Recombination of radicals in the high-pressure and high-temperature limit Part 1 Reaction CH3 + CH3

Abstract

The reaction CH₃ + CH₃ –(k)→ C₂H₆ is used in a theoretical study of radical recombination in the gas phase. Long-range attractive forces are studied with the use of adiabatic curves that describe weak reactant interactions. Provided that the kinetic rotational energy is sufficiently large, so that the orbital momentum remains smaller than the boundary value l_lim, the radicals form loosely bound states CH₃···CH₃. Such loose states are of crucial importance in the postulated reaction mechanism. At higher pressures and temperatures (T > 200 K) the maximum asymptotical rate constant for the recombination of methyl radicals is given by the expression k_rec(T) = Lg_e√[h/(RT)](ℏ/μ)^3/2〈l_lim〉²where L = 1/2 is the ratio of the symmetry coefficients for the reagents and transition state, g_e = 1/4, µ is the reduced mass and 〈l_min〉 is the threshold value for the orbital quantum number l, averaged over all the states. For two methyl radicals 〈l_lim〉 = 39 ± 2. This simple analytical formula reproduces experimental results very well. The contribution of direct recombination involving a strongly bound complex is smaller by about an order of magnitude than that involving loosely bound CH₃···CH₃ complexes exclusively.