Interplay of Different Moieties in the Binary System 1-Ethyl-3-methylimidazolium Trifluoromethanesulfonate/Water Studied by Raman Spectroscopy and Density Functional Theory Calculations

Abstract

The present work reports new insights into specific interactions in aqueous solutions of the ionic liquid (IL) 1-ethyl-3methylimidazolium trifluoromethanesulfonate (C 2 mimTfO). A systematic investigation based on a combination of Raman spectroscopy and density functional theory (DFT) calculations shows evidence of self-encapsulation of the ionic moiety. Raman spectroscopy reveals preferred interactions between water molecules and the TfO − anions. The comparison of the experimental results with dispersion-corrected DFT calculations, which yield the predictions of the possible conformers of the cation−water, anion−water, and cation−anion−water structures, strongly supports the hypotheses of site-selective IL/water interactions. The obtained results allow for a detailed discussion of the nature and strength of the molecular interactions. It is shown that the TfO − anion establishes a preferred interaction with water, whereas the vibrational band at 3118 cm −1 for C− H motion at the C(2) position, the most acidic site for cation and anion interaction, does not indicate any specific energy shift, when adding water to the IL. This finding gives evidence for a self-protective microstructure of the molecules of C 2 mimTfO in an aqueous environment. In contrast to other ILs reported in the literature, there is no evidence of an increasing cation−anion distance in the IL ion-pair when increasing the water content. Instead, the C 2 mimTfO molecules undergo a perfect rearrangement, allowing interactions at other molecular sites with higher selectivity. A direct exposure to water at the cation− anion interacting site (C(2) position) is avoided. Ultimately, we show that clusters of ion-pair dimers solvated with water exhibit a more stable geometry compared to the hydrated single ion-pairs, and our calculations correctly reproduce the experimental findings.

An ionic hydrogel with stimuli-responsive, self-healable and injectable characteristics for the targeted and sustained delivery of doxorubicin in the treatment of breast cancer

“…17,14,43 Further, we initiated quantum chemical calculations based on DFT to perceive a greater insight into the intermolecular interaction in the hydrogel matrix and contribution of weak non-covalent interaction in the drug-hydrogel interaction with different anions and molecular systems in imidazolium cations. [44][45][46][47][48][49][50][51][52] In order to investigate the strength of ion-pair interaction, the BEs for C 16 MeImSal was calculated. The calculated optimized energies of cation, anion, DOX, IL (both in gas and solvent phase), the dimers of IL and IL-Drug complex with BEs are listed in Table 1.…”

Understanding the Photoexcitation of Room Temperature Ionic Liquids

“…The importance of understanding the microstructure of RTILs was also the topic of a combined study using Raman spectroscopy and DFT calculations on a mixture of an imidazolium‐based RTIL with water [99] . Due to red‐shifts of some Raman peaks while increasing the amount of water, the authors proposed some kind of self‐encapsulation of the RTIL as further supported by DFT calculations, i. e. formation of clusters of ion pair dimers that proved to be more geometrically stable compared to hydrated single ion pairs.…”

Impact of water on the hydrogen bonding between halide‐based ion‐pairs investigated by Raman scattering and density functional theory calculations

“…The vibrational bands below ~3,000 cm −1 were assigned to the alkyl chain of the cation and those at higher wavenumbers to the ring chain (C 2 ─H, C 4/5 ─H) modes . In the following, we discuss the analysis of the alkyl chain and ring chain vibrations separately.…”

“…For the DFT calculations, the hybrid exchange functional wB97X‐D and the Gaussian type split basis set 6‐311++G(d,p) were used. DFT calculations have been demonstrated to contribute substantially for the understanding of experimental findings of ILs . The studied ion pairs and their water‐associated clusters were simulated without having any restrictions in the input geometries.…”

“…Most recently, Zheng et al investigated the structural and interaction properties of the C 2 mim + cation with BF 4 − and NTf 2 − anions using FTIR spectroscopy and DFT calculations contributing to a better understanding of hydrogen bonds formation in these complexes . Also very recently, our group reported new insights into specific interactions in aqueous solutions of C 2 mim TfO gained from a combined application of Raman spectroscopy and DFT calculations . Here, interestingly at high dilution, the cation–anion interaction was shown to still play a role, and the vibrational features of the ν (C 2 ─H) wavenumber was not altered in aqueous solution.…”