Location of Energy Barriers. I. Effect on the Dynamics of Reactions A + BC

Abstract

The dynamics of exchange reactions A+BC→AB+C have been examined on two types of potential-energy hypersurfaces that differed in the location of the energy barrier along the reaction coordinate. On “surface I” the barrier was in the entry valley of the energy surface, along the approach coordinate. On “surface II” the barrier was in the exit valley of the energy surface, along the retreat coordinate. The classical barrier height was Ec = 7.0 kcal mole−1 on both surfaces, and was displaced from the corner of the energy surface by the same amount; on surface I, r1‡ = 1.20 Å, r2‡ = 0.80 Å; on surface II, r1‡ = 0.80 Å, r2‡ = 1.20 Å (r1 ≡ rAB, r2 ≡ rBC, and the superscript ‡ refers to the location of the crest of the barrier). Three-dimensional (3D) classical trajectory calculations were performed for the mass combination mA = mB = mC at several reagent energies. The reagent energy took the form of translation, vibration or an equilibrium distribution of the two. The main findings were that translation was markedly more effective than vibration in promoting reaction on surface I, and vibration markedly more effective than translation in promoting reaction on surface II. The total reactive cross section with the entire reagent energy vested in translation was symbolized ST, with the reagent energy (but for 1.5 kcal) in vibration, SV, and with an equilibrium distribution over reagent translation and vibration, Seq. On surface I ST ≫ SV: on surface II SV ≫ ST. Close to the threshold for ST on surface I, ST / Seq ∼ 10; close to the threshold for SV, on surface II, SV / Seq ∼ 10. At high reagent energies (2 × threshold) on surface I ST / Seq fell to 2, whereas on surface II SV / Seq increased to extremely large values. Product energy and angular distributions were recorded for two reagent energies. On surface I with low translational energy in the reagents a major part of the available energy appeared as vibration in the molecular product. At higher collision energy this fraction decreased. On surface II with low vibrational energy in the reagents only a small part of the available energy appeared as vibration in the product. At higher vibrational energy this fraction increased. The product angular distribution at low reagent translational energy on surfaces I and II corresponded to backward-peaked scattering of the molecular product. At increased reagent energy on both surfaces the distribution shifted forward (this is a novel phenomenon in the case of increased reagent vibration; surface II).