Intrinsic steric 2H/1H isotope effects on 13C shieldings: dihedral angular dependence of shifts over three bonds in saturated systems

This report examines the stereochemical dependence of vicinal deuterium-induced 13C isotope shifts, a phenomenon of both theoretical interest and practical significance for spectral assignments. Results indicated that the longer range isotope shifts induced in 13C spectra by 2H are dependent on the spacial proximity of the interacting nuclei. The fact that the magnitude of three-bond isotope shifts can closely approach those of two-bond effects requires that results from deuterium labeling for spectral assignments be applied with due caution although vicinally shifted components can be distinguished from geminal effects if thetaapprox.0 or 1800 for which J/sub cccd/>J/sub ccd/. These results can aid the theoretical interpretations of isotope shifts in general, but it must be noted that the stereochemical dependence of deuterium-induced isotope shifts in 19F spectra has been found to be quite different, with maximal vicinal shifts found for orientations that maximize the separation between 2H and 19F