An interesting theoretical insight into CO2 capture of phosphonium-based ionic liquids with aprotic heterocyclic anions

Abstract

In this investigation, a systematic understanding of the nature of molecular interactions between aprotic heterocyclic anions (AHAs) and typically phosphonium-based cation named as [Pnnnm]+ (n = 6, and m = 14), has been carried out using density functional theory (DFT) method and Gaussian 09 program at B3LYP/6-311G** basis sets. Multiple stable conformers were determined between the heterocyclic N atoms of the AHA anions that geometrically positioned around the [P66614]+ (C-H(α) bond) so as to formation of maximum non-covalent interactions. According to the Mulliken charge distributions on atoms in gas phase, the computational results show that the [P66614]+[3-Triaz]− ion pair possess of −394.6 kJ mol−1 as highest negative interaction energy, with six hydrogen bonds between N and H atoms of [3-Triaz]− and [P66614]+, respectively. According to the solvent effect results, the charge distributions and interaction energy is more affected by solvent than geometrical conformations. Also, for addition of CO2 molecules to multiple [P66614]+[AHA]−, our investigations indicated that, chemical interacts of one CO2 to the isolated AHA anions clearly resulted in the formation of a covalently bound carbamate with the high strength of binding, while the addition of two or more CO2 molecules leads to non-covalent bond due to the steric effect. Our results reveals the [Inda]− position rather than cation is so that five fairly strong interactions create between ion pairs in “[P66614]+[Inda]- –CO2” conformer and leads to the formation most stable conformer with highest negative interaction energy. Moreover, to the best of our knowledge, this is the first theoretical report to deeply explore the molecular interactions in the [P66614]+[AHA]− ion pairs, as well as interaction strength with the CO2 molecules.