Unimolecular dynamics of Cl^-⋯CH₃Cl intermolecular complexes formed by Cl^-+CH₃Cl association

A previous trajectory study of the dissociation of Cl ^-⋯CH₃Cl complexes formed by Cl^-+CH ₃Cl association is further analyzed to determine (1) the relationship between classical and quantum Rice-Ramsperger-Kassel-Marcus (RRKM) rate constants for Cl^-⋯CH₃Cl→Cl^-+CH ₃Cl dissociation; (2) the importance of anharmonicity in calculating the RRKM dissociation rate constant; (3) the role of angular momentum in interpreting the trajectory distribution N(t)/N(0) of Cl ^-⋯CH₃Cl complexes versus time; and (4) the pressure-dependent collision-averaged rate constant k(ω,E) for Cl ^-⋯CH₃Cl dissociation. It is found that only the low-frequency intermolecular modes of Cl^-⋯CH₃Cl are initially excited by Cl^-+CH₃Cl association. Classical and quantum RRKM rate constants for dissociation of this intermolecular complex are in excellent agreement. Anharmonicity lowers the rate constant by a factor of 4-8 from its harmonic value. The dissociation rate for the long-time tail of the trajectory N(t)/N(0) distribution is much smaller than predicted by a RRKM model, which accurately treats angular momentum. It is suggested that the long-lived trajectories may arise from motion on vague tori. The trajectory collision-averaged rate constant k(ω,E) is in agreement with an experimental study at 300 K.