Electron Decoupling with Chirped Microwave Pulses for Rapid Signal Acquisition and Electron Saturation Recovery

Dynamic nuclear polarization (DNP) increases NMR sensitivity by transferring polarization from electron to nuclear spins. Herein, we demonstrate that electron decoupling with chirped microwave pulses enables improved observation of DNP-enhanced ¹³ C spins in direct dipolar contact with electron spins, thereby leading to an optimal delay between transients largely governed by relatively fast electron relaxation. We report the first measurement of electron longitudinal relaxation time (T_1e ) during magic angle spinning (MAS) NMR by observation of DNP-enhanced NMR signals (T_1e =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. MAS at 4.3 K, DNP, electron decoupling, and short recycle delays improve the sensitivity of ¹³ C 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.