Structural perturbations on the bridging oxygen 17O NMR EFG parameters in ultraphosphates:: an ab initio study

Ab initio molecular orbital calculations (Hartree-Fock, HF and density functional theory, DFF) of the O-17 NMR electrical field gradient (EFG) parameters for the bridging oxygen (O-B) in the model cluster H4P2O7 are presented. The variation of the quadrupolar coupling constant (C-Q) and asymmetry parameter (eta(Q)) for the EFG of the bridging oxygen was shown to be a function of both the bridging P-O-B-P bond angle and the O-P-O-B-P=O conformation. These ab initio calculations allowed empirical relationships between the O-17 NMR EFG parameters and the P-OB-P bond angle to be tabulated for a wide range of O=P-O-B-P=O conformations. For experimentally realistic ranges of the P-O-B-P bond angle, the O-17 NMR EFG parameters are dominated by variations in the P-O-B-P bond angle, with minor changes due to the O=P-O-B-P=O conformation. These calculations also demonstrated systematic differences in the predicted O-17 EFG parameters obtained using HF and DFT level of theory. (C) 2004 Elsevier B.V. All rights reserved.