A computational study of the interaction of [7]-helicene with alkali cations and benzene

Abstract

Ab initio calculations, using restricted Hartree-Fock, density-functional (DFT) and Moeller-Plesset perturbation theories, have been performed to study the cation-π interactions between alkali cations and [7]-helicene as well as the interaction between [7]-helicene and benzene. The helical molecule's two terminal benzene rings give the profile of 'crocodile's jaws' which open up to receive the cations, exhibiting the stability of the complexes increasing in the order Na⁺>K⁺>Cs⁺. The binding energies in these cation-π complexes comprise mainly electrostatic, polarization (including dispersion) and charge transfer contributions. According to DFT, the electrostatic contribution alone is 43% for Na⁺, 60% for K⁺ and 75% for Cs⁺. However, even DFT shows very little stabilization for the helicene-benzene complex so that this interaction between a helical and a neutral molecule cannot serve as a model for DNA-drug interactions due to very little overlap between the π clouds of the two molecules.