P-31 SOLID-STATE NMR OF A PHOSPHINE BORANE ADDUCT - PHOSPHORUS CHEMICAL SHIELDING TRENDS IN THE ISOELECTRONIC SERIES R(3)PX, WHERE X=BH3, CH2, NH, O

The orientation of the phosphorus chemical shift tenser and the magnitudes of its principal components in solid triphenylphosphine-borane adduct were determined from the P-31 chemical shift B-11-dipolar-coupled NMR powder lineshape. Although the P-31 isotropic chemical shift of the adduct was similar to that of triphenylphosphine oxide, the anisotropic chemical shift parameters revealed substantial differences in the local electronic environment surrounding the phosphorus nucleus. The orientation-dependence of the P-31-B-11 dipolar coupling indicated that the most shielded component of the phosphorus chemical shift tenser lies close to the P-B bond and provided a P-B bond length of 1.95 + 0.02 Angstrom, in good agreement with diffraction data. Ab initio calculations of the phosphorus chemical shielding tensors for the isoelectronic series of trimethylphosphine derivatives, (CH3)(3)PX, X = BH3, CH2, NH, O, provide considerable detail concerning the sensitivity of the three principal components to changes in the X group. Only the most shielded component, sigma(33), which lies along the P-X bond, is dominated in an exclusive and consistent fashion by the X group, linearly with respect to the P-X bond length.