Electron Coupling of Nuclear Spins. V. Vibrational Effects in CHX Groups

Abstract

Previous calculations of the H–H coupling in CH₂ groups assumed a static H–C–H bond angle. It is shown here that, because the predicted angular dependence of the coupling is nonlinear, the bond‐bending vibrations should give an averaged value appreciably larger than that for the corresponding equilibrium, but static, angle. The contribution of excited vibrational states is very small and hence the averaged coupling constant is effectively temperature independent. There should be a small but detectable difference in the coupling constants for CH₂ and CHD groups because of the isotopic differences in vibrational amplitude. The experimental coupling constants exhibit an apparent linear dependence upon the equilibrium H–C–H angle, and careful measurements of the isotope effects could be useful in establishing whether or not there is any explicit curvature in the angular dependence, and thus whether the observed linearity is real or due to the neglect of substituent effects in the calculations. In any case, comparison between experiment and the predicted angular dependence suggests that too low a value may have been used previously for one of the exchange integrals, K(C₁, C₂) in the calculations for both the CH₂ and the HCCH groups. An attempt to detect an isotopic difference between the geminal H–F and D–F coupling in n—C₃F₇H and n—C₃F₇D gave negative results.