Large 31P-NMR enhancements in liquid state dynamic nuclear polarization through radical/target molecule non-covalent interaction

Dynamic nuclear polarization (DNP) is a method to enhance the low sensitivity of nuclear magnetic resonance (NMR) via spin polarization transfer from electron spins to nuclear spins. In the liquid state, this process is mediated by fast modulations of the electron-nuclear hyperfine coupling and its efficiency depends strongly on the applied magnetic field. A peculiar case study is triphenylphosphine (PPh3) dissolved in benzene and doped with BDPA radical because it gives 31P-NMR signal enhancements of two orders of magnitude up to a magnetic field of 14.1 T. Here we show that the large 31P enhancements of BDPA/PPh3 in benzene at 1.2 T (i) decrease when the moieties are dissolved in other organic solvents, (ii) are strongly reduced when using a nitroxide radical, and (iii) vanish with pentavalent 31P triphenylphosphine oxide. Those experimental observations are rationalized with numerical calculations based on density functional theory that show the tendency of BDPA and PPh3 to form a weak complex via non-covalent interaction that leads to large hyperfine couplings to 31P (ΔAiso ≥ 13 MHz). This mechanism is hampered in other investigated systems. The case study of 31P-DNP in PPh3 is an important example that extends the current understanding of DNP in the liquids state: non-covalent interactions between radical and target can be particularly effective to obtain large NMR signal enhancements.