Direct and indirect substituent effects on 1J(CH) in vinyl derivatives and related compounds

The variation in the one–bond couplings ¹J(CH) in vinyl derivatives with substituent has been examined. For the geminal proton ¹J correlates very badly with substituent electronegativity but extremely well with σ_I, if conjugating substituents are excluded. In the case of halogen substituents the marked stereospecificity of ¹J(CH) for the cis and trans protons can be rationalised in terms of an intrinsic dependence of π_CH on the dihedral angle between the coupling atoms and the perturbing substituent, with an additional positive increment to the cis coupling due to direct interaction of the substituent non-bonding electrons or to orbital circulation of the substituent electrons. The intrinsic specificity of β-substituent effects on ¹J(CH) is also found in analogous compounds containing C?N and C?O bonds.