Dirac vector model and variational treatments of proton spin-spin coupling across three bonds

Abstract

The Dirac vector model and variational treatments have been used to study the angular dependence of proton spin couplings across three bonds in fragments of the type H-X-Y-H (X, Y = C, N, O). These couplings can be expressed by means of the relation J(φ) = A + B cos φ + C cos2 φ, where φ is the dihedral angle and A, B and C are constants. It is seen that the behaviour of various fragments towards substituents can be qualitatively predicted by looking at the sign of the constant B.