Dynamic Nuclear Polarization Enhanced Solid-State NMR Spectroscopy at Very High Field and Fast MAS
Final Report Abstract
Dynamic Nuclear Polarization (DNP) has emerged as the most promising approach to overcome the sensitivity issues of NMR spectroscopy. However, the sensitivity gain achieved with binitroxides, which are the most commonly used polarizing agents today, suffers from an unfavorable field and Magic Angle Spinning (MAS) frequency dependence. During this project, we have first introduced a series of hybrid biradicals relying on the Cross Effect (CE) and soluble in organic solvents, by tethering a BDPA to a nitroxide moiety. The best radicals in this series outperform today’s most efficient binitroxides and yield enhancements of over 180 at 18.8 T and 40 kHz MAS frequency. By rationally fine tuning of the chemical structures of these biradicals, their electronic properties could be significantly modulated. Higher enhancements are obtained by decreasing the length of the linker, leading to stronger magnetic interactions between the two unpaired electrons, and by introducing bulky substituents on the nitroxide, which increase the electronic relaxation times of this moiety. In parallel, we have explored the performance of Overhauser Effect (OE) DNP at high magnetic field (18.8 T) and fast MAS using α,γ-bisdiphenylene-β-phenylallyl (BDPA) in 95 % deuterated orthoterphenyl (OTP). High enhancements of over 100 were achieved at this field and at 40 kHz MAS. However, the nuclear relaxations times were on the order of 40 s, which limits to some extent the applicability of this formulation. In both cases, i.e. for hybrid biradicals as well as for BDPA in OTP, the nuclear relaxation times and the enhancements were found to increase with the spinning speed, in contrast to binitroxides, where stable enhancements have been observed before above 5 kHz MAS. We introduced a model of proton spin diffusion dynamics which can effectively explain this behaviour, and which relies on the presence of separate polarization sources (radicals) and a small concentration of polarization sinks (paramagnetic impurities) in the system. Fast sample spinning decouples the relaxation sinks from the bulk solution, which increases the overall apparent build-up time constant, and in turn leads to more efficient propagation of the hyperpolarization from the polarization agents throughout the bulk. The relevance of this model is further supported by the fact that when introducing additional relaxation sinks, the overall nuclear relaxation times as well as the enhancements drop. We impregnated different functional materials, namely silica, γ-alumina and aluminosilicates, with the above-mentioned radical formulations. We were able to record spectra on quadrupolar nuclei, 29Si and 27Al, with unprecedented surface enhancements of up to 46 at 18.8 T at 40 kHz MAS, yielding well resolved spectra free from spinning sideband overlap. The development of these new formulations led to a deeper understanding of the polarization transfer mechanisms and constitutes an important step forward for a broad applicability of MAS DNP on challenging samples at high magnetic field.
Publications
- Dynamic Nuclear Polarization Efficiency Increased by Very Fast Magic Angle Spinning. J. Am. Chem. Soc. 2017, 139, 10609- 10612
S. Chaudhari, D. Wisser, A. Pinon, P. Berruyer, D. Gajan, P. Tordo, O. Ouari, C. Reiter, F. Engelke, C. Copéret, M. Lelli, A. Lesage, L. Emsley
(See online at https://doi.org/10.1021/jacs.7b05194)
