A computational analysis of some diaryl ureas in relation to their observed crystalline hydrogen bonding patterns

We have carried out anab initio self-consistent-field molecular orbital analysis of the structures, atomic charges and electrostatic potentials of 1,3-bisphenylurea, 1,3-bis(m-nitrophenyl)urea and 1,3-bis(p-nitrophenyl)urea. Our results provide insight into the contrasting hydrogen bond patterns and preferences of these molecules in crystalline environments. The tendency for 1,3-bisphenylurea to form homomeric rather than heteromeric (i.e. host-guest) crystals is attributed to a relatively strong and extended homomeric attractive electrostatic interaction between two like molecules. Our calculated internuclear distances and atomic charges indicate that these molecules have some degree of H...O intramolecular hydrogen bonding between the carbonyl oxygen and the nearest aromatic hydrogens when the aromatic and the urea portions of the molecules are coplanar. This interaction is strongest for the meta nitro derivative, consistent with the latter remaining very nearly planar in its cocrystal structures. Our surface electrostatic potentials for the three diaryl ureas are used to interpret their differing crystallization properties and tendencies to form cocrystals with guest molecules.