive to the steric configuration of reaction intermediate, thus it may be called as the concerted stereodynamics.