Reaction mechanism of the gas-phase pyrolysis of N – Acetylthiourea and N, N’–diacetylthiourea: A theoretical study based in density functional theory

In the present study, we propose a reaction mechanism for the decomposition of N -Acetylthiourea and N, N '-Diacetylthiourea using density functional quantum chemical calculations. For both reactions the level of theory that gave us the better agreement between theory and experiment was MPW1PW91/6-311G (d, p). Thermodynamic and kinetic parameters were evaluated at 600 K and 1 atm. For N -Acetylthiourea decomposition reaction, the products are ketene and thiourea, while for are N, N '-Diacetylthiourea, the product are ketene and N–Acetylthiourea. Our calculations suggest that the mechanism proceeds stepwise, where the rate-determining step is promoted by an intramolecular nucleophilic attack of the sulfur atom to the carbonyl carbon in the acetyl group. The kinetics of these reactions is influenced by the electron-withdrawing effect of the acetyl group. Population analysis shows that the mechanism is predominantly synchronous as demonstrated by the high values ​​of the Wiberg synchronicity index. A non-covalent interactions study was also carried out in each stage of the reaction, as well as the analysis of the binding forces of the structures was followed by the independent gradient model (IGM) and its descriptor δg.