Vibrational analysis of a model for the hydroxide ion in aqueous solution

Abstract

A vibrational analysis of a model for the solvated hydroxide ion in which OH^– is hydrogen-bonded to three H₂O molecules has been carried out. Force constants were assigned to give calculated frequencies consistent with: (a) i.r. and Raman measurements of alkali metal hydroxide solutions and solid hydrates; (b) the magnitude and temperature dependence of the equilibrium constant K_G for the isotope exchange reaction, 2DO^–(D₂O)₃+ 7H₂O(g)⇌ 2HO^–(H₂O)₃+ 7D₂O(g); and (c) alternative combinations of fractionation factors, ϕ_a for the hydroxide ion and ϕ_b for its three hydrogen bonded water hydrogens, consistent with the experimental value of K_G. Best agreement of calculated with observed hydroxide stretching frequencies (3 600 cm^–1) was achieved with ϕ_a 1.15 and ϕ_b 0.57, implying that hydroxide solvent isotope effects are dominated by contributions from the solvation shell; however, the calculated librational frequency was high. A lower librational frequency was obtained with ϕ_a 0.57 and ϕ_b 0.91, but then the stretching frequency was low (ca. 3 000 cm^–1). The results are discussed in the light of current interpretations of hydroxide spectra in condensed phases.