István Fábián

NMR relaxation studies in solution of transition metal complexes. V. Proton exchange reactions in aqueous solutions of VO2+-oxalic acid, -malonic acid systems

Abstract The equilibria existing in the VO 2+ -oxalic acid and -malonic acid systems have been studied pH-metrically and the proton exchanges between the bulk water and the different paramagnetic species have been insvestigated by measuring the T 2 relaxation time of water protons at 298 K, I = 1.0 M NaClO 4 . Mainly VOL and VOL 2− 2 type complexes are formed in both systems, the relative amounts of the protonated and mixed hydroxo complexes being very small up to pH 5. The first order rate constants of the proton exchange between the bulk water and the paramagnetic species are given. The surprisingly high value for the VOL 2− 2 type complexes is interpreted by a continuous intramolecular rearrangement of the ligands from two equatorial positions to axial-equatorial position and vice versa. The unexpectedly high rate constant for the protonated complex VOmalH + is interpreted by its fast acid dissociation process.

A common misconception about the Eyring equation

Linearization and direct fitting to the Eyring equation both give the entropy of activation with the same reliability as that of the enthalpy of activation.

Speciation and NMR relaxation studies of VO(IV) complexes with several O-donor containing ligands: oxalate, malonate, maltolate and kojate

Abstract Potentiometric, spectral and 1H NMR relaxation studies are reported on the complex formation of VO(IV) with the bidentate chelating ligands oxalic and malonic acids, of a dicarboxylic nature, and maltol and kojic acid, of the hydroxy-4-pyrone type. Complexes with stochiometries VOA and VOA2 were characterised in aqueous solution. Two main features were established for the complex systems. It was found that five-membered ring chelation favours the cis arrangement in the bis complexes of maltol and kojic acid to a higher extent than in those of oxalic and malonic acids. Moreover, previous investigations of VO(IV) binding to maltol or its analogues did not consider the formation of hydroxo species. Formation of dihydroxo bridged dimeric complexes of stoichiometry (VOAH−1)2 and monomeric hydroxo species VOA2H−1 or VOA2(OH), are now taken into account in order to explain the equilibrium and spectroscopic results. 1H NMR relaxation measurements strongly suggest an equilibrium between the two isomers of the bis complexes.

One- versus two-electron oxidation with peroxomonosulfate ion: reactions with iron(II), vanadium(IV), halide ions, and photoreaction with cerium(III).

The kinetics of the redox reactions of the peroxomonosulfate ion (HSO(5)(-)) with iron(II), vanadium(IV), cerium(III), chloride, bromide, and iodide ions were studied. Cerium(III) is only oxidized upon illumination by UV light and cerium(IV) is produced in a photoreaction with a quantum yield of 0.33 +/- 0.03. Iron(II) and vanadium(IV) are most probably oxidized through one-electron transfer producing sulfate ion radicals as intermediates. The halide ions are oxidized in a formally two-electron process, which most likely includes oxygen-atom transfer. Comparison with literature data suggests that the activation entropies might be used as indicators distinguishing between heterolytic and homolytic cleavage of the peroxo bond in the redox reactions of HSO(5)(-).

Air pollution assessment based on elemental concentration of leaves tissue and foliage dust along an urbanization gradient in Vienna

Foliage dust contains heavy metal that may have harmful effects on human health. The elemental contents of tree leaves and foliage dust are especially useful to assess air environmental pollution. We studied the elemental concentrations in foliage dust and leaves of Acer pseudoplatanus along an urbanization gradient in Vienna, Austria. Samples were collected from urban, suburban and rural areas. We analysed 19 elements in both kind of samples: aluminium, barium, calcium, copper, iron, potassium, magnesium, sodium, phosphor, sulphur, strontium and zinc. We found that the elemental concentrations of foliage dust were significantly higher in the urban area than in the rural area for aluminium, barium, iron, lead, phosphor and selenium. Elemental concentrations of leaves were significantly higher in urban than in rural area for manganese and strontium. Urbanization changed significantly the elemental concentrations of foliage dust and leaves and the applied method can be useful for monitoring the environmental load.

Numerical Simulations of Aqueous Ozone Decomposition

Abstract The decomposition of ozone in basic aqueous solutions was simulated on the basis of two previously reported detailed mechanisms. According to the calculations, the mechanism given by Tomiyasu, Fukutomi and Gordon5 is an appropriate model for the ozone decomposition, both in the presence and the absence of hydrogen peroxide and/or carbonate ion in basic solution. On the basis of this mechanism the main chain carrier is the ozonide ion radical .

Experimental Aspects of Mechanistic Studies on Aqueous Ozone Decomposition in Alkaline Solution

Abstract Ozone decomposition in aqueous solution was studied by the stopped - flow method over the pH 10.4 - 13.2 range at 25 ± 0.1 °C and I = 0.5 M NaClO4. At 260 nm the molar absorptivity of aqueous ozone was determined to be 3135 ± 22 M−1cm−1. It was shown that various experimental factors may significantly alter the course of the reaction. Even small amounts of H2O2 absorbed by the plastic parts of the stopped-flow instrumént can affect the kinetic features of the reaction for an extended period of time. Under strictly controlled experimental conditions sufficiently reproducible data could be obtained for the decomposition. The data were evaluated by comparing experimental and simulated kinetic traces. A detailed kinetic model was developed which is able to predict the decay and life-time of ozone as well as the formation and decomposition of the ozonide ion radical (O3 −) over the pH 10.4 - 13.2 range.

NMR relaxation studies in solutions of transition metal complexes. VI. Equilibria and proton exchange processes in aqueous solutions of VO2+-glycine system

The equilibria in aqueous solution of the VO2+-glycine system has been studied by pH-metry in very high ligand excess, to avoid the hydrolysis of vanadyl ions. The complexes VOG+, VOG2, VOGH2+, VOG2− H+ and VOG2H−−1 = VOG2(OH)− are dominating in the system; their formation constants are given. The formation of complexes of VOGH−1 = VOG(OH), VOGH−−2 = VOG(OH)−2 and (VO)2G2H−2 = (VO)2G2(OH)2 have also been detected. It is stated that the VO2G2(OH)2 is not the only binuclear (polynuclear) complex in the system; the composition of the other polynuclear complexes, however, cannot be stated unambiguously because of their very low concentrations. The rate constants of the proton exchange between the bulk water and the different complexes have been determined by measuring the T2 relaxation time of the water protons. In contrast to the oxalate, and some other vanadyl complexes, the rate constant decreases by the decrease of the number of water molecules remaining in the first coordination sphere of the vanadyl ion. The exceedingly high proton exchange rate constant for the VOG2(OH)− mixed hydroxo complex is interpreted as being due to the direct proton exchange between the bulk water and the coordinated OH group.

Trace element concentrations in soils along urbanization gradients in the city of Wien, Austria

Urban soil is an important component of urban ecosystems. This study focuses on heavy metal contamination in soils of Wien (Austria) and results are compared to those for a few large European cities. We analysed the elemental contents of 96 samples of topsoil from urban, suburban and rural areas in Wien along a dynamic (floodplain forest) and a stable (oak–hornbeam forest) urbanization gradient. The following elements were quantified using ICP-OES technique: Al, As, Ba, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Pb, P, S and Zn. For heavy metals PI (pollution index) values were used to assess the level of pollution. The PI values indicated high level of pollution by Pb in the suburban and rural area of stable gradient and in the urban area of dynamic gradient; moderate level of pollution was indicated for Cd in the urban area of stable gradient. The level of pollution was moderate for Co in the suburban and rural area of the stable gradient, and for Cu in suburban area of stable gradient, and urban area of dynamic gradient. The pollution level of Zn was moderate in all areas. Urban soils, especially in urban parks and green areas may have a direct influence on human health. Thus, the elemental analysis of soil samples is one of the best ways to study the effects of urbanization. Our results indicated that the heavy metal contamination was higher in Wien than in a few large European cities.

Reactive intermediates in aqueous ozone decomposition: A mechanistic approach

This paper provides a detailed account of recent developments in the mechanistic interpretation of aqueous ozone decomposition. Experimental complications with collecting reliable kinetic information for this system are illustrated via a few examples. Classical and comprehensive data treatment methods are compared, and an advanced kinetic model is presented on the basis of simultaneous evaluation of about 60 kinetic traces recorded at 260, 430, and 600 nm. These are the wavelengths of the absorbance maxima of O3, O3-, and CO3-, respectively. The model is used to explore specific features of the initiation sequence, the role of reactive intermediates, and the carbonate ion inhibition. It is also demonstrated how a complex kinetic model can be validated by studying the system under specific experimental conditions.