Protonated aromatic aldehydes and methyl ketones 1a-10a, carrying initially the proton at the carbonyl group, are prepared by electron impact-induced loss of a methyl radical from 1-arylethanols and 2-aryl-2-propanols, respectively. The aryl moiety of the ions corresponds to a benzene group, a naphthalene group, a phenanthrene group, a biphenyl group, and a terphenyl group, respectively, each substituted by a CH3OCH2 side-chain as remote from the acyl substituent as possible. The characteristic reactions of the metastable ions, studied by mass-analyzed ion kinetic energy spectrometry, are the elimination of methanol, the formation of CH3OCH2+ ions, and the elimination of an ester RCOOCH3 (R = H and CH3). The mechanisms of these fragmentations were studied by using D-labeled derivatives. Confirming earlier results, it is shown that the ester elimination, at least from the protonated aryl methyl ketones, has to proceed by an intermediate [acyl cation/arylmethyl methyl ether]-complex. The relative abundances of the elimination of methanol and of the ester decrease and increase, respectively, with the size of the aromatic system. Clearly, the fragmentation via intermediate ion-neutral complexes is favored for the larger ions. Furthermore, the acyl cation of these complexes can move unrestricted over quite large molecular distances to react with the remote CH3OCH2-side-chain, contrasting the restricted migration of a proton by 1,2-shifts ("ring walk") in these systems.
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