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

The dissociation/recombination reaction CH(4) (+M) ⇔ CH(3) + 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 K(c) = k(rec)/k(dis) = exp(52,044 K/T) [10(-24.65) (T/300 K)(-1.76) + 10(-26.38) (T/300 K)(0.67)] cm(3) molecule(-1), high pressure recombination rate constants between 130 and 3000 K as k(rec,∞) = 3.34 × 10(-10) (T/300 K)(0.186) exp(-T/25,200 K) cm(3) molecule(-1) s(-1). Low pressure recombination rate constants for M = Ar are represented by k(rec,0) = [Ar] 10(-26.19) exp[-(T/21.22 K)(0.5)] cm(6) molecule(-2) s(-1), for M = N(2) by k(rec,0) = [N(2)] 10(-26.04) exp[-(T/21.91 K)(0.5)] cm(6) molecule(-2) s(-1) between 100 and 5000 K. Weak collision falloff curves are approximated by asymmetric broadening factors [J. Troe and V. G. Ushakov, J. Chem. Phys. 135, 054304 (2011)] with center broadening factors of F(c) ≈ 0.262 + [(T - 2950 K)/6100 K](2) for M = Ar. Expressions for other bath gases can also be obtained.