A quasiclassical trajectory study of reagent vibrational excitation effects in the OH+H2→H2O+H reaction

Abstract

Quasiclassical trajectories have been used to determine reaction rate constant enhancements and product state energy partitioning in the OH(v)+H2(v′)→H2O+H reaction for (v,v′)= (0,0), (1,0), (0,1), and (1,1). An analytical fit to the accurate ab initio potential surface of Walch and Dunning was used in the Monte Carlo calculations. Final H2O vibrational states were assigned using the histogram method to bin the good action variables governing H2O vibrational motions. These actions were calculated by using second order classical perturbation theory to solve the vibrational Hamilton–Jacobi equation. The resulting integral reaction cross sections and thermal rate constants, indicate that OH vibrational excitation leads to a very small enhancement (only a factor of 1.28) in the thermal rate constant at 300 K. H2 excitation, on the other hand, causes a large reduction in the reaction activation energy (from 0.18 eV to 0.03 eV) and a large enhancement in the rate constant (a factor of 393 at 300 K). These results, as well as the ground reagent state thermal rate constants are in good agreement with experiment. We also find that simultaneous H2 and OH vibrational excitation gives a result which is just the superposition of the separate excitation results. Reagent rotational excitation causes a decrease in the reaction cross section, with the rate of decrease larger when H2 is excited than OH. An examination of product state energy partitioning indicates that nearly all of the additional energy coming from reagent vibrational excitation ends up as product vibration, with all of it going to the H2O stretch modes when OH is initially excited, and a more random distribution of the three H2O modes when H2 is excited. This mode specificity of energy flow contrasts with the nonspecific vibrational mode distributions obtained for the reaction from the ground state reagents.