Dissociation of acetone radical cation (CH3COCH3+˙ → CH3CO++CH3˙): An ab initio direct classical trajectory study

Abstract

The dissociation of acetone radical cation (CH₃COCH₃⁺˙ → CH₃CO⁺ + CH₃˙) has been studied by ab initio direct classical trajectory calculations at the MP2/6-31G(d) level of theory. A total of 247 trajectories were initiated at the transition state of the keto–enol tautomerization. A microcanonical ensemble using quasiclassical normal mode sampling was constructed by distributing 10 kcal mol⁻¹ of excess energy above the barrier. The dissociation is found to favor the loss of the newly formed methyl group in agreement with experiments. The branching ratio of methyl loss was calculated to be 1.53 ± 0.20 which is fortuitously in very good agreement with the experimental ratio of 1.55 for 8–12 kcal mol⁻¹ excess energy. Nearly 50% of the available energy is retained by the acetyl fragment as vibrational energy. The methyl fragment has very little vibrational energy but receives ca. 25% of the available energy in translation. The translational energy distribution of the methyl radicals is bimodal with the newly formed methyl having higher average translational energy than that the methyl derived from the spectator group.