A SLOW-COOLING MONTE-CARLO CONFORMATIONAL SPACE STUDY OF 18-CROWN-6 AND ITS ALKALI-METAL CATION COMPLEXES

A ''slow-cooling'' Monte Carlo technique is utilized in a search of th e low-energy structures of ionophore 18-crown-6 (18C6) and its complex es with alkali metal cations in vacuo. The potential energy is approxi mated by the AMBER force field with a new set of atomic point-charges and corrected alkali metal cation van der Waals radii, which are consi stent with high-level ab initio calculations. Twelve conformers of the uncomplexed ligand are obtained; eight of them have not been characte rized before. Five new low-energy conformers of complexed 18C6 are fou nd. An alkali metal cation enclosed inside 18C6 dramatically changes t he low-energy conformational space of the ligand. In all of the comple xed structures the metal cation is located in a cavity formed by oxyge ns of the crown ether, while the empty cavity of the uncomplexed 18C6 is filled by hydrogen atoms. The global-minimum-energy structures of t he complexes with Li+ and Na+ belong to C1 and C2 symmetry point-group s, respectively. C2 is found to be the only ligand conformer capable o f 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 i ncrease and a decrease of the cation size extends the energy-acceptabl e conformational space of the ligand by decreasing the energy gap betw een the two lowest-energy structures. The binding affinities of Li+, N a+, K+, Rb+ and Cs+ to 18C6 are 411, 349, 310, 283 and 226 kJ mol-1, r espectively. These values are in good agreement with ab initio counter poise corrected calculations of the 18C6 complexes with Li+, Na+ and K + at the HF/6-31 + G(2d,2p) level.