The involvement of ion-neutral complexes in ethylene loss from [PhC(CH₃)₂]⁺ and its isomers

High-level ab initio calculations at the G2(MP2,SVP) level have been used to characterize the potential energy surface for ethylene loss from various [C₉H₁₁]⁺ ions. There is good agreement between the theoretical predictions and available experimental thermochemical data. We have characterized an alternative pathway to the phenylated cyclopropane mechanism originally proposed to explain the results of ¹³C-labeling studies of ethylene elimination from [PhC(CH₃)₂]⁺. This alternative mechanism is found to be consistent with experimental results of both ¹³C- and deuterium-labeling experiments. We also examine the mechanism for ethylene loss and label exchange for several other isomeric [C₉H₁₁]⁺ ions. It is found that the ¹³C-label exchange observed in protonated allylbenzene and some of the deuterium-labeling results for other ions can be explained by the intervention of intermediate ion-neutral complexes. Comparisons are made with previous theoretical work on related [C₃H₆X]⁺ ions (X = Ph, OH, SH and NH₂). (C) 2000 Elsevier Science B.V.