Hydrogen bond acceptor capability of carbonyl π-electrons—case study of the hydrogen-bonded urea dimer

Abstract

Due to hydrogen bonding interactions, the urea–urea dimer has two relatively stable rotamers, having dihedral angles of either 0° (coplanar) or 90° (perpendicular). These two rotamers may be respectively explained by contributions to the bridging hydrogen bonds by either electrons from the nonbonding orbital or from the π orbital of oxygen. We find that the π-donor substituents can enhance effectively the hydrogen bond acceptor capability for the carbonyl π-electrons. The urea dimer has nearly equal bonding energies for the two rotamers with θ = 0° and θ = 90°. For the dimer series R₂CO·urea (R = H, F, Cl, NH₂, NHCH), the energy difference between the two conformers can be accounted for by the HOMO–LUMO type non-Coulombic interaction.