Fragmentations of protonated benzaldehydes via intermediate ion/molecule complexes

Ions a_p and a_m corresponding to protonated p- and m-methoxymethylbenzaldehydes have been generated in a mass spectrometer by electron impact fragmentation of the correspondingly substituted 1-phenylethanols (1 and 2). Metastable ions a_p and a_m (2nd FFR of a VG-ZAB-2F mass spectrometer) react by elimination of CH₃OH, loss of HCOOCH₃, formation of ions CH₂=OCH₃ and to a small extent by loss of CH₂O and CH₃OCH₃, respectively. The mechanisms of these reactions have been studied by extensive D-labelling, and it is shown that these fragmentations are initiated by a transfer of the proton located originally at the carbonyl group onto the aromatic ring. The formation of ions \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_2 = \mathop {{\rm OCH}_3 }\limits^ + $\end{document} and the loss of CH₃OH occurs via π-and σ-complexes. The elimination of HCOOCH₃ from ions a_p and a_m corresponds to a functional group interaction between distal side chains and occurs via intermediate ion/molecule complexes formed by a protolytic cleavage of the formyl group. The loss of CH₂O and CH₃OCH₃ proceeds also by intermediate ion/molecule complexes which are generated by a protolytic cleavage of the methoxymethyl side chain in ions a_p and a_m.