Jesús Ágreda

Photophysics and Photochemistry of 1-Nitropyrene

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.

Understanding the induction period of the Belousov-Zhabotinsky reaction

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.

Bursting in the Belousov-Zhabotinsky reaction added with phenol in a batch reactor

4+ to Ce 3+ and in the removal of molecular bromine of the reaction mixture. The oscillating reaction of two substrates exhibited burst firing and an oscillatory period of long duration. Analysis of experimental data shows an increasing of the bursting phenomenon, with a greater spiking in the burst firing and with a longer quiescent state, as a function of the initial phenol concentration increase. It was hypothesized that the bursting phenomenon can be explained introducing a redox cycle between the reduced phenolic species (hydroxyphenols) and the oxidized ones (quinones). The hypothesis was experimentally and numerically tested and from the results it is possible to conclude that the bursting phenomenon exhibited by the oscillating reaction of two substrates is mainly driven by a p-di-hydroxy-benzene/p-benzoquinone redox cycle.

Induction period in the BrO3-, Ce(III), H2SO4, oxalic acid and ketone oscillating reaction

Contrary to what has been thought, systems classified as bromine-hydrolysis-controlled (BHC) show an induction period. In studies on the effect of a family of ketones upon this type of oscillators, it was found that an induction period appears in an interval of concentrations of the ketone, or in an interval of the value of the enolization constant. Simulations of the processes involved and a corresponding explanation are shown in this paper.

CALORIMETRIC STUDY OF THE COMPONENT STEPS OF OSCILLATING CHEMICAL REACTIONS

The complexity of oscillating chemical reactions makes difficult a direct calorimetric study of them. It is more advantageous to carry out studies of the component steps and then try to put the parts together. Here, a mass-flow heat conduction calorimeter was used to study component reactions of two of the principal chemical oscillators. The studied reactions were: the net reaction of the inorganic set of the Belousov-Zhabotinsky reaction BrO3-+4Ce3++5H+D4≤;8805;Ce4++HOBr+2H2O), and the Dushman reaction IO3-+5I-+6H+≤;8805;3I2+3H2O), which is a component of the Bray-Liebhafsky oscillator. The experimental values of the enthalpies of these two reactions are reported in this work.

On the linear algebra of biological homochirality

We look for structural properties of chemical networks giving place to homochiral phenomena. We found a necessary condition for homochirality that we call Frank inequality, and which is a linear inequality related to the entries of the jacobian matrices that occur at racemic steady states. We also investigate the existence of stronger conditions that can be formulated in a similar algebraic way. Those investigations lead us to introduce a homochirality degree for the racemic states of chiral neworks, which is intended to measure the probability of observing homochiral dynamics after perturbing those states. It is important to stress that all the introduced concepts and degrees are effective. The later fact allows us to develop an algorithm that can be used to, given a chiral network as input, compute large samples of steady states of different degrees.

Entropy production in the Oregonator model perturbed in a calorimeter with a chemical pulse

The classical Oregonator model of the Belousov–Zhabotinsky reaction is used to examine the response of an oscillating reaction to a chemical perturbation in a batch calorimeter. To do this, a reaction step for the perturbing chemical substance and the energy balance equation are added to the model. Changes in the frequency and amplitude of the oscillations occur as a function of the concentration of the perturbing substance, and these changes are quantified by applying the thermodynamics of irreversible processes in order to determine the rate of entropy production. Various criteria are proposed to evaluate the time series and measurable quantities with linear correlations between the entropy production due to transfer and the concentration of the chemical perturbation are obtained, showing that it is possible to implement experimental protocols in the proposed methodology.