COMPARISON OF MODELS FOR CALCULATING THE RRKM UNIMOLECULAR RATE CONSTANT-K(E,J)

Both adiabatic and active treatments of the rotational quantum number K are considered in evaluating the RRKM unimolecular rate constant k(E, J). If K is treated adiabatically, k(E, J) is a weighted average of individual k(E, J, K) unimolecular rate constants. For non-variational transition states, this k(E, J) is identical to the one which results by treating K as an active degree of freedom with the proper limits placed on K. Another model for treating K as active is to let energy exchange statistically between the vibrational modes and the K-dependent term of the rotational energy. If a separable rigid rotor/harmonic oscillator model is used to calculate the sum and density of states for the transition state and reactant, respectively, the latter K active model is found to give a substantially smaller k(E, J). © 1990.