Phosphorus chemical shift tensors for tetramethyldiphosphine disulfide: A P-31 single-crystal NMR, dipolar-chemical shift NMR, and ab initio molecular orbital study

Phosphorus chemical shift and spin-spin coupling tensors have been characte rized for tetramethyldiphosphine disulfide (TMPS) by analysis of P-31 CP NM R spectra obtained at 4.7 T for a single crystal. In addition, P-31 CP NMR spectra of stationary powder and magic angle spinning (MAS) samples have be en acquired at two applied magnetic fields (4.7 and 9.4 T) and analyzed ind ependently using the dipolar-chemical shift method. A 2D spin-echo NMR spec trum was also obtained to independently determine the effective P-31-P-31 d ipolar coupling constant. The crystal structure of TMPS (space group C2/m) consists of six molecules per unit cell. For two of the six molecules, the two phosphorus nuclei are related by an inversion center (site 1), while th e remaining four molecules possess mirror planes containing the S-P-P-S bon ds (site 2). The differences between the two sites are very subtle, as reve aled by a redetermination of the X-ray crystal structure. The phosphorus ch emical shift tensors obtained from both single-crystal and dipolar-chemical shift NMR methods are in excellent agreement. For site 1, delta(11) = 91 p pm, delta(22) = 75 ppm, and delta(33) = -63 ppm with an error of +/-2 ppm f or each component. The principal components of the phosphorus chemical shif t tensor at site 2 are very similar; delta(11) = 92 ppm, delta(22) = 74 ppm , and delta(33) = -59 ppm, again with errors of +/-2 ppm. The phosphorus ch emical shift tensors for both sites are oriented such that the direction of highest shielding is closest to the P-S bond while the direction of least shielding is perpendicular to the plane containing the S-P-P-S bonds. Ab in itio (RHF and DFT) calculations of the phosphorus chemical shift tensors fo r both sites are in good agreement with experiment.