³¹P NMR Chemical Shifts in Hypervalent Oxyphosphoranes and Polymeric Orthophosphates

We report the first quantum chemical investigation of the solid- and solution-state ³¹P NMR chemical shifts in models for phosphoryl transfer enzyme reaction intermediates and in polymeric inorganic phosphates. The ³¹P NMR chemical shifts of five- and six-coordinate oxyphosphoranes containing a variety of substitutions at phosphorus, as well as four-coordinate polymeric orthophosphates and four-coordinate phosphonates, are predicted with a slope of 1.00 and an R² = 0.993 (N = 34), corresponding to a 3.8 ppm (or 2.1%) error over the entire 178.3 ppm experimental chemical shift range, using Hartree−Fock methods. For the oxyphosphoranes, we used either experimental crystallographic structures or, when these were not available, fully geometry optimized molecular structures. For the four-coordinate phosphonates we used X-ray structures together with charge field perturbation, to represent lattice interactions. For the three-dimensional orthophosphates (BPO₄, AlPO₄, GaPO₄), we again used X-ray structures, but for these inorganic systems we employed a self-consistent charge field perturbation approach on large clusters, to deduce peripheral atom charges. For pentaoxyphosphoranes, the solvent effect on ³¹P NMR chemical shieldings was found to be very small (<0.5 ppm). The ³¹P NMR chemical shielding tensors in the pentaoxyphosphoranes were in most cases found to be close to axially symmetric and were dominated by changes in the shielding tensor components in the equatorial plane (σ₂₂ and σ₃₃). The isotropic shifts were highly correlated (R² = 0.923) with phosphorus natural bonding orbital charges, with the larger charges being associated with shorter axial P−O bond lengths and, hence, more shielding. Overall, these results should facilitate the use of ³¹P NMR techniques in investigating the structures of more complex systems, such as phosphoryl transfer enzymes, as well as in investigating other, complex oxide structures.