Transient Optical Absorption Bands Research Articles

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 (λmax = 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 (λmax = 490 nm, ɛ = 10.4 × 103 dm3 mol−1 cm−1) is assigned to radical anion, AD*−, which has a pKa 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.

Pulse Radiolysis Investigations On the Nature of Oh Radical Reactions With 1-Fluoro-2-Iodobenzene in Aqueous Solutions

The transient optical absorption bands (λ max = 310 nm, 330-350 nm, k = 3.2×10 9 dm 3 mol -1 s -1 ) formed on reaction of'OH radicals with 1-fluoro-2-iodobenzene in neutral aqueous solutions are assigned to OH-adduct, which decayed by second order kinetics with 2k = 2.9×10 9 dm 3 mol -1 s -1 . The absorption band (λ max = 630 nm) observed in acidic solutions (pH = 3) is assigned to solute radical cation formed on removal of an electron from iodine, which decayed by first order kinetics with k = 5.6×10 5 s -1 . At pH = 1, the OH-adduct was found to react with H + (k = 4.7×10 5 s -1 ) and formed a broad absorption band in 350 - 400 nm region. It is assigned to solute radical cation formed on removal of an electron from fluorine. Acid-catalyzed dehydration of OH-adduct is observed with ~85% at iodine and ~15% at fluorine. The iodine centered radical cation is a strong one-electron oxidant and is able to oxidize I- and Br- with rate constant in the diffusion controlled range. SO 4 * - radical anion is able to undergo electron transfer with the solute and absorption bands due to iodine centered radical cation (λ = 630 nm), and OH-adduct (λ = 310, 330-370 nm) are observed.

Pulse Radiolysis Investigations on Acidic Aqueous Solutions of Benzene: Formation of Radical Cations

The transient optical absorption bands (λmax = 310, 350−500 nm, k = 5.6 × 109 dm3 mol-1 s-1) formed on pulse radiolysis of an acidic (HClO4 = 7.8 mol dm-3) aqueous solution of benzene are assigned to the solute radical cation formed on acid-catalyzed dehydration of the OH adduct. The solute radical cation is able to undergo an electron transfer reaction with Br- with a bimolecular rate constant of 8 × 109 dm3 mol-1 s-1. On the other hand, the OH adduct (λmax = 310 nm) was oxidized by Fe(CN)63- with a bimolecular rate constant of 6.6 × 107 dm3 mol-1 s-1. The radical cation of benzene is a strong one-electron oxidant with oxidation potential in the range 2.1−2.4 V vs NHE.

One-Electron Reduction of 3,3,6,6-Tetramethyl-3,4,6,7,9,10-hexahydro-(1,8)-(2H,5H)-acridinedione: A Pulse Radiolysis Study

The radical anion formed on reaction of e aq ¯ with acridinedione dye (ADH) exhibits transient optical absorption bands at λ max = 315, 460, and >780 nm. The bimolecular rate constant for the reaction of e aq ¯ with the dye was determined to be 2.7 × 10 10 dm 3 mol -1 s -1 . It is converted to (ADH 2 • ) on protonation and showed an absorption band at 310 nm. At pH = 1, acetone ketyl radicals are observed to react with the dye with a bimolecular rate constant of 2.3 × 10 8 dm 3 mol -1 s -1 , and the transient optical absorption bands ( λ max = 315, 540 nm) are assigned to (ADH 3 ) •+ . The radical anion (ADH •- ) is a strong reducing agent, with reduction potential = -1.37 V vs NHE, and is able to undergo electron-transfer reactions with methyl viologen (MV 2+ ) and p-nitroacetophenone (p-NAP) with bimolecular rate constants of 4.9 × 10 9 and 6.1 × 10 9 dm 3 mol -1 s -1 , respectively.

Spectral, Kinetic, and Redox Studies on the Transients Formed on Reaction of Hydroxyl Radicals with Thioanisole

The hydroxyl radicals are observed to react with thioanisole (TA) with a bimolecular rate constant of 3.5 × 109 dm3 mol-1 s-1 and form transient optical absorption bands with λmax = 310, 365, 530, and 740 nm. The transient absorption bands at 310, 530, and 740 nm are also formed on reaction of specific one-electron oxidants such as CCl3OO•, Br2•-, and Cl2•- with thioanisole and are assigned to solute radical cations with positive charge on the benzene ring. Based on the studies with specific one-electron oxidants, ≈73% of •OH radicals are inferred to react with thioanisole by electron transfer and 27% by OH-adduct formation. The oxidation potential for the TA/TA•+ couple, based on the equilibrium established with the Br2•- / 2Br- couple, is determined to be 1.53 V vs NHE. The OH-adduct (λmax = 365 nm) is observed to undergo acid-catalyzed dehydration and form a solute radical cation. A small fraction of it is converted to sulfur-centered dimer radical cations and the major fraction into the monomer radical cations with the positive charge on the benzene ring. The monomer radical cations, formed in neutral aqueous solutions, are a strong one-electron oxidant and are able to oxidize I- with a bimolecular rate constant value of 6.4 × 109 dm3 mol-1 s-1.

One-Electron Reduction of Acridine-1,8-dione in Aqueous Solution: A Pulse Radiolysis Study

In a neutral aqueous solution of acridine-1,8-dione (AD) the eaq- was observed to react with a bimolecular rate constant of 1.5 × 1010 dm3 mol-1 s-1 and form transient optical absorption bands with λmax = 305, 475 nm, a shoulder at 680 nm, and increasing absorption with λ > 800 nm. The entire spectrum from λ > 450 nm decayed by first-order kinetics with k = 1.5 × 105 s-1, whereas the absorption at 315 nm showed growth with k = 1.6 × 105 s-1. It decayed by second-order kinetics with 2k/εl = 1 × 104 s-1. The transient optical absorption bands at 305, 475, 680, and >800 nm are assigned to the electron adduct, AD•-, which on protonation forms H adduct (ADH•) with λmax = 315 nm and a pK value of 4.7. In basic solutions (pH = 13), AD•- does not show slow transformation to ADH• while in acidic solutions (pH = 3), only ADH• was observed (λmax = 315 nm). ADH• in highly acidic solutions (pH < 3) protonates to form (ADH2)•+ (λmax = 315, 560 nm) with a pK value of 2.6. AD/AD•- established an equilibrium with MV2+/MV•+ and the reduction potential for AD/AD•- couple was determined to be −0.586 ± 0.03 V. α-Hydroxyalkly radicals (CH3)2C•OH, CH3C•HOH, and •CH2OH) were able to reduce AD with bimolecular rate constant values of 3.2 × 108, 2.3 × 108, and 1.2 × 108 dm3 mol-1 s-1, respectively. The radical anion AD•- was able to transfer electron to p-nitroacetophenone with a bimolecular rate constant of 6 × 109 dm3 mol-1 s-1.

Pulse radiolysis studies on .OH radical induced reactions with substituted iodobenzenes in aqueous solutions

The transient optical absorption band formed on pulse radiolysis of N 2O saturated neutral aqueous solution of substituted iodobenzenes (C 6H 4XI, X = H, Br, CH 3, CF 3) has been assigned to hydroxy cyclohexadienyl radical, which absorbs in the region of 320–325 nm. In acidic solutions, the hydroxyl radicals react to form two transient optical absorption bands ( λ max = 310–320, 640–650 nm). The bands are assigned to solute radical cation formed on H +-catalyzed dehydration of OH-adduct. SO 4 .−, specific one-electron oxidant, is found to react with substituted iodobenzenes and form transient optical absorption bands, similar to those formed on reaction of .OH radicals in acidic solutions and are assigned to solute radical cations.

Reaction of hydroxyl radicals with 1-bromo- N-iodoalkanes in aqueous solution: 2C–3E bonded radical cations

The transient optical absorption bands ( λ max = 310, 350 nm) formed on reaction of OH radicals with 1-bromo-4-iodobutane (pH = 6) are assigned to OH-adduct. In acidic solutions (pH = 3.0), the OH adduct is converted to a transient band with λ max = 440 nm. This band is assigned to intramolecular radical cation formed on p-orbital overlap of oxidized iodine with bromine. In case of 1-bromo-5-iodopentane, the transient optical absorption bands formed on the reaction of OH radicals at pH = 6.0 are at λ max = 310 and 350 nm. In acidic aqueous solution (pH = 3.0), the OH adducts are converted to intramolecularly coordinated solute radical cation ( λ max = 410 nm) through acid catalysed dehydration reaction. In case of 1-bromo-3-iodopropane, the hydroxyl radicals react to form absorption band with λ max = 400 nm and a small shoulder in 310–330 nm region at neutral pH. In acidic solutions (pH = 3.0), the absorption of 310–330 nm band decreases and that of 400 nm band increases. The transient band at 400 nm is assigned to intramolecular radical cation. A fraction of OH radical is able to react by electron transfer mechanism at neutral pH. The fraction of OH radical forming OH-adduct at neutral pH decreases and is converted to radical cation in acidic pH.

Optical studies of self-trapped holes and excitons in beryllium oxide

Time-resolved spectra, decay kinetics, degree of polarization of the luminescence and transient optical absorption induced by irradiation of BeO with an electron, synchrotron and subsequent laser pulses have been studied. It is found that the two pairs of slowly and fastly decaying intrinsic luminescence bands at 6.7, 4.9 and 4.4 eV are associated with the two different types of the self-trapped excitons. The 1.7 eV transient optical absorption band and the ultra-violet absorption band at 3.6 eV are attributed to the electron and hole components of the self-trapped excitons, respectively. The transient optical absorption band with decay time of 6.5 ms is found to be due to the self-trapped holes. The models of self-trapped excitons and holes are discussed on the basis of the present experimental results.

The experimental observation of the potential barrier for self-trapped exciton decay into F-H pair in KCl-Na crystals

Abstract The optical absorption induced by the electron pulse irradiation of Na+ doped KCl has been measured. Transient optical absorption band of FA centers was observed at 80 K (LNT). The temperature dependence of FA center formation was studied. It is proposed that the obtained activation energy originates from the potential barrier between the STE perturbed by the cation impurity and the nearest neighbour FA-H pair. The mechanism of the suppression of the defect formation by the monovalent cation impurity in alkali halide crystals is discussed.

Formation of radical cations of chlorobenzene in aqueous solutions. A pulse radiolysis study

The transient optical absorption bands ( λ max = 290, 475 nm) formed on the reactions of OH radicals in an acidic aqueous solution of chlorobenzene are assigned to the solute radical cation. It decays (475 nm) by first-order kinetics with k = 1.8 × 10 4s −1. The extinction coefficient (475 nm) is determined to be 7.87 × 10 3dm 3mol −1cm −1.

Effect of fluorine substitution on the [rad]OH-radical-induced electron-transfer reaction in aqueous solutions of fluorinated alkyl iodides

The transient optical absorption bands formed at λ max=340 and 435 nm, on reaction of OH radicals in aerated acidic aqueous solutions of 1,1,1-trifluoro-2-iodoethane at low and high solute concentration, have been assigned to monomer and dimer radical cations, respectively. The deprotonation of the solute radical cations is the rate-determining step for the decay of the dimer radical cations. The stability constant for the dimer radical cation is determined to be 50 dm 3 mol −1 at 25°C. The dimer radical cation is a strong one-electron oxidant. Quantum chemical calculations and experimental results confirm that fluorine reduces the electron density at iodine and the OH-radical-induced oxidation of fluoroiodoalkanes becomes a difficult process compared to iodoalkanes.

Redox reactions of dopamine transients in aqueous solution: a pulse radiolysis study

On reaction of eaq– with protonated dopamine (D) a transient optical absorption band [formed at λmax= 355 nm (ε= 5100 dm3 mol–1 cm–1)] has been assigned to an ewaq adduct of the dopamine ring (D˙–)(the first site of attack of eaq–), which subsequently goes to the amine site to cleave the C–N bond. The bimolecular rate constant for the reaction of eaq– with protonated dopamine has been determined to be 2.5 × 108 dm3 mol–1 s–1. The transient optical absorption band (λmax= 355 nm, ε= 3760 dm3 mol–1 cm–1) formed on reaction of H atoms with protonated dopamine has been assigned to a H-adduct of dopamine. The bimolecular rate constant for the reaction has been determined to be 4.2 × 109 dm3 mol–1 s–1 from the build-up kinetics of the 355 nm band. The transient H-adduct decays with the formation of ammonia. The isopropanol radical is unable to undergo a one electron transfer reaction with dopamine, whereas an equilibrium is established between the benzamide radical anion and dopamine from which the redox potential for the D/D˙– couple has been determined to be –1.91 V. The dopamine radical anion (D˙–) is a strong reducing agent and is able to reduce methyl viologen with a bimolecular rate constant of 3.3 × 1010 dm3 mol–1 s–1. Specific one electron oxidants are able to undergo one electron transfer reactions forming a dopamine semiquinone radical (DSQ˙)(λmax= 290 nm, ε= 7750 dm3 mol–1 cm–1).

Characterization of transients formed in aqueous solutions of substituted alkyl sulfides: a pulse radiolysis study

The transient optical absorption band (λmax= 340 nm) formed on reaction of OH radicals with substituted alkyl sulfides, S(CH2CO2R)2, R = CH3, C2H5, C3H7, C4H9 in neutral aqueous solutions has been assigned to the OH adduct. The yield, lifetime and molar absorption coefficient of this band is found to decrease with increasing chain length of R. The OH adduct of S(CH2CO2CH3)2 is able to undergo an electron transfer reaction with Br–, I– and SCN–. in acidic solutions, dimer radical cations are formed at high solute concentration. The observed rate constant for the decay of dimer radical cations is deconvoluted to different processes and the deprotonation is found to be the rate-determining step. The yield and lifetime are found to decrease with increasing chain length of R. The high concentration of acid required for their formation is discussed. Cl˙–2 and SO˙–4 are able to undergo one-electron transfer reactions with S(CH2CO2CH3)2 with rate constants of 1.9 × 109 and 1.4 × 1010 dm3 mol–1 s–1, respectively. The dimer radical cation is found to be a strong oxidizing agent and is able to oxidize Br– with a rate constant of 2.2 × 109 dm3 mol–1 s–1.

Pulse radiolysis study on radical cations of alkyl bromides in aqueous solutions

The transient optical absorption bands formed on reaction of ˙OH radicals with 1,m-dibromoalkanes and n-alkyl bromides in acidic aqueous solutions have been assigned to intra-, [graphic omitted], and inter-molecular, (RBr∴BrR)+, radical cations, respectively. Depending upon R, these radical cations exhibit an optical absorption band in the 385–450 nm region. Increases in the chain length and electron-releasing power of R results in a red shift of λmax and a decrease in the lifetime. These radical cations are strong oxidants and oxidize various substrates with bimolecular rate constants of the order of 109 dm3 mol–1 s–1.

Inter- and intra-molecular radical cations of alkyl bromides in aqueous solution

The transient optical absorption bands with λ max=410 and 385 nm, formed on OH radical-induced reaction with ethyl bromide and 1,3-dibromopropane in 6.5 mol dm −3 perchloric acid have been assigned to inter- and intra-molecular radical cations, respectively.

Structure–reactivity studies on the nature of transients formed on reaction of ˙OH radicals with chloroiodoalkanes in aqueous solution

The transient optical absorption bands (λmax= 315 and 345 nm) formed on reaction of ˙OH radicals with 1-chloro-3-iodopropane in N2O-saturated neutral aqueous solutions have been assigned to an OH-adduct formed between iodine atom and ˙OH radicals (t½= 1.5 µs, ε350= 2.46 × 103 dm3 mol–1 cm–1). In acidic solutions, iodine-centred radical cations (λmax= 345 nm, t½= 4.0 µs, ε345= 2.41 × 103 dm3 mol–1 cm–1) are formed at low solute concentrations and dimer radical cations (λmax= 430 nm, t½= 16.0µs, ε430= 6.7 × 103 dm3 mol–1 cm–1) at high solute concentrations. In chloroiodomethane, the ˙OH radicals do not seem to form OH-adducts in neutral aqueous solutions. In acidic solutions, intramolecular radical cation (λmax= 430 nm, t½= 8.0 µs, ε430= 2.63 × 103 dm3 mol–1 cm–1) with oxidized iodine and chlorine is observed. The intermolecular dimer radical cations are formed at high solute concentration (λmax= 500 nm).

Pulse radiolysis investigations on electron-transfer reactions in aqueous solutions of substituted alkyl sulfides

The transient optical absorption band (λmax= 345 nm, t½= 4.0 µs, Iµ= 2.6 × 103 dm3 mol–1 cm–1) formed on pulse radiolysis of N2O-saturated dilute neutral aqueous solutions of dimethyl 3,3′-thiodipropionate (DTDP) is assigned to an ˙OH-adduct. The rate constant for the reaction of ˙OH radicals has been determined to be 1.4 × 1010 dm3 mol–1 s–1. At higher solute concentrations the ˙OH-adduct is converted to sulfur-centred dimer radical cations (λmax= 510 nm, Iµ= 4.9 × 103 dm3 mol–1 cm–1, t½= 14.0 µs). In acidic solutions, only dimer radical cations are formed. In neutral aqueous solutions of 3,3′-thiodipropanol (TDP)˙OH radicals are able to bring about one-electron oxidation, forming intramolecular radical cations with p-orbital overlap between the oxidized sulfur and oxygen (λmax= 420 nm, t½= 17 µs, Iµ= 2.6 × 103 dm3 mol–1 cm–1). In acidic solutions, sulfur-centred dimer radical cations are formed (λmax= 510 nm, t½= 12 µs, Iµ= 6.3 × 103 dm3 mol–1 cm–1). In the presence of halide ions, the oxidized sulfur in DTDP and TDP stabilizes by formation of an S∴X type species.

Electron transfer reactions in aqueous solutions of 2,2′-thiodiethanol: A pulse radiolysis study

The transient optical absorption band ( λ max =290 nm, ε = 2020 dm 3 mol −1 cm −1) produced on pulse radiolysis of N 2O saturated aqueous solution ( pH = 6.0−10.0) of 2,2′-thiodiethanol (TDE) has been assigned to the product of reaction of OH radicals with the solute. It decayed by second order kinetics with bimolecular rate constant of 1.7 × 10 9 dm 3 mol −1 s −1. The absolute rate constant for the reaction of OH radicals with the solute has been determined to be 1.4 × 10 10 dm 3 mol −1 s −1. The transient optical absorption band ( λ max=505 nm, t 1 2 =35 μs, ε = 6255 dm 3 mol −1 cm −1 ) formed on pulse radiolysis of O 2 saturated aqueous solution of the solute (1.4 × 10 −2 mol dm −3, pH = 1.5) has been assigned to dimer radical cation. Br − 2 and Cl − 2 are able to oxidize TDE with a bimolecular rate constant of 4.8 × 10 8 and 1.1 × 10 9 dm 3 mol −1 s −1 respectively. The dimer radical cation of TDE oxidizes I − with a bimolecular rate constant of 3.6 × 10 9 dm 3 mol −1 s −1.

ChemInform Abstract: Formation of Radical Cations of Polyiodomethanes: A Pulse Radiolysis Study.

AbstractThe transient optical absorption bands observed on pulse γ‐radiolysis of CH2I2 and CHI3 in 1,2‐dichloroethane are assigned to CH2I2+ and CHI3+, respectively.