Anion binding to polar molecules

The interaction of phosphoric acid and chlorine-substituted phosphoric acid with hydroxyl and chlorine anions yields a number of hydrogen-bonded and five-coordinate adducts. Stable hydrogen-bonded structures are found both for adducts with OH^- and Cl^- and for proton-transferred structures involving H₂O in the complexes H₄PO₅^-, H₃PO₄Cl^-, and H₂PO₃Cl₂^-. The hydrogen bond energy of the OH^- and Cl^- adducts is calculated to be large due to the polar character of the P-O and O-H bonds. Ligand-field analysis of the hydrogen bonding determines the electrostatic interaction as dominant for these adducts. Five-coordinate phosphorus structures corresponding to reaction intermediates are also found at a higher energy than the hydrogen-bonded structures. These adducts have very polar bonds. Axial Cl ligands are completely dissociative in all studied structures. Stability and binding energy trends upon chlorine substitution are related to the local polarity of the bonds.