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

An intergrown crystal of two phases of bis(dineopentoxyphosphorothioyl) dis elenide 1 was investigated by goniometer P-31 NMR. From the angular depende nce of the chemical shift, the tensors of a triclinic and a monoclinic phas e were determined. The principal values sigma (11), sigma (22), and sigma ( 33) of the absolute nuclear magnetic shielding tensors for the triclinic ph ase 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 P-31 tensor is directed nearly along the P=S bond and the principal ax is 2 is nearly perpendicular to the S=P-Se plane. Calculations of the P-31 and Se-77 nuclear magnetic shielding tensors were performed for molecules o f both phases of 1 and for model compounds by the sum-over-states density f unctional perturbation theory IGLO method. The rms distances between calcul ated and experimental P-31 NMR icosahedral tensor values sigma (1)(j = 1,.. ..,6) amount to 17-21 ppm. The calculated and experimental orientations of the P-31 principal axes show a maximum difference of 5 and rms distances of 3.2 and 3.3. For the principal value sigma (33) of the selenium shielding tensor the agreement between calculated and experimental values is satisfac tory, but the calculated values sigma (11) and rr,, are distinctly too smal l. Calculations for a model compound in which the methyl groups of tht neop entoxy residue are substituted by protons lead practically to the same resu lts, (C) 2000 Academic Press.