XANES in the C K, N K, O K, and F K regions of ionic liquids composed of imidazolium-based cations with bis(trifluoromethylsulfonyl)amine anion or bromide anion

Abstract

To clarify the electronic structures and obtain the association structures of ionic liquids (ILs), X-ray absorption near-edge structure (XANES) spectra in the C K, N K, O K, and F K regions of a systematic series of ILs were measured. The C K-XANES spectra were theoretically analyzed using density functional theory (DFT) calculations. Focusing on the cation-size-effects on the electronic structure, target ILs were chosen as pairs of imidazolium (Im) cations with various alkyl substituents (Cx,y) and anions of bromide (Br) or bis(trifluoromethylsulfonyl)amine anion (TFSA). Subtracting the measured XANES as [CxCyIm]TFSA – [CxCyIm]Br suggested that multi-ion interactions on the C atoms in Im with shorter substituents were stronger than those in Im with longer substituents. Additionally, fewer multi-ion interactions were observed on the N, O, and F atoms than the C atom. By comparing the calculated XANES spectra of the cation-anion pair models, the measured C K-XANES profiles were assigned to the corresponding component C atoms. However, the measured strong multi-ion interactions on the C atoms could not be reproduced by the DFT calculations of the simple cation-anion pair models. This means that XANES analysis for C atoms in smaller Im cations should be considered as aggregated structures composed of various multi-ion clusters.