Overtone NMR spectroscopy

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Abstract
The theoretical background and experimental implementation of overtone NMR spectroscopy of quadrupolar nuclei, i.e., the direct observation of NMR signals at multiples of the Larmor frequency, are described. Perturbation theory is used to derive expressions for the dependence of overtone NMR frequencies, transition moments, and dipole–dipole splittings on molecular orientation for a nucleus with arbitrary spin. Experimental techniques for overtone cross-polarization and for two-dimensional spectroscopic measurements of transition moments and dipole–dipole splittings are demonstrated with 14N overtone spectra of single crystals. Overtone powder pattern line shapes are analyzed, both for spin S=1 and spin S=3/2 nuclei, and examples of 14N overtone powder patterns are presented. Overtone spectroscopy can be used to characterize quadrupole and dipole–dipole interactions in polycrystalline samples. The sensitivity, resolution, and spectral width characteristics of overtone NMR spectra are discussed and contrasted with those of conventional NMR spectra. The sensitivity and resolution of overtone spectra are shown to be comparable to those of conventional spectra, while the reduced spectral width of overtone spectra is a significant advantage in studies of single crystals and, in particular, polycrystalline materials.