Characterization of biochars by nuclear magnetic resonance

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RMN haute résolution de molécules paramagnétiques à l'état solide

Cette thèse porte sur la RMN haute résolution de composés paramagnétiques solides. Elle présente de nouvelles méthodes permettant d'obtenir de tels spectres, ainsi que l'exploitation des spectres ainsi obtenus pour la caractérisation structurale de complexes paramagnétiques. Nous avons dans un premier temps développé des méthodes qui permettent de surmonter les difficultés liées au paramagnétisme. Ces méthodes inclues du transfert d'aimantation par recouplage dipolaire, l'adaptation d'impulsions adiabatiques pour les solides en rotation à très haute vitesse et la compréhension des phénomènes mis en cause dans leur efficacité. Nous présentons ensuite l'exploitation des données ainsi obtenues. Notamment pour la caractérisation de la structure électronique d'un catalyseur à base de fer à haut spin, la démonstration de l'absence de relaxation dite "de Curie" dans les solides ainsi que le développement d'un nouvel outil de cristallographie par RMN de poudres.This thesis is about high-resolution solid-state NMR of paramagnetic molecules. It exposes new methods to obtain high-resolution NMR spectra of paramagnetic solids. These methods gave us access to the structural information born by the electronic paramagnetism. In the first part, we propose new tools to overcome the difficulties associated with NMR of paramagnetic solids . These methods include proton to carbon magnetization transfer via dipolar recoupling, the use of adiabatic pulses with paramagnetic solids rotating at high MAS speeds, the development of a theory for a better understanding of the physics of such pulses. The second part exposes the interpretation of the high quality spectra obtained throught those methods. We characterized the electronic structure of high-spin iron (II) catalyst, we tackled the absence of the so-called "Curie relaxation" mechanism in the solid-state an we developed a new tool for crystallography thanks to proton NMR of paramagnetic powedrs.LYON-ENS Sciences (693872304) / SudocSudocFranceF

Chemo-Mechanical Couplings characterisation by Nuclear Magnetic Resonance in Lime-Treated Soils

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Fast adiabatic pulses for solid-state NMR of paramagnetic systems

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Absence of curie relaxation in paramagnetic solids yields long H-1 coherence lifetimes

NMR of paramagnetic systems in solution is hampered by relaxation enhancement (PRE). Here we present a method to access paramagnetic line widths in microcrystalline samples under magic angle spinning. We demonstrate that a CPMG technique may advantageously be combined with fast MAS of paramagnetic solids and suitable adiabatic pulses both to provide increased sensitivity and to allow experimental determination of the homogeneous coherence lifetimes T (2)'. In this way, we show that the Curie contribution to PRE is absent in solids and, therefore, that the lifetimes of nuclear coherences may be longer than in liquids for paramagnetic systems, even for protons

Absence of Curie Relaxation in Paramagnetic Solids Yields Long 1 H Coherence Lifetimes

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Probing the Dynamics of Layered Double Hydroxides by Solid-State 27 Al NMR Spectroscopy

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Utilisation de la RMN du solide paramagnétique pour déterminer le tenseur de susceptibilité magnétique local

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Utilization of solid-state NMR to determine the local magnetic susceptibility

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Superadiabaticity in magnetic resonance

Adiabaticity plays a central role in modern magnetic resonance experiments, as excitations with adiabatic Hamiltonians allow precise control of the dynamics of the spin states during the course of an experiment. Surprisingly, many commonly used adiabatic processes in magnetic resonance perform well even though the adiabatic approximation does not appear to hold throughout the process. Here we show that this discrepancy can now be explained through the use of Berry's superadiabatic formalism, which provides a framework for including the finite duration of the process in the theoretical and numerical treatments. In this approach, a slow, but finite time-dependent Hamiltonian is iteratively transformed into time-dependent diagonal frames until the most accurate adiabatic approximation is obtained. In the case of magnetic resonance, the magnetization during an adiabatic process of finite duration is not locked to the effective Hamiltonian in the conventional adiabatic frame, but rather to an effective Hamiltonian in a superadiabatic frame. Only in the superadiabatic frame can the true validity of the adiabatic approximation be evaluated, as the inertial forces acting in this frame are the true cause for deviation from adiabaticity and loss of control during the process. Here we present a brief theoretical background of superadiabaticity and illustrate the concept in the context of magnetic resonance with commonly used shaped radio-frequency pulses