Extending ¹⁷O transverse relaxation measurement to satellite transitions as a direct probe of molecular dynamics in...

We report utilization of transverse relaxation rate (R₂) of ¹⁷O (I = 5/2) satellite transitions (STs) as a probe of molecular dynamics in solids. A simple theoretical model using spectral density functions is proposed to describe the general R₂ behaviors of half-integer quadrupolar nuclei in solids in the presence of molecular motion (or chemical exchange). Experimental ¹⁷O R₂ data recorded for both CT and ST from ¹⁷O-labeled NaNO₂ over a large temperature range are used to verify the theoretical predictions. Our theoretical model is shown to be fully consistent with a full quantum mechanical treatment of the chemical exchange problem involving half-integer quadrupolar nuclei in solids by numerically solving the Liouville-von Neumann equation. The new ¹⁷O ST R₂ method was also applied to study the carboxylate flipping motion in two [¹⁷O]carboxylic acid-pyridine adducts in the solid state. The advantages of the ST R₂ approach are discussed. This ST R₂ approach adds a new dimension to the currently available CT-based solid-state NMR techniques for probing molecular motion in solids.