UV–visible spectrum of the phenyl radical and kinetics of its reaction with NO in the gas phase

Pulse radiolysis transient UV–visible absorption spectroscopy was used to study the UV–visible absorption spectrum (225–575 nm) of the phenyl radical, C₆H₅(), and kinetics of its reaction with NO. Phenyl radicals have a strong broad featureless absorption in the region of 225–340 nm. In the presence of NO phenyl radicals are converted into nitrosobenzene. The phenyl radical spectrum was measured relative to that of nitrosobenzene. Based upon σ(C₆H₅NO)_{270 nm}=3.82×10⁻¹⁷ cm² molecule⁻¹ we derive an absorption cross-section for phenyl radicals at 250 nm, σ(C₆H₅())_{250 nm}=(2.75±0.58)×10⁻¹⁷ cm² molecule⁻¹. At 295 K in 200–1000 mbar of Ar diluent k(C₆H₅()+NO)=(2.09±0.15)×10⁻¹¹ cm³ molecule⁻¹ s⁻¹.     

Reactive oxygen and nitrogen species (RONS) scavenging reactions of o-vanillin: Pulse radiolysis and stopped flow studies

Reactions of peroxyl radicals and peroxynitrite with o-vanillin (2-hydroxy 3-methoxy benzaldehyde), a positional isomer of the well-known dietary compound vanillin, were studied to understand the mechanisms of its free radical scavenging action. Trichloromethylperoxyl radicals (CCl₃O ₂ ^· ) were used as model peroxyl radicals and their reactions with o-vanillin were studied using nanosecond pulse radiolysis technique with absorption detection. The reaction produced a transient with a bimolecular rate constant of approx. 10⁵ M⁻¹s⁻¹, having absorption in the 400–500 nm region with a maximum at 450 nm. This spectrum looked significantly different from that of phenoxyl radicals of o-vanillin produced by the one-electron oxidation by azide radicals. The spectra and decay kinetics suggest that peroxyl radical reacts with o-vanillin mainly by forming a radical adduct. Peroxynitrite reactions with o-vanillin at pH 6.8 were studied using a stopped-flow spectrophotometer. o-Vanillin reacts with peroxynitrite with a bimolecular rate constant of 3 × 10³ M⁻¹s⁻¹. The reaction produced an intermediate having absorption in the wavelength region of 300–500 nm with a absorption maximum at 420 nm, that subsequently decayed in 20 s with a first-order decay constant of 0.09 s⁻¹. The studies indicate that o-vanillin is a very efficient scavenger of peroxynitrite, but not a very good scavenger of peroxyl radical. The reactions take place through the aldehyde and the phenolic OH group and are significantly different from other phenolic compounds.     

Spectral and kinetic characteristics of the α-propionic acid methyl ester radical in cyclohexane and water

The -propionic acid methyl ester radical was produced in dissociative electron capture reaction of 2-chloropropionic acid methyl ester. The absorption maxima of the radical are at 310 and 300 nm in cyclohexane and water with extinction coefficients of 440±50 and 400±50 mol^–1 dm³ cm^–1. The second order decay rate parameter in water is (2.3±0.5)×10⁹ mol^–1 dm³ s^–1. The peroxy radicals have the characteristics: _max=265–270 nm, _max=700–900 mol^–1 dm³ and 2k=(7±2)·10⁸ mol^–1 dm³ s^–1.     

Pulse radiolysis investigations on the reactions of primary radiolytic species of water with atropine

On pulse radiolysis of N₂O saturated aqueous solutions of atropine, an optical absorption band (_max at 320 nm,e=2.81·10³ dm³·mol^–1·cm^–1) was observed, which is assigned to the product of reaction of OH radicals with the solute. This absorption decayed following second order kinetics with a rate constant of 4.5·10⁸ dm³·mol^–1·s^–1. The rate constant for the reaction of OH radicals with atropine as estimated by following the build-up kinetics is 2.7·10⁹ dm³·mol^–1·s^–1. The H atoms also reacted with this compound to produce a transient absorption band behaving similarly to the one observed in the case of reaction with OH radicals. The transient species formed in both cases is assigned to a radical derived by H atom abstraction by H/OH radicals from the parent compound. This radical was unreactive towards 2-mercaptoethanol. e _aq ^– was found to react with atropine forming a transient band with _max at 310 nm (=3.55·10³ dm³·mol^–1). Its decay was also second order with a rate constant of 1.64·10⁹ dm³·mol^–1·s^–1. The bimolecular rate constant for the reaction of e _aq ^– with atropine as estimated from the decay of e _aq ^– absorption at 720 nm is 3.9·10⁹ dm³·mol^–1·s^–1. Specific one-electron oxidizing and reducing agents (such as Cl ₂ ^– , Tl²⁺, SO ₄ ^– and (CH₃)₂COH, CO ₂ ^– , respectively) failed to oxidize or reduce this compound in aqoues solutions. The radical anion of atropine formed by its reaction with e _aq ^– was found to reduce thionine and methyl viologen with bimolecular rate constant of 3.8·10⁹ and 3.2·10⁹ dm³·mol^–1·s^–1, respectively.     

Tunable Diode Laser Absorption Studies of Hydrocarbons in RF Plasmas Containing Hexamethyldisiloxane

Tunable infrared diode laser absorption spectroscopy has been used to detect the methyl radical and three stable molecules, CH₄, C₂H₂ and C₂H₆, in radio frequency plasmas (f=13.56 MHz) containing hexamethyldisiloxane (HMDSO). The methyl radical concentration and the concentration of the stable hydrocarbons, produced in the plasma, have been measured in pure HMDSO discharges and with admixtures of Ar, while discharge power (P=20–200 W), total gas pressure (p=0.08–0.6 mbar), gas mixture and total gas flow rate (=1–10 sccm) were varied. The methyl radical concentration was found to be in the range of 10¹³ molecules cm^-3, while methane and ethane are the dominant hydrocarbons with concentrations of 10¹⁴–10¹⁵ mol cm^-3. Conversion rates to the measured stable hydrocarbons (RC(C_xH_y): 2×10¹²–2×10¹⁶ molecules J^-1 s^-1) could be estimated in dependence on power, flow, mixture and pressure. Under the used experimental conditions a maximum deposition rate of polymer layers of about 400 nm min^-1 has been found.     

Reaction of oxide radical ion (O.–) with substituted pyrimidines

Pulse radiolysis technique has been used to investigate the reaction of oxide radical ion (O^.–) with 4,6-dihydroxy-2-methyl pyrimidine (DHMP), 2,4-dimethyl-6-hydroxy pyrimidine (DMHP), 5,6-dimethyl uracil (DMU) and 6-methyl uracil (MU) in strongly alkaline medium. The second-order rate constants for the reaction of O^.– with these compounds are in the range 2-5 × 10⁸ dm³ mol^–1 s^–1. The transient absorption spectra obtained with DHMP have two maxima at 290 and 370 nm and with DMHP have maxima at 310 and 470 nm. The transient spectrum from DMU is characterized by its absorption maxima at 310 and 520 nm and that of MU by its single maximum at 425 nm. The intermediate species were found to react with N,N,N,N-tetramethyl-p-phenylenediamine (TMPD) with high G(TMPD^.+) values ranged between 3.9 × 10^–7 molJ^–1 and 4.8 × 10^–7 molJ^–1. These radicals undergo decay by second-order kinetics (2k/ = 1.0-1.7 × 10⁶ s^–1). The reaction of O^.– with the selected pyrimidines is proposed to proceed through a hydrogen abstraction from the methyl group forming allyl type radicals. These are mainly oxidizing radicals and hence readily undergo electron transfer reactions with TMPD.     

Rate Coefficient for Self-Association Reaction of CF2 Radicals Determined in the Afterglowof Low-Pressure C4F8 Plasmas

We have analyzed decay kinetics of CF₂ radicals in the afterglow of low-pressure, high-density C₄F₈ plasmas. The decay curve of CF₂ density has been approximated by the combination of first- and second-order kinetics. The surface loss probability evaluated from the frequency of the first-order decay process has been on the order of 10^–4. This small surface loss probability has enabled us to observe the second-order decay process. The mechanism of the second-order decay is self-association reaction between CF₂ radicals (CF₂+CF₂C₂F₄). The rate coefficient for this reaction has been evaluated as (2.6–5.3)×10^–14 cm³/s under gas pressures of 2 to 100 mTorr. The rate coefficient was found to be almost independent of the gas pressure and has been in close agreement with known values, which are determined in high gas pressures above 1 Torr.     

Mechanism of OH radical-induced oxidation of <Emphasis Type="Italic">p</Emphasis>-cresol to <Emphasis Type="Italic">p</Emphasis>-methylphenoxyl radical

The OH radical-induced oxidation of p-cresol to p-methylphenoxyl radical was studied in aqueous solution in a wide pH range by means of pulse radiolysis combined with optical spectroscopy. OH-adduct cyclohexadienyl type radicals were identified as intermediates of the reaction. In the acidic pH range the first-order rate coefficient of phenoxyl radical formation was found linearly dependent on the H₃O⁺ concentration yielding a bimolecular rate coefficient of 1.8 × 10⁸ mol^–1 dm³ s^–1. In the alkaline range a linear dependence was found on the OH^– concentration with rate coefficient of 4.9 × 10¹⁰ mol^–1 dm³ s^–1. These findings were interpreted in terms of acid-base catalysis of the H₂O elimination from the OH-adduct. With the time resolution applied, 30 ns, the radical cation p-CH₃C₆H₄OH^+. was not observed as intermediate.     

Ion-molecule reactions of primary radical cations in the liquid-phase radiolysis of <Emphasis Type="Italic">n</Emphasis>-alkanes: An EPR study

Radiation-chemical yields the liquid-phase radiolysis of C₅–C₁₂ n-alkanes were measured using the spin trap technique. The yields of n-alkyl radicals depended only slightly on the chain length in C₅–C₉ alkanes and amounted up to 30% of the total yield of trapped radicals; they were inhibited by the addition of charge scavengers. An analysis of the experimental results together with data on radicals in irradiated crystalline alkanes and radical cations in freon matrices showed that n-alkyl radicals results from the ion-molecule reactions of primary radical cations, whereas the protonated ions RH₂⁺ as products of these reactions are a source of sec-alkyl radicals. At least 60% of primary radical cations are consumed via these reaction pathways. A part of sec-alkyl radicals is due to gauche-conformers. The relative amount of primary alkyl radicals formed in the degradation of excited states and the subsequent charge neutralization processes should be insignificant.Translated from Khimiya Vysokikh Energii, Vol. 39, No. 1, 2005, pp. 5–14.Original Russian Text Copyright © 2005 by Belevskii, Belopushkin.     

Radiation-Induced Oxidation Reactions of 2-Selenouracil in Aqueous Solutions: Comparison with Sulfur Analog of Uracil

One-electron oxidation of 2-selenouracil (2-SeU) by hydroxyl (^●OH) and azide (^●N₃) radicals leads to various primary reactive intermediates. Their optical absorption spectra and kinetic characteristics were studied by pulse radiolysis with UV-vis spectrophotometric and conductivity detection and by the density functional theory (DFT) method. The transient absorption spectra recorded in the reactions of ^●OH with 2-SeU are dominated by an absorption band with an λ_max = 440 nm, the intensity of which depends on the concentration of 2-SeU and pH. Based on the combination of conductometric and DFT studies, the transient absorption band observed both at low and high concentrations of 2-SeU was assigned to the dimeric 2c-3e Se-Se-bonded radical in neutral form (2^●). The dimeric radical (2^●) is formed in the reaction of a selenyl-type radical (6^●) with 2-SeU, and both radicals are in equilibrium with K_eq = 1.3 × 10⁴ M⁻¹ at pH 4 (below the pK_a of 2-SeU). Similar equilibrium with K_eq = 4.4 × 10³ M⁻¹ was determined for pH 10 (above the pK_a of 2-SeU), which admittedly involves the same radical (6^●) but with a dimeric 2c-3e Se-Se bonded radical in anionic form (2^●−). In turn, at the lowest concentration of 2-SeU (0.05 mM) and pH 10, the transient absorption spectrum is dominated by an absorption band with an λ_max = 390 nm, which was assigned to the ^●OH adduct to the double bond at C5 carbon atom (3^●) based on DFT calculations. Similar spectral and kinetic features were also observed during the ^●N₃-induced oxidation of 2-SeU. In principle, our results mostly revealed similarities in one-electron oxidation pathways of 2-SeU and 2-thiouracil (2-TU). The major difference concerns the stability of dimeric radicals with a 2c-3e chalcogen-chalcogen bond in favor of 2-SeU.     

Photosensitivity of paramagnetic centers in radiolyzed solid hydrogen cyanide

The photolysis of solid hydrogen cyanide and the effects of UV light on⁶⁰Co--irradiated HCN at 77 K were studied using an ESR technique. As in the case of radiolysis, the HZ₂C=N radical formed due to sticking of the H atom to the triple bond of the HCN molecule is the main radical product of low-temperature HCN photolysis. The C=N^– radicals are accumulated at 77 K in insignificant amounts (3 %). It was established that radical and ionic products stabilized in y-irradiated HCN possess photochromism and under the action of UV light enter photochemical reactions leading to their decomposition. Upon photobleaching, the concentration of H₂C=N^– radicals first increases two- to threefold because of the decomposition of H₂C=N^– ions and then decreases. The presence of radicals and ions formed upon the low-temperature radiolysis of HCN broadens the optical absorption band of the system, and the boundary of the action spectrum shifts from 280 nm (for nonirradiated HCN) to the visible region at 400–440 nm.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 859–863, April, 1996.     

Temperature effect on quenching of CH(AΔ)

The quenching rate constants of CH(A²Δ) radicals by alcohol, alkane, O₂, and C₂H₄ molecules over the temperature range 297–653 K have been measured using laser photolysis of CHBr₃ at 266 nm to produce CH(A) radical and time-resolved fluorescence measurements. Under the simultaneous effects of multiple attractive potentials and repulsive barrier, the temperature dependence of the quenching process of CH(A²Δ) is discussed qualitatively based on a modified collision complex model.     

Radical reactions of C84

Pulse radiolysis and steady-state radiolysis experiments describing the radical and electron transfer reactions of C₈₄ are reported here for the first time. C₈₄ reacts readily with radiolytically generated chloromethyl (^•CCl₃) and trichloromethylperoxyl (CCl₃OO^•) radicals in CCl₄. The formation of the radical adduct has been confirmed from its characteristic absorption in the UV (320 nm) and visible (480 nm). Radical-induced oxidation in 1,2-dichloroethane (1,2-DCE) resulted in a short lived transient absorbing at 920 nm. Reduction of C₈₄ in toluene/2-propanol/acetone could be conveniently followed by formation of an absorption band with an absorption maximum at 960 nm.     

An ESR and Optical Spectroscopic Study of the Mechanism of Photostimulated Reactions in Hydrogen Cyanide γ-Irradiated at 77 K

Changes in the electronic absorption spectra and ESR spectra in the course of photobleaching of radiolyzed solid HCN with light of different wavelengths (236–600 nm) were studied by ESR and optical spectroscopy. Two bands at 270 and 290 nm in the optical spectrum were attributed to the presence of H₂C=N and HC=NH radicals, respectively (the molar absorption coefficients are k ₂₇₀ 2.7 × 10²l mol^–1cm^–1and k ₂₉₀ 1.5 × 10²l mol^–1cm^–1, respectively). Structureless broad bands with maximums at 313 and 465 nm, which were detected after the exposure of a sample to light with 300 nm, can belong to the cyanide ions (CN^–) and H₂C=N⁺cations (the molar absorption coefficients of the ions are k _ion= (0.4–1.0) × 10²l mol^–1cm^–1). In the photobleaching of -irradiated HCN ( = 236–280 nm), H₂C=N⁺radicals were additionally formed by the photoinduced reaction of electron transfer from the CN^–anion to the H₂C=N⁺cation. The amount of these radicals generated in the course of photobleaching is several times greater than that of the same radicals formed in the radiolysis via hydrogen atom addition to the multiple bond of HCN molecules.     

Tetracene oxygenation caused by infrared excitation of molecular oxygen in air-saturated solutions: the photoreaction action spectrum and spectroscopic parameters of the transition in oxygen molecules

The action spectrum of tetracene photooxygenation was measured in air-saturated carbon tetrachloride in the wavelength range of 1220–1290 nm using a wavelength-tunable forsterite laser. The data show that the photoreaction occurs due to laser excitation of the transition in oxygen molecules. The molar absorption coefficient (₁₂₇₃) and the cross section of light absorption (σ₁₂₇₃) corresponding to the spectral maximum of this transition were calculated from the observed photoreaction rates. The obtained values ε₁₂₇₃ = 0.003 M⁻¹ cm⁻¹ and σ₁₂₇₃ = 10⁻²³ cm² (±20%) reasonably correlate with those extrapolated from the high-pressure oxygen absorption spectra.     

Formation and decay of phenoxyl radicals: Variation with pH and pulse dose

Phenoxyl type radicals were produced from tyrosine methyl ester (TME) using azide (N ₃ ^. ) radicals. The rate constant of formation increased from 2·10⁸ dm³·mol^–1·s^–1 at pH 7 to 4·10⁹ dm³·mol^–1·s^–1 at pH 11, whereas that of the decay, 2k=(6±1)·10⁸ dm³·mol^–1·s^–1, remained constant. The maximum yield of the radicals varied with pH and pulse dose consistently with the kinetic scheme, which involved a competition of the oxidation of TME by azide radicals with the natural decay of N ₃ ^. .     

Pulse radiolysis studies on the polymerization of 1,6-hexanediol diacrylate in cyclohexane solvent

The mechanism of initiation and kinetics of radiation induced polymerization of 1,6-hexanediol diacrylate (HDDA) was studied by pulse radiolysis with optical detection in cyclohexane solution and in pure HDDA at room temperature. In both cases a transient light absorption decreasing with the wavelength in the 270–400 nm wavelength range, but possessing a shoulder at 310 nm was observed. The spectrum did not change with time after the pulse. The absorption is due to α-carboxyalkyl radicals: the assignment was supported by the results obtained from the reaction of electrons with methyl 2-chloropropionate leading to dechlorination. The radicals produced from HDDA or from the chloro compound decayed in a second order process with rate parameters of 5 · 10⁸ and of 4 · 10⁹ mol^-1 dm³ s^-1 in cyclohexane. In the presence of oxygen the corresponding peroxy radicals decaying in very slow process (several milliseconds) were observed. The results are interrupted in terms of radical polymerization mechanism.     

PRIMARY INTERMEDIATES IN THE PULSED IRRADIATION OF RETINOIDS

Abstract Laser flash photolysis and pulse radiolysis have led to the characterisation of several shortlived intermediates formed after irradiation of retinoic acid and retinyl acetate in hexane or methanol. For retinoic acid, the triplet state, wavelength maximum 440 nm, extinction coefficient 7.3 × 10⁴ dm³ mol^?1 cm^?1, decay constant 6.2 × 10⁵ s^?1, is formed with a quantum yield of 0.012 for 347 nm excitation. The radical cation, absorption maximum 590 nm, extinction coefficient ～7 × 10⁴ dm³mol^?1 cm^?1, is formed in a biphotonic process. The radical anion, absorption maximum 510nm in hexane, 480 nm in methanol where its extinction coefficient is 1.2 × 10⁵ dm³mol^?1 cm^?1, appears to decay partially in methanol into another longer-lived neutral radical, wavelength maximum 420 nm, by loss of OH^?. For retinyl acetate, the triplet state, absorption maximum 395 nm, extinction coefficient 7.9 × 10⁴dm³mol^?1 cm^?1, decay constant 1.2 × 10⁶s^?1 is formed with a quantum yield of 0.025 for 347 nm excitation. Monophotonic photoelimination of OCOCH₃? in methanol produces the retinylic carbenium ion, wavelength maximum 590 nm, whose decay is enhanced by ammonia, k ～ 2 × 10⁶ dm³ mol^?1 s^?1 and retarded by water. The radical cation also has a wavelength maximum at 590 nm, its extinction coefficient being ～ 1.0 × 10⁵ dm³mol¹ cm^?1. The long-lived transient absorption with maximum at 385 nm, extinction coefficient 1.0 × 10⁵ dm³mol^?1 cm^?1, obtained from the reaction of the solvated electron with retinyl acetate in methanol may be due to either the radical anion itself or more likely the radical resulting from elimination of OCOCH₃? from this anion. These results suggest that skin photosensitivity caused by retinyl acetate might be greater than that due to retinoic acid.     

Long-wave light absorption and photochemical decomposition of peroxide radicals

Photochemical decomposition of alkylperoxide radicals in glassy matrices at 77 K was experimentally studied. Irradiation with light up to 405 nm leads to the photodecomposition of peroxide radicals. The quantum yield of the reaction was estimated to be 10^–2. A light-induced angular dependence of the ESR spectra of peroxide radicals resulting from photoselection was detected. The photoselection found proves that the photodecomposition is induced by the absorption of light in the inherent absorption band of RO₂.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1074–1077, June, 1995.The work was carried out with the financial support of the Russian Foundation for Basic Research (Project No. 95-03-08505) and the International Science Foundation (Grants NBU 000 and NBU 300).     

Photochemical reactions of radical cations of dimethylformamide in freon matrices at 77 K

Spectral characteristics of the radical cations (RC) of DMF (λ_max = 415 nm, ε_max = (2.6±0.8)·10³ L mol^?2 cm^?2) stabilized in an irradiated glassy freon mixture (CFCl₃ and CF₂BrCF₂Br) at 77 K were determined. Amide type radicals and RC of the matrix were shown to be formed by irradiation (λ=365–436 nm) of the radical cations of DMF in freon matrices using ESR and UV spectroscopy. The quantum yields of photoconversion of the DMF radical cations are independent of the wavelength of exciting light. It was found that the matrix structure affects the processes stabilizing the products of photoconversion of the DMF radical cations.