DOES DISSOCIATION OF AMINE WATER COMPLEXES DEPEND UPON AMINE BASICITY - PROTON-EXCHANGE BETWEEN QUINUCLIDINIUM IONS AND SOLVENT

The rate of proton exchange between 3-X-quinuclidinium ions (X = H, OH, Cl, and CN) and solvent water was studied by dynamic H-1 NMR techniques. The rate constants show a pH dependence expected for the Swain-Grunwald mechanism, with a pH-independent region above pH almost-equal-to 1 and the characteristic inhibition at lower pH values. The rate constants for breaking the R3N.HOH hydrogen bond, k(H), range from 1.9 x 10(9) s-1 for X = H to 1.2 x 10(10) s-1 for X = CN, and show a dependence on the base strength of R3N with a Bronsted plot slope of beta-diss = -0.25 +/- 0.04. A molecular mechanics computation on complexes of representative model compounds and solvent molecules of different sizes indicates that the London dispersion contribution to the interaction enthalpy only accounts for a minor fraction of the dissociation barrier, at variance with earlier findings of Grunwald and co-workers. It is suggested that the rate of dissociation of amine-water or amine-alcohol complexes is primarily determined by the Gibbs free energy contributions from the pK(a)-dependent hydrogen-bond breaking, dispersion forces between acceptor and donor, and the energy required to create a cavity prior to dissociation.