Microscopic interactions of the imidazolium-based ionic liquid with molecular liquids depending on their electron-donicity

Abstract

Microscopic interactions of an imidazolium-based ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (C2mimTFSI), with dimethyl sulfoxide (DMSO), methanol (MeOH), and acetonitrile (AN) have been analyzed by means of Raman, attenuated total reflectance infrared (ATR-IR), (1)H and (13)C NMR spectroscopy techniques. The magnitude of the red-shift of the C(2)-H vibration mode of the imidazolium ring and the deshielding of the C(2)-H hydrogen and carbon atoms, compared with that of the other atoms of the ring or the anion, indicated a strong interaction between the C(2)-H hydrogen atom and the molecular liquids in the following order; DMSO ≫ MeOH > AN. This correlates with the order of the electron donicities of these molecular liquids which allows us to suggest a hydrogen bonding character of these interactions. The behavior of S= O vibration of DMSO as a function of the DMSO molar fraction xDMSO also suggested that DMSO molecules are stoichiometrically hydrogen-bonded with the three hydrogen atoms, C(2,4,5)-H, of the ring. In contrast, the hydrogen bonding between MeOH and the C(4,5)-H atoms is much weaker than that in DMSO. AN hardly forms hydrogen bonds with the C(4,5)-H atoms. Instead, AN molecules may interact with the imidazolium ring through the π-π interaction. The interactions between the imidazolium ring and the molecular liquids lead to the loosening of the TFSI anion from the cation; this correlates with both the blue-shift of the S=O stretching vibration of TFSI and the deshielding of the trifluoromethyl carbon atoms with an increase in the molar fraction of the molecular liquid xML. The latter is weak in the MeOH solutions, and may be explained by the possible hydrogen bonding of the MeOH hydroxyl group as an electron-acceptor with the TFSI anion. Furthermore, the organization of MeOH molecules around the ethyl and methyl groups of the cation is discussed in terms of the chemical shift of the hydrogen and carbon atoms in these groups as a function of xML.

Local Structure in Terms of Nearest-Neighbor Approach in 1-Butyl-3-methylimidazolium-Based Ionic Liquids: MD Simulations

“…3,[27][28][29][30] Regarding the use of molecular simulations in explaining various spectroscopic observations one has to keep in mind that the vast majority of such experimental information reports the local molecular environment around specific sites of IL cations, e.g., 1 H NMR signals, CH vibrational bands, etc. [22][23][31][32][33][34] The importance of studying the influence of the anions on the microscopic structure around a reference cation is evident in this context.…”

“…This claim has been experimentally confirmed in neat ILs via the isotopic shift of 19 F NMR signals after selective deuteration of the C 4 mim + cation in C 4 mimBF 4 and C 4 mimPF 6 , 75 and also in their solutions in various molecular solvents, from concentration dependent chemical shifts. 17,[22][23] Similarly to Figure 4, we present in Figure 7 the CDFs relating the H-bonding distance and the corresponding pseudovalent angle between different alkyl hydrogen atoms of the C 4 mim + cation and the nearest neighboring H-bond accepting atom of the anions. Not surprisingly, the H-bonds involving the alkyl hydrogen atoms are less frequent (note the scale used in Figure 7 is half of that in Figure 4), longer, and deviate more from the linear arrangement than those involving the imidazolium ring C-H sites.…”

“…Here the term 'detailed' refers to the atomic-level description of the local structure around particularly relevant interaction sites, whereas 'systematic' implies a thoughtful selection of systems to be studied so that the eventual conclusions could be of use for the subsequent design of novel better performing ILs. [1][2][3]20 Despite the fact that even the crudest MD force-fields describe the bulk thermodynamic properties of ILs relatively well, 21 it is the description of the local microscopic structure that is of interest for deciphering some spectroscopic findings 17,[22][23][24] or rationalizing their performance in such applications as catalysis 25 or cellulose dissolution 26 .…”

Microscopic study of binary mixtures between pyrrolidinium bis(triflorosulfonyl)imide and dimethyl sulfoxide/acetonitrile

“…Negative peaks show complexes in pure ILs, while positive peaks are attributed to new complexes in the binary mixtures. According to literature work, stretching mode regarding ν(S= O) moves to higher wavenumber and that concerning ν(S-N-S) shifts to lower wavenumber with increasing strength of hydrogen bond[6,43,50]. It means that hydrogen bonds concerning S=O and S-N-S are getting stronger and weaker, respectively, upon mixing in both systems in this work.3.1.4 ATR-FTIR analysis of ν(C-D) in molecular solventsATR-FTIR spectra of ν as (C-D), ν s (C-D) from DMSO/AN are displayed in…”

“…However, the knowledge about ILs, particularly their mixtures with molecular solvents, is still incomplete. So gathering data of physicochemical properties of ILs and their binary solutions with other solvents is proceeding currently in many laboratories [6][7][8][9][10].…”