The 1:1 gallic acid– <i>N</i> -methylurea cocrystal: QTAIM and NBO studies of the O—H...O and N—H...O hydrogen-bonded supramolecular carboxylic acid–urea heterosynthon

The structure of the 1:1 gallic acid (3,4,5-trihydroxybenzoic acid)-N-methylurea cocrystal, C₇H₆O₅·C₂H₆N₂O, (I), has been determined by single-crystal X-ray diffraction. In the crystal, molecules of both components form O-H...O and N-H...O hydrogen-bonded complexes. These complexes are further linked by O-H...O, N-H...O and C-H...O hydrogen bonds, together with aromatic π-π stacking interactions, to generate a triperiodic supramolecular network. Energy framework analysis shows that electrostatic contributions predominate over dispersive ones in (I). Quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analyses indicate that the O-H...O and N-H...O interactions are strong, with homonuclear hydrogen bonds being stronger than heteronuclear ones. Stabilization of the carboxylic acid-urea complexes arises primarily from charge transfer resulting from orbital interactions between the lone electron pairs of hydrogen-bond acceptors (O atoms) and the empty antibonding orbitals of the hydrogen-bond donor [LP(O) → BD*(D-H)].