Double-rotation NMR using spherical rotors

Double-rotation (DOR) solid-state NMR spectroscopy (ssNMR) enables higher-resolution spectra of quadrupolar nuclei to be acquired as compared to the ubiquitous magic-angle spinning (MAS). However, due to low filling factors, slow spinning speeds and the cost and complexity of the specialised probes required, multiple quantum MAS (MQMAS) has almost completely superseded DOR for acquiring high-resolution spectra of quadrupolar nuclei. Herein, we investigate the use of spherical rotors to achieve DOR. Such rotors overcome the filling-factor issue, and the stator can be 3D-printed. Further, since there is only a single spinning surface and no nested rotors, the spinning speeds can, in principle, approach that of conventional MAS rotors. We present a proof-of-concept stator and sphere undergoing double rotation driven by pulsed gas streams, precessing at low tens of Hz, from which NMR data is acquired. We discuss the weight distribution in spheres for DOR and the implications for the ratio of the spinning and precession frequency, and conclude by outlining a potential avenue for achieving faster DOR using piezoelectric valves.