The reaction of F + H2→ HF + H. A case study in reaction dynamics
Concepts developed in recent years as a result of experimental and theoretical studies of the dynamics of reactions A + BC → AB + C are examined here in trajectory studies of the reaction F + H2→ HF + H—currently the prototype of exothermic reaction. (i) An “early” barrier, characteristic of substantially exothermic reaction, has the consequence that reagent translation is much more efficient than vibration in promoting reaction, even for energies well in excess of the barrier. (ii) High vibrational excitation in the molecular product stems from the release of H·H repulsion while the new bond, F––H, is still extended (termed “mixed energy-release”); the large zero point vibration in H2 introduces variability in the F––H extension and H·H repulsion, and consequently in the breadth of the observed product vibrational distribution. (iii) Since the product vibrational excitation is governed by the attractive plus mixed energy-release, the slope of the outrun of the energy surface is not, per se, a dominant factor governing product vibrational excitation. (iv) Enhanced reagent vibration, 〈ΔV〉, tends to be channelled into enhanced product vibration, 〈ΔV′〉; the efficiency (〈ΔV′〉= 0.81 ΔV) is dependent on the form of energy surface in the region of “corner-cutting”, corresponding to reaction through extended intermediates F––H–H. (v) Enhanced reagent translation is channelled into product translation plus rotation (〈ΔT′〉+〈ΔR′〉= 1.12 〈ΔT〉); the efficiency provides an indication of the energy required to bend and compress the intermediate FH·H. (vi) The rotational dependence of the reactive cross section, σ(J), provides a sensitive probe of the region of the potential-energy hyper-surface along the entry valley up to the barrier. (vii) Enhanced reagent rotational excitation at first decreases product vibrational excitation and then increases it according to experiment; this effect has not been reproduced theoretically. (viii) Product rotational excitation derives in large part from the release of repulsion in bent configurations: this gives rise predominantly to coplanarity and opposed directions for the product rotational and orbital motions. (ix) Enhanced reagent excitation of all sorts results in significant enhancement in product rotation, due to reaction through more-compressed and bent configurations; the most efficient conversion is from reagent rotation into product rotation (〈ΔR′〉= 1.2 〈ΔR〉). (x) For thermal (300 K) reaction the computed centre-of-mass angular distribution is sharply backward-peaked, and similar for all product vibrational levels. (xi) Enhanced reagent vibration or translation shifts the computed mean scattering angle forward, with ΔT being markedly more effective than ΔV; the “stripping threshold energy” is high, as anticipated for these masses reacting on a strongly repulsive potential-energy surface favouring collinear approach.