Transition-state control of product rotational distributions in H + RH → H2+ R reactions (RH = HCl, HBr, HI, CH4, C2H6, C3H8)

Abstract

Measurements of the rotational state distributions for the H₂ product of the H + RH → H₂+ R reactions (RH = HCl, HBr, HI, CH₄, C₂H₆, C₃H₈) at a collision energy of 1.6 eV are reported. For the reactions with the hydrogen halides we also have carried out quasiclassical trajectory calculations for conditions that mimic those of the experiments. We find that the rotational state distributions are quite characteristic and distinguishing in these systems. The combination of experiments and calculations allows us to provide some compelling explanation of how the structure and dynamics in the transition state are coupled to the product rotational state distributions. Not surprisingly, mere measurement of the rotational state distributions is insufficient to establish this connection, as illustrated by the results of the state-to-state experiments with the alkanes as reactants. These reactions give anomalous rotational state distributions, showing rotational excitation that actually increases with increasing product vibrational energy, rather than decreasing, as has been observed for all other reactions whose rotational and vibrational state distributions we know have been measured.