New solid-state NMR methods for exciting and separating anisotropic interactions of spin I=1 nuclei

Sammanfattning: Solid-state NMR has become an essential tool for structural characterisation of materials, in particular systems with poor crystallinity and structural disorder. In recent years, a surge of interest has been observed for the study of paramagnetic systems, in which the interaction between nuclei and unpaired electrons allows to probe the electronic structure and properties of materials more directly. However, simultaneously this interaction leads to very broad resonances, which are difficult to acquire and interpret. While significant advancements in both NMR instrumentation and methodology have paved the way for the study of spin I=1/2 nuclei in these systems, still many issues remain to be resolved for routine investigation of quadrupolar nuclei I>1/2. In this Thesis we focus on improving both the excitation of the broad resonances and the resolution in the spectra of spin I=1 nuclei. The latter problem is addressed by developing methods for separation of the shift and the quadrupolar interactions. We introduce two new methods under static conditions, which have the advantage over previous experiments of both suppressing spectral artefacts and exhibiting a broader excitation bandwidth. Furthermore, we demonstrate for the first time an approach for separation of the anisotropic parts of the shift and quadrupolar interaction under magic-angle spinning. Secondly, to achieve broadband excitation we develop a new theoretical formalism for phase-modulated pulse sequences in rotating solids, which are applicable to nuclear spins with anisotropic interactions substantially larger than the spinning frequency, under conditions where the radio-frequency amplitude is smaller than or comparable to the spinning frequency. We apply the framework to the excitation of double-quantum spectra of 14N and design new pulse schemes with γ-encoded properties. Finally, we employ some of the new sequences together with density functional theory calculations to resolve the electronic structure of barium titanium oxhydride.