Experimental and theoretical 31P and 77Se nuclear magnetic shielding tensors for bis(dineopentoxyphosphorothioyl)...

An intergrown crystal of two phases of bis(dineopentoxyphosphorothioyl) diselenide 1 was investigated by goniometer 31P NMR. From the angular dependence of the chemical shift, the tensors of a triclinic and a monoclinic phase were determined. The principal values sigma11, sigma22, and sigma33, of the absolute nuclear magnetic shielding tensors for the triclinic phase are 134.1, 227.2, and 375.5 ppm and for the monoclinic phase are 132.4, 227.8, and 374.2 ppm, respectively. In both cases, the principal axis 3 of the 31P tensor is directed nearly along the P=S bond and the principal axis 2 is nearly perpendicular to the S=P-Se plane. Calculations of the 31P and 77Se nuclear magnetic shielding tensors were performed for molecules of both phases of 1 and for model compounds by the sum-over-states density functional perturbation theory IGLO method. The rms distances between calculated and experimental 31P NMR icosahedral tensor values sigma(j) (j = 1, ..., 6) amount to 17-21 ppm. The calculated and experimental orientations of the 31P principal axes show a maximum difference of 5 degrees and rms distances of 3.2 and 3.3 degrees. For the principal value sigma33 of the selenium shielding tensor the agreement between calculated and experimental values is satisfactory, but the calculated values sigma11 and sigma22 are distinctly too small. Calculations for a model compound in which the methyl groups of the neopentoxy residue are substituted by protons lead practically to the same results.