Abstract
Abstract. The use of spherical rotors for magic angle spinning offers a number of advantages, including improved sample exchange, efficient microwave coupling for dynamic nuclear polarization nuclear magnetic resonance (NMR) experiments, and, most significantly, high frequency and stable spinning with minimal risk of rotor crash. Here we demonstrate the simple retrofitting of a commercial NMR probe with MAS spheres for solid-state NMR. We analyze a series of turbine groove geometries to investigate the importance of the rotor surface for spinning performance. Of note, rotors lacking any surface modification spin rapidly and stably even without feedback control. The high stability of a spherical rotor about the magic angle is shown to be dependent on its inertia tensor rather than the presence of turbine grooves.
Highlights
The use of spherical rotors for magic angle spinning offers a number of advantages, including improved sample exchange, efficient microwave coupling for dynamic nuclear polarization nuclear magnetic resonance (NMR) experiments, and, most significantly, high frequency and stable spinning with minimal risk of rotor crash
We show that the spinning performance of spherical rotors can be improved by using a turbine groove geometry to the drive tips used in conventional cylindrical rotor Magic angle spinning (MAS) systems, which are themselves based on the Pelton impulse turbine (Wilhelm et al, 2015)
While turbine grooves can help to increase MAS rates for spherical rotors, the inertia tensor is responsible for its spinning stability
Summary
Magic angle spinning (MAS) nuclear magnetic resonance (NMR) is usually used for high-resolution analysis of the local chemical environments of nuclear spins within biomolecular and inorganic solids (Schaefer and Stejskal, 1976; McDermott, 2009; Doty and Ellis, 1981; Knight et al, 2012; Retel et al, 2017; Theint et al, 2017; Petkova et al, 2005; Kong et al, 2013; Wang et al, 2013; Cegelski et al, 2002; Bougault et al, 2019; Clauss et al, 1993; Trebosc et al, 2005; Lesage et al, 2008). We recently showed that it is possible to spin samples via a different paradigm, namely using spherical rotors spun using a single gas stream for both the bearing and drive (Chen et al, 2018; Gao et al, 2019). This approach allows highly stable rotor spinning about a single axis inclined at the magic angle, with record rates as high as 4.6 kHz (N2, 4.1 bar) and 10.6 kHz (He, 11 bar) for 9.5 mm diameter rotors and 11.4 kHz (N2, 3.1 bar) and 28 kHz (He, 7.6 bar) for 4 mm diameter rotors. We show that a spherical rotor attains its stability from its inherent shape and mass distribution (i.e., its inertia tensor) and that turbine grooves are not essential for stable spinning
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