From Ultraslow to Extremely Fast Dynamics in Sodium Nitrate: an 17O NMR Study

J Beerwerth,R Böhmer,L S Plaga,J Senker,M Storek,L Hoffmann,W Hiller,R Siegel,B Bojer

doi:10.1007/s00723-020-01201-5

J Beerwerth, R Böhmer + Show 7 more

Open Access

https://doi.org/10.1007/s00723-020-01201-5

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Abstract

Increasing dynamics in solids featuring nuclei subjected to second-order quadrupolar interactions lead to central-transition spectra that undergo two consecutive line-shaped transitions. Conventional motional narrowing occurs when the molecular exchange rate is on the order of the strength of the dominant interaction. In a second step, the resulting intermediately narrowed spectra change further when the motion becomes faster than the Larmor precession rate, leading to terminally narrowed spectra that can display a residual quadrupolar shift. We derive analytic expressions for this shift and analyze the quadrupolar central-transition spectra in terms of CN symmetrical cone models. Increasing the number of sites to N ≥ 3, the terminally narrowed spectra remain unaltered, while the intermediately narrowed spectra remain unaltered only for N ≥ 5. This finding relates to the different (cubic vs. icosahedral) symmetries that are required to average out the spatial second- and fourth-rank terms in the second-order quadrupolar interaction. Following recent work (Hung et al., Solid State Nucl Magn Reson 84:14–19, 2017), 17O NMR is applied to examine the three-site rotation of the nitrate group in NaNO3. Line shapes are measured and analyzed, and in addition to prior work, satellite-transition and stimulated-echo experiments are carried out. The final-state amplitudes extracted from the latter are reproduced using model calculations. It is shown how two-dimensional exchange spectra relating to N-site cone motions can be decomposed in terms of effective two-site-jump spectra. This latter approach is successfully tested for NaNO3.

Highlights

For many nuclear magnetic resonance (NMR) studies, observing motion-induced spectral narrowing is an indispensable source of information to map out the molecular dynamics in solid- and liquid-like materials [1]
By averaging over the sites accessible in the course of the molecular jump process prior to applying the secular approximation, we obtain the averaged quadrupolar coupling CQ and the averaged asymmetry parameter Q, recovering results analogous to those known in the context of first-order anisotropies
We calculated the residual quadrupolar-induced shift which in the regime of extreme narrowing can be expressed in terms of CQ and Q

Summary

Introduction

For many nuclear magnetic resonance (NMR) studies, observing motion-induced spectral narrowing is an indispensable source of information to map out the molecular dynamics in solid- and liquid-like materials [1]. Tracing the dynamics of such materials using, e.g., popular spin-1/2 probes, the motional narrowing observed when increasing the sample temperature is typically found complete if the molecular reorientation rates exceed the spectral width accessible via the probe nucleus. Motional narrowing effects usually lead to narrow, "fully averaged" spectra. If the molecular motion is anisotropic, a significant residual line width usually remains. In the presence of first-order anisotropies originating, e.g., from chemical shift or relatively small quadrupolar interactions, many scenarios of molecular motions and their impact on the resulting high-temperature NMR spectra were considered [2,3,4]. The exploitation of nuclear probes with spins I > 1 is of particular interest when studying molecular or ionic dynamics since many applications in the life and materials sciences require alkali, oxygen, and many other nuclear probes with half-integer spin I > 1 [5,6,7,8,9,10]

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