Reaction of H with highly vibrationally excited water: Activated or not?

The dynamics of the collisions of H atoms with vibrationally excited H₂O were studied using quasiclassical reactive and quantum mechanical non-reactive scattering calculations using the recently developed potential surface from Ochoa and Clary (OC). The trajectory calculations show that this endoergic reaction is activated for water in its vibrational ground state and with one quantum of OH stretch, but excitation by two or more OH stretch quanta results in diverging cross-sections at low translational energy. The reactive rate coefficients are very large for these states, being a significant fraction of the gas kinetic rate coefficient. This behavior is qualitatively different from what is obtained using the 15 surface of Isaacson, which shows activated behavior even for excitation as high as (04) . To verify the accuracy of the OC surface, we have performed high quality ab initio calculations for H + H₂O, considering geometries in the reagent region that correspond to high OH stretch excitation, and we find that the OC surface is qualitatively correct, but with too long a range in its attractive tail. Our quantum calculations of the total inelastic rate coefficients for collisions with water initially excited in OH stretch overtone states give values that are comparable in magnitude with the reactive rate coefficients for the same states. This suggests that in the recent measurements by Smith and coworkers of the rate coefficients for total loss of excited water, reaction and vibrational energy transfer are of comparable importance.