Quantification of the interactions in halide-anion-based imidazolium ionic liquids

Non-covalent interactions between constituent ions of ionic liquids (ILs) define their distinctive physicochemical properties and are critical for understanding their micro and bulk structures as well as designing new task-specific ILs. Therefore, the quantification of these interactions can improve our understanding of this unique class of materials. A set of 20 halide-anion-based imidazolium ILs (C_nmim X, X = Cl, Br, I, and BF₄; n = 2, 4, 6, 8, and 10) was selected for this study. The variation of the anion (hydrogen-bond-acceptor property and size), presence of water, and the alkyl chain length of the imidazolium ring have all been considered as parameters when attempting to address mainly the following two fundamental questions: How does the strength of hydrogen bonding in these selected Coulomb systems change? How much do dispersion forces play a role in the net attractive interaction energy despite being weak? To this end, dispersion-corrected density functional theory (DFT) computations were executed to acquire the optimized structures of all the considered ILs, followed by an ensemble of NCI and QATIM analyses to explore the strength of non-covalent interactions. Further, quantum mechanical energy decomposition analysis (QM-EDA) based on symmetry-adapted-perturbation-theory (SAPT) has been employed to dissect the total interaction energy into its components. An assessment of QM-EDA demonstrates that the electrostatic interaction dominates the intermolecular attraction, although induction and dispersion components also play a substantial role. The dispersion energies are amplified when water is present, as well as when anion size and alkyl chain length increase. We quantified the non-covalent interactions between ion pairs using NCI-RDG and Bader's QTAIM analyses. Strong hydrogen bonding with a partial covalent character was observed between monoatomic anions and C2-proton of the imidazolium ring, but for the multiatomic anion (BF₄⁻) and interaction between imidazolium ring alkyl groups and anions, a weak electrostatic hydrogen bonding was found. The hydrogen bonding strength decreases with increasing anion size; it is strongest in the C₂mim Cl ion pair. NBO analysis gives a clear indication of $n_X\to {\sigma }_{C-H}^{*}(X=\mathrm{Cl},\mathrm{Br},I,\mathrm{and}F)$ type intermolecular interaction with highest stabilization energy ($E_{n\to \sigma *}^{\left(2\right)}$). A clear correlation between $E^{\left(2\right)}$ and hydrogen bond lengths were observed. Overall, the current study illuminates the intricate network of covalent-like and non-covalent interactions in selected imidazolium ILs based on halide anions.