Kinetic, spectral and redox properties of the transients formed on one-electron reduction of 3, 3, 6, 6-tetramethyl-3, 4, 6, 7, 9-pentahydro-10-phenyl (1, 8) (2H, 5H) acridinedione in aqueous-organic mixed solvent system: A pulse radiolysis study

The phenyl substituted acridine-1,8-dione (AD) dye reacts with (CH₃)₂*COH radicals with a bimolecular rate constant of 0.6 × 10⁸ dm³ mol⁻¹ s⁻¹ in acidic aqueous-organic mixed solvent system. The transient optical absorption band (λ_max = 465 nm, ɛ = 6.8 × 10² dm³ mol⁻¹ cm⁻¹) is assigned to ADH* formed on protonation of the radical anion. In basic solutions, (CH₃)₂*COH radicals react with a bimolecular rate constant of 4.6 × 10⁸ dm³ mol⁻¹ s⁻¹ and the transient optical absorption band (λ_max = 490 nm, ɛ = 10.4 × 10³ dm³ mol⁻¹ cm⁻¹) is assigned to radical anion, AD*⁻, which has a pK_a value of 8.0. The reduction potential value of the AD/AD*⁻ couple is estimated to be between −0.99 and −1.15 V vs NHE by pulse radiolysis studies. The cyclic voltammetric studies showed the peak potential close to −1.2 V vs Ag/AgCl.     

Radical scavenging reactions of chlorogenic acid: A pulse radiolysis study

At near neutral pH (approx. 5.5), the OH-adduct of chlorogenic acid (CGA), formed on pulse radiolysis of N₂O-saturated aqueous CGA solutions (λ _max = 400 and 450 nm) with k = 9 × 10⁹ dm³ mol⁻¹ s⁻¹, rapidly eliminates water (k = 1 × 10³ s⁻¹) to give a resonance-stabilized phenoxyl type of radical. Oxygen rapidly adds to the OH-adduct of CGA (pH 5.5) to form a peroxyl type of radical (k = 6 × 10⁷ dm³ mol⁻¹ s⁻¹). At pH 10.5, where both the hydroxyl groups of CGA are deprotonated, the rate of reaction of · OH radicals with CGA was essentially the same as at pH 5.5, although there was a marked shift in the absorption maximum to approx. 500 nm. The CGA phenoxyl radical formed with more specific one-electron oxidants, viz., Br ₂ ^·− and N ₃ ^· radicals show an absorption maximum at 385 and 500 nm, k ranging from 1–5.5 × 10⁹ dm³ mol⁻¹ s⁻¹. Reactions of other one-electron oxidants, viz., NO ₂ ^· , NO^· and CCl₃OO^· radicals, are also discussed. Repair rates of thymidine, cytidine and guanosine radicals generated pulse radiolytically at pH 9.5 by CGA are in the range of (0.7–3) × 10⁹ dm³ mol⁻¹ s⁻¹.     

Radiation chemical investigations on aqueous solutions of C60(OH)18

The ultraviolet-visible absorption spectrum of C₆₀(OH)₁₈ in water showed an absorption band with λ_max = 215 nm and other characteristic absorption bands of C₆₀ are not observed. The singlet-singlet and triplet-triplet absorption bands are not observed in the 400–900 nm region. It has low reactivity with e_aq⁻ and formed an absorption band with λ_max = 580 nm. The hydroxyl radicals react with a bimolecular rate constant of 2.4×10⁹ dm³ mol⁻¹ s⁻¹ and showed an absorption band at 540 nm.     

Pulse radiolytic investigation of the reaction of hydroxyl radicals with gentamycin

Hydroxyl radicals were generated radiolytically in N₂O-saturated aqueous solutions of the aminoglycoside antibiotic, gentamycin. Using the pulse radiolysis technique, the rate constant of OH radicals with gentamycin determined was 1.2·10⁹ dm³·mol⁻¹·s⁻¹. Upon^.OH attack a transient species with an absorption maximum at 270 nm is observed which decays by second-order kinetics within the solute concentration range of 3.2·10⁻⁵ to 1·10⁻³mol·dm⁻³. Transient species undergoes transformation to a permanent product absorbing between 260 and 340 nm with maximum absorption at 300 nm. Rate constant of the reaction of bimolecular decay of gentamycin radicals, k (Gen^.+Gen^.) was found to be ≈ 1.4·10⁷ dm³·mol⁻¹·s⁻¹.     

Nature of the transient species formed in pulse radiolysis of n-allylthiourea in aqueous solutions

Reactions of e⁻_aq, OH radicals and H atoms were studied with n-allylthiourea (NATU) using pulse radiolysis. Hydrated electrons reacted with NATU (k = 2.8×10⁹ dm³ mol⁻¹ s⁻¹) giving a transient species which did not have any significant absorption above 300 nm. It was found to transfer electrons to methyl viologen. At pH 6.8, the reduction potential of NATU has been determined to be −0.527 V versus NHE. At pH 6.8, OH radicals were found to react with NATU, giving a transient species having absorption maxima at 400–410 nm and continuously increasing absorption below 290 nm. Absorption at 400–410 nm was found to increase with parent concentration, from which the equilibrium constant for dimer radical cation formation has been estimated to be 4.9×10³ dm³ mol⁻¹. H atoms were found to react with NATU with a rate constant of 5 × 10⁹ dm³ mol⁻¹ s⁻¹, giving a transient species having an absorption maximum at 310 nm, which has been assigned to H-atom addition to the double bond in the allyl group. Acetoneketyl radicals reacted with NATU at acidic pH values and the species formed underwent reaction with parent NATU molecule. Reaction of Cl^.−₂ radicals (k = 4.6 × 10⁹ dm³ mol⁻¹ s⁻¹) at pH 1 was found to give a transient species with λ_max at 400 nm. At the same pH, reaction of OH radicals also gave transient species, having a similar spectrum, but the yield was lower. This showed that OH radicals react with NATU by two mechanisms, viz., one-electron oxidation, as well as addition to the allylic double bond. From the absorbance values at 410 nm, it has been estimated that around 38% of the OH radicals abstract H atoms and the remaining 62% of the OH radicals add to the allylic double bond.     

Alkyl radical adducts of aromatic N-oxides as hydrogen-abstracting agents: The reactivity of phenazine-N,N′-dioxide-methyl radical adduct

An O-methylated analog of protonated phenazine-di-N-oxide radical anion abstracts hydrogen from primary and secondary alcohols in a slow (k ₁ < 500 M⁻¹ s⁻¹) bimolecular reaction. No kinetic evidence has been found for the unimolecular release of free methoxyl radicals through the homolytic N-OMe bond cleavage in these species. DFT calculations at the UB3LYP 6-31G(d) level indicate that protonated and O-alkylated radical anions of pyrazine, quinoxaline and phenazine di-N-oxides are close analogues of aromatic nitroxyl radicals with the highest spin density localized on the oxygen and nitrogen of the nitrone moiety.     

Nature of the transient species formed during pulse radiolysis of thioacetamide in aqueous solutions

Thioacetamide (TA) is an organic compound having thioamide group similar to that in thiourea derivatives. Its reactions with e_aq⁻, H-atom and OH radicals were studied using the pulse radiolysis technique at various pHs and the kinetic and spectral characteristics of the transient species were determined. The initial adduct formed by the reaction of TA with OH radicals at pH 7 does not absorb light in the 300–600 nm region but reacts with the parent compound to give a transient species with an absorption maximum around 400 nm. At pH 0, the reaction of OH radicals with TA directly gives a similar transient species with absorption maximum at 400 nm. Transient species formed by H-atom reaction with TA and pH 0 has no absorption in the 300–600 nm region but at higher acidity a new transient species is formed which has absorption maximum at 400 nm. This transient absorption observed in the case of both OH and H atom reaction with TA is ascribed to the formation of a resonance stabilized radical similar to that obtained in the case of thiourea derivatives. The species produced by electron reaction viz. electron adduct was found to be a strong reductant and could reduce MV²⁺ with a high rate constant. H₂S was produced as a stable product in the reaction of e_aq⁻ and its G-value was determined to be about 0.8.     

Bisphenol A reactions with hydroxyl radicals: diverse pathways determined between deionized water and tertiary treated wastewater solutions

The oxidative reactions of bisphenol A (BPA) with radiolytically generated hydroxyl radicals were studied in both deionized water and tertiary treated wastewaters. In deionized water, bisphenol A reacts with the hydroxyl radical by addition to the aromatic ring, k = 6.9 × 10⁹ (±0.2) M⁻¹ s⁻¹, to eventually form the prominent, long-lived, hydroxylated intermediate product. In contrast, in tertiary treated water solutions, although the initial hydroxyl radical addition reaction occurs, the hydroxylation is averted, and a different mechanistic pathway ensues. The removal constant for the hydroxyl radical reaction with BPA is 0.45 ± 0.04 μmol/kGy, corresponding to an overall degradation efficiency of 76%.     

Kinetics of the reactions of catechins with hypochlorite,peroxynitrite, and amino acid–derived peroxyl- radicals

Catechins, one of the class of flavonoids, are known as very efficient antioxidants. Here we investigated the kinetics of the reactions of three catechins, namely, catechin, epigallocatechin, and epigallocatechin gallate (EGCG) with some oxidants, which are formed in vivo under oxidative stress, hypochlorite, peroxynitrite, and amino acid peroxyl radicals. Stopped-flow spectrophotometry and pulse radiolysis technique with absorption detection were used to observe the formation of intermediate products of oxidized catechins. We found that catechins react with hypochlorite with the rate constant of the order of 10⁵–10⁶ M⁻¹ s⁻¹ at pH 7.4. Experimental kinetic traces of the reaction of EGCG with valine peroxyl radicals were fitted using chemical simulation, and the rate constant of this reaction was found to be 5 × 10⁵ M⁻¹ s⁻¹. The rate constants of the formation of unstable catechin quinones in the reaction with peroxynitrite were comparable to that of spontaneous peroxynitrite isomerization, which indicates that catechins are oxidized indirectly by peroxynitrite. Biological consequences of these reactions are discussed.     

Effect of electron-withdrawing power of the substituted group on?OH radical reaction with substituted aryl sulphides: A pulse radiolysis study

In neutral aqueous solution of (phenylthio)acetic acid, hydroxyl radical is observed to react with a bimolecular rate constant of 7.2 × 10^-1 dm³ mol⁻s⁻ and the transient absorption bands are assigned to^•OH radical addition to benzene and sulphur with a rough estimated values of 50 and 40% respectively. The reaction of the^•OH radical with diphenyl sulphide (k = 4.3 × 10⁸ dm³ mol⁻¹ s⁻¹) is observed to take place with formation of solute radical cation, OH-adduct at sulphur and benzene with estimated values of about 12, 28 and 60% respectively. The transient absorption bands observed on reaction of^•OH radical, in neutral aqueous solution of 4-(methylthio)phenyl acetic acid, are assigned to solute radical cation (λ_max = 550 and 730 nm), OH-adduct at sulphur (λ_max = 360 nm) and addition at benzene ring (λ_max = 320 nm). The fraction of^•OH radical reacting to form solute radical cation is observed to depend on the electron-withdrawing power of substituted group. In acidic solutions, depending on the concentration of acid and electron-withdrawing power, solute radical cation is the only transient species formed on reaction of^•OH radical with the sulphides studied.     

Photochromic transformations in solutions of 8,8′-diquinolyl disulfide and di(mercaptoquinolinato)nickel(<Emphasis Type="SmallCaps">II</Emphasis>)

The nature of intermediate species and their reactions were studied by laser pulse photolysis for a photochromic system consisting of 8,8′-diquinolyl disulfide (RSSR) and a planar Ni^II complex di(mercaptoquinolinato)nickel(II) (Ni(SR)₂) in toluene and benzene solutions. Under exposure to laser radiation, disulfide RSSR dissociates to two RS^· radicals, whose spectrum has an intense absorption band with a maximum at λ = 400 nm (ε = 8400 L mol⁻¹ cm⁻¹). The radicals disappear by recombination (2k _rec = 4.6 · 10⁹ L mol⁻¹ s⁻¹). In the presence of the Ni(SR)₂ complex, coordination of the radical (k _coord = 4.4 · 10⁹ L mol⁻¹ s⁻¹) competes with recombination to form a radical complex RS^· Ni(SR)₂ having an intense absorption band with a maximum at 460 nm (ε = 16 600 L mol⁻¹ cm⁻¹). This species decays in the second-order reaction (2k = 4.6 · 10⁴ L mol⁻¹ s⁻¹). Since the photochromic system returns to the initial state, the reaction of two radical complexes is assumed to produce radical recombination and reduction of the disulfide and Ni(SR)₂ complex. Analysis of the kinetic data showed that some RS^· radicals decay in the microsecond time interval due to the reaction with the RS^· Ni(SR)₂ radical complex (k = 3.1 · 10⁹ L mol⁻¹ s⁻¹). Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2291–2300, October, 2005.     

Redox reactions of 3'-azido-3'-deoxythymidine (AZT) and interaction of its OH-derived radical with bilirubin and riboflavin: A pulse radiolysis study

Using the technique of pulse radiolysis, redox studies of 3-azido-3-deoxythymidine (azidothymidine or AZT) with hydrated electron and hydroxyl radicals, generated in phosphate-buffered aqueous medium, are reported. The hydrated electron reacts with AZT (k = 1.9 × 10¹⁰ dm³ mol^-1 s^-1) to generate transients with absorption maxima at 300 and 340 nm. The hydroxyl radical adds to AZT to generate transients with absorption maxima at 310 and 365 nm, with formation rate constant of 9.0 × 10⁹ dm³ mol^-1 s^-1 as observed at 310 and 365 nm. The secondary radical 6-hydroxy-5-yl-azidothymidine, formed in the reaction of AZT with hydroxyl radical, reacts with bilirubin to give a transient of bilirubin with bimolecular rate constant of 1.8 × 10⁸ dm³ mol^-1 s^-1. In the reaction of hydroxyl radical with AZT/riboflavin pair, an ^. H atom transfer from riboflavin to the 6-hydroxy-5-yl-azidothymidine is observed. Reactions of OH-derived radicals of thymine with bilirubin and riboflavin are similar to that of AZT. Possible mechanisms are proposed for the observed reactions.     

Scavenging of reactive oxygen radicals by resveratrol: antioxidant effect

Pulse radiolysis of resveratrol was carried out in aqueous solutions at pH ranging from 6.5 to 10.5. The one-electron oxidized species formed by the N^•₃ radicals at pH 6.5 and 10.5 were essentially the same with λ_max at 420 nm and rate constant varying marginally (k = (5−6.5) × 10⁹ dm³ mol⁻¹ s⁻¹). The nature of the transients formed by NO^•₂, NO^• radical reaction at pH 10.5 was the same as that with N^•₃, due to the similarity in decay rates and the absorption maximum. Reaction of ^•OH radical with resveratrol at pH 7 gives an absorption maximum at 380 nm, attributed to the formation of carbon centered radical. The repair rates for the thymidine and guanosine radicals by resveratrol were approx. 1 × 10⁹ dm³ mol⁻¹ s⁻¹, while the repair rate for tryptophan was lower by nearly an order of magnitude (k = 2 × 10⁸ dm³ mol⁻¹ s⁻¹). The superoxide radical anion was scavenged by resveratrol, as well as by the Cu–resveratrol complex with k = 2 × 10⁷ and 1.5 × 10⁹ dm³ mol⁻¹ s⁻¹, respectively. Its reduction potential was also measured by cyclic voltammetry.     

Evidence of neutral radical induced analyte ion transformations in APPI and Near-VUV APLI

We report on the reactions of neutral radical species [OH, Cl, O(³P), H], generated in a typical atmospheric pressure ionization (API) source upon irradiation of the sample gases with either 193 nm laser radiation or 124 nm VUV light, the latter commonly used in atmospheric pressure photoionization (APPI). The present investigations focus on the polycyclic aromatic hydrocarbon pyrene as representative of the aromatic compound class. Experimental results are supported by computational methods: simple kinetic models are used to estimate the temporal evolution of the concentrations of reactants, intermediates, and final products, whereas density functional theory (DFT) energy calculations are carried out to further elucidate the proposed reaction pathways. The neutral radicals are generated upon photolysis of background water and oxygen always present in appreciable mixing ratios in typical API sources. Substantial amounts of oxygenated analyte product ions are observed using both techniques. In contrast, upon atmospheric pressure laser ionization (APLI) with 248 nm radiation, oxygenated products are virtually absent. In addition, kinetic data evaluation yielded a bimolecular rate constant of k=(1.9±0.9)×10⁻⁹ cm³ molecule⁻¹ s⁻¹ for the reaction of the pyrene radical cation with OH radicals.     

Kinetics and mechanism of protection of thymine from sulphate radical anion under anoxic conditions

The rates of photooxidation of thymine in presence of peroxydisulphate (PDS) have been determined by measuring the absorbance of thymine at 264 nm spectrophotometrically. The rates and the quantum yields (φ) of oxidation of thymine by sulphate radical anion have been determined in the presence of different concentrations of caffeic acid. Increase in [caffeic acid] is found to decrease the rate of oxidation of thymine suggesting that caffeic acid acts as an efficient scavenger of SO ₄ ^•- and protects thymine from it. Sulphate radical anion competes for thymine as well as for caffeic acid. The rate constant of sulphate radical anion with caffeic acid has been calculated to be 1.24 x 10¹⁰ dm³ mol^-1s^-1. The quantum yields of photooxidation of thymine have been calculated from the rates of oxidation of thymine and the light intensity absorbed by PDS at 254 nm, the wavelength at which PDS is activated to sulphate radical anion. From the results of experimentally determined quantum yields (φ_expt1) and the quantum yields calculated (φ_cl) assuming caffeic acid acting only as a scavenger of SO ₄ ^•- radicals show that φ_expt1 values are lower than φ_cl values. The φ ’ values, which are experimentally found quantum yield values at each caffeic acid concentration and corrected for SO ₄ ^•- scavenging by caffeic acid, are also found to be greater than φ_expt1 values. These observations suggest that the thymine radicals are repaired by caffeic acid in addition to scavenging of sulphate radical anions.     

Studies on reactivity of benzoyl and benzoylperoxy radicals produced by laser flash photolysis of dibenzoyldiazene in aerated solutions

Photolysis of dibenzoyldiazene gives benzoyl radicals. In aerated solutions, the benzoyl radicals react with oxygen to yield benzoylperoxy radicals. Spin trapping studies indicate that 5,5′dimethyl-1-pyrroline N-oxide reacts with the benzoylperoxy radicals to produce the adduct which exhibits ESR parameters, A_N = 13.8 G and A_Hβ = 10.1 G. Laser photolysis studies reveal that the rate constants for the reaction between the benzoyl radical and oxygen are ca. 4 × 10⁹ M^-1 s^-1 in toluene, acetone, and ethyl acetate. The benzoylperoxy radicals undergo one-electron oxidation of tetramethyl-p-phenylenediamine, TMPD, to give an ion pair. The ion pair has an absorption spectrum similar to that of the TMPD cation radical. The formation of the ion pair is detected by monitoring the absorbance change at 600 nm after laser pulsing. From the kinetic studies for the formation of the ion pair in the presence of olefins, the bimolecular rate constants for reactions between several olefins and the benzoylperoxy radical are determined. The electrophilic addition of the benzoylperoxy radicals to olefins is discussed in comparison with the addition reactions of thiyl radicals to olefins. The detection and determination of the dipole moments of both the benzoylperoxy radicals and the ion pair are carried out with the use of the time-resolved microwave dielectric absorption technique. The distance between the positive and negative ions in the ion pair is estimated as 0.20 nm.     

The reactivity of ketyl and alkyl radicals in reactions with carbonyl compounds

A parabolic model of bimolecular radical reactions was used for analysis of the hydrogen transfer reactions of ketyl radicals: >C·OH+R¹COR²→>C=O+R¹R²C·OH. The parameters describing the reactivity of the reagents were calculated from the experimental data. The parameters that characterize the reactions of ketyl and alkyl radicals as hydrogen donors with olefins and with carbonyl compounds were obtained: >C·OH+R¹CH=CH₂→>C=O+R¹C·HCH₃; >R¹CH=CH₂+R²C·HCH₂R³→R²C·HCH₃+R²CH=CHR³. These parameters were used to calculate the activation energies of these transformations. The kinetic parameters of reactions of hydrogen abstraction by free radicals and molecules (adelhydes, ketones, and quinones) from the C−H and O−H bonds were compared. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2178–2184, November, 1998.     

Kinetics of radical formation and decay in photooxidation of 4-halophenols sensitized by 4-carboxybenzophenone in aqueous solutions

Kinetics of formation and recombination of radicals formed by quenching of the triplet state of 4-carboxybenzophenone (CB) with para-substituted phenol derivatives RC₆H₄OH (R = OMe, H, Cl, Br, I) in aqueous solutions was studied by nanosecond laser photolysis. At pH ≥ 5.4, quenching proceeds with high rate constants ((1–3)⋅10⁹ L mol⁻¹ s⁻¹) through electron transfer to form the radical anion CB^⋅− and radical cation RC₆H₄OH^⋅+. The latter is transformed into the phenoxyl radical within ≤10 ns. At pH ≤ 8, the CB^⋅− radical anion is protonated in a phosphate buffer with the rate constant increasing from 4⋅10⁶ to 15⋅10⁶ s⁻¹ with a decrease in the pH from 8 to 5.4. The yield of radicals decreases from 100 to 13% as the atomic weight of halogen in the RC₆H₄OH molecule increases due to an increase in the probability of recombination of the primary triplet radical pair in the solvent cage and partial intersystem crossing in an encounter complex (³CB, RC₆H₄OH). The effect of heavy atom is also observed in the kinetics of volume recombination of the radicals, the magnitude of effect corresponds to the acceleration of the primary recombination of the triplet radical pair. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1397–1402, June, 2005.     

Dichlorosilylene: Rate constant for the gas-phase reaction with nitric oxide

Using the recently detected intense UV absorption spectrum of it has been established that SiCl₂ reacts with nitric oxide in the gas phase with a bimolecular rate constant, k(SiCl₂+NO), of (1.6±0.1)×10⁹ M⁻¹s⁻¹ at 25°C.     

Pulse radiolysis study of dithio-oxamide in aqueous solutions

The reactions of e⁻ _aq, H-atoms, OH radicals and some one electron oxidants and reductants were studied with dithio-oxamide (DTO) in aqueous solutions using pulse radiolysis technique. The transient species formed by the reaction of e⁻ _aq with DTO at pH 6.8 has an absorption band with λ _max at 380 nm and is reducing in nature. H-atom reaction with DTO at pH 6.8 also produced the same transient species. The semi-reduced species was found to be neutral indicating that the electron adduct gets protonated quickly. However at pH 1, the species produced by H-atom reaction had a different spectrum with λ _max at 360 and 520 nm. Reaction of acetone ketyl radicals and CO₂ ⁻ radicals with DTO at pH 6.8 gave transient spectra which were identical to that obtained by e⁻ _aq reaction. However at pH 1, the spectrum obtained by the reaction of acetone ketyl radicals with DTO was similar to that obtained by H-atom reaction at that pH. The transient species formed by OH radical reaction with DTO in the pH range 1–9.2 also has two absorption maxima at 360 and 520 nm. This spectrum was identical with the spectrum obtained by H-atom reaction at pH 1. This means that all these radicals viz. OH, H-atom and (CH₃)₂COH radicals react with DTO at pH 1 by H-abstraction mechanism. The transient species produced was found to be sensitive to the presence of oxygen. One-electron oxidizing radicals such as Br₂ ^−· and SO₄ ^−· radicals reacted with DTO at neutral pH to give the same species as produced by OH radical reaction having absorption maxima at 360 to 520 nm. At acidic pHs, only Br₂ ^−· and Cl₂ ^−· radicals were able to oxidize DTO to give the same species as produced by OH radical reaction. The semioxidized species is a resonance stabilized species with the electron delocalized over the-N-C-S bond. This species was found to be neutral and non-oxidizing in nature.