Loss of Methyl Radical From Some Small Immonium Ions: Unusual Violation of the Even-electron Rule

Article abstract of DOI:10.1002/oms.1210160408

Several small immonium ions of general formula R¹R²C=N(+)R³CH3 (R¹, R², R³=H or alkyl) eliminate <*>CH3; this reaction occurs in the mass spectrometer in both fast (source) and slow (metastable) dissociations.Such behaviour violates the even-electron rule, which states that closed-shell cations usually decompose to give closed-shell daughter ions and neutral molecules.The heats of formation of the observed product ions (for example, <(CH3)2C=NH>^+*) can be bracketed using arguments based on energy data.Deuterium labelling results reveal that the methyl group originally bound to nitrogen is not necessarily lost in the course of dissociation.Thus, for instance, (CH3)2=N(+)HCD3 eliminates both CH3<*> and CD3<*>, via different mechanisms, but very little CH2D<*> or CHD2<*> loss occurs.