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 CH 3 OH, loss of HCOOCH 3 , formation of ions CH 2 =OCH 3 and to a small extent by loss of CH 2 O and CH 3 OCH 3 , 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 CH 3 OH occurs via π‐and σ‐complexes. The elimination of HCOOCH 3 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 CH 2 O and CH 3 OCH 3 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 .