Interionic Interactions in Imidazolic Ionic Liquids Probed by Soft X-ray Absorption Spectroscopy

Abstract

This work investigates pure ionic liquids (ILs) derived from an imidazolium ring with different carbonic chains and halides or bis(trifluoromethanesulfonilimide) (TFSI(-)) as anions, using X-ray absorption near edge spectroscopy (XANES) at different energies (N, S, O, F, and Cl edges) to probe the interionic interactions. XANES data show that the interaction with the anion is weaker when the cation is an imidazolium than when the salt is formed by smaller cations, as lithium, independently of the length of the carbonic chain attached to the imidazolium cation. The results also show that, for all studied ILs, it is not observed any influence of the anion on the XANES spectra of the cation, nor the opposite. 1-Methylimidazolium with Cl(-), a small and strongly coordinating anion, presents in the N K XANES spectrum a splitting of the band corresponding to nitrogen in the imidazolic ring, indicating two different chemical environments. For this cation with TFSI(-), on the contrary, this splitting was not observed, showing that the anion has a weaker interaction with the imidazolic ring, even without a lateral carbonic chain.

A detailed assignment of NEXAFS resonances of imidazolium based ionic liquids

“…2(a)) contributes significantly to the NEXAFS signal in the region slightly above signal I (red curve). Similar results can be found in the paper of Rodrigues et al, 12 where N K NEXAFS spectra of [C 4 C 1 im] + [Br] À and [C 4 C 1 im] + [NTf 2 ] À are presented.…”

X-ray-Induced Fragmentation of Imidazolium-Based Ionic Liquids Studied by Soft X-ray Absorption Spectroscopy

“…Recently, XAS/XES has received considerable attention as a powerful tool to characterize the electronic structure of ILs. 20,[23][24][25][26] It is noteworthy that many of the above-mentioned techniques require highly energetic excitation sources such as x-rays, electrons, and neutrons, which are known to cause excitation-induced side reactions, particularly for organic compounds. 27 During in-situ and operando experiments, such side reactions and the resulting products may have a significant impact on the performance of the electrochemical system as well as the interpretation of reaction mechanisms.…”

“…The imidazolium-based ILs are among the most studied in the IL family and 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 have been recently examined using various characterization techniques including XAS/XES. 24,26,28 In these experiments, thanks to their extremely low vapor pressure, ILs can be introduced directly into the high vacuum (HV) chamber in the form of droplets supported on a substrate. In this paper, we compare the XAS spectra of…”

“…28 The N K-edge XAS spectrum collected in the open geometry (bottom curve in Figure 2A and Figure S1B) agrees very well with reported XAS spectra in the literature. 24,26 The sharp peak at 402 eV is characteristic of the transition from N1s to the π* orbital of the imidazolium ring. 24,26 The other broader features shown in the peak deconvolution in Figure S1B originate from transitions to σ* orbitals of both 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 cations and anions.…”

“…24,26 The sharp peak at 402 eV is characteristic of the transition from N1s to the π* orbital of the imidazolium ring. 24,26 The other broader features shown in the peak deconvolution in Figure S1B originate from transitions to σ* orbitals of both 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 cations and anions. 24,26 Even after ~60 min exposure to the x-ray beam at BL8.0.1.3, the N K-edge spectrum remains unchanged with time within the probing depth of the TFY signal (~µm).…”