Pilar Perez-Tejeda

Electron transfer reactions in micellar systems: Separation of the true (unimolecular) electron transfer rate constant in its components

Abstract The intramolecular electron transfer reaction within the binuclear complex [(en) 2 Co III (μ-2-pzCO 2 )Fe II (CN) 5 ] − has been studied in micellar (dodecylsulphate sodium salt (SDS) and hexadecyltrimethylammonium chloride (CTACl)) solutions. The true (unimolecular) electron transfer rate constant as well as the parameters controlling this rate constant, the reorganization free energy and the reaction free energy were obtained. This has permitted us to establish that, in spite of the similar behaviour observed for k et in both kinds of micellar solutions, in the case of SDS solutions the kinetics is controlled by both the driving force and the reorganization energy. In the case of CTACl, only the reorganization energy controls the k et variations.

Nonfunctionalized Gold Nanoparticles: Synthetic Routes and Synthesis Condition Dependence

Since Faraday first described gold sol synthesis, synthetic routes to nanoparticles, as well as their applications, have experienced a huge growth. Variations in synthesis conditions such as pH, temperature, reduction, and the stabilizing agent used will determine the morphology, size, monodispersity, and stability of nanoparticles obtained, allowing for modulation of their physical and chemical properties. Although many studies have been made about the synthesis and characterization of individual nanosystems of interest, to our knowledge the common, general traits that all those synthesis share have not been previously compiled. In this review, we aim to offer a global vision of some of the most relevant synthetic procedures reported up to date, with a special focus on nonfunctionalized gold nanoparticle synthetic routes in aqueous media, and to display a broad overview of the influence that synthesis conditions have on the shape, stability, and reactivity of nanoparticle systems.

On the equivalence of the pseudophase related models and the Brönsted approach in the interpretation of reactivity under restricted geometry conditions

The equivalence of the Pseudophase and related models, frequently used in the interpretation of kinetic data under restricted geometry conditions, and the well known Brönsteds equation, used in the interpretation of the reactivity in homogeneous media, has been proven in several cases. The generalisation in relation to other non-kinetic properties such as reaction free energy and redox potentials has also been considered.

ELECTRON TRANSFER REACTIONS IN MICELLAR SYSTEMS

The influence of micelles on electron transfer processes is reviewed. The micelles modify the rate of electron transfer reactions by producing changes in all the relevant parameters controlling this rate; that is, through modification of the reorganization free energy, the reaction free energy, the nuclear dynamics and the strength of the coupling between the donor and the acceptor. Applications of studies on electron transfer reactions in micellar systems in different fields are presented.

Solvent effects on the Co(NH3)4(pzCO2)2+–Fe(CN)64– reaction. An interpretation based on spectroscopic data

The reduction of Co(NH3)4(pzCO2)2+(pzCO2= pyrazinecarboxylate) by Fe(CN)64– has been studied in several isodielectric binary aqueous mixtures (water–tert-butyl alcohol, water–ethylene glycol and water–glycerol) at 298.2 K. Spectroscopic data have been obtained for the metal-to-metal charge-transfer band within the ion-pair, Co(NH3)4(pzCO2)2+/Ru(CN)64–. From these data, the Gibbs energies of activation for the thermal electron-transfer reaction have been calculated and compared with those obtained from a kinetic study. The two series of data corresponding to the different water–cosolvent mixtures are of the same order of magnitude but quantitative agreement between calculated and experimental data is not found. This discrepancy is discussed with the main conclusions being: (i) The classical model of the solvent, with a linear response, does not give correct predictions for the Gibbs energy of reorganization: the trend in changes of this magnitude calculated from the model is opposite to that found experimentally. (ii) Differences between the experimental and calculated Gibbs energies of activation arise from (1) the distance effect and (2) a failure of Hushs formulae based on linear response of the medium.

DNA conformational changes induced by cationic gemini surfactants: the key to switching DNA compact structures into elongated forms

The DNA conformational changes induced by different members of the N,N′-bis(dimethyldodecyl)-α-ω-alkanediammonium dibromide series (m-s-m, m = 12, s = 3 and 6) and the analogous series of hexadecyl gemini surfactants (m = 16, s = 3 and 6) were investigated in aqueous media by means of circular dichroism (CD), zeta potential, dynamic light scattering (DLS), viscometry, and atomic force microscopy (AFM) methods. The measurements were carried out by varying the gemini surfactant–DNA molar ratio, R = Cm-s-m/CDNA. For the conditions investigated two significantly different conformational changes were observed, the second of them being worth noting. At low molar ratios, all methods concurred by showing that gemini surfactants were able to form ordered aggregates which precedes DNA compaction. The second effect observed, at high molar ratios, corresponds to the transition from the compact state to a new more extended conformation. The degree of decompaction and the morphologies of the visualized structures are different not only depending on the surfactant tails length, but also on the spacers length. The results obtained for the 16-3-16/DNA and 16-6-16/DNA systems point out that the compaction/decompaction processes are somewhat different to those previously visualized for the analogous monoquaternary chain surfactant CTAB.

Improving the understanding of DNA–propanediyl-1,3-bis(dodecyldimethylammonium) dibromide interaction using thermodynamic, structural and kinetic approaches

A kinetic, thermodynamic and structural study of the interaction of the gemini surfactant propanediyl-1,3-bis(dimethyldodecylammonium dibromide) (12-3-12.2Br) with calf thymus DNA was carried out at several ionic strengths (NaCl) in aqueous solutions. A new 12-3-12(2+)-selective membrane was prepared in order to gain insight into the factors that control the binding of 12-3-12.2Br to DNA. We used ethidium bromide (EB) as a fluorescence probe to follow the kinetics of the interaction by using the stopped-flow fluorescence technique. The results can be explained in terms of a reaction mechanism involving two consecutive reversible (fast and slow) steps. The fast step was attributed to the union/separation of the surfactant with/from the DNA polynucleotide. Changes in the kinetic constants in the forward and backward directions were discussed in terms of the Brönsted-Pitzer equation and of the increase in hydrophobic interactions of the surfactant tails as a consequence of salting-out effects, respectively. The slow step corresponds to a conformational change of the surfactant-DNA complex to a more compacted form. The equilibrium constant, calculated from the forward and reverse rate constants of these steps, agrees with the results obtained from potentiometric titration using a 12-3-12-(2+) selective electrode.

Estimation of electron transfer rate constants by static (optical and electrochemical) measurements

Abstract A kinetic study of the reduction of Co(NH 3 ) 5 (DMSO) 3+ (DMSO=dimethylsulfoxide) by Fe(CN) 6 4− has been carried out in several water–cosolvent mixtures at 298.2 K. The cosolvents used were ethylene glycol, acetonitrile, methanol and glucose. The free energies of activation for this thermal electron-transfer reaction have been calculated from a combination of spectroscopic and electrochemical data and compared with those obtained from the kinetic study. Quantitative agreement is found between both series of data. This shows the possibility of estimating activation free energies for electron transfer reactions from these (static) measurements.

Setschenow coefficients for caffeine, theophylline and theobromine in aqueous electrolyte solutions

Setschenow coefficient measurements have been made on methylated xanthines (caffeine, theophylline and theobromine) in several inorganic and organic aqueous salt solutions at 298 K. In general, the order of salting effect is similar for the three solutes and can be rationalized as a consequence of structural salt–water interactions. Scaled particle theory and the Cross and McTigue equation have been employed to compare theoretical and experimental salting constants.

CALCULATION OF RATE CONSTANTS FROM UV-VIS SPECTROSCOPIC DATA : AN APPLICATION OF THE MARCUS-HUSH MODEL

Abstract The MMCT band in the binuclear complex μ -cyano pentacyano ruthenium(II) pentaamminecobalt(III) was obtained in different salt solutions. A correlation of optical data with rate constants of the thermal electron transfer in a related binuclear compound was found. Combination of the spectroscopic data with the results of quantum yield measurements allows an estimation of the backward (Co → Ru) electron transfer rate constant, k b . The value of k found is about 10 12 s −1 , and is shown to be equal to the preexponential factor of other rate constants corresponding to other electron transfer reactions in the binuclear complex. This analysis allows determination of all magnitudes characterizing the different optical and thermal electron transfer processes involved. Determination of these parameters is usually not possible from experiments. The present work shows how to overcome difficulties that arise when not all the required experimental information is available.