A Theoretical investigation of the shape and hydration properties of aqueous urea: Evidence for nonplanar urea geometry

To model the structure of a urea molecule in aqueous solution including adaption of the solute electronic structure, we have used the reference interaction site model - self-consistent-field (RISM-SCF) method, describing the solute electronic structure at the ab initio level and hydration via an integral equation for the solvent distribution. The RISM-SCF model for aqueous urea gives a clearly nonplanar urea structure, with more than seven waters located within a contact distance defined for hydrogen bonding. The pyramidalization at the urea nitrogen sites is reduced in water relative to the gas phase, but the structure is closer to the gas phase structure than to the planar crystal structure. It is further shown that the solvent produces substantial polarization of the urea solute, with the dipole moment increasing from 4 to 7 D as the geometry and electronic structure are optimized to the aqueous environment. The overall result is to favor a high density of hydrogen bonds between urea and the surrounding solvent. The geometry obtained is intermediate between the previously known nonplanar gas phase structure and well-characterized planar crystal structure. An uncommon hemispherical shape for the electrostatic potential of around the carbonyl of urea is found, which affects the solvation structure.