On the evaluation of quasi-thermodynamic magnitudes from rate constant values. Influence of the variational and tunnelling contributions

Quasi-thermodynamic magnitudes obtained from three different analytical fits to the experimental rate constants of the CH4 ± OH reaction are compared to the values obtained from theoretical rate constants calculated using canonical variational transition state theory plus multidimensional tunnelling contributions. A right decomposition of ΔGtot,0 into its enthalpic and entropic contributions is not experimentally feasible because it depends on the particular analytical expression used for the rate constants. Then, theoretical calculation of the rate constants at all the required temparatures becomes the only way to get reliable values of ΔHtot,0 (and Ea) and ΔStot,0. Our results show that both variational and tunnelling nonsubstantial contributions to the quasi-thermodynamic magnitudes are significant for the CH4 ± OH reaction and, probably, for a wide range of gas-phase chemical reactions. © 2002 Elsevier Science B.V. All rights reserved.