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

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) calcula-tions. Focusing on the cation-size-effects on the electronic structure, target ILs were chosen as pairs of imida-zolium (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.