DFT investigations on the structure and properties of MBP dimers and crystal with strong hydrogen-bonding interactions

Theoretical studies on the monomers, dimers, and crystal of the prototypical bisphosphonic acid, methylenebisphosphonic acid (MBP), were performed at different density functional theory levels. The hydrogen bonding, interaction energy, thermodynamic property, lattice energy, and electronic structure were investigated. Five stable dimers were identified through the intermolecular hydrogen-bonding interaction, and the stability order was estimated by the interaction energies. For the most stable dimer, the interaction energy is -170.86 kJ/mol at the M06-2X/6-311++G** level, while that for the least stable dimer is -46.74 kJ/mol. At 298.15 K, the changes of Gibbs free energies (a dagger G) for the dimerization processes of five dimers are all negative (-122.72, -85.09, -8.46, -114.02, and -100.70 kJ/mol), suggesting these dimers can be spontaneously produced from the isolated monomer at room temperature. The stability order of dimers derived from the a dagger G (T) values agrees well with that determined by the interaction energies. The lattice energy for the crystalline MBP was predicted to be -828.90 and -899.79 kJ/mol by GGA/PBE and GGA/PW91, respectively, whereas it was overestimated by LDA/CA-PZ (-1319.72 kJ/mol). The band structure calculations indicate that MBP is a wide-gap insulator with a band gap of more than 6.0 eV. The charge distribution and bonding overlap populations show that the bond strength of O-H is less than other bonds due to taking part in the formation of intermolecular hydrogen bonds.