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The conformational features and anion-binding properties of a series of β-octasubstituted calix(4)pyrroles have been investigated by the BLYP method of density functional theory with the 6-31+G** basis set both in the gas phase and in CH2Cl2 solution. The calculations demonstrated that adjusting the electronic properties of β-substituents on the pyrrole rings do change the anion-binding ability of calix(4)pyrroles. With the BLYP/6-31+G** method in CH2Cl2 solution, the relative binding energies follow the order of electron withdrawing abilities of the substituents, i.e. CN (18kcal/mol) >> Cl (7 kcal/mol) > Br (4 kcal/mol) > H (0 kcal/mol). Calculations also indicate that the energy difference between the most stable 1,3-alternate conformation and the least stable cone-conformation that is for anion-binding is increased by electron-withdrawing β-substituents CN, Cl, and Br. Further analysis on dipyrromethane models reveals that the destabilization of the cone-conformation is mainly caused by electrostatic interactions between the β-substituents on the adjacent pyrrole rings. Our results thus provide useful information for designing stronger and more efficient calix(4)pyrrole-based anion binding receptors.

Density functional theory studies of β-substituent effect on conformational preference and anion binding ability of calix[4]pyrroles