Fragmentations of protonated acetophenones via intermediate ion–molecule complexes

Protonated acetophenones, substituted with a methoxymethyl group in the para and meta positions, have been generated by electron impact induced fragmentation of the correspondingly substituted 2-phenylpropan-2-ols. The metastable ions, formed in the second field-free region of a VG ZAb 2F mass spectrometer, react unimolecularly by elimination of CH₃OH, formation of CH₃CO⁺ and ions, loss of CH₃COOCH₃, and loss of CH₂O. The mechanisms of these fragmentations have been elucidated with the aid of deuterated analogues of the protonated acetophenones. It is shown that these reactions are initiated by an endothermic transfer of the proton at the carbonyl group of the protonated acetophenones to the benzene ring. A further migration of the proton to the ether O atom of the methoxymethyl side-chain leads eventually to the elimination of CH₃OH. Protolytic bond cleavages of either side-chain gives rise to the CH₃CO⁺ and ions. At low internal energies both these ions may be trapped by the aromatic neutral fragment in ion-molecule complexes. Reactions within these complexes result in the energetically favourable losses of CH₃COOCH₃ and CH₂O, respectively. With respect to these reactions, the protonated acetophenones behave analogously to the correspondingly substituted and protonated benzaldehydes.