Does Dissociation of Amine-Water Complexes Depend upon Amine Basicity? Proton Exchange between Quinuclidinium Ions and Solvent

Article abstract of DOI:10.1021/ja00018a043

The rate of proton exchange between 3-X-quinuclidinium ions (X = H, OH, Cl, and CN) and solvent water was studied by dynamic 1H NMR techniques.The rate constants show a pH dependence expected for the Swain-Grunwald mechanism, with a pH-independent region above pH ca. 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 * 10⁹ s^-1 for X = H to 1.2 * 10¹⁰ s^-1 for X = CN, and show a dependence on the base strength of R3N with a Broensted plot slope of β_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.