T. R. Carver

Bio: T. R. Carver is an academic researcher from University of Illinois at Urbana–Champaign. The author has contributed to research in topics: Spins. The author has an hindex of 1, co-authored 1 publications receiving 480 citations.

Dynamic nuclear polarization at high magnetic fields

Abstract: Dynamic nuclear polarization (DNP) is a method that permits NMR signal intensities of solids and liquids to be enhanced significantly, and is therefore potentially an important tool in structural and mechanistic studies of biologically relevant molecules. During a DNP experiment, the large polarization of an exogeneous or endogeneous unpaired electron is transferred to the nuclei of interest (I) by microwave (microw) irradiation of the sample. The maximum theoretical enhancement achievable is given by the gyromagnetic ratios (gamma(e)gamma(l)), being approximately 660 for protons. In the early 1950s, the DNP phenomenon was demonstrated experimentally, and intensively investigated in the following four decades, primarily at low magnetic fields. This review focuses on recent developments in the field of DNP with a special emphasis on work done at high magnetic fields (> or =5 T), the regime where contemporary NMR experiments are performed. After a brief historical survey, we present a review of the classical continuous wave (cw) DNP mechanisms-the Overhauser effect, the solid effect, the cross effect, and thermal mixing. A special section is devoted to the theory of coherent polarization transfer mechanisms, since they are potentially more efficient at high fields than classical polarization schemes. The implementation of DNP at high magnetic fields has required the development and improvement of new and existing instrumentation. Therefore, we also review some recent developments in microw and probe technology, followed by an overview of DNP applications in biological solids and liquids. Finally, we outline some possible areas for future developments.

Dynamic nuclear polarization surface enhanced NMR spectroscopy.

Abstract: Many of the functions and applications of advanced materials result from their interfacial structures and properties. However, the difficulty in characterizing the surface structure of these materials at an atomic level can often slow their further development Solid-state NMR can probe surface structure and complement established surface science techniques, but its low sensitivity often limits its application. Many materials have low surface areas and/or low concentrations of active/surface sites. Dynamic nuclear polarization (DNP) is one Intriguing method to enhance the sensitivity of solid-state NMR experiments by several orders of magnitude. In a DNP experiment, the large polarization of unpaired electrons is transferred to surrounding nuclei, which provides a maximum theoretical DNP enhancement of similar to 658 for H-1 NMR. In this Account, we discuss the application of DPIP to enhance surface NMR signals, an approach known as DNP surface enhanced NMR spectroscopy (DNP SENS). Enabling DNP for these systems requires bringing an exogeneous radical solution into contact with surfaces without diluting the sample. We proposed the incipient wetness impregnation technique (IWI) a well-known method in materials science, to impregnate porous and particulate materials with just enough radical containing solution to fill the porous volume. IWI offers several advantages: it is extremely simple, provides a uniform wetting of the surface, and does not increase the sample volume or substantially reduce the concentration of the sample. This Account describes the basic principles behind DNP SENS through results obtained for mesoporous and nanoparticulate samples impregnated with radical solutions. We also discuss the quantification of the overall sensitivity enhancements obtained with DNP SENS and compare that with ordinary room temperature NMR spectroscopy. We then review the development of radicals and solvents that give the best possible enhancements today. With the best polarizing mixtures, DNP SENS enhances sensitivity by a factor of up to 100, which decreases acquisition time by five orders of magnitude. Such enhancement enables the detailed and expedient atomic level characterization of the surfaces of complex materials at natural isotopic abundance and opens new avenues for NMR. To illustrate these improvements, we describe the successful application of DNP SENS to characterize hybrid materials, organometallic surface species, and metal-organic frameworks.

TOTAPOL: a biradical polarizing agent for dynamic nuclear polarization experiments in aqueous media.

Abstract: In a previous publication, we described the use of biradicals, in that case two TEMPO molecules tethered by an ethylene glycol chain of variable length, as polarizing agents for microwave driven dynamic nuclear polarization (DNP) experiments. The use of biradicals in place of monomeric paramagnetic centers such as TEMPO yields enhancements that are a factor of approximately 4 larger (e ∼ 175 at 5 T and 90 K) and concurrently the concentration of the polarizing agent is a factor of 4 smaller (10 mM electron spins), reducing the residual electron nuclear dipole broadening. In this paper we describe the synthesis and characterization by EPR and DNP/NMR of an improved polarizing agent 1-(TEMPO-4-oxy)-3-(TEMPO-4-amino)propan-2-ol (TOTAPOL). Under the same experimental conditions and using 2.5 mm magic angle rotors, this new biradical yields larger enhancements (e ∼ 290) at lower concentrations (6 mM electron spins) and has the additional important property that it is compatible with experiments in aqueous medi...

Surface Enhanced NMR Spectroscopy by Dynamic Nuclear Polarization

Abstract: It is shown that surface NMR spectra can be greatly enhanced using dynamic nuclear polarization. Polarization is transferred from the protons of the solvent to the rare nuclei (here carbon-13 at natural isotopic abundance) at the surface, yielding at least a 50-fold signal enhancement for surface species covalently incorporated into a silica framework.