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 o f substituted 1-(phenyl)ethanone oximes and diphenylmethanone oximes h as 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-methy lbenz-1,2-isoxazole and 3-phenylbenz-1,2-isoxazole radical cation stru ctures 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 t he hydrogen of the hydroxyl group is proposed for the elimination of R H. Substituent scrambling in the distonic ion has been invoked for the loss of RH from certain para substituted derivatives. An important fr agment ion at m/z 181 has been noticed in the mass spectra of (2-metho xyphenyl)phenylmethanone oxime due to ortho interaction during the sec ondary fragmentation process. The proposed mechanism for these process es and the ion structures are supported by high-resolution data, B/E a nd B-2/E linked-scan spectra and CAD/B/E spectra.