RESUMO
The decomposition mechanism of ammonium nitrate in the gas phase was investigated and fully characterized by means of CBS-QB3 calculations. Five reaction channels were identified, leading to the formation of products (N2, H2O, O2, OH, HNO, NO3) found in the experimental works. The identified mechanism well underlines the origin of the chemical hazard of ammonium nitrate which is related to the exothermicity of the lowest decomposition channels. Furthermore, the high barrier to overcome in the rate determining step well explained the fact that the reaction is not usually spontaneous and requires a significant external stimulus for its onset. An accurate DFT benchmark study was then conducted to determine the most suitable exchange-correlation functional to accurately describe the reaction profile both in terms of structures and thermochemistry. This evaluation supports the use of the M06-2X functional as the best option for the study of ammonium nitrate decomposition and related reactions. Indeed, this level of theory provided the lowest deviations with respect to CBS-QB3 reference values, outperforming functionals especially developed for reaction kinetics.
RESUMO
The [1,2] and [2,3] migration steps in the Stevens and Sommelet-Hauser rearrangements which occur in the ylides of quaternary ammonium salts have been studied at M05-2x levels. The Stevens migration has been found to take place through a diradical pathway in several cases (tetramethylammonium, benzyltrimethylammonium, benzylphenacyldimethylammonium ylides). By contrast, in the phenyltrimethylammonium ylide this reaction takes place through a concerted process. The Sommelet-Hauser rearrangement takes place through a concerted transition structure. The most important factor determining the extent of competition with the Stevens rearrangement is the difference in the reaction energies as the formation of the Sommelet-Hauser intermediate is significantly less endoergic.
