Contributed
Hydrogen-detected relaxation time measurements of "invisible" quadrupolar spins by solid-state nuclear magnetic resonance under magic angle spinning

Over 74% of the nuclear magnetic resonance (NMR)-active elements in the periodic table have nuclei with a spin greater than one-half. Such nuclei are usually influenced by a strong coupling to the electric field gradient at the site of the nucleus. When exists, this `quadrupolar` interaction is anisotropic and very large (~MHz), making quadrupolar solid state NMR (ssNMR) challenging and in many cases inapplicable, as the available power levels for irradiation are much smaller (~kHz). This "broad-band" excitation problem brings several severe complications to quadrupolar relaxation measurements as well. If developed, such experiments might be applied to a large variety of fields or systems, such as G-quadruplex metals-containing DNA structures, MOFs, polymorphism differentiation, imaging, and more.

Our efforts, latest achievements and ongoing work on development of NMR experiments and methodology for measurements of quadrupolar relaxation times of nuclei with very large quadrupolar couplings, that prevent their detection by NMR techniques (hence: "invisible"), will be described. We will show experiments conducted on ¹⁴N, a highly abundant (99.6%) spin-1 and a highly common nucleus in biological systems that is rarely studied due to its large quadrupolar coupling. In addition, first attempts to measure ⁸¹Br relaxation times will be presented as well. We will show that the inability to directly detect the quadrupolar spins can be overpowered by indirect detection via protons.

Additional work on development of other ssNMR experiments (such as distance measurement experiments and polarization transfer experiments) will be mentioned as well, as part of a general journey towards making quadrupolar NMR feasible and widespread.

References:

• M. Makrinich, R. Gupta, T. Polenova, and A. Goldbourt, Solid State Nucl. Magn. Reson., vol. 84, 2017.
• M. Makrinich, E. Nimerovsky, and A. Goldbourt, Solid State Nucl. Magn. Reson., vol. 92, 2018.