Distribution of reaction products (theory). XI. H + F 2

Abstract

The availability for the first time of detailed rate constants k( V′, R′, T′) (where V′, R′ and T′ are product vibrational, rotational and translational excitation) for the highly exothermic reaction H + F 2 → HF( V′, R′) + F has prompted the 3D classical-trajectory study reported here. The potential-energy surface is found to be predominantly repulsive ( A ⊥ ≈ 42%, R ⊥ ≈ 58%) corresponding to the rather low fractional conversion of reaction energy into vibration (( f′V) = 0.58 from experiment, and 0.56 from theory). In the homologous series of reactions H + X 2 (X  F, Cl, Br, I) the percentage of repulsive energy-release decreases for X  Cl, Br, I, but increases from X  F to Cl. It is shown that this cannot be due to charge in mass-combination, but can plausibly be explained by the anomolously short range of interaction between the separating X atoms in the case X  F. It is predicted that the more-forward scattered HF will be more rotationally excited. The form of the cross section function S r( T) (where T is reagent translation) is analysed. In accordance with the expectation for a strongly exothermic reaction, it is found that S r( T) rises more steeply than S r( V) (where V is reagent vibrational energy). The effect on the product energy distribution conforms qualitatively to the “adiabatic” behaviour noted in previous work: Δ T → Δ T′ + Δ R′; Δ V → Δ V′. The explanation is to be found in reaction through more-compressed or more-extended intermediate configurations than are characteristic of room temperature reaction. We note the existence of an amplification effect: (Δ T′ + Δ R′)/Δ T ≈ 2, and Δ V′/Δ V ≈ 2.