Evidence for an ortho specific loss of OH· in some aromatic aldoximes from examination of metastable peak shapes

Abstract The mechanism for loss of an OH radical from the molecular ions of the ortho substituted compounds 2‐methyl‐, 2‐hydroxy‐, 2‐amino‐ and 2‐methoxybenzaldoxime has been studied. It is inferred from the examination of the first field free region (composite) metastable peak shapes that two distinct mechanisms are operating. Based upon additional deuterium labelling experiments it is proposed that loss of OH involves both participation of the ortho substituent in the formation of a 5‐membered heterocyclic ring and the formation of a protonated isocyanide type ion via a 3‐membered ring transition state. Loss of OCH3 from the corresponding O‐methyl ethers probably occurs by similar mechanisms. The formation of a 5‐membered ring is also proposed to be the driving force for the loss of H2NO in the spectrum of 2‐methoxybenzaldoxime.