Theoretical study of substituent effects on Cs+/Sr2+-dibenzo-18-crown-6 complexes

The effect of electron-donating and -withdrawing substituents of Cs+/Sr2+-dibenzo-18-crown-6 complexes was studied by density functional theory (DFT) calculations. For the investigations the interaction energies were correlated to the second-order interaction energies, charge-transfer effects, and HOMO energies. Electron-donating substituents increase the crown ethers' ability to complex metal ions, whereas electron-withdrawing substituents have the opposite effect. Natural bond orbital analysis indicates a small contribution of these substituents to the complex stability in the gas phase. In the condensed phase, the interaction energy of the complexes is significantly reduced because of solvation effects. The results demonstrate a discrepancy between gas and condensed phase in which Cs+ is bound to dibenzo-18-crown-6 better than Sr2+. We speculate that the species in water form 2:1 complexes consisting of two dibenzo-18-crown-6 molecules and one cation.