Theoretical and Experimental Investigation of Crown/Ammonium Complexes in Solution

The Gibbs energies of association Delta G(sol)(T) between primary alkyl ammonium ions and crown ethers in solution are measured and calculated. Measurements are performed by isothermal titration calorimetry and revealed a strong solvent-dependent ion pair effect. Calculations are performed with density functional theory including Grimme's dispersion correction D3(BJ). The translational, rotational, and vibrational contributions to the Gibbs energy of association Delta G(sol)(T) sol are taken into account by a rigid-rotor-harmonic-oscillator approximation with a free-rotor approximation for low lying vibrational modes. Solvation effects delta G(se)(T) are taken into account by applying the continuum solvation model COSMO-RS. Our study aims at finding a suitable theoretical method for the evaluation of the host guest interaction in crown/ammonium complexes as well as the observed ion pair effects. A good agreement of theory and experiment is only achieved, when solvation and the effects of the counterions are explicitly taken into account. (C) 2015 Wiley Periodicals, Inc.