Collisional Energy Transfer in C2H2

Energy transfer via bath gas collisions is an important factor in controlling the kinetics of many combustion reactions. Previous work [1, 2] demonstrated the feasibility and accuracy of trajectory calculations for predicting energy transfer parameters for use in the master equation calculations of pressure dependent rate coefficients. Here we use these methods to study collisions between acetylene and rare gas atoms. Dai and coworkers [3] have observed a dramatic increase in energy transfer efficiency when acetylene is vibrationally excited above the threshold for vinylidene isomerization well beyond the effect of increase in internal energy. Using the C2H2 potential energy surface of Bowman and co-workers [4] and an ab initio based He interaction potential, our trajectory calculations confirm the experimental trend and provide insight into the energy transfer mechanism. Above the isomerization threshold, the majority of trajectories (~70%) sample the acetylene well almost exclusively and the average energy transferred per collision increases weakly with internal energy. The surprising increase in energy transfer efficiency above the vinylidene threshold can be associated with the remaining trajectories, which feature non-statistical internal energy distributions and frequent interconversions between acetylene and vinylidene. Slow IVR and more efficient energy transfer are observed for the low frequency orbiting H atom motion.