Durant la nuit, le radical nitrate (NO3) est le principal oxydant troposphérique des composés organiques. La compréhension de l'implication des composés organiques dans les processus de chimie troposphérique exige donc une connaissance des constantes cinétiques de leurs réactions avec le radical NO3. Toutefois, au regard du nombre considérable de composés organiques émis ou formés dans la troposphère, il est difficilement envisageable d'appréhender la réactivité de chaque composé en nous reposant exclusivement sur des études de laboratoire. Celles-ci se doivent d'être complétées par l'usage de méthodes prédictives. Nous avons donc, au cours de ce travail, développé une relation de type structure-réactivité (SAR) qui permet le calcul des constantes de vitesse des réactions des composés organiques avec le radical nitrate. Cette méthode prédictive empirique permet d'estimer la réactivité d'un composé à partir de sa structure moléculaire et a été élaborée à partir de constantes cinétiques expérimentales publiées dans la littérature. De plus, conjointement au développement de cette SAR, les constantes cinétiques des réactions d'aldéhydes et d'éthers insaturés avec le radical nitrate ont été mesurées au laboratoire. Ces études expérimentales ont ainsi contribué à étoffer la base de données cinétiques sur laquelle repose cette SAR afin de permettre son parachèvement. Cette SAR reproduit, à un facteur deux près, plus de 90% des constantes cinétiques des alcènes et des composés aliphatiques oxygénés saturés et insaturés / The nitrate radical (NO3) is the main oxidant of organic compounds in the night-time troposphere. Thus, comprehension of organic compounds involvement in tropospheric chemical processes requires the knowledge of the rate coefficients for their reactions with the nitrate radical. Nevertheless, considering the wide range of organic compounds emitted or formed in the atmosphere, it is difficult to determine the reactivity of each compound only with laboratory studies. Thereby, these experimental studies have to be completed by predictive methods. In this study, a group-additivity method is therefore used to develop a new Structure-Activity Relationship (SAR) which allows prediction of the rate constants for reactions of organic compounds with the NO3 radical. This empirical method is based on the prediction of a rate constant leaning only on the molecular structure of the organic compound. It relies on experimental rate constants available in the literature. Moreover, the rate constants of unsaturated aldehydes and ethers with the nitrate radical have been measured. Thereby, these experimental studies contribute to expend the kinetic database used for the SAR development and allow its improvement. For saturated and unsaturated oxygenated compounds, more than 90% of the rate constants are reproduced within a factor of two

Ce travail de thèse porte sur la synthèse de carbures de molybdène sur support afin de tester leurs performances catalytiques dans des réactions d’hydrogénation. Dans ce but, la conversion d'acide succinique en phase aqueuse dans un réacteur discontinu et du dioxyde de carbone en phase gazeuse dans un réacteur à flux continu, ont été effectuées. Les catalyseurs ont été préparés par la méthode de carburation par réduction en température programmée, pendant laquelle des paramètres ont été modifiés conduisant à différents rapports molybdène/carbone. Les différents catalyseurs testés dans cette réaction étaient actifs pour la conversion de l'acide succinique en gamma-butyrolctone et, plus remarquablement, en acide butyrique, ce dernier n'était pas obtenu en quantités significatives avec des catalyseurs à base de métaux précieux. L’augmentation de la conversion d’acide succinique avec une sélectivité plus élevée en acide butyrique a été faite avec les catalyseurs contenant plus de carbone, préparés en augmentant la vitesse spatiale horaire gazeuse. Les intermédiaires ont été ensuite convertis en tétrahydrofurane, butanol, 1,4-butanediol et butane. La désactivation observée lors de recyclage du catalyseur a été principalement attribuée à une diminution de la quantité de molybdène et de carbone carbidique, démontrée par l'analyse XPS. Des essais préliminaires de ces catalyseurs dans l'hydrogénation du dioxyde de carbone ont montré qu'ils fonctionnaient principalement en tant que catalyseurs pour la réaction du gaz à l’eau inverse, et l'excès de carbone entrave l'activité catalytique d'une manière opposée à la réaction en phase aqueuse. Le support semble jouer un rôle dans la réactivité des catalyseurs, la conversion du dioxyde de carbone ainsi que la sélectivité en méthane et méthanol, qui ont augmenté dans l'ordre suivant: carbure de molybdène sur support DT51 TiO2> P25 TiO2 ˜ ZrO2 / This work focuses on the synthesis of supported molybdenum carbides and evaluating their catalytic performance in succinic acid hydrogenation reactions in aqueous phase using batch reactor and carbon dioxide hydrogenation in gas phase using continuous flow reactor. The catalysts were prepared by the temperature programmed reduction carburization method, where the parameters were modified leading to different molybdenum to carbon ratios. The different catalysts tested were active in converting succinic acid to gamma butyrolctone and more remarkably butyric acid which is not reported in significant quantities in this reaction with precious metal based catalysts. The catalysts containing more carbon contents that were prepared by increasing the gas hourly space velocity showed higher activity in converting succinic acid and higher selectivity to butyric acid. The intermediates were then converted to tetrahydrofuran, butanol, 1,4-butanediol and butane gas. The deactivation observed while recycling the catalyst was mainly attributed to a decrease in the amounts of carbidic molybdenum and carbidic carbon, as demonstrated by XPS analysis. Preliminary tests for these catalysts in carbon dioxide hydrogenation showed that they functioned mainly as reverse water gas shift catalysts, and the excess of carbon hinders the catalytic activity in an opposite manner of the reaction in aqueous phase. The support seems playing a role in the reactivity of the catalysts, carbon dioxide conversion as well as methane and methanol selectivity increased in the order: molybdenum carbide supported on DT51 TiO2 > P25 TiO2 ˜ ZrO2

Je présente ici mes travaux de thèseque j’ai réalisé ces trois dernières années au seindu Laboratoire ATMosphères et Observations Spa-tiales (LATMOS) de l’Université de Versailles St-Quentin-en-Yvelines (UVSQ) et du Jet PropulsionLaboratory (JPL), California Institute of Technol-ogy. Pendant ces 3 ans je me suis intéressé à la réac-tivité chimique des composés organiques en phasegaz et solide, en utilisant des expériences de labo-ratoire simulant les conditions de l’ionosphère et dela basse atmosphère de Titan, le plus gros satellitede Saturne. Titan est la seule lune du Système So-laire qui possède sa propre atmosphère. Cette atmo-sphère est principalement composée d’azote molécu-laire (N2). Le méthane (CH4) forme le gaz sec-ondaire. D’une part, j’ai analysé les composés neu-tres et les composés chargés (ions) présents dansdes mélanges gazeux simulant la haute atmosphèrede Titan. Ces composés sont considérés commeprécurseurs chimique à la brume organique observéeentourant Titan. C’est-à-dire qu’ils forment les pre-mières étapes d’une succession de réactions chim-iques de plus en plus élaborées formant plus bas dansl’atmosphère des particules solides complexes. Lanature de ces particules dans l’atmosphère de Titanreste encore à élucider complètement. Mon travailpendant cette thèse a été d’utiliser des expériencesde laboratoire pour investiguer la réactivité chim-ique en phase gaz (Chapitres 3 & 4), précurseurs àla formation d’aérosols, ainsi que le vieillissement deces composés plus bas dans l’atmosphère lorsqu’ilsforment les premiers condensats de nucléation à laformation de nuages (Chapitre 5). / Titan is the only moon in the SolarSystem to possess its own dense and gravitationallybound atmosphere, and is even larger than planetMercury. Its rocky diameter is a mere 117 km shy ofGanymede’s. If we were to scoop up a 1 cm3 sam-ple from Titan’s upper atmosphere, we would findtwo dominant molecules: molecular nitrogen N2 andmethane CH4. Should we look a bit more carefully,we would find many neutral molecules and positiveand negative ion compounds. These chemical speciesare the outcome of processes resulting from ener-getic radiation reaching Titan’s upper atmosphere,breaking apart the initial N2 and CH4. A cascadeof subsequent reactions will trigger the formationof new gas phase products more and more com-plex. Eventually, these products mainly contain-ing hydrogen, carbon and nitrogen will form largefractal aggregates composing the opaque haze en-shrouding the surface of Titan. This haze is whatgives Titan such a unique brownish hue. Most ofthe photochemically-produced volatiles will eventu-ally condense in the lower atmosphere, where theymay aggregate to form micrometer-sized icy parti-cles and clouds. During my PhD, I have focusedmy studies on (i) the gas phase reactivity of aerosolprecursors in experimental conditions analogous toTitan’s upper atmosphere (Chapters 3 & 4), and (ii)the end of life of some of the products as they con-dense in the lower and colder atmosphere (Chapter5). I used two experiments to address these respec-tive issues: the PAMPRE plasma reactor, located atLATMOS, UVSQ, Guyancourt, France, and the Ac-quabella chamber at the Jet Propulsion Laboratory,California Institute of Technology, Pasadena, USA.In this manuscript, I present my work on the neutraland positive ion reactivity in the PAMPRE plasmadischarge, as well as ice photochemistry results usinglaser irradiation in near-UV wavelengths.

La déshydratation du glycérol en acroléine en phase gazeuse sur catalyseur solide a été étudiée pour comprendre la formation des sous-produits, déterminer un mécanisme réactionnel et expliquer la désactivation du catalyseur. Les expériences ont été réalisées dans un réacteur isotherme à lit fixe sous différentes conditions opératoires. Pour déterminer les chemins réactionnels de la réaction, de nombreux sous-produits ont été passés séparément sur le catalyseur. Une chromatographie gazeuse en ligne, en continu et originale a été utilisée pour analyser tous les produits de la réaction simultanément. Deux produits ont été quantifiés par chromatographie liquide et de nouveaux composés ont été identifiés par chromatographie gazeuse couplée à un spectromètre de masse. Un mécanisme réactionnel a été proposé. Pour comprendre la désactivation du catalyseur, les produits responsables du dépôt carboné ont été identifiés et la chute de la conversion du glycérol au cours du temps a été modélisée / Glycerol dehydration to form acrolein in gas phase over a solid acid catalyst was studied to understand by-products formation, to determine a detailed mechanism and to explain the deactivation process. Experiments were run in an isothermal fixed bed reactor under different operating conditions. To understand the multiple pathways of the glycerol dehydration mechanism, many by-products of the reaction were passed separately over the catalyst. An original multivalve on-line gas chromatography was used to analyze the whole reaction products continuously and simultaneously. Two products were quantified by high performance liquid chromatography, and the unknown products were identified by gas chromatography-mass spectrometry. A detailed reaction mechanism was then proposed. In order to understand the deactivation process, compounds responsible of carbon deposit were identified and the glycerol conversion fall during an experiment was modelled

L'objectif de cette thèse était d’évaluer quelques Metal-Organic Frameworks (MOFs), choisis en fonction de leur taille de pores, de leur volume poreux et de leur stabilité thermique, en adsorption et séparation des hydrocarbures. Pour étudier le comportement général des MOFs nous avons choisi des MOFs avec des centres métalliques insaturés, des MOFs à charpente anionique et des ZIFs neutres et avons étudié leur sélectivité en séparation de trois familles d'hydrocarbures, à savoir alcanes, alcènes, aromatiques. Les MOFs à centre métallique insaturé se comportent généralement comme des zéolithes polaires, les ZIFs comme des zéolithes apolaires et/ou comme des tamis moléculaires. Les adsorbants les plus prometteurs sont testés sur des séparations d’intérêt industriel telles que la séparation des isomères de xylène, la séparation des paraffines linéaires, monobranchées et di-branchées et l’adsorption sélective du thiophène en vu de l’évaluation de ces adsorbants en désulfuration des essences. / The aim of this thesis was to evaluate several Metal Organic Frameworks (MOFs), selected based on criteria of pore size, pore volume and thermal stability, in adsorption and separation of hydrocarbons. For studying the general behavior of MOFs in hydrocarbon adsorption, we have chosen MOFs with open metal sites, MOFs with anionic frameworks and neutral ZIFs. The MOFs with open metal sites behave similar to polar zeolites, the ZIFs behave like apolar zeolites and/or like molecular sieves. Finally we selected the most interesting MOFs and tested them in several separations with industrial interest: xylene isomers, paraffin isomers and selective adsorption of thiophene for the purpose of fuel desulfuration.

Halogenated heterocycles are common in pharmaceutical and natural products and there is a need to develop a better understanding of processes used to synthesize them. Although the halogenation of simple aromatic molecules is well understood, the mechanisms behind the halogenation of aromatic heterocycles have been more problematic to elucidate because multiple pathways are possible. Recently, new, radical-based mechanisms have been proposed for heterocycle halogenation. In this study, we examine and test the viability of possible nucleophilic substitution, SN2@X, mechanisms in the halogenation of anions derived from the deprotonation of aromatic heterocycles. All the experiments were done in a modified Thermo LCQ Plus equipped with ESI. The modifications allow a neutral reagent to be added to the helium buffer gas in the 3D ion trap. In this system, it is possible to monitor ion/molecule reactions over time periods up to 10 seconds. A variety of aromatic heterocyclic nucleophiles were chosen based on their inclusion of nitrogen and or sulfur as the heteroatoms. In addition to this, the halogenating molecules chosen included traditional halobenzenes and a new class of perfluorinated alkyl iodides. It was found that, experimentally, the SN2@X path is the likely mechanism in the halogenation of deprotonated heterocycles. With computational modeling, we have additional support for this substitution mechanism. From this original study, two more studies were developed to look at the competing nucleophilic aromatic substitution reaction, SNAr. In the first of these studies, the focus was to look at how electron withdrawing substituents about an aromatic ring affect the ratio of SN2@X verses SNAr. As nucleophiles, 2-thiophenide and 5-thiazolide were used. The neutral reagents focus on trifluorobromobenzene derivatives along with pentafluorobromo- and -iodobenzene, and a two trifluoroiodobenzenes. What was found was that the ratio of the reactions depends on where the fluorines, or electron withdrawing substituents are in relation to the bromine or iodine on the ring. If the fluorines are in a close location to stabilize the resulting ionic product, SN2@X proceeds easily. However, the fluorines directly adjacent to the bromine or iodine also provide steric hinderance in the SNAr reaction. In the final project, arylation and benzylation of bromopyridines was examined. The nucleophiles used were benzyl and phenyl anions as well as 5-thiazolide, and the neutral reagents were bromopyridines, with fluorines used as an electron withdrawing groups to help stabilize the transition state. In these experiments, steric hinderance highly affected the results between the phenyl and benzyl nucleophiles. With benzylic anions, the nucleophile is able to reach the aromatic ring with less steric interference and therefore can proceed with an SNAr reaction. In addition to this, with mono and difluorinated pyridine substrates, the nitrogen in the ring activated the ring yielding nucleophilic aromatic substitution losing fluoride rather than bromide in many cases.

The convective drying of thin layers of multicomponentliquid mixtures into an inert gas, and the influence ofdifferent process controlling mechanisms on drying selectivityis studied. Drying experiments under gas-phase-controlledconditions are performed by low intensity evaporation, fromfree liquid surfaces, of ternary mixtures without non-volatilesolutes. Liquid-side-controlled experiments are carried out bydrying a multicomponent polymeric solution containing twovolatile components, one non-volatile polymer and an optionalnonvolatile softening substance. Mathematical models to describe gas- andliquid-side-controlled drying based on interactive diffusion inboth liquid and gas phases as the main mechanisms for masstransfer are developed. For gas-phase-controlled drying, astability analysis of the ordinary differential equations thatdescribes the evaporation process is performed. Isothermal andnon-isothermal drying processes are considered in batch andcontinuous modes. The mathematical model to describe thecomposition profiles during batch drying of the polymeric film,considering liquid resistance, is solved numerically. Due tothe lack of experimental data, properties for this polymericsystem are estimated by using established methods. Ananalytical solution of the diffusion equation, by assuming anisothermal drying process and a constant matrix ofmulticomponent diffusion coefficients is developed. For thecontinuous case, liquid-side resistance is studied by modellingevaporation of a multicomponent falling liquid film into aninert gas including indirect heating. The results of the gas-phase-controlled model are in goodagreement with experimental results. For the polymeric film,the agreement is only qualitative since the model does notaccount for a membrane that develops on the film surface. Thestability analysis permits the prediction of trajectories andfinal state of a liquid mixture in a gas-phase-controlleddrying process. For isothermal evaporation of ternary mixturesinto pure gas, the solutions are trajectories in the phaseplane represented by a triangular diagram of compositions. Thepredicted ternary dynamic azeotropic points are unstable orsaddle. On the other hand, binary azeotropes are stable whenthe combination of the selectivities of the correspondingcomponents is negative. In addition, pure component singularpoints are stable when they are contained within theirrespective isolated negative selectivity zones. Undernon-isothermal conditions, maximum temperature valuescharacterise stable azeotropes. Incremental loading of the gaswith one or more of the components leads to a node-saddlebifurcation, where a saddle azeotrope and a stable azeotropecoalesce and disappear. For continuous drying, the singularpoints are infinite and represent dynamic equilibrium pointswhose stability is mainly dependent on the ratio of inletgas-to-liquid flow rates. As long as the process isgas-phasecontrolled, these results also apply to a porous solidcontaining a liquid mixture. In general, liquid-side control makes the drying processless selective but it is difficult to maintain this conditionduring the whole process. Under the influence of its owndynamics, a process starting as liquid-side-controlled tendstowards a gas-phase-controlled process. The presence ofnon-volatile components and indirect heating may delay thisdevelopment. Considering the evolution of the processcontrolling steps and its influence on selectivity, a modelaimed at describing the complete trajectory of a drying orevaporation process must include the coexistence of allrelevant mechanisms. Keywords:ternary mixture, falling film, diffusionequation, gas-phase control, liquid-phase control, selectivity,stability analysis, polymeric solution, evaporation, azeotrope,batch drying, continuous drying.

Knowledge about structure and dynamics of the important biological macromolecules — proteins, nucleic acids, lipids and sugars — helps to understand their function. Atomic-resolution structures of macromolecules are routinely captured with X-ray crystallography and other techniques. In this thesis, simulations are used to explore the dynamics of the molecules beyond the static structures. Viruses are machines constructed from macromolecules. Crystal structures of them reveal little to no information about their genomes. In simulations of empty capsids, we observed a correlation between the spatial distribution of chloride ions in the solution and the position of RNA in crystals of satellite tobacco necrosis virus (STNV) and satellite tobacco mosaic virus (STMV). In this manner, structural features of the non-symmetric RNA could also be inferred. The capsid of STNV binds calcium ions on the icosahedral symmetry axes. The release of these ions controls the activation of the virus particle upon infection. Our simulations reproduced the swelling of the capsid upon removal of the ions and we quantified the water permeability of the capsid. The structure and dynamics of the expanded capsid suggest that the disassembly is initiated at the 3-fold symmetry axis. Several experimental methods require biomolecular samples to be injected into vacuum, such as mass-spectrometry and diffractive imaging of single particles. It is therefore important to understand how proteins and molecule-complexes respond to being aerosolized. In simulations we mimicked the dehydration process upon going from solution into the gas phase. We find that two important factors for structural stability of proteins are the temperature and the level of residual hydration. The simulations support experimental claims that membrane proteins can be protected by a lipid micelle and that a non-membrane protein could be stabilized in a reverse micelle in the gas phase. A water-layer around virus particles would impede the signal in diffractive experiments, but our calculations estimate that it should be possible to determine the orientation of the particle in individual images, which is a prerequisite for three-dimensional reconstruction. / BMC B41, 25/5, 9:15

The transport in oxides is studied by the use of gas phase analysis (GPA). An experimental method to identify transported species of gases and their contribution to the overall transport of gases in oxides and an experimental method to evaluate the parameters diffusivity, concentration, permeability of gases in oxides and effective pore size in oxides are developed, respectively. Pt has two effects on the thermal oxidation of metals. One is to enhance the oxidation of metals which takes place at the oxide-metal interface by promoting a high concentration gradient of dissociated oxygen across the oxide layer. The other effect is to suppress the oxidation of metals by decreasing the contact area between metal and oxygen. The overall effect of Pt on the oxidation of metals depends on the mechanism of oxide growth in the absence of Pt. It is suggested that an appropriate amount of Pt coating induces a balanced oxide growth resulting from stoichimetrical inward oxygen flux to outward metal flux, which leads to a reduced oxidation rate. The diffusion of diatomic gases in oxides such as vitreous silica and yttria stabilized zirconia (YSZ) takes place in both molecular and dissociated (atomic or/and ionic) form. The fraction of transport of molecular species decreases with temperatures, and the fraction of transport of dissociated species increases with temperatures. Measured permeabilities of diatomic gases in vitreous silica are higher than the expected permeabilities of their molecules, which are explained by diffusion of molecules combined with a retardation of dissociated species in reversible traps. The diffusion of hydrogen in vitreous silica is concentration dependent and increases with local concentration. Transport paths are shared among transported species and gases at all temperatures in YSZ. Helium shares transport path with molecular oxygen and nitrogen at low temperatures; whereas helium shares transport path with dissociated oxygen and also dissociated nitrogen at high temperatures. / QC 20100705

The Earth's atmosphere is a large photochemical reactor consisting primarily of N2 (~78%) and O2 (~21%) with Ar and water vapor being the next most abundant constituents. All of the remaining gases in the atmosphere are referred to as 'trace gases', and they play a critical role in understanding climate change, urban air quality, ozone production and depletion, and in determining the overall 'health' of the atmosphere. These trace components are present in our atmosphere with mixing ratios, i.e., mole fractions, ranging from sub parts per trillion to several hundred parts per million. One class of trace constituents that play a critical role in atmospheric chemistry are free radicals. Free radicals are highly reactive, often initiating the oxidation of natural and anthropogenic atmospheric species, thereby often controlling the fate and lifetimes of these species. The research comprising this dissertation focuses on laboratory studies of the kinetics and mechanisms of free radical (atomic halogen) reactions that can impact the levels of important trace atmospheric species. In the studies reported herein, laser flash photolysis (LFP) was coupled with time resolved atomic resonance fluorescence (RF) spectroscopic detection of Cl or Br atoms to investigate halogen atom chemistry. The research addresses three groups of reactions: Cl atom reactions with alkyl bromides, Cl and Br-initiated oxidations of small (C2-C6) alkenes, and Cl reactions with CH3SCH3 (DMS, dimethylsulfide) and CH3SeCH3 (DMSe, dimethylselenide). The alkyl bromide reactions were experimentally unique in that we were able to deduce kinetics of the Cl atom reaction with bromoethane, n-bromopropane, and 1,2-dibromoethane by monitoring the appearance of the Br product by LFP-RF. The Br is formed via elimination that occurs essentially instantaneously following β-H abstraction by the Cl atom. All three of the bromoalkanes investigated are emitted into the atmosphere primarily from anthropogenic sources and all three have been identified by the World Meteorological Organization (WMO) as very short-lived (lifetime less than 6 months) source gases with significant ozone depletion potentials (ODPs). Additionally, the bromoalkanes mentioned above have been of interest as model compounds for larger partially halogenated organics found in the atmosphere, and they have been considered as potential replacement compounds for chlorofluorocarbons (CFCs) that have been banned as a result of the Montreal Protocol. Brominated very short-lived compounds are thought to contribute 20-25% of total stratospheric bromine. Thus, there is considerable interest in understanding the atmospheric chemistry of even the most short-lived organic bromine compounds. Temporal profiles of Br atoms provided important kinetic and mechanistic insight for the reactions over a wide range of temperature and pressure. Temperature-dependent rate coefficients are determined for the alkyl bromides of interest for the first time, and the potential importance of the Cl reaction as an atmospheric degradation pathway for each alkyl bromide is qualitatively assessed. The studies of halogen atom reactions with alkenes focused on formation of weakly-bound adducts where kinetics of adduct formation and dissociation as well as non-adduct forming channels were evaluated. The elementary steps in the Br initiated oxidation of the alkenes 2-methyl-1,3-butadiene (isoprene), 2,3-dimethyl-2-butene (tetramethylethylene, TME), and 1,3-butadiene have been investigated. The experimental kinetic database for these reactions is quite sparse. The kinetic results reported herein, suggests that Br reaction with the above olefins is much faster than previously thought. Analysis of the temperature dependence of the "approach to equilibrium" kinetic data in conjunction with electronic structure calculations allows for determination of enthalpy and entropy changes associated with each addition reaction. Where possible, both forward addition and reverse dissociation channels as well as H-abstraction pathways were characterized. The enthalpy change associated with the addition reaction to give the Br−isoprene and Br−1,3-butadiene adducts has been determined for the first time and the bond dissociation enthalpy obtained for the Br−TME adduct is in reasonable agreement with the only other previously reported value. It should be noted that in the case of isoprene and 1,3-butadiene, there are multiple possible adducts that could be formed. In order to help clarify which adducts are more or less likely to be formed, we rely on electronic structure calculations (see Chapter 5) to aid in our overall understanding of the adduct forming channels. Furthermore, for the Br reactions with the three alkenes above, atomic Br kinetics have been monitored directly both in the absence and in the presence of O2 which allowed, for the first time, determination of rate coefficients for the elementary steps in the overall complex mechanism including determination of the Br−olefin + O2 rate coefficient. Also included in this group of reactions is the chlorine reaction with isoprene. In addition to the well-known fact that isoprene is emitted into the atmosphere from vegetation, a potentially significant marine source of isoprene has received considerable attention. Chlorine has long been thought to exist primarily in marine environments, however, recent findings also suggest a significant Cl production rate in the middle of the continental United States. There are numerous room temperature kinetic studies for the Cl + isoprene reaction in the literature, however, there is only one temperature dependent study reported. Current recommended 298 K rate coefficients for isoprene reactions suggest the Cl reaction is ~ 4x faster than the analogous OH reaction. If indeed this is the case, the Cl reaction could play a non-neglibible role in isoprene oxidation in atmospheric locales where Cl concentrations are relatively high. In addition, the C−Cl bond strength in Cl−C5H8 is obtained from direct measurements of the forward and reversible addition rate coefficients. Our results are compared with the literature data, and the potential importance of Cl-initiated oxidation as an atmospheric sink for isoprene is assessed. The final group of reactions investigated involves reactions of Cl with DMS and DMSe. DMS and DMSe are the most prevalent sulfur and selenium compounds emitted to the atmosphere from the oceans. The oxidation of DMS has been studied extensively due to the interest in the possible role of DMS oxidation in the formation of sulfate aerosols, however, DMSe oxidation processes have hardly been studied at all. And, DMSe oxidation products are likely to be less volatile than the analogous DMS species. Selenium is an essential nutrient for many plants and animals; however, there is a fine line between enough and excess selenium which can be toxic. Most studies suggest that atmospheric deposition is an important source of Se contamination, and it is therefore critical to evaluate the source emissions and fate of Se in the atmosphere. Since the majority of atmospheric Se exists in the form of DMSe, determination of the kinetics and oxidation mechanisms of DMSe will go a long way towards understanding the global biogeochemical cycle of Se. Both reversible addition and H-abstraction pathways have been characterized, and the first experimental determination of bond strength of the gas-phase DMS−Cl and DMSe−Cl adducts have been obtained.