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

Abstract

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. Polarization build-up times with MAS DNP (T_{1DNP, 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 T_1DNP (¹H) of 18.7 s and the T_1DNP (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 polarization periods.

Keywords: 2-D NMR; Cryogenic magic angle spinning; Dynamic nuclear polarization; Electron decoupling.