Is CHNC 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 CHNC. Microcanonical ensembles of CHNC, excited with 150, 120, and 100 kcal/mol of vibrational energy, isomerized to CHCN nonexponentially, indicative of intrinsic non-Rice-Ramsperger-Kassel-Marcus (RRKM) dynamics. The distribution of surviving CHNC 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 CHNC isomerization. Classical power spectra calculated for vibrationally excited CHNC, at the experimental average energy of isomerizing molecules, show that the intramolecular dynamics of CHNC are not chaotic and the C-N≡C and CH units are weakly coupled. The biexponential N(t)/N(0) at 100 kcal/mol is used as a model to study CHNC → CHCN isomerization with biexponential dynamics. The Hinshelwood-Lindemann rate constant k(ω,E) found from the biexponential N(t)/N(0) agrees with the Hinshelwood-Lindemann-RRKM k(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(ω,E) curves may be brought into agreement by scaling ω in the Hinshelwood-Lindemann-RRKM k(ω,E) by a collisional energy transfer efficiency factor β. The interplay between the value of β, for the actual intermolecular energy transfer, and the ways the treatment of the rotational quantum number K and nonexponential unimolecular dynamics affect β suggests that the ability to fit an experimental k(ω,T) with Hinshelwood-Lindemann-RRKM theory does not identify a unimolecular reactant as an intrinsic RRKM molecule.