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 complexes with alkali cations in water using hybrid quantum mechanics/molecular mechanics (QM/MM) potentials. The approach allows us, for the first time in this kind of systems, to make a detailed discussion on the role of electronic polarization. Such an effect had been omitted in previous studies of calixarene-alkali cation complexes, although it is known to be important in many host-guest systems. The macrocycle, calix[4]arene-bis-crown6 (BC6), is treated at the semiempirical AM1 level whereas solvent water molecules are treated using the TIP3P model. The alkali metal cations (Na+ and Cs') are described as classical point charges with a set of Lennard-Jones parameters developed in this work. The interaction energy is analysed in terms of strain, electrostatic, polarization and van der Waals contributions. The polarization component is shown to be substantial and may represent 20% of the total electrostatic energy. We show that instantaneous fluctuation of the net atomic charge on O atoms and aromatic rings are quite large. The structural results predicted by the AM1/TIP3P model are shown to agree reasonably well with X-ray data. Comparison with previous MM simulations using effective pairwise additive potentials is made. Some differences found in the case of the BC6/Na+ system, namely for the solvation number, are discussed. (C) 2001 Elsevier Science B.V. All rights reserved.