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.

Abstract

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.