Etude de la structure de matériaux paramagnétiques par RMN du solide.

Unravelling the structure of paramagnetic materials by solid-state NMR.

• Auteur / Autrice : Gabriel Balavoine
• Direction : Andrew Pell
• Type : Projet de thèse
• Discipline(s) : Chimie
• Date : Inscription en doctorat le 01/10/2022
• Etablissement(s) : Lyon, École normale supérieure
• Ecole(s) doctorale(s) : Chimie de Lyon
• Partenaire(s) de recherche : Laboratoire : CRMN - CENTRE DE RMN À TRÈS HAUTS CHAMPS DE LYON

Mots clés

Mots clés libres

Rmn

Matériaux

Paramagnétisme

Calculs quantiques

Modélisation

Nmr

Materials

Paramagnetism

Quantum computations

Modelling

Résumé

My PhD is about the study of paramagnetic materials using solid-state NMR spectroscopy and quantum chemistry computations with Pr Andrew J. Pell. Paramagnetic NMR is challenging because the coupling between the nuclear and the electronic spins strongly affects the NMR spectra. Therefore, the computations offer a complementary study to the experiment and can enable the assignment and interpretation of the signals. During the first year, we have mainly worked on the computational study of cerium-based photoanode CeTiNO2. The averaged structure was determined by powder X-ray diffraction (XRD) but the nitrogen and oxygen anions cannot be differentiated. Solid-state 14N (spin I=1) NMR is a complementary technique for probing the local environment(s) of the nitrogen ions. The solid-state NMR data currently available in the literature shows neither a quadrupolar broadening nor a paramagnetic shift of the signal and hypothesized not to correspond to material at all, but rather to an impurity that is easier to detect. In order to test our hypothesis, we have computed the paramagnetic shift and quadrupolar parameters, to whether or not they are consistent with the currently available data. These computations are challenging on several levels. First, because of the strong correlation effects and strong relativistic effects in lanthanide ions, density functional theory (DFT) based methods have comparatively poor accuracy for predictions of the electric and magnetic properties. Moreover, the studied system is an infinite periodic solid whereas multireference methods are only available with molecular codes. The computations were performed on a single cerium molecular model complex extracted from the crystallographic structure. The lowest-energy anion distributions were determined using a Monte-Carlo simulation. The electronic g-tensor of the cerium ions was determined with the Spin-Orbit Complete Active Space Self-Convergence Field (SO-CASSCF) multireference method that considers the degeneracy of the 4f orbitals manifold. The SO-CASSCF magnetic susceptibility fits the available experimental data well. Hyperfine coupling tensors were obtained with DFT or with CASSCF in order to evaluate the contribution of the excited states. The 14N paramagnetic shifts were computed from these two tensors and are in the range of 100-200 ppm. Therefore, the computed parameters are not consistent with the experimental data, so a new sample will be synthetized and new NMR experiments with improved method will be performed.