Levien, M., M. Hiller, I. Tkach, M. Bennati, and T. Orlando. “Nitroxide Derivatives for Dynamic Nuclear Polarization in Liquids: The Role of Rotational Diffusion.” The Journal of Physical Chemistry Letters 11, no. 5 (March 5, 2020): 1629–35.
Polarization transfer eﬃciency in liquid-state dynamic nuclear polarization (DNP) depends on the interaction between polarizing agents (PAs) and target nuclei modulated by molecular motions. We show how translational and rotational diﬀusion diﬀerently aﬀect the DNP eﬃciency. These contributions were disentangled by measuring 1HDNP enhancements of toluene and chloroform doped with nitroxide derivatives at 0.34 T as a function of either the temperature or the size of the PA. The results were employed to analyze 13C-DNP data at higher ﬁelds, where the polarization transfer is also driven by the Fermi contact interaction. In this case, bulky nitroxide PAs perform better than the small TEMPONE radical due to structural ﬂuctuations of the ring conformation. These ﬁndings will help in designing PAs with features speciﬁcally optimized for liquid-state DNP at various magnetic ﬁelds.