LAUSR

Abstract We performed a DFT calculation of the cationic selectivity for 2:1 complexes of two dibenzo-18-crown-6-ethers (DB18C6s) and one alkaline-earth cation with the M06-2X functional and IEFPCM/SMD solvation models. In Ba2+-(DB18C6)2 or Sr2+-(DB18C6)2, the ion is capsulated by two symmetrically positioned DB18C6s. A smaller ion such as Ca2+ or Mg2+ interacts exclusively with one of the two DB18C6s. The order of the calculated binding energies in the gas phase follows Mg2+ > Ca2+ > Sr2+ > Ba2+. In water, the differences of Gibbs energy change in binding become smaller among these complexes and the affinity is predicted to follow the order of Ba2+ > Mg2+ > Sr2+ > Ca2+. By breaking down the cation-binding into two consecutive bindings, it turns out that the addition of the DB18C6 molecule to the A2+-DB18C6 complex is the most favorable in the case of Ba2+. The enthalpies of the dication exchange reaction between DB18C6 dimer and water cluster also confirm the selective binding of Ba2+ to DB18C6.