Photochemische Carbonylierung und Oxydation von wässerigem Methanol bei 1470 Å

Zusammenfassung Das photochemische Verhalten von wässerigem Methanol, gesättigt mit Argon, CO bzw. O₂ wurde bei 1470 Å als Funktion der VUV-Dosis und der Methanolkonzentration untersucht. Als Reaktionsprodukte wurden Wasserstoff, Formaldehyd, Glykolaldehyd, Glykol, Ameisensäure und in Gegenwart von Sauerstoff auch Peroxyd nachgewiesen. In 10^–2 m-Methanol, gesättigt mit Argon, betrug (H₂)=0,69 und (HCHO)=0,22, in Gegenwart von 7·10^–4 m-CO war (H₂)=0,10 und (HCHO)=0,29, während bei Sättigung mit O₂ (10^–3 mO₂) (H₂)=0,01, (HCHO)=0,33, (HCOOH)=0,20 und (H₂O₂)=0,22 gefunden wurde. Die -Werte der übrigen Produkte, die unter den angegebenen Bedingungen erhalten wurden, waren <0,07. Durch Erhöhung der Methanolkonzentration bis zu reinem Methanol, gesättigt mit Argon, stieg die Ausbeute von H₂, HCHO und (CH₂OH)₂ entsprechend an, während die von HCOOH und CH₂OHCHO sich praktisch nicht änderte. Bei reinem, mit Argon gesättigtem Methanol war (H₂)=0,89 und (HCHO)=0,73. Das Glykol erreichte bei etwa 2m-Methanol eine Höchstausbeute von =0,38. Reaktionsmechanismen werden diskutiert.

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Photochemical carbonylation and oxidation of aqueous methanol at 1470 Å

The photochemical behaviour of aqueous methanol saturated with argon, CO and O₂, resp., has been investigated at 1470 Å as a function of the v.u.v. dose and the methanol concentration. Hydrogen, formaldehyde, glycolaldehyde, glycol, formic acid and, in the presence of oxygen, also peroxide were detected. In 10^–2 M-CH₃OH saturated with argon (H₂)=0,69 and (HCHO)=0,22 were found; in the presence of 7·10^–4 M-CO the yields are (H₂)=0,10 and (HCHO)=0,29, whereas under saturation with oxygen (10^–3 M O₂) (H₂)=0,01, (HCHO)=0,33, (HCOOH)=0,20 and (H₂O₂)=0,22. The quantum yields of all other products obtained under the conditions mentioned above are <0.07. With increasing methanol concentration up to pure argon saturated methanol, the quantum yields of H₂, HCHO and (CH₂OH)₂ increase correspondingly, whereas the yields of HCOOH and CH₂OHCHO remain practically unchanged. When irradiating pure, argon saturated methanol, (H₂)=0,89 and (HCHO)=0,73 were found. A maximum yield of (CH₂OH)₂=0,38 was achieved using 2M-CH₃OH. Reaction mechanisms are discussed.

     

Chloride-catalyzed oxidation of phenol in pulsed-laser irradiation titanium dioxide sols

The kinetics of photolysis of phenol in presence of two kinds of TiO₂ colloid in acid aqueous solution medium was studied by transient absorption spectroscopy. The absorbance and quantum yield of the phenoxyl radicals is strongly influenced by the chloride ions. The process of laser flash photolysis of phenol in the presence of chloride has been discussed.     

Pulsradiolytische Untersuchungen der Reaktion des e aq − mit halogenierten aromatischen Verbindungen

Kinetic pulse radiolysis experiments (1 s pulse duration) on the reaction of e _aq ^– with halogenated aromatic compounds (fluoro-, chloro-, bromobenzene, benzylchloride and phenethylchloride were carried out in order to check the existence and to investigate the fate of an electron adduct. The measured absorption spectra, being identical with those previously observed for the phenyl and benzyl radical, and thepH dependence of the formation of the H-adducts, indicating no protonation of an intermediateRX ^–, can be explained simply by quantitative elimination of the halide. No evidence for the existence of an electron adduct as an intermediate could be obtained under the applied experimental conditions.

     

PATHWAYS FOR FORMATION OF HYDRATED ELECTRONS FROM EXCITED PHENOL AND RELATED COMPOUNDS

Abstract. The dependence of primary photophysical and photochemical processes, especially of electron photoejection, in phenol and related compounds in aqueous solution on excitation intensity and excitation energy is examined. Theoretical and experimental evidence is presented for the possibility of three pathways for electron ejection: (1) A monophotonic pathway via the fluorescent state, which most probably does not involve the lowest triplet state; (2) a monophotonic pathway requiring higher excitation energies, which takes place in competition with internal conversion to the fluorescent state; and (3) a consecutive biphotonic pathway in which the lowest triplet state absorbs the second photon, and which can become predominant at high intensities, e.g. under flash conditions. It is shown that this model reconciles apparently conflicting results published in the literature.     

Photolyse von Monophenylphosphat und e aq − -Bildung in wäßriger Lösung

At photolysis (253.7 nm) of monophenylphosphate (10^?3 m) in O₂-free neutral aqueous solution were determined: orthophosphate (Φ=0.006), phenol (Φ=0.0029), besides of small amounts of phosphorous acid, benzene, 2.2′-dihydroxybiphenyl and traces of 2.4′- and 4.4′-dihydroxybiphenyl. The yield of the main products is smaller at pH 2 and 12. Polymers were formed at u.v.-doses >2·10¹⁹ hv/ml. The electron yield determined by means of N₂O increases from Φ (N₂)= Φ(e _aq ^? )=0.012 to 0.019 changing the ester concentration from 0.001 to 0.1m. Φ (N₂)-value rises by addition of methanol or increasing pH. As electron ejecting state an excited complex is postulated. The effective ionization potential of phenylphosphate in aqueous solution is ≦4.9 e.v.     

Photo-induced electron emission from 17beta-estradiol and progesterone and possible biological consequences

It was established for the first time, that the sexual hormones 17beta-estradiol (17betaE2) and progesterone (PRG) are able to emit electrons from their excited single state in water-ethanol mixtures. The yield of the "solvated electrons" (e(s)(-)) depends on the substrate concentration, the ratio of water-alcohol-mixtures and the temperature. The e(s)(-) yield obtained from 17betaE2 is by two orders of magnitude higher than this of PRG. The possible relationship of the resulting hormone transients from 17betaE2 leading via specific metabolites to breast cancer is discussed.     

WAVELENGTH EFFECTS ON THE PHOTOPROCESSES OF INDOLE AND DERIVATIVES IN SOLUTION

Abstract— The two main primary photoprocesses (electron ejection and H-atom release) for indole, 5-methoxyindole and N-methylindole in various polar and nonpolar solvents were studied as a function of the excitation energy and were correlated with the corresponding fluorescence quantum yields. In hydrocarbon solvents, N–H bond cleavage is the main primary photoprocess from the ¹B_b band of the substrates with the exception of N-methylindole. In alcohols, both processes are of negligible importance. Hydrated electrons (e⁻_aq) are ejected from the relaxed singlet states of all three compounds in aqueous solutions with a similar yield for excitation at 280 and 254 nm (¹L_a and ¹L_b states). The yield increases when the excitation is into the ¹B_b band. The quantum yields of the two primary processes from the higher excited states are generally lower than the fraction of molecules not converting to the fluorescent state. This is explained by an efficient back reaction in competition with a thermally activated radical release from an intermediate state or radical pair formed from the S₂ (¹B_b) state. The non-occurrence of a photoionization energy threshold is discussed.     

VUV-Photolyse von Methanol bei 1236 Å

Zusammenfassung Es wird über die indirekte und die direkte VUV-Photolyse von luftfreiem Methanol in flüssiger Phase bei 1236 Å berichtet. Bei der Bestrahlung von wäßr. 0,01m-Methanol wurden folgende Produkte und Ausbeuten erhalten: (H₂)=1,00, (HCHO)=0,27, (CH₂OH)₂=0,12, (CH₂OH·CHO)=0,04 und (HCOOH)=0,02. Die Photolyse von flüssigem Methanol lieferte die gleichen Produkte, jedoch mit höheren Ausbeuten: (H₂)=1,50, (HCHO)=0,98, (CH₂OH)₂=0,28 und (CH₂OH·CHO)=0,06. Unter Anwendung von Methanol als Radikalfänger konnte auch die Quantenausbeute der Wasserphotolyse bei 1236 Å, (H,OH)=1,025 verifiziert werden. Wahrscheinliche Reaktionsmechanismen werden diskutiert.

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V.U.V.-Photolysis of methanol at 1236 Å

The indirect and the direct v.u.v.-photolysis of air free methanol in liquid phase at 1236 Å is reported. After irradiation of aqueous 0.01M-methanol the following products were obtained: (H₂)=1.00, (HCHO)=0.27, (CH₂OH)₂=0.12, (CH₂OH·CHO)=0.04 and (HCOOH)=0.02. The photolysis of liquid methanol yielded the same compounds, however with higher amount as follows: (H₂)=1.50, (HCHO)=0.98, (CH₂OH)₂=0.28 and (CH₂OH·CHO)=0.06. Using methanol as a scavenger for the H and OH radicals the quantum yield of the water photolysis at 1236 Å could be verified to be (H,OH)=1.025. Probable reaction mechanisms are discussed.

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Mit 3 Abbildungen

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Herrn Prof. Dr.E. Broda zum 60. Geburtstag gewidmet.     

Reactivity of OH radicals with chlorobenzoic acids—A pulse radiolysis and steady-state radiolysis study

The reactions of OH radicals with 2-, 3-, 4-chlorobenzoic acids (ClBzA) and chlorobenzene (ClBz), k(OH+substrates)=(4.5?6.2)×10⁹ dm³ mol^?1 s^?1, have been studied by pulse radiolysis in N₂O saturated solutions. The absorption maxima of the OH-adducts were in the range of 320?340 nm. Their decay was according to a second-order reaction, 2k=(1?9)×10⁸ dm³ mol^?1 s^?1. In the presence of N₂O/O₂ the formation of peroxyl radicals was detectable for 2-, 4-ClBzA and ClBz, k(OH-adduct+O₂)=(2?4)×10⁷ dm³ mol^?1 s^?1, while this reaction for 3-ClBzA was too slow to be registered. In the presence of N₂O the degradation rates induced by gamma radiation were very similar for all chlorobenzoic acids, yet the chloride formation was distinctly higher for 3-ClBzA. In the presence of oxygen the initial degradation of 2-and 4-ClBzA equaled the OH-radical concentration, whereas in case of 3-ClBzA only ～60% of OH led to degradation. The order for the efficiency of dehalogenation was 4->2->3-ClBzA. Several primary radiolytic products could be detected by HPLC. To evaluate the toxicity of final products a bacterial bioluminescence test was carried out.