A study of gaseous benzenium and toluenium ions generated from 1,4- dihydro- and 1-methyl-1,4-dihydro-benzoic acids

Abstract

Gaseous benzenium C₆H₇⁺(1) and toluenium C₇H₉⁺(2) ions have been generated by mass spectrometric loss of ˙CO₂H from the corresponding 1,4-dihydrobenzoic acids (3) and (4), and their fragmentations after ca. 10 µs have been investigated by means of mass-analysed ion kinetic energy (MIKE) spectrometry of some ²H and ¹³C labelled analogues. Metastable C₆H₇⁺ ions eliminate H₂ after proton randomization, whereas metastable C₇H₉⁺ ions expel both H₂ and CH₄ after incomplete proton equilibration. In particular, 40% of C₇H₉⁺ ions randomize all their carbon and hydrogen atoms prior to loss of CH₄, and 60% of C₇H₉⁺ ions lose the original methyl group along with a hydrogen atom from the (proton-equilibrated) benzenium ring, accompanied by a slow and incomplete exchange between the hydrogen atoms of the ring and the methyl group. It is suggested that loss of both CH₄ and H₂ occur via the (ipso-)toluenium ion (2). The role of a non-classical C₇H₉⁺ isomer, phenylmethonium ion (6), is discussed since striking similarities are found compared with [C₆H₆·CH₃⁺]* adducts from ion–molecule reactions described in the literature.