The results of the theoretical study of the mechanism of thermal decomposition of α-nitroolefines are discussed in the work. It is noted, that for some of the derivatives of nitroethylene the process connected with isomerization and the further destruction on the biradical mechanism is the basic channel of thermal decomposition. For the compounds in which molecules there are aqueous sub-stituents, the minimal activation enthalpy is observed for process of 1,5-sigmatrop shift of hydrogen to the NO_2 group. It is also established for the first time in the work, that the barriers of bimolecular decomposition of nitroolefines reactions are energetically more favorable in comparison with reactions of monomolecular decomposition. Taking these data into consideration the opportunity of realization of biradical stages in multi-stage mechanisms of thermal decomposition of olefins is considered. Besides the significance of this type of processes for the thermal decomposition of aliphatic C-nitrocompounds in general is shown. So, for example, the new channel of nitro-nitrite rearrangement in nitromethane through the singlet biradical transitive state has been found out. According to calculation (B3LYP/6-31G(d)) the length of C-N connection is strongly extended (up to 300 pm). The barrier of this process (256.8 kJ/mol) is essentially less than the barrier of the classical nitro-nitrite rearrangement (278.3 kJ/mol), and activation entropy (23.4 J/mol*K) exceeds activation entropy of radical decomposition. Therefore with the temperature increasing the pointed out mechanism can compete with the process of hemolytic break of C-N connection (dissociation energy of C-N connection according to B3LYP/6-31G(d) data is 233.6 kJ/mol). The conclusion about an opportunity of participation of nitro-nitrite rearrangement in the competition of mechanisms gas-phase decomposition of nitromethane proves to be true also experimentally with infrared multiphoton dissociation.
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