Overhauser dynamic nuclear polarisation studies in solution-state at 3.4T

Abstract

Studies of Overhauser DNP in liquids are presented in this thesis, where the polarisation is achieved in-situ using TEMPO-derived radicals at a magnetic field of 3.4 T (143 MHz/94 GHz 1H NMR/EPR frequency). The dielectric heating of lossy water solvent is unavoidable at high field, and so knowledge of temperature effects is important to properly compare enhancement results. It is shown that the temperature dependent DNP enhancement of water protons can be determined provided that the 1H NMR shift is sufficiently resolved and the nuclear relaxation T1I is sufficiently fast. Considerable sensitivity gains are made at modest temperatures, e.g. [E] ~ 40 at ~40 degrees C, and much greater enhancements are achievable at elevated temperatures, e.g. [E]~ 130 at ~ 100 degrees C. Since high radical concentrations (100 mM TEMPOL) are used, the leakage and saturation factors approach 1, enabling an experimental determination of the coupling factor from the enhancement. A value of E = 0:055+0:003 is found at 25 degrees C, which agrees well with values in the literature calculated from molecular dynamics simulations. The DNP enhancement is measured as a function of temperature for three organic compounds dissolved in water: glycine, L-proline and acrylic acid; with enhancements of -17, -16 and -11 at ~40 degrees C. To the author's knowledge, this is the first report of solute molecule enhancements for direct in-situ liquid DNP at this field. Significant enhancements are obtained, however, further analysis of the results reveals significantly weaker coupling of the electron spin to the solute molecule protons than to the solvent molecule protons. Discrepancies between experimental coupling factor ratios and those calculated from a force-free hard-sphere model suggest that the classical analytical models used to describe Overhauser DNP may require refinement. In addition to these temperature studies, simultaneous saturation of two EPR hyperfine lines is investigated and achieved, resulting in an increase in observed DNP enhancement