Reactions of Sulphate Radicals with Substituted Pyridines: A Structure–Reactivity Correlation Analysis

The kinetics of the oxidation of pyridine, 3‐chloropyridine, 3‐cyanopyridine, 3‐methoxypyridine and 3‐methylpyridine mediated by SO₄ ^{< M ‐>} radicals are studied by flash photolysis of peroxodisulphate, S₂O₈^2?, at pH 2.5 and 9. The absolute rate constants for the reactions of both, the basic and acid forms of the pyridines, are determined and discussed in terms of the Hammett correlation. The monosubstituted pyridines react about 10 times faster with sulphate radicals than their protonated forms, the pyridine ions. The organic intermediates are identified as the corresponding hydroxypyridine radical adducts and their absorption spectra compared with those estimated employing the time‐dependent density functional theory with explicit account for bulk solvent effects. A reaction mechanism which accounts for the observed intermediates and the pyridinols formed as products is proposed.     

A kinetic study of the reactions of sulfate and dihydrogen phosphate radicals with epicatechin,epicatechingallate, and epigalocatechingallate

The bimolecular rate constants for the reactions of sulfate radicals with epicatechin (EC), epicatechingallate (ECG), and epigallocatechingallate (EGCG) were found to be (1.46 ± 0.06) × 10⁹, (1.20 ± 0.08) × 10⁹, and (1.04 ± 0.07) × 10⁹, respectively. The activation energy [E_A = 9 ± 3 kJ mol^?1] and preexponential factor [A = (4.8 ± 0.6) × 10¹⁰] for the reaction of EC with the sulfate radical were measured in the temperature range 288–303 K. The phenoxyl radicals of EC (λ_max = 310 nm) were obtained both by the reaction of this flavonoid with the sulfate radicals and by photoionization. The measured bimolecular rate constants for the reactions of the dihydrogen phosphate radicals with EC, ECG, and EGCG were (7.8 ± 0.9) × 10⁸, (8.5 ± 0.4) × 10⁸, and (6.8 ± 0.4) × 10⁸, respectively. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 391–396, 2010     

Reduction of mercury(II) by the carbon dioxide radical anion: a theoretical and experimental investigation

The laser flash photolysis technique (λ(exc) = 266 nm) was used to investigate the mechanism of the HgCl(2) reduction mediated by CO(2)(·-) radicals in the temperature range 291.7-308.0 K. For this purpose, the CO(2)(·-) radicals were generated by scavenging of sulfate radicals by formic acid. The absorbance of the reduced radical of methyl viologen, a competitive scavenger of CO(2)(·-), was monitored at 390 nm. Moreover, theoretical calculations, including solvent effects, were also performed within the framework of the density functional theory for various chemical species of Hg(I) and Hg(II) to aid in the modeling of the reaction of reduction of HgCl(2) by CO(2)(·-).     

Kinetics of O·− and O3·− in alkaline aqueous solutions of hydrogen peroxide

The photolysis of strong alkaline (pH>12.7) solutions of H₂O₂ yields O^·−, which in the presence of molecular oxygen forms the ozonide radical ion, O₃^·−. A detailed kinetic study on the reaction mechanisms involved during formation and decay of O₃^·− radical ions in these solutions, in the presence and absence of added O^·−/HO^· scavengers is reported. In order to obtain a complete interpretation of the experimental data, kinetic computer simulations were done using a complete set of reactions. A very good agreement between experimental and computer simulated data is obtained. The following simplified mechanism accounts for the observed first-order decay of O₃^·− in alkaline hydrogen peroxide solutions: O^·− + O₂ → O₃^·− O₃^·− → O^·− + O₂ O^·− + S → OH^· + S → HO^· + HO₂⁻ → O₂^·− + H₂O O^·− + HO₂⁻ → O₂^·− + HO⁻ with S: O^·−/HO^· scavengers. © 1997 John Wiley & Sons, Inc.     

A kinetic study of the aqueous phase oxidation of di-μ-oxo-bis(L-cysteine)-oxomolybdenum(V) by molecular oxygen

The molecular oxygen-mediated decomposition of the binuclear complex, prepared from oxomolybdate(V) and L-(+)-cysteine, was studied spectrophotometrically at pH 3.5–5.6. The formation of MoVI was detected. The effects of pH and [O2] on the decomposition kinetics are given by the equation:

Two choices for the functionalization of silica nanoparticles with gallic acid: characterization of the nanomaterials and their antimicrobial activity against <Emphasis Type="Italic">Paenibacillus larvae</Emphasis>

Silica nanoparticles attached to gallic acid were synthesized from 7-nm diameter fumed silica particles by different functionalization methods involving the condensation of hydroxyl or carboxyl groups. The particles were characterized by thermal analyses and UV–vis, FTIR, NMR, and EPR spectroscopies. In comparison to free gallic acid, enhanced stability and increased antimicrobial activity against Paenibacillus larvae were found for the functionalized nanoparticles. Thus, both derivatization strategies result in improved properties of the natural polyphenol as antimicrobial agent for the treatment of honeybee pathologies.     

Photodegradation of soil organic matter and its effect on gram-negative bacterial growth

To learn more about the role of the reactive oxygen species (ROS) in the production of bioavailable products of the dissolved organic matter, we investigate here the effect of the photolysis (lambda(exc) > 320 nm) of a soil extract (SE) on the growth of bacteria isolated from the same soil as used for obtaining the extract. Comparative experiments with Aldrich humic acid (AHA) as substrate were performed. The photodegradation of the SE was evaluated with different techniques-UV-visible absorption spectroscopy, fluorescence excitation emission matrices (EEM) and Fourier transform infrared spectroscopy (FTIR). Known ROS scavengers were employed to study the effect of photochemically produced ROS on the photodegradation of the substrates. To evaluate the effect of irradiation on the bioavailability of the SE and AHA, photolyzed and nonphotolyzed substrates were added to different culture media and the growth of Pseudomonas sp. isolated from the soil and a strain of Escherichia coli were studied. The different results obtained were assigned to the dissimilar metabolisms of both bacteria.     

Photolytic and radiolytic oxidation of humic acid

The reactions of Br(2)(˙-), Br˙, HO˙ and N(3)˙ with Aldrich humic acid (AHA) were investigated. The Br/Br(2)(˙-) radicals were obtained in flash-photolysis experiments (λ(exc) = 266 nm) performed with NaS(2)O(8) solutions in the presence of bromide ions. HO˙ and N(3)˙ radicals were generated by pulse radiolysis of N(2)O-saturated solutions. From the combination of a bilinear analysis and computer simulations of the absorbance traces, it was possible to obtain information on the rate constants for the reactions of Br(2)(˙-), Br˙, HO˙ and N(3)˙ with AHA and on the intermediate species involved in the mechanism. Evidence for the participation of phenoxyl radicals (λ(max) = 410 nm) is given.     

Reactions of Cl*/Cl2*- radicals with the nanoparticle silica surface and with humic acids: model reactions for the aqueous phase chemistry of the atmosphere

Reactions of chlorine radicals might play a role in aqueous aerosols where a core of inorganic components containing insulators such as SiO2 and dissolved HUmic-LIke Substances (HULIS) are present. Herein, we report conventional flash photolysis experiments performed to investigate the aqueous phase reactions of silica nanoparticles (NP) and humic acid (HA) with chlorine atoms, Cl*, and dichloride radical anions, Cl2*-. Silica NP and HA may be taken as rough models for the inorganic core and HULIS contained in atmospheric particles, respectively. Both Cl* and Cl2*- were observed to react with the deprotonated silanols on the NP surface with reaction rate constants, k +/- sigma, of (9 +/- 6) x 10(7) M(-1) s(-1) and (7 +/- 4) x 10(5) M(-1) s(-1), respectively. The reaction of Cl* with the surface deprotonated silanols leads to the formation of SiO* defects. HA are also observed to react with Cl* and Cl2*- radicals, with reaction rate constants at pH 4 of (3 +/- 2) x 10(10) M(-1) s(-1) and (1.2 +/- 0.3) x 10(9) M(-1) s(-1), respectively. The high values observed for these constants were discussed in terms of the multifunctional heterogeneous mixture of organic molecules conforming HA.     

Kinetic studies on the sulfate radical-initiated polymerization of vinyl acetate and 4-vinyl pyridine in the presence of silica nanoparticles

The photolysis of silica suspensions of pH approximately 8 containing peroxodisulfate ions leads to the generation of two "surface transients" with a distinct spectrum and reactivity. Time-resolved and continuous irradiation experiments of similar dispersions also containing variable concentrations of vinyl acetate (VA) or 4-vinyl pyridine (4-VP) allowed the evaluation of the contribution of silica/water interfacial reactions to the kinetics and structural pattern of polymers synthesized using sulfate radicals as initiators. The rate constants measured for the reactions of the surface transients with 4-VP are 1 order of magnitude higher than those of VA, despite the fact that both species show similar reactivity in homogeneous solution toward sulfate radicals. It is suggested that both the sorption capacity and the different specific interactions with the silica surface of 4-VP and VA contribute to the observed reaction rates. Micrometer-sized latex particles of 4-VP and VP showed higher stability and more homogeneous size distributions when obtained in the presence of silica nanoparticles. Under the experimental conditions required for obtaining polymer particles, both the contribution of the described interfacial reactions and the effect of silica adsorbed monomer on the initiation steps of the polymerization may be neglected. The importance of in situ adsorption of the oligomer/polymer chains to silica NP during the polymerization propagation steps in determining the particle morphology is discussed.