Metal ion binding of s-block cations and nanotubular cyclic (proline)4: A theoretical study

Abstract

The binding interaction of alkali metal ions within the cavity of nanotubular cyclic (proline)4 [Cyclo(Pro)4] has been studied using quantum chemical density functional theory. The Cyclo(Pro)4 and its alkali metal ionic complexes were optimized at B3LYP/6-31+G(d) and CAM-B3LYP/6-31+G(d) levels of theory. For each alkali metal ion, two binding modes of complexation with Cyclo(Pro)4 were considered: tetradentate series of (a) and bidentate series of (b). The binding energies and various thermodynamic parameters of free Cyclo(Pro)4 and its alkali metal ion complexes were determined. In series of (a), the binding energy of Cyclo(Pro)4 toward metal ions increases as Li+ > K+ > Na+ > Rb+. In series of (b), the binding energy order is obtained as Li+ > Na+ > K+ > Rb+ > Cs+. The optimized structures are used to perform natural bond orbital analysis. The results indicate that the electron-donating oxygen offers lone pair electrons to the LP* orbitals of metal cations except in the tetradentate Li+ and Na+ complexes, where the electron-donating nitrogen offers lone pair electrons to the LP* orbitals of metal cations. The strength and nature of interactions between alkali metal ions and macrocyclic Cyclo(Pro)4 was studied using topological parameters at bond critical points (BCP) by AIM analysis. Consequently, these interactions were closed-shell interactions due to their positive ∇2 ρ(r) values at corresponding BCP. The bulk solution effect on the binding interaction between alkali metal ions and Cyclo(Pro)4 was evaluated using PCM-SCRF optimization calculations at the same level of theory. The obtained optimized geometries and binding energies of gas and solution phases were compared. In addition, the bulk solution effect reduced the binding energies of Cyclo(Pro)4···M+ complexes.