Dynamic Nuclear Polarization of ¹³C Nuclei in the Liquid State over a 10 Tesla Field Range

Abstract

Nuclear magnetic resonance (NMR) techniques play an essential role in natural science and medicine. In spite of the tremendous utility associated with the small energies detected, the most severe limitation is the low signal‐to‐noise ratio. Dynamic nuclear polarization (DNP), a technique based on transfer of polarization from electron to nuclear spins, has emerged as a tool to enhance sensitivity of NMR. However, the approach in liquids still faces several challenges. Herein we report the observation of room‐temperature, liquid DNP 13C signal enhancements in organic small molecules as high as 600 at 9.4 Tesla and 800 at 1.2 Tesla. A mechanistic investigation of the 13C‐DNP field dependence shows that DNP efficiency is raised by proper choice of the polarizing agent (paramagnetic center) and by halogen atoms as mediators of scalar hyperfine interaction. Observation of sizable DNP of 13CH2 and 13CH3 groups in organic molecules at 9.4 T opens perspective for a broader application of this method.

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

“…[11][12][13][14][15] Furthermore, higher NMR enhancements could boost the applications of OE-DNP at low fields (o2 T), which include in-flow hyperpolarization for magnetic resonance imaging 16,17 or chromatography, 18,19 NMR relaxometry of low-g nuclei, 20 hydration dynamic studies, 21,22 and DNP-NMR spectroscopy. [23][24][25] In the experimental practice, nitroxide derivatives (NODs) have been established as optimal PAs for OE-DNP in the liquid state at room temperature and ambient pressure. 26,27 In water, they perform better than trityl radicals at various fields (from 0.34 T to 3.4 T), 28,29 and they are the benchmark for 1 H-DNP at low fields (enhancements e = À178 AE 13 for water doped with TEMPONE).…”

“…Despite the available data, it is difficult to compare the PA's performance independently of the experimental conditions, such as mw power and resonant cavity, magnetic field, radical concentration, and target nuclei. 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.…”

“…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.…”

“…2a) are negative, a fact which indicates a scalar-dominated polarization transfer, 8,40 and show an interesting, quite unexpected behaviour. Indeed, in CCl 4 , x is strongly dependent on the PA, and goes from the least efficient BDPA (|x| o 0.12) to the most efficient fullerene nitroxide (FN-2a), 24,32 with |x| = 0.65 AE 0.1. This indicates a factor of B5 variation in DNP efficiency.…”