Rate constant for the reaction C _2H _5 + HBr → C _2H _6 + Br

摘要

RRKM theory has been employed to analyze the kinetics of the title reaction, in particular, the once-controversial negative activation energy. Stationary points along the reaction coordinate were characterized with coupled cluster theory combined with basis set extrapolation to the complete basis set limit. A shallow minimum, bound by 9.7 kJ-mol ~(-1) relative to C _2H _5 + HBr, was located, with a very small energy barrier to dissociation to Br + C _2H _6. The transition state is tight compared to the adduct. The influence of vibrational anharmonicity on the kinetics and thermochemistry of the title reaction were explored quantitatively. With adjustment of the adduct binding energy by ～4 kJ-mol ~(-1), the computed rate constants may be brought into agreement with most experimental data in the literature, including new room-temperature results described here. There are indications that at temperatures above those studied experimentally, the activation energy may switch from negative to positive.