Szentpály, L.V.,; Shamovsky, I.L.; Nefedova, V.; Zubkus, V.E.
Author Affiliation, Ana.
Chemistry; Faculty of Pure and Applied Sciences
A 'slow-cooling' Monte Carlo conformational space study of 18-Crown-6 and its complexes with alkali metal cations
Journal of Molecular Structure
Date of Publication
A 'slow-cooling' Monte Carlo technique is utilized in a search of the low-energy structures of inophore 19-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 an initio calculations. Twelve conformers of the uncomplexed ligand are obtained; eight of them have not been characterized before. Five new low energy conformers of complexed 18C6 are found. An alkali metal cation enclosed inside 19C6 dramatically changes the low-energy conformational space of the ligand. In all of the complexed stuctures 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 C1 and C2 symmetry point-groups, respectively. C2 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 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....