H+ and OH− ions in aqueous solutions vibrational spectra of hydrates

The ions (H₂O ... H ... OH₂)⁺ and (HO ... H ... OH)^? are the simplest stable H⁺ and OH^? hydrates in aqueous acid and base solutions, respectively. Using the attenuated total reflection method, the IR spectra of aqueous HCl and KOH solutions are obtained and the assignment of the H₅O₂⁺ and H₃O₂^? vibrational frequencies is performed. The absorption spectrum of the OHO fragment is separated from the spectra of the solutions investigated. This spectrum exhibits a broad continuous band and two rather narrow bands at its background which are assigned to the antisymmetrical stretching vibration and to the bending vibrations of the fragment. A theoretical model is suggested which explains the origin of the continuum by a strong proton-phonon coupling. The model takes into account the large number of low-frequency vibrational modes of the system; the frequency dispersion for these modes is assumed to be sufficiently large. The continuous absorption bandshape is calculated in the Condon approximation. The theoretical absorption curve is in good agreement with experiment at reasonable values of the parameters involved.