Final state-resolved mode specificity in HX + OH → X + H2O (X = F and Cl) reactions: a quasi-classical trajectory study.

The state-to-state dynamics of the title reactions are investigated using a quasi-classical trajectory method on recently developed accurate global potential energy surfaces. Although both produce the H2O product, these two reactions have very different characteristics in the reaction energy, barrier location, and barrier height. It is shown that the H2O product is moderately excited in its three vibrational modes in the HF + OH reaction, but its stretching modes are highly excited in the HCl + OH reaction. For both reactions, the OH vibrational degree of freedom is essentially a spectator, which sequesters its energy throughout the reaction. On the other hand, the HF vibrational excitation has almost no impact on the H2O vibrational distribution while HCl converts some of its vibrational energy into the stretching modes of H2O. These mode specific correlations can be rationalized by the recently proposed Sudden Vector Projection model.