Determination of relative configuration is frequently a rate-limiting step in the characterization of small organic molecules. Solution NMR-based nuclear Overhauser effect and scalar J-coupling constants can provide useful spatial information but often fail when stereocenters are separated by more than 4–5 Å. Residual dipolar couplings (RDCs) can provide a means of assigning relative configuration without limits of distance between stereocenters. However, sensitivity limits their application. Chemical shift is the most readily measured NMR parameter, and partial molecular alignment can reveal the anisotropic component of the chemical shift tensor, manifested as residual chemical shift anisotropy (RCSA). Hence, 13C RCSAs provide information on the relative orientations of specific structural moieties including nonprotonated carbons and can be used for stereochemical assignment. Herein, we present two robust and sensitive methods to accurately measure and apply 13C RCSAs for stereochemical assignment. The complementary techniques are demonstrated with five molecules representing differing structural classes.
Experimental details, alignment tensors, RCSA and RDC data for all investigated molecules, a description of gel preparation, derivation of the isotropic shift correction formula, comparison of RCSA data with stretching and compression devices, structural coordinates, and chemical shift tensors (PDF)