Double nuclear quadrupole resonance in biological compounds

• Main Author: Mackay, A. L.
• Published: University of Oxford 1975
• Subjects: 541.2
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.464261

This thesis describes the mechanisms, applications and limitations of the technique of nuclear quadrupole resonance carried out using double resonance with spin mixing in the laboratory frame. In the first chapter the basic elements of the theory of quadrupole resonance are outlined and the quadrupole Hamiltonian is solved for spin 1 nuclei. The second chapter describes some of the ways in which quadrupole parameters may be used to extract information about the electronic structure of molecules. In chapter III the most common techniques for the measurement of NQR are discussed and some of their advantages and limitations are noted. The next two chapters give a detailed description of the technique of double resonance with spin mixing in the laboratory frame as applied to nitrogen and deuterium nuclei. The events of an experimental cycle are related and the layout of the apparatus is presented. The inherent high sensitivity of the method is attributed to the large population differences given to the quadrupolar nuclei by level crossing processes with the protons. For nitrogen, a simple model where the protons transfer alignment in mutual spin flip transitions involving a single proton and a single nitrogen is shown to predict a lower sensitivity for the technique than that found experimentally The relative sensitivities to the three transitions in nitrogen NQR, as predicted from the simple model, are in qualitative agreement with experiment. In chapter VII the solid effect transitions occuring when a nitrogen is bonded to one, two, or three protons are discussed. The energy of the two proton system, which determines the frequencies of NH₂ solid effect transitions, is solved as a function of applied magnetic field. The fine structure in deuterium pure quadrupole resonance, which occurs when nearby deuterons have nearly degenerate energy levels, is theoretically analysed in chapter VIII. The heights of the subsidiary lines in the fine structure are predicted and the quadrupole lines are assigned from a calculation of the magnetic dipolar interaction between the two nearby deuterons. In chapter IX, the characteristics of the quadrupole spectra from OHD and NHD sites have been qualitatively accounted for by attributing them to the magnetic dipolar interaction between the deuteron and the proton. Since this technique is quite new and the methods of running the apparatus and interpreting the spectra have not yet become well established, a fairly detailed section on operational procedures and the assignment of nitrogen and deuterium quadrupole lines has been included. In the literature there has been some controversy over the assignment of quadrupole constants to the two nitrogen sites in imidazole. In chapter XI the set of lines with e²qQ = 1418 KHz and η = 0.997 is positively assigned to the NH site and the e²qQ = 3253 KHz and η = 0.135 to the N with no covalently bonded protons. In chapter XII the quadrupole resonance technique is applied to the study of hydrogen bonding in amino acids. Three relations of the form e²qQ = (e²qQ)_o - A/R(H...O)³ have been found. (e²qQ)_o represents the e²qQ, of a non-hydrogen bonded site. For O-D...O bonds (e²qQ)_o and A were found to be 328 KHz and 643 KHz-angstrom³. The OD(e²qQ)_o is in good agreement with experimentally determined values. For N±D...O and N±D...Cl bonds the values were 252 KHz and 572 KHz-angstrom³ and 239 KHz and 728 KHz-angstrom³. The average (e²qQ)_o value for a non-hydrogen bonded N⁺D is in excellent agreement with that predicted by a theoretical calculation in the literature. The quadrupole resonance of the water molecule has been carried out in a number of different environments including ice Ih, ice II, clathrate hydrates and molecular complexes. The ice Ih spectrum has been shown to be the sum of two components, one from the HDO molecule and the other from the D₂O molecule. Although this thesis is largely devoted to the quadrupole resonance of ¹⁴N and ²H, a preliminary research project was undertaken to apply the technique to a number of different nuclei with quadrupole transitions in the frequency range around a few MHz. This project was quite successful with quadrupole resonance having been observed in ²³Na, ³⁹K, ¹¹B, ¹⁷O, ²⁶Al and ¹⁰B. This quadrupole data, along with the solutions of the quadrupole Harniltonian for spins ³⁄₂, ⁵⁄₂ and 3 is written up in chapter XIV. Because the processes of laboratory frame double resonance depend on the nuclear spin and the quadrupole frequencies, the observations in chapter V on the mechanisms of the method for nitrogen and deuterium resonance do not necessarily apply to the new nuclei. A comprehensive list of quadrupole data for nitrogen and deuterium is presented in appendices I and II.