Theoretical study of kinetic isotope effects on rate constants for the H2+C2H→H+C2H2 reaction and its isotopic variants

Abstract

Thermal rate constants have been calculated for the H2+C2H→H+C2H2 reaction (1) and its isotopic variants: HD+C2H→H+C2HD (2); DH+C2H→D+C2H2 (3); D2+C2H→D+C2HD (4); H2+C2D→H+C2HD (5) using variational transition state theory with the multidimensional semiclassical tunneling correction. The geometries were optimized at the MP2(full)/cc-pVTZ level and the potential energy curves for these reactions were calculated at the PMP4(SDTQ,full)/cc-pVTZ and QCISD(T,full)/cc-pVTZ levels. It was thus revealed that these reactions have “early” potential barriers. The calculated rate constants for reactions (1) and (5) were found to be comparable and the largest among these reactions. The calculated rate constants for reactions (1) and (4) showed good agreement with experiment at relatively low temperatures. The reaction-path-curvature effects and secondary kinetic isotope effects [the effects of change in zero-point energies (ZPEs) along the reaction path relative to the reactant ZPEs] were predicted not to be so large in these reactions. Rather, the primary isotope effects (the effects of change in the effective mass for the reaction coordinate) were found to be the main contributions to tunneling. This is because these reactions have “early” characters.