engleski

Dimerization mechanism of aromatic nitroso compounds in solid state

Kinetics of the solid-state reactions can be described by using the classical Avrami’s- Erofeev approach.[1] Avrami’s model represents only empirical description of the reaction kinetics, thus it’s inconvenient for the deeper mechanistic interpretations. Recently, Finney and Finke have proposed a new four-step autocatalytic model (FF model) and applied it for explanation of particular solid-state reactions mechanisms.[2] However, we found that neither Avrami nor Finke models are adequate for analyzing the kinetic data for dimerization of aromatic nitroso compounds in the solid state, the reaction that we use as a model for studying solid-state reaction mechanisms. The starting nitroso monomers are obtained by photolysis of the corresponding dimers (azodioxides) under cryogenic conditions. Re- dimerization of these compounds occurs upon heating and affords data that can be interpreted as a combination of sigmoidal and exponential behavior.[3] The former is characteristic for solid-state transformations and can be described by Avrami’s, A→B→C or by FF models. The latter represents chemical reactions accompanied by (bi-)exponential growth curves. Dimerizations of the same compounds in different molecular arrangement, i.e. in crystals of monomers obtained by mild sublimation, is for several orders of magnitude slower. This observation can be explained by different distances between nitrogen atoms of the starting neighbouring nitroso molecules that appear in sublimed or photolysed crystals.[4] Bis(4-nitrosophenyl) derivatives with various spacers between the phenyl rings follow different kinetics. Dimerization of the methane derivative at 200 K occurs in parallel reactions yielding cis¬ and trans-isomers. At higher temperature (270 K) trans-isomers transform to cis isomers via monomer intermediates. On the other hand, derivative with the -CH2CH2- spacer follows standard exponential-like dimerization to trans- isomers.

Kinetics, solid state, nitroso, aromatic, photolysis, dimers, models

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