STABILITY AND STRUCTURE OF FORMAMIDE AND UREA DIMERS IN AQUEOUS SOLUTION A THEORETICAL STUDY

The microscopic characteristics of concentrated aqueous solutions of formamide and urea have been investigated through energy minimization of clusters consisting of one or two solute molecules surrounded by up to 19 water molecules. The computations performed for single solute molecules correctly reproduce the pattern of solvent molecules in the first hydration shell found by molecular-dynamics simulations and lead to reasonable solvation enthalpies. The computations performed for two solute molecules indicate that direct CO---NH bridges are not to be expected in aqueous solution. Solute molecules are, instead, linked by water chains, leading to compact structures which are also compatible with the local tetrahedral environment of the solvent molecules.