Novel Biradicals for Direct Excitation Highfield Dynamic Nuclear Polarization

Synthesis of novel trityl-nitroxyl biradicals and their performance as polarization agents in DNP-enhanced solid-state MAS NMR spectroscopy is presented. Signal enhancements in 1H, 1H → 13C CP MAS, and 13C MAS experiments obtained with these radicals dissolved in 1,1,2,2-tetrachloroethane (TCE) solution are compared with the enhancements obtained from TCE solutions of binitroxyl radicals. The signal enhancements are correlated with the distance between the radical centers of the biradicals, as determined by theoretical structure calculations. Some of the biradical TCE solutions display direct-channel resonances in 13C MAS experiments as well as indirect channel resonances induced via the proton spin reservoir. Differential scanning calorimetry reveals that only these solutions do not form any solid crystalline phases upon rapid cooling, suggesting that molecular motions needed for polarization transfer from radicals to 13C via the proton spin reservoir remain active at the experimental low temperatures of nominal 120 K. DNP magnetic field sweep enhancement profiles for selected new biradicals are presented as well. These indicate that the DNP transfer is dominated by the cross-effect mechanism.