Neutral thallium clusters in aqueous solutions. Pulse radiolysis study

The formation of neutral Tl₂ (λ_max = 390 nm) and Tl₄ (λ_max = 360 nm) clusters in dilute aqueous solutions of Tl₂SO₄ containing formate ions was found by pulse radiolysis. The rate constants for the recombination of Tl⁰ atoms and Tl₂ clusters are equal to 1.5·10¹⁰ L mol⁻¹ s⁻¹ and 1.0·10¹⁰Lmol⁻¹ s⁻¹ (±30%), respectively, and the extinction coefficient of Tl₂ at 390 nm is −6.0·10³ L mol⁻¹ cm⁻¹ Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2367–2369, December, 1999.     

Pulse radiolysis study of one-electron oxidation of thionine in aqueous solutions

One-electron oxidation of thionine has been studied using specific oxidizing radicals such as Cl⁻Tl(II) and N₃ generated by pulse radiolysis of aqueous solutions. The semioxidized thionine exhibited threepK’s indicating four conjugate acid-base forms. N₃ radicals were found to be less efficient in oxidizing thionine as compared to Cl ₂ ⁻ , Tl²⁺ and Tl(OH)⁺. The rate constants for electron abstraction from thionine by Cl ₂ ⁻ , Tl²⁺, Tl(OH)⁺, Tl(OH)₂ and N₃ were evaluated. The spectra of different protonated forms of semioxidized thionine and the extinction coefficients at λ_max are presented. Reaction of OH radicals with thionine gave transient products whose spectra and acid-base properties were different from those of semioxidized thionine. The rate constant for formation of the product transient agrees well with competition kinetic value for reaction of OH with thionine reported earlier.     

Pulse radiolysis of adrenaline in acid aqueous solutions

Pulse radiolysis of adrenaline in acid aqueous solutions (pH 1–3) was carried out. The rate constants for the reactions of adrenaline with H and OH were determined: k(H + adr.) = (0·9±0·1) × 10⁹ dm³ mol⁻¹ s⁻¹; k(OH + adr.) = (1·65±0·15) × 10¹⁰ dm³ mol⁻¹ s⁻¹. The H-adduct of adrenaline has two λ_max, at 280 and 355 nm, with ϵ₂₈₀ = 420 m² mol⁻¹ and ϵ₃₅₅ = 390 m² mol⁻¹, which disappears according to a first order reaction, k₁ = 1·4 × 10³ s⁻¹. The spectra formed by OH attack was assigned to the corresponding benzoxy radical with absorption maxima at 285 and 365 nm and ϵ₂₈₅ = 620 m² mol⁻¹ and ϵ₃₆₅ = 105 m² mol⁻¹. Due to the overlapping of the intermediates, no decay kinetics could be obtained.     

Pulse radiolysis of aqueous solutions of tetraphenylphosphonium ions

The reactions of tetraphenylphosphonium ion (Ar₄P⁺) with e_aq⁻, H atoms and OH radicals have been investigated. The absorption spectra of three transient species were obtained. Ar₄P· radicals formed by the reaction: Ar₄P⁺ + e_aq⁻ → Ar₄P· have a maximum absorption at 305 nm [ε₃₀₅ = (9400 ± 300) dm³ mol⁻¹ cm⁻¹] and decays by second-order kinetics with the rate constant 2k = (2·7 ±0·4) × 10⁹ dm³ mol⁻¹ s⁻¹. H atoms and OH radicals form transient adducts to the phenyl groups of the Ar₄P⁺ ion with the rate constants of (1·5 ± 0·3) × 10⁹ dm³ mol⁻¹ s⁻¹ and (3·0±0·3) × 10⁹ dm³ mol⁻¹ s⁻¹, respectively. Both adducts have broad absorption spectra at 300≈380 nm (λ_max = 340 nm) with the molar extinction coefficients ε₃₄₀ = 5400±300 dm³ mol⁻¹ cm⁻¹ for the H adduct and ε₃₄₀ = 3500±200 dm³ mol⁻¹ cm⁻¹ for the OH adduct.     

Pulse radiolysis of pyridine and methylpyridines in aqueous solutions

The radicals formed from pyridine, 3-methylpyridine, 3,5-dimethylpyridine, 2,6-dimethylpyridine and 2,4,6-trimethylpyridine by attack of H, e^-_aq, OH and O^•- in acqueous solutions were investigated by pulse radiolysisin the pH-range 1–13.8. The UV-vis. absorption spectra as well as the formation and decay kinetics for the protonated and unprotonated forms of the methylpyridine radicals studied are presented. The pK_a-values for the OH-adducts were determined.     

4-氯酚稀水溶液的脉冲辐解研究

利用脉冲电子束进行了多种条件下4－氯酚稀水溶液的脉冲辐解研究,对其瞬态光谱中的主要吸收峰作了归属,并初步考察了这些瞬态物种的生长、衰减等行为,研究表明,·OH基与4－氯酚在碱性条件下反应生成氯代酚氧基,速率常数为4.14×10^9L/(mol·s),在酸性条件下要经过OH-adducts;H原子与4-氯酚反应生成H-adducts的速率常为2.0×10^9L/(mol·s),产物可通过双分子二级反应逐步脱氯;eaq^-可直接从4-氯酚分子夺氯,反应速率常数为1.82×10^9/L(mol·s)。     

Pulse radiolysis investigations of one-electron reduction of terephthalonitrile in aqueous solutions

Pulse radiolysis of aqueous solutions of terephthalonitrile (TPN) was carried out to study the redox properties of anions of TPN, by determining the rate constants for the reaction of TPN with specific one-electron reducing species formed on reaction of OH radicals with methyl,ethyl,isopropyl alcohols,tetrahydrofuran and cytosine. Formation of anions of TPN, which absorb at wavelength 340 nm, was followed. From the graph of rate constants and maximum absorbance vs reduction potential of reducing species, reduction potential of TPN was found out to be —0.85 V vs NHE. Reduction of thionine (TH⁺) and methyl viologen (MV²⁺) with the help of radical anions of TPN was carried out to establish its high reduction potential.     

Pulse radiolysis of pentacyanonitrosylferrate/II/ ions in aqueous solutions

Pulse radiolysis of aqueous solutions of sodium pentacyanonitrosylferrate/II/ Na₂[Fe/CN/₅NO] /sodium nitroprusside/ was studied in the presence of ethylene glycol as an OH radical scavenger. The rate constants of the one-electron reduction of nitroprusside ion were measured with e _q ^– and with radicals derived from some alcohols /ethylene glycol, ethanol, 2-propanol/ as reducing species. The results show that the transition state for the reduction by alcohol radicals is polar. The only observed product of reduction is the Fe/CN/₅NO^3– ion, which then undergoes a slow dissociation to form Fe/CN/₄NO^2–. Only a small isotope effect k_H/k_D=1.08 was observed in D₂O solutions for the dissociation reaction. This suggests an intramolecular electron transfer as rate-determining step for the dissociation reaction.Dedicated to Professor Schulte-Frohlinde on the occasion of his 60th birthday.     

Pulse radiolysis of aqueous diphenyloxide

Pulse radiolysis of aqueous diphenyloxide (DPO) has been performed under various experimental conditions. The OH radicals react with DPO on various positions of the molecule with a rate constant, k=2.1×10¹⁰ l mol⁻¹ s⁻¹. The major reaction step appears to be a cleavage of the C–O bond of DPO resulting into C₆H₄OH (λ=285 nm) and C₆H₅O(λ=325 nm) radicals in addition to DPO–OH adducts. They disappear according to a second-order reaction. In the presence of air or in a gas mixture of N₂O:O₂=4:1 the DPO–OH adducts are scavenged by oxygen, resulting into peroxyl radicals, which are long-lived species. For the reaction of e_aq⁻ with DPO a rate constant, k=2×10¹⁰ l mol⁻¹ s⁻¹ was found.     

Pulse radiolysis study on one-electron oxidation of 1-naphthylamine-4-sulphonic acid in aqueous solutions

Reactions of 1-naphthylamine-1-sulphonic acid (NASA) with hydroxyl radicals and oneelectron oxidants such as N₃, Br₂ ^- and Cl₂ ^- radicals have been studied at various pHs using pulse radiolysis technique. Rate constants for the reaction of N₃ and Br₂ ^-. radicals with NASA at neutral pH were found to be 5 × 10⁹ and 4 × 10⁸ dm³ mol^-1 s^-1 respectively. These reactions led to the formation of a cation radical (semi-oxidized species). OH radical reaction with NASA (k = 7.2 × 10⁹ dm³ mol^-1 s^-1) at neutral pH gave a mixture of species, namely, a semi-oxidized species as well as an adduct species. Cl₂ ^-. radicals reacted with NASA rather slowly (k = 7 × 10⁷ dm³ mol^-1 s^-1) at pH 1 to give the semioxidised species. However, even at pH 1, OH radical reaction with NASA gave a mixture containing semi-oxidized as well as an adduct species. The OH-adduct species having _max at 340 nm decays at acidic pHs to give semi-oxidized species having _max at 370 nm. Electron adduct of NASA was found to be a strong reducing radical.     

Radiolytic degradation of 4-nitrophenol in aqueous solutions: Pulse and steady state radiolysis study

The radiation induced degradation of 4-nitrophenol (4-NP) has been studied by gamma irradiation, while the reactivity and spectral features of the short lived transients formed by reaction with primary transient radicals at different pHs has been investigated by pulse radiolysis technique. In steady state radiolysis a dose of 4.4 k Gy is able to degrade 98% of 1×10⁻⁴ mol dm⁻³ 4-NP. 4-NP has pK_a at 7.1, above which it is present in the anionic form. At pH 5.2, ^•OH and N₃^• radicals were found to react with 4-NP with rate constants of 4.1×10⁹ dm³ mol⁻¹ s⁻¹ and 2.8×10⁸ dm³ mol⁻¹ s⁻¹, respectively. Differences in the absorption spectra of species formed in the reactions of 4-NP with ^•OH and N₃^• radicals suggested that ^•OH radicals add to the aromatic ring of 4-NP along with electron transfer reaction, whereas N₃^• radicals undergo only electron transfer reaction. At pH 9.2, rate constants for the reaction of ^•OH radicals with 4-NP was found to be higher by a factor of 2 compared to that at pH 5.2. This has been assigned to the deprotonation of 4-NP at pH 9.2.     

Pulse radiolysis study of electrons in frozen alkaline solutions

The decay kinetics of e _t ^– optical absorption has been analyzed for 10 M frozen alkaline solutions of Na⁺, K⁺ and Rb⁺ cations. Samples were irradiated with 8 MeV electron pulses at temperatures in the range 92 to 160 K. The change of absorption with time depends on wavelength, temperature and cation used. To interprete the influence of cations and temperature on behaviour of electrons in the system examined a first-order kinetics with time dependent rate constant k(t)=B·t^–1 was used. The dependence of parameters and B on the kind of cation and irradiation conditions is discussed.     

Pulse radiolysis study of redox reactions of safranine T in aqueous solutions: One electron oxidation

One electron oxidation of safranine T by specific oxidizing radicals such as Cl^-₂, Tl²⁺, Tl(OH)⁺, N^.₃, Br^-₂ etc. has been studied using the nanosecond pulse radiolysis technique. Reaction of free Br^. atom has also been investigated at neutral pH. The semioxidized safranine species formed by these reactions have been shown to exist in two conjugate acid-base forms with pK_a=4.0. Their spectral and kinetic parameters have been evaluated. Using N^.₃/N^-₃ and I^-₂/2I^- as reference couples, the one electron reduction potential of the semioxidized safranine has been determined to be 1.13±0.02 V vs NHE. The absorption spectra, second order decay rate constant and the pK_a of the OH-reaction product revealed features quite different from that of the semioxidized species suggesting that the mode of OH reaction is not via electron abstraction.     

Pulse radiolysis of epicatechin in aqueous solution

Pulse radiolysis of epicatechin in aqueous solution has been done to investigate the reactions of epicatechin derived phenoxy radical (EpO) at neutral pH. EpO was generated by N₃ reacting toward EpOH, the rate constant was measured to be 3 × 10⁸ dm³ mol⁻¹ s⁻¹. The biomolecular termination of EpO is rather slow ((2k < × 10⁶ dm³ mol⁻¹ s⁻¹) and results in products exhibiting strong visible absorption around 450 nm. No reactions have been observed for EpO with O₂ and O₂ in the time scale of pulse radiolysis (0.01 s), suggesting the bimolecular rate constant are less than 10⁴ and 5 × 10⁶ dm³ mol⁻¹ s⁻¹, respectively.     

Pulse radiolysis of aqueous thiocyanate solution

The pulse radiolysis of N(2)O saturated aqueous solutions of KSCN was studied under neutral pH conditions. The observed optical absorption spectrum of the SCN(*) radical in solution is more complex than previously reported, but it is in good agreement with that measured in the gas phase. Kinetic traces at 330 and 472 nm corresponding to SCN(*) and (SCN)(2)(*-), respectively, were fit using a Monte Carlo simulation kinetic model. The rate coefficient for the oxidation of SCN(-) ions by OH radicals, an important reaction used in competition kinetics measurements, was found to be (1.4 +/- 0.1) x 10(10) M(-1) s(-1), about 30% higher than the normally accepted value. A detailed discussion of the reaction mechanism is presented.     

Pulse radiolysis of 3-pyridine methanol and 3-pyridine carboxaldehyde in aqueous solutions

Reactions of e_aq^-, H-atom and OH radicals with 3-pyridine methanol (3-PM) and 3-pyridine carboxaldehyde (3-PCA) have been studied at various pHs using pulse radiolysis technique. e_aq^- was found to be highly reactive with both 3-PM and 3-PCA (k approx. 10¹⁰ dm³ mol¹ s^-1). Semi-reduced species formed in both cases were strongly reducing in nature. In the case of 3-PM, electron addition leads to the formation of pyridinyl radicals whereas in the case of 3-PCA, PyCHOH type radicals are formed. At pH 6.8, H-atom reaction with 3-PCA also gives semi-reduced species (PyCHOH), whereas at pH 1, H-atoms add to the ring. (CH₃)₂˙COH radicals were found to transfer electron to 3-PCA at all the pH values tested and by making use of changes in the absorption spectra, pK_a values of the semi-reduced species were determined to be 4.5 and 10.6. OH radicals were found to undergo addition reaction with 3-PCA, whereas in the case of 3-PM they reacted by H-abstraction as well as addition reaction. By following the yield of methylviologen radical cation formed by electron transfer reaction, it was estimated that approx. 50% of OH radicals react with 3-PM by H-atom abstraction at pH 6.8, giving reducing radicals, whereas at pH 3.2, where 3-PM is in the protonated form, the same is only about 10%. At pH 13, O^-˙ radical anions were found to react exclusively by H-atom abstraction. Reaction of SO₄^-˙ radicals with 3-PCA was found to give a species identical to the one formed by one electron reduction of nicotinic acid at acidic pH values.