Nitroxide Derivatives for Dynamic Nuclear Polarization in Liquids: The Role of Rotational Diffusion

Abstract

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 1 H-DNP 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 13

Spin density localization and accessibility of organic radicals affect liquid-state DNP efficiency

“…This is particularly relevant for the specific case of FN-2a, whose outstanding performance is likely caused by a particularly favourable collision time scale (t cont B 4-12 ps) which maximizes J cont (o e ,t cont ) at this magnetic field (1.2 T). In our previous report, 39 this was attributed to the transition of the six-membered ring from a chair to an unstable half-chair conformation, enabled by the asymmetry of the backbone linker. 39 Similar dynamics are not expected in TL, TN, and DTBN but cannot be excluded for TN-py.…”

“…In our previous report, 39 this was attributed to the transition of the six-membered ring from a chair to an unstable half-chair conformation, enabled by the asymmetry of the backbone linker. 39 Similar dynamics are not expected in TL, TN, and DTBN but cannot be excluded for TN-py.…”

“…Although several mechanistic studies on 1 H 27,35,36 and 13 C 24,37,38 have been reported, the detailed role of the PA remains unclear. Very recently, we investigated the case of fullerene nitroxides in comparison to TEMPONE, 24,39 and found that small structural reorientations can impact the DNP efficiency at both low and high magnetic fields. 39 Therefore, we proposed that the chemical structure of the PA molecule must play an essential role within the OE-DNP mechanism.…”

“…51 We also consider the fullerene-nitroxide FN-2a, which has been already reported as a PA in the context of 1 H and 13 C DNP. 24,32,39 Finally, we compared NODs with BDPA. The organic radicals were dissolved in toluene (C 7 H 8 ), chloroform (CHCl 3 ) and tetrachloromethane (CCl 4 ), with concentrations in the range 1.5-10 mM.…”

“…Two stable conformations were considered also for FN-2a, the most stable one (chair) and the closest in energy (boat), although the latter is not accessible at room temperature. 39 Fig. 4 Hyperfine coupling computed for geometry optimized complexes of PA/CHCl 3 .…”