The effect of side chain conformation on the carbon-13 substituent chemical shifts of N-substituted benzamides

Abstract

The carbon-13 substituent chemical shifts of the ipso and carbonyl atoms in the para-substituted series Z-C₆H₄CONY₂ (where Y = Me, Prⁱ, Ph) have been measured at low dilution in (D)chloroform, and dimethyl sulfoxide. The DSP equation has been shown to hold for varying side chain conformations with high precision when the same resonance substituent parameter is used. Analysis of these data by the DSP equation indicates that the 'reverse substituent chemical shift effect' for the carbonyl atom is substantially due to a localized polarization of the π-electron system of the amide side chain. The ratio of localized to extended π-polarization of the β position of the amide side chain has been estimated. It is suggested that the effective dielectric constant term for the transmission of field effects by the cavity model varies with side chain conformation. The Cl substituent shifts have been discussed in terms of the shift-charge ratio. A variable-temperature study of the DSP parameters indicates an insignificant temperature effect provided rotation around the benzene ring to amide side chain bond is rapid.