The defendence of the reaction rate constant on reagent excitation: the implications of detailed balance

Simple but useful relations between the detailed rate constants in the forward and reversed directions of a given collision are noted and applied to the determination of vibrational (and rotational) energy requirements and disposal. The discussion is conveniently cast in terms of the 'reaction probability' matrix which is identical for the forward and reversed reactions. It is argued that it is preferable (and easier) to measure (or compute) the rate for a given collision in the exothermic direction. As an illustration, the vibrational energy requirements of several endothermic reactions [S + CO(υ); I + HCl(υ); Cl+HCl(υ); H + HF(υ) and their isotopic variants] are determined from the measurements of vibrational energy disposal in their reversed, ??? counterparts. Similarly, vibrational energy disposal for endothermic reactions [e.g., Cl+HF→F+HCl(υ); Cl+HBr→ Br+HCl(υ), υ ≥ 2] is determined from the measurements of vibrational energy requirements of the reversed, exothermic processes. Additional evidence in support of the diminishing effect of reagent vibrational energy on the reaction rate, at higher excitations and particularly in the exothermic regime, is considered. Finally, rotational energy requirements are determined for the D+DF(υ,J) reaction.