A predictive model for unimolecular reactions of organic ions

The slow unimolecular dissociations of six members of the [C_nH_2n-3]⁺ (n = 3-8) series of unsaturated carbonium ions are explained in terms of a potential surface approach together with some concepts of mechanistic organic chemistry. The occurrence of some dissociations is shown to be precluded because either the reacting configurations or product combinations are inaccessible at energies appropriate to metastable transitions. The approach permits correct predictions to be made concerning the shapes of metastable peaks for dissociations which occur without σ-bond formation in the final step. In particular, the observation of a composite peak, thus indicating two channels for reaction, for C₂H₄ loss from [C₇H₁₁]⁺ is naturally accommodated.