Microscopic hydration of the fluoride anion

A combined experimental and theoretical investigation of the step-wise hydration of the fluoride ion has been performed in order to characterize the details of its solvation at the microscopic level. The comparable anion–water and water–water interactions pose a challenging experimental/theoretical problem due to competing intermolecular forces in these small ionic clusters. Vibrational spectra of size-selected F−(H2O)3−5 in the O–H stretching region, coupled with high level ab initio calculations, have been used to analyze the spectra and assign the structures of these species. The interaction between the fluoride anion and water plays the dominant role, resulting in internally solvated clusters. The microhydration of fluoride ion is thus qualitatively different from the other halide ions.