Scott, Faith J., Erika L. Sesti, Eric J. Choi, Alexander J. Laut, Jagadishwar R. Sirigiri, and Alexander B. Barnes. “Magic Angle Spinning NMR with Metallized Rotors as Cylindrical Microwave Resonators.” Magnetic Resonance in Chemistry, May 16, 2018.
We introduce a novel design for millimeter wave electromagnetic structures within magic angle spinning (MAS) rotors. In this demonstration, a copper coating is vacuum deposited onto the outside surface of a sapphire rotor at a thickness of 50 nanometers. This thickness is sufficient to reflect 197 GHz microwaves, yet not too thick as to interfere with radiofrequency fields at 300 MHz or prevent sample spinning due to eddy currents. Electromagnetic simulations of an idealized rotor geometry show a microwave quality factor of 148. MAS experiments with sample rotation frequencies of ωr/2π = 5.4 kHz demonstrate that the drag force due to eddy currents within the copper does not prevent sample spinning. Spectra of sodium acetate show resolved 13C J-couplings of 60 Hz and no appreciable broadening between coated and uncoated sapphire rotors, demonstrating that the copper coating does not prevent shimming and high-resolution NMR spectroscopy. Additionally, 13C Rabi nutation curves of ω1/2π = 103 kHz for both coated and uncoated rotors indicate no detrimental impact of the copper coating on radiofrequency coupling of the nuclear spins to the sample coil. We present this metal coated rotor as a first step towards an MAS resonator. MAS resonators are expected to have a significant impact on developments in electron decoupling, pulsed DNP, room temperature DNP, DNP with low power microwave sources, and EPR detection.