The potassium fluoride–boric acid system. Hydrogen bonding in KF·H3BO3

Abstract

Ab initio LCAO–MO–SCF calculations have been performed on the most likely interactions between F– and H₃BO₃. These show that the monofluoroborate ion [BF(OH)₃]- is the most stable, but only by 33 kJ mol^–1 relative to the hydrogen-bonded system F^–⋯ HOB(OH)₂. Infrared analysis of the solid phase of composition KF.H₃BO₃, which grows from an aqueous solution of KF and H₃BO₃, shows the hydrogen-bonded structure to be the preferred form. The results of ¹⁹F and ¹¹B n.m.r. studies on aqueous solutions of KF and H₃BO₃ are also consistent with hydrogen bonding being the chief interaction although it is possible that an equilibrium exists involving [BF(OH)₃]^– since [BF₃(OH)]^– is detectable in such solutions.