Two millimeter diameter spherical rotors spinning at 68 kHz for MAS NMR
Pin-Hui Chen, Chukun Gao, Lauren E. Price, Michael A. Urban, Thomas M. Osborn Popp, Alexander B. Barnes
Abstract
Spherical rotors are a new paradigm in magic angle spinning (MAS) solid-state nuclear magnetic resonance (NMR). The simple geometry makes smaller diameter spheres and their utilization within narrow-bore NMR probes feasible. Here we report a 68 kHz spinning frequency of 2 mm diameter spheres using helium spinning gas outside the magnet and demonstrate the use of KMnO 4 to adjust the magic angle at a spinning frequency of 59.3 kHz for MAS NMR. We observe third-order spinning sidebands in the 55 Mn spectrum clearly showing the MAS frequency of 59.3 kHz, with KBr showing nearly no first-order spinning sidebands at a similar frequency. The spinning stability was ±0.5% during data acquisition without spinning regulation. To address concerns about the low NMR filling factor of MAS spheres, we employ a modified stator and a smaller coil and achieve three times higher NMR sensitivity then our previous coil geometries for MAS spheres. Advanced coil and rotor fabrication technologies are expected to further increase the spinning frequency and NMR sensitivity of MAS spheres.