Thesis of Luna Cartayrade

Defense of thesis of Luna Cartayrade - laboratory PC2A

Abstract :

Selenium (Se), the third chalcogen, plays essential biological roles across organisms from fish to mammals. Its biogeochemical cycle has originally been restrained to soil and water, where it undergoes speciation, adsorption, and microbially-mediated redox transformations, that dictates its bioavailability and mobility between these two reservoirs. Recently, however, the detection of Se in precipitation has revealed a previously overlooked atmospheric pathway. This phenomenon paints a Se actively cycling through atmosphere rather than being confined to Earth’s floor compartments alone. This discovery has prompted a revision of the classical Se cycle model and has opened an emerging research field focused on atmospheric selenium sources, transport, and deposition, with important implications for how we assess Se fluxes at regional and global scales. This thesis aims to provide thermochemical and reactive insights of atmospheric relevance. First, thermochemical data (DfH°298K, S°298K, and Cp = f(T)) together with bond dissociation energies at 298 K were determined for five emitted organic Se species and their radicals, using computational chemistry combined with statistical thermodynamics at the CCSD(T)/aug-cc-pVTZ level. We then investigated the HO• radical-initiated transformations of dimethyl selenide, dimethyl diselenide, and methyl selenol at the M06-2X-D3/aug-cc-pVTZ level of theory. The thermodynamics, mechanisms, and kinetics of these reactions, as well as the atmospheric lifetimes of these species, were characterized over 253–323 K. We show how Se's reactivity compares to that of its sulfur analogs, in a field where Se is often assumed to behave like the second chalcogen. The key distinction lies in the faster oxidation of Se and its greater propensity to form weak bonds relative to S. These data help identify trends across the chalcogen series and provide input for chemical transport models constraining the fate of atmospheric Se.

Keywords : Selenium, Molecular modelisation, Atmosphere, Reactivity, Thermodynamics,