On the optimal spectral resolution in quadrupole central transition NMR at ultrahigh magnetic fields

Quadrupole central transition (QCT) NMR has recently been shown to be an effective way of obtaining high spectral resolution for half-integer quadrupolar nuclei in slowly tumbling molecules in liquids. QCT NMR for slowly tumbling molecules shares many common characteristics with conventional CT-based solid-state NMR for half-integer quadrupolar nuclei. As a result, QCT NMR can be considered to be a cousin of solid-state NMR. Experimental QCT NMR data reported so far in the literature strongly indicate that the optimal resolution achievable in QCT NMR increases with the strength of the applied magnetic field (B₀). In this study, we showed that, if the nuclear quadrupole interaction is the predominant relaxation mechanism, the minimal line width (expressed in ppm) obtained in QCT NMR is proportional to B₀^-3. In comparison, the corresponding field dependence in CT-based solid-state NMR is only B₀^-2. We also demonstrated that the presence of shielding anisotropy (SA) would significantly reduce the B₀^-3 dependence in QCT NMR. We presented new ¹⁷O (I = 5/2) QCT NMR results obtained at multiple magnetic fields up to 35.2 T and carefully examined a wide range of previously reported QCT NMR data from the literature for ²⁷Al (I = 5/2), ³⁹K (I = 3/2), ⁴⁵Sc (I = 7/2), ⁵⁹Co (I = 7/2), ⁷¹Ga (I = 3/2), and ⁸⁷Rb (I = 3/2) nuclei. Our findings provide a general guideline for future QCT NMR applications especially at ultrahigh magnetic fields.