Relative signs of six proton coupling constants in isopropenylacetylene. The effect of the acetylenic group on the chemical shift of the protons

Abstract

The relative signs of the six proton coupling constants in isopropenylacetylene have been determined by multiple resonance techniques. The signs of the long-range couplings agree with those predicted by the Dirac vector model. The geminal ethylenic proton coupling is taken as positive. It is concluded that the methyl group does not interact with the triple bond via a hyperconjugative mechanism. The ethylenic protons are less shielded than in propene, whereas the opposite is expected on the basis of the magnetic anisotropy of the triple bond. This apparent anomaly can be rationalized quantitatively with a model given by Castellano and Lorenc and all four shifts relative to propene are predicted satisfactorily.