Is CH₃NC isomerization an intrinsic non-RRKM unimolecular reaction?

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 k_uni(ω,E) found from the biexponential N(t)/N(0) agrees with the Hinshelwood-Lindemann-RRKM k_uni(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 k_uni(ω,E) curves may be brought into agreement by scaling ω in the Hinshelwood-Lindemann-RRKM k_uni(ω,E) by a collisional energy transfer efficiency factor β_c. The interplay between the value of β_c, for the actual intermolecular energy transfer, and the ways the treatment of the rotational quantum number K and nonexponential unimolecular dynamics affect β_c suggests that the ability to fit an experimental k_uni(ω,T) with Hinshelwood-Lindemann-RRKM theory does not identify a unimolecular reactant as an intrinsic RRKM molecule.