Mass spectral study of O- and S-aryl dimethylthiocarbamates under electron impact conditions: Newman-Kwart rearrangement in the gas phase

Abstract

The electron impact (EI) mass spectra of isomeric S- and O-aryl (2-naphthyl, R-Ph-= aryl, where R = H, NO₂, COOCH₃, COCH₃ and OCH₃) dimethylthiocarbamates have been studied in detail. The EI spectra of O-aryl dimethylthiocarbamates include some of the fragment ions that are characteristic of the corresponding S-aryl isomer, which could be explained by a Newman-Kwart rearrangement in the ion source of the mass spectrometer, after ionization. The EI mass spectra of S-aryl dimethylthiocarbamates showed specific fragment ions resulting from cleavage of the molecular ion involving the bond between sulfur and the carbonyl group of the carbamate moiety (α-cleavage). The EI mass spectra of O-aryl dimethylthiocarbamates, however, showed the fragmentation of the molecular ion by α-cleavage involving the bonds on both sides of oxygen atom. Further, the EI mass spectra of O-aryl dimethylthiocarbamates containing an electron-withdrawing substituent (-R) on the aryl group showed a characteristic fragment ion corresponding to [M - R]⁺.