EPR Characterization of a Rigid Bis-TEMPO–Bis-Ketal for Dynamic Nuclear Polarization

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.