Reaction pathway and free energy barrier for urea elimination in aqueous solution

Abstract

To accurately predict the free energy barrier for urea elimination in aqueous solution, we examined the reaction coordinates for the direct and water-assisted elimination pathways, and evaluated the corresponding free energy barriers by using the surface and volume polarization for electrostatics (SVPE) model-based first-principles electronic-structure calculations. Based on the computational results, the water-assisted elimination pathway is dominant for urea elimination in aqueous solution, and the corresponding free energy barrier is 25.3 kcal/mol. The free energy barrier of 25.3 kcal/mol predicted for the dominant reaction pathway of urea elimination in aqueous solution is in good agreement with available experimental kinetic data.

Predicting the Mechanism and Products of CO2 Capture by Amines in the Presence of H2O

“…The FPD/CCSD­(T) values tend to give slightly lower barrier heights and more exothermic (less endothermic) reaction energies. Lower level predictions − ,,− are in general agreement with our values, and a complete comparison table to other work is given in the SI. We describe the NH 3 reactions in more detail as they are easier to visualize before examining the role of substituent effects.…”

On the influence of water on urea condensation reactions: a theoretical study

“…These workers found a significant decrease of the energy barrier when one or several water molecules were involved in the H-atom transfer. Several other quantum chemical studies on urea decomposition arrive at similar conclusions [29][30][31][32][33][34].…”

YAG:Ce3+ Phosphor: From Micron-Sized Workhorse for General Lighting to a Bright Future on the Nanoscale

“…However, according to Xu et al and Sordelet et al urea decomposes as follows: While Lv et al describe a hydrolysis reaction of urea in both acidic (, ) and basic environment (, ): and conclude from the initial drop in the reaction pH that the acidic hydrolysis reaction is taking place. However, studies focusing specifically on urea degradation in water conclude that hydrolysis of urea only takes place when specific enzymes are present, and a water assisted NH 3 elimination reaction has a lower activation barrier (): , For the decomposition of NH 4 HCO 3 , too, different reaction equilibria are reported, as shown above in eqs – and below (– and ): The section above makes clear that although the main reaction products of the decomposition reactions of urea and NH 4 HCO 3 are similar, the exact concentrations might differ from one reaction to another, depending on the concentration of reactants added, pH, reaction temperature and duration, and other chemical present (e.g., surfactants). However, since some of the ions are basic (carbonate, ammonium) and others acidic (carbon dioxide) understanding the decomposition reactions is inevitable in understanding the reaction mechanisms of the precipitation and thus for the design of reaction conditions.…”

“…and conclude from the initial drop in the reaction pH that the acidic hydrolysis reaction is taking place. However, studies focusing specifically on urea degradation in water conclude that hydrolysis of urea only takes place when specific enzymes are present, and a water assisted NH 3 elimination reaction has a lower activation barrier (12): 72,73 NH CO HNCO NH ( )…”