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Behaviour of 2-nitrosopyridines in solution and in solid state

It is already well known that nitrosobenzenes in solution at room temperature appear exclusively as monomers, but 2-nitrosopyridines prefer to exist in a form of Z-dimers. In addition, whereas nitrosobenzenes are mostly present as E-dimers in solid state, 2- nitrosopyridines are present like Z-stereomers as well. In the present work we were investigating monomer-dimer equilibrium of 2- nitrosopyridine (1) and its 4-methyl- (2) and 4-chloro- (3) derivatives both in solution and in solid state.The equilibrium constants and thermodynamic parameters of three 2- nitrosopyridine derivatives were determined by UV-Vis spectroscopy. By using variable temperature 1H NMR spectroscopy it was found that compound 1 is able to form the heterodimers with 4-nitronitrosobenzene. Solid- state photo-thermal studies of 2- nitrosopyridine derivatives were studied by IR spectroscopy at cryogenic conditions indicating that both, the Z- and E-dimers can be photolysed to corresponding monomers which re- dimerize by warming to room temperature. For the first time, monomers of 2-nitrosopyridine derivatives were detected in solid state enabling analysis of their IR spectra. Furthermore, the kinetics of the re- dimerization of monomers of 2-nitrosopyridines prepared by photolysis of corresponding dimers at 13 K and by cryogenic deposition were investihated as well. Estimated activation parameters were enabled from obtained reaction rates at different temperatures. The results of this study demonstrate that different topochemical conditions affect the kinetics of solid-state dimerization of 2-nitrosopyridine derivatives.

2-nitrosopyridine derivatives ; Monomer-dimer equilibrium ; Solid-state photo-thermal study ; Thermodynamic study ; Topochemical principle

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