Thermal Mixing via Cross-Talk between Spin Reservoirs In Dynamic Nuclear Polarization

Phantom IRM Typical 13C multiplet of a hyperpolarized 13CD3 group in DMSO-d6. The lines of the septet deviate from the normal high-temperature intensity distribution 1:3:6:7:6:3:1. The inner line intensities are attenuated compared to the outer lines. The energy-level diagram comprises 27 states. By DNP, states of symmetry A can be overpopulated, while states of symmetry B and E are partly depleted, thus boosting the outer lines in the multiplet.

Phantom IRM Time evolution of inverse nuclear spin temperatures. First, a polarization P(1H) is generated by DNP. After the microwave irradiation is interrupted, the 13C spins are saturated, and a spontaneous flow of polarization from 1H to 13C reservoirs is observed. Via triple spins flips of two electrons and one 1H, the polarization flows from 1H to the non-Zeeman dipolar electron reservoir NZ, and is then transferred to 13C spins via further triple spin flips of two electrons and one 13C.
Dynamic nuclear polarization (DNP) embraces a family of methods to increase signal intensities in nuclear magnetic resonance (NMR) spectroscopy. Despite extensive theoretical work that allows one to distinguish at least five distinct mechanisms, it remains challenging to determine the relative weights of the processes that are responsible for DNP in state-of-the-art experiments operating with stable organic radicals like nitroxides at high magnetic fields and low temperatures. Specifically, determining experimental conditions where DNP involves thermal mixing, which denotes a spontaneous heat exchange between different spin reservoirs, remains challenging. We propose an experimental approach to ascertain the prevalence of the thermal mixing regime by monitoring characteristic signature properties of the time evolution of the hyperpolarization. We find that thermal mixing is the dominant DNP mechanism at high nitroxide radical concentrations, while a mixture of different mechanisms prevails at lower concentrations.

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