High-resolution nitrogen-14 overtone spectroscopy: an approach to natural abundance nitrogen NMR of oriented and polycrystalline systems

Abstract

In this paper, the authors present a novel approach to nitrogen NMR spectroscopy of both oriented and polycrystalline samples. The method employs the direct excitation and detection of /sup 14/N overtone NMR transitions at approximately twice the /sup 14/N Larmor frequency. New approaches to nitrogen NMR are needed both because of the chemical and biochemical importance of nitrogen and because of the limitations of current methods. In the solid state, /sup 15/N NMR typically requires isotopic labeling to achieve sufficient sensitivity. Through cross-polarization, good sensitivity is achieved in /sup 14/N NMR of single crystals. The large quadrupole interactions of /sup 14/N nuclei provide high resolution in single-crystal spectra but at the expense of a spectral width of several megahertz. Since the experimentally observable spectral range is only about 200 kHz, it is impossible to acquire a complete spectrum without repeated returning of the spectrometer. /sup 14/N powder pattern spectra have only been obtained with indirect detection or when the quadrupole couplings are unusually small. Large quadrupole couplings also limit the applicability of double-quantum and zero-field NMR techniques. For these reasons, the authors become interested in the insightful proposal by Bloom, and the experimental demonstration by Legros and Bloom, that /supmore » 14/N overtone NMR transitions can be directly excited and detected in the solid state.« less