Does interaction between an amino acid anion and methylimidazolium cation lead to a nanostructured ion pairs of [Mim][AA] as an ionic liquid?

Molecular interactions between six amino acids based anions ([AA]− (AA = Gly, Cys, Ser, His, Phe and Tyr)) and methylimidazolium cation ([Mim]+) have been investigated using B3LYP, and MP2 methods in combination with a wide range of basis sets. According to the five preferential binding sites around the [Mim]+ cation, upon interaction with [Gly]− anion, five ion pairs nominated as [Mim-Gly]S1-[Mim-Gly]S5 were found on the potential energy surface, including two intermolecular hydrogen bonds of the type N‐‐‐HO and O‐‐‐HC. These hydrogen bond interactions were analyzed with the help of the quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analysis. Interaction energies of all studied ion pairs were calculated using various levels of theory. The results showed that the interaction energies (IEs) for [Mim-Gly]S1-[Mim-Gly]S5 are − 125.36, − 126.5, − 99.77, − 92.82 and − 89.15 kcal/mol at MP2/6–311 ++G(d,p) level, respectively. At the same level of theory, the interaction energy of [Mim-AA]S2, which is the most stable ion pair, is − 126.50, − 120.89, − 116.65, − 126.10, − 122.95 and − 123.3 kcal/mol for AA = Gly, Cys, Ser, His, Phe and Tyr respectively. Finally, some correlations between electron densities of hydrogen bonds, critical points and interaction energies, as well as vibrational frequencies among studied ion pairs have been analyzed.