Solid-state 31P NMR chemical shielding tensors in phosphonates and bisphosphonates:: A quantum chemical investigation

We report the results of a quantum chemical investigation of the IT nuclear magnetic resonance (NMR) isotropic chemical shieldings (sigma(iso)) and the P-31 NMR chemical shielding tensor principal values (sigma(ii), i = 1-3) in a series of eight different phosphonates, including the bisphosphonates pamidronate and risedronate currently in use in bone resorption therapy. We used primarily Hartree-Fock methods with a 6-311++G(2d,2p) basis set and the CSGT (continuous set of gauge transformations) formalism to predict the experimental observables, using various approaches to incorporating intermolecular, crystal lattice effects. Good predictions of the (31)p NMR isotropic chemical shielding, shielding tensor principal values, and tensor orientations were obtained, with R-2 = 0.95 and similar to7% root-mean-square error from experiment. In the zwitterionic aminophosphonates, electrostatic (lattice) interactions were found to be strong but could be well accounted for by incorporating charge lattice effects into the calculations. The ability to now predict both isotropic and anisotropic shielding (shift) tensors in phosphonates and bisphosphonates should open the way to the determination of their protonation states when bound to proteins, information which is not accessible from crystallographic studies.