Canonical Unified Statistical Rate Constants Using a High-Level Composite Coupled Cluster Energy Scheme for the CH4 + CH Reaction

Abstract

We report an ab initio study on the kinetics and chemical dynamics of the CH4 + CH reaction using a coupled cluster based composite scheme that includes unrestricted coupled cluster singles and doubles with perturbative connected triples method, UCCSD(T), energies extrapolated to the complete basis set limit, in addition to core-valence, higher-order correlation, and relativistic corrections. With this protocol, the reaction enthalpies for the two main reaction channels, C2H4 + H and CH2 + CH3, are reproduced with an absolute deviation of 0.04 kcal mol–1 relative to experimental data. At the UCCSD(T)/complete basis set (CBS) level of theory, the formation of the C2H5 intermediate is barrierless, in contrast with the low submerged barrier (–1.87 kcal -mol–1 relative to the reactants) found at the UCCSD(T)/augmented correlation-consistent polarized double-zeta (aug-cc-pVDZ) level of theory. With the correlation-consistent polarized triple-zeta (cc-pVTZ) basis set, we describe structural variations for the reaction bottleneck along the reaction path, finding at least three canonical variational transition states for the temperature range from 20 to 800 K. The thermal rates were obtained via the canonical unified statistical theory (CUS), using the canonical variational transition state theory (VTST) for the inner-transition state and long-range transition state theory (LRTST) for the outer-transition state. Our calculations agree with literature measurements and show inverse Arrhenius behavior, as observed experimentally. At 298 K, the computed rate constant is 2.65 × 10–10 cm3 molecule–1 s−1 and reported experimental measurements range from 2.5 × 10−12 to 3.0 × 10–10 cm3 molecule–1 s−1. Our theoretical study represents an improvement on previous computational investigations and highlights that even relatively simple gas-phase reactions can require high levels of theory to be modeled accurately.