1-Nitropyrene (1NPy) is the most abundant nitropolycyclic aromatic contaminant encountered in diesel exhausts. Understanding its photochemistry is important because of its carcinogenic and mutagenic properties, and potential phototransformations into biologically active products. We have studied the photophysics and photochemistry of 1NPy in solvents that could mimic the microenvironments in which it can be found in the atmospheric aerosol, using nanosecond laser flash photolysis, and conventional absorption and fluorescence techniques. Significant interactions between 1NPy and solvent molecules are demonstrated from the changes in the magnitude of the molar absorption coefficient, bandwidth at half-peak, oscillator strengths, absorption maxima, Stokes shifts, and fluorescence yield. The latter are very low (10 (-4)), increasing slightly with solvent polarity. Low temperature phosphorescence and room temperature transient absorption spectra demonstrate the presence of a low energy (3)(pi,pi*) triplet state, which decays with rate constants on the order of 10 (4)-10 (5) s (-1). This state is effectively quenched by known triplet quenchers at diffusion control rates. Intersystem crossing yields of 0.40-0.60 were determined. A long-lived absorption, which grows within the laser pulse, and simultaneously with the triplet state, presents a maximum absorption in the wavelength region of 420-440 nm. Its initial yield and lifetime depend on the solvent polarity. This species is assigned to the pyrenoxy radical that decays following a pseudo-first-order process by abstracting a hydrogen atom from the solvent to form one the major photoproducts, 1-hydroxypyrene. The (3)(pi,pi*) state reacts readily ( k approximately 10 (7)-10 (9) M (-1) s (-1)) with substances with hydrogen donor abilities encountered in the aerosol, forming a protonated radical that presents an absorption band with maximum at 420 nm.

We apply stoichiometric network analysis (SNA) to study enantioselective chemical reaction schemes, subject to various thermodynamic architectures, which may lead to spontaneous mirror symmetry breaking (SMSB). Stoichiometric matrices are used to calculate extreme currents or fluxes: the vector basis for the convex polyhedral cone of all stationary reaction rates. A major emphasis is given to the constraints that the rate constants must obey and how to express these in terms of the convex parameters and stationary inverse concentrations. We evaluate the corresponding Jacobians in terms of the constrained convex parameters and the inverse stationary concentrations and carry out stability analyses for the steady-state racemic configurations. A geometric visualization of SMSB is proposed, based on the structures of the convex cones, the angles between currents, and the cone's subspaces that result from enforcing the pertinent thermodynamic and chiral constraints.

When a methyl ketone is added to the Belousov-Zhabotinskii (BZ) reaction, an increase of the induction period and in the number of oscillations is observed. This behavior can be explained by the competition of malonic acid and the methyl ketone for molecular bromine. We studied the effect of four methyl ketones in the BZ reaction and found a direct relationship between the induction period, the concentration, and the enolization constant of the ketone. A discussion of the results with regard to the crucial concentration of bromomalonic acid is given. The experimental results are supported by simulations based upon the GTF mechanism of the reaction.

No presente estudo apresentamos a dependência do tempo de indução da reação BelousovZhabotinsky (BZ) em função da concentração inicial do ácido malonico, bromato e cerium. Os resultados mostram que o tempo de indução aumenta com o aumento da concentração de bromato e tal comportamento não coincide com o mecanismo proposto para as reações BZ. Propomos então que uma competição cinética entre a bromação do ácido malonico e a oxidação dos ácidos bromomalonico e malonico seja o caminho para entender este mecanismo. Cáculos usando modelos GTF e MBM apoiam a proposição sugerida.In this paper we present the dependence of the induction time of the Belousov-Zhabotinsky reaction (BZ) on the initial concentrations of malonic acid, bromate and cerium. The experimental results show that the induction time gets larger with bromate increasing and this behaviour does not agree with the mechanistic explanations based on the models proposed for the BZ reaction. We propose that a kinetic competition between the bromination of malonic acid and the oxidation of bromomalonic and malonic acids is a way to understand this behaviour. Model calculations using the GTF and MBM models support the propose explanation.

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The goal of our research is the development of algorithmic tools for the analysis of chemical reaction networks proposed as models of biological homochirality. We focus on two algorithmic problems: detecting whether or not a chemical mechanism admits mirror symmetry-breaking; and, given one of those networks as input, sampling the set of racemic steady states that can produce mirror symmetry-breaking. Algorithmic solutions to those two problems will allow us to compute the parameter values for the emergence of homochirality. We found a mathematical criterion for the occurrence of mirror symmetry-breaking. This criterion allows us to compute semialgebraic definitions of the sets of racemic steady states that produce homochirality. Although those semialgebraic definitions can be processed algorithmically, the algorithmic analysis of them becomes unfeasible in most cases, given the nonlinear character of those definitions. We use Clarke’s system of convex coordinates to linearize, as much as possible, those semialgebraic definitions. As a result of this work, we get an efficient algorithm that solves both algorithmic problems for networks containing only one enantiomeric pair and a heuristic algorithm that can be used in the general case, with two or more enantiomeric pairs.

A kinetic study of the uncatalyzed bromate oscillator (UBO) using phenol was carried out by chemometric analysis of the UV/Vis data. The reaction was studied with different starting concentrations of the reagents (sulfuric acid and potassium bromate) and by changing phenol for 2-bromophenol and 4-bromophenol. The number of chemical absorbing species involved in the UV/Vis data was established by singular value decomposition and by the residual fit parameter. Multivariate curve resolution with alternating least squares generated valuable information about the different chemical species along the process (spectral and concentration profiles). These results show the presence of four principal components: two intermediates and two products. The products can be assigned as o-quinone and 4,4 -diphenoquinone, and the principal components associated with the intermediates represent the linear combination of all the intermediates in the reaction mixture. These data were used to generate a model of the UBO-phenol oscillator that reproduces qualitatively the experimental results. C

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