A combined quantum chemical and transition state theory study of the C2H2++CH4 reaction dynamics

The stationary points of importance in the initial branching between complex formation and H transfer in the reaction of acetylene cation with methane are studied at the Gaussian-2 level of theory. Three separate C2H2+... CH4 ion-neutral complexes are found. One complex leads smoothly to the bridged form of C2H3+ plus CH3. A slightly exothermic saddlepoint also connects this ''bridged'' complex to a stable C(3)H6(+) species. A ''classical'' complex leads to the more endothermic classical form of C2H3+. These classical and bridged ion-neutral complexes are nearly degenerate and are directly connected via a low energy saddlepoint which is substantially exothermic relative to reactants. The third, more weakly bound complex, is of a more electrostatic nature and appears to be of no importance in the subsequent branching between the C3H6+ and C2H3+ + CH3 products. Transition state theory calculations employing the ab initio determined energetics and quadratic force fields provide estimated reactive cross sections which are in qualitative accord with recent experimental data. The quantitative discrepancies may be explained on the basis of the neglect of anharmonicites and/or errors in the estimated energetics. Consideration of the vibrational frequencies for the various stationary points provides a qualitative explanation for the observed variations in the reaction cross sections with initial vibrational excitations in the CC stretching and bending modes of C2H2+. (C) 1996 American Institute of Physics.