Ab Initio Calculations of 31P 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-31) nuclear magnetic resonance (NMR) chemical shielding anisotropy (CSA) tensor. Ab initio calculations of the P-31 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-31 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-31 CSA tensor anisotropy, and the isotropic chemical shielding can be used for the identification of cyclic phosphates. The differences between the P-31 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-31 CSA tensor shows some variation due to cyclization, most notably with the smaller, highly strained ring systems.