Mechanistic origins of methyl-driven Overhauser DNP

Frédéric A Perras; Yoh Matsuki; Scott A Southern; Thierry Dubroca; Dragos F Flesariu; Johan Van Tol; Christos P Constantinides; Panayiotis A Koutentis

doi:10.1063/5.0149664

Mechanistic origins of methyl-driven Overhauser DNP

J Chem Phys. 2023 Apr 21;158(15):154201. doi: 10.1063/5.0149664.

Authors

Frédéric A Perras^{1

2}, Yoh Matsuki^{2

3}, Scott A Southern¹, Thierry Dubroca⁴, Dragos F Flesariu⁵, Johan Van Tol⁴, Christos P Constantinides⁶, Panayiotis A Koutentis⁵

Affiliations

¹ Chemical and Biological Sciences Division, Ames National Laboratory, Ames, Iowa 50011, USA.
² Department of Chemistry, Iowa State University, Ames, Iowa 50011, USA.
³ Institute for Protein Research, Osaka University, Suita, Osaka 565-0871, Japan.
⁴ National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, USA.
⁵ Department of Chemistry, University of Cyprus, P.O. Box 20537, 1678 Nicosia, Cyprus.
⁶ Department of Natural Sciences, University of Michigan-Dearborn, Dearborn, Michigan 48128, USA.

PMID: 37093991
DOI: 10.1063/5.0149664

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.