A new polarizing agent with superior performance in dynamic nuclear polarization experiments is introduced, and utilizes two TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) moieties connected through a rigid spiro tether (see structure). The observed NMR signal intensities were enhanced by a factor of 1.4 compared to those of TOTAPOL, a previously described TEMPO-based biradical with a flexible tether.

Dynamic nuclear polarization with a rigid biradical

Rapid progress has been witnessed in the past decade in the fields of covalent organic frameworks (COFs) and dynamic nuclear polarization (DNP). In this contribution, we bridge these two fields by constructing radical-embedded COFs as promising DNP agents. Via polarization transfer from unpaired electrons to nuclei, DNP realizes significant enhancement of NMR signal intensities. One of the crucial issues in DNP is to screen for suitable radicals to act as efficient polarizing agents, the basic criteria for which are homogeneous distribution and fixed orientation of unpaired electrons. We therefore envisioned that the crystalline and porous structures of COFs, if evenly embedded with radicals, may work as a new “crystalline sponge” for DNP experiments. As a proof of concept, we constructed a series of proxyl-radical-embedded COFs (denoted as PR(x)-COFs) and successfully applied them to achieve substantial DNP enhancement. Benefiting from the bottom-up and multivariate synthetic strategies, proxyl radicals ...

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Exploring Applications of Covalent Organic Frameworks: Homogeneous Reticulation of Radicals for Dynamic Nuclear Polarization

Dynamic Nuclear Polarization Surface Enhanced NMR spectroscopy has been demonstrated to significantly improve NMR sensitivity on materials by 1 to 2 orders of magnitude at high magnetic fields. The preferential surface enhancement also allows for selectively probing the solid surface. In this review, we will briefly describe the main mechanism used nowadays, i.e. cross effect, for DNP enhanced solid-state NMR. We will show the typical protocols of sample formulation leading to effective DNP surface enhancements and the key experimental factors in performing DNP SENS experiments. Other important developments in DNP, i.e. shielded polarizing agents for reactive surfaces, hyperpolarizing solid matrices, and high-temperature and high-field DNP, will be discussed as well. Finally, we close the review with a short summary and our perspectives on the directions of future developments in this field.

Dynamic Nuclear Polarization Surface Enhanced NMR spectroscopy (DNP SENS): Principles, protocols, and practice

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.

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

The invention provides a method for measuring alkali metal atomic polarizability of a nuclear magnetic resonance gyro in real time and belongs to the field of atomic physics. The method includes: measuring alkali metal atomic densities of an atomic pool at different temperatures, measuring atomic nuclear magnetic resonance frequency of inert gas caused by polarization of alkali metal atoms, and thus acquiring polarizability of the alkali metal atoms in the atomic pool without changing the optical path structure of the nuclear magnetic resonance gyro and the magnetic field environment; building a three-dimensional model of changes in nuclear magnetic resonance frequency of the inert gas in the atomic pool along with temperature and the alkali metal atomic polarizability, measuring the frequency shift of atomic NMR (nuclear magnetic resonance) of the inert gas at any temperature point while the nuclear magnetic resonance frequency normally runs, and thus calculating the polarizability of the alkali metal atoms in the atomic pool in real time. The method has the advantages that the method is simple, no influence is caused to the optical path structure of the nuclear magnetic resonance gyro and the method is of great significance to improving the performance of the nuclear magnetic resonance gyro.

Method for measuring alkali metal atomic polarizability of nuclear magnetic resonance gyro in real time

The present invention concerns materials consisting in a porous and structured network, this network being at least in part formed by Si atoms, or Si atoms and metal atoms, linked to each other's via siloxy bonds, the amount of radical ranging from 0.50 to 0.03 mmol of radical per gram of material, and in that the network is formed with a sol-gel step using an organosilane for the introduction of the organic molecules allowing their regular distribution within the porous structured material. The invention also concern a process for the preparation of such material and a method of analysis by Nuclear Magnetic Resonance (NMR) of an analyte wherein it uses dynamic nuclear polarization generated with a material according to the invention.

Porous and structured materials for dynamic nuclear polarization, process for their preparation and nmr analysis method

Hyperpolarization of NMR-active nuclei is key to gather high quality spectra of rare species and insensitive isotopes. We have recently established that silica-based materials containing regularly distributed nitroxyl radicals connected to the silica matrix by flexible linkers can serve as promising polarization matrices for dynamic nuclear polarization (DNP). Here we investigate the influence of the linker on the efficiency of the polarization. The materials were fully characterized and exhibit high surface areas and narrow pore size distributions with a tunable amount of phenyl azide groups over a broad range of concentrations. The phenyl azide groups can be easily functionalized via a two-step procedure with 4-carboxy-2,2,6,6-tetramethyl-1-oxylpiperidine (TEMPO) to give polarizing matrices with controllable radical content. The DNP efficiency was found to be similar as in materials with flexible linkers, both for magic angle spinning at 105 K and dissolution DNP at 4 K.

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Phenylazide Hybrid-Silica – Polarization Platform for Dynamic Nuclear Polarization at Cryogenic Temperatures

Non aqueous solvents or mixtures for DNP NMR spectroscopy, method to prepare said solvents or mixtures and use of said solvents or mixtures The present invention concerns a method to determine the efficiency of a solvent to impregnate a sample material for use in a dynamic nuclear polarization (DNP) nuclear magnetic resonance (NMR) experiments, characterized in that the method comprising: —selecting a non-aqueous solvent, —providing a polarizing agent that is soluble in the non-aqueous solvent, —dissolving the polarizing agent in the non-aqueous solvent, —impregnating or dissolving the sample material with the non-aqueous solvent containing the polarizing agent, —performing a solid state DNP NMR experiment on the impregnated sample material, —determining a DNP enhancement factor of the DNP NMR experiment

Non aqueous solvents or mixtures for dnp nmr spectroscopy, method to prepare said solvents or mixtures and use of said solvents or mixtures

The present invention concerns a method to determine the efficiency of a solvent to impregnate a sample material for use in a dynamic nuclear polarization (DNP) nuclear magnetic resonance (NMR) experiments, characterized in that the method comprising: - selecting a non-aqueous solvent, - providing a polarizing agent that is soluble in the non-aqueous solvent, - dissolving the polarizing agent in the non-aqueous solvent, - impregnating or dissolving the sample material with the non-aqueous solvent containing the polarizing agent, - performing a solid state DNP NMR experiment on the impregnated sample material, - determining a DNP enhancement factor of the DNP NMR experiment.

David Gajan

Papers

Porous and structured materials for dynamic nuclear polarization, process for their preparation and nmr analysis method

Phenylazide Hybrid-Silica – Polarization Platform for Dynamic Nuclear Polarization at Cryogenic Temperatures

Non aqueous solvents or mixtures for dnp nmr spectroscopy, method to prepare said solvents or mixtures and use of said solvents or mixtures

DNP NMR spectroscopy with non-aqueous solvents