Spin decoupling of high resolution N.M.R. spectra of the type AKX2

Abstract

The determination of relative signs of proton spin coupling constants by double irradiation of high resolution nuclear magnetic resonance spectra is discussed with particular emphasis on AKX2 type molecules. A knowledge of theoretical double irradiation patterns proves to be a useful guide to the analysis of decoupled spectra and indicates that one of the two methods of attacking the problem, instead of causing the spin multiplets to coalesce to a single line, leaves a residual splitting, making interpretation difficult or even ambiguous. Spin decoupling experiments on 2-butenoic acid y-lactone demonstrate that the long-range coupling (2·1 c/s) is opposite in sign to the two vicinal couplings and thus presumably negative. This is in agreement with theoretical predictions.