Phosphorus Chemical Shift Tensors for Tetramethyldiphosphine Disulfide: A 31P Single-Crystal NMR, Dipolar-Chemical Shift NMR, and Ab Initio Molecular Orbital Study

Abstract

Phosphorus chemical shift and spin−spin coupling tensors have been characterized for tetramethyldiphosphine disulfide (TMPS) by analysis of 31P CP NMR spectra obtained at 4.7 T for a single crystal. In addition, 31P CP NMR spectra of stationary powder and magic angle spinning (MAS) samples have been acquired at two applied magnetic fields (4.7 and 9.4 T) and analyzed independently using the dipolar-chemical shift method. A 2D spin-echo NMR spectrum was also obtained to independently determine the effective 31P−31P dipolar 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 the remaining four molecules possess mirror planes containing the S−P−P−S bonds (site 2). The differences between the two sites are very subtle, as revealed by a redetermination of the X-ray crystal structure. The phosphorus chemical shift tensors obtained from both single-crystal and dipolar-chemical shift NMR methods are in excellent agreement. For site 1, δ11 = 91 ppm, δ22 = 75 ppm, and δ33 = −63 ppm with an error of ±2 ppm for each component. The principal components of the phosphorus chemical shift tensor at site 2 are very similar; δ11 = 92 ppm, δ22 = 74 ppm, and δ33 = -59 ppm, again with errors of ±2 ppm. The phosphorus chemical 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 initio (RHF and DFT) calculations of the phosphorus chemical shift tensors for both sites are in good agreement with experiment.