Benchmark rate constants by the hyperquantization algorithm. The FÂ +Â H2 reaction for various potential energy surfaces: features of the entrance channel and of the transition state, and low temperature reactivity

In this work, we present quantum-mechanical rate constants for the prototypical reaction F (2P3/2) + H2 (v=0,j=0,…,5) → HF (v′, j′) + H for temperatures ranging from ∼10 up to 350 K. Rate constants have been obtained by an essentially exact time-independent calculation of cross-sections on a very fine grid of collision energy, permitting accurate Boltzmann averaging including all the contributing partial waves. Quantum effects play a crucial role at the investigated temperatures, where the reactivity is essentially under-barrier and shows a non-Arrhenius behavior. The reaction is thus found to proceed efficiently by tunnel effect and to be enhanced by the presence of resonances. As for the interaction potential, we have used first the benchmark ab initio potential energy surface by Stark and Werner, and then we have properly modified it in the entrance channel according to molecular beam scattering experiments carried out in our laboratory. This allowed us to provide a realistic description of the height and width of the reaction barrier, of the long-range portion of the interaction and to include the fluorine spin-orbit coupling contribution to the entrance channel energetics. Rate constants calculated with such a potential energy surface (PES III) agree within uncertainties with experimental data in the temperature range where they are available (i.e. down to ∼200 K) thus permitting the assessment of the reliability of calculated reactivity properties in the cold (i.e. down to ∼1 K) - and possibly ultra-cold (i.e. ≪1 K) - regimes. Briefly discussed are also threshold behavior and some resonance features, particularly the previously overlooked ones associated to entrance channel characteristics.