Gas-phase titration of C7H9+ ion mixtures by FT-ICR mass spectrometry: Semiquantitative determination of ion populations generated by CI-induced protonation of C7H8 isomers and by EI-induced fragmentation of some monoterpenes

The composition of various mixtures of isomeric gaseous C7H9+ ions, generated from four different C7H8 precursors by CI(methane) and CI(isobutane) and by EI-induced fragmentation of two monoterpenes, α-pinene (5) and limonene (6), in the external ion source of an FT-ICR mass spectrometer, has been detected by means of proton transfer reactions to selected bases (“gas-phase titration”) within the ICR cell. Protonation of toluene (1) was found to give a uniform C7H9+ ion population containing exclusively the toluenium ions [1 + H]+ under both CI conditions. By contrast, protonation of 1,3,5-cycloheptatriene (2), 2,5-norbornadiene (3) and 6-methylfulvene (4) was found to give mixtures of constitutionally isomeric C7H9+ ions, containing ions [1 + H]+ and protonated 6-methylfulvene, [4 + H]+, in all of the cases. Protonated norbornadiene, or its valence isomer, the 3-nortricyclyl ion [3′ + H]+, was detected only in minor amounts by CI(CH4) of 2 and by CI of the neutral precursor, 3, itself. Protonation of 6-methylfulvene (4) by CI(CH4) gave rise to partial isomerization to ions [1 + H]+ only, but CI(i-C4H10) of 4 generated also protonated cycloheptatriene, [2 + H]+. Both of the C7H9+ ion mixtures formed by EI-induced fragmentation of α-pinene (5) and limonene (6) were found to consist mainly (≥85%) of toluenium ions, [1 + H]+, with only very small fractions (≤10%) of dihydrotropylium ions, [2 + H]+, in contrast to previous reports in the literature. The gas-phase titration experiments of the C7H9+ mixtures was found to reflect the relative thermodynamic stabilities of the constitutional C7H9+ isomers and the exothermicity of the proton transfer step. The results are also discussed in view of the energy hypersurface of the isomeric C7H9+ ions, parts of which have been calculated by ab initio methods [MP2(full)/6-311+G(3df,2p)//MP2(full)/6-31G(d)].