Ion Pair Structures in Aqueous KSCN Solution: Classical and Quantum Mechanical/Molecular Mechanical Molecular Dynamics Simulation Study

Ion pair formation and dissociation are fundamental processes in electrolyte solutions so that understanding thermodynamic stabilities and dynamic aspects of ion pairs is of great importance. The structures of various ion pair states are here studied by carrying out classical and quantum mechanical/molecular mechanical (QM/MM) molecular dynamics (MD) simulations of aqueous KSCN solutions. A few different solvent potential models are considered and the resulting ion pair structures are directly compared. In particular, when KSCN is treated quantum mechanically and effective fragment potential (EFP) model for water is used, we found two stable forms of ion pairs that can be considered as contact ion pair (CIP) and solvent separated ion pair (SSIP), where the interionic distances between K⁺ and SCN^- are found to be 4.0 and 5.5 Å, respectively. QM/EFP-MD further indicates that the nitrogen side of SCN^- is preferentially interacting with K⁺ in CIP. However, the corresponding CIP appears at 3.0 Å interionic distance in fully classical MD simulation at a higher KSCN concentration. Nonetheless, both QM/EFP and classical MD simulation results show that the CIP state appears to be more stable than SSIP. From the site-site radial distribution functions (RDF) calculated from the QM/MM and classical MD trajectories, detailed ion pair structures and surrounding solvent configurations are further elucidated.