EVALUATION OF NITRIC-OXIDE CHEMICAL IONIZATION MASS-SPECTRA OF SUBSTITUTED BENZENES

Nitric oxide chemical ionization mass spectra of substituted benzenes obtained with the Townsend discharge technique were studied. There were four kinds of base peaks in the mass spectra, i.e. [M + NO]+., M+., [M - H]+ and [M - OR]+ (R = H, CH3). The formation of the specific ion [M + NO]+. was highly dependent on the kind of substituent, and it was produced more abundantly in the case of substitutions involving electron-accepting groups. The measure of [M + NO]+. production was evaluated from the value of the ratio [M + NO]+./M+.. In monosubstitutions, it was clarified that the ratios of [M + NO]+./M+. were correlated with the Hammett substituent constant sigma-p or the electrophilic substituent constant sigma-p+. Monosubstitutions (C6H5R) and toluene substitutions (CH3C6H4R) could be classified into six groups in terms of base peak species, [M + NO]+./M+. ratios and substituents. In disubstitutions, the mass spectral patterns were governed by the combination of substituents. Mass spectral distinctions among ortho, meta and para isomers could be made in many cases.