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31P NMR Chemical Shifts in Hypervalent Oxyphosphoranes and Polymeric Orthophosphates

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posted on 12.01.2006, 00:00 by Yong Zhang, Eric Oldfield
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.