The pyrolysis of dimethyl ether in the presence of a methane bath, over the temperature range 790-850K, follows the simplified reaction scheme numbered 1-5: CH3OCH3 CH3O + CH3 (l) CH3O + CH4 CH3OH + CH3 (2), CH3O + M CH2O + H + M (3),H + CH4H2 + CH3 (4),CH3 + CH3 C2H6 (5) Thus monitoring the level of ethane, or ethylene derived from ethane, yields a measure of the rate of dimethyl ether decomposition, (l) Results obtained from the current work, together with those of other workers obtained over a wider temperature range, yields the rate constant for reaction (l): k1 = 10l6.3 1 .3 exp-81.4 0.3 k cal mol-1. Hence a value for the heat of formation of the methoxy radical is calculated to be 4.1 kcal mol-1. However, both computer modelling of the full reaction scheme and further experimental work has highlighted the importance of sensitised decomposition reactions within this system, reactions (6), (7): CH3 + CH3OCH3 CH4 + CH2OCH3 (6), CH2OCH3 + M CH2O + CH3 (7) For the case where neat dimethyl ether is pyrolysed the reaction is dominated by sensitised decomposition. Computer modeling successfully resolved the mechanism of reaction under the experimental conditions. Attempts to obtain an experimental rate constant for the decomposition of the methoxy radical have proved unsuccessful due to the complex nature of this reaction system. This also proved to be the case where dimethyl ether-d6 acted as the precursor of methoxy-d3. The decomposition of the trifluoromethoxy radical was examined using an RRKM computer model, reconciling the calculated parameters for this radical with the pressure dependent data of Descamps and Forst. This study of the trifluoromethoxy radical yielded data which enabled application of Unimolecular Gas Kinetic Theory to the similar methoxy radical. With so few experimental data on the decomposition of the methoxy radical it was essential to reduce the number of variables prior to applying the RRKM model to this reaction (3). Formaldehyde pyrolysis in a methane bath, over the temperature range 740-910K, was examined to resolve controversy over the mode of formaldehyde decomposition: whether the reaction proceeds by a molecular elimination path, A, or via a radical chain mechanism, B. Although ethane was not observed as a principle product, computer modelling of this reaction indicated that the combination of methyl radicals was the principle termination step. To test for the participation of the molecular elimination pathway (15), the ratio CD2O + M CO + D2 + M (15) RD2/RHD was followed as a fuction of both the formaldehyde-d2 concentration and temperature. As (D) is a function of the concentration of formaldehyde-d2, an intercept will be obtained upon plotting RD2/RHD versus (CD2O)/(CH4) where the molecular elimination path participates. No evidence of the molecular elimination route was obtained. Reaction data for steps (10) and (11) were obtained together with evidence of heterogeneous reaction contributing to the rate of formaldehyde decay.

The pyrolysis of methyl nitrite (1 torr) in the presence of nitrogen dioxide (1 torr) was studied at 458K over the pressure range 0-760 torr of carbon tetrafluoride. The only detectable products were methyl nitrate and formaldehyde. The decomposition can be described by the following simple mechanism: CH₃ONO + M → CH₃O + NO + M (1); CH₃O + NO₂ → CH₃ONO₂ (2); CH₃O + NO₂ → HCHO + HONO (3). Step (1) was found to be stongly pressure dependent with a P½ value of 760 torr. The rate constant for decomposition k₁ was found from RRKM modelling to be given by the following expression: log (k₁/s⁻¹) = 15.89 - 8879/T. The ratio k₃/k₂ was determined over the pressure range and was also found to be pressure dependent. This was attributed to the pressure dependence of step (2). Estimates were also made for the ratio of disproportionation to combination for the reaction between nitric oxide and the methoxy radical. This ratio was also found to be strongly pressure dependent. The pyrolysis of perfluoroazo-2-propane, PAP, (25 torr) was studied over the temperature range 450-514K. The products were nitrogen and perfluorohexane, PFH, which were produced in equal amounts. The production of nitrogen was found to be first order with respect to the azo compound. First order kinetics were observed even for extents of reaction exceeding 60%. No surface effects were observed. The reaction was pressure independent. i-C₃F₇N²i-C₃F₇ → i-C₃F₇N₂ + i-C₃F₇ (4). The rate constant for decomposition, k₄, was found to be given by the following expression: log(k₄) = 16.74 - 9856/T. The pyrolysis of formaldehyde (4-10 torr) was studied using a static system over the temperature range 705-773K and 150-760 torr of carbon dioxide. Methane (4-10 torr) was used as an inert marker. Preliminary experiments showed that methane did not decompose under these experimental conditions. The only measurable products were hydrogen and carbon monoxide. No pressure dependence was observed, even at the highest temperatures studied. The rate of formation of products was found to be 1.04 ± 0.05 with respect to formaldehyde. From this the reaction was taken to be first order. The addition of small concentrations of toluene was found to markedly reduce the rate of formation of products. There did not appear to be any surface effects, indicating that the reaction was homogeneous.

Lee Kim-sze. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1979. / Bibliography: leaves 84-86.

La réponse cellulaire aux dommages de l'ADN provoqués par l'irradiation (IR) est relativement bien étudiée mais de nombreuses observations montrent l'implication de l'expression de nombreux gènes. Nous souhaitons identifier les différentes formes de la réponse transcriptionnelle à l'IR et reconstruire un réseau de régulation génique impliqué dans son contrôle. La problématique réside dans l'exploitation de dynamiques d'expression de gènes dans des conditions de perturbations génétiques et dans l'intégration d'informations biologiques systémiques. Nous définissons une approche constituée d'une étape automatisée de déduction de régulations à partir de perturbations et de deux étapes d'induction qui permettent d'analyser la dynamique d'expression des gènes et d'extraire des régulations des données additionnelles. Cela nous a permis d'identifier, chez la levure, une réponse complexe à l'IR et de proposer un modèle de régulation dont certaines relations ont été validées expérimentalement. / The cellular response to the DNA damage provoked by irradiation (IR) is relatively well studied, however, many observations show the involvement of the expression of many genes. We propose to identify the different potential patterns of the transcriptional response to IR and to reconstruct a gene regulatory network involved in its control. The first point of this work lies in the exploitation of the gene expression dynamics in conditions of genetic perturbations. The second point lies in the integration of systemic biological informations. We define an approach composed of one step of automated logical deduction of regulations from a strategy of perturbations and two induction steps that allow the analysis of the gene expression dynamics and the extraction of potential regulation from additional data. This approach allowed to identify, for the yeast, a complex response to IR and allowed to propose a regulation model which some relations have been experimentally validated.

No description available.

Treating the activated complex for unimolecular dissociation as uncoupled fragments simplifies the calculation of transition-state frequencies in the quasi-equilibrium-theory formulation of an isotopic rate-constant ratio. The general method is applied to the specific case of the intramolecular kinetic isotope effect in decarboxylation of malonic acid. The fragment models are further simplified by means of the “cut-off” approximation of Sterm and Wolfsberg. These model calculations are in agreement with the presently available experimental results. Applicability to reactions involving simultaneous multiple bond rupture or formation is noted.

Polysulfides, produced from olefin sulfurization, are common additives to various petroleum-derived products such as engine oils, hydraulic fluids, rubbers and polymeric materials in an effort to protect these materials from oxidative degradation. The mechanisms by which organosulfur compounds act as antioxidants are often described as ‘secondary antioxidants’ since it is believed that they slow the rate of radical initiation by decomposing hydroperoxides. Organosulfurs also figure in various natural products from the genus Allium, of which garlic is a prominent species. Our group previously shown that thiosulfinates (RS(O)SR), key components of garlic, are potent radical-trapping antioxidants (RTAs) through the intermediacy of sulfenic acids (RSOH). In an effort to shed light on the antioxidant mechanisms of other organosulfur constituents of garlic and sulfurized olefins, in Chapter 2, we reported on the antioxidant activity of trisulfide-1-oxides (RS(O)SSR) at 37 °C. In that study, we found these moieties to be effective RTAs, equal in potency to hindered phenols, the industry standard. Trisulfide-1-oxides were shown to react with peroxyl radicals via a concerted bimolecular homolytic substitution reaction, forming a perthiyl radical as product. In Chapter 3, inspired by the findings at 37 °C, we investigated the RTA activity of polysulfides at elevated temperatures, from 100 °C to 160 °C, and found that tetrasulfides were surprisingly potent RTAs at and above 100 °C. Paralleling the reactivity of trisulfide-1-oxides, tetrasulfides were found to react with peroxyl radicals via a concerted bimolecular homolytic substitution reaction, also forming a perthiyl radical as an initial product. Perthiyl radicals are formed in multiple reactions described in the abovementioned studies and although they were shown to rapidly recombine, their apparent stability, as well as preliminary computational investigations into the S-H bond strength in hydropersulfides (RSSH), prompted us to investigate hydrogen-atom transfers from hydropersulfides. As reported in Chapter 4, we found that hydropersulfides (RSSH) are very potent hydrogen-atom donors, making them highly effective RTAs in organic and aqueous media, even besting thiols in many H-atom transfer reactions with alkyl, alkoxyl and peroxyl radicals. The one-electron oxidation exhibited by hydropersulfides parallels the one we previously reported for sulfenic acids (RSOH). Previous successful studies on the one-electron oxidation of sulfenic acids prompted us to make use of the persistence imparted by a triptycene backbone to study two-electron processes, namely - thiol oxidation by hydrogen peroxide (H2O2). Indeed, the reaction of thiols with H2O2 is central to many processes essential to life, from protein folding to redox signaling. However, kinetic and mechanistic characterization of their subsequent reactions had not been reported due to the instability of the sulfenic acid intermediate under study. Gratifyingly, as reported in Chapter 5, using our triptycene model with an appended fluorine atom, we were able to investigate the complete series of consecutive reactions with hydrogen peroxide, from thiol to sulfenic acid to sulfinic acid to sulfonic acid, which has hitherto not been possible, and found that the reaction is specific base-catalyzed. Sulfinic acids (RSO2H), the primary product of sulfenic acid oxidation, have been used increasingly in recent years in many synthetic applications such as sulfonylation reactions. As described in Chapter 6, using a similar approach to previous studies, we synthesized a triptycene sulfinic acid and demonstrated that the triptycene backbone is also effective a rendering the sulfinic acid and sulfonyl radical (RSO2•) persistent enough to study H-atom transfer reactions. We showed that sulfinic acids are good H-atom donors to alkyl and alkoxyl radicals but they are not effective RTAs due to the propensity of sulfinic acids to autoxidize through the reaction of sulfonyl radicals with oxygen.

Thesis (Ph. D.)--Washington State University, May 2007. / Includes bibliographical references.

Ph.D. / Environmental Science and Engineering / In the past decade, permeable reactive barriers containing zero-valent iron metal (FePRBs) have emerged as the most significant new technology for the treatment of groundwaters contaminated with chlorinated organic compounds and, more recently, other organic contaminants such as 2,4,6-trinitrotoluene (TNT). Principle issues relating to the design, implementation, and monitoring of FePRBs include the rates of contaminant transformation, the resulting distribution of products, and the potential changes in FePRB performance due to aging of the iron material. Each of these issues is, at its root, a problem of chemical kinetics. In this thesis, commonly observed kinetic expressions for contaminant transformation are derived. Analyses of the simplifications involved in these derivations indicate that the forms of the rate laws are correct (either exactly or approximately) over a wider range of conditions than previously expected and that reaction rates may respond in unexpected fashion to changes in concentrations of reacting species or iron loading. These theoretical developments are applied to experimental investigations of product distribution and FePRB longevity for the treatment of TNT contaminated groundwaters.

Sedimentary basins can contain close to 20% by volume pore fluids that are commonly classified as brines. These fluids can become undersaturated with respect to calcite as a result of processes such as migration, dispersive mixing, or anthropogenic injection of CO2. This study measured calcite solubility and dissolution rates in geologically relevant Na-Ca-Mg-Cl synthetic brines (35 to 200 g L-1 TDS). In brines < 50 g L-1 TDS, the EQPITZER calculated calcium carbonate ion activity product (IAP) at steady-state was in reasonable agreement (±10%) with the thermodynamic solubility constant for calcite (Kc). However, the IAP systematically exceeded Kc in more concentrated brines. The deviation was strongly correlated with calcium concentration and also was observed in magnesium-free solutions. This is interpreted as an uncertainty in the carbonate ion activity coefficient, and minor adjustment in stoichiometric association constants (K*M2+CO30) for the CaCO30 or MgCo30 ion pairs would correct for the error. The dissolution rate dependency on brine composition, pCO2 (0.1 to 1 bar), and temperature (25.0 to 82.5 °C) was modeled using the empirical rate equation ()nkRΩ−=1 where R is the rate, k and n are empirical fitting terms, and Ω the degree of disequilibrium with respect to calcite. When Ω was defined relative to an apparent kinetic solubility, n could be assumed first-order over the range of Ω investigated (Ω = 0.2 to 1.0). Rates increased with increasing pCO2 as did the sensitivity to brine concentration. At 0.1 bar, rates were nearly independent of concentration (k = 13.0 ±2.0 x 10-3 moles m-1 hr-1). However, at higher CO2 partial pressures rates became composition dependent and the rate constant, k, was shown to be a function of temperature, pCO2, ionic strength, and calcium and magnesium activity. The rate constant (k) can be estimated from a multiple regression (MR) model of the form k = B0 + B1(T) + B2(pCo2) + B4(aCa2+) + B5(aMg2+). A relatively high activation energy (Ea = 20 kJ mol-1) was measured, along with a stirring rate independence suggesting the dissolution is dominated by surface controlled processes at saturation states Ω > 0.2 in these calcium-rich brines. These findings offer important implications to reaction-transport models in carbonate-bearing saline reservoirs.