Solid-State NMR Analyses of Molecular Structure and Dynamics in Hydrogen-Bonded Materials

Abstract

This thesis presents analyses of hydrogen-bonded materials using solid-state nuclear magnetic resonance (NMR) spectroscopy. Proton dynamics were investigated in two classes of phosphate-based proton conductors: phosphate solid acids and tin pyrophosphates. These materials have the potential to be used as solid state proton conductors in fuel cells. Proton dynamics in phosphate solid acids were probed based on the attenuation of homonuclear dipolar coupling with increasing temperature. These studies showed that homonuclear dipolar recoupling NMR techniques can be employed in complex multi-spin systems. Additionally, two pathways for proton hopping in monoclinic RbH2PO4, a sample with two proton environments, were identified and quantified for the first time using a combination of dipolar recoupling and proton exchange NMR methods. Tin pyrophosphates, another class of solid-state proton conductor with analogous phosphate tetrahedral structure, were studied. Proton dynamics had to be analyzed via exchange-based NMR techniques as a result of low proton concentration in these materials. Proton mobility in tin pyrophosphate was found to increase with increased protonation. Furthermore, hydrogen bonding was investigated as a coordination mode in silicone boronic acid (SiBA) elastomers, potential materials for contact lens manufacture. As in the phosphate-based proton conductors, hydrogen bonding played an important role in the structure of the SiBA elastomers as one of the mechanisms through which these materials crosslink. In addition to hydrogen bonding, covalent bonding between boronic acids was found to occur at three- and four-coordinate boron centers. The purpose of this study was to determine the influence of boronic acid loading and packing density on crosslinking in SiBA elastomers. Boron coordination environments were investigated by 11B quadrupolar lineshape analysis. The incidence of four-coordinate dative bonding, a predictor of the stress-strain response in these materials, increased with boronic acid loading but was most heavily influenced by boronic acid packing density.