A new transient absorption spectrum observed in the flash photolysis of thiophene

In the flash photolysis of thiophene diluted with argon, a transient absorption spectrum in the region 3770 Å and 4170 Å has been observed. The new absorption spectrum which consists of about 30 narrow and sharp red degraded bands could be analysed into an electronic system with the origin band at 4113.8 Å. The bands could tentatively assigned to the C₄H₃ radical.     

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.     

On the far ultraviolet absorption spectrum of ethyl fluoride

The absorption spectrum of C₂H₅F has been measured in the region down to 1150 Å with absorption maxima at 1290, 1240 and 1180 Å and has been interpreted with Rydberg transitions from the σ(CC) and pseudo π(CH₃) orbitals to the carbon 3s and 3p orbitals.     

Formation of free radicals in photoirradiated cellulose. I. Effect of wavelength

Formation of free radicals in photoirradiated cellulose has been studied by means of ESR spectroscopy at 77°K. Three kinds of light sources with different wavelengths between 2500 and 4000 ÅR were employed. No radicals detectable by ESR were recorded when cellulose was irradiated with light of wavelength longer than 3300–3400 ÅR. Hydrogen atoms that generated a doublet spectrum (ΔH = 508 G) were observed when cellulose was irradiated with light longer than 2800 ÅR. Hydrogen atoms and formly radicals that generated doublet spectra with splitting constants of 508 and 129 G, respectively, were observed when irradiated with light shorter than 2800 ÅR. The scission of the polymer chain in cellulose is evident from decrease of the degree of polymerization, and the results of mass spectrometric analysis indicated H₂, CO, CO₂, and H₂O to be the main volatile products of cellulose upon photoirradiation.     

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.     

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.     

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.     

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.     

Structure of the complex of dimethylphenyl betaine with dichloroacetic acid studied by X-ray diffraction,DFT calculations,infrared and Raman spectra

The structure of the complex of dimethylphenyl betaine (DMPB) with dichloroacetic acid (DCA) (1) has been investigated by X-ray diffraction, FTIR and Raman spectroscopy, and B3LYP/6-311 + + G(d,p) calculations. The crystal is monoclinic, space group P2₁. The acid is connected with betaine through the OH⋯O hydrogen bond of 2.480(2) Å. In the optimized structure the short, asymmetric O⋯O distance is 2.491 Å. FTIR spectrum shows a broad absorption in the 1500–400 cm⁻¹ region characteristic of very short OH⋯O hydrogen bond caused by Fermi resonance between νOH and overtones of δOH and γOH. In the Raman spectrum this broad absorption is not observed. The potential energy distributions (PED) were used for the assignments of IR and Raman frequencies in the experimental and calculated spectra. The FTIR and Raman spectra of the crystal complex are consistent with the X-ray results.     

Spectral,kinetics, and redox properties of the transients formed on one‐electron oxidation of 4,4′‐thiodiphenol: A pulse radiolysis study

The transient absorption bands (λ_max = 330, 525 nm, k_f = 5 × 10⁹ dm³ mol⁻¹ s⁻¹) obtained on pulse radiolysis of N₂O‐saturated neutral aqueous solution of 4,4′‐thiodiphenol (TDPH) are due to the reaction of TDPH with ^·OH radicals and are assigned to phenoxyl radical formed on fast deprotonation of the solute radical cation. The reaction of specific one‐electron oxidants (Cl₂^·−, Br₂^·−, N₃^·, TI²⁺, CCl₃OO^·) with TDPH also produced similar transient absorption bands. The phenoxyl radicals are also produced on pulse radiolysis of N₂‐saturated solution of TDPH in 1,2‐dichloroethane. The nature of transient absorption spectrum obtained on reaction of ^·OH radicals with TDPH is not affected in acidic solutions, showing that OH‐adduct is not formed in neutral solutions. The oxidation potential for the formation of phenoxyl radical is determined to be 0.98 V. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 603–610, 1999     

Resonance raman and infrared spectra of ClO3 radicals in electron irradiated NaClO3

Resonance Raman and infrared absorption spectra of ClO₃ radicals have been observed from measurements of crystalline NaClO₃ irradiated at 77 K with 1.5 MeV electrons. The ClO₃ radicals occupy two sets of nonequivalent sites; radicals at one site are observed from resonance scattering using 6328 Å excitation while those at the other site are observed from resonance scattering with 5145 Å excitation. The red sensitive radicals decompose at 77 K with a half-life of ≈29 min while the green sensitive radicals are more stable at this temperature.     

Observation of hydrated electron, (SCN)2 .- and CO3 .- radical in high temperature and supercritical water

Transient radicals (hydrated electron, (SCN)₂ ^.- and CO₃ ^.-) formed in supercritical water have been observed by the pulse radiolysis technique. The change of spectra of these radicals with temperature has been measured. It was found that the spectra and absorption coefficients of the radicals, e^- _aq and (SCN)₂ ^.-, are strongly dependent on the temperature of the water. Since it was found that the absorption spectrum and molar absorption coefficient of CO₃ ^.- radical seem to be almost independent of temperature, G-values of OH and e^- _aq could be derived. Then, the absolute values of the absorption coefficients for the radicals could be calculated. The G-values of the radical products in water radiolysis tend to increase with increasing temperature up to 400°C. Based on the above observation, radiolysis of supercritical water is discussed.     

Electronic absorption spectrum of 4-methylindole

The near u.v. absorption spectrum of 4-methylindole has been recorded. The observed spectrum lying in the region λ 2960-2600 Å has been identified as a π* ← π transition, analogous to the λ 2850 Å system of indole. A vibrational analysis has been proposed for the band system.     

High‐temperature shock tube study of the reactions CH3 + OH → products and CH3OH + Ar → products

The reaction between methyl and hydroxyl radicals has been studied in reflected shock wave experiments using narrow‐linewidth OH laser absorption. OH radicals were generated by the rapid thermal decomposition of tert‐butyl hydroperoxide. Two different species were used as CH₃ radical precursors, azomethane and methyl iodide. The overall rate coefficient of the CH₃ + OH reaction was determined in the temperature range 1081–1426 K under conditions of chemical isolation. The experimental data are in good agreement with a recent theoretical study of the reaction. The decomposition of methanol to methyl and OH radicals was also investigated behind reflected shock waves. The current measurements are in good agreement with a recent experimental study and a master equation simulation. © 2008 Wiley Periodicals, Inc. 40: 488–495, 2008     

High resolution spectrum of the xenon molecule in the vacuum ultraviolet region (1150–1300 Å)

The absorption spectrum of the Van der Waals dimer Xe₂ has been photographed in the region 1150–1130 Å with a ten-metre spectrograph. The vibronic analysis of twelve molecular systems provides information on the ground state and on several excited molecular states. Some of the numerous bands appearing in the Xe-rare-gas mixture spectra are examined and compared to those of Xe₂.     

ESR single-crystal study of radical and radical-pair formation in some γ-irradiated vinyl monomers

Single crystals of methyl methacrylate (MMA), methyl acrylate (MA), and acrolein (A) have been prepared by a low-temperature technique. After irradiation with γ-rays at 77°K the paramagnetic species were identified by ESR spectroscopy. MMA gave a seven-line single spectrum from radicals formed by hydrogen addition. The hyperfine coupling constants are slightly anisotropic with a mean value of 22 G. Radical pairs were observed as ΔM_s = 1 and ΔM_s = 2 transitions; the hyperfine coupling was 11 G. From the strongly anisotropic dipolar interaction, upper limits for the distances between the pair components were calculated to be 5.45 Å and 6.3 Å. MA gave a five-line main spectrum with the same hyperfine coupling values and two radical pairs, one with a distance 5.9 Å between the components. In a there was also a strongly anisotropic interaction. The hyperfine coupling of the ΔM_s = 2 transition was 9.8 G. The number of radical pairs compared to the total number of radicals increases only slightly with the radiation dose. This makes it likely that pair formation occurs in the spurs and blobs formed by the γ-radiation. At an increased temperature the radical pairs disappeared; the spectrum of MMA changed to that characteristic of propagating polymer radicals.     

Location of the first 3B2 ← 3B+2 transition in phenanthrene

The polarization of the triplet-triplet absorption spectrum of phenanthrene-d₁₀ in the region from 3500 to 8500 A has been recorded. The ³B₂^? state is located at about 4250 Å. The possibility that phenanthrene might be a suitable chromophore for laser dye synthesis is discussed.     

ESR. Study of an X-Irradiated Single Crystal of Methylene Diarsonic Acid

X-Irradiation of single crystals of methylene diarsonic acid gives rise to a variety of radicals which have been identified by ESR. spectroscopy; (OH)₂As(O)CHAs(O) (OH)₂, ^?OÅs(OH)₂CH₂As(O)(OH)₂, AsO₃^– and (OH)Ås(O)CH₂As(O)(OH)₂. The spin densities have been obtained from the ⁷⁵As-hyperfine tensors. An additional species - probably a radical anion localized on a pentacoordinated arsenic - is also detected after annealing at room temperature. The radiation damage process is discussed.     

Formation of free radicals in photoirradiated cellulose. VII. Radical decay

The ESR spectra of untreated and photosensitized celluloses irradiated with three different ultraviolet light sources, i.e., λ > 2537 Å, λ > 2800 Å, λ > 3400 Å, at 77°K under vacuum were studied. Based on the warm-up process, that is, warming the sample from 77°K to 273°K for a certain time and recorded at 77°K, the decay behavior of free radicals of celluloses was examined for changes of the pattern and the intensities of ESR spectra. For the untreated samples irradiated with light of λ > 2537 Å and λ > 2800 Å, beside the two doublet spectra originating from hydrogen atoms (508 gauss splitting) and formyl radicals (129 gauss splitting), the observed sevenline spectrum was resolved to be a superposition of a singlet (ΔH_msl = 16 gauss), a doublet (24 gauss splitting), a triplet (34 gauss splitting), and a quartet (overall width, 88 gauss) spectrum. For the photosensitized samples irradiated with light of λ > 3400 Å, the 1:1:1 three-line spectrum was resolved to be a superposition of a singlet (ΔH_msl = 27 gauss), a doublet (43 gauss splitting), and a triplet (34 gauss splitting) spectrum. The five-line spectra of the photosensitized samples irradiated with light of λ > 2537 Å and λ > 2800 Å were resolved to be a superposition of a singlet (ΔH_msl = 27 gauss), a doublet (43 gauss splitting), and a triplet (34 gauss splitting) spectrum. Based on these findings, the conclusion was drawn that at least six kinds of spectra, generated from six kinds of radical species, were formed in cellulose irradiated with ultraviolet light under appropriate experimental conditions.     

A picosecond transient absorption study of the intramolecular excited state proton transfer in 3-hydroxyflavone: assignment of the observed bands

The time-resolved transient absorption and gain of 3-hydroxyflavone in ethanol ( T = 300 and 173 K) and MCH ( T = 300 and 198 K) has been investigated. Two absorption bands (A and B) and two gain bands (E₁ and E₂) have been observed. Band A (lying in the 5200 Å spectral region) appears during and just after the excitation step and is assigned to the normal excited form N_x Band B (lying in the 6050 Å spectral region) appears after band A and is assigned to the tautomer form T. A gain band E₂ (lying at λ=5400 Å) appears at the same time as the B band and is attributed to the T→T emission. A gain band E₁ (lying at λ = 5730 Å) appears earlier than the E₂ band. Its origin is discussed and two possible assignments are presented: vibronic progression of the emission spectrum of T or formation of an intermediate species on the proton transfer pathway from N_x to T.