Theoretical analysis of scalar relaxation in 13C-DNP in liquids

Dynamic nuclear polarization in the liquid state via Overhauser effect is enabled by the fluctuations of the electron-nuclear hyperfine interaction. Fermi contact (or scalar) hyperfine coupling can be modulated by molecular collisions on timescales of a few picoseconds and shorter, enabling an effective polarization transfer even at high magnetic fields. However, only a few studies have presented a theoretical analysis of the scalar mechanism. Here we report the current understanding of the scalar relaxation in liquid-state DNP and present different modeling strategies based on analytical relaxation theory and numerical calculations from molecular dynamics simulations. These approaches give consistent results in identifying the timescale of the fluctuations of the scalar interaction that drives C-DNP in the model system of CHCl doped with nitroxide radical. Subpicosecond fluctuations arise not only from random molecular collisions but are also present when target molecule and polarizing agent form a transient complex that persists for tens of picoseconds. We expect that these kind of interactions, possibly based on hydrogen bond-like complexations, might be present in a large variety of compounds