13C NMR of organosulphur compounds: I—the effects of sulphur substituents on the 13C chemical shifts of alkyl chains and of S-heterocycles

The ¹³C NMR spectra have been determined of: (i) aliphatic compounds having at one end a functionalized sulphur atom (? SH, ? S^?, ? SMe, ? S(O)Me, ? SO₂Me and ? S⁺Me₂) and (ii) saturated sulphur heterocycles variously substituted at the S-atom . The results are discussed in terms of the familiar deshielding effects for α- and β-carbons and shielding effects for γ-carbons, exerted by the sulphur atom itself and/or by the atoms or groups of which the sulphur function is made up. The γ-effect of the S-atom appears to be nearly independent of the nature of the S-function and of comparable magnitude to that of an aliphatic carbon (?2·5 + ?3·0 ppm). Surprisingly, however, a S? CH₃ group shields the carbon in γ position with respect to CH₃ by an amount (?5·4 ppm) which is more than twice that (?2·5 ppm) exerted by the aliphatic γ-carbon on the S-CH₃ carbon itself. As to the cyclic compounds, the shieldings of the α- and β-carbons can be rationalized in terms of the conformational orientation of the substituent at sulphur, and the equilibrium distribution of the conformers. The results confirm the great value of ¹³C NMR for configurational and conformational assignment of S-heterocycles.