Is CH3NC isomerization an intrinsic non-RRKM unimolecular reaction?

Abstract

Direct dynamics simulations, using B3LYP/6-311++G(2d,2p) theory, were used to study the unimolecular and intramolecular dynamics of vibrationally excited CH3NC. Microcanonical ensembles of CH3NC, excited with 150, 120, and 100 kcal/mol of vibrational energy, isomerized to CH3CN nonexponentially, indicative of intrinsic non-Rice-Ramsperger-Kassel-Marcus (RRKM) dynamics. The distribution of surviving CH3NC molecules vs time, i.e., N(t)/N(0), was described by two separate functions, valid above and below a time limit, a single exponential for the former and a biexponential for the latter. The dynamics for the short-time component are consistent with a separable phase space model. The importance of this component decreases with vibrational energy and may be unimportant for energies relevant to experimental studies of CH3NC isomerization. Classical power spectra calculated for vibrationally excited CH3NC, at the experimental average energy of isomerizing molecules, show that the intramolecular dynamics of CH3NC are not chaotic and the C-N≡C and CH3 units are weakly coupled. The biexponential N(t)/N(0) at 100 kcal/mol is used as a model to study CH3NC → CH3CN isomerization with biexponential dynamics. The Hinshelwood-Lindemann rate constant kuni(ω,E) found from the biexponential N(t)/N(0) agrees with the Hinshelwood-Lindemann-RRKM kuni(ω,E) at the high and low pressure limits, but is lower at intermediate pressures. As found from previous work [S. Malpathak and W. L. Hase, J. Phys. Chem. A 123, 1923 (2019)], the two kuni(ω,E) curves may be brought into agreement by scaling ω in the Hinshelwood-Lindemann-RRKM kuni(ω,E) by a collisional energy transfer efficiency factor βc. The interplay between the value of βc, for the actual intermolecular energy transfer, and the ways the treatment of the rotational quantum number K and nonexponential unimolecular dynamics affect βc suggests that the ability to fit an experimental kuni(ω,T) with Hinshelwood-Lindemann-RRKM theory does not identify a unimolecular reactant as an intrinsic RRKM molecule.