An 17O NMR and Quantum Chemical Study of Monoclinic and Orthorhombic Polymorphs of Triphenylphosphine Oxide

Solid-state 17O NMR spectroscopy is employed to characterize powdered samples of known monoclinic and orthorhombic modifications of 17O-enriched triphenylphosphine oxide, Ph3PO. Precise data on the orientation-dependent 17O electric field gradient (EFG) and chemical shift (CS) tensors are obtained for both polymorphs. While the 17O nuclear quadrupolar coupling constants (CQ) are essentially identical for the two polymorphs (CQ = -4.59 (plus-minus) 0.01 MHz (orthorhombic); CQ = -4.57 (plus-minus) 0.01 MHz (monoclinic)), the spans (omega) of the CS tensors are distinctly different ((omega)= 135 (plus-minus) 3 ppm (orthorhombic);(omega)= 155 (plus-minus) 5 ppm (monoclinic)). The oxygen CS tensor is discussed in terms of Ramseyʹs theory and the electronic structure of the phosphorus-oxygen bond. The NMR results favor the hemipolar -bonded R3P^+-O^- end of the resonance structure continuum over the multiple bond representation. Indirect nuclear spin-spin (J) coupling between 31P and 17O is observed directly in 17O magic-angle -spinning (MAS) NMR spectra as well as in 31P MAS NMR spectra. Ab initio and density-functional theory calculations of the 17O EFG, CS, and 1J(31P,17O) tensors have been performed with a variety of basis sets to complement the experimental data. This work describes an interesting spin system for which the CS, quadrupolar, J, and direct dipolar interactions all contribute significantly to the observed 17O NMR spectra and demonstrates the wealth of information which is available from NMR studies of solid materials.