Characterization of stereochemistry and molecular conformation using solid-state NMR tensors.

A solid-state NMR technique is described for establishing stereochemistry using the natural product terrein as a model compound. This method involves comparison of experimental (13)C tensor principal values with ab initio computed values for all possible computer-generated stereoisomers. In terrein the relative stereochemistry is confirmed by NMR to be 2R*,3S with high statistical probability (>99.5%). The proposed approach also simultaneously verifies the molecular conformation of the two hydroxy groups in terrein established by X-ray data. It is sufficient to use only shift tensor values at carbons 2 and 3, the stereocenters, to characterize both the stereochemistry and molecular conformations. The solid-state NMR method appears to be especially useful for determining relative stereochemistry of compounds or their derivatives that are difficult to crystallize.

Fitting of low-intensity wide-line spectra dominated by chemical shift anisotropy.

How experimental parameters affect the appearance, and consequently the fitting, of signals having very wide lines is discussed. As these spectra usually have very low intensities, pulse sequences that remove acoustic ringing and other experimental artefacts have to be used. These pulse sequences introduce further spectral distortions. A FORTRAN 77 program was developed that accounts for these effects. The fitting of these broad spectra is demonstrated by application to 207Pb solid-state nuclear magnetic resonance (NMR) spectra.