77Se Solid-State NMR of Inorganic and Organoselenium Systems: A Combined Experimental and Computational Study

Experimental Se-77 NMR parameters for 17 selenium-containing compounds have been determined by analysis of Se-77 solid-state NMR spectra. These are compared to values obtained from first-principles gauge including projector augmented wave (or GIPAW) calculations performed on geometry-optimized crystal structures. Good agreement is observed between experimental and calculated values across a wide chemical shift range, enabling assignment of the experimental Se-77 NMR spectra for compounds containing more than one crystallographically distinct selenium site. Calculations for isolated molecules extracted from the optimized structure reveal that intermolecular interactions have a relatively small effect on isotropic shifts in general, but larger effects on the chemical shift anisotropy are observed for some compounds. Further calculations for a model structure give insight into the effects of local bonding geometry on the Se-77 chemical shift in a diselenide linkage. The Se-77 chemical shift is found to be highly sensitive to torsional angles that define the geometry of the diselenide linkage, and this leads to an understanding of the origins of the large chemical shift differences observed between chemically equivalent selenium sites for one of the compounds studied in this work.