Multispin order relaxation study of the hypophosphite ion

The ³¹P nucleus of the hypophosphite ion has been found to possess a complex NMR relaxation mechanism composed of chemical shift anisotropy, dipole-dipole, and spin-rotation interactions. The proton-coupled ³¹P resonances of the hypophosphite ion display differential spin-lattice (longitudinal) relaxation which is indicative of the presence of chemical shift anisotropy-dipole-dipole cross-correlation. ³¹P spin-lattice and two- and three-spin order relaxation of the hypophosphite ion were measured in D₂O solution. In addition, ³¹P spin-lattice relaxation of the mono- and dideuterated analogues of HP were measured in D₂O solution. The structure of hypophosphite was determined by using ab initio molecular orbital calculations. The chemical shift shielding tensor of the ³¹P nucleus was determined from analysis of cross-polarization magic angle sample spinning spectra of solid sodium hypophosphite to be σ₁₁ = -105, σ₂₂ = -7, and σ₃₃ = 112 ppm (with respect to the isotropic chemical shift). Multispin operator theory, in combination with the structure and ³¹P chemical shift shielding tensor of hypophosphite, was used to analyze the relaxation data. The spin-rotation coupling constant of the ³¹P nucleus was determined to be (1.92 ± 0.10) × 10⁵ s^-1, and the orientation of the chemical shift shielding tensor was found to be with a tilt angle of 6.0 ± 1.0° between σ₂₂ and the O-P-O plane. The model derived successfully accounts for the relaxation properties of the ion. This work provides a clear example of the utility of multispin order relaxation methods to the study of molecular dynamics and of its sensitivity to reorientational correlation times.