New NMR methods for studying dynamics in solids.
PhD thesis, University of Glasgow.
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There is currently much interest in the investigation of dynamics in solids and the primary goal of this thesis is to present well-known and new NMR methods used for studying motion on Larmor and spectral timescale. The quadrupolar interaction usually dominates solid-state NMR spectra of quadrupolar nuclei. When the magnitude of quadrupolar interaction is large then the second-order correction to the dominant Zeeman Hamiltonian must be considered. Owing to this second-order quadrupolar effect, NMR peaks can be displaced from their chemical shift positions by a second-order shift. When considering motional averaging of the second-order shift, the critical frequency is the Larmor frequency, 0. In the case of motion that is faster than the Larmor precession, the isotropic quadrupolar shift is affected. This analogous phenomenon in solution-state NMR is known as the "dynamic shift". In Chapter 4, it will be shown that multiple-quantum NMR measurements of isotropic second-order quadrupolar shifts are a simple way to probe nanosecond timescale motions in solids. An analysis of one- and two-dimensional 11B MAS NMR spectra of three isomers of the closo-carboranes gives the results that provide the first evidence for the presence of solid-state dynamic shifts. There are several experiments that provide a sensitive test for the presence of dynamics on spectral timescale. One piece of evidence for dynamics on the spectral timescale is a motional broadening of quadrupolar satellite-transition spinning sidebands. Therefore, it is possible to investigate the influence of dynamic reorientation on satellite-transitions MAS spectra by recording variable-temperature one-dimensional spectra with wide spectral width or by comparing two-dimensional STMAS spectra with MQMAS spectra. These methods can be extended to 2H NMR spectroscopy as the sidebands observed in the magic angle spinning (MAS) NMR spectrum of a spin I = 1 2H nucleus may be very strongly broadened due to interference between the line-narrowing effects of MAS and the dynamics-driven reorientation of the 2H quadrupole tensor, if motion is present in the solid. In the last chapter, the 27Al, 31P and 2H NMR study of AlPO-34 type materials with the topology of chabazite are undertaken and the use of the full range of NMR methods to develop a structure and dynamic behaviour of these materials is presented. In addition, the NMR calculations are performed in order to combine DFT calculations with experimental data. Finally, GaPO-34 samples were investigated to extract information about the effects of Ga substitution in AlPO-34 on dynamical behaviour.
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