Ab Initio Calculations of P NMR Chemical Shielding Anisotropy Tensors in Phosphates: Variations Due to Ring Formation

Ring formation in phosphate systems is expected to influence both the magnitude and orientation of the phosphorus (P) nuclear magnetic resonance (NMR) chemical shielding anisotropy (CSA) tensor. Ab initio calculations of the P CSA tensor in both cyclic and acyclic phosphate clusters were performed as a function of the number of phosphate tetrahedral in the system. The calculation of the P CSA tensors employed the GAUSSIAN 98 implementation of the gauge-including atomic orbital (GIAO) method at the Hartree-Fock (HF) level. It is shown that both the P CSA tensor anisotropy, and the isotropic chemical shielding can be used for the identification of cyclic phosphates. The differences between the P CSA tensor in acyclic and cyclic phosphate systems become less pronounced with increasing number of phosphate groups within the ring. The orientation of the principal components for the P CSA tensor shows some variation due to cyclization, most notably with the smaller, highly strained ring systems.