VIBRATIONAL SPECTRA OF $H_{3}O^{+}$ IONS IN AQUEOUS ACIDS

Abstract

The infrared and Raman spectra of aqueous solutions of the four hydrohalic acids have been reexamined with improved techniques and over wider concentration ranges than heretofore. The results confirm definitely the three characteristic vibrations of pyramidal $H_{3}O^{+}$ ions at about $2800 cm^{-1}$ (O-H stretching), $1750 cm^{-1}$ (OH bending), and $1200 cm^{-1}$ (breathing). In hydrofluoric acid the OH bending frequency is shifted up to $1830 cm^{-1}$ due to formation of strongly hydrogen-bonded ion pairs, $H_{3}O+...F^{-}$. This is consistent with the surprising weakness of that acid. The contention of Ackermann that the above features are also present in solutions of strong bases is unfounded. Very concentrated (under pressure) solutions of HCl and HBr show the fundamental frequency of undissociated, solvent-complexed molecules of hydrogen halides, shifted some $200 cm^{-1}$ lower than in the gas.