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.

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.

Modeling chemical incompatibility : the ammonium nitrate and sodium salt of dichloroisocyanuric acid as a case study. AbstractThe dramatic accident involving ammonium nitrate (AN) that took place at Toulouse in September 2001 has once again focused attention on the hazards pertaining to chemical incompatibility in industrial environment. To complete the experimental results, a detailed theoretical study was performed in order to better understand the involved mechanisms, considering the reaction between ammonium nitrate and the sodium salt of dichloroisocyanuric acid (SDIC). Starting from theoretical results obtained for the pure reactants, the gas-phase decomposition mechanism of the mixture was investigated and fully characterized by means of Density Functional Theory (DFT) calculations. Beyond the complete characterization, in terms of intermediate structures and energies, of the decomposition pathways, our results evidenced as anticipated the role of water in catalyzing the decomposition reaction, through a significant decrease of the activation energy of the rate determining step. These results, in qualitative agreement with the calorimetric experiments, pointed out the instability of the AN-SDIC wet mixture and the underpinning incompatibility mechanism between these two chemicals.

Hazards posed by chemical incompatibility, especially in a large-scale industrial environment, warrant a deeper understanding of the mechanisms of the reactions involved in these phenomena. In this study, reactions between ammonium nitrate and two sodium salts, namely, sodium nitrate and sodium nitrite, have been studied by ab initio calculations (with density functional theory, DFT) and experimental calorimetric methods (with differential scanning calorimetry, DSC, and heat flux calorimetry, HFC). The agreement between theoretical and experimental results allows an understanding of the thermal decomposition behaviors of the two sodium salts when exposed to ammonium nitrate. Moreover, this study highlighted the critical role of the water that appears to promote the incompatibility between ammonium nitrate and sodium nitrite.

This study aims at validating a multi-scale modeling methodology based on an implicit 12 solvent model for urea thermal decomposition pathways in aqueous solutions. The influence 13 of the number of cooperative water molecules on kinetics was highlighted. The obtained 14 kinetic model is able to accurately reproduce urea decomposition in aqueous phase under a 15 variety of experimental conditions from different research groups. The model also highlights 16 the competition between HNCO desorption to gas phase and hydrolysis in aqueous phase, 17 which may influence SCR depollution process operation.18 19 20 * (ammonia elimination, hydrolysis and tautomerization), Alexandrova and Jorgensen [6] found 32 the first path to have the lowest activation energy, partly resulting from the resonance 33 stabilization in the first transition state. However, their study mainly focused on the solvent 34 effects on the potential energy surface (PES), but not on the corresponding kinetic rate 35 constants. These authors did not investigate the subsequent hydrolysis of isocyanic acid 36 leading to an additional ammonia production. In the present study, we demonstrated the 37 feasibility of a multiscale first-principle based kinetic modeling of urea decomposition in 38 aqueous solution. We performed a high-level electronic structure study on the main ammonia 39 production paths from urea decomposition including an implicit solvent model. Based on 40 these new results, we herein derived the corresponding phenomenological rate constants and 41 thermokinetic data to build a macrokinetic mechanism, which was subsequently validated 42 against experimental data, allowing rate-of-production studies of urea decomposition under 43 realistic operating conditions. acid, which can in turn decompose through either intramolecular or assisted mechanism. In 105 the present study, we focused on the addition of water across the C=N bond of HNCO, as it is 106 energetically more favorable [24] than addition across the C=O bond due to the extended 107 concentration of the electron density on nitrogen [25]. Although a competitive bicarbonate 108 mechanism could also be considered in this study, it is not expected to change HNCO 109 hydrolysis kinetics since the transition state (TS) structures involved are very similar [26]. As 110 could be anticipated [27], the six-membered-ring TS involving two water molecules results in 111 a lower barrier (19.7 kcal/mol) compared to a four-membered-ring TS (38.9 kcal/mol).

The design of innovative combustion processes relies on a comprehensive understanding of biodiesel oxidation kinetics. The present study aims at unraveling the reaction mechanism involved in the epoxidation of a realistic biodiesel surrogate, methyl trans-3-hexenoate, by hydroperoxy radicals using a bottom-up theoretical kinetics methodology. The obtained rate constants are in good agreement with experimental data for alkene epoxidation by HO. The impact of temperature and pressure on epoxidation pathways involving H-bonded and non-H-bonded conformers was assessed. The obtained rate constant was finally implemented into a state-of-the-art detailed combustion mechanism, resulting in fairly good agreement with engine experiments.