Observation of a four-spin solid effect

Abstract

The two-spin solid effect (2SSE) is one of the established continuous wave dynamic nuclear polarization (DNP) mechanisms that enables enhancement of NMR signals. It functions via a state-mixing mechanism that mediates the excitation of forbidden transitions in an electron-nuclear spin system. Specifically, microwave irradiation at frequencies ᵱ4;μw~ᵱ4;0S {plus minus} ᵱ4;0I, where ᵱ4;0S and ᵱ4;0I are electron and nuclear Larmor frequencies, respectively, yields enhanced nuclear spin polarization. Following the recent rediscovery of the three-spin solid effect (3SSE) (Tan et al., Sci. Adv, aax2743), where the matching condition is given by ᵱ4;μw = ᵱ4;0S {plus minus} 2ᵱ4;0I,we report here the first direct observation of the four-spin solid effect (4SSE) at ᵱ4;μw = ᵱ4;0S {plus minus} 3ᵱ4;0I . The forbidden double- and quadruple-quantum transitions were observed in samples containing trityl radicals dispersed in a glycerol-water mixture at 0.35 T / 15 MHz / 9.8 GHz and 80 K. We present a derivation of the 4SSE effective Hamiltonian, matching conditions, and transition probabilities. Finally, we show that the experimental observations agree with the results from numerical simulations and analytical theory.

Relaxation enhancement by microwave irradiation may limit dynamic nuclear polarization

“…Microwave irradiation will decouple some of these spins and make them visible and contribute to the observable nuclear signal intensity. 23,24 Irradiation closer to the center of the EPR line would result in a stronger decoupling effect as more electrons are affected by the decoupling.…”

Observing Nearby Nuclei on Paramagnetic Trityls and MOFs via DNP and Electron Decoupling

“…This configuration was chosen because the frequency of the latter μ w channel can be directly configured in the PulseSPEL pulse program, which facilitates the acquisition of the electron decoupling profile. The RF circuitry was also improved for better sensitivity, [ 39 ] and 128 saturation pulses were employed to ensure that any enhanced NMR signal from previous scans was fully saturated. This is crucial because the signal intensity gain (≤ 1.8×) observed in electron-decoupled signal could be overshadowed by the sensitivity gain from DNP (≥100×).…”

“…Note that the DNP enhancement factor represents only an estimated value based on previous studies performed on partially protonated DNP juice. [ 17 , 39 ] The actual DNP enhancement on the nearby nuclei could not be determined because the 1 H thermal equilibrium signal was not observable despite long acquisition periods. To directly observe only the nearby nuclei, a short τ DNP time (≤25 ms) and a recycle delay of ~0.25 s was used.…”

“…This is crucial because the signal intensity gain (≤ 1.8x) observed in electron-decoupled signal could be overshadowed by the sensitivity gain from DNP (≥ 100x). Note that the DNP enhancement factor represents only an estimated value based on previous studies performed on partially protonated DNP juice [17,35] . The actual DNP enhancement on the nearby nuclei could not be determined because the 1 H thermal equilibrium signal was not observable despite long acquisition periods.…”