Christian Csongár

Photochemistry of arylsydnones part VI. Peculiarities of the photolysis of N-phenylsydnones☆

Abstract The photolysis of sydnones and glycines with appropriate 15 N and/or D labelling was investigated in situ using electron spin resonance (ESR) spectroscopy in fluid solution. In contrast with other sydnones, e . g . 3,4-diphenylsydnone, radical intermediates were observed and identified as PhNCHNO CHNPh. The proposed radical structure was confirmed by analysis of the spectral pattern of the isotopically labelled radicals produced by photolysis of the corresponding starting materials. Mechanistically these radicals are of importance in the photolysis of N -arylsydnones. This is supported by the fact that it has so far been impossible to detect nitrile imines, reflecting the peculiarities of the photolysis of N -arylsydnones. Bearing in mind the experimental conditions for the registration of the ESR spectra, it is not possible to give an exact number of radicals (PhNCHNO CH=NPh) generated in the photochemical process. Therefore we cannot rule out the formation of nitrile imine intermediates.

Bis-2H-tetrazoles VII: Quantum yields of bis-2H-tetrazoles and studies of thermal consecutive reactions of bis-nitrile imines by time-resolved spectroscopy

Abstract The photogeneration mechanism of a series of 21 bis-nitrile imines with different types of linkage, and their subsequent thermal reactions, especially cycloadditions, are investigated by means of stationary and time-resolved methods. The short-living nitrile imines are efficiently photo-generated from the corresponding bis-2H-tetrazoles. Directly linked bis-2H-tetrazoles decompose according to a two-photon mechanism with different quantum yields of both steps. Both the transient bis-nitrile imines and the tetrazolyl-nitrile imines are indirectly detectable as their cycloaddition products tetrazolyl-Δ 2 -pyrazolines and bis-Δ 2 -pyrazolines. Exclusively, the formation of bis-Δ 2 -pyrazolines is detectables with all other bis-2H-tetrazoles. Quantum yields depend on the type of linkage. The decay kinetics of the bis-nitrile imines were investigated with the help of conventional flash photolysis. The rate of the cycloaddition to methylmethacrylate is discussed as a function of the linkage type and of substitution. Activation parameters of these reactions were determined. The Arrhenius parameters determined for the carbonyl-linked bis-nitrile imines under the variation of the solvent may suggest a non-concerted mechanism.

Competition between back electron transfer and bond fission in photoinduced ether cleavage

Abstract The dependence of the quantum yields of the electron-transfer-sensitized photolysis of substituted phenyl(trimethylsilyl)methyl ethers on the substituent is explained by the dependence of the rate of back electron transfer on the exothermicity of the reaction (Marcus-inverted region). In order to confirm the mechanism, the ion yields of the forward electron transfer were determined using photoconductivity measurements with dicyanoanthracene as acceptor. The ion yields correspond to the quantum yields of ether cleavage. For N -methylquinolinium perchlorate as acceptor, the quantum yields of ether cleavage were used to calculate the rate constants of back electron transfer. The rate constants showed a marked decrease with increased Δ G −et values. Semiquantum mechanical theory of electron transfer was used to analyse the data and to compare with results in the literature. The data agree with those in the literature obtained from photoconductivity measurements. Deprotonation of the ether radical cation within the radical pair (formed after electron transfer to the quinolinium salt) plays the role of an energy wasting process with ethers of low donor capacity.