Barnes, Alexander, Nicholas Alaniva, Edward P. Saliba, Erika L. Sesti, and Patrick T. Judge. “Electron Decoupling with Chirped Microwave Pulses for Rapid Signal Acquisition and Electron Saturation Recovery.” Angewandte Chemie, March 28, 2019.
Dynamic nuclear polarization (DNP) increases NMR sensitivity by transferring polarization from electron to nuclear spins. Here we demonstrate that electron decoupling enables improved observation of DNP-enhanced 13C spins in direct dipolar contact with electron spins, thereby leading to an optimal delay between transients largely governed by relatively fast electron relaxation. Signal acquisition constitutes 12% of the total experimental time, significantly increasing signal-to-noise per unit time. We report the first measurement of electron longitudinal relaxation (T1e) during magicangle spinning (MAS) NMR through observation of DNP-enhanced NMR (T1e = 40 ± 6 ms, 40 mM trityl 4.0 kHz MAS, 4.3 K). With a 5 ms DNP period, electron decoupling results in a 195% increase in signal intensity. Chirped microwave pulses and MAS at 4.3 K are achieved with a custom spectrometer. MAS at 4.3 K, DNP, electron decoupling, and short recycle delays improves the sensitivity of 13C in the vicinity of the polarizing agent. This is the first demonstration of recovery times between MAS-NMR transients being governed by short electron T1 and fast DNP transfer.