Theoretical Probing of Weak Anion-Cation Interactions in Certain Pyridinium-Based Ionic Liquid Ion Pairs and the Application of Molecular Electrostatic Potential in Their Ionic Crystal Density Determination: A Comparative Study Using Density Functional Approach.

A comprehensive study on the structure, nature of interaction, and properties of six ionic pairs of 1-butylpyridinium and 1-butyl-4-methylpyridinium cations in combination with tetrafluoroborate (BF4 - ), chloride (Cl- ), and bromide (Br- ) anions have been carried out using density functional theory (DFT). The anion-cation interaction energy (ΔEint ), thermochemistry values, theoretical band gap, molecular orbital energy order, DFT-based chemical activity descriptors [chemical potential (μ), chemical hardness (η), and electrophilicity index (ω)], and distribution of density of states (DOS) of these ion pairs were investigated. The ascendancy of the -CH3 substituent at the fourth position of the 1-butylpyridinium cation ring on the values of ΔEint , theoretical band gap and chemical activity descriptors was evaluated. The ΔEint values were negative for all six ion pairs and were highest for Cl- containing ion pairs. The theoretical band gap value after -CH3 substitution increased from 3.78 to 3.96 eV (for Cl- ) and from 2.74 to 2.88 eV (for Br- ) and decreased from 4.9 to 4.89 eV (for BF4 - ). Ion pairs of BF4 - were more susceptible to charge transfer processes as inferred from their significantly high η values and comparatively small difference in ω value after -CH3 substitution. The change in η and μ values due to the -CH3 substituent is negligibly small in all cases except for the ion pairs of Cl- . Critical-point (CP) analyses were carried out to investigate the AIM topological parameters at the interionic bond critical points (BCPs). The RDG isosurface analysis indicated that the anion-cation interaction was dominated by strong Hcat ···Xani and Ccat ···Xani interactions in ion pairs of Cl- and Br- whereas a weak van der Waal's effect dominated in ion pairs of BF4 - . The molecular electrostatic potential (MESP)-based parameter ΔΔVmin measuring the anion-cation interaction strength showed a good linear correlation with ΔEint for all 1-butylpyridinium ion pairs (R2 = 0.9918). The ionic crystal density values calculated by using DFT-based MESP showed only slight variations from experimentally reported values.