Rotational isomerism. Part VII. Effect of substituents on vicinal coupling constants in XCH2·CH2Y fragments

Abstract

The vicinal H–H couplings of the individual rotational isomers of a number of 1,2-dihalogenoethanes are obtained from the solvent dependence of the coupling constants. The analyses of the ¹H n.m.r. spectra of the CH₂·CH₂ groups in 2,2,5,5-tetramethylhexane and 2,2-dimethylbutyltrimethylammonium iodide are given. These results are combined with those for a number of CH₂·CH₂ groups in a staggered conformation in cyclic and acyclic compounds to examine the influence of substituents on the individual H–H coupling constants.

All the couplings depend primarily on the electronegativity of the substituents, but whereas most of the couplings decrease with increasing electronegative substituents the gauche-coupling in the trans-isomer (J_t^g) increases. Although this result is contrary to all explicit theories of these couplings there is some recent theoretical evidence to support this trend.

The significance of these results on other experimental rationalisations of the orientation dependence of electronegative substituents, and on the theoretical cos²ϕ relationship, is considered.

The predictable values of the couplings in the individual isomers of any CH₂X·CH₂Y compound provides a simple method for the determination of the energy difference between the gauche-and the trans-isomer from the measured H–H couplings. Some examples of this method are given.