Cation-π Interactions in Gaseous ω-Phenylalkyloxonium Ions

The gas phase reactivity behavior of -phenylalkyloxonium ions obtained by protonation or alkylation of -phenylalkanols (C6H5(CH2)nOH with n = 1–5, denoted as 1–5, respectively) has been investigated by FT–ICR mass spectrometry and by a radiolytic approach operating at nearly atmospheric pressure. The radiolytic methylation reaction is directed exclusively at the O-atom, yielding oxonium ions whose reactivity behavior varies along the series 1–5. The oxonium ions from the lower members (1, 2) are prone to undergo a nucleophilic displacement process in competition with an exothermic deprotonation reaction at variance with the higher members which are unreactive towards this process. This feature is paralled by their increased stability towards CI(i-C4H10) induced dissociation, both effects assigned to the increased stabilization gained by the electrostatic interaction of the oxonium part with the -electron density of the phenyl group allowed by the folding of the aliphatic chain. The occurrence of cycloalkylation processes and peculiar extent of H/D exchange sequences within protonated 3–5 lend support to this effect.