Molecular dynamics simulations of ring inversion in RDX

Molecular dynamics simulations, using the finite volume method of Murrell and co-workers [J. Chem. Phys. 94, 3908 (1991)], have been carried out to study conformational changes in hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) in isolation and in dense Xe gas. The configurational distributions for RDX in a Xe bath and in the gas-phase are markedly different. The results show that as the solvent concentration increases, the concentration of RDX molecules in the boat conformation increases by a factor of about 4. The rate constant for the chair→boat ring inversion was calculated as a function of the xenon concentration [Xe]. The rate constant obeys Lindemann behavior at low concentrations, i.e., it increases with increasing solvent density. At [Xe]∼6.2 mol dm−3, the rate constant reaches a maximum (Kramer’s turnover) and becomes a decreasing function of the solvent concentration. For [Xe] above 16.2 mol dm−3, the rate constant again increases as a function of the solvent density.