Kundu, Krishnendu, Frédéric Mentink‐Vigier, Akiva Feintuch, and Shimon Vega. “DNP Mechanisms.” In EMagRes, 295–338. American Cancer Society, 2019.
This article presents a comprehensive description of the spin dynamics underlying the main DNP mechanisms leading to nuclear signal enhancements in glassy amorphous solids containing free radicals. The emphasis of the article to derive quantum mechanically based formalisms that enable us to analyze experimental DNP data. After a short review of the history of DNP, rate equations of the eigenstate populations of static coupled electron–nuclear spin systems are introduced, based on their spin-Hamiltonians and including spin-lattice and cross-relaxation mechanisms. They are used to simulate the dynamics of small spin systems under microwave (MW) irradiation and the basic Solid Effect (SE), Cross Effect (CE), and Overhauser DNP (O-DNP) enhancement mechanisms are presented. These calculations are then extended to systems containing up to 10 spins and are used to calculate EPR, ELDOR, and DNP spectra. Plots showing the population of the eigenstates vs energy are used to demonstrate the conditions for the thermal mixing mechanism and the corresponding EPR and ELDOR spectra are discussed. Following these calculations, the electron spectral diffusion (eSD) and the indirect Cross Effect (iCE) numerical models are introduced and used to analyze EPR and DNP spectra of real samples. In the last section, the basic theory of magic angle spinning (MAS) DNP on small spin systems is summarized and the influence of the rotor events on the quasiperiodic steady-state DNP enhancements discussed. The origins of depolarization effects occurring when no MW is applied are described. Finally, the nuclear spin diffusion process inside the diffusion barrier is studied using multielectron and multinuclear calculations.