Theoretical Study on the Biodegradable Aliphatic Choline Based Naphthenic Acid Ionic Liquids

Choline based naphthenic acid ionic liquids (CBNAILs) are a new family of ILs withsuitable properties such as null toxicity, high biodegradability and low cost which makes them asreal green ILs [1,2]. In this study, we focus on the molecular interactions of two CBNAILs,namely, cholinium cyclopentane carboxylate ([CH][CPC]) and cholinium cyclohexanecarboxylate ([CH][CHC]). The effect of the size of anions on the nature of interactions has beenstudied using quantum mechanical methods. Initial optimization of the geometries of ILs wasperformed at the M06-2X method using Dunning cc-pVDZ basis set [3,4]. All calculations werecarried out with the Gaussian 09 package [5]. The results showed that the interaction energybetween [CH]+ and [CPC]- is greater than that of the [CH]+ and [CHC]-. The increase in anionsize from CPC to CHC results in a decrease in the binding energies due to the increase indelocalization of charges. Analysis of frontier orbitals for isolated anions and cations shows thatthe HOMO orbitals for the studied CBNAILs come from the corresponding anion HOMOs,whereas LUMOs come from the cation LUMOs. Atoms in molecule (AIM) analysis and thereduced density gradient (RDG) iso-surfaces indicated that in both CBNAILs, the interactions ofoxygens of anions with the hydrogen of hydroxyl group of cation is stronger than the interactionswith the rest of hydrogens and the strength of these interactions for [CH][CHC] is weaker thanthat of the [CH][CPC] in agreement with its weaker interaction energies. As the ring size of theanion goes up further, the hydrogen bonds between the anion and cation are weakened due to thegrowing of non-polar part of anion.

کلیدواژه ها:

Aliphatic CBNAILs ، Interaction energies ، AIM analysis ، RDG analysis ، HOMOLUMO gap