Ionic Recognition Controlled by Conformational Change: A DFT Investigation

Ions play a crucial role in the production of important materials and are associated with various health and environmental issues. Noncovalent interactions serve as fundamental tools for controlling the availability of cations and/or anions. Herein, we investigate the ability of two conformations of the 2,6-bis(1,2,3-triazol-4-yl)pyridine molecule to recognize cations (1), such as Li⁺, Na⁺, or K⁺, and anions (2), including F^-, Cl^-, or Br^-. EDA-NOCV analysis demonstrates that the conformers preferentially recognize ions based on the size of the cations (K⁺ → Na⁺ → Li⁺) and anions (Br^- → Cl^- → F^-). The preferential interaction with smaller cations (and anions) arises from the more attractive electrostatic and orbital interactions (N^···.cation and C-H^···.anion bonds). The presence of electron-donor groups (-NH₂) in the first conformer (1) enhances cation recognition through stronger electrostatic N^···.cation interactions. Conversely, the presence of electron-acceptor groups (-NO₂) in the second conformer (2) facilitates anion recognition via more favorable electrostatic, orbital, and dispersion C-H^···.anion interactions. Cation recognition is found to be more favorable in the first conformer than anion recognition in the second due to more attractive electrostatic energy and/or less Pauli repulsive energy associated with (O or primarily N)^···.cation interactions in 1 ^···.cations compared to (N or mainly C)-H^···.anion bonds in 2 ^···.anions. These findings provide significant insights into the mechanisms of cation and/or anion recognition through different conformations using the same base structure and can inform the design of molecules with enhanced functionalities.