Co-crystal/salt crystal structure disorder of trichloroacetic acid-N-methylurea complex with double system of homo- and heteronuclear O-H•••O/N-H•••O hydrogen bonds: X-ray investigation, ab initio and DFT studies

The X-ray diffraction studies revealed disorder of a trichloroacetic acid-N-methylurea complex crystal structure, connected with a proton transfer via O-H center dot center dot center dot O hydrogen bond. The observed structure corresponds to a co-existence of ionic (salt) and neutral (co-crystal) forms of the complex in the solid state in ratio 3:1, respectively. The geometrical analysis based on ab initio and density functional theory methods combined with the experimental research indicated that two different N-methylurea molecular conformations, defined by CNCN torsion angle, correspond to the neutral and the ionic form of the complex, respectively. The conformational changes seem to be connected with stabilization of the ionic structure after a proton transfer, as according to theoretical calculations this form of the complex (the ionic one) was unstable in the gas phase. A particular attention was focused on a system of a double intermolecular hydrogen bonds, O-H center dot center dot center dot O and N-H center dot center dot center dot O which join molecules into the title complex. The analysis of these interactions performed in terms of their geometry, energetic and topological electron density properties let for their classification into strong and medium strength hydrogen bonds. It was also found that the antibonding hydrogen bonding donor orbital occupation corresponded to the stabilization energy resulting from charge transfer in hydrogen bonds. Hence, it is postulated as a possible indicator of interaction strength.