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 (Delta E-int), thermochemistry values, theoretical band gap, molecular orbital energy order, DFT-based chemical activity descriptors [chemical potential (mu), chemical hardness (eta), and electrophilicity index (omega)], 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 Delta E-int , theoretical band gap and chemical activity descriptors was evaluated. The Delta E-int 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 eta values and comparatively small difference in w value after -CH3 substitution. The change in eta and mu 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 H-cat...X-ani and C-cat...X-ani 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 Delta Delta V-min measuring the anion-cation interaction strength showed a good linear correlation with Delta E-int for all 1-butylpyridinium ion pairs (R-2 = 0.9918). The ionic crystal density values calculated by using DFT-based MESP showed only slight variations from experimentally reported values.