Journal of Photochemistry最新文献

The photochemical reduction of uranyl ions by triphenylphosphine, tri-p-tolylphosphine, triphenylarsine and tri-p-tolylarsine in a dry acetone medium in the presence of trimethylacetic acid, using radiation in the visible region (λ > 365 nm), was investigated. The electron spin resonance and electronic spectral data supported the proposal that uranium(V) was formed. The quantum yields and Stern—Volmer quenching constants were also calculated for all four types of substrate. Keeping in view the available evidence, an electron transfer mechanism was postulated for the formation of uranium(V) which disproportionates into uranium(IV) and uranium(VI) in aqueous acidic media.

Relative values of the gas phase absorption cross-sections σ for hydroperoxy (HO₂) and methylperoxy (CH₃ O₂) radicals were determined at 298 K over the wavelength λ ranges 210 – 250 nm and 210 – 280 nm respectively. These determinations were based on measurements of the initial optical density (OD)⁰ of each radical vs. wavelength immediately following its flash photolytic generation. Chlorine atoms produced by the photolysis (λ ⩾ 300 nm) of Cl₂ were stoichiometrically converted into the desired peroxy radical via the reaction sequences

The vacuum UV photolysis of ethyl bromide at 121.6 nm (235.2 kcal mol⁻¹) has been studied over the pressure range 0.5 – 100 Torr at room temperature using a hydrogen atom resonance lamp. The scavenger effect of the reaction was observed by adding NO gas as a radical scavenger. The pressure effect was also investigated with CF₄ as an additive. The major products of the reaction were CH₄, C₂H₄, C₂H₆, C₂H₂ and CH₃CHBr₂ via a two-channel competition between molecular elimination and the formation of radicals. The decomposition modes in the primary process were 58.7% molecular elimination and 41.3% radical reaction. It was estimated that the portion of hot C₂H₄ molecules produced through the molecular elimination process with more than the threshold energy (E₀ = 80 kcal (mol⁻¹) for decomposition to C₂H₂ was 25% of the total energy distribution.

The photochemical cis—trans isomerization of 1-(9-anthryl)-2-phenylethylenes (9-styrylanthracenes) has been studied by performing steady state and time-resolved fluorescence measurements. It has been found that singlet excited trans isomers are formed from their photoexcited cis precursors and this is explained in terms of an adiabatic photoreaction. Polar solvents and/or polar substituents on the styrylanthracene markedly enhance the adiabatic isomerization.

Transient triplet—triplet absorption spectra and decay curves of 2-styrylanthracene and four related compounds were determined with conventional microsecond flash spectroscopy over a wide range of temperatures between room temperature and −170 °C, in decalin and methyltetrahydrofuran as solvents. The main anomalies observed were as follows: pronounced variation in the triplet absorption spectra with the temperature and, to some extent, with the wavelength of the exciting light; “growing-in”; initial fast decay stages; non-monoexponential decay; variation in the decay rate with the wavelength of measurement. All these were observed at certain temperatures and wavelengths for each compound. Some of these anomalies may be due to the fact that 2-styrylanthracene and its derivatives exist in solution as an equilibrium mixture of two modifications, possibly rotational conformers, both in the ground and in the excited state. Some trans—cis photoisomerization, both direct and sensitized, takes place in all compounds, except possibly in 2-styrylanthracene, and is more pronounced in bromobenzene solutions.

Irradiation of methyl 2-chloro-3-oxobutanoate (I) in 50% ethanol yielded methyl 3-oxobutanoate, 2-methyl-2-pentanol-4-one, 2,5-dioxohexane-3,4-dicarboxylic acid dimethyl ester, 4-hydroxy-4,5-dimethyltetra-hydrofuran-2-one, 2,4-dimethylfuran-3,5-dicarboxylic acid dimethyl ester, 2,4-dimethyl-3-acetyl-2,3-dihydrofuran-3,5-dicarboxylic acid dimethyl ester and 1,4-dimethyl-6-acetyl-3,5,6-trimethoxycarbonyl-2-oxabicyclo[3.1.0]hex-3-ene.

The photolysis of I involved homolytic cleavage of the CCl bond, α-cleavage and α-elimination of hydrogen chloride, the last step resulting in the intermediate formation of carbomethoxyacylcarbene.

A possible route for the formation of the reaction products is discussed.

The photochemistry of cyclopentene and cyclohexene has been restudied in a region close to their absorption threshold and in the presence of oxygen. In the 203 – 214 nm region, the irradiation of cyclopentene results in the formation of methylenecyclobutane and bicyclo[2.1.0]pentane provided a high pressure of propane is added (P > 50 Torr). The products are similar to those reported at 184.9 nm; isomers and 1,4-pentadiene as well as vinylcyclopropane are the major ones. The lifetime of the excited methylenecyclobutane molecules is close to 0.16 ns. As far as the 1,3-pentadiene formation is concerned, that of the cis isomer is favoured in the high pressure region in agreement with the structure of the starting alkene.

The cyclohexene photoisomerization observed between 184.9 and 213.8 nm is similar to that of cyclopentene. However, owing to the high quantum values of the retro-Diels-Alder process, the observed isomers have low quantum values.