The 14N and 2H quadrupole coupling tensors in sodium nitroprusside

Abstract

Measurements by straightforward FT NMR at room temperature in a field of 8.4 T are reported of the principal values and directions of the quadrupole coupling (QC) tensors at all nitrogen sites in sodium nitroprusside (SNP), Na 2Fe(CN) 5NO × 2H 20. The assignment of the measured QC tensors to the 14N sites in SNP is discussed. All 14N QC tensors in SNP are nearly axially symmetric. The unique axes of these tensors are parallel to the bond directions of the respective CN and NO ligands within the accuracy of this and the X-ray experiment by which the bond directions were established. The feasibility is discussed of measuring 14N QC tensors in the so-called new state of SNP discovered by U. Hauser and coworkers. The QC tensors at the hydrogen sites in SNP were measured using deuterated crystals of SNP. These measurements show that the water molecules in SNP undergo a flip process which at room temperature is fast on the time scale of deuteron quadrupole couplings (≈.5 μsec) whereas it is slow on this time scale at 77 K. The unique directions of the deuteron QC tensors are off the O-D bond directions established by X-ray scattering by as much as 11°. It is argued that the measurement of deuteron QC tensors provides much more accurate information about the orientation of water molecules in hydrates than can be inferred from X-ray scattering data.