Modification of the Duchovic–Hase–Schlegel potential energy function for H+CH3↔CH4. Comparison of canonical variational transition state theory, trajectory, and experimental association rate constants

Abstract

The Duchovic–Hase–Schlegel analytic potential energy function for H+CH3→CH4 association is modified to have the correct asymptotic behavior at large separations. Canonical variational transition state theory calculations performed on this modified potential yield the same association transition state structures and rate constants as reported previously for the unmodified potential. This modified potential is also used in a classical trajectory calculation of the association rate constant. The canonical variational transition state theory and trajectory association rate constants agree for both H+CH3 and D+CH3. Trajectories are used to study the unimolecular decomposition of the excited methane molecules formed by association. Most of the molecules decay exponentially. However, an initial nonrandom transient constitutes ∼5%–7% of the decomposition. Trajectory rate constants, derived from the exponential decay, are compared with harmonic Rice–Ramsberger–Kassel–Marcus (RRKM) rate constants to derive anharmonic correction factors.