Frequency swept microwaves for hyperfine decoupling and time domain dynamic nuclear polarization.
Hyperfine decoupling and pulsed dynamic nuclear polarization (DNP) are promising techniques to improve high field DNP NMR. 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 198GHz MAS DNP probe. Our calculations show that a microwave power input of 17W is required to generate an average EPR nutation frequency of 0.84MHz. 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. Voltage tunable gyrotron oscillators are proposed as a class of frequency agile microwave sources to generate microwave frequency sweeps required for the frequency modulated cross effect, electron spin inversions, and hyperfine decoupling. 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 5kHz. Finally, we demonstrate gyrotron acceleration potentials required to generate swept microwave frequency profiles for the frequency modulated cross effect and electron spin inversions.
Department of Chemistry, Washington University in St. Louis, One Brookings Drive, St. Louis, MO 63130, USA.
Daniel E M
DE
Hoff
Department of Chemistry, Washington University in St. Louis, One Brookings Drive, St. Louis, MO 63130, USA.
Brice J
BJ
Albert
Department of Chemistry, Washington University in St. Louis, One Brookings Drive, St. Louis, MO 63130, USA.
Edward P
EP
Saliba
Department of Chemistry, Washington University in St. Louis, One Brookings Drive, St. Louis, MO 63130, USA.
Faith J
FJ
Scott
Department of Chemistry, Washington University in St. Louis, One Brookings Drive, St. Louis, MO 63130, USA.
Eric J
EJ
Choi
Department of Chemistry, Washington University in St. Louis, One Brookings Drive, St. Louis, MO 63130, USA.
Michael
M
Mardini
Department of Chemistry, Washington University in St. Louis, One Brookings Drive, St. Louis, MO 63130, USA. Electronic address: barnesab@wustl.edu.
Alexander B
AB
Barnes
D000498:Allyl Compounds / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008872:Microwaves; D008958:Models, Molecular; D008968:Molecular Conformation; D013451:Sulfonic Acids / Q000737:chemistry
D000498:Allyl Compounds; D013451:Sulfonic Acids; C416323:bis-diphenylene-phenyl-allyl
Adiabatic inversion; Dielectric heating; Dynamic nuclear polarization; Electron decoupling; Frequency modulated cross effect; Hyperfine decoupling; Magic angle spinning; Pulsed DNP; Voltage tunable gyrotrons