Role of electronic polarization on the liquid phase affinity of calixarene-crown-ethers towards alkali cations: a QM/MM molecular dynamics simulation

We report molecular dynamics simulations of calixarene-crown-ether complexe s with alkali cations in water using hybrid quantum mechanics/molecular mec hanics (QM/MM) potentials. The approach allows us, for the first time in th is kind of systems, to make a detailed discussion on the role of electronic polarization. Such an effect had been omitted in previous studies of calix arene-alkali cation complexes, although it is known to be important in many host-guest systems. The macrocycle, calix[4]arene-bis-crown6 (BC6), is tre ated at the semiempirical AM1 level whereas solvent water molecules are tre ated using the TIP3P model. The alkali metal cations (Na+ and Cs') are desc ribed as classical point charges with a set of Lennard-Jones parameters dev eloped in this work. The interaction energy is analysed in terms of strain, electrostatic, polarization and van der Waals contributions. The polarizat ion component is shown to be substantial and may represent 20% of the total electrostatic energy. We show that instantaneous fluctuation of the net at omic charge on O atoms and aromatic rings are quite large. The structural r esults predicted by the AM1/TIP3P model are shown to agree reasonably well with X-ray data. Comparison with previous MM simulations using effective pa irwise additive potentials is made. Some differences found in the case of t he BC6/Na+ system, namely for the solvation number, are discussed. (C) 2001 Elsevier Science B.V. All rights reserved.