Improving Sensitivity in Solid State NMR : Surfaces, Quadrupolar Nuclei and Dynamic Nuclear Polarization

Sensitivity in Nuclear Magnetic Resonance (NMR), especially in solid-state NMR, has always been a challenging and important issue and thus a motivation for new developments. The magnetic field B0, the gyromagnetic ratios of the observed nuclei, as well as the preparation of the samples are important for optimizing the sensitivity. Often only a small amount of sample is available and the nuclei of interest have low gyromagnetic ratios, which makes solid-state NMR extremely time consuming and, in some cases, practically impossible. In addition, strong interactions can cause extensive line-broadening, thus making it difficult to excite or detect high resolution spectra. In this thesis two main aspects concerning sensitivity in solid-state NMR are addressed. The first part reveals the difficulties of the excitation and detection of quadrupolar nuclei, in particular of the highly abundant nucleus nitrogen-14, and presents an indirect detection method combined with a more efficient excitation and reconversion scheme using Delays Alternating with Nutations for Tailored Excitation (DANTE). The second part presents a new experimental set-up for Dynamic Nuclear Polarization (DNP), which has evolved into a very successful method at the Ecole Polytechnique Fédérale de Lausanne. It is shown in this work that the DNP method combined with solid-state NMR allows one to enhance NMR signal intensities by one or two orders of magnitude, thus opening the doors towards a great variety of new exciting applications. In solids spinning at the magic angle, the indirect detection of single-quantum (SQ) and double-quantum (DQ) 14N spectra (I = 1) via spy nuclei S = 1/2 such as protons can be achieved in the manner of heteronuclear single- or multiple-quantum correlation (HSQC or HMQC) spectroscopy. The coherence transfer is achieved by recoupling the heteronuclear dipolar interactions, i.e., with symmetry-based sequences. The line broadening caused by the homogeneous decay of the transverse terms of the spy nuclei S can be reduced by implementing suitable dipolar decoupling schemes during the evolution period. The lack of efficiency of the excitation and reconversion can be overcome by replacing the conventional rectangular pulses applied to the nitrogen-14 nuclei by a train of short rotor-synchronized pulses in the manner of DANTE. These pulse trains excite a large number of crystallites in the sample uniformly and allow one to efficiently excite magnetization over large bandwidths, limited only by the quality factor of the probe. When combining N interleaved DANTE schemes, obtained by applying N pulses per rotor period, leading to so-called "DANTE-N" sequences, one obtains methods that greatly improve the uniformity of the excitation and lead to higher signal intensity in less rotor periods than basic DANTE. Applications to nitrogen-14 with fast magic angle spinning (typically νrot ≥ 60 kHz), using either direct or indirect detection, are backed up by simulations that pr