Studies in selenium-77 and tellurium-125 nuclear magnetic resonance. Substituent effect and polarizability

Article abstract of DOI:10.1016/0022-328X(88)80638-7

Selenium-77 substituent-caused chemical shifts, where the substituents are alkyl groups, have been studied for a series of selenols, selenides and diselenides, respectively.An understanding of the origin of these shifts was obtained by examining solvent effects on 77Se chemical shifts of dialkyl selenides and dialkyl diselenides in ten halocarbon solvents of varying molecular polarizabilities as well as in 2,2,2-trifluoroethanol.Based on these results of intermolecular polarizability, an intramolecular polarizability concept is proposed whereby dispersion forces within a molecule exerted by neighboring alkyl and haloalkyl groups influence selenium shielding.This effect along with the well-known γ effect offer a clear understanding of a large number of 77Se chemical shifts.The same intramolecular polarizability concept can be used when examining the chemical shifts of tellurides and ditellurides.Here, even larger substituent caused chemical shifts are observed.Solvent effects on 125Te chemical shifts of dimethyl telluride were also examined in solvents of varying polarizability.As in the case of several selenides and diselenides studied, the 125Te chemical shift of dimethyl telluride can be correlated with the polarizability of the solvent.