A new potential energy surface is reported for the gas-phase reaction ${\mathrm{Cl+CH}}_{\mathrm{4}}{\mathrm{\to HCl+CH}}_{\mathrm{3}}.$ It is based on the analytical function of Jordan and Gilbert for the analog reaction ${\mathrm{H+CH}}_{\mathrm{4}}{\mathrm{\to H}}_{\mathrm{2}}{\mathrm{+CH}}_{\mathrm{3}},$ and it is calibrated by using the experimental thermal rate coefficients and kinetic isotope effects. The forward and reverse thermal rate coefficients were calculated using variational transition state theory with semiclassical transmission coefficients over a wide temperature range, 200–2500 K. This surface is also used to analyze dynamical features, such as reaction-path curvature, the coupling between the reaction coordinate and vibrational modes, and the effect of vibrational excitation on the rate coefficients. We find that excitation of C–H stretching modes and Cl–H stretching modes enhances the rate of both the forward and the reverse reactions, and excitation of the lowest frequency bending mode in the ${\mathrm{CH}}_4$ reactant also enhances the rate coefficient for the forward reaction. However, the vibrational excitation of the ${\mathrm{CH}}_3$ umbrella mode (lowest frequency mode in products) slows the reaction at temperatures below 1000 K, while above 1000 K it also accelerates the reaction.

In recent years, many chemical reactions have been studied by means of the quasiclassical trajectory (QCT) method within the Gaussian binning (GB) procedure. The latter consists of “quantizing” the final vibrational actions in Bohr spirit by putting strong emphasis on the trajectories reaching the products with vibrational actions close to integer values. A major drawback of this procedure is that if N is the number of product vibrational modes, the amount of trajectories necessary to converge the calculations is ∼10N×larger than with the standard QCT method. Applying it to polyatomic processes is thus problematic. In a recent paper, however, Czakó and Bowman propose to quantize the total vibrational energy instead of the vibrational actions [G. Czakó and J. M. Bowman, J. Chem. Phys. 131, 244302 (2009)], a procedure called 1GB here. The calculations are then only ∼10 times more time consuming than with the standard QCT method, allowing thereby for considerable numerical saving. In this paper, we propose some theoretical arguments supporting the 1GB procedure and check its validity on model test cases as well as the prototype four-atom reaction OH+D2→HOD+D.

The potential energy surface (PES) for the gas-phase NH₃+H↔NH₂+H₂ reaction is constructed with suitable functional forms to represent the stretching and bending modes, and using as calibration criterion the reactant and product experimental properties and the ab initio saddle point properties. This surface is then used to calculate rate constants with variational transition-state theory over the temperature range 300–2000 K. While the forward rate constants agree with experimental results, the reverse ones are lower by factors of between 4 and 6. Since the same PES is used and these rates are related by detailed balance, this disagreement could indicate an uncertainty in the few available experimental studies for the reverse reaction. We also provide a detailed analysis of the equilibrium constants and of the kinetic isotope effects and compare the results of this analytical PES with earlier ab initio reaction-path calculations. Finally, for the vibrational frequency calculations, we analyze the consequences of the choice of different coordinate systems (curvilinear or rectilinear) on various kinetic magnitudes.

A modified and recalibrated potential energy surface for the gas-phase ${\mathrm{CH}}_4+{\mathrm{H\to CH}}_3+{\mathrm{H}}_2$ reaction and its deuterated analogs is reported and tested, which is completely symmetric with respect to the permutation of the four methane hydrogen atoms, and is calibrated with respect to updated experimental and theoretical stationary point (reactants, products, and saddle point) properties, and experimental forward thermal rate constants. The forward and reverse rate constants are calculated using variational transition-state theory with multidimensional tunneling effect over a wide temperature range, 300–2000 K. The theoretical results reproduce the available experimental data, with a small curvature of the Arrhenius plot which indicates the role of the tunneling in this reaction. Five sets of kinetic isotope effects are also calculated. In general, they agree with experimental values within the experimental errors. This surface is then used to analyze dynamical features, such as reaction-path curvature, the coupling between the reaction-coordinate and vibrational modes, and the effect of the vibrational excitation on the rate constants. It is found qualitatively that excitation of the ${\mathrm{CH}}_4$ stretching and umbrella modes enhance the forward rate constants, and only the ${\mathrm{CH}}_3$ umbrella mode in the product appear vibrationally excited.