Impact of Size and Electronegativity of Halide Anions on Hydrogen Bonds and Properties of 1-Ethyl-3-methylimidazolium-Based Ionic Liquids

Abstract

The effect of the anion size and electronegativity of halide-based anions (Cl − , Br − , I − , and BF 4 − ) on the interionic interaction in 1-ethyl-3-methylimidazolium-based ionic liquids (ILs) C 2 mim X (X = Cl, Br, I, and BF 4 ) is studied by a combined approach of experiments (Raman, IR, UV−vis spectroscopy) and quantum chemical calculations. The fingerprint region of the Raman spectra of these C 2 mim X ion-pairs provides evidence of the presence of the conformational isomerism in the alkyl chain of the C 2 mim + cation. The Raman and IR bands of the imidazolium C 2 −H stretch vibration for C 2 mim X (X = Cl, Br, I, and BF 4 ) were noticeably blue-shifted with the systematic change in size of anions and the electronegativity. The observed blue shift in the C 2 −H stretch vibration follows the order C 2 mim BF 4 > C 2 mim I > C 2 mim Br > C 2 mim Cl, which essentially indicates the strong hydrogen bonding in the C 2 mim Cl ion-pair. DFT calculations predict at least four configurations for the cation−anion interaction. On the basis of relative optimized energies and basis-setsuperposition-error (BSSE) corrected binding energies for all ion-pair configurations, the most active site for the anion interaction was found at the C 2 H position of the cation. Besides information about the C 2 H position, our DFT results give insights into the anion interaction with the ethyl and methyl chain of the cation, which was also confirmed experimentally [Chem. Commun. 2015, 51, 3193]. The anion interaction at the C 2 H site of the cation favors a planar geometry in C 2 mim X for X = Cl, Br, and I; however, for BF 4 , the system prefers a nonplanar geometry where the anion is located over the imidazolium ring. TD-DFT results were used to analyze the observed UV−vis absorption spectra in a more adequate way giving insights into the electronic structure of the ILs. Overall, a reasonable correlation between the observed and the DFT-predicted results is established.

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.…”

“…In contrast to this, a significant change was observed for the ν (C 4/5 ─H) stretching wavenumber. Our group has also investigated the electronic structures and molecular interactions in various possible conformers of the cation–anion ion pair in C 2 mim X (X = Cl, Br, I, and BF 4 ) ILs using Raman, IR, and ultraviolet–visible (UV–Vis) absorption spectroscopy and DFT calculations . There, the main emphasis was put on the better understanding of the effect, which the anion size (Cl − , Br − , I − , and BF 4 − ) and its electronegativity have on the hydrogen bond properties of the C 2 mim + cation‐based ILs …”

Metal‐Halide/Ionic‐Liquid Oxidative Carbonylation of Ethanol to Synthesize Diethyl Carbonate with High Activity and Low Corrosion

“…then vacuum dried for 24 h and characterized by FTIR (Nicolet 6700). As illustrated in Figure (a), the characteristic peaks of C=C of pure [BMIm]BF 4 around 1628 cm −1 , 1575 cm −1 and C=N of [BMIm]BF 4 around 1468 cm −1 , 1431 cm −1 are observed, which is consistent with the results reported by literature . However, a red‐shift was found in the spectrum of C=C of CuCl 2 ‐[BMIm]BF 4 around 1574 cm −1 , 1533 cm −1 , and C=N around 1415 cm −1 belongs to the anti‐symmetric stretching vibration of C=N (Figure (b)).…”

“…However, a red‐shift was found in the spectrum of C=C of CuCl 2 ‐[BMIm]BF 4 around 1574 cm −1 , 1533 cm −1 , and C=N around 1415 cm −1 belongs to the anti‐symmetric stretching vibration of C=N (Figure (b)). The red‐shift was attributed to the formation of a covalent bond (Cu‐N=C) …”