The dissociation/recombination reaction CH4 (+M) CH3 + H (+M): A case study for unimolecular rate theory

The dissociation/recombination reaction CH4 (+M) ⇔ CH3 + H (+M) is modeled by statistical unimolecular rate theory completely based on dynamical information using ab initio potentials. The results are compared with experimental data. Minor discrepancies are removed by fine-tuning theoretical energy transfer data. The treatment accounts for transitional mode dynamics, adequate centrifugal barriers, anharmonicity of vibrational densities of states, weak collision and other effects, thus being “complete” from a theoretical point of view. Equilibrium constants between 300 and 5000 K are expressed as Kc = krec/kdis = exp(52 044 K/T) [10−24.65 (T/300 K)−1.76 + 10−26.38 (T/300 K)0.67] cm3 molecule−1, high pressure recombination rate constants between 130 and 3000 K as krec,∞ = 3.34 × 10−10 (T/300 K)0.186 exp(−T/25 200 K) cm3 molecule−1 s−1. Low pressure recombination rate constants for M = Ar are represented by krec,0 = [Ar] 10−26.19 exp[−(T/21.22 K)0.5] cm6 molecule−2 s−1, for M = N2 by krec,0 = [N2] 10−26.04 e...