CONFORMATIONAL STUDIES OF α-SUBSTITUTED CARBONYL COMPOUNDS BY I.R. SPECTROSCOPY. III. S-ETHYL α-(ALKYLTHIO)-THIOESTERS

Abstract

The carbonyl stretching frequencies and the apparent molar absorptivities are reported for some α-(alkylthio)-thioacetates, thiopropionate and thioisobutyrate, in the fundamental and in the first overtone regions and compared to those for the corresponding unsubstituted thioesters. In the α-(alkylthio)-thioacetate series the data indicate the presence of two rotamers: cis and gauche. The unusual solvent effect and δ ν_CO shifts for the cis conformers as compared to other α-hetero-substituted carbonyl compounds are interpreted in terms of a decrease in polarity in relation to the gauche conformers. The larger negative δν_CO values for the gauche conformers, which are still more increased by the increased bulkiness of the α-alkylthio group in comparison with the other α-sulfur substituted carbonyl compounds, are attributed to the gauche repulsion between two alkylthio-groups. The presence of two gauche rotamers in the α-(ethylthio)-thiopropionate and one in the α-(ethylthio)-thioisobutyrate is observed and the δν_CO values discussed in terms of change in the mechanical coupling and carbonyl angle. Charge transfer from π_CO to 3d_(S) and hyperconjugative σ_C–S/π_CO interaction are invoked to explain the obtained results.