Président du jury Buess Herman, Claudine
Promoteur Liévin, Jacques
Publication Non publié, 2014-11-24
| Résumé : | This thesis is dedicated to the theoretical study of the methyl cation CH3+ electronic states and, globally, falls within the study of the interstellar clouds molecular synthesis and the chemistry of the hydrocarbons which are present in high-energy plasmas such as in the experimental nuclear fusion reactor ITER. Among the different possible formation reactions, we chose two reactions involved in these fields: the ionization of the methyl radical CH3 ground state and the reactive collisions between simple carbonated or hydrocarbonated (C, CH+ and CH2+) and hydrogen species (H3+, H2 and H, respectively). As this cation is characterized by 8 electrons and 4 nuclei, this allowed us to perform high-level ab initio calculations using the CASSCF/MRCI method with the Dunning aug-cc-p(C)VXZ basis sets. These calculations were completed by a study of different methodological effects such as the core-valence electronic correlation, the complete basis set extrapolation and the basis set superposition error. We calculated equilibrium geometries (precision within 10^-5 angtröm and 10^-2° and their energies for the methyl radical and cation, studied the potential energy surfaces involved by the Jahn-Teller effect targeting the methyl cation E' states and achieved frequencies calculations. From these values were derived ionization potentials (IP) from the methyl radical ground state towards the methyl cation lowest-lying states (precision within 10^-2 eV). Vibrational corrections and nuclear relaxation effects were taken into account. The calculated IPs and frequencies should facilitate the analysis of methyl radical threshold photoelectron spectra leading to the methyl cation lowest-lying states, recorded at the synchrotron facility Soleil by the Dr. Alcaraz group from the Université Paris-Sud. Through reaction paths calculations using a quadratic steepest-descent method, we have proposed new reactional pathways enabling the connections between the different dissociation channels correlated to the cation lowest-lying triplet states. The absence of potential barriers in the energy profiles allows us to propose these reactions as sources, in interstellar clouds, of hydrocarbonated molecules whose stability increases with growing size according to the series C -> CH+ -> CH2+ -> CH3+. The groups of Pr. Urbain from UCL and Dr. Savin from Columbia University studied the collisions in copropagating beams of C and H3+ leading to the formation of these hydrocarbonated species. The proposed reactional mechanisms are in good agreement with the experimental observations, what permits a better understanding of the chemistry behind these collisional processes of astrochemical interest. |
