Pyramidal inversion in the solid state

Abstract

Pyramidal inversion is a stereochemical phenomenon that describes the interconversion between two equivalent pyramidal configurations of the same chemical species. Using the IO₃ molecule as a prototypical trigonal pyramidal unit, pyramidal inversion has been observed in the solid state by applying hydrostatic pressure to crystals of barium di-iodate monohydrate, Ba(IO₃)₂·H₂O, without chemical reaction or the introduction/elimination of guest molecules. The pyramidal inversion was identified by high-pressure single-crystal synchrotron X-ray diffraction from the appearance of charge density on the unoccupied side of the IO₃ pyramid at pressures above 5 GPa. The percentage of inverted pyramids increases with pressure, from 2.5% at 5.10(5) GPa to 17.5% at 14.84(5) GPa. The energetic competition between the original and inverted IO₃ pyramids as a function of pressure is investigated by density functional theory calculations, finding the two configurations to be very close in energy. Factors contributing to the observation of pyramidal inversion in barium iodate monohydrate are discussed and it is suggested that hydrogen bonding due to the presence of water may play a significant role.