Griffin, Robert G., Timothy M. Swager, and Richard J. Temkin. “High Frequency Dynamic Nuclear Polarization: New Directions for the 21st Century.” Journal of Magnetic Resonance 306 (September 2019): 128–33.
Dynamic nuclear polarization (DNP) is a technique that permits the sensitivity of nuclear magnetic resonance (NMR) experiments to be enhanced by a factor of (γe/γn) where the γ’s are the gyromagnetic ratios of the electron and a nuclear spin, respectively. When the nuclear spin is 1H, then optimally (γe/γH) ∼ 660. At present, ε ∼ 100 is readily achieved but even this “modest” enhancement means that the experimental acquisition time is reduced by a factor of 104. Thus, an experiment can be done in a single day that would otherwise require ∼ 30 years of signal averaging. Accordingly, the incorporation of DNP into MAS and solution experimental protocols has enabled many experiments that are otherwise simply not possible. Furthermore, DNP experiments are in principle quite straightforward to perform and involve introducing a paramagnetic polarizing agent such as a bisnitroxide biradical into a glassy matrix containing the solute molecule of interest. Subsequently, the sample is irradiated with high frequency microwaves that excite electron-nuclear spin transitions, and, via a number of mechanisms discussed below, the large polarization in the electron spin reservoir is transferred to the nuclei.