Effect of changing reagent energy on reaction dynamics: VI. Dependence of reaction rate on vibrational excitation in substantially endothermic reactions, XH(υ′) + Y → X + HY

Abstract

The chemiluminescence depletion (CD) method has been applied to three substantially endothermic reactions to obtain approximate relative rates of reaction, k endo (υ′), out of specified reagent vibrational levels, υ′, for a range of vibrational energies, V′, extending from below the barrier energy ( V′ < Q ‡) to well above it. The three reactions were: (1) HCl (υ′ = 1–4) + Br → Cl + HBr ( Q ‡ ≈ −16.5 kcal mole −1), (2) HF(υ′ = 1–6) + Cl → F + HCl ( Q ‡ ≈ −34 kcal mole −1), (3) HF(υ′ = 1–6) + Br → F + HBr ( Q ‡ ≈ −49 kcal mole −1). The molecular reagents for the endothermic reactions were formed in a variety of prior reactions. The rate of the endothermic reaction was obtained from the measured depletion of resolved vibrational-rotational states of the molecular reagent. Experiment and theory (a classical trajectory study, reported here) are in agreement that the major part of the depletion from vibrational levels having V′ > Q ‡ is by way of reaction, rather than by relaxation. For all three reactions, (1)–(3) (irrespective of pre-reaction), k endo (υ′) for reaction (1) on an absolute scale; all led to reactive cross sections S(υ′ = 4) = 1–4 A 2. Comparable depletions, and hence similar reactive cross sections, were obtained for the highest υ′ levels of reactions (2) and (3). Since the reagent translational energy is thermal (300 K), approximately 96% of the reagent energy is present as vibration for the highest reagent vibrational levels of reactions (1)–(3), nonetheless the reactive cross sections exceed 1 A 2, corresponding to collision efficiencies for reaction approaching 0.1. Highly efficient utilisation of reagent vibration in endothermic barrier crossing has previously been noted in trajectory studies of reactions which have the crest of their barrier in the coordinate of product separation, and has been postulated as being a common feature of substantially endothermic reaction. The present experimental study lends support to this view.