Quantum chemical calculations were employed to examine the structures, binding energies and vibrational spectra of complexes formed by Li+, Na+, and K+ with diphenylmethane, diphenylethane, diphenylpropane, [2.2]paracyclophane and [3.3]paracyclophane molecules. The effect of heteroatom substitution in the connecting chains of diphenylpropane and [3,3]paracyclophane molecules on cation-π interactions was also examined. Full geometry optimizations and harmonic vibrational frequency calculations were performed using M06-2X/6-311+G(2d,2p) level. The data of binding energies, distances between two six membered rings in the ligands and complexes were analyzed. In the present work, the cage-type molecules, which possess linear chain connectors at para-positions of six membered aromatic rings, seems to have weak cation-π interactions especially with metal ions Na+ and K+ when they are placed in between two rings. This is due to insufficient space available to accommodate those metal ions in the cage. On the other hand, Li+ shows strong cation-π interactions when it binds between two rings of the cage-type molecules. Our aim is to understand how the binding affinity of the metal ion varies by changing from single fusion to double fusion of alkyl chain (or hetero substituted alkyl chain) with two benzene rings. In addition, the preference of metal ion positions ‘inside’ or ‘outside’ in binding with ligands was identified. Our study reveals that Li+ binds notably stronger than Na+ and K+ for all ligands. It is found that the lone pair-cation interaction dominates when the metal ion is placed in between two benzene rings in case of ligands containing hetero substituted alkyl chains. Vibrational frequency calculations reveal that all of the ligands and complexes are minima on the potential energy surface. Furthermore, the infrared spectrum could be used to characterize the metal ion binding with the ligands and distinguish the binding of metal ion in between the rings or above single benzene ring.

DFT study on cation-π interactions of Li+, Na+ and K+ with two benzene rings connected through linear chains

256th National Meeting (2018)

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