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


Troe,  J.
Emeritus Group of Spectroscopy and Photochemical Kinetics, MPI for Biophysical Chemistry, Max Planck Society;

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Troe, J., & Ushakov, V. G. (2012). The dissociation/recombination reaction CH4 (+M) ⇔ CH3 + H (+M): A case study for unimolecular rate theory. The Journal of Chemical Physics, 136(21): 214309. doi:10.1063/1.4717706.

Cite as: https://hdl.handle.net/11858/00-001M-0000-000E-EDAA-C
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 exp[−(T/21.91 K)0.5] cm6 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 Fc ≈ 0.262 + [(T − 2950 K)/6100 K]2 for M = Ar. Expressions for other bath gases can also be obtained.