Rb(+)/Cs(+) selectivity of benzo and tribenzo derivatives of the 21C7 crown ether. A density functional study

A density functional study has been performed on Rb(+) and Cs(+) complexes of mono and tribenzo derivatives of the 21Crown7 crown ether. Calculations have been carried out using the B3LYP functional together with a split valence basis set with additional polarization and diffuse functions. The cations have been described with effective core potentials. Structures of the tribenzo-21Crown7 are compared to X-ray data. The Rb(+)/Cs(+) affinity of the benzo and tribenzo derivatives in gas phase is discussed on the basis of binding energies, strain energies, and electrostatic contributions. For the first time, a detailed description of the effect arising from the presence of benzo groups is described. On one hand, the benzo groups decrease the electronic density on the ether oxygen atoms. On the other hand, they confer a stronger electronic polarizability to the ligand. The energetic analysis shows that the polarization contributions play a crucial role and reach up to 29% of the total electrostatic energy.