Spontaneous fragmentations of protonated benzaldimines mediated by intermediate ion‐neutral complexes

Abstract

Abstract4‐Methoxymethylbenzaldimmonium ions (a) and the corresponding N‐methylated ions (b) and N,N‐dimethylated ions (c) were easily generated in the ion source by electron impact‐induced dissociation from 1‐(4‐methoxymethylphenyl)ethylamine and its N‐methylated derivatives. The spontaneous fragmentations of metastable ions a‐c and of specifically deuterated derivatives in the second field‐free region of a VG ZAB‐2F mass spectrometer were studied by mass‐analysed ion kinetic energy Spectrometry. The formation of an amino‐p‐quinodimethane radical cation by loss of the methoxy group is observed for all ions. In the case of a and b carrying at least one proton at the immonium group, competing fragmentations are the loss of CH2O and CH3OH, respectively, and the formation of ions CH3OCH2 +, m/z 45, and C7H7 +, m/z 91. Deuterium‐labelling experiments indicated the migration of a proton from the protonated imino group of a and b to the aromatic ring followed by the loss of methanol from the methoxymethyl side‐chain or protolysis of the bond to either side‐chain to form ion‐neutral complexes, in close analogy with the reactions of the corresponding protonated benzaldehydes. The intermediate ion‐neutral complexes dissociate eventually by internal ion‐neutral reactions resulting in the loss of CH2 O and the formation of C7H7 +, respectively.