Energy distribution among reaction products. IX. F + H 2, HD and D 2

Abstract

The infrared chemiluminescence technique has been used to obtain detailed rate constant k( V′, R′, T′) ( V′, R′, T′ are product vibrational, rotational and translational energies) for four isotopic reactions; (1a) F + HD → HF + D (−Δ H 0 0 = 31.1 kcal mole −1), (1b) F + HD → DF + H (−Δ H 0 0 = 32.8 kcal mol −1), (2) F + H 2 → HF + F (−Δ H 0 0 = 31.9 kcal mole −1), and (3) F + D 2 → DF + D (−Δ H 0 0 = 31.8 kcal mole −1). The mean fraction of the available energy which becomes vibration and rotation in the molecular product, 〈 f » V〉 and 〈 f » R〉, listed in this sequence for the four reactions is: (1a) 0.59 and 0.125; (1b) 0.63 and 0.066; (2) 0.66 and 0.083; (3) 0.67 and 0.076. The changes in mean product rotational excitation along the series are correctly predicted by (prior) classical trajectory studies. These studies do not, however, account for 〈 f » V〉 1a < 〈 f » V〉 1b. The “anomalously” low vibrational excitation for reaction (1a) is likely to be linked to the fact that this reaction liberates an energy barely sufficient to populate the (highest) vibrational level, HF(υ′ = 3); classical mechanics is unsuited to the study of processes near to threshold. The effect of the product vibrational and rotational distributions of variation in the temperature of the molecular reagent in the range from 77—1315 K, has been determined for reactions (1a), (2) and (3). The changes in the mean product distributions are in accord with our earlier finding, based on both theory and experiment, that enhanced reagent translation results in enhanced product translation and rotation, 〈Δ T〉 → 〈Δ T′〉 + 〈Δ R′〉. The detailed rate constant k(υ′ = 3) for the HF product of F + HD [reaction (1a)] showed a marked increase with reagent energy at low energy (77—400 K), levelling off at higher reagent energy (⪆ 600 K). This threshold-type behaviour contrasts with that observed in the other systems for highly vibrationally excited product. For (1a) the energy release measured off the energy surface, E a − Δ H 0 0, (where E a is the activation energy) exceeds the energy of HF (υ′ = 3) by only ∼0.2 kcal mole −1. For reactions (2) and (3) E a - Δ H 0 0 exceeds the energy of the highest-populated υ′-level by ⪆ 1 kcal mole −1. Triangle plots, recording k( V′, R′, T′), are given for both paths of F + Hd [i.e., (1a) and (1b) proceeding in the exothermic direction] and for both paths (HF + D and DF + H, each yielding F + HD) in the endothermic direction.