MONTE-CARLO VARIATIONAL TRANSITION-STATE THEORY STUDY OF THE UNIMOLECULAR DISSOCIATION OF RDX

Monte Carlo variational transition-state theory (MCVTST) has been used to calculate unimolecular dissociation sates for RDX (hexahydro-1,3,6 -trinitro-1,3,5-triazine) for total energies over the range 170-450 kc al/mol. The calculations were done using the potential energy surface (PES) developed by Chambers and Thompson (J. Phys. Chem. 1996, 99, 158 81). This PES allows for dissociation to occur by bond fission (energy required: 48 kcal/mol) and by concerted triple bond fission (energy b arrier: 37 kcal/mol); these are the dominant primary dissociation chan nels consistent with the results of the molecular beam infrared multip hoton dissociation (MB-IRMPD) experiments of Zhao, Hintsa, and Lee (J. Chem. Phys. 1988, 88, 801). The computed branching ratio for ring to simple bond fission at 170 kcal/mol is in good agreement with the valu e (similar to 2) determined from the MB-IRMPD data. The rates for the two reaction channels and the ratio of the rates are compared to class ical trajectory results; the agreement is good, as expected, at the lo wer energies, but diverges after the total energy exceeds about 250 kc al/mol. However, the ratio of the rates is comparable for the entire e nergy range. We find that the TST dividing surface for the concerted m olecular elimination (i.e., ring fission) is correlated with the ring opening, the initial stage of the reaction, thus simplifying the defin ition of the surface dividing reactants and products defined by the mi nimum flux. We also show how importance sampling can be used to facili tate the computations.