Dissolution DNP reveals a long-lived deuterium spin state imbalance in methyl groups

International audienceWe report the generation and observation of long-lived spin states in deuterated methyl groups by dissolution DNP. These states are based on population imbalances between manifolds of spin states corresponding to irreducible representations of the C3v point group and feature strongly dampened quadrupolar relaxation. Their lifetime depends on the activation energies of methyl group rotation. With dissolution DNP, we can reduce the deuterium relaxation rate by a factor up to 20, thereby extending the experimentally available time window. The intrinsic limitation of NMR spectroscopy of quadrupolar spins by short relaxation times can thus be alleviated.Dissolution dynamic nuclear polarization (D-DNP) aims at overcoming the intrinsically low sensitivity of NMR spectroscopy and MRI.1–3 This enables an exciting range of previously inaccessible applications.4–6 D-DNP relies on the coupling between nuclear and electron spins, as well as on the much larger magnitude of the latter’s magnetic moments. By saturating the electron paramagnetic resonance (EPR) transitions of unpaired electrons by microwave (μw) irradiation, the electron polarization is readily transferred to nearby nuclei, most efficiently to protons.7 Other nuclear spins can be enhanced by cross polarization (CP).8 In our laboratory polarizations P(1H) > 90% and P(13C) > 60% can be achieved routinely at cryogenic temperatures of 1.2 K in a magnetic field strength of 6.7 T. The rapid dissolution of the frozen hyperpolarized sample with overheated water and its transfer in 3-6 s to solution-state 800 or 400 MHz NMR systems allows us to achieve signal enhancements by factors up to 104–105 for 13C nuclei.9 The process is schematically depicted in Fig. 1