Ab initio studies on organophosphorus compounds. Part 4. Intramolecular hydrogen bonding and water interactions of bisphosphonates

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Abstract
Molecular properties of two clodronate-type bisphosphonates have been studied in the gas-phase by ab initio molecular orbital methods. Both compounds have P–C–P backbone structure and divalent localized negative charge on oxygen atoms. Molecular structures, charge distributions and energetics of the molecules were calculated with the 3-21G(*) basis set. Internal hydrogen bonds were found between the monophosphonic acid ends of the bisphosphonate molecules. A gas-phase ab initio study has also been made of complexes of the bisphosphonate molecule with one to six explicit water molecules. Interactions between the bisphosphonate and water molecules took place through intermolecular hydrogen bonding. Finally, a self-consistent reaction field (SCRF) model was applied to the interaction models to complement the explicit study. The results of the study provide a quantum chemical reference for future experimental and theoretical studies of bisphosphonate compounds.