Indirect detection of nitrogen-14 in solid-state NMR spectroscopy

This thesis describes new methods for the detection of 14N nuclei by solid-state nuclear magnetic resonance. So far, the low natural abundance (0.4 %) 15N isotope has been widely used to study nitrogen-containing samples because of its spin-1/2 nature. The limited use of the spin-1 isotope 14N (natural abundance 99.6 %) in NMR is due to its strong quadrupolar coupling constant, which leads to very broad spectra that are difficult to excite uniformly and equally difficult to detect, requiring broad receiver bandwidths. The new methods presented in this work result from two main projects covering the indirect detection of 14N via carbon and via protons. The indirect detection of 14N can be achieved by heteronuclear multiple- or single-quantum correlation (HMQC or HSQC) experiments, which rely on the transfer of coherence from a neighboring spin S = 1/2 (13C or 1H) to single- (SQ) or double-quantum (DQ) transitions of 14N nuclei. These methods allow one to observe 14N powder patterns with enhanced sensitivity. These spectra depend on the quadrupolar coupling constant CQ (typically a few MHz), the asymmetry parameter ηQ, and on the orientation of the internuclear vector rIS between the I (14N) and S (13C or 1H) nuclei with respect to the quadrupolar tensor. These parameters reveal valuable information about the structure and dynamics of nitrogen-containing solids. The indirect detection methods are discussed in detail, covering the optimization of experimental parameters; subsequently, applications to the study of amino acids at different magnetic fields are demonstrated.


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