A “slow-cooling” Monte Carlo conformational space study of 18-crown-6 and its alkali metal cation complexes

Abstract

A “slow-cooling” Monte Carlo technique is utilized in a search of the low-energy structures of ionophore 18-crown-6 (18C6) and its complexes with alkali metal cations in vacuo. The potential energy is approximated by the AMBER force field with a new set of atomic point-charges and corrected alkali metal cation van der Waals radii, which are consistent with high-level ab initio calculations. Twelve conformers of the uncomplexed ligand are obtained; eight of them have not been characterized before. Five new low-energy conformers ofcomplexed 18C6 are found. An alkali metal cation enclosed inside 18C6 dramatically changes the low-energy conformational space of the ligand. In all of the complexed structures the metal cation is located in a cavity formed by oxygens of the crown ether, while the empty cavity of the uncomplexed 18C6 is filled by hydrogen atoms. The global-minimum-energy structures of the complexes with Li + and Na + belong to C 1 and C 2symmetry point-groups, respectively. C 2 is found to be the only ligand conformer capable of changing its cavity size when forming stable complexes with Li +, Na + or K +. The 18C6:Rb + complex is the most rigid in vacuo, and both an increase and a decrease of the cation size extends the energy-acceptable conformational space of the ligand by decreasing the energy gap between the two lowest-energy structures. The binding affinities of Li +, Na +, K +, Rb + and Cs + to 18C6 are 411, 349, 310, 283 and 226 kJ mol −1, respectively. These values are in good agreement with ab initio counterpoise corrected calculations of the 18C6 complexes with Li +, Na + and K + at the HF/6-31 +G(2d,2p) level.