Influence of anionic species on the molecular structure, nature of bonding, reactivity, and stability of ionic liquids-based on 1-butyl-3-methylimidazolium

The unique physicochemical properties of ionic liquids (ILs) have positioned them as a significant class of solvents, making them a subject of considerable interest. In this investigation, the experimental analysis involved solvothermal reflux techniques, while theoretical approaches utilized density functional theory (DFT) with the def2-SVP/ωB97XD computational approach to explore the electronic properties, reactivity, stability, and structural aspects of ILs based on the combinations of [BMIM]+ cation with [MeSO3]-, [BF4]-, [PF6]-, and [C2H3O2]- anions. The experimental and theoretical data exhibited good agreement, demonstrating that the choice of cation and anion greatly influences the stability of ILs. By employing Frontier molecular orbital analysis, the reactivity and stability of the studied ionic species were observed to follow the order of [BMIM]+[MeSO3]- > [BMIM]+[BF4]- > BMIM > [BMIM]+[PF6]- > [BMIM]+[C2H3O2]-, with corresponding energy gaps of 4.8474 > 3.1971 > 2.013 > 1.8830 and 1.3276 eV, respectively. The Natural Bond Orbital (NBO) analysis, encompassing second-order perturbation energy, revealed the strength of interactions within the investigated systems in the following order: [BMIM]+[PF6]- < [BMIM]+[MeSO3]- < [BMIM]+[BF4]- < [BMIM]+[C2H3O2]-. Topology analysis indicated a higher electron density (ρ(r)) value of 0.8966 a.u., indicating a denser distribution of electrons. Furthermore, the nature of bonding in the studied ion pairs displayed the following order: [BMIM]+[MeSO3]- > [BMIM]+[BF4]- > [BMIM]+[PF6]- > [BMIM]+[C2H3O2]-, signifying that [BMIM]+[MeSO3]- exhibited stronger interactions compared to the other combinations. The vibrational spectra and UV–vis results of the investigated ILs underscored the sensitivity of characteristic peaks in frequency and intensity to the specific cation–anion combination. These findings have the potential to guide experimental researchers in exploring the applications of the studied ionic species in diverse areas such as energy storage and catalysis.