Revisited vibrational assignments of imidazolium‐based ionic liquids

Abstract

Imidazolium-based ionic liquids (ILs) involving anions of variable coordinating strength have been investigated using infrared (IR) andRaman spectroscopies, density functional theory (DFT) calculations and selective deuteration of the imidazolium CH groups. Particular emphasis has been placed on the vibrational assignments of the anion and cation internal vibrations, a prerequisite before any interpretation of spectral changes due to ion-ion interactions in these unconventional liquids. The vibrations of highly symmetric and weakly coordinating anions, such as PF 6 − , have unperturbed wavenumbers, but unexpected IR or Raman activity for some modes, showing that the anion is subjected to an anisotropic electric field. The stretching as well as the in-plane and out-of-plane bending modes of the imidazolium CH groups are anharmonic. They give broad bands that reflect a large distribution of interactions with the surrounding anions. All the bending modes are mixed with ring vibrations and the stretching modes are complicated by Fermi resonance interactions with overtones and combination of in-plane ring modes. However, the stretching vibration of the quasi-diatomic C (2) -D bond appears to be a good spectroscopic probe of the increasing cation-anion interactions when the coordinating strength of the anion increases. The broad absorption observed in the far IR with weakly coordinating anions remains practically unchanged when the acidic C (2) -H imidazolium bond is methylated and even when the imidazolium cation is substituted by tetra-alkyl ammonium or pyrrolidinium cations. It is concluded that this absorption is a general feature of any IL, coming from the relative translational and librational motions of the ions without needing to invoke C (2) -H anion hydrogen bonds.

Effects of C(2) Methylation on Thermal Behavior and Interionic Interactions in Imidazolium-Based Ionic Liquids with Highly Symmetric Anions

“…In contrast, the C(2)-H stretches appear at 2852, 2849, and 3004 cm −1 , indicating an involvement in strong hydrogen bonding interactions. For completeness, we note that assignment of the CH stretching bands of imidazolium-based ILs is a subject of ongoing debate (see, for example, [3,22,[33][34][35]) and a definitive conclusion has not yet been reached. The assignment used in the present study is consistent with our previous work; all IL data we have recorded so far point in this direction.…”

“…On the other hand, interactions with highly symmetrical anions have been studied less. Examples include the halogenides [17][18][19], tetrafluoroborate [20,21], and hexafluorophosphate [15,22]. However, systematic studies with varied anions are rare [23] or have had a different focus, e.g., on the rotational isomerism of the cation [24].…”

Interactions in 1-ethyl-3-methyl imidazolium tetracyanoborate ion pair: Spectroscopic and density functional study

“…31 Instead, the γ (CH) mode in the range of 800-850 cm −1 has been suggested to be used as direct spectroscopic probes of the cation-anion interactions since it changes noticeably in different ILs. 32 which is from planar conformer. 33 All three peaks (598 cm −1 , 622 cm −1 , and 704 cm −1 ) are …”

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

“…(7)],w hich has been successfully used to correlate the viscosity of IL-solvent mixtures, [39] was employed to predict the viscosity of the studied systems. It can be found from Equation (7), that there are two unknown parameters in the Eyring-UNIQUAC equation, namely,( g 21 -g 11 ) and (g 12 -g 22 ). According to the description in Section3.2, (g 21 -g 11 )a nd (g 12 -g 22 )o ft he studied systems can be obtained from 1 …”

“…[33][34][35][36] There are two adjustable parameters in the Equation (7), namely (g 21 -g 11 ) and (g 12 -g 22 ), and other physicalq uantities can be obtained from the literature or calculations. 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.…”