Frequency Swept Microwaves for Hyperfine Decoupling and Time Domain DNP in Rotating Solids

Edward Saliba, Brice Albert, Faith Scott, Nicholas Alaniva, Michael Mardini, Eric Choi, Seong Ho Pahng, Alexander B. Barnes

Washington University in St. Louis, Department of Chemistry MO 63130, USA

Hyperfine decoupling and pulsed dynamic nuclear polarization (DNP) are promising techniques to improve DNP performance at room temperature.[1-3] We explore experimental and theoretical considerations to implement them with magic angle spinning (MAS). Microwave field simulations using the high frequency structural simulator (HFSS) software suite are performed to characterize the inhomogeneous phase independent microwave field throughout a 198 GHz MAS DNP probe. Our calculations show that a microwave power input of 17 W is required to generate an average EPR nutation frequency of 0.84 MHz. We also present a detailed calculation of microwave heating from the HFSS parameters and find that 7.1% of the incident microwave power contributes to dielectric sample heating. Adiabatic electron spin inversions of stable organic radicals are simulated with SPINEVOLUTION using the inhomogeneous microwave fields calculated by HFSS. We calculate an electron spin inversion efficiency of 56% at a spinning frequency of 5 kHz.

Voltage tunable gyrotron oscillators are proposed as a class of frequency agile microwave sources to generate microwave frequency sweeps required for time domain DNP transfers (i.e., the integrated solid effect),[4] adiabatic electron spin inversions, and hyperfine decoupling. Cyclotron resonance masers (a.k.a. gyrotrons) are vacuum electron devices capable of generating >100 W of millimeter radiation. However, commercially available gyrotrons cannot be tuned quickly. The lack of frequency agility of gyrotrons often restricts solid state DNP spectroscopists to a short list of continuous wave DNP transfer mechanisms. Extending these experiments to the time-domain could have substantial advantages such as improved performance at higher temperatures and employing mounted mono-radicals as polarizing agents, but requires pulsed or frequency swept microwaves. We demonstrate gyrotron acceleration potentials required to generate swept microwave frequency profiles for the frequency modulated cross effect, adiabatic electron spin inversions, hyperfine decoupling, and the integrated solid effect.

Acknowledgements: This research was supported by the NIH Director’s New Innovator Award number DP2GM119131.

References: 1. Corzilius, B., Andreas, L. B., Smith, A. A., Ni, Q. Z., & Griffin, R. G. (2014). Paramagnet induced signal quenching in MAS–DNP experiments in frozen homogeneous solutions. Journal of Magnetic Resonance, 240, 113-123. 2. Can, T. V., Walish, J. J., Swager, T. M., & Griffin, R. G. (2015). Time domain DNP with the NOVEL sequence. The Journal of chemical physics, 143(5), 054201. 3. Hoff, D. E., Albert, B. J., Saliba, E. P., Scott, F. J., Choi, E. J., Mardini, M., & Barnes, A. B. (2015). Frequency swept Microwaves for hyperfine decoupling and time domain dynamic nuclear polarization. Solid state nuclear magnetic resonance, 72, 79-89. 4. Eichhorn, T. R., Brandt, B. V. D., Hautle, P., Henstra, A., & Wenckebach, W. T. (2014). Dynamic nuclear polarisation via the integrated solid effect II: experiments on naphthalene-h 8 doped with pentacene-d 14. Molecular Physics, 112(13), 1773-1782.

Figure 1. Innovations in DNP NMR methods. a) Energy splitting diagram illustrating strong

hyperfine interactions active for DNP transfers, but inactive during the NMR experiment. b) Pulse

sequence for time-domain DNP followed by NMR measurements under hyperfine decoupling.

Figure 2. Custom spectrometer for hyperfine decoupling and time

domain DNP. A new fast-tuning gyrotron (left) will deliver >50 W of microwave power to the sample via microwave waveguides. The

frequency sweeps are controlled through an arbitrary wave generator (10 ns stepsize) integrated directly into the NMR

spectrometer. Novel cryogenics will cool the sample to <25 Kelvin.