Conformational analysis of 9-crown-3, 9-thiacrown-3 and 9-azacrown-3.

Crown ethers are an important class of molecules with wide applications. Crown ethers are large ring flexible molecules with a large number of possible conformations. In the current manuscript, we report the conformational analysis results of one of the smallest crown ether systems, namely, 9-crown-3, 9c3, 9-thiacrown-3, 9t3, and 9-azacrown-3, 9a3. The conformational search is performed using the CONFLEX conformational search method utilizing the MMFF94s force field. 8, 11 and 62 conformations were predicted for 9c3, 9t3 and 9a3, respectively. The ab initio computations were performed at B3LYP and MP2 levels using the 6-311G** basis set. For the accurate prediction of the ground state conformation and the energy order of the low-lying energy conformations, the computations were performed at the G4 level for some selected conformations. Factors affecting the stability of different conformations of 9c3, 9t3 and 9a3 are discussed. These factors are also discussed with respect to larger crown ether systems. The ab initio computations were performed also for water and chloroform as solution phases. The same ground state predicted in the gas phase was also predicted in the solution water and chloroform phases.