Theoretical study of the dynamics of the H+CH4 and H+C2H6 reactions using a specific-reaction-parameter semiempirical...

We present a theoretical study of the reactions of hydrogen atoms with methane and ethane molecules and isotopomers. High-accuracy electronic-structure calculations have been carried out to characterize representative regions of the potential-energy surface (PES) of various reaction pathways, including H abstraction and H exchange. These ab initio calculations have been subsequently employed to derive an improved set of parameters for the modified symmetrically-orthogonalized intermediate neglect of differential overlap (MSINDO) semiempirical Hamiltonian, which are specific to the H+alkane family of reactions. The specific-reaction-parameter (SRP) Hamiltonian has then been used to perform a quasiclassical-trajectory study of both the H+CH4 and H+C2H6 reactions. The calculated values of dynamics properties of the H+CH4-->H2+CH3 reaction and isotopologues, including alkyl product speed distributions, diatomic product internal-state distributions, and cross sections, are generally in good agreement with experiment and with the results provided by the ZBB3 PES [Z. Xie et al., J. Chem. Phys. 125, 133120 (2006)]. The results of trajectories propagated with the SRP Hamiltonian for the H+C2H6-->H2+C2H5 reaction also agree with experiment. The level of agreement between the results calculated with the SRP Hamiltonian and experiment in both the H+methane and H+ethane reactions indicates that semiempirical Hamiltonians can be improved for not only a specific reaction but also a family of reactions.