Rationalization of scrambling processes among [C2H3O]+ ions

Scrambling data for the three observed C₂H₃O]⁺ isomers, namely CH₃CO]⁺ (a), CH₂COH]⁺ (b) and (c), are rationalized by using ab initio molecular orbital calculations. For ions a and c, processes leading to scrambling of the carbon atoms require substantially more energy than the threshold for decomposition to CH₃]⁺ + CO. Accordingly, little or no carbon scrambling is predicted nor is any observed in the metastable dissociation of a and c. The observed carbon scrambling in b prior to metastable dissociation to CH₃]⁺ + CO has previously been explained in terms of a mechanism involving the oxiranyl cation (c). However, this mechanism is shown to be unlikely because of the high energies involved. An alternative lower-energy pathway involving the intermediacy of protonated oxirene (h) is proposed. Such a mechanism is fully compatible with the experimental data.