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

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*])approximate to$$ \approx $$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 mu s 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 + H-2 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.