19F Chemical Shift Tensor in Group II Difluorides

Abstract

The multiple pulse nuclear magnetic resonance techniques have been used to measure the ¹⁹F chemical shift, or nuclear magnetic shielding, tensor in a series of Group II difluorides. For the cubic difluorides measured values relative to a C₆F₆ reference are: CaF₂, −61 ppm; SrF₂, −82; BaF₂, −154 ppm; CdF₂, +33 ppm; and HgF₂, +32 ppm. For the two noncubic difluorides principal values of the chemical shift tensors relative to C₆F₆ are; for MgF₂, +13, +28, and +43 ppm; and for ZnF₂, +15, +38, and +59 ppm. The chemical shifts from the cubic difluorides were found to correlate well with electronegativities and a covalency parameter calculated from electron spin resonance superhyperfine interaction parameters. A theoretical calculation of the chemical shift tensor for MgF₂ is presented and give good agreement with the experimental data, accounting for 90% of the chemical shift anisotropy.