Reactions of carbonate radical with cobalt(II) aminopolycarboxylates

Reactions of carbonate (CO ₃ ^–· ) and bicarbonate (HCO ₃ ^· ) radicals generated by photolysis of a carbonate or bicarbonate solution at pH 11.2 and 8.5, respectively, with Co(II) complexes of iminodiacetic acid (IDA) and ethylenediaminetetraacetic acid (EDTA) have been studied. The rate constants for the reactions were in the order of 10⁶–10⁷ dm³mol^–1s^–1. From the time-resolved spectroscopy of the products formed after reaction of CO _3· ^–· or HCO ₃ ^· , it is observed that CO ₃ ^–· or HCO ₃ ^· oxidize the metal center to its higher oxidation state.     

Picosecond flash photolysis: Transient absorption in Sn(IV), Pd(II), and Cu(II) porphyrins

From picosecond flash photolysis and spectroscopy, the absorption spectra of the first excited singlet S₁ and of the first excited triplet T₁ of (OEP)SnCl₂ are determined; we also determine triplet quantum yield, φ_t ≈ 0.80. Similar S₁ and T₁ phototransient absorption spectra occur in (OEP)Pd. However, two Cu porphyrins show only one phototransient absorption spectrum. Our T₁ absorption results are generally consistent with the available earlier spectroscopic studies made with other techniques. Our value for φ_t lies intermediate between values given by two earlier studies, which were based on two different methods.     

Carbonate radical induced polymerisation of pyrrole: A steady state and flash photolysis study

A study was undertaken to determine experimentally the uptake of pollutants into of the different parts of the water hyacinth (Eichhornia crassipes) found in “José Antonio Alzate” dam in the State of México, México. There is evidence for efficient and significant root accumulation of Ti, Mn, Fe, and Ba; but in the upper parts concentrations was consistently determined by the degree of watering. However, a significant input could by derived from a common generic source, such as the atmospheric deposition. The experimental study would, therefore, indicate that water hyacinth species can be highly effective in providing a control and treatment buffer for toxic discharges to the dam.     

Photolytic degradation of poly(olefin sulphones): A laser flash photolysis study

Poly(styrene sulphone) (PSS) and poly(butene-1 sulphone) (PBS) were irradiated in dilute solutions (dioxane and other solvents) with single 15-ns flashes of 265-nm light. Changes in the light scattering intensity (LSI) and the optical density (OD) after the flash were recorded with aerated solutions. It turned out that, with both polymers, the rate of LSI decrease (due to main-chain scission) is correlated to the rate of fragment diffusion. Optical density measurements with PSS revealed the existence of a short-lived species (λ_max = 500 nm), probably a singlet excimer (τ < 25 ns) which is considered a precursor in the main-chain scission process. The existence of macroradicals, even after fragment separation, was inferred from an absorption band below 400 nm decaying with a half-life of 40 ms. In the absence of oxygen, main-chain scission and cross-linking occur simultaneously. Therefore, the presence of oxygen, which acts as a fixing agent for main-chain breaks, is prerequisite for using the photodegradation of poly(olefin sulphones) as a probe for studying the dynamics of polymers in solution.     

A laser flash photolysis study of dibenzocycloheptadienylidene

Laser flash photolysis of diazodibenzocyclohaptadiene gives the title carbene which is readily observed and which reacts by hydrogen abstraction to give the corresponding radical.     

The acetyl radical studied by flash photolysis and kinetic spectroscopy

The acetyl radical absorption spectrum is a broad band with maximum decadic extinction coefficient of (1.0 ± 0.1) × 10⁴ ? mole^?1 cm^?1 at 215 nm and an oscillator strength of 0.23 ± 0.03. Absolute rate constants were estimated as 4.5 × 10¹⁰ ? mole^?1 s^?1 for the mutual interaction of acetyl radicals, and as 7.5 × 10¹⁰ ? mole^?1 s^?1 for the cross interaction of acetyl and methyl radicals.     

On the electrophilicity of hydroxyl radical: a laser flash photolysis and computational study

The rate coefficients for reactions of hydroxyl radical with aromatic hydrocarbons were measured in acetonitrile using a novel laser flash photolysis method. Comparison of kinetic data obtained in acetonitrile with those obtained in aqueous solution demonstrates an unexpected solvent effect on the reactivity of hydroxyl radical. In particular, reactions of hydroxyl radical with benzene were faster in water than in acetonitrile, and by a significant factor of 65. Computational studies, at the B3LYP and CBS-QB3 levels, have confirmed the rate enhancement of hydroxyl radical addition to benzene via calculation of the transition states in the presence of explicit solvent molecules as well as a continuum dielectric field. The origin of the rate enhancement lies entirely in the structures of the transition states and not in the pre-reactive complexes. The calculations reveal that the hydroxyl radical moiety becomes more anionic in the transition state and, therefore, looks more like hydroxide anion. In the transition states, solvation of the incipient hydroxide anion is more effective with water than with acetonitrile and provides the strong energetic advantage for a polar solvent capable of hydrogen bonding. At the same time, the aromatic unit looks more like the radical cation in the transition state. The commonly held view that hydroxyl radical is electrophilic in its reactions with DNA bases is, therefore, strongly dependent on the ability of the organic substrate to stabilize the resulting radical cation.     

Tris(trimethylsilyl)silane (TTMSS)-derived radical reactivity toward alkenes: a combined quantum mechanical and laser flash photolysis study

The reactivity of the tris(trimethylsilyl)silane (TTMSS)-derived radical is studied through an approach combining laser flash photolysis and quantum mechanical calculations. The results obtained for TTMSS are compared both to a classical silyl radical derived from triethylsilane and to a previously studied carbon-centered structure. Different worthwhile results are obtained: (i) the addition and hydrogen abstraction rate constants are directly measured, (ii) the high reactivity and the low selectivity of TTMSS toward the addition to alkenes are perfectly explained by antagonist polar and enthalpy effects, (iii) efficient hydrogen abstraction reactions from antioxidants such as vitamin E are observed, and (iv) TTMSS is seen to also act as an efficient initiator for the polymerization of an acrylate monomer.     

A quantitative study of alkyl radical reactions by kinetic spectroscopy. IV. The flash photolysis of azopropanes

The flash photolysis of azo?n?propane and of azoisopropane has been studied by kinetic spectroscopy. Transient absorption spectra in theregion of 220–260 nm have been assigned to the n-propyl and isopropyl radicals. For the n-propyl radical, ?_max = 744 ± 39 l/mol cm at 245 nm and the rate constants for the mutual reactions were measured to be k_c = (1.0 ± 0.1) × 10¹⁰ l/mol sec (combination) and k_d = (1.9 ± 0.2) × 10⁹ l/mol sec (disproportionation). For the isopropyl radical, ?_max = 1280 ± 110 l/mol cm at 238 nm, with k_c = (7.7 ± 1.6) × 10⁹ l/mol sec and k_d = (5.0 ± 1.2) × 10⁹ l/mol sec The rate constant for the dissociation of the vibrationally excited triplet state of the azopropanes into radicals was measured from the variation in the quantum yield of radicals with pressure. For azo-n-propane k = (6.6 ± 1.3) × 10⁷ sec^?1, and for azoisopropane k = (1.6 ± 0.4) × 10⁸ sec^?1. Collisional deactivation of the vibrationally excited singlet and triplet states was found to occur on every collision for n-pentane; but nitrogen and argon were inefficient with a rate constant of 1.1 × 10¹⁰ l/mol sec. Spectra observed in the region of 220–260 and 370–400 nm areattributed to the cis isomers of the parent trans-azopropanes. These are formed, as permanent products, in increasing amounts as the pressure is increased.     

Laser flash photolysis study on the retinol radical cation in polar solvents

Laser flash photolysis (LFP) of retinol in argon-saturated methanol gives rise to a transient at 580 nm (transient A). Formation of transient A is accompanied by a transient growth at 370 nm. The rate of this growth is retinol concentration-dependent. The transient growth at 370 nm was removed in the presence of N(2)O, which is known to scavenge solvated electrons. These results can be interpreted by formation of retinol˙(+) (λ(max) = 580 nm) and solvated electrons following LFP of retinol. Subsequently, the solvated electrons are rapidly scavenged by retinol to form retinol˙(-) (λ(max) = 370 nm in methanol). On the other hand, transient A is not ascribed to the retinyl cation, as was previously proposed, because the retinyl cation, generated from LFP of retinyl acetate, and transient A show different reactivities towards halide ions (e.g. k(Br) = 1.7 × 10(9) and 1.51 × 10(10) M(-1) s(-1) respectively, in acetonitrile). After demonstrating the identity of transient A as retinol˙(+), its reactions with carotenoids were examined in air-saturated polar solvents. In the presence of carotenoids, an enhancement in the decay of retinol˙(+) was observed and was accompanied by formation of the corresponding carotenoid radical cations via electron transfer from carotenoids to retinol˙(+). Furthermore, the reactivity of retinol˙(+) towards pyridine derivatives was investigated in air-saturated polar solvents. It was found that the decay of retinol˙(+) was accelerated with concomitant formation, with the same rate, of a transient at 370 nm. Similar observations were obtained with increasing pH of air-saturated aqueous 2% Triton X-100 of retinol˙(+). The 370 nm (or 380 nm in the case of Triton X-100) transient is attributed to the base adducts or deprotonated neutral radicals. On the basis of these results, the reactivities of the retinyl cation and retinol˙(+) are compared and the consequences of retinol˙(+) formation within biological environments are discussed.     

Photodissociation of naphthalene dimer radical cation during the two-color two-laser flash photolysis and pulse radiolysis-laser flash photolysis

Photodissociation of naphthalene (Np) dimer radical cation (Np2*+) to give naphthalene radical cation (Np*+) and Np and the subsequent regeneration of Np2*+ by the dimerization of Np*+ and Np were directly observed during the two-color two-laser flash photolysis in solution at room temperature. When Np2*+ was excited at the charge-resonance (CR) band with the 1064-nm laser, the bleaching and recovery of the transient absorption at 570 and 1000 nm, assigned to the local excitation (LE) and CR bands of Np2*+, respectively, were observed together with the growth and decay of the transient absorption at 685 nm, assigned to Np*+. The dissociation of Np2*+ proceeds via a one-photon process within the 5-ns laser flash to give Np*+ and Np in the quantum yield of 3.2 x 10(-3) and in the chemical yield of 100%. The recovery time profiles of Np2*+ at 570 and 1000 nm were equivalent to the decay time profile of Np*+ at 685 nm, suggesting that the dimerization of Np*+ and Np occurs to regenerate Np2*+ in 100% yield. Similar experimental results of the photodissociation and regeneration of Np2*+ were observed during the pulse radiolysis-laser flash photolysis of Np in 1,2-dichloroethane. The photodissociation mechanism can be explained based on the crossing between two potential surfaces of the excited-state Np2*+ and ground-state Np*+.     

One electron oxidation of Ni(II)-iminodiacetate by carbonate radical

Reactions of carbonate radical (CO₃ ^–), generated by photolysis or by radiolysis of a carbonate solution with nickel(II)-iminodiacetate (Ni(II)IDA) were studied at pH 10.5 and ionic strength (I)==0.2 mol·dm^–3. The stable product arising from the ligand degradation in the complex is mainly glyxalic acid. Time-resolved spectroscopy and transient kinetics were studied using flash photolysis. From the kinetic data it was suggested that the carbonate radical initially reacts with Ni(III)IDA with a rate constant (2.4±0.4)·10⁶ dm³·mol^–1·s^–1 to form a Ni(II)IDA species which, however, undergoes a first-order transformation (k=2.7·10²·s^–1) to give a radical intermediate of the type Ni(II)RNHCHCO ₂ ^– ) which rapidly forms an adduct containing a Ni–C bond. This adduct decays very slowly to give rise to glyoxalic acid. From a consideration of equilibrium between Ni(II)IDA and Ni(III)IDA, the one electron reduction potential for the Ni(III)IDA/Ni(II)IDA couple was determined to be 1.467 V.     

Photophysical properties of 5-hydroxyindole (5HI): Laser flash photolysis study

Steady state fluorescence emission and transient absorption spectra of 9-fluorenone (9FL) were measured in the presence of 5-hydroxyindole (5HI) in highly polar acetonitrile (ACN) environment at ambient temperature. Cyclic voltammetry measurements demonstrate that ground state 5HI as a donor could take part in highly exothermic electron transfer (ET) reactions with excited 9FL, which should serve as electron acceptor. From the transient absorption measurements it is inferred that in geminate ion-pair (GIP) (or contact ion pair), formed initially due to photoinduced ET, the decay of this contact ion-pair occurs not only through ion recombination (back electron transfer to ground state of reactants), but through the other processes also such as proton-transfer (hydrogen abstraction) from radical cation to anion and separation of ion-pair producing the free ions. From the computed reorganisation energy parameter (λ) and experimentally observed -‡ _ET ⁰ values it is hinted that there is a possibility that highly exothermic forward electron transfer reactions in the singlet stateS ₁ occur, within present reacting systems, in Marcus inverted region. Back transfer seems to follow the same path. Investigations with similar other reacting systems are underway.     

Laser flash photolysis study of methanol addition to the C60 radical cation

Stable alcohol adducts of buckminsterfullerene (C₆₀) can be created via addition to C₆₀ radical cations. The radical cations were generated by photosensitized electron transfer from C₆₀ in a solution of N-methylacridinium hexafluorophosphate and biphenyl. Growth and decay of the C₆₀ radical cation population was monitored by transient absorption spectroscopy at 980 nm. The lifetime of the transient decreases in the presence of methanol, supporting trapping of the radical cation.     

Laser flash photolysis study of triphenylimidazole

Laser flash photolysis of the photocyclization of triphenylimidazole (TPI) in ethyl alcohol at 308 nm. indicates that the dihydrophenanthroimidazole (DHPI) intermediate is produced rapidly, has a lifetime of 0.25 ms, and returns predominantly back to triphenylimidazole. Analysis of the decay channels for this intermediate indicates two rate constants: (1) k1 = 3.3 x 10(3) s(-1), associated with reversion back to triphenylimidazole and (2) k2 = 0.67 x 10(2) s(-1), which is associated with the conversion of the dihydrophenanthroimidazole to the photoproduct, 2-phenyl-9,10-phenanthroimidazole. The photoproduct is readily observed as an increasing component in the biexponential fluorescence decay data. Fluorescence lifetimes for triphenylimidazole and 2-phenyl-9,10-phenanthroimidazole (PPI) in ethyl alcohol were determined to be 1.76 and 8.21 ns, respectively, with no additional components in the fluorescence decay as the photochemistry proceeds. An additional transient absorption observed in the 450 nm. region, with a lifetime of 0.7 micros, decaying faster than the dihydrophenanthroimidazole intermediate, is assigned to the triplet state of triphenylimidazole.     

Laser flash photolysis study of carboethoxynitrene

[reaction: see text] Laser flash photolysis (LFP, 266 nm) of carboethoxyazide produces a mixture of the ethoxycarbonyl radical (lambda(max) = 333 nm, tau = 0.4 micros, CF(2)ClCFCl(2), ambient temperature) and triplet carboethoxynitrene (lambda(max) = 400 nm, tau = 1.5 micros, CF(2)ClCFCl(2), ambient temperature). The carbon-centered radical is selectively scavenged by oxygen allowing sole observation of the triplet nitrene. We deduce that the singlet nitrene has a lifetime between 2 and 10 ns in CF(2)ClCFCl(2) at ambient temperature.     

Reaction of hydroxyl radical with aromatic hydrocarbons in nonaqueous solutions: A laser flash photolysis study in acetonitrile

Laser flash photolysis (LFP) of acetonitrile solutions of N-hydroxypyridin-2-thione in the presence of trans-stilbene generates a transient absorbance at 392 nm, attributed to the addition of hydroxyl radical to stilbene. The observed transient absorbance was used in competitive LFP experiments to determine relative rates of reaction for hydroxyl radical with a range of aromatic hydrocarbons in acetonitrile. Structure-reactivity relationships for the reaction of hydroxyl radical with arenes are derived. With these aromatic hydrocarbons, we observe a good correlation between the rates of hydroxyl-radical reaction and the ionization potential of the arene. Kinetic isotope effects are consistent with hydroxyl-radical addition being the dominant reaction pathway with the arene.     

Reactivities of sulfur-centered radical toward polyisoprenes and polybutadienes studied by flash photolysis method

The kinetic behavior of the thiyl radical in the solution containing polyisoprenes and polybutadienes has been studied by the flash photolysis method. For benzothiazole-2-thiyl radical, the addition rate constants toward these polymers and the model compounds of the polymers were evaluated. The relative reverse rate constants and equilibrium constants were also estimated. The addition rate constants decrease with an increase in the degree of polymerization; the ratio of the addition rate constant for polyisoprene (3.1 × 10⁴ M^?1 s^?1 (in monomer unit); M_v = 674,000) to that for 2-methyl-2-butene (1.5 × 10⁵ M^?1 s^?1) is about 1/5. This indicates that the polymer chain effect appears in the free-radical addition reaction. The relative reverse rate constants for the polymers are also smaller than those for 2-methyl-2-butene, suggesting a kind of polymer effects; i.e., it can be presumed that the bonded-thiyl radicals migrate very rapidly to the neighboring double bonds in the polymer. Significant differences in the rate parameters were observed between polyisoprene and polybutadiene, between cis- and trans-polyisoprenes, and between 1,4- and 1,2-polybutadienes.     

Photosensitized oxidation of tryptophan and tyrosine by aromatic ketones: A laser flash photolysis study

Photochemical processes of benzophenone (BP) and xanthone (XT) with tryptophan (TrpH) and tyrosine (TyrOH) were studied using the laser flash photolysis technique.It has been observed that the triplet state of BP and XT reacted with TrpH and TyrOH by hydrogen transfer with the formation of BP and XT ketyl radicals and oxidized radicals of TrpY and TyrOY.The related rate constants of these reactions were determined in this paper.The free energy changes (G) of these reactions suggested that the proposed hydrogen transfer mechanism was thermodynamically feasible.These results provide theoretical foundation for further studying structural effects on the photochemical behaviors of proteins with triplet state BP and XT.