17-O NMR on Crystalline Hydrades Hydrates: Impact of Hydrogen Bonding

Abstract

The water molecules in inorganic hydrate salts adopt different geometries and are involved in different hydrogen bond interactions. In this work, magic-angle spinning (MAS) and static 17O solid-state NMR experiments are performed to characterize the 17O electric field gradient (EFG) and chemical shift (CS) tensors of the water molecules in a series of inorganic salt hydrates which include: oxalic acid hydrate, barium chlorate hydrate, sodium perchlorate hydrate, lithium sulphate hydrate, and potassium oxalate hydrate, which were all enriched with 17O water. Data were acquired at magnetic field strengths of 9.4, 11.75, and 21.1 T. Gauge-including projector-augmented-wave density functional theory (GIPAW DFT) calculations are performed on barium chlorate hydrate and oxalic acid hydrate where structural changes including the Ow-H•••O distance, H-O-H angle, and O-H distance are employed to understand their impact on the NMR parameters. Furthermore, simplified molecular models consisting of a metal cation and a water molecule were built to establish the effect the M-Ow distance has on the parameters. The computational studies are then used to understand the experimental results. The 17O quadrupolar coupling constant ranged from 6.75 MHz in K2C2O4•H2O to 7.39 MHz in NaClO4•H2O while the asymmetry parameter ranged from 0.75 in NaClO4•H2O to 1.0 in K2C2O4•H2O and the isotropic chemical shift ranged from -15.0 ppm in NaClO4•H2O to 19.6 ppm in BaClO3•H2O. The computational results revealed the trends for each parameter, where there is an increasing trend for quadrupolar coupling constant and span as a function of increasing hydrogen bond distance, decreasing trend for the three chemical shift tensors as a function of increasing M-Ow distance and unclear trends for asymmetry parameter and skew due to competing electronic factors. Overall, this study provides benchmark 17O NMR data for water molecules in crystalline hydrates, including the first measurement of 17O chemical shift anisotropy for such materials.