La dissociation unimoléculaire de systèmes chimiques faiblement liés (molécules de van der Waals, HeBr2, NeBr2, ArBr2) et covalents (NCO, CH2CO) est étudiée au moyen de la méthode des trajectoires classiques et d’une approche statistique basée sur la théorie de l’état de transition. D’une part, l’apport de la procédure de pondération gaussienne appliquée à la détermination de distribution ro-vibrationnelles et d’énergie de translation, est illustrée. D’autre part, une transformation des coordonnées angle-action aux coordonnées cartésiennes, dans le cas général de fragments polyatomiques, ainsi que qu’une correction quasi-classique permettant d’incorporer la structure rotationnelle aux distributions résolues vibrationnellement, sont développées. Sur la base de ces développements, une méthodologie applicable aux processus polyatomiques indirects conduisant à une résolution ro-vibrationnelle complète sans nécessiter de procédures de pondération ou de binning, est présentée. / The quasi-classical trajectory method and statistical assumptions from the transition state theory are employed in the investigation of the unimolecular dissociation of weakly (van der Waals aggregates, i.e. HeBr2, NeBr2, ArBr2) and conventionally bound molecular systems (NCO, CH2CO). The capabilities of the Gaussian weighting procedure are illustrated through the reproduction of ro-vibrational and translational energy distributions. A transformation from angle-action variables to Cartesian coordinates is derived for the general case of polyatomic fragments. An alternative methodology is developed to study indirect polyatomic processes which provides complete ro-vibrational resolution while effectively avoids any binning or weighting procedure. The new algorithm is based on the transformation previously derived and an approximate formula developed to incorporate the rotational structures on the vibrationally-resolved quasi-classical distributions / El método de trayectorias cuasi-clasicas e hipotesis estadisticas de la teoria del estado de transicion, son empleados en la investigacion de la disociacion unimolecular de sistemas débil (agregados de van der Waals, i.e. HeBr2, NeBr2, ArBr2) y convencionalmente enlazados (NCO, CH2CO). Se demuestran las potencialidades del procedimiento de ponderacion gaussiana para la reproduccion de distribuciones ro-vibracionales y de energia de traslacion, en comparacion con las obtenidas por los métodos convencionales. Son desarrolladas la transformacion desde variables angulares y de accion a coordenadas cartesianas para el caso general de fragmentos poliatomicos, asi como un conjunto de expresiones cuasi-clasicas para incorporar estruturas rotacionales en las distribuciones resueltas vibracionalmente. Sobre la base de estos desarrollos, se demuestra y aplica una nueva metodologia para estudiar procesos poliatomicos indirectos con la que resulta posible obtener distribuciones téoricas con resolucion ro-vibracional total, evitando el uso de métodos de binning o ponderaciones.

The present work aims at studying the coupling between hydrodynamic fingering instabilities and chemical reactions at the interface between two miscible solutions. Hydrodynamic deformations of interfaces between two reactive fluids as well as flows induced by chemical reactions at the front between two initially steady fluids are encountered frequently in combustion, petroleum, chemical and pharmaceutical engineering. Most of the time, concrete applications imply a very large number of variables so that an understanding of the fundamental processes of chemo-hydrodynamic coupling is out of reach. Our goal is here to analyze a much simpler model system in which only one mechanism of hydrodynamic instability is at play and for which the chemical reactions can be modeled by a one or two-variable model. <p><p>Buoyantly unstable, autocatalytic chemical fronts, are one such model system, which can be used as prototype to study the effects of the coupling between chemical reactions and hydrodynamic fingering instabilities. Fingering processes occur whenever a fluid of high mobility displaces a less mobile one in a porous medium. The initially planar interface looses then stability and a cellular fingering deformation of the interface is observed. Such an instability has been observed, for instance, in the iodate-arsenous acid and chlorite-tetrathionate reactions, autocatalytic redox reactions known to produce a change of density across a traveling front. Fingering happens there when the heavier solution lies on top of the lighter one in the gravity field. <p> <p>Our theoretical contribution to the analysis of fingering of chemical fronts focuses on different points which we detail in this thesis along the following outline. In chapter 2, we introduce fingering phenomena occurring in porous media and distinguish the situation of viscous and density fingering of pure non reactive fluids. Chapter 3 reviews the literature on coupling between fingering and chemical reactions before studying the linear stability conditions as well as nonlinear dynamics of density fingering of isothermal iodate-arsenous acid fronts. This prototype nonlinear redox reaction is the first one on which experimental results on fingering in spatially extended set-ups have been obtained. We next analyze in chapter 4 the density fingering of another front producing autocatalytic system i.e. the chlorite-tetrathionate reaction in order to address the influence of the chemical kinetics on the dynamics observed. The influence of the exothermicity of the reaction is then presented in chapter 5. Eventually, chapter 6 analyzes what happens if the kinetics is now bistable and further compares the situation of both viscous and density fingering of bistable fronts. We then conclude and present suggestions for future work in this subject at the frontier between nonlinear chemistry, hydrodynamics and engineering. <p> / Agrégation de l'enseignement supérieur, Orientation sciences / info:eu-repo/semantics/nonPublished