Probing local magnetic structure with high-resolution solid-stateNMR of paramagnetic species

Le but de ce travail est de mettre en place un nouvel outil de détermination experimentale du tenseur de susceptibilité magnétique locale. Nous avons travaillé sur une série isostructurale d'oxalate de lanthanide [Ln2(C2O4)3, 9,5H2O] avec Ln = La, Ce, Pr, Nd, grâce à la RMN du solide paramagnétique afin de démontrer la faisabilité de ce type d'étude par RMN des solides. Pour déterminer la susceptibilité magnétique locale, nous proposons une approche semi-empirique basée sur un modèle de dipôle ponctuel paramétré pour le magnétisme local. Le modèle est basé sur la connaissance de la structure cristalline et l'approximation empirique du dipôle ponctuel pour chaque centre paramagnétique de la structure. L'interaction hyperfine dipolaire est calculée pour chaque noyau observable par RMN en additionnant l'effet de chaque centre paramagnétique sur le spectre RMN dans le rayon de convergence de notre modèle. Le spectre résultant est ensuite comparé aux données expérimentales et les paramètres du modèle sont optimisés pour obtenir le meilleur ajustement. La rapidité de ce calcul semi-empirique ouvre des portes en termes de mesures rendues accessibles par cette méthode. Elle nous a par exemple permis d'effectuer une analyse statistique de chaque paramètre du tenseur de susceptibilité magnétique locale. Nous avons comparé nos résultats avec des mesures de la susceptibilité magnétique macroscopique obtenues grâce à un SQUID et nous avons trouvé un excellent accord entre cette méthode et la nôtre pour la composante isotrope de cette mesure. Nous avons exploré d'autres possibilités offertes par cette méthode en nous intéressant à la possibilité d'accéder aux composantes diamagnétiques du tenseur de déplacement chimique en faisant varier la température, de nos échantillons. Ou encore, nous avons étudié des systèmes désordonnés afin de pousser notre modèle vers des systèmes plus difficiles à atteindre par des méthodes de diffraction.The purpose of this work is to try and set up a new tool for the experimental determination of local magnetic susceptibility tensor. We have worked on an isotructural series of lanthanide oxalate [Ln2(C2O4)3, 9,5H2O] with Ln = La, Ce, Pr, Nd, using NMR of the paramagnetic solid in order to show thefeasability of this kind of studies by solid-state NMR.To determine the local magnetic susceptibility, we propose a semi-empirical approach based on a parameterized point dipole model for local magnetism. The model is based on the knowledge of the crystal structure and the empirical approximation of the point dipole for each paramagnetic center in the structure. The hyperfine dipole interaction is calculated for each NMR observable nucleus by summing the effect of each paramagnetic center on the NMR spectrum within the radius of convergence of our model. The resulting spectrum is then compared to the experimental data and the model parameters are optimized to obtain the best fit.The fact that this semi-empirical model is very fast to calculate opens a wide panel of accessible measurements through this method. For instance, we were able to perform a statistical analysis of each parameter of the local magnetic susceptibility tensor.We have compared our results with SQUID measurements of the macroscopic susceptibility and it showed a good agreement between the isotropic component obtained by both measurements.We explored several possibilities offered by this method in order to access the diamagnetic components of the chemical shift tensor by changing the temperature of our samples. We also tried to push ourstudies towards disordered systems for which diffraction methods have more difficulties to access local magnetic informations

Étude du magnétisme moléculaire local par RMN des solides paramagnétiques à haute résolution

The purpose of this work is to try and set up a new tool for the experimental determination of local magnetic susceptibility tensor. We have worked on an isotructural series of lanthanide oxalate [Ln2(C2O4)3, 9,5H2O] with Ln = La, Ce, Pr, Nd, using NMR of the paramagnetic solid in order to show thefeasability of this kind of studies by solid-state NMR.To determine the local magnetic susceptibility, we propose a semi-empirical approach based on a parameterized point dipole model for local magnetism. The model is based on the knowledge of the crystal structure and the empirical approximation of the point dipole for each paramagnetic center in the structure. The hyperfine dipole interaction is calculated for each NMR observable nucleus by summing the effect of each paramagnetic center on the NMR spectrum within the radius of convergence of our model. The resulting spectrum is then compared to the experimental data and the model parameters are optimized to obtain the best fit.The fact that this semi-empirical model is very fast to calculate opens a wide panel of accessible measurements through this method. For instance, we were able to perform a statistical analysis of each parameter of the local magnetic susceptibility tensor.We have compared our results with SQUID measurements of the macroscopic susceptibility and it showed a good agreement between the isotropic component obtained by both measurements.We explored several possibilities offered by this method in order to access the diamagnetic components of the chemical shift tensor by changing the temperature of our samples. We also tried to push ourstudies towards disordered systems for which diffraction methods have more difficulties to access local magnetic informations.Le but de ce travail est de mettre en place un nouvel outil de détermination experimentale du tenseur de susceptibilité magnétique locale. Nous avons travaillé sur une série isostructurale d'oxalate de lanthanide [Ln2(C2O4)3, 9,5H2O] avec Ln = La, Ce, Pr, Nd, grâce à la RMN du solide paramagnétique afin de démontrer la faisabilité de ce type d'étude par RMN des solides. Pour déterminer la susceptibilité magnétique locale, nous proposons une approche semi-empirique basée sur un modèle de dipôle ponctuel paramétré pour le magnétisme local. Le modèle est basé sur la connaissance de la structure cristalline et l'approximation empirique du dipôle ponctuel pour chaque centre paramagnétique de la structure. L'interaction hyperfine dipolaire est calculée pour chaque noyau observable par RMN en additionnant l'effet de chaque centre paramagnétique sur le spectre RMN dans le rayon de convergence de notre modèle. Le spectre résultant est ensuite comparé aux données expérimentales et les paramètres du modèle sont optimisés pour obtenir le meilleur ajustement. La rapidité de ce calcul semi-empirique ouvre des portes en termes de mesures rendues accessibles par cette méthode. Elle nous a par exemple permis d'effectuer une analyse statistique de chaque paramètre du tenseur de susceptibilité magnétique locale. Nous avons comparé nos résultats avec des mesures de la susceptibilité magnétique macroscopique obtenues grâce à un SQUID et nous avons trouvé un excellent accord entre cette méthode et la nôtre pour la composante isotrope de cette mesure. Nous avons exploré d'autres possibilités offertes par cette méthode en nous intéressant à la possibilité d'accéder aux composantes diamagnétiques du tenseur de déplacement chimique en faisant varier la température, de nos échantillons. Ou encore, nous avons étudié des systèmes désordonnés afin de pousser notre modèle vers des systèmes plus difficiles à atteindre par des méthodes de diffraction

A facile method for fabrication of responsive micropatterned surfaces

Responsive micropatterned surfaces are fabricated using a facile, one-step method that allows for the separate control of topography and surface chemistry. Temperature responsive poly(N-isopropylacrylamide) (pNIPAAm), and amphiphilic poly(hydroxyethyl methacrylate-co-perfluorodecylacrylate) (p(HEMA-co-PFA)) polymer thin films are deposited on prestrained polydimethylsiloxane (PDMS) substrates using the initiated chemical vapor deposition (iCVD) technique. Subsequent release of the strain results in the formation of periodic wrinkle structures on the surface of polymer thin films. The iCVD technique allows control of the chemical composition while preserving the functional groups of the polymers intact. Surface topography is controlled separately by tuning elastic modulus of the polymer coatings and substrates. Highly ordered, well-defined wrinkle structures are obtained on pNIPAAm surfaces whereas wrinkles on the amphiphilic surfaces are less ordered due to the difference in elastic moduli of the polymers. Furthermore, process temperature is observed to have detrimental effects on the ordering of the wrinkles

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

International audienc

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

International audienc

Acquired epileptiform opercular syndrome: F-18 fluorodeoxyglucose positron emission tomography (FDG-PET) findings and efficacy of levetiracetam therapy

We report a five-year-old girl presenting with dysphagia, dysarthria, drooling, and generalized tonic convulsions in whom the final diagnosis was acquired epileptiform opercular syndrome. Levetiracetam monotherapy at a dosage of 40 mg/kg/day improved the clinical findings, and seizures were controlled at the end of the first month of treatment. Six months after the initial diagnosis, she presented with speech deterioration and dysarthria. At this time, although sleep and awake electroencephalography ( EEG) were normal, FDG-PET showed hypometabolic and hypermetabolic regions in the anterior/inferior and anterior regions of the right frontal lobe, respectively. By increasing before levetiracetam dosage to 50 mg/kg/day, the clinical findings resolved and the patient is still seizure free. Acquired epileptiform opercular syndrome is a rare epileptic disorder in which the seizures are resistant to conventional antiepileptic drugs. Levetiracetam may be an effective antiepileptic drug in controlling seizures and other clinical findings in acquired opercular epileptiform syndrome. Hypometabolic and hypermetabolic regions in FDG-PET study may be due to ongoing seizure activity or impaired glucose metabolism in this disorder. (c) 2012 Elsevier Inc. All rights reserved

Détermination d'un tenseur de susceptibilité magnétique local dans une poudre microcristalline de complexes paramagnétiques de lanthanides par analyse des anisotropies de déplacement chimique en RMN des solides.

International audienceExploring magnetic properties at the molecular level is a challenge that have been met by developing many experimental and theoretical solutions such as polarized neutron diffraction (PND), muon-spin rotation (µ-SR), electron paramagnetic resonance (EPR), SQUID-based magnetometry measurements and advanced modelling on openshell systems and relativistic calculations. These methods are powerful tools that shed light on the local magnetic response in specifically designed magnetic materials such as contrast agents, for MRI, molecular magnets, magnetic tags for biological NMR etc.All of these methods have their advantages and disadvantages. In order to complement the possibilities offered by these methods, we propose a new tool that implements a new approach combining simulation and fitting for high-resolution solid-state NMR spectra of lanthanide-based paramagnetic species.This method relies on a rigorous acquisition thanks to Short Highpower Adiabatic Pulses (SHAP) of high-resolution solid-state NMR isotropic and anisotropic data on a powdered magnetic material. It is also based on an efficient modelling of this data thanks to a semi-empirical model based on a parametrization of the local magnetism and the crystal structure provided by diffraction methods. The efficiency of the calculation relies on a thorough simplification of the electron-nucleus interactions (point-dipole interaction, no Fermi-contact) which is validated by experimental analysis. By taking advantage of the efficient calculation possibilities offered by our method, we can compare a great number of simulated spectra to experimental data and find the best-matching local magnetic susceptibility tensor. This method was applied to a series of isostructural lanthanide oxalates which are used as a benchmark system for many analytical methods. We present the results of thorough solid-state NMR and extensive modelling of the hyperfine interaction (including up to 400 paramagnetic centers) that yields local magnetic susceptibility tensors measurements that are self-consistent aswell as consistent with bulk susceptibility measurements