Variable reaction coordinate direct RRKM theory

Various aspects of the direct implementation of the variable reaction coordinate formalism for evaluating the kinetics of barrierless reactions are considered. A new discussion of the decoupling of the conserved and transitional modes within this formalism is provided with sample results for NCNO suggesting an optimum simple procedure for implementing this decoupling. The effort involved in replacing analytic potentials with direct ab initio determinations in the Monte Carlo-based evaluation of the transitional mode partition functions is illustrated through sample calculations for Cl-+CH3Cl, CN+O-2, CN+NO, and (CH2)-C-1+CO. Some procedures for improving the convergence properties of such direct statistical evaluations are also suggested. A brief summary is given of a procedure for directly evaluating the effects of anharmonicities and certain rovibrational couplings on the density of states for the complex. The results from our previous direct variable reaction coordinate RRKM theory study for the dissociation of singlet ketene are slightly revised on the basis of the improved decoupling scheme. The results for both the dissociation rate constant and vibrational distributions are in quantitative agreement (i.e., within 20% throughout the energy ranges considered) with the corresponding recent experimental results. Finally, a possible procedure for making greater use of the ab initio data, and perhaps also obtaining improved convergence, is summarized.