Origin of Bath Gas Dependence in Unimolecular Reaction Rates

The bath gas dependence of thermal unimolecular reaction rates arises from different rates and efficiencies of collisions between reactant and third-body molecules. This study aims to unravel the mechanistic origin of this dependence based on the classical trajectories of methyl isocyanide (CH₃NC) colliding with 15 different bath gas molecules (CH₃NC, He, Ar, H₂, N₂, CO, CO₂, HCN, NH₃, CH₄, CH₃F, CF₄, C₂H₂, C₂H₄, and C₂H₆). The collision frequencies, energy transfer parameters, and relative third-body efficiencies are evaluated from the trajectory calculations. The relative third-body efficiencies of the studied bath gases are found to be in good agreement with available experimental data. The results indicate that differences in collision frequencies are the primary source of the bath gas dependence of the low-pressure rate constants. The nature of the long-range intermolecular interaction, particularly, its anisotropy, is suggested to play a key role in determining the collision frequency.