On the nature of fluoride ion hydration

Hydration of aqueous fluoride ions has been studied by theoretical ab initio calculations in an attempt to understand the experimental Raman spectrum. Calculations for hydrated fluoride, F- (H2O), where n = 1-10, have been performed at the RHF/6-31 + G* level. A relatively stable geometry exists for n = 6; above this number, additional waters hydrogen bond to water of the hydrated fluoride. On the long time scale of the ab initio calculation or experimental diffraction studies, the average coordination of fluoride is 6. However, it has been possible to interpret the low-frequency Raman spectrum on the basis of a single hydrogen-bonded water molecule, F- . . . HOH. To rationalize these results, it is proposed that the average coordination of fluoride is 6, but on the time scale of the Raman experiment the fluoride is symmetrically bonded to only one hydrogen of one water molecule.