Carbon‐13 magnetic resonance: γ‐anti substituent effects

Abstract γ‐anti Substituent effects of the first row elements (CH3, NH2, OH, F) and the higher‐row halogens on 13C chemical shifts are shown to have separate linear relationships with element electronegativity. The halogens generally show an increasing upfield shift with increasing electronegativity, but the response of the effect of the first‐row elements to changing electronegativity is dependent upon the substitution pattern of the atoms involved. In molecules with the substituent at a bridgehead, increasing electronegativity causes increasing downfield shift of the γ‐anti carbon, whereas in other systems which have the substituent at a secondary carbon, increasing electronegativity causes an increasing upfield shift. Examples are cited to show that the γ‐anti effect of a hydroxyl group is more shielding (by about 2 ppm) when a 1,3‐diaxial interaction between an α and a γ hydrogen is present, than when this interaction is removed by replacement of either of the hydrogens. This correlation is taken as evidence of this hydrogen‐hydrogen interaction as one pathway for the transmission of the γ‐anti substituent effects of first‐row elements.