BDPA-Nitroxide Biradicals Tailored for Efficient Dynamic Nuclear Polarization Enhanced Solid-State NMR at Magnetic Fields up to 21.1 T

Dynamic nuclear polarization (DNP) solid-state nuclear magnetic resonance (NMR) has developed into an invaluable tool for the investigation of a wide range of materials. However, the sensitivity gain achieved with many polarizing agents suffers from an unfavorable field and magic angle spinning (MAS) frequency dependence. We present a series of new hybrid biradicals, soluble in organic solvents, that consist of an isotropic narrow electron paramagnetic resonance line radical, alpha,gamma-isdiphenylene-beta-phenylallyl (BDPA), tethered to a broad line nitroxide. By tuning the distance between the two electrons and the substituents at the nitroxide moiety, correlations between the electron-electron interactions and the electron spin relaxation times on one hand and the DNP enhancement factors on the other hand are established. The best radical in this series has a short methylene linker and bears bulky phenyl spirocyclohexyl ligands. In a 1.3 mm prototype DNP probe, it yields enhancements of up to 185 at 18.8 T (800 MHz H-1 resonance frequency) and 40 kHz MAS. We show that this radical gives enhancement factors of over 60 in 3.2 mm sapphire rotors at both 18.8 and 21.1 T (900 MHz H-1 resonance frequency), the highest magnetic field available today for DNP. The effect of the rotor size and of the microwave irradiation inside the MAS rotor is discussed. Finally, we demonstrate the potential of this new series of polarizing agents by recording high field Al-27 and Si-29 DNP surface enhanced NMR spectra of amorphous aluminosilicates and O-17 NMR on silica nanoparticles.

TinyPols: a family of water-soluble binitroxides tailored for dynamic nuclear polarization enhanced NMR spectroscopy at 18.8 and 21.1 T

David Lyndon Emsley, David Benjamin Roger Antoine Gajan, Georges Guévork Jean-Jacques Menzildjian, Gabriele Stevanato

Dynamic Nuclear Polarization (DNP) has recently emerged as a key method to increase the sensitivity of solid-state NMR spectroscopy under Magic Angle Spinning (MAS). While efficient binitroxide polarizing agents such as AMUPol have been developed for MAS DNP NMR at magnetic fields up to 9.4 T, their performance drops rapidly at higher fields due to the unfavorable field dependence of the cross-effect (CE) mechanism and AMUPol-like radicals were so far disregarded in the context of the development of polarizing agents for very high-field DNP. Here, we introduce a new family of water-soluble binitroxides, dubbed TinyPols, which have a three-bond non-conjugated flexible amine linker allowing sizable couplings between the two unpaired electrons. We show that this adjustment of the linker is crucial and leads to unexpectedly high DNP enhancement factors at 18.8 T and 21.1 T: an improvement of about a factor 2 compared to AMUPol is reported for spinning frequencies ranging from 5 to 40 kHz, with epsilon(H) of up to 90 at 18.8 T and 38 at 21.1 T for the best radical in this series, which are the highest MAS DNP enhancements measured so far in aqueous solutions at these magnetic fields. This work not only breathes a new momentum into the design of binitroxides tailored towards high magnetic fields, but also is expected to push the application frontiers of high-resolution DNP MAS NMR, as demonstrated here on a hybrid mesostructured silica material.

ROYAL SOC CHEMISTRY2020

Chemical Developments in DNP-NMR

Pascal Miéville

The Nuclear Magnetic Resonance (NMR) is an extraordinary tool to understand molecular structures. Its main limitation resides in its relatively low sensitivity when compared with other physico-chemical methods such as Mass Spectrometry (MS) or Optical Spectroscopy. Dynamic Nuclear Polarization (DNP), invented in the 50's but only widely applied in the last ten years, allows, by transferring the high polarization of paramagnetic centers to surrounding nuclei at low temperature, to enhance NMR signals by several orders of magnitude. This technique can be applied to solids at 100 K using Magic Angle Spinning (DNP-MAS) or in liquid phase through Dissolution-DNP as described by Ardenkjaer-Larsen in 2003. The efficiency of the polarization transfer is strongly dependent on the nature of the paramagnetic centers and on the way they are mixed with the target molecules. Part of this work will consist in the study of new ways to integrate paramagnetic centers into the matrix and also to limit their negative effects on NMR signals in liquid phase after dissolution. Because DNP-NMR has reached a stage where it should become a standard chemical method, a second objective of this thesis is to propose new chemical applications such as kinetic measurements that may be of interest in other domain of chemistry.

EPFL2011

Tailored Polarizing Hybrid Solids with Nitroxide Radicals Localized in Mesostructured Silica Walls

Mathieu Baudin, David Baudouin, Pierrick Berruyer, Geoffrey Bodenhausen, Aurélien Bornet, Matthieu Cavaillès, David Lyndon Emsley, David Benjamin Roger Antoine Gajan, Sami Jannin, Basile Vuichoud

Hyperpolarization by dynamic nuclear polarization relies on the microwave irradiation of paramagnetic radicals dispersed in molecular glasses to enhance the nuclear magnetic resonance (NMR) signals of target molecules. However, magnetic or chemical interactions between the radicals and the target molecules can lead to attenuation of the NMR signal through paramagnetic quenching and/or radical decomposition. Here we describe polarizing materials incorporating nitroxide radicals within the walls of the solids to minimize interactions between the radicals and the solute. These materials can hyperpolarize pure pyruvic acid, a particularly important substrate of clinical interest, while nitroxide radicals cannot be used, even when incorporated in the pores of silica, because of reactions between pyruvic acid and the radicals. The properties of these materials can be engineered by tuning the composition of the wall by introducing organic functionalities.

2017