EPR Characterization of a Rigid Bis-TEMPO–Bis-Ketal for Dynamic Nuclear Polarization
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We have characterized the rigid binitroxide radical bis-TEMPO–bis-Ketal (bTbK) by continuous-wave (CW) and pulsed electron paramagnetic resonance (EPR) spectroscopy performed at X-band (9 GHz) and G-band (180 GHz) frequencies. bTbK has been successfully used for dynamic nuclear polarization (DNP)-enhanced solid-state nuclear magnetic resonance (SS-DNP) experiments based on the cross-effect, which involves two electrons and one nuclear spin, and gave very high signal enhancements. For a quantitative description of the polarization enhancements and their excitation frequency profile, a detailed information about the values and relative orientation of the magnetic hyperfine-, dipolar-, g-tensors and the exchange interaction of the two unpaired electron spins within the molecule is mandatory. We have determined these tensors and their relative orientation by CW-EPR spectra and pulsed electron double resonance experiments in frozen solution. The potential of using the cross-effect also for DNP in liquid solutions has been experimentally investigated by room-temperature high-field DNP experiments performed at 9.2 T.
KeywordsElectron Paramagnetic Resonance Electron Paramagnetic Resonance Spectrum Dynamic Nuclear Polarization Hyperfine Line Dynamic Nuclear Polarization Enhancement
We want to thank Burkhard Endeward for experimental support, Deniz Sezer, Mark Prandolini for many fruitful discussions and Dominik Margraf for comments on this paper. This project is funded by the European Commission in the design study project BIO-DNP.
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