Nature of the Hydrogen Bond in Sodium Acid Fluoride

Abstract

Neutron diffraction data, obtained from single crystals of NaHF2 and NaDF2, have been analyzed in terms of several models for the bifluoride ion geometry. In sodium acid fluoride the bifluoride ion must be linear and the two fluorine atoms equivalent. The F–H–F distance is 2.264±0.003 Å, the F—D—F distance is 2.265±0.007 Å. From the diffraction data alone it is impossible to distinguish between a symmetric model for the bifluoride ion and one in which the potential for the hydrogen (or deuterium) along the bond has two equal minima on either side of the center. However from a comparison between the differences of mean-square amplitudes of vibration of hydrogen (or deuterium) and fluorine as obtained from the analysis of the diffraction data and as calculated from the spectroscopic frequencies it is concluded that the bifluoride ion is indeed symmetric.