Date of Graduation

1996

Document Type

Dissertation/Thesis

Abstract

This dissertation focused on two aspects of Nuclear Magnetic Resonance (NMR) techniques in the solid state: (i) the possibility of enhancing the NMR spectral resolution via utilization of single crystals (instead of powders) in the magic angle spinning experiments, and (ii) application of this methodology as a new probe in the studies of the microscopic mechanism of solid-solid phase transitions in hydrogen-bonded solids. The bulk of the studies employed a CMX 360 NMR spectrometer with multinuclear double-resonance probes, operating at 360 MHz for protons, and variable temperatures over the range of {dollar}-{dollar}150 to 250{dollar}\\sp\\circ{dollar}C. The samples investigated were squaric acid and ammonium dihydrogen phosphate. These intermolecularly hydrogen-bonded compounds were chosen because they were known to exhibit antiferroelectric phase transition at about 100{dollar}\\sp\\circ{dollar}C and -125{dollar}\\sp\\circ{dollar}C respectively, but the atomistic mechanism underlying these transitions were not fully understood. For additional insight, studies were made on naphthazarin which contains a network of intramolecular hydrogen bonds. It was demonstrated that in the case of squaric acid, the utilization of single crystals in magic angle spinning (MAS) measurements of {dollar}\\sp{lcub}13{rcub}{dollar}C NMR peaks leads to about a four-fold decrease in the signal linewidth, as compared to the usual case of using powders in MAS measurements. This enhancement in spectral resolution enabled us to measure the isotropic chemical shifts from all of the four carbons of squaric acid for the first time. Also, the high precision thus obtained enabled us to construct a plot of the {dollar}\\sp{lcub}13{rcub}{dollar}C chemical shift ({dollar}\\sigma{dollar}) vs. OH distance (r), and to propose an empirical mathematical expression: exponential dependence of {dollar}\\sigma{dollar} on the r. Measurements on partially deuterated samples strongly supported this proposed relationship. Variable temperature measurements showed that average sigma for all four carbons increases gradually as the sample is heated, with a finite jump in the vicinity ({dollar}\\pm3\\sp\\circ{dollar}C) of the transition temperature, T{dollar}\\rm\\sb{lcub}c{rcub}\\sim100\\sp\\circ{dollar}C. The squaric acid studies were extended to ammonium dihydrogen phosphate (ADP). Here we investigated the feasibility of utilizing the changes in the isotropic chemical shift of {dollar}\\sp{lcub}31{rcub}{dollar}P. In contrast to earlier studies, this investigation showed that sigma contrast to earlier studies, this investigation showed that the {dollar}\\sp{lcub}31{rcub}{dollar}P chemical shift is a sensitive probe of the antiferroelectric phase transition of phosphorus-containing hydrogen-bonded solids. The change in the {dollar}\\sp{lcub}31{rcub}{dollar}P chemical shift was interpreted in terms of the change in the electron density at the {dollar}\\sp{lcub}31{rcub}{dollar}P nucleus, as calculated by model (ab-initio) electronic structure calculations. The data analysis again suggested that the phase transition in ADP also can be better described as a cross-over type, rather than the currently accepted model of a pure order-disorder displacive one. The final portion of the dissertation discusses similar studies on naphthazarin, a compound containing intramolecular hydrogen bonds. Variable temperature {dollar}\\sp{lcub}13{rcub}{dollar}C CPMAS measurements on this compound showed that the changes in {dollar}\\sigma{dollar} are much larger here than for the above discussed cases that involved intermolecular hydrogen bonds. Finally, a detailed discussion is presented on the possible utility of squaric acid as an internal standard for temperature measurements in CPMAS studies involving {dollar}\\sp{lcub}13{rcub}{dollar}C. (Abstract shortened by UMI.).

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