Fractionation factors for the aqueous hydroxide ion and solvent isotope effects on the ionisation of 1,8-bis(dimethyamino)naphthalene (proton sponge)

Abstract

Isotopic fractionation factors ϕ_a and ϕ_b for the hydroxy hydrogen and hydrogen-bonded water hydrogens of the solvated hydroxide ion are important in controlling solvent isotope effects upon reactions involving the hydroxide ion, and were required for a vibrational analysis of the solvated ion. The product ϕ_aϕ_b³= 0.434 is accurately available from e.m.f. measurements corrected for free energies of ion transfer between H₂O and D₂O, but separation of ϕ_a and ϕ_b, which requires measurements in H₂O–D₂O mixtures, is known to be subject to considerable uncertainty. Thus careful measurements with 1,8-bis(dimethylamino)naphthalene gave K_D2O/K_H2O= 0.420 ± 0.006 for the ratio of basic ionisation constants in H₂O and D₂O and ϕ_P= 0.901 ± 0.014 for the fractionation factor of the protonated base, but measurements in 1 : 1 H₂O–D₂O gave an ionisation constant too small to be consistent with any reasonable fractionation factor model for the hydroxide ion. Alternative methods of dissecting ϕ_a and ϕ_b, from measurements of isotope separation factors between hydroxide solutions and water vapour and autoprotolysis constants of H₂O–D₂O mixtures, are critically reviewed and optimum values are assessed. The results are shown to be sensitive to experimental error and to medium effects but not to departures from the Rule of the Geometric Mean.