Intramolecular aromatic substitution and ortho interactions in substituted 1-phenyl ethanone and diphenylmethanone oximes on electron Impact⁺

The formation of the ions at m/z 133 and 195 from the molecular ions of substituted 1-(phenyl)ethanone oximes and diphenylmethanone oximes has been identified to be due to the expulsion of RH, wherein R is the substituent present in the phenyl ring, for which are assigned 3-methylbenz-1,2-isoxazole and 3-phenylbenz-1 ,2-isoxazole radical cation structures respectively based on CAD spectral study. A mechanism involving an initial intramolecular aromatic substitution of the oxygen atom of the oxime hydroxyl group on the ortho carbon leading to the distonic ion from which a 1,2-elimination of the ortho substituent along with the hydrogen of the hydroxyl group is proposed for the elimination of RH. Substituent scrambling in the distonic ion has been invoked for the loss of RH from certain para substituted derivatives. An important fragment ion at m/z 181 has been noticed in the mass spectra of (2-methoxyphenyl)phenylmethanone oxime due to ortho interaction during the secondary fragmentation process. The proposed mechanism for these processes and the ion structures are supported by high-resolution data, B/E and B2/E linked-scan spectra and CAD/B/E spectra.