IR and UV Spectroscopy of Vapor-Phase Jet-Cooled Ionic Liquid [emim]+[Tf2N]−: Ion Pair Structure and Photodissociation Dynamics

Stepwise dissociation of ion pairs by water molecules: cation-dependent separation mechanisms between carboxylate and alkali-earth metal ions

Microhydrated H₂-tagged ion pairs (Ca²⁺, AcO^-)(H₂O)_n=0-8 and (Ba²⁺, AcO^-)(H₂O)_n=0-5 are investigated by IR photodissociation laser spectroscopy and DFT-D frequency calculations. The detailed picture of the first steps of ion dissociation reveals two mechanisms, where water molecules promote dissociation either directly or indirectly depending on the nature of the cation.

On the Behavior of Newly Synthesized Functionalized Imidazolium-Based Ionic Liquids for Highly Efficient Extraction and Separation of Pirimicarb from Orchard Real Wastewater

The presence of pirimicarb compounds as pollutants in orchard wastewater has sparked rising worries about their detrimental impacts on the ecosystem and human health, and their removal is critical for Pakistan’s aquatic environment. It not only contaminates fruit, but it also leaches into the soil and contaminates groundwater. However, there is little data on the effective removal of pirimicarb from orchard wastewater. The main purpose of this study is to create a novel family of functionalized imidazolium-based ionic liquids (ILs) using a simple chemical process, which will be utilized for the first time to extract pirimicarb from orchard wastewater in an efficient, cost-effective, and environmentally acceptable manner. FTIR, SEM, XRD, TGA, BET, and 1H NMR spectroscopy were used to characterize the functionalized samples. The impact of the IL substituent on the separation capacity was studied. In addition, the extraction and separation of pirimicarb from orchard wastewater were investigated under a variety of conditions (time, concentration, and temperature) in order to better understand the adsorption behaviors of distinct ILs in an aqueous solution. The adsorption equilibrium was reached in 30 minutes, and the maximum removal of pirimicarb was achieved utilizing the synthesized [C2im][C3H6NH2]Br-, according to the data. The pseudo-first-order model and the Langmuir model both suit well with the adsorption mechanism of pirimicarb with very good adsorptive capacities. Thermodynamic analyses indicated spontaneous, endothermic, and entropy-driven adsorption processes. The synthesized imidazolium-based ILs have good regeneration capability and recycling at least for six adsorption-desorption runs and have also been used to successfully detect pirimicarb orchard wastewater samples. The superior safety of the proposed method nominates it as a promising future strategy for pollution prevention. Consequently, this work has proven that the pirimicarb adsorption to various imidazolium-based ILs was dependent on the structures of the produced imidazolium-based ILs, which specifies its potential for practical applications in water pollutant removal and environmental remediation.

1-Alkyl-3-methylimidazolium cation binding preferences in hexafluorophosphate ionic liquid clusters determined using competitive TCID measurements and theoretical calculations

Ionic liquids (ILs) exhibit unique properties that have led to their development and widespread use for a variety of applications. Development efforts have generally focused on achieving desired macroscopic properties via tuning of the IL through variation of the cations and anions. Both the macroscopic and microscopic properties of an IL influence its tunability and thus feasibility of use for selected applications. Works geared toward a microscopic understanding of the nature and strength of the intrinsic cation-anion interactions of ILs have been limited to date. Specifically, the intrinsic strength of the cation-anion interactions in ILs is largely unknown. In previous work, we employed threshold collision-induced dissociation (TCID) approaches supported and enhanced by electronic structure calculations to determine the bond dissociation energies (BDEs) and characterize the nature of the cation-anion interactions in a series of four 2 : 1 clusters of 1-alkyl-3-methylimidazolium cations with the hexafluorophosphate anion, [2C_nmim:PF₆]⁺. To examine the effects of the 1-alkyl chain on the structure and energetics of binding, the cation was varied over the series: 1-ethyl-3-methylimidazolium, [C₂mim]⁺, 1-butyl-3-methylimidazolium, [C₄mim]⁺, 1-hexyl-3-methylimidazolium, [C₆mim]⁺, and 1-octyl-3-methylimidazolium, [C₈mim]⁺. The variation in the strength of binding among these [2C_nmim:PF₆]⁺ clusters was found to be similar in magnitude to the average experimental uncertainty in the measurements. To definitively establish an absolute order of binding among these [2C_nmim:PF₆]⁺ clusters, we extend this work again using TCID and electronic structure theory approaches to include competitive binding studies of three mixed 2 : 1 clusters of 1-alkyl-3-methylimidazolium cations and the hexafluorophosphate anion, [C_n-2mim:PF₆:C_nmim]⁺ for n = 4, 6, and 8. The absolute BDEs of these mixed [C_n-2mim:PF₆:C_nmim]⁺ clusters as well as the absolute difference in the strength of the intrinsic binding interactions as a function of the cation are determined with significantly improved precision. By combining the thermochemical results of the previous independent and present competitive measurements, the BDEs of the [2C_nmim:PF₆]⁺ clusters are both more accurately and more precisely determined. Comparisons are made to results for the analogous [2C_nmim:BF₄]⁺ and [C_n-2mim:BF₄:C_nmim]⁺ clusters previously examined to elucidate the effects of the [PF₆]^- and [BF₄]^- anions on the binding.

Nature and strength of intrinsic cation-anion interactions of 1-alkyl-3-methylimidazolium hexafluorophosphate clusters

Imidazolium-based cations and the hexafluorophosphate anion are among the most commonly used ionic liquids (ILs). Yet, the nature and strength of the intrinsic cation-anion interactions, and how they influence the macroscopic properties of these ILs are still not well understood. Threshold collision-induced dissociation is utilized to determine the bond dissociation energies (BDEs) of the 2 : 1 clusters of 1-alkyl-3-methylimidazolium cations and the hexafluorophosphate anion, [2Cnmim:PF6]+. The cation, [Cnmim]+, is varied across the series, 1-ethyl-3-methylimidazolium [C2mim]+, 1-butyl-3-methylimidazolium [C4mim]+, 1-hexyl-3-methylimidazolium [C6mim]+, 1-octyl-3-methylimidazolium [C8mim]+, to examine the structural and energetic effects of the size of the 1-alkyl substituent of the cation on the binding to [PF6]-. Complementary electronic structure methods are employed for the [Cnmim]+ cations, (Cnmim:PF6) ion pairs, and [2Cnmim:PF6]+ clusters to elucidate details of the cation-anion interactions and their impact on structure and energetics. Multiple levels of theory are benchmarked with the measured BDEs including B3LYP, B3LYP-GD3BJ, and M06-2X each with the 6-311+G(d,p) basis set for geometry optimizations and frequency analyses and the 6-311+G(2d,2p) basis set for energetic determinations. The modest structural variation among the [Cnmim]+ cations produces only minor structural changes and variation in the measured BDEs of the [2Cnmim:PF6]+ clusters. Present results are compared to those previously reported for the analogous 1-alkyl-3-methylimidazolium tetrafluoroborate IL clusters to compare the effects of these anions on the nature and strength of the intrinsic binding interactions.

Gas-Phase Binding Energies and Dissociation Dynamics of 1-Alkyl-3-Methylimidazolium Tetrafluoroborate Ionic Liquid Clusters

Ionic liquids (ILs) have become increasingly popular due to their useful and unique properties, yet there are still many unanswered questions regarding their fundamental interactions. In particular, details regarding the nature and strength of the intrinsic cation-anion interactions and how they influence the macroscopic properties of ILs are still largely unknown. Elucidating the molecular-level details of these interactions is essential to the development of better models for describing ILs and enabling the purposeful design of ILs with properties tailored for specific applications. Current uses of ILs are widespread and diverse and include applications for energy storage, electrochemistry, designer/green solvents, separations, and space propulsion. To advance the understanding of the energetics, conformations, and dynamics of gas-phase IL clustering relevant to space propulsion, threshold collision-induced dissociation approaches are used to measure the bond dissociation energies (BDEs) of the 2:1 clusters of 1-alkyl-3-methylimidazolium cations and tetrafluoroborate, [2C_nmim:BF₄]⁺. The cation, [C_nmim]⁺, is varied across the series, 1-ethyl-3-methylimidazolium [C₂mim]⁺, 1-butyl-3-methylimidazolium [C₄mim]⁺, 1-hexyl-3-methylimidazolium [C₆mim]⁺, and 1-octyl-3-methylimidazolium [C₈mim]⁺, to examine the structural and energetic effects of the size of the 1-alkyl substituent on binding. Complementary electronic structure calculations are performed to determine the structures and energetics of the [C_nmim]⁺ and [BF₄]^- ions and their binding preferences in the (C_nmim:BF₄) ion pairs and [2C_nmim:BF₄]⁺ clusters. Several levels of theory, B3LYP, B3LYP-GD3BJ, and M06-2X, using the 6-311+G(d,p) basis set for geometry optimizations and frequency analyses and the 6-311+G(2d,2p) basis set for energetics, are benchmarked to examine their abilities to properly describe the nature of the binding interactions and to reproduce the measured BDEs. The modest structural variation among these [C_nmim]⁺ cations produces only minor structural changes and variation in the measured BDEs of the [2C_nmim:BF₄]⁺ clusters. Present findings indicate that the dominant cation-anion interactions involve the 3-methylimidazolium moieties and that these clusters are sufficiently small that differences in packing effects associated with the variable length of the 1-alkyl substituents are not yet significant.

Photoinduced Intermolecular Electron Transfer in Gas Phase Ion Pairs of the 1-Ethyl-3-methylimidazolium Cation and the Bis(trifluoromethylsulfonyl)imide Anion

In this study, the UV photodissociation of gas phase ion pairs of the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [emim]⁺[tf2n]^-, is shown to proceed primarily through radical intermediates. [emim]⁺[tf2n]^- ion pairs have been shown previously to undergo two-photon-dependent dissociation, but the mechanisms of this have not been probed in detail. By employing a two-laser pump probe spectroscopy and time-dependent density functional theory (TD-DFT) calculations, we have illustrated that one of the major UV photodissociation pathways in [emim]⁺[tf2n]^- ion pairs is an intermolecular electron transfer wherein the anion transfers an electron to the cation resulting in two neutral open-shelled products. These products were observable for at least 1.6 μs post photodissociation, the experimental limit, via detection of the [emim]⁺ cation. This data demonstrates that the likely photoproducts of [emim]⁺[tf2n]^- UV photodissociation are two neutral species that separate spatially, demonstrated through lack of observed relaxation pathways such as electron recombination. TD-DFT and frontier molecular orbital analysis calculations at the MN15/6-311++G(d,p) level are employed to aid in identifying excited state characteristics and support the interpretations of the experimental data. The energetic onset of the intermolecular electron transfer is consistent with previously observed [emim]⁺[tf2n]^- absorption spectra in the bulk and gas phases. The similarities between bulk and gas phase UV spectra imply that this electron-transfer pathway may be a major photodissociation channel in both phases.

Electrospray ionization enters the final frontier: Mass spectrometry's role in understanding electrospray thrusters and their plumes

Electrospray thrusters using ionic liquid (IL)-based propellants are quickly gaining popularity in spacecraft design. Mass spectrometry is especially well-suited to provide important knowledge on the fundamentals of how these systems work and on evaluating their efficiencies and impacts, given that the operating principles of electrospray thrusters closely mimics the mass spectrometry experiment - in both ions are generated by electrospray and then enter a vacuum. Here, electrospray thruster technology and IL-based propellants are briefly introduced. This introduction is then followed by a discussion of mass spectrometry's current contribution to the study of IL-based electrospray thrusters - with a focus on electrospray, dissociation, and spectroscopy studies - and a brief discussion of areas ripe for immediate contributions from the mass spectrometry community.

The infrared spectra of protic ionic liquids: performances of different computational models to predict hydrogen bonds and conformer evolution

The temperature dependence of the far- and mid-infrared spectrum of two prototypical protic ionic liquids (PILs) sharing a common trialkylammonium cation, but having different anions, is investigated. The exploitation of both the FIR and MIR ranges provides complementary information about the microscopic configurations and the intermolecular interactions, which determine the structure and the properties of ILs. The analysis of the data collected for all the measured frequencies in a wide temperature range reveals several phase transitions and allows the evaluation of the conformer distribution in the different physical states. The difference in the average energy between the H-bonded configurations and the dispersion-governed ones was also determined for the two PILs. Moreover, a computational model for ionic couples based on the ωB97X-D functional and a polar solvent is here successfully exploited for the description of the hydrogen bonding between anion and cation. For the attribution of vibrational lines of the conformers of the cation, the picture based on single ion calculations at the B3LYP level is more valuable and provides better agreement with the experiments.

Ultraslow Relaxation in Aprotic Double Salt Ionic Liquids

A mixture of two pure ionic liquids (ILs) or double salt ILs (DSILs) can push the limits of ILs in terms of unraveling their unique physicochemical properties and potential in clean technology. While the correlated ion dynamics and heterogeneity in the bulk of pure ILs have been reported, such a phenomenon at longer timescales in DSILs has never been elucidated. Here, a combination of temperature-dependent polarized dynamic light scattering and rheological measurements has been employed to reveal the presence of structural and ultraslow relaxation in three DSILs, each containing a 1-ethyl-3-methylimidazolium cation and two different anions. The slow relaxation caused by Brownian diffusion of cluster-like arrangements occurs at a timescale of a few to several hundred milliseconds; both the relaxation processes, nevertheless, are Arrhenius in nature. Notably, slow relaxation in the DSILs is much different compared to that in the pure ILs. The decay of intensity correlation functions (ICFs) and average hydrodynamic correlation length of the clusters and their response to temperature markedly vary with the nature of the two anions present in the DSILs. Stretched exponential analyses of the ICFs disclose the cluster-to-cluster transfer of ionic species as well as percolation dynamics among clusters. The identity of anions also governs whether the DSILs follow or violate the Stokes-Einstein relationship or not.

Identification of ion pairs in solution by IR spectroscopy: crucial contributions of gas phase data and simulations

In a context where structure elucidation of ion pairs in solution remains a contemporary challenge, this work explores an original approach where accurate gas phase spectroscopic data are used to refine high level quantum chemistry calculations of ion pairs in solution, resulting in an unprecedented level of accuracy in vibrational frequency prediction. First, gas phase studies focus on a series of isolated contact ion pairs (M⁺, Ph-CH₂-COO^-, with M = Li, Na, K, Rb, Cs) for which conformer-selective IR spectra in the CO₂^- stretch region are recorded. These experiments reveal the interactions at play in isolated contact ion pairs, and provide vibrational frequencies enabling us to assess the accuracy of the theoretical approach used, i.e., mode-dependent scaled harmonic frequency calculations at the RI-B97-D3/dhf-TZVPP level. This level of calculation is then employed on large water clusters embedding either a free acetate ion or its contact or solvent-shared pairs with a sodium cation in order to simulate the individual vibrational spectra of these species in solution. This study shows that the stretching modes of carboxylate are sensitive to both solvent-shared and contact ion pair formation. FTIR spectra of solutions of increasing concentrations indeed reveal several spectral changes consistent with the presence of specific types of solvent-shared and contact ion pairs. By providing relevant guidelines for the interpretation of solution phase IR spectra, this work illustrates the potential of the approach for the elucidation of supramolecular structures in electrolyte solutions.

Thermal stability of dialkylimidazolium tetrafluoroborate and hexafluorophosphate ionic liquids: ex situ bulk heating to complement in situ mass spectrometry

Thermal decomposition (TD) products of the ionic liquids (ILs) [CnC1Im][BF4] and [CnC1Im][PF6] ([CnC1Im]+ = 1-alkyl-3-methylimidazolium, [BF4]- = tetrafluoroborate, and [PF6]- = hexafluorophosphate) were prepared, ex situ, by bulk heating experiments in a bespoke setup. The respective products, CnC1(C3N2H2)BF3 and CnC1(C3N2H2)PF5 (1-alkyl-3-methylimidazolium-2-trifluoroborate and 1-alkyl-3-methylimidazolium-2-pentafluorophosphate), were then vaporized and analyzed by direct insertion mass spectrometry (DIMS) in order to identify their characteristic MS signals. During IL DIMS experiments we were subsequently able, in situ, to identify and monitor signals due to both IL vaporization and IL thermal decomposition. These decomposition products have not been observed in situ during previous analytical vaporization studies of similar ILs. The ex situ preparation of TD products is therefore perfectly complimentary to in situ thermal stability measurements. Experimental parameters such as sample surface area to volume ratios are consequently very important for ILs that show competitive vaporization and thermal decomposition. We have explained these experimental factors in terms of Langmuir evaporation and Knudsen effusion-like conditions, allowing us to draw together observations from previous studies to make sense of the literature on IL thermal stability. Hence, the design of experimental setups are crucial and previously overlooked experimental factors.

Structural Motifs in Cold Ternary Ion Complexes of Hydroxyl-Functionalized Ionic Liquids: Isolating the Role of Cation-Cation Interactions

We address the competition between intermolecular forces underlying the recent observation that ionic liquids (ILs) with a hydroxyl-functionalized cation can form domains with attractive interactions between the nominally repulsive positively charged constituents. Here we show that this behavior is present even in the isolated ternary (HEMIm⁺)₂NTf₂^- complex (HEMIm⁺ = 1-(2-hydroxyethyl)-3-methylimidazolium) cooled to about 35 K in a photodissociation mass spectrometer. Of the three isomers isolated by double resonance techniques, one is identified to exhibit direct contact between the cations. This linkage involves a cooperative H-bond wherein the OH group on one cation binds to the OH group on the other, which then attaches to the basic N atom of the anion. Formation of this motif comes at the expense of the usually dominant interaction of the acidic C₍₂₎H group on the Im ring with molecular anions, as evidenced by isomer-dependent shifts in the C₍₂₎H vibrational fundamentals.

Theoretical and Experimental Insights into the Dissociation of 2-Hydroxyethylhydrazinium Nitrate Clusters Formed via Electrospray

Ionic liquids are used for myriad applications, including as catalysts, solvents, and propellants. Specifically, 2-hydroxyethylhydrazinium nitrate (HEHN) has been developed as a chemical propellant for space applications. The gas-phase behavior of HEHN ions and clusters is important in understanding its potential as an electrospray thruster propellant. Here, the unimolecular dissociation pathways of two clusters are experimentally observed, and theoretical modeling of hydrogen bonding and dissociation pathways is used to help rationalize those observations. The cation/deprotonated cation cluster [HEH₂ - H]⁺, which is observed from electrospray ionization, is calculated to be considerably more stable than the complementary cation/protonated anion adduct, [HEH + HNO₃]⁺, which is not observed experimentally. Upon collisional activation, a larger cluster [(HEHN)₂HEH]⁺ undergoes dissociation via loss of nitric acid at lower collision energies, as predicted theoretically. At higher collision energies, additional primary and secondary loss pathways open, including deprotonated cation loss, ion-pair loss, and double-nitric-acid loss. Taken together, these experimental and theoretical results contribute to a foundational understanding of the dissociation of protic ionic liquid clusters in the gas phase.

Valence electronic structure of [EMIM][BF4] ionic liquid: photoemission and DFT+D study

The ultraviolet photoelectron spectrum (UPS) of the [EMIM][BF₄] ionic liquid was recorded and analyzed. Together with the gas-phase UPS spectrum of the [EMIM][BF₄] vapor and ab initio calculation methods, detailed insight into the electronic structure of this simple ionic liquid is possible. The low binding energy tail in the UPS spectrum is about 7.4 eV, in agreement with previous estimations of the HOMO–LUMO gap of the [EMIM][BF₄] ion-pair. The bulk ab initio calculations are able to explain most of the features in the spectrum. However, DFT consistently lacks accuracy in the description of the top of the valence band. The dispersion corrected PBE calculation (PBE-D3) did offer very good agreement with the experimental structure, but the recently-developed vdW-DF functionals C09, optPBE, optB88 and CX were found to offer the best agreement in terms of the electronic structure.

The ultraviolet photoelectron spectrum (UPS) of the [EMIM][BF₄] ionic liquid was recorded and compared to previously measured vapor phase UPS spectrum.

Structure and lifetimes in ionic liquids and their mixtures

With the aid of molecular dynamics simulations, we study the structure and dynamics of different ionic liquid systems.

Quantifying intermolecular interactions of ionic liquids using cohesive energy densities

For ionic liquids (ILs), both the large number of possible cation + anion combinations and their ionic nature provide a unique challenge for understanding intermolecular interactions. Cohesive energy density, ced, is used to quantify the strength of intermolecular interactions for molecular liquids, and is determined using the enthalpy of vaporization. A critical analysis of the experimental challenges and data to obtain ced for ILs is provided. For ILs there are two methods to judge the strength of intermolecular interactions, due to the presence of multiple constituents in the vapour phase of ILs. Firstly, ced_IP, where the ionic vapour constituent is neutral ion pairs, the major constituent of the IL vapour. Secondly, ced_C+A, where the ionic vapour constituents are isolated ions. A ced_IP dataset is presented for 64 ILs. For the first time an experimental ced_C+A, a measure of the strength of the total intermolecular interaction for an IL, is presented. ced_C+A is significantly larger for ILs than ced for most molecular liquids, reflecting the need to break all of the relatively strong electrostatic interactions present in ILs. However, the van der Waals interactions contribute significantly to IL volatility due to the very strong electrostatic interaction in the neutral ion pair ionic vapour. An excellent linear correlation is found between ced_IP and the inverse of the molecular volume. A good linear correlation is found between IL ced_IP and IL Gordon parameter (which are dependent primarily on surface tension). ced values obtained through indirect methods gave similar magnitude values to ced_IP. These findings show that ced_IP is very important for understanding IL intermolecular interactions, in spite of ced_IP not being a measure of the total intermolecular interactions of an IL. In the outlook section, remaining challenges for understanding IL intermolecular interactions are outlined.

Quantum Chemical Modeling of Hydrogen Bonding in Ionic Liquids

Hydrogen bonding (H-bonding) is an important and very general phenomenon. H-bonding is part of the basis of life in DNA, key in controlling the properties of water and ice, and critical to modern applications such as crystal engineering, catalysis applications, pharmaceutical and agrochemical development. H-bonding also plays a significant role for many ionic liquids (IL), determining the secondary structuring and affecting key physical parameters. ILs exhibit a particularly diverse and wide range of traditional as well as non-standard forms of H-bonding, in particular the doubly ionic H-bond is important. Understanding the fundamental nature of the H-bonds that form within ILs is critical, and one way of accessing this information, that cannot be recovered by any other computational method, is through quantum chemical electronic structure calculations. However, an appropriate method and basis set must be employed, and a robust procedure for determining key structures is essential. Modern generalised solvation models have recently been extended to ILs, bringing both advantages and disadvantages. QC can provide a range of information on geometry, IR and Raman spectra, NMR spectra and at a more fundamental level through analysis of the electronic structure.

Vibrational Spectroscopy of Ionic Liquids

Vibrational spectroscopy has continued use as a powerful tool to characterize ionic liquids since the literature on room temperature molten salts experienced the rapid increase in number of publications in the 1990's. In the past years, infrared (IR) and Raman spectroscopies have provided insights on ionic interactions and the resulting liquid structure in ionic liquids. A large body of information is now available concerning vibrational spectra of ionic liquids made of many different combinations of anions and cations, but reviews on this literature are scarce. This review is an attempt at filling this gap. Some basic care needed while recording IR or Raman spectra of ionic liquids is explained. We have reviewed the conceptual basis of theoretical frameworks which have been used to interpret vibrational spectra of ionic liquids, helping the reader to distinguish the scope of application of different methods of calculation. Vibrational frequencies observed in IR and Raman spectra of ionic liquids based on different anions and cations are discussed and eventual disagreements between different sources are critically reviewed. The aim is that the reader can use this information while assigning vibrational spectra of an ionic liquid containing another particular combination of anions and cations. Different applications of IR and Raman spectroscopies are given for both pure ionic liquids and solutions. Further issues addressed in this review are the intermolecular vibrations that are more directly probed by the low-frequency range of IR and Raman spectra and the applications of vibrational spectroscopy in studying phase transitions of ionic liquids.

Gas-Phase Spectroscopic Signatures of Carboxylate-Li(+) Contact Ion Pairs: New Benchmarks For Characterizing Ion Pairing in Solution

The coexistence of several types of ion pairs in solution together with their elusive nature hampers their experimental characterization, which relies in practice on theoretical models resorting to numerous approximations. In this context, a series of isolated contact ion pairs between a lithium cation and phenyl-tagged carboxylate anions of various lengths (Ph-(CH2)n-COO(-), n = 1-3) has been investigated in a conformer-selective manner by IR and UV laser spectroscopy, in conjunction with quantum chemistry calculations. The typical gas-phase IR signature of the bidentate structure formed between the carboxylate moiety and Li(+) has thus been obtained in the CO2(-) stretch region. In addition to the cation-anion interaction, a cation-π interaction occurs simultaneously in the largest system investigated (n = 3). The resulting distorted ion pair structure has been evidenced from both the IR signature of the CO2(-) stretches and the unique vibrationally resolved UV spectroscopy of a phenyl ring interacting with a cation. Such specific spectroscopic signatures of contact ion pairs provide experimental benchmarks, alternative to theoretical predictions, that can assist the assignment of vibrational spectra in solution.

Interaction between ionic liquid cation and water: infrared predissociation study of [bmim]+·(H2O)n clusters

The infrared predissociation spectra of [bmim]⁺·(H₂O)_n, n = 1–8, in the 2800–3800 cm⁻¹ region are presented and analyzed with the help of electronic structure calculations.

Identification of multiple conformers of the ionic liquid [emim][tf2n] in the gas phase using IR/UV action spectroscopy

We have identified three families of conformers in gas phase ion pairs of [emim][tf2n] using IR/UV spectroscopy.

Triphilic Ionic-Liquid Mixtures: Fluorinated and Non-fluorinated Aprotic Ionic-Liquid Mixtures

We present here the possibility of forming triphilic mixtures from alkyl- and fluoroalkylimidazolium ionic liquids, thus, macroscopically homogeneous mixtures for which instead of the often observed two domains-polar and nonpolar-three stable microphases are present: polar, lipophilic, and fluorous ones. The fluorinated side chains of the cations indeed self-associate and form domains that are segregated from those of the polar and alkyl domains. To enable miscibility, despite the generally preferred macroscopic separation between fluorous and alkyl moieties, the importance of strong hydrogen bonding is shown. As the long-range structure in the alkyl and fluoroalkyl domains is dependent on the composition of the liquid, we propose that the heterogeneous, triphilic structure can be easily tuned by the molar ratio of the components. We believe that further development may allow the design of switchable, smart liquids that change their properties in a predictable way according to their composition or even their environment.

Understanding the ionic liquid [NC4111][NTf2] from individual building blocks: an IR-spectroscopic study

This study explores the interactions underlying the IR spectra of the ionic liquid [NC4111][NTf2] and its deuterated isotopomer [d9-NC4111][NTf2] by first isolating the spectra of charged ionic building blocks using mass-selective CIVP spectroscopy and then following the evolution of these bands upon sequential assembly of the ionic constituents. The spectra of the (1,1) and (2,2) neutral ion pairs are recorded using superfluid helium droplets as well as a solid neon matrix, while those of the larger charged aggregates are again obtained with CIVP. In general, the cluster spectra are similar to that of the bulk, with the (2,2) system displaying the closest resemblance. Analysis of the polarization-dependent band intensities of the neutral ion pairs in liquid droplets as a function of external electric field yields dipole moments of the neutral aggregates. This information allows a coarse assessment of the packing structure of the neutral pairs to be antiparallel at 0.37 K, in contrast to the parallel arrangement found for the assembly of small, high-dipole neutral molecules with large rotational constants (e.g., HCN). The role of an extra anion or cation attached to both the (1,1) and the (2,2) ion pairs to form the charged clusters is discussed in the context of an additional remote, more unfavorable binding site intrinsic to the nature of the charged IL clusters and as such not anticipated in the bulk phase. Whereas for the anion itself only the lowest energy trans conformer was observed, the higher clusters showed an additional population of the cis conformer. The interactions are found to be consistent with a minimal role of hydrogen bonding.

Non-covalent interactions in ionic liquid ion pairs and ion pair dimers: a quantum chemical calculation analysis

Weak non-covalent interactions were studied by means of QTAIM and NCI approaches in ion pairs and ion pair dimers of 1-alkyl-3-methylimidazolium cations coupled with perfluorinated anions.

Free electrons and ionic liquids: study of excited states by means of electron-energy loss spectroscopy and the density functional theory multireference configuration interaction method

Collisions of slow electrons with ionic liquids and DFT/MRCI calculations reveal triplet states and interesting physics at low energies.

Inter- and intramolecular interactions in imidazolium protic ionic liquids

The interactions of alkyl substituted imidazolium bis(trifluoromethanolsulfonyl)imide protic ionic liquids (PILs) HCnImNTf2 (n = 0-12) were studied using vibrational spectroscopy (FT-IR/ATR and FT-Raman) and differential scanning calorimetry (DSC). The effect of alkyl substituent length (n = 0-12) and temperature on the relative magnitude of the different interactions is elucidated. For short carbon alkyl chains (n < 3), the PIL structure is affected from intramolecular interaction caused from the induction effect (+I) due to the chain substituent of the imidazolium ring, while for PILs with n > 3 the van der Waals forces between the chains and π-π interaction between neighboring imidazolium rings become important. The tendency of reducing the melting point and increasing glass transition values with the lengthening of the alkyl chain was also noticed as a result the increasing contribution of the van der Waals forces to the overall interactions. Finally, we also show that the conformational isomerism of the anion (expressed by ΔHeq) is a good indicator of the relative magnitude of the interactions. When Coulombic interactions are predominant, the trans conformer is the most probable, while when other type of interactions (HB, vdW, etc.) become important the cis conformer is favored.

Vaporization of the prototypical ionic liquid BMImNTf₂ under equilibrium conditions: a multitechnique study

The vaporization behaviour and thermodynamics of the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethyl)sulfonylimide (BMImNTf2) were studied by combining the Knudsen Effusion Mass Loss (KEML) and Knudsen Effusion Mass Spectrometry (KEMS) techniques. KEML studies were carried out in a large temperature range (398-567) K by using effusion orifices with 0.3, 1, and 3 mm diameters. The vapor pressures so measured revealed no kinetically hindered vaporization effects and provided second-law vaporization enthalpies at the mean experimental temperatures in close agreement with literature. By exploiting the large temperature range covered, the heat capacity change associated with vaporization was estimated, resulting in a value of -66.8 J K(-1) mol(-1), much lower than that predicted from calorimetric measurements on the liquid phase and theoretical calculations on the gas phase. The conversion of the high temperature vaporization enthalpy to 298 K was discussed and the value Δ(l)(g)H(m)(298 K) = (128.6 ± 1.3) kJ mol(-1) assessed on the basis of data from literature and present work. Vapor pressure data were also processed by the third-law procedure using different estimations for the auxiliary thermal functions, and a Δ(l)(g)H(m)(298 K) consistent with the assessed value was obtained, although the overall agreement is sensitive to the accuracy of heat capacity data. KEMS measurements were carried out in the lower temperature range (393-467) K and showed that the largely prevailing ion species is BMIm(+), supporting the common view of BMImNTf2 vaporizing as individual, neutral ion pairs also under equilibrium conditions. By monitoring the mass spectrometric signal of this ion as a function of temperature, a second-law Δ(l)(g)H(m)(298 K) of 129.4 ± 7.3 kJ mol(-1) was obtained, well consistent with KEML and literature results. Finally, by combining KEML and KEMS measurements, the electron impact ionization cross section of BMIm(+) was estimated.