Structural Studies of the Ionic Liquid 1-Ethyl-3-methylimidazolium Tetrafluoroborate in Dichloromethane Using a Combined DFT-NMR Spectroscopic Approach

Abstract

DFT methods in combination with NMR spectroscopy are used to investigate possible variants of the molecular structure of the ion pairs of the ionic liquid (IL) 1-ethyl-3-methylimidazolium tetrafluoroborate, [EMIM][BF(4)], in dichloromethane. According to the computations of the chemical shifts, experimental NMR spectra can be rationalized by an equilibrium between ca. 70-80% of structures with the anion positioned near to the C2 atom of the imidazolium ring and ca. 20-30% of structures with the anion close to the C5 and/or C4 atoms. The content of the latter structures, according to the computed Gibbs free energies, does not exceed 10%. Both the computations and the experimental NMR data suggest that the ratio of the two above-mentioned types of structures of the imidazolium-based ILs is influenced by the concentration/polarity of their dichloromethane solutions.

An Internal Fluorescent Probe Based on Anthracene to Evaluate Cation–Anion Interactions in Imidazolium Salts

“…Combined, these data indicate the ion pairing persists in the CD 3 CN solvent, at least to some extent, and the main contact is between the anion and the ring proton at the 2-position as expected. [30,31] In addition, the IR spectra of salts 2-7 show characteristic absorptions of the imidazolium ring. It has been reported that ionic liquids tend to form cation-anion aggregates and these aggregates have been investigated by ESIMS.…”

Towards a Molecular Understanding of Cation–Anion Interactions—Probing the Electronic Structure of Imidazolium Ionic Liquids by NMR Spectroscopy, X‐ray Photoelectron Spectroscopy and Theoretical Calculations

“…[28,29,73,80] As the imidazolium cation is surrounded by anions, the 1 H NMR shift of H 2 , H 4 and H 5 is a result of an equilibrium of several ion-pair structures. [81] Increased p stacking-as predicted for an increasing concentration of ILs diluted in common solvents-generally leads to a shift towards lower ppm values. [28] In our case of pure ILs it appears unlikely that the very large and weakly coordinating [Pf 2 …”

The Effect of C4H and C5H on the Microstructure of Aqueous Solutions of 1‐Alkyl‐3‐methylimidazolium Tetrafluoroborate Ionic Liquids

“…Accordingt oE quations (1)-(6), the two interaction-energy parameters (g 21 4 -water mixtures are all shifted upfield upon dilution with water,w hereas those of OÀ Ha lways shift downfield, thus indicatingt hat the ILs dissociate gradually with increasing water contenta nd that the self-association interactions of water gradually replace the interactions between water andt he cations and anions,w hich is consistent with previousr eports. [12,19] According to the literature, [12,15,[40][41][42] ( Figure 1 (Figure 2). For the same system, the range of Dd for C2ÀHi ss lightly smaller than that for C4ÀHa nd C5ÀH; this indicates that the C2ÀH···Oi nteraction is stronger than the C4ÀH···O and C5ÀH···O interactions.…”

“…[26][27][28] It is well-known that the ability of the C2ÀHp roton of the cation of [C n mim]BF 4 to form hydrogen bonds with protona cceptors, such as BF 4 À and water,i sm uch greater than that of C4ÀHa nd C5ÀH. [15] Therefore, the hydrogen-bonding interaction of C2ÀHo f[ C n mim]BF 4 is often used to represent the intermolecular interaction of the cation. [14,19] For example, in our previousw ork, [14] the local compositiono f1 -ethyl-3-methylimidazolium tetrafluoroborate ([Emim]BF 4 )i n[ Emim]BF 4 -water mixture has been calculated by using the interaction-energy parameter obtained from the 1 HNMR chemical shifts of C2ÀH and hasb een compared with the bulk composition.…”

“…Actually,b esides the C2ÀHp roton, the C4ÀHa nd C5ÀHp rotons of [C n mim]BF 4 could also form hydrogen-bonding interactions with BF 4 À and water. [12,15] Unfortunately,u ntil now the influence of the interactions of the C4ÀHa nd C5ÀHp rotons on the microstructure and physicochemical properties was poorly understood.T herefore, in this work, the microstructures of [Emim]BF 4 -watera nd [Bmim]BF 4 -water mixtures were studied throught he concentration-dependent 1 HNMR chemicals hifts including three aromatic protons (C sp 2ÀH) of the ILs and ah ydroxy proton (OÀH) of water.M oreover,t he relative 1 HNMR chemicals hifts and local composition (LC) model [29] were employed to evaluate the effect of the interactions of C4ÀHa nd C5ÀHo nt he microstructure of the systems. Finally,t ov erify the influence of the interactions of C4ÀHa nd C5ÀHo nt he microstructure, as well as to better understand the relationship between the microstructure andp hysicochemical properties, the viscosity of the studied systems, which can be related to hydrogen-bonding interactions, [8] was predicted with the Eyring-UNIQUAC equationb yu sing the interaction-energy parameters obtained from the 1 HNMR chemical shifts of the studied C sp 2ÀHa nd OÀH, and the prediction results of viscosity were used to confirm the influence of C4ÀHa nd C5ÀHo nt he microstructure.…”