Gertraud Mark

OH-radical formation by ultrasound in aqueous solution--Part II: Terephthalate and Fricke dosimetry and the influence of various conditions on the sonolytic yield.

Terephthalate and Fricke dosimetry have been carried out to determine the sonolytic energy yields of the OH free radical and of its recombination product H2O2 in aqueous solutions under various operating conditions (nature of operating gas, power, frequency, temperature). For example, in the sonolysis of Ar-saturated terephthalate solutions at room temperature, a frequency of 321 kHz, and a power of 170 W kg-1, the total yield [G(.OH) + 2 G(H2O2)], equals 16 x 10(-10) mol J-1. This represents the total of .OH that reach the liquid phase from gas phase of the cavitating bubble. The higher the solute concentration, the lower the H2O2 production as more of the OH free radicals are scavenged, in competition with their recombination. Fricke dosimetry, in the absence and presence of Cu2+ ions, shows that the yield of H atom reaching the liquid phase is much lower, with G(H.) of the order of 3 x 10(-10) mol J-1. These sonolytic yields are smaller in solutions that are at the point of gas saturation, and increase to an optimum as the initial sonication-induced degassing and effervescence subsides. The probing of the sonic field has shown that the rate of sonolytic free-radical formation may vary across the sonicated volume depending on frequency and power input.

OH radical formation by ultrasound in aqueous solutions Part I: the chemistry underlying the terephthalate dosimeter

Abstract The terephthalate dosimeter is widely used in sonochemical studies. Details of the underlying chemistry have been elucidated using ionizing radiation techniques. Hydroxyl radicals were generated radiolytically in N 2 O-saturated aqueous solutions of terepthalate ions (10 −3 mol dm −3 ). The products were studied after γ-radiolysis and the kinetics were followed by pulse radiolysis. The OH radicals add rapidly to the ortho - and to a much lesser extent to the ipso -positions of the terephthalate ions. The resulting hydroxycyclohexadienyl radicals display a strong absorption at 350 nm ( e ≈4000 dm 3 mol −1 cm −1 ). They decay bimolecularly (2 k = 2.5 × 10 8 dm 3 mol −1 s −1 at pH 5 and 2 k = 4 × 10 7 dm 3 mol −1 s −1 at pH 10.2). They can be rapidly oxidized by IrCl 6 2− ( k = 7.7 × 10 7 dm 3 mol −1 s −1 ), yielding 2-hydroxyterephthalate ions in 84% yield ( G =4.9 × 10 −7 mol J −1 ). As further minor products 4-hydroxybenzoic acid ( G =0.2 × 10 −7 mol J −1 ) and 3-hydroxybenzoic acid ( G =0.1 × 10 −7 mol J −1 ) were also observed. When O 2 is used as oxidant the yield of 2-hydroxyterephthalic acid as much less [ G =2.1 × 10 −7 mol J −1 in N 2 OO 2 (4:1)-saturated solutions]. Pulse radiolysis revealed that O 2 adds to the hydroxycyclohexadienyl radicals with a rate constant k f =1.6×10 7 dm 3 mol −1 s −1 . The O 2 addistion is however, reversible ( k r =3.4 × 10 3 s −1 ), the equilibrium constant being K = 4800 dm 3 mol −1 . The hydroxycyclohexadienylperoxyl radicals undergo an HO 2 . elimination (leading to 2-hydroxyterephthalate ions) in competition with other (ring fragmentation) reactions. These reactions occur with an overall rate constant of 390s −1 . 2-Hydroxyterephthalate ions are readily detected by their fluorescence ( λ exc =315 nm, λ em =425 nm). Since other products do not interfere, this assay can be used for the determination of OH radical production not only in the radiolysis but also in the sonolysis of water.

The photochemistry of aqueous nitrate ion revisited

Aqueous nitrate solutions were photolysed at 254 nm in the absence of oxidizable additives, in the presence of methanol or propan-2-ol and oxygen and in the presence of cyclopentane under anaerobic conditions. The main nitrogen-containing products are nitrite and peroxynitrite. The quantum yields depend on the pH, nitrate concentration, nature of the additive and the light intensity. The intrinsic nitrite yield in alkaline solutions could not be determined directly because, under the conditions of the nitrite assay, the accompanying peroxynitrite decomposes to form nitrite and nitrate; it is smaller than the apparent nitrite yield. In the acidic (pH 4–7) range, the intrinsic nitrite quantum yield is equal to the apparent nitrite yield because there is no buildup of peroxynitrite under these conditions. The apparent nitrite quantum yield increases from 0.01 (no oxidizable additive) to approximately 0.03 (cyclopentane (millimolar range), oxygen free) to 0.06 (methanol (millimolar range), air saturated). At pH 13 and in the absence of oxidizable additives, the apparent nitrite quantum yield increases to about 0.1, whereas from material balance considerations the intrinsic nitrite quantum yield is estimated to be 0.06, twice the oxygen quantum yield of 0.03. Spectrophotometrically, peroxynitrite is detected in the alkaline range only, because its protonated form is unstable. In the absence of oxidizable additives, the quantum yield of peroxynitrite is about 0.1, i.e. only about two-thirds of the quantum yield in the presence of oxidizable additives. Mechanistic considerations on the basis of the pH dependence of the quantum yields of the products nitrite, peroxynitrite and oxygen, as well as their dependence on the kind of additive, indicate that the decisive factor of photolysis in the absence of additives is the formation of the nitric oxide peroxyl radical, ONOO, formed by reaction of peroxynitrite with the primarily generated OH radical. The decay of ONOO is the source of O2 in this system. Nitric oxide, NO, the other fragment of this decay reaction, reacts with nitrogen dioxide, which is one of the primarily formed intermediates. The latter reaction is one of the pathways to the product nitrite, particularly in the alkaline range. The formation of NO during photolysis has been verified by electron spin resonance (ESR) spectroscopic detection of the nitroxide 1,1,3,3-tetramethyl-isoindolin-2-oxyl, the NO adduct to 7,7,8,8-tetramethyl-o-quinodimethane. Of the three primary processes discussed in the literature, we conclude that reactions (1) and (2) occur with quantum yields of approximately 0.09 and 0.1 respectively NO3−+hv→NO2+O−(O−+H2O→OH+OH−)(1)NO3−+hv→ONOO−(2) It appears that none of the peroxynitrite anion is formed in a cage reaction through the recombination of the primary fragments from reaction (1). The primary process shown in reaction (3) is of relatively minor importance, with a quantum yield of no more than 0.001 NO3−+hv→NO2−+O(3) In the presence of methanol (or propan-2-ol) and oxygen under acidic conditions, formaldehyde (or acetone) is formed in an amount equivalent to nitrite via peroxyl radical reactions (quantum yield of approximately 0.06 for both alcohols). In the alkaline range, the apparent formaldehyde quantum yield decreases with increasing pH, while formic acid is produced in increasing amounts. The formation of formic acid is ascribed to the reaction of peroxynitrite anion with photolytically generated formaldehyde. The acetone quantum yield does not decrease with increasing pH over the whole alkaline pH range. In the presence of cyclopentane under oxygen-free conditions, apart from nitrite (and peroxynitrite when alkaline), the compounds nitrocyclopentane, cyclopentyl nitrate, cyclopentene, cyclopentanol and cyclopentanone are produced. The formation of the organic nitrogen compounds leads to an increase in the pH as photolysis proceeds. This pH shift is particularly pronounced in the neutral range.

The photolysis of potassium peroxodisulphate in aqueous solution in the presence of tert-butanol : a simple actinometer for 254 nm radiation

Aqueous potassium peroxodisulphate (0.01 mol dm−3) together with tert-butanol (0.1 mol dm−3) was photolysed with 254 nm light at 20 °C. The peroxodisulphate ion is cleaved, giving rise to sulphate radicals SO4−. These attack the tert-butanol under hydrogen abstraction and hydrogen sulphate formation. In deoxygenated solutions, Φ(H+) = 1.4, Φ(SO42−) = 1.4 and Φ((HOC(CH3)2CH2)2) is approximately 0.7; in the presence of oxygen, Φ(H+) = 1.8. The enhancement of Φ(H+) by oxygen comes about through the production of superoxide via one of the bimolecular decay channels of the tert-butanol-derived peroxyl radical. The superoxide radical reduces S2O82− under liberation of further SO4−. The oxygenated peroxodisulphate-tert-butanol system is shown to constitute a straightforward actinometer for 254 nm light. Some data and observations regarding the tert-butanol-free peroxodisulphate system are also presented.

Sonolysis of tert-butyl alcohol in aqueous solution

A product study of the sonolysis of the volatile substrate t-butanol in aqueous solution indicates that substrate decomposition is practically completely determined, even at concentrations as low as millimolar, by oxidative pyrolysis going on in the gas phase within the collapsing cavitational bubble. OH-Radical-induced reactions in solution are insignificant since the volatility of this substrate, its gas-phase concentration within the bubble enhanced by a certain degree of hydrophobicity, causes OH radicals generated thermolytically from water vapour to be intercepted before they can reach the aqueous phase. The nature of the products, as well as the t-butanol-concentration dependence of the product yields, can be qualitatively explained on the basis of the t-butanol-pyrolysis mechanism. Kinetic considerations involving the relative yields of the pyrolysis products ethane, ethylene and acetylene lead to an estimate of a value of 3600 K for the average pyrolysis temperature at a t-butanol bulk concentration of 10–3 molar.

The Reactions of Nitrite Ion with Ozone in Aqueous Solution – New Experimental Data and Quantum-Chemical Considerations

Ozone reacts with nitrite mainly to nitrate and singlet oxygen (1O2, 96% yield), but peroxynitrite (2.6%) and •OH radicals (∼8%) are also formed. The latter are generated in the course of the reactions towards peroxynitrite and upon its decay. They also result from the reaction of ozone with nitrite by electron transfer ( , ∼4%). Ozone adducts (O2NOOO− en route to nitrate, ONOOO− en route to peroxynitrite) are likely intermediates as evaluated by Density Functional Theory (DFT) calculations (Jaguar program).

Zur Photodimerisierung von 3-Methyl-2-cyclopentenon

ZusammenfassungBei der Belichtung von 3-Methyl-2-cyclopentenon in Lösung entstehen die Isomeren Cyclobutane I, II, III und IV sowie zwei weitere Dimere, V und VI, die ein Oxocyclopentylcyclopentenon-Gerüst besitzen. Das Verhältnis der gebildeten Dimeren ist abhängig vom Lösungsmittel und von der Konzentration der belichteten Lösung. Die Logarithmen der Dimerenverhältnisse korrelieren linear mit demKirkwood-Onsager-Parameter des Lösungsmittels; der Konzentrationseinfluß läßt sich durch den gemessenenKirkwood-Onsager-Parameter der Lösung erfassen.AbstractIrradiation of 3-methyl-2-cyclopentenone in solution yields the isomeric cyclobutanes I, II, III and IV, and two other dimers with oxocyclopentyl-cyclopentenone structure (V and VI, resp.). The ratio of the dimers is dependent on the solvent and on the concentration of the irradiated solution. The logarithms of the dimer ratios are related linearly to theKirkwood-Onsager parameter of the solvent; the concentration effect can be correlated with the measuredKirkwood-Onsager parameter of the solution.

The Reaction of Ozone with Bisulfide (HS−) in Aqueous Solution – Mechanistic Aspects

The ozone demand to oxidize HS−/H2S [pKa(H2S) = 6.9, k(HS− + O3) = 3 × 109 M−1 s−1, k(H2S + O3) = 3 × 104 M−1 s−1] to SO4 2− is only 2.4 mol ozone per mol SO4 2− formed, much lower than stoichiometric 4.0 mol/mol if a series of O-transfer reactions would occur. As primary step, the formation of an ozone adduct to HS−, HSOOO–, is suggested that decomposes into HSO– and singlet oxygen (16%) or rearranges into peroxysulfinate ion, HS(O)OO– (84%). Potential reactions of the above intermediates are discussed. Some of these can account for the low ozone demand.