Revisiting chemical kinetics for H + CH4 → H2 + CH3 using variational transition state theory: comparison with global dynamics calculations.

CONTEXT: For both the forward and reverse reaction, there are no accurate rate constants at the VTST/MT level using potential energy surfaces built with the density functional theory (DFT) approach. Calculations of rate constants in function of temperature are carried out for the elementary reaction H + CH4 → H2 + CH3 using variational transition state theory with multidimensional tunneling corrections (VTST/MT) with density functional theory (DFT) and are compared with previous global dynamics calculations reported in the literature. Calculations show that the dual-level method based on an M06-2X PES is capable of providing an accurate framework to compute thermal rate constants in comparison with global dynamics methods. METHODS: The CCSD(T) method with the aug-cc-pVnZ (n = 3-5) basis set is employed to characterize the stationary states for the forward and reverse reactions and the M06-2X and ωB97X-D DFTs are used to generate potential energy surfaces (PESs) in the VTST/MT calculations. At the CVT/μOMT level and using the M06-2X PES, the rate constants at 300 and 600 K are 2.54 × 10-19 and 2.92 × 10-15 cm3 molecule-1 s-1, which are in excellent agreement with the multi-configurational time-dependent Hartree (MCTDH) calculations (1.9 × 10-19 and 2.9 × 10-15 cm3 molecule-1 s-1). For the reverse H2 + CH3 reaction, CVT/μOMT shows good agreement with results using CCSD(T) and analytical PESs. The kinetic isotope effect (KIE) (kCH4+H/kCH4+D) at 500, 600, and 700 K are 0.65, 0.71, and 0.77 in good agreement with experimental data, respectively equal to 0.8 ± 0.2, 0.9 ± 0.2, and 1.0 ± 0.2.