Electronic structure investigation of the stability, reactivity, NBO analysis, thermodynamics, and the nature of the interactions in methyl-substituted imidazolium-based ionic liquids

Abstract

This work focuses on the stability, reactivity and electronic interionic interactions of methyl-substituted imidazolium (MMMIM) based ionic liquids (ILs); 1,2,4-trimethylimidazolium nitrate [MMMIM][NO3], hydrogen carbonate [MMMM][HCO3], and hydrogen sulfate [MMMIM][HSO4]. We have performed a high-level quantum study to provide a comprehensive insight into the stability of these ILs. Interestingly, it is shown that these ILs are stabilized through N…H and H…O hydrogen bonds (HBs) formed between the cation–anion pairs of the complexes. The nature and strength of the interactions were determined by atoms in molecules (AIM) topological parameters and natural bond orbital (NBO) analysis. These HBs are classified as medium-to-large and are driven by electrostatic interactions. The stability of the ILs was further investigated using energy decomposition analysis (EDA) and single point energy calculation. In any case, [MMMIM][HSO4] was shown to be the most stable of the ILs under consideration. The results of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) show [MMMIM][HSO4] with energy gap of 6.0488 eV is the most stable but kinetically less reactive. It is worthy to mention here that the density of state plots clearly shows a greater contribution of carbon, oxygen and nitrogen fragments to the frontier molecular orbital. The addition of anions (carbonate, sulfate and nitrate) to cation (MMMIM) to form ILs reduces the absorption maxima (absorption wavelength) of the compounds and leads to a Bathochromic shift of the ILs. The thermodynamic study of the ILs suggest that the formation of [MMMIM][HSO4] with free energy value of 231.47 kcal/mol is highly endothermic while that of [MMMIM][NO3] is more exothermic than [MMMIM][HCO3].