Category Archives: Decoupling

Electron decoupling with cross polarization and dynamic nuclear polarization below 6 K #DNPNMR

Sesti, Erika L., Edward P. Saliba, Nicholas Alaniva, and Alexander B. Barnes. “Electron Decoupling with Cross Polarization and Dynamic Nuclear Polarization below 6 K.” Journal of Magnetic Resonance 295 (October 2018): 1–5.

https://doi.org/10.1016/j.jmr.2018.07.016

Dynamic nuclear polarization (DNP) can improve nuclear magnetic resonance (NMR) sensitivity by orders of magnitude. Polarizing agents containing unpaired electrons required for DNP can broaden nuclear resonances in the presence of appreciable hyperfine couplings. Here we present the first cross polarization experiments implemented with electron decoupling, which attenuates detrimental hyperfine couplings. We also demonstrate magic angle spinning (MAS) DNP experiments below 6 K, producing unprecedented nuclear spin polarization in rotating solids. 13C correlation spectra were collected with MAS DNP below 6 K for the first time. Longitudinal magnetization recovery times with MAS DNP (T1DNP, 1H) of urea in a frozen glassy matrix below 6 K were measured for both the solid effect and the cross effect. Trityl radicals exhibit a T1DNP (1H) of 18.7 s and the T1DNP (1H) of samples doped with 20 mM AMUPol is only 1.3 s. MAS below 6 K with DNP and electron decoupling is an effective strategy to increase NMR signal-to-noise ratios per transient while retaining short recovery periods.

Frequency swept microwaves for hyperfine decoupling and time domain dynamic nuclear polarization

Hoff, D.E., et al., Frequency swept microwaves for hyperfine decoupling and time domain dynamic nuclear polarization. Solid State Nucl Magn Reson, 2015. 72: p. 79-89.

http://www.ncbi.nlm.nih.gov/pubmed/26482131

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.

Have a question?

If you have questions about our instrumentation or how we can help you, please contact us.