The dissociationrecombination reaction CH _4 (+M) ? CH _3 + H (+M): A case study for unimolecular rate theory

摘要

The dissociationrecombination 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 KT) 10 ~(-24.65) (T300 K) ~(-1.76) 10 ~(-26.38) (T300 K) ~(0.67) cm ~3 molecule ~(-1), high pressure recombination rate constants between 130 and 3000 K as k _(rec), 3.34 × 10 ~(-10) (T300 K) ~(0.186) exp(-T25 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-(T21.22 K) ~(0.5) cm ~6 molecule ~(-2) s ~(-1), for M N 2 by k rec,0 N 2 10 -26.04 exp-(T21.91 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)10.1063/1.3615542 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.