Reactant vibrational excitations are more effective than translational energy in promoting an early-barrier reaction F + H2O → HF + OH.

The exothermic F + H(2)O → HF + OH reaction has a decidedly "early" or "reactant-like" barrier. According to a naïve interpretation of the Polanyi's rules, translational energy would be more effective than vibrational energy in promoting such reactions. However, we demonstrate here using both quasi-classical trajectory and full-dimensional quantum wave packet methods on an accurate global potential energy surface that excitations in the H(2)O vibrational degrees of freedom have higher efficacy in enhancing the reactivity of the title reaction than the same amount of translational energy, thus providing a counter-example to Polanyi's rules. This enhancement of reactivity is analyzed using a vibrational adiabatic model, which sheds light on the surprising mode selectivity in this reaction.