Structure of crown ethers

Abstract

We use the combined conformational and vibrational analysis methodology to predict in what conformation different crown ethers and some of their complexes exist. Comparison between the structure of some of the low and high energy conformations allows the prediction of the factors that affects the conformational stability of the studied crown ether. Using the conformational analysis, it was predicted that 12-crown-4 (12c4) and 18-crown-6 (18c6) have S4 and S6 conformations, respectively, as the gas phase ground state structure. This was rationalized since in both of the S4 and S6 conformations there is one hydrogen bond for each oxygen atom and at distances shorter than any other predicted conformation of either molecule. This suggests that crown ethers adopt the conformation that maximizes the hydrogen bond rather than to have an endodentate structure. Using vibrational analysis, it was concluded that both of 12c4 and 18c6 have Ci conformation in the solid phase. Using the combined conformation and vibrational analysis, it was concluded that 12-thiacrown-4 (12t4) exists in the D4 conformation in the gas and solid phases. Conformational analysis of 18-thiacrown-6 (18t6) predicted a new C2 conformation as the ground state gas phase conformation which is lower by 4.67 kcal/mol at the MP2/6-311G** level than the solid state structure. The ground state C2 conformation of 18t6, and also the D4 conformation of 12t4 have all the SCCS angles adopt an exodentate structure without any exception. It is concluded that for the stability of thiacrown ethers a SCCS dihedral angle of 180° requirement is more important than a gauche CSCC dihedral angle requirement.