THEORETICAL-STUDY ON THE MECHANISM CH4- MODE-ENHANCEMENT EFFECT(C2H2+REACTION )

High level nb initio calculations have been performed to investigate t he mechanism of the ion-molecule reaction of CH4 + C2H2+. Except for s ome subtle differences, the profile for the H-abstraction channel obta ined here at the G2M//B3PW91/6-311 G(d,p) level is very similar to tha t found in a previous study at the G2//MP2/6-31G(d) level. For the com plex formation channel, however, a different transition slate has been located; the geometry and energetics of which are more consistent wit h experimental findings. Calculations of a few direct trajectories hav e been carried out to investigate the possible reason for the signific ant mode enhancement observed experimentally for the H-abstraction cha nnel. Although none of them is reactive, a trajectory with an asymmetr ic C2H bend excitation exhibits a clear signature for being more react ive than those without vibrational excitation or with a symmetric bend excitation. (C) 1998 American Institute of Physics. [S0021-9606(98)30 125-7].