CHAPTER 4-2 - VICINAL INTERPROTON COUPLING

Abstract

This chapter describes vicinal coupling as the interaction between nuclei bound to contiguous atoms, that is, a coupling across three bonds. The vast majority of vicinal inter-proton coupling constants involve the systems H—C—C—H and H—C = C—H. The absolute magnitude of vicinal coupling constants in saturated systems varies from 0 to 16 Hz. Dependence of the vicinal coupling constants on factors other than the dihedral angle is implicit in the original treatment. The magnitude of the other influences is such that they can be regarded as perturbations of the Karplus rule. These perturbations concern the electronegativities of substituents, the orientation of substituents, hybridization of the carbon atoms, the bond angles, and the bond lengths. The existence of an approximately linear relation between substituent electronegativities and the vicinal coupling constants in ethyl derivatives was realized in 1956. However, the analysis of a large number of data clearly showed that the effect of electronegativity, which may be expressed in the form J = A — B.E . where A and B are constants and E is the Huggins electronegativity, is different in various systems, that is, A and B are not general constants.