Cyclic Crown Ether Traps for Alkali Ions: IQA Portrayed Reaction Pathways

Abstract

Sodium and potassium ions have crucial roles in various physiological processes within the human body, and their imbalance has been linked to a range of problematic health conditions. Detection of these ions in blood samples relies on optical and electrochemical sensing. These sensors need to meet specific criteria, including high sensitivity and strong selectivity. Crown ethers are recognized as effective traps for alkali ions due to their oxygen atoms oriented inward, attracting positively charged particles. Cyclic crown ethers that demonstrate selective preference for Na+ and K+ ions are already synthesized. [1] Furthermore, notable spectral changes are observed upon ion capture. Apart from the favorable cavity dimensions, the remarkable selectivity of these ethers arises from significant interactions between the metal ions and polar solvent molecules. [2] While long-range electrostatic forces facilitate ion-trap recognition, different characteristics define the interaction between the ions and their respective hosts. In this sense, Na+ resembles a point charge, whereas the high polarizability of K+ results in pronounced quantum effects. This research aims at providing comprehensive mechanistic insights as well as energetic changes along ion capture reaction pathways. The reaction pathways are resolved by constructing minimum energy paths with constraints imposed in the form of the coordination number (CN) of the alkali cation with respect to crown’s coordinating oxygen atoms. Afterwards, Interacting Quantum Atoms (IQA) methodology is employed to examine intra- and inter-fragment interactions along the reaction pathways. IQA has been proven successful in describing various systems featuring non-covalent interactions. Additionally, we utilize Relative Energy Gradient (REG) analysis which helps in identification of the driving forces for ion-trap complex formation. In this particular case, dominant Na+-crown interaction is electrostatic, while the nonclassical interaction plays a significant role for K+-crown complexes. However, REG analysis reveals that classical contributions govern not just Na+ complexation (CN = 4.8), but also K+-crown formation in the small region along the reaction path close to the final point (CN = 5.4).