NMRcrystallography of sodium diphosphates: combining dipolar, shielding, quadrupolar, diffraction, and computational information

Abstract

The crystal structures of two sodium diphosphates (Na₄P₂O₇ and Na₃HP₂O₇·H₂O) that were solved using diffraction methods are refined with the use of ²³Na and ³¹P NMR data in conjunction with projector-augmented-wave density functional theory (DFT) calculations. The sodium cations' NMR parameters are shown to be particularly well suited for the investigation of crystalline structures as they are largely dictated by the crystalline network instead of local directional bonding interactions. High-resolution spin-diffusion ²³Na correlation spectra were acquired with the use of double rotation (DOR) and the spin diffusion build-up curves were modeled with the use of the rate matrix method. The availability of dipolar-based information from these measurements is important in validating the assignment of the resonances prior to performing structural refinements using the ²³Na EFG tensor parameters. The NMR-refined structures of Na₄P₂O₇ and Na₃HP₂O₇·H₂O are then cross-validated with the use of diffraction and magnetic shielding tensor information and are compared to the pure DFT and X-ray diffraction structures. A new crystal structure of Na₃HP₂O₇·9H₂O is solved using single-crystal X-ray diffraction and is compared to the refined crystal structures of Na₃HP₂O₇·H₂O. The combination of methods used in this work ensures consistency of the final structures with all available data.