Molecular Interactions in Binary Mixtures of n-Alkylmethylimidazolium bis(trifluoromethylsulfonyl)imide + Acetonitrile: Thermophysical and Density Functional Theory Studies

This work is based on the investigation of thermophysical properties of pure ionic liquids {ILs; 1-ethyl-/1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide; [EMIM][(NTf)₂], [BMIM][(NTf)₂], solvent acetonitrile (ACN), and its binary mixtures. Under these investigations, density (ρ) and ultrasonic velocity (u) were measured using high-precision vibrating-tube densitometer and viscosity (η) with an automated falling ball microviscometer for all components as functions of the mole fraction of ILs ( \({x}_{1}\) $x_1$ ) at T = 298.15–323.15 K and p = 0.1 MPa. ρ, u, and η data of pure and binary components were used to evaluate excess/deviation parameters, and these parameters are correlated utilizing the extended form of Redlich–Kister equation. Interactions inside the ion pair of ILs and ILs–solvent are well discussed in terms of various specific/nonspecific forces of attractions. The interactions between the ion pair \(({[\text{EMIM}]}^{+}\) $({[\text{EMIM}]}^{+}$ / \(\left[\text {BMIM}\right]^{+}\) ${\left[\text{BMIM}\right]}^{+}$ and \({\left[{\text{NTf}}_{2}\right]}^{-})\) ${\left[{\text{NTf}}_2\right]}^{-})$ as well as IL solvent was calculated using Density Functional Theory (DFT) in terms of various parameters at the D3-B3LYP/6–311 +  + G(d,p) level of theory. Moreover, various molecular properties, including structures, frontier molecular orbitals, electrostatic potentials, atomic charges, dipole moments, interaction energies, reactivity descriptors, zero-point energy (ZPE), and heat capacity, were obtained at the same level of theory. Thereafter, the natural bond orbital (NBO) analyses were performed to see all the interactions between donor–acceptor atoms at molecular level.