Mechanistic origins of methyl-driven Overhauser DNP

Abstract

The Overhauser effect in the dynamic nuclear polarization (DNP) of non-conducting solids has drawn much attention due to the potential for efficient high-field DNP as well as a general interest in the underlying principles that enable the Overhauser effect in small molecules. We recently reported the observation of 1H and 2H Overhauser effects in H3C- or D3C-functionalized Blatter radical analogs, which we presumed to be caused by methyl rotation. In this work, we look at the mechanism for methyl-driven Overhauser DNP in greater detail, considering methyl librations and tunneling in addition to classical rotation. We predict the temperature dependence of these mechanisms using density functional theory and spin dynamics simulations. Comparisons with results from ultralow-temperature magic angle spinning-DNP experiments revealed that cross-relaxation at temperatures above 60 K originates from both libration and rotation, while librations dominate at lower temperatures. Due to the zero-point vibrational nature of these motions, they are not quenched by very low temperatures, and methyl-driven Overhauser DNP is expected to increase in efficiency down to 0 K, predominantly due to increases in nuclear relaxation times.

Temperature-Dependent Antiferromagnetic Exchange along 1D Linear Regular Chains of the Phthalonitrile Blatter Radical

“…Thin films based on Blatter-type radicals can act as metal-free spin-sources in purely organic spintronic devices . Blatter radicals can also be used as polarizing agents of Overhauser DNP for high-field MAS-DNP. , …”

“…66 Blatter radicals can also be used as polarizing agents of Overhauser DNP for high-field MAS-DNP. 67,68 The application of Blatter radicals in cutting-edge devices requires the development of "structure−activity" relationships. Such studies can help understand how intrinsic microscopic and macroscopic properties can be "customized" to prepare materials with the desired properties.…”