Theoretical study on the ion-pair recognition of Na+/X- (X=F-, Cl-, Br-) by urea calix[4]bis crown-3 derivative

The structure of tetraurea calix[4]bis crown-3 derivative (Ht) and its complexes with ion-pair Na+/X- (X=F-, Cl-, Br-) were studied using quantum mechanical (QM) methods (at the B3LYP/6-31G(d,p) level) and molecular dynamics (MD) simulations under various conditions. Two types of complexes (host-separated type (S) and contact type (C)) were found based on QM calculations in the gas phase. The ion-binding sites in Ht, the interaction forces between Na+/X- and Ht and the recognition selectivity for ion pairs were discussed in detail. In the case of host-separated type (S), the MD simulation results showed that the halogen anion gradually broke away from the host molecule in water. However, the anion got into the calix[4]arene subunit and interacted with sodium cation in chloroform solvent. For the contact type, anion and cation were always embraced by the host molecule during a 1-ns simulation both in water and chloroform. These results indicated that Ht could be a potential selective ion-pair receptor, and the QM+MD strategy could provide reasonable descriptions for ion-pair recognitions.