Quantum mechanical insights into the structural and conformational features of benzimidazolium ionophore and its ion pairs

In this study, we identified seven minimum stationary points on the potential energy surface of the benzimidazolium ionophore (BII) using quantum mechanical methods. These minima were further analyzed through energy decomposition analysis employing the sobEDA method, alongside population analysis utilizing the atoms in molecules (AIM) and natural bond orbital (NBO) approaches. Notably, the most stable conformer of BII remains unchanged in the presence of a counterion and its interaction with a cation; however, the optimal positioning of the counterion varies in the presence of a cation. Interactions that influence the binding of the counterion to the BII were identified through AIM analysis. The nature of these interactions was evaluated by establishing a linear relationship between the electron density (ρ), calculated at bond critical points (BCPs) between the two components of the ion pair, and their corresponding interaction energies. The interaction with the benzimidazolium skeleton (BIS) is primarily electrostatic, while both attractive and repulsive weak interactions are observed with the side chains (SC). These interactions are strongly supported by reduced density gradient (RDG) scatter plots and low RDG isosurfaces. Furthermore, sobEDA analysis, utilizing model systems that focus solely on the BIS. X and SC. X interactions, indicates that the nature of the predominant interaction is dependent on the specific anion involved. Charge transfer occurs from the anion to the BS and from the side chains to the anion, as demonstrated by NBO analysis in conjunction with the aforementioned model systems. This comprehensive evaluation enhances our understanding of the binding mechanisms within this system.