| Abstrakt: |
Monte Carlo variational transition-state theory (MCVTST) has been used to calculate unimolecular dissociation rates for RDX (hexahydro-1,3,6-trinitro-1,3,5-triazine) for total energies over the range 170−450 kcal/mol. The calculations were done using the potential energy surface (PES) developed by Chambers and Thompson (J. Phys. Chem. 1996, 99, 15881). This PES allows for dissociation to occur by bond fission (energy required: 48 kcal/mol) and by concerted triple bond fission (energy barrier: 37 kcal/mol); these are the dominant primary dissociation channels consistent with the results of the molecular beam infrared multiphoton 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 value (~2) determined from the MB-IRMPD data. The rates for the two reaction channels and the ratio of the rates are compared to classical trajectory results; the agreement is good, as expected, at the lower energies, but diverges after the total energy exceeds about 250 kcal/mol. However, the ratio of the rates is comparable for the entire energy range. We find that the TST dividing surface for the concerted molecular elimination (i.e., ring fission) is correlated with the ring opening, the initial stage of the reaction, thus simplifying the definition of the surface dividing reactants and products defined by the minimum flux. We also show how importance sampling can be used to facilitate the computations. |
