Pulsed Electron Decoupling and Strategies for Time Domain Dynamic Nuclear Polarization with Magic Angle Spinning

Abstract

Magic angle spinning (MAS) dynamic nuclear polarization (DNP) is widely used to increase nuclear magnetic resonance (NMR) signal intensity. Frequency-chirped microwaves yield superior control of electron spins and are expected to play a central role in the development of DNP MAS experiments. Time domain electron control with MAS has considerable promise to improve DNP performance at higher fields and temperatures. We have recently demonstrated that pulsed electron decoupling using frequency-chirped microwaves improves MAS DNP experiments by partially attenuating detrimental hyperfine interactions. The continued development of pulsed electron decoupling will enable a new suite of MAS DNP experiments that transfer polarization directly to observed spins. Time domain DNP transfers to nuclear spins in conjunction with pulsed electron decoupling is described as a viable avenue toward DNP-enhanced, high-resolution NMR spectroscopy over a range of temperatures from <6 to 320 K.

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

“…In the case of dynamic nuclear polarization (DNP), hyperfine interactions are required to transfer spin polarization from electron to nuclear spins. [30,31] DNP experiments typically require seconds to hours to spread enhanced polarization via nuclear spin diffusion to chemical sites of interest. [26] Here, we use as table organic monoradical, trityl (Finland radical), which has ar elatively narrow EPR lineshape.…”

“…This could be due to spins that were narrowed sufficiently for their signal to be distinguished from the noise, but not enough to decrease the overall linewidth, thus adding broader components to the spectrum. [30,49] …”

“…[27] Then arrow electron spin resonance and high-power microwave chirps generated by ac ustom-built frequency-agile gyrotron enable time-domain manipulation of electronic and hyperfine interactions. [30,31] DNP experiments typically require seconds to hours to spread enhanced polarization via nuclear spin diffusion to chemical sites of interest. [30,31] DNP experiments typically require seconds to hours to spread enhanced polarization via nuclear spin diffusion to chemical sites of interest.…”

“…[22,[40][41][42][43][44] Short recovery periods also significantly improve the sensitivity per unit time of MAS NMR, and are expected to play aprominent role in the future of magnetic resonance experiments.T he manipulation of electron spins in the time domain, as demonstrated here,is an important step towards implementing pulsed DNP MAS. [30,49] [30,49] …”

“…[28,29] We have shown that chirped microwave pulses can partially decouple the electron-nuclear spin interaction after DNP in am ethod known as electron decoupling,w ith amore pronounced effect on observed nuclear spins in direct proximity to electron spins. [30,31] DNP experiments typically require seconds to hours to spread enhanced polarization via nuclear spin diffusion to chemical sites of interest. [32][33][34] This slow spin diffusion significantly limits experiments.D NP is faster if nuclear spins proximal to the electron are polarized directly through hyperfine interactions.H owever,s uch direct DNP requires electron decoupling after the polarization transfer period to mitigate the detrimental effects of hyperfine interactions on NMR signatures.…”