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Persistent Radicals of Self‐assembled Benzophenone bis ‐Urea Macrocycles: Characterization and Application as a Polarizing Agent for Solid‐state DNP MAS Spectroscopy
Author(s) -
DeHaven Baillie A.,
Tokarski John T.,
Korous Arthur A.,
MentinkVigier Frederic,
Makris Thomas M.,
Brugh Alexander M.,
Forbes Malcolm D. E.,
van Tol Johan,
Bowers Clifford R.,
Shimizu Linda S.
Publication year - 2017
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201701705
Subject(s) - radical , electron paramagnetic resonance , benzophenone , chemistry , photochemistry , spectroscopy , magic angle spinning , nuclear magnetic resonance spectroscopy , stereochemistry , nuclear magnetic resonance , organic chemistry , physics , quantum mechanics
UV‐irradiation of a self‐assembled benzophenone bis ‐urea macrocycle generates μ m amounts of radicals that persist for weeks under ambient conditions. High‐field EPR and variable‐temperature X‐band EPR studies suggest a resonance stabilized radical pair through H‐abstraction. These endogenous radicals were applied as a polarizing agent for magic angle spinning (MAS) dynamic nuclear polarization (DNP) NMR enhancement. The field‐stepped DNP enhancement profile exhibits a sharp peak with a maximum enhancement of ϵ on/off =4 superimposed on a nearly constant DNP enhancement of ϵ on/off =2 over a broad field range. This maximum coincides with the high field EPR absorption spectrum, consistent with an Overhauser effect mechanism. DNP enhancement was observed for both the host and guests, suggesting that even low levels of endogenous radicals can facilitate the study of host–guest relationships in the solid‐state.