Open and Closed Radicals: Local Geometry around Unpaired Electrons Governs Magic-Angle Spinning Dynamic Nuclear Polarization Performance

未成对电子 化学 构象异构 魔角纺纱 激进的 电子 魔术(望远镜) 极化(电化学) 结晶学 分子 核磁共振波谱 立体化学 物理化学 物理 有机化学 量子力学
作者
Gabriele Stevanato,Gilles Casano,Dominik J. Kubicki,Yu Rao,Laura Esteban Hofer,Georges Menzildjian,Hakim Karoui,Didier Siri,Manuel Cordova,Maxim Yulikov,Gunnar Jeschke,Moreno Lelli,Anne Lesage,Olivier Ouari,Lyndon Emsley
出处
期刊:Journal of the American Chemical Society [American Chemical Society]
卷期号:142 (39): 16587-16599 被引量:50
标识
DOI:10.1021/jacs.0c04911
摘要

The development of magic-angle spinning dynamic nuclear polarization (MAS DNP) has allowed atomic-level characterization of materials for which conventional solid-state NMR is impractical due to the lack of sensitivity. The rapid progress of MAS DNP has been largely enabled through the understanding of rational design concepts for more efficient polarizing agents (PAs). Here, we identify a new design principle which has so far been overlooked. We find that the local geometry around the unpaired electron can change the DNP enhancement by an order of magnitude for two otherwise identical conformers. We present a set of 13 new stable mono- and dinitroxide PAs for MAS DNP NMR where this principle is demonstrated. The radicals are divided into two groups of isomers, named open (O-) and closed (C-), based on the ring conformations in the vicinity of the N–O bond. In all cases, the open conformers exhibit dramatically improved DNP performance as compared to the closed counterparts. In particular, a new urea-based biradical named HydrOPol and a mononitroxide O-MbPyTol yield enhancements of 330 ± 60 and 119 ± 25, respectively, at 9.4 T and 100 K, which are the highest enhancements reported so far in the aqueous solvents used here. We find that while the conformational changes do not significantly affect electron spin–spin distances, they do affect the distribution of the exchange couplings in these biradicals. Electron spin echo envelope modulation (ESEEM) experiments suggest that the improved performance of the open conformers is correlated with higher solvent accessibility.
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