NMR of group 2 element quadrupolar nuclei and some applications in materials science and biology

For many years, NMR has provided an easy access for chemists to perform structural and kinetic studies on a whole variety of systems. To a great extent, these investigations have been restricted to non-quadrupolar nuclei. The study of quadrupolar nuclei (I > 1/2) offers the potential to gain insight into important problems in material science and biology. In addition to the large quadrupole moment associated with the spin active nuclei of interest, several of the most interesting species also possess an extremely low natural abundance. My recent research focuses on 87Sr NMR, which has been cited by earlier workers as being limited to only ionic species. Several strontium-containing compounds have been synthesized and characterized by single crystal x-ray diffraction. 87Sr NMR signals were determined for these compounds in a series of aprotic polar solvents. The chemical shift variation was found to be consistent with linen free energy relationship, which can be very useful in helping to elucidate mechanism, in predicting reaction rates, and the extent of reaction at equilibrium, and in discovering under what conditions a change in mechanism occurs. Control over symmetry of the compound was found to be the key to obtain the good NMR signals.; One application of the new technique that has been developed was in the area of material science. An observation relative to sol-gel derived ionic conductors (La_0.8Sr_0.2Co_0.8Fe_0.2O _3.2) was that films often formed cracks upon pyrolysis. By careful examination of the sol-gel process by 87Sr NMR, a model for the structure of the sol was developed. Through the relaxation rate study of the strontium sites, the polymerization mechanism was determined to be predominantly bimolecular within the concentration region studied. The kinetic study of the fast cation exchange between two strontium sites indicated that the inhomogeneity of the polymeric network lads to the film cracking during pyrolysis. As a consequence of understanding the fundamental coordination chemistry of strontium present in the sol, the homogeneity of the ceramic was substantially enhanced, and the resulting electronic properties were improved. In another application of the new technique which have been developed, 87Sr NMR was employed to study the metal ion binding properties of α-Lactalbumin. The NMR data was fitted into a model with one high affinity and one low affinity site in α-Lactalbumin. The calculation of the correlation time, quadrupolar coupling constant provided the opportunity to study the location and the structure of the binding sites.; In the application of the new technique in the area of biology, 25Mg NMR was employed to study the metal ion binding properties of bacteriorhodopsin. We presented the results of our studies regarding to two important questions in the photo conversion process: (1) The number of metal binding sites and their binding constants; and (2) What is the role of metal cations in the proton dissociation process/proton transfer (e.g., the key issue of the switch of the proton pump)? 113Cd NM of metal-bR complexes was employed as a probe of the chemical structure of bR calcium binding sites. We discuss the results in terms of the possible involvement of the metal cations in the bR function.