Trajectory and Model Studies of Collisions of Highly Excited Methane with Water Using an ab Initio Potential

Quasi-classical trajectory studies have been performed for the collision of internally excited methane with water using an accurate methane-water potential based on a full-dimensional, permutationally invariant analytical representation of energies calculated at a high level of theory. The results suggest that most energy transfer takes place at impact parameters smaller than about 8 Bohr; collisions at higher impact parameters are mostly elastic. Overall, energy transfer is fairly facile, with values for \uab\u394Edown\uab and \uab\u394Eup\uab approaching almost 2% of the total excitation energy. A classical model previously developed for the collision of internally excited molecules with atoms (Houston, P. L.; Conte, R.; Bowman, J. M. J. Phys. Chem. A 2015, 119, 4695-4710) has been extended to cover collisions of internally excited molecules with other molecules. For high initial rotational levels, the agreement with the trajectory results is quite good (R2 48 0.9), whereas for low initial rotational levels it is only fair (R2 48 0.7). Both the model and the trajectories can be characterized by a four-dimensional joint probability distribution, P(J1,f,\u394E1,J2,f,\u394E2), where J1,f and J2,f are the final rotational levels of molecules 1 and 2 and \u394E1 and \u394E2 are the respective changes in internal energy. A strong anticorrelation between \u394E1 and \u394E2 is observed in both the model and trajectory results and can be explained by the model. There is evidence in the trajectory results for a small amount of V \u8660 V energy transfer from the water, which has low internal energy, to the methane, which has substantial internal energy. This observation suggests that V \u8660 V energy transfer in the other direction also occurs