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Article Dans Une Revue The Journal of Chemical Physics Année : 2018

N-14 overtone nuclear magnetic resonance of rotating solids

Résumé

By irradiating and observing at twice the 14N Larmor frequency, overtone (OT) NMR is capable of obtaining 14NOT spectra without first-order quadrupolar broadening. Direct excitation and detection of the usually “forbidden” double-quantum transition is mediated by the perturbation from the large quadrupole interaction to the spin states quantized by the Zeeman interaction. A recent work (L.A. O’Dell, C.I. Ratcliffe, Chem. Phys. Lett. 514, 168, 2011) has shown that 14NOT NMR under magic-angle spinning (MAS) can yield high-resolution spectra with typical second- order quadrupolar line shapes allowing the measurement of 14N chemical shift and quadrupolar coupling parameters. This article has also shown that under MAS the main 14NOT peak is shifted by twice the sample spinning frequency with respect to its static position. We present the theory of 14NOT NMR of static or rotating samples and the physical picture of the intriguing spinning- induced shift in the second case. We use perturbation theory for the case of static samples and Floquet theory for rotating samples. In both cases, the results can be described by a so-called overtone parameter that scales down the 14NOT radio-frequency (rf) excitation and signal detection. This overtone parameter shows that the components of the rf field, which are transverse and longitudinal with respect to the magnetic field, are both effective for 14NOT rf excitation and signal detection. In the case of magic-angle spinning at angular frequency ωr, the superposition of the excitation and detection components in the overtone parameter makes either the +2ωr or −2ωr term the dominant 14NOT signal, depending on the sense of sample spinning with respect to the magnetic field. This leads to an apparent 14NOT signal shifted at twice the spinning frequency. Features of 14NOT NMR spectra for both static and rotating samples are illustrated with simulations. The spinning induced shift and its dependence on spinning direction are confirmed experimentally by reversing the spinning direction and the field of the 36 Tesla series-connected hybrid magnet at the US National High Magnetic Field Laboratory.
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Dates et versions

hal-04313341 , version 1 (29-11-2023)

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Zhehong Gan, Ivan Hung, Yusuke Nishiyama, Jean-Paul Amoureux, Olivier Lafon, et al.. N-14 overtone nuclear magnetic resonance of rotating solids. The Journal of Chemical Physics, 2018, Journal of Chemical Physics, 149, ⟨10.1063/1.5044653⟩. ⟨hal-04313341⟩
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