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

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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.

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