| Abstract: | ‘Unexpected’ second-order effects encountered in proton-coupled 13C NMR spectra where relative proton shifts are large compared to proton-proton coupling constants are illustrated. The observed phenomenon is explained, with reference to the ABX spin system as a model, as being due to the near equality of the relative proton shift, in frequency units, to the difference between one-bond and long-range 13CH couplings. The effect may be removed by a change in operating field strength or addition of a shift reagent. |
