Theoretical analysis of the polar morphology and absolute polarity of crystalline urea

Abstract

Changes to the molecular polarisation of urea associated with its crystallisation are considered through ab initio quantum-mechanical calculations starting with the isolated molecule with increasing complexity up to a full 3D calculation using periodic boundary conditions. The calculations accurately reveal the electrostatic nature of the intermolecular forces associated with the hydrogen-bonding network in the solid state. The resulting charge densities are used together with the force field of Lifson, Hagler and Dauber (J. Am. Chem. Soc., 1979, 101, 5111) in an attachment energy calculation to predict the polar morphology of the material. The resultant simulations are in good agreement with the morphology of crystals prepared from the vapour phase and are used to absolutely assign the polar forms as {1 1 1}. Habit modification effects associated with re-crystallisation from polar solvents are also discussed from a structural perspective.