Relationship of 13C NMR chemical shift tensors to diffraction structures.

Abstract

13C chemical shift tensor measurements on single crystals provide a powerful method to study changes in the electron environment of nuclei with changes in molecular structure. Thus, diffraction structures are critical to an understanding of chemical shift tensors. This work explores the general reliability of using structural data to predict components of the symmetrical chemical shift tensor. Imprecision in the hydrogen positions introduces considerable scatter in the simulated 13C shift tensors, and optimized C-H bond distances in methyl-beta-D-glucopyranoside used with the X-ray positions of the heavier C and O atoms greatly improve the simulated chemical shifts. Acenaphthene, with two crystallographically different molecules per unit cell, offers an excellent example for comparing and contrasting structural differences in the two molecules. A recently improved X-ray structure of naphthalene obtained at low temperature provides chemical shift simulations which are comparable to those from neutron diffraction methods and appear to reflect breaks in the D2h symmetry measured in the NMR chemical shift tensors. These data illustrate the close relationship between NMR and diffraction structures.