Effect of structure and interaction on physicochemical properties of new [Emim][BF₃X] complex anion ionic liquids studied by quantum chemistry

Context: One of the key challenges in the industrial application of ionic liquids (ILs) is their extreme characteristics, such as viscosity, glass transition temperatures, and conductivity. Understanding the relationship between ILs structure and physicochemical property is a crucial aspect of the directed design of ILs with good properties, which is a prerequisite for their successful implementation in industrial processes. In this work, high-level quantum-chemical research with for four pairs ionic liquids, [Emim][X] and [Emim][BF₃X] (X = CH₃SO₃, EtSO₄, HSO₄, Tos), was performed, to analyze the stable structure, interionic interaction, and charge transfer and provide a new insight into the property variances at the molecular level. The result shows that the overall structural stability of ionic liquids is contributed with hydrogen bonding network between the protons in the C-H and N-H of the cation and oxygen atoms of the anion, as well as fluorine atoms. The nature and strength of the interionic interaction were measured via atoms in molecule analysis and sobEDAw method and results suggested that BF₃ could waning interionic interaction of ion pairs. Moreover, a close relation between the binding energies of ion pairs and physicochemical properties was established: the weaker the interionic interaction, the lower is the viscosity and glass transition, and the higher is the conductivity.

Methods: Quantum chemistry calculations were performed under B3LYP-D3/aug-cc-pVTZ level of DFT functional using the Gaussian 16 package (version C01). The Multiwfn 3.7 program was used to calculate the electrostatic potential, interaction region indicator, the information of bond critical points, core-valence bifurcation index, and ADCH charge. Visualization of structure and the region of interaction were achieved using VESTA and VMD.