Absorption spectrum of electrons trapped in single crystals of sucrose.: A pulse-radiolysis study in the temperature range 6–270 K

Pulse radiolysis of single crystals of sucrose at 6 K yields an absorption spectrum with λ_maxat 450–475 nm which is attributed to the trapped electron. The yields and decay of this species are reported for the temperature range 6–270 K. It is concluded that there is only one form of e_t^? in these crystals and that it is deeply trapped.     

Kinetic and spectral properties of the intermediates formed in the pulse radiolysis of 2-mercaptobenzimidazole

The reactions of primary species of water radiolysis such as e^- _aq, H* and *OH, and some specific one electron reductants and oxidants with 2-mercaptobenzimidazole have been studied at various pHs. *OH radical reaction with MBZ at pH 7 gave a transient species having absorption maxima (λ_max) at 330 and 590 nm. The transient species (pK_a = 3.6) was found to be neutral at this pH and was a mild oxidant. The initial transient species formed by the reaction of MBZ with e^{- aq} at pH 7 and with H atom at pH 0 were found to react with the parent molecule to form another transient species which has an absorption spectrum similar to that obtained by *OH radical reaction with λ_max at 590 nm. However the reaction is not quantitative. The kinetic, spectral, acid-base and redox properties of the transient species are reported.     

Reactions of e aq' − H and OH with picolinic acid studied by pulse radiolysis

Rate constants for the reaction of the primary species of water radiolysis, viz. e _aq' ^– , H and OH, with picolinic acid have been determined at various pHs. The semi-reduced species exhibits _max at 305 and 350 nm. It is a strong reductant and has two pK_a values of 1 and 5.7. The OH adduct of this compound also exhibits two pK_a s at 2.1 and 5.2. H-atom reaction with picolinic acid gives rise to a mixture of species.     

On the blue-colored species,possibly assignable to a radical cation,formed by the aromatic hydrocarbon–sulfur dioxide–oxygen system: The polymerization of olefins initiated by aromatic hydrocarbons–oxygen in liquid sulfur dioxide

It was found that styrene in liquid sulfur dioxide polymerizes, giving polystyrene readily and quantitatively by addition of such aromatic hydrocarbons as anthracene and trans- or cis-stilbene in the presence of oxygen, and the polymerization proceeds via a cationic process. The observations on the electronic spectra and kinetics in the system suggested that the polymerization was initiated by an electron transfer from the aromatic hydrocarbon to oxygen, followed by the formation of styrene radical cation. Supporting evidence of the radical cation is that 1,1-diphenylethylene in liquid sulfur dioxide in the presence of oxygen shows a peak at λ_max = 605 mμ and reacts to give benzophenone and 1,1,3,3-tetraphenyl-butene-1, which are eliminated by addition of a radical or cation inhibitor.     

A PREVIOUSLY UNREPORTED INTENSE ABSORPTION BAND AND THE pK, OF PROTONATED TRIPLET METHYLENE BLUE

Abstract— Excitation by a Q-switched giant ruby laser (1.2 J output at 694 nm ?50 ns flash) of 2–10 µM solutions of methylene blue in water, 30% ethanol in water or 50 v/v% water-CH₃CN at pH values in the range 2.0–9.3 converted the dye essentially completely to its T₁ state. The absorption spectrum of T₁ dye was measured in different media at pH 2.0 and 8.2 by kinetic spectrophotometry. Previously reported T-T absorption in the violet in acidic and alkaline solutions and in the near infrared in alkaline solution was confirmed. Values found for these absorptions in the present work with 30% ethanol in water as solvent are λ_max - 370nm, ε_max, - 13,200 M^-1 cm^-1 at pH 2 and λ_max,?420nm, ε_max 9000 M^-1 cm^-1, λ_max, - 840 nm, ε_max - 20,000 m ^-1 cm^-1 at pH 8.2. Long-wavelength T-T absorption in acidic solution is reported here for the first time: λ_max, ? 680 nm, e_max? 19,000 M cm^-1 in 30% ethanol in water at pH 2. Observation of a pH-independent isobestic point ? 720 nm confirms that the long-wavelength absorptions are due to different protonated states of the same species, MB⁺(T₁) and MBH²⁺(T₁). The pK_a of MBH²⁺(T₁) in water was determined from the dependence on pH of absorption at 700 and 825 nm to be 7.1₄± 0.1 and from the kinetics of decay of triplet absorption to be 7.2. The specific rate of protonation of MB⁺(T₁) by H₂PO₄ in water at pH 4.4 was found to be 4.5 ± 0.4 times 10⁸M^-1s^-1.     

Contrasting photochemical behavior between meta-substituted and para-substituted aromatic polycarbonyl compounds

The meta-substituted polyketones 1b - f, including the parent ketone 1a, afforded approximately similar values for the quantum yield of benzocyclobutenol formation (Φ_CB), the triplet lifetime (τ), E_T and λ_max, respectively. In contrast, Φ_CB, τ, E_T, and λ_max of the para-substituted polyketones 2a - c depended on their molecular structures.     

Pulse radiolysis studies of vitaminK1 in an aqueous–organic mixed solvent

Pulse radiolysis studies of VK₁ were carried out in a nitrogen-saturated aqueous–isopropanol–acetone mixed solvent. We studied the spectroscopic characteristics of semiquinone radicals of VK₁ and determined the kinetic parameters of formation and decay of the species. It was found that the decay process of semiquinone radicals of VK₁ was according to a first-order reaction, this was different from the case in ethanol solution, possible reasons were also analyzed.     

Interaction of organo-alkali compounds with unsaturated hydrocarbons

The stability of disodium tetramer of α-methylstyrene (“living” polymer) in THF and in a THF-α-methylstyrene mixture has been investigated by spectrophotometry. It was found that at 25°C and at concentrations lower than the equilibrium concentration α-methylstyrene greatly stabilizes the process leading to disappearance of the main absorption band (λ_max = 340 mμ) of “living” polymer. In this case isomerization of “living” polymer is accompanied by quantitative conversion of the compound having λ_max = 340 mμ into a new compound with λ_max = 430 mμ. The constants of the disappearance rate D₃₄₀ and the activation energies of the process were determined in THF and in a THF-α-methylstyrene mixture. The introduction of small amounts of α-methylstyrene into living polymer at 25°C markedly increases its activity in the course of propagation. The experimental results are considered from the standpoint of formation of complexes of living polymer with α-methylstyrene.     

Pulse Radiolysis Study of Initiation,Dimerization, and Propagation Steps of 3,3-Dimethylacrylic Acid in Aqueous Medium

Rates of initiation, dimerization, and first propagation steps originating from the hydrated electron and hydroxyl radical reactions with 3,3-dimethylacrylic acid monomer have been investigated using the technique of pulse radiolysis. The initiation step for the protonated monomer is found to be faster than that of the deprotonated form. The dimerization step is about two orders of magnitude faster than the first propagation step. Radical species are more reactive than the corresponding radical-anionic species, and the β-protonated electron adduct to the monomer is found to propagate faster than other electron adduct species. The different transient species have been characterized by their absorption spectra, λ_max, extinction coefficient, pK, and redox behavior. Based on these observations, the most probable structures are suggested.     

Absorption spectrum,lifetime and reactivity of the excited triplet state of dimesitylcarbene

The first excited triplet state of dimesitylcarbene has been generated in solution at room temperature. It has a lifetime of 60 ns and shows fluorescence with λ_max = 501 nm and absorption with λ_max = 360 nm. This species is quenched by oxygen and carbon tetrachloride with rate constants of (4.4 ± 0.8) × 10¹⁰ and (7.3 ± 0.6) × 10⁹ M⁻¹ s⁻¹, respectively.     

Electron-beam initiated crosslinking in poly(N-isopropylacrylamide) aqueous solution

The reactions between the OH, H and e_aq⁻ transients of water radiolysis and a linear poly(N-isopropylacrylamide) were identified by the spectral and kinetic properties of intermediates. The radical responsible for crosslink formation is the isopropyl-centered radical that forms mainly in OH radical attack on the polymer. Below pH 3 this radical undergoes reversible protonation with pK_a≈2.1. The radical decay is composed of fast and slow parts. The initial fast decay is attributed to intramolecular reactions of radicals on the same chain (loop formation), the following slow decay to competition between further intramolecular and intermolecular (H-type crosslinks) termination processes. The differences in the network formation during irradiation of aqueous monomer and polymer solutions are discussed.     

A wavelength effect on urocanic acid E/Z photoisomerization

Abstract E-Urocanic acid exhibits a single, featureless, long-wavelength absorption band with λ_max˜258 nm in water. However, the quantum efficiency for E→Z photoisomerization is wavelength dependent in this region, with the maximum value at the low energy edge of the band (e.g. 313 nm) and appreciably lower efficiencies measured at ≦ 300 nm.     

Pulse radiolysis study on the initial processes of radiation-induced cationic polymerization of styrene and α-methylstyrene

Initial processes of radiation-induced cationic polymerization of styrene and α-methylstyrene have been studied by means of microsecond pulse radiolysis. For styrene, absorption bands caused by the monomer cation radical St⁺? appear at 630 and 350 nm in a mixture of isopentane and n-butyl chloride at about ?165°C. In parallel with the decay of St⁺?, three absorption bands appear in the near-infrared (IR) region, and at 600 and 450 nm. The IR and 600 nm bands are assigned to the associated dimer cation radical St₂⁺?, and the 450 nm band to the bonded dimer cation radical St-St⁺?. The kinetic behavior of these species shows that reaction of St⁺? with styrene monomer forms both St₂⁺? and St-St⁺?. With the decay of St-St⁺?, another absorption band appears at 340 nm, and the lifetime of this band is relatively long. The 340 nm band may be due to carbonium ions of the growing polystyrene. For α-methylstyrene, the monomer cation radical (at 690 and 350 nm), the associated dimer cation radical (in the near-IR region and at 620 nm) and the bonded dimer cation radical (at 480 nm) behave in a manner similar to that of the corresponding styrene species. The absorption band caused by carbonium ions of growing poly(α-methylstyrene) appears at 340 nm.     

Determination of Toosendanin Extracted from Traditional Chinese Medicine: the Fruit of Melia azedarach LC–FD Detection

A rapid and selective reversed-phase liquid chromatography with fluorescence detection was developed and validated for the determination of toosendanin in the fruit of Melia azedarach. After extraction, the extracts were analysed at 25 °C using Eclipse XDB-C18 column (4.6 mm × 150 mm; 5 μm) and a mobile phase consisting of methanol–water at a flow rate of 1.0 mL min^?1. Detection wavelength was set at λ_Ex = 230 nm, λ_Em = 320 nm. The linearity, accuracy, and precision of the method were good, and the method can be successfully used to investigate the level of toosendanin component in herb samples with satisfactory results.     

Anionic polymerization of N-ethyl-2-vinylcarbazole and N-ethyl-3-vinylcarbazole

The polymerization of N-ethyl-2-vinylcarbazole and N-ethyl-3-vinylcarbazole by an anionic mechanism has been demonstrated. Polymerization reactions were monitored by ultraviolet/visible spectroscopy and λ_max and ε values for the propagating carbanions determined. The 2-vinyl monomer exhibits all the features of a standard “living” polymer; the carbanion is stable at ambient temperatures and high molecular weight, M?_n ? 10⁶, narrow distribution polymers and block copolymers with styrene have been prepared. The carbanion from the 3-vinyl monomer is much less stable and a clean polymerization can only be conducted at temperatures below -60°C. A comparison of the anionic polymerization characteristics of the N-, 2-, and 3-vinyl carbazole monomer series is presented.     

Esr studies on the γ-irradiated acetone-CCl3F system

γ-irradiated acetone in polycrystalline CCl₃F at 77 K has been studied by ESR to reveal that the acetone molecules tend to be oriented in CCl₃F Matrix upon freezing the solution in ESR tubings. At higher concentrations both monomer and dimer radical cations of acetone are produced. The dimer cation is responsible for the optical absorption with λ_max ≈ 740 nm.     

About the OH yield in the radiolysis of an aqueous/H2O2 system. Its optimisation for water treatment

Unless the radiolytic reducing species are neutralised or converted into oxidising species, an EB remediation system cannot be considered a true Advanced Oxidation Processes (AOP). A water/H₂O₂ system irradiated by UVC mercury lamps constitutes a widely used OH production method. Employing H₂O₂ in radiolysis as well, an enhancement of the oxidative efficiency of an EB treatment can be obtained. Pulse radiolysis measurements of an aerated aqueous/H₂O₂/KSCN system have been systematically undertaken to assess the optimal H₂O₂ concentration. By linearly fitting a competition kinetics relationship, it is found that the scavengeable extra-yield of OH is ΔG(OH)=0.24 μmol J^?1 (R=0,9958), while the maximum experimental yield is measured G(OH)_max=(0.52±0.02) μmol J^?1 when [H₂O₂]=5–10 mM. Exceeding these concentrations the OH yield drops off.     

Reaktionsanalyse durch nichtisotherme Kalorimetrie: Lösungsreaktionen von Peroxiden,Ozoniden und Trioxiden

Homogeneous solution reactions (half-life-time for 1 M solutionsτ ₂₀° > ≈ 10^?4 sec, [ΔH] > 0.5 kcal/mole) have been investigated by non-isothermal differential calorimetry. By application of a new diagnostic procedure developed by use of an analogue computer, it can be decided whether the reactions of the substances tested are formal one-step processes, or what types of reaction-mechanisms must be considered. For the decompositions of some peroxide compounds the basic mechanism types, activation energies and action constants or entropies of the starting processes were determined. The cyclic furan ozonides have similar stabilities to that of the trioxide of di-isopropyl ether. On the other hand, triphenyl phosphite ozonide is considerably more unstable and can transfer oxygen to various acceptors below its decomposition temperature.     

Phosphorescence at Low Temperature by External Heavy-Atom Effect in Zinc(II) Clusters

Luminescent Zn^II clusters [Zn₄L₄(μ₃-OMe)₂X₂] (X=SCN ( 1 ), Cl ( 2 ), Br ( 3 )) and [Zn₇L₆(μ₃-OMe)₂(μ₃-OH)₄]Y₂ (Y=I⁻ ( 4 ), ClO₄⁻ ( 5 )), HL=methyl-3-methoxysalicylate, exhibiting blue fluorescence at room temperature (λ_max=416≈429 nm, Φ_em=0.09–0.36) have been synthesised and investigated in detail. In one case the external heavy-atom effect (EHE) arising the presence of iodide counter anions yielded phosphorescence with a long emission lifetime (λ_max=520 nm, τ=95.3 ms) at 77 K. Single-crystal X-ray structural analysis and time-dependent density-functional theory (TD-DFT) calculations revealed that their emission origin was attributed to the fluorescence from the singlet ligand-centred (¹LC) excited state, and the phosphorescence observed in 4 was caused by the EHE of counter anions having strong CH−I interactions.     

The Fluorescence of native DNA at room temperature

The fluorescence spectra (300–500nm) quantum yields (φ_f) and excitation spectra (240–285 nm) are reported for neutral aqueous sloutions of purified native DNA from calf thymus, E. coli bacterium, and hen erythrocyte near 20°C. The same properties were also measured for a reference solution of mononucleotides, and direct comparisons were made. Whether purified or not, the DNA spectra all closely resemble that from the monomer mixture between 300 and 360 nm but shows a broad, low level shoulder at λ_max ≈ 450nm which is absent in the monomer spectrum. The φ_f for the purified DNA is (4 = 1) x 10^-5, about half that of the monomer reference solution and unpurified DNA. The excitation spectrum is slightly red-shifted from the absorption for both the DNA and the monomer mixture, but not for the individual monomers. The fluorescence exhibits abrupt changes associated with the denaturation of DNA at ≈ 80°C and pII) <4 or > 11.