Kuzhelev, Andrei A., Danhua Dai, Vasyl Denysenkov, Igor A. Kirilyuk, Elena G. Bagryanskaya, and Thomas F. Prisner. “Influence of Rotational Motion of Nitroxides on Overhauser Dynamic Nuclear Polarization: A Systematic Study at High Magnetic Fields.” The Journal of Physical Chemistry C, November 11, 2021, acs.jpcc.1c06979.
https://doi.org/10.1021/acs.jpcc.1c06979.
TEMPOL nitroxide radicals are frequently used as a polarizing agent for liquid-state Overhauser dynamic nuclear polarization. To achieve large signal enhancements at high magnetic fields (9.4 T), fast picosecond to sub-picosecond dynamics between nitroxide and the target solvent molecule are required. Such dynamics have been predicted by molecular dynamic studies and attributed to fast inner-sphere motions of a transient radical−solvent complex. Here, we systematically study a series of nitroxide radicals with different substituents around the electron spin-bearing NO moiety and different overall sizes to quantify the contribution of the rotational dynamics of the radical to these inner-sphere dynamics. The experiments are performed at a 9.4 T magnetic field, which exhibits high sensitivity to rotational motion contributions in a low picosecond time range. We can show that the observed enhancements can be quantitatively predicted taking the rotational motion of the radical into account.