THEORETICAL-ANALYSIS OF THE POLAR MORPHOLOGY AND ABSOLUTE POLARITY OFCRYSTALLINE UREA

Changes to the molecular polarisation of urea associated with its crys tallisation are considered through ab initio quantum-mechanical calcul ations starting with the isolated molecule with increasing complexity up to a full 3D calculation using periodic boundary conditions. The ca lculations accurately reveal the electrostatic nature of the intermole cular forces associated with the hydrogen-bonding network in the solid state. The resulting charge densities are used together with the forc e field of Lifson, Hagler and Dauber (J. Am. Chem. Soc., 1979, 101, 51 11) in an attachment energy calculation to predict the polar morpholog y of the material. The resultant simulations are in good agreement wit h the morphology of crystals prepared from the vapour phase and are us ed to absolutely assign the polar forms as {1 1 1}. Habit modification effects associated with re-crystallisation from polar solvents are al so discussed from a structural perspective.