Theoretical investigation on the geminal proton-carnon-13 Coupling constants

Abstract

It has been concluded from the present study that the geminal proton coupling of carboxyl (carbonyl) carbon-13 has an angular dependence of the form: A+Bcos?2?A + B \cos^2 \thetaA+Bcos2? based on the relative orientation of the ?-orbital at the coupled carbon. The magnitude of the coupling is maximum when the C-H bond bearing the coupled proton lies perpendicular to the ?-nodal plane. In general, the trends in the effect of substituents on the geminal proton-carbon-13 coupling constants can be explained qualitatively in the same manner as the Pople-Bothner-By treatment for geminal proton-proton coupling constants. However, when there is a ?-orbital on the coupled carbon-13, it is necessary to consider the extent of core polarization in explaining substituent effects. Both exchange polarization and electron transfer mechanisms contribute to nuclear spin coupling. The extent to which these mechanisms contribute varies from system to system. From the present calculations, it appears that the electron transfer contribution increases with unsaturation. For unsubstituted and saturated systems, exchange polarization makes the dominant contribution, provided interbond spin exchanges are properly accounted for in calculations. The present calculations indicate that both the Dirac vector model and Penney-Dirac bond order formulations are useful as models for calculating nuclear spin coupling constants. If exchange integrals of reasonable magnitudes are used, satisfactory results can be obtained for most cases. However, it is necessary to include ionic VB structures in more complete calculations. For substituted systems, the Penney-Dirac and “effective” Penney-Dirac bond orders derived from observed nuclear spin coupling constants can be employed. Wherever suitable choices of coupling constants can be made for the bond order approach, this study shows that it is possible to reproduce the observed trends qualitatively, if not quantitatively. Because of inherent approximations and limitations in the various theoretical approaches, it is not always possible to obtain calculated values in quantitative agreement with experimental values.