Mass spectral study of O- and S-aryl dimethylthiocarbamates under electron impact conditions: Newman-Kwart rearrangement in the gas phase ² S. Prabhakar 1 *, Paramita Kar 2 , S. P. Mirza 1 , V. V. S. Lakshmi 1 , K. Nagaiah 2 and M. Vairamani 1 1 National Centre for Mass Spectrometry, Indian Institute of Chemical Technology, Hyderabad 500 007, India 2 Organic Division III, Indian Institute of Chemical Technology, Hyderabad 500 007, India Received 3 August 2001; Revised 20 September 2001; Accepted 21 September 2001 The electron impact EI) mass spectra of isomeric S- and O-aryl 2-naphthyl, R-Ph- = aryl, where R=H,NO 2 , COOCH 3 , COCH 3 and OCH 3 ) 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 a-cleavage). The EI mass spectra of O-aryl dimethylthiocarbamates, however, showed the fragmentation of the molecular ion by a-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]  . Copyright # 2001 John Wiley & Sons, Ltd. Mass spectrometry continues to be a sensitive tool not only for the characterization of compounds of different nature, but also for the study of gas-phase reactions. Rearrangement reactions play a major role in the solution phase, which can sometimes be a direct pathway for critical transformations. Many solution-phase rearrangements are also studied in the ion source of a mass spectrometer, a procedure suitable for exploring the fundamental nature of the rearrangements in the absence of solvent. 1±5 The electron impact EI) source is useful for the study of intramolecular rearrangements, because the fragmentation processes under EI conditions are unimolecular processes. One such intramolecular re- arrangement is the Newman-Kwart rearrangement of O-aryl dialkylthiocarbamates found under solution-phase condi- tions. 6±14 A number of phenols have been converted to the corresponding thiol compounds by exploiting the Newman- Kwart rearrangement Scheme 1). Mass spectral analyses of reaction products of the Newman-Kwart rearrangement have not been reported in the literature, to our knowledge. Hence, a detailed study of the behaviour of isomeric S-aryl and O-aryl dimethylthiocarbamates under EI conditions was undertaken, and the results are presented. EXPERIMENTAL The S-aryl dimethylthiocarbamates 1, 3, 5, 7 and 9; Scheme 2) were prepared from the corresponding O-aryl di- methylthiocarbamates 2, 4, 6, 8 and 10; Scheme 2) under microwave irradiation. 15 The structures of all the com- pounds were characterized by IR, NMR and mass spectro- metry. Mass spectra were recorded using a Micromass Autospec M mass spectrometer equipped with an OPUS V3.IX data system, using a dedicated EI source. The source conditions were: accelerating voltage 7 kV, electron energy 70 eV, trap current 200 mA, and source temperature 250 °C. Spectra were acquired using the data system with a scan time of 1s/decade and 0.5 s interscan delay. All the samples were introduced through an HP 5890 series-II gas chromato- graph using an HP-5 length, 30m; film thickness, 0.25 mm; i.d., 0.32 mm) capillary column for all the experiments. The injector and interface temperatures were kept at 250 °C and 220 °C, respectively. The column temperature for all the compounds was programmed to start at 140 °C and allowed to rise after 2 min at the rate of 8 °C/min to 200 °C. Helium was used as the carrier gas at a flow rate of 1 mL/min. The CID fragment-ion spectra were obtained by the linked scan technique at constant B/E under computer control. The collision gas, helium, was admitted into the collision cell in the first field-free region to reduce the main beam signal by 30±40% of its original intensity. Precursor ion spectra were obtained using the linked scan technique at constant B 2 /E. *Correspondence to : S. Prabhakar, National Centre for Mass Spectrometry, Indian Institute of Chemical Technology, Hyderabad 500 007, India. ² IICT Communication No.4857 DOI:10.1002/rcm.490 Copyright # 2001 John Wiley & Sons, Ltd. RAPID COMMUNICATIONS IN MASS SPECTROMETRY Rapid Commun. Mass Spectrom. 2001; 15: 2127±2134