QUANTITATIVE STRUCTURE STABILITY RELATIONSHIP FOR POTASSIUM-ION COMPLEXATION BY CROWN-ETHERS - A MOLECULAR MECHANICS AND AB-INITIO STUDY

Molecular mechanics calculations with the MM2 program were used to exa mine the structures and strain energies 11 hexadentate crown ether lig ands and their potassium complexes. With the exception of K-O stretchi ng parameters, all force field parameters for interactions involving t he potassium ion were obtained by fitting to ab initio potential energ y surfaces for selected distortions in K-O(Me)2 and K-O(Me)(Et). The K -O stretching parameters were optimized relative to crystallographic d ata. Comparison of the molecular mechanics results to experimental log K values that were available for all 11 crown ethers revealed the com plex stability to be strongly correlated with the difference in strain energy between the uncomplexed ligand and its potassium complex. The results establish that both the K-O length preference and bonding dire ctionality at the ether oxygen donor atom are important factors in the determination of complex stability. It is concluded that the failure to consider the orientation of the ether C-O-C moiety, relative to the metal ion, is a serious flaw in the size-match selectivity theory.