TY - JOUR

T1 - The effects of nitroxide structure upon 1H Overhauser dynamic nuclear polarization efficacy at ultralow-field

AU - Fehling, Paul

AU - Buckenmaier, Kai

AU - Dobrynin, Sergey A.

AU - Morozov, Denis A.

AU - Polienko, Yuliya F.

AU - Khoroshunova, Yulia V.

AU - Borozdina, Yulia

AU - Mayer, Philipp

AU - Engelmann, Jörn

AU - Scheffler, Klaus

AU - Angelovski, Goran

AU - Kirilyuk, Igor A.

N1 - Funding Information: The financial support of the ERA.Net RUS+ project (No. ST2017-382), NanoHyperRadicals (including RFBR 18-53-76003-ERA-A and BMBF, FKZ: Grant No. 01DJ18009), and the Shanghai Municipal Science and Technology Major Project (Grant No. 2019SHZDZX02) is acknowledged. The authors thank the MultiAccess Chemical Research Center SB RAS for spectral and analytical measurements of nitroxides (supplementary material). P.F. acknowledges the financial support from the Cusanuswerk e.V. The authors thank the DEAL project for promoting open access to research publications. Publisher Copyright: © 2021 Author(s).

PY - 2021/10/14

Y1 - 2021/10/14

N2 - The efficacy in 1H Overhauser dynamic nuclear polarization in liquids at ultralow magnetic field (ULF, B0 = 92 ± 0.8 µT) and polarization field (Bp = 1-10 mT) was studied for a broad variety of 26 different spin probes. Among others, piperidine, pyrrolidine, and pyrroline radicals specifically synthesized for this study, along with some well-established commercially available nitroxides, were investigated. Isotope-substituted variants, some sterically shielded reduction-resistant nitroxides, and some biradicals were included in the measurements. The maximal achievable enhancement, Emax, and the radio frequency power, P1/2, needed for reaching Emax/2 were measured. Physico-chemical features such as molecular weight, spectral linewidth, heterocyclic structure, different types of substituents, deuteration, and 15N-labeling as well as the difference between monoradicals and biradicals were investigated. For the unmodified nitroxide radicals, the Emax values correlate with the molecular weight. The P1/2 values correlate with the spectral linewidth and are additionally influenced by the type of substituents neighboring the nitroxide group. The nitroxide biradicals with high intramolecular spin-spin coupling show low performance. Nitroxides enriched with 15N and/or 2H afford significantly higher |Emax| and require lower power to do so, compared to their unmodified counterparts containing at natural abundance predominantly 14N and 1H. The results allow for a correlation of chemical features with physical hyperpolarization-related properties and indicate that small nitroxides with narrow spectral lines have clear advantages for the use in Overhauser dynamic nuclear polarization experiments. Perdeuteration and 15N-labeling can be used to additionally boost the spin probe performance.

AB - The efficacy in 1H Overhauser dynamic nuclear polarization in liquids at ultralow magnetic field (ULF, B0 = 92 ± 0.8 µT) and polarization field (Bp = 1-10 mT) was studied for a broad variety of 26 different spin probes. Among others, piperidine, pyrrolidine, and pyrroline radicals specifically synthesized for this study, along with some well-established commercially available nitroxides, were investigated. Isotope-substituted variants, some sterically shielded reduction-resistant nitroxides, and some biradicals were included in the measurements. The maximal achievable enhancement, Emax, and the radio frequency power, P1/2, needed for reaching Emax/2 were measured. Physico-chemical features such as molecular weight, spectral linewidth, heterocyclic structure, different types of substituents, deuteration, and 15N-labeling as well as the difference between monoradicals and biradicals were investigated. For the unmodified nitroxide radicals, the Emax values correlate with the molecular weight. The P1/2 values correlate with the spectral linewidth and are additionally influenced by the type of substituents neighboring the nitroxide group. The nitroxide biradicals with high intramolecular spin-spin coupling show low performance. Nitroxides enriched with 15N and/or 2H afford significantly higher |Emax| and require lower power to do so, compared to their unmodified counterparts containing at natural abundance predominantly 14N and 1H. The results allow for a correlation of chemical features with physical hyperpolarization-related properties and indicate that small nitroxides with narrow spectral lines have clear advantages for the use in Overhauser dynamic nuclear polarization experiments. Perdeuteration and 15N-labeling can be used to additionally boost the spin probe performance.

UR - http://www.scopus.com/inward/record.url?scp=85117116119&partnerID=8YFLogxK

U2 - 10.1063/5.0064342

DO - 10.1063/5.0064342

M3 - Article

C2 - 34654311

AN - SCOPUS:85117116119

VL - 155

SP - 1ENG

JO - Journal of Chemical Physics

JF - Journal of Chemical Physics

SN - 0021-9606

IS - 14

M1 - 144203

ER -