Implications of Berthelot's Description of Associative Reactions

Berthelot's equation, k,(T) = A,exp{-Dlc,T}, provides a good description of the high-pressure rate coefficient for associative reactions such as CH3 + CH3, CH3 + OH, and CH3+ H. The parameter D may be used to characterize an effective temperature-dependent barrier, AE,ff = DkiT2, which is due to the internal energy, i.e. rotational and vibrational energy, or the centrifugal barrier of the associating species. The parameter A, may be identified as the rate coefficient for association when the species approach one mother with zero translational and internal energy. A microcanonical rate expression is given which may be used to directly compare experimental results with theoretical calculations. In addition, this approach suggests that the Arrhenius expression for the high-pressure rate coefficient for dissociation should contain both the change of enthalpy of the reaction and this effective barrier.