Specific solvation effects on the formation of urea anion in alkaline solution.

The reaction between urea and HO^- in alkaline solution was investigated at B3LYP and MP2 levels. The PCM solvent model was used to include the implicit solvent effects. To estimate the specific solvation effects, explicit water molecules were added stepwise to form molecular clusters. The results show that the OH^- anion can abstract a proton from urea, forming the urea anion. The calculated energies indicate the hydrogen bonds formed between the reactants and solvent water molecules affect both the potential and thermodynamic energy barriers. The changing trend of the barriers appears to be 1W > 2W > 3W > 0W. A fast equilibrium can be established between the reactants and the products but the reverse reaction is both kinetically and thermodynamically more favourable, suggesting that the concentration of urea anion in alkaline solution would be low, in agreement with known weak acidity. The results of this study suggest that for reaction systems involving strongly polar reactants and solvent, the employment of only implicit solvent model is not enough, and explicit solvent molecules must be included. [ABSTRACT FROM AUTHOR]