Thermal Decomposition Mechanisms of Alkylimidazolium Ionic Liquids with Cyano-Functionalized Anions

Structural Properties of HEHN- and HAN-Based Ionic Liquid Mixtures: A Polarizable Molecular Dynamics Study

Molecular dynamics simulations of binary mixtures comprising 2-hydroxyethylhydrazinium nitrate (HEHN) and hydroxylammonium nitrate (HAN) were conducted using the polarizable APPLE&P force field to investigate fundamental properties of multimode propulsion (MMP) propellants. Calculated densities as a function of temperature were in good agreement with experiments and similar simulations. The structural properties of neat HEHN and HAN-HEHN provided insights into their inherent, protic nature. Radial distribution functions (RDFs) identified key hydrogen bonding sites located at N-H···O and O-H···O within a first solvation shell of approximately 2 Å. Angular distribution functions further affirmed the relatively strong nature of the hydrogen bonds with nearly linear directionality. The increased hydroxylammonium cation (HA+) mole fraction shows the influence of competitively strong hydrogen bonds on the overall hydrogen bond network. Dominant spatial motifs via three-dimensional distribution functions along with nearly nanosecond-long hydrogen bond lifetimes highlight the local bonding environment that may precede proton transfer reactions.

Unraveling the initial steps of the ignition chemistry of the hypergolic ionic liquid 1-ethyl-3-methylimidazolium cyanoborohydride ([EMIM][CBH]) with nitric acid (HNO3) exploiting chirped pulse triggered droplet merging

The composition of the products and the mechanistic routes for the reaction of the hypergolic ionic liquid (HIL) 1-ethyl-3-methylimidazolium cyanoborohydride ([EMIM][CBH]) and nitric acid (HNO₃) at various concentrations from 10% to 70% were explored using a contactless single droplet merging within an ultrasonic levitation setup in an inert atmosphere of argon to reveal the initial steps that cause hypergolicity. The reactions were initiated through controlled droplet-merging manipulation triggered by a frequency chirp pulse amplitude modulation. Utilizing the high-speed optical and infrared cameras surrounding the levitation process chamber, intriguing visual images were unveiled: (i) extensive gas release and (ii) temperature rises of up to 435 K in the merged droplets. The gas development was validated qualitatively and quantitatively with Fourier Transform Infrared Spectroscopy (FTIR) indicating the major gas-phase products to be hydrogen cyanide (HCN) and nitrous oxide (N₂O). The merged droplet was also probed by pulsed Raman spectroscopy which deciphered features for key functional groups of the reaction products and intermediates (-BH, -BH₂, -BH₃, -NCO); reaction kinetics revealed that the reaction was initiated by the interaction of the [CBH]^- anion of the HIL with the oxidizer (HNO₃) through proton transfer. Computations indicate the formation of a van-der-Waals complex between the [CBH]^- anion and HNO₃ initially, followed by proton transfer from the acid to the anion and subsequent extensive isomerization; these rearrangements were found to be essential for the formation of HCN and N₂O. The exoergicity observed during the merging process provides a molar enthalpy change up to 10 kJ mol^-1 to the system, which could be sufficient for a significant fraction of the reactants of about 11% to overcome the reaction barriers in the individual steps of the computationally determined minimum energy pathways.

Thermal and Catalytic Decomposition of 2-Hydroxyethylhydrazine and 2-Hydroxyethylhydrazinium Nitrate Ionic Liquid

To develop chemical kinetics models for the combustion of ionic liquid-based monopropellants, identification of the elementary steps in the thermal and catalytic decomposition of components such as 2-hydroxyethylhydrazinium nitrate (HEHN) is needed but is currently not well understood. The first decomposition step in protic ionic liquids such as HEHN is typically the proton transfer from the cation to the anion, resulting in the formation of 2-hydroxyethylhydrazine (HEH) and HNO₃. In the first part of this investigation, the high-temperature thermal decomposition of HEH is probed with flash pyrolysis (<1400 K) and vacuum ultraviolet (10.45 eV) photoionization time-of-flight mass spectrometry (VUV-PI-TOFMS). Next, the investigation into the thermal and catalytic decomposition of HEHN includes two mass spectrometric techniques: (1) tunable VUV-PI-TOFMS (7.4-15 eV) and (2) ambient ionization mass spectrometry utilizing both plasma and laser ionization techniques whereby HEHN is introduced onto a heated inert or iridium catalytic surface and the products are probed. The products can be identified by their masses, their ionization energies, and their collision-induced fragmentation patterns. Formation of product species indicates that catalytic surface recombination is an important reaction process in the decomposition mechanism of HEHN. The products and their possible elementary reaction mechanisms are discussed.

The Composition of Saturated Vapor over 1-Butyl-3-methylimidazolium Tetrafluoroborate Ionic Liquid: A Multi-Technique Study of the Vaporization Process

A multi-technique approach based on Knudsen effusion mass spectrometry, gas phase chromatography, mass spectrometry, NMR and IR spectroscopy, thermal analysis, and quantum-chemical calculations was used to study the evaporation of 1-butyl-3-methylimidazolium tetrafluoroborate (BMImBF₄). The saturated vapor over BMImBF₄ was shown to have a complex composition which consisted of the neutral ion pairs (NIPs) [BMIm⁺][BF₄^-], imidazole-2-ylidene C₈N₂H₁₄BF₃, 1-methylimidazole C₄N₂H₆, 1-butene C₄H₈, hydrogen fluoride HF, and boron trifluoride BF₃. The vapor composition strongly depends on the evaporation conditions, shifting from congruent evaporation in the form of NIP under Langmuir conditions (open surface) to primary evaporation in the form of decomposition products under equilibrium conditions (Knudsen cell). Decomposition into imidazole-2-ylidene and HF is preferred. The vapor composition of BMImBF₄ is temperature-depended as well: the fraction ratio of [BMIm⁺][BF₄^-] NIPs to decomposition products decreased by about a factor of three in the temperature range from 450 K to 510 K.

Experimental measurement and prediction of ionic liquid ionisation energies

Ionic liquid (IL) valence electronic structure provides key descriptors for understanding and predicting IL properties. The ionisation energies of 60 ILs are measured and the most readily ionised valence state of each IL (the highest occupied molecular orbital, HOMO) is identified using a combination of X-ray photoelectron spectroscopy (XPS) and synchrotron resonant XPS. A structurally diverse range of cations and anions were studied. The cation gave rise to the HOMO for nine of the 60 ILs presented here, meaning it is energetically more favourable to remove an electron from the cation than the anion. The influence of the cation on the anion electronic structure (and vice versa) were established; the electrostatic effects are well understood and demonstrated to be consistently predictable. We used this knowledge to make predictions of both ionisation energy and HOMO identity for a further 516 ILs, providing a very valuable dataset for benchmarking electronic structure calculations and enabling the development of models linking experimental valence electronic structure descriptors to other IL properties, e.g. electrochemical stability. Furthermore, we provide design rules for the prediction of the electronic structure of ILs.

Physics-based, neural network force fields for reactive molecular dynamics: Investigation of carbene formation from [EMIM+][OAc-]

Reactive molecular dynamics simulations enable a detailed understanding of solvent effects on chemical reaction mechanisms and reaction rates. While classical molecular dynamics using reactive force fields allows significantly longer simulation time scales and larger system sizes compared with ab initio molecular dynamics, constructing reactive force fields is a difficult and complex task. In this work, we describe a general approach following the empirical valence bond framework for constructing ab initio reactive force fields for condensed phase simulations by combining physics-based methods with neural networks (PB/NNs). The physics-based terms ensure the correct asymptotic behavior of electrostatic, polarization, and dispersion interactions and are compatible with existing solvent force fields. NNs are utilized for a versatile description of short-range orbital interactions within the transition state region and accurate rendering of vibrational motion of the reacting complex. We demonstrate our methodology for a simple deprotonation reaction of the 1-ethyl-3-methylimidazolium cation with acetate to form 1-ethyl-3-methylimidazol-2-ylidene and acetic acid. Our PB/NN force field exhibits ∼1 kJ mol^-1 mean absolute error accuracy within the transition state region for the gas-phase complex. To characterize the solvent modulation of the reaction profile, we compute potentials of mean force for the gas-phase reaction as well as the reaction within a four-ion cluster and benchmark against ab initio molecular dynamics simulations. We find that the surrounding ionic environment significantly destabilizes the formation of the carbene product, and we show that this effect is accurately captured by the reactive force field. By construction, the PB/NN potential may be directly employed for simulations of other solvents/chemical environments without additional parameterization.

Molecular Dynamics Simulations and Product Vibrational Spectral Analysis for the Reactions of NO2 with 1-Ethyl-3-methylimidazolium Dicyanamide (EMIM+DCA-), 1-Butyl-3-methylimidazolium Dicyanamide (BMIM+DCA-), and 1-Allyl-3-methylimidazolium Dicyanamide (AMIM+DCA-)

Direct dynamics trajectory simulations were carried out for the NO₂ oxidation of 1-ethyl-3-methylimidazolium dicyanamide (EMIM⁺DCA^-), which were aimed at probing the nature of the primary and secondary reactions in the system. Guided by trajectory results, reaction coordinates and potential energy diagrams were mapped out for NO₂ with EMIM⁺DCA^-, as well as with its analogues 1-butyl-3-methylimidazolium dicyanamide (BMIM⁺DCA^-) and 1-allyl-3-methylimidazolium dicyanamide (AMIM⁺DCA^-). Reactions of the dialkylimidazolium-dicyanamide (DCA) ionic liquids (ILs) are all initiated by proton transfer and/or alkyl abstraction between 1,3-dialkylimidazolium cations and DCA^- anion, of which two exoergic pathways are particularly relevant to their oxidation activities. One pathway is the transfer of a H^β-proton from the ethyl, butyl, or allyl group of the dialkylimidazolium cation to DCA^- that results in the concomitant elimination of the corresponding alkyl as a neutral alkene, and the other pathway is the alkyl abstraction by DCA^- via a second order nucleophilic substitution (SN₂) mechanism. The intra-ion-pair reaction products, including [dialkylimidazolium⁺ - H_C2⁺], alkylimidazole, alkene, alkyl-DCA, HDCA, and DCA^-, react with NO₂ and favor the formation of nitrite (-ONO) complexes over nitro (-NO₂) complexes, albeit the two complex structures have similar formation energies. The exoergic intra-ion-pair reactions in the dialkylimidazolium-DCA ILs account for their significantly higher oxidation activities over the previously reported 1-methyl-4-amino-1,2,4-triazolium dicyanamide [Liu, J.; J. Phys. Chem. B 2019, 123, 2956-2970] and for the relatively higher reactivity of BMIM⁺DCA^- vs AMIM⁺DCA^- as BMIM⁺ has a higher reaction path degeneracy for intra-ion-pair H^β-proton transfer and its H^β-transfer is more energetically favorable. To validate and directly compare our computational results with spectral measurements in the ILs, infrared and Raman spectra of BMIM⁺DCA^- and AMIM⁺DCA^- and their products with NO₂ were calculated using an ionic liquid solvation model. The simulated spectra reproduced all of the vibrational frequencies detected in the reactions of BMIM⁺DCA^- and AMIM⁺DCA^- IL droplets with NO₂ (as reported by Brotton et al. [ J. Phys. Chem. A 2018, 122, 7351-7377] and Lucas et al. [ J. Phys. Chem. A 2019, 123, 400-416]).

Thermal Decomposition and Hypergolic Reaction of a Dicyanoborohydride Ionic Liquid

In this study, in situ infrared spectroscopy techniques and thermogravimetric analysis coupled with mass spectrometry (TGA-MS) are employed to characterize the reactivity of the ionic liquid, 1-butyl-3-methylimidazolium dicyanoborohydride (BMIM⁺DCBH^-), in comparison to the well-characterized 1-butyl-3-methylimidazolium dicyanamide (BMIM⁺DCA^-) ionic liquid. TGA measurements determined the enthalpy of vaporization (ΔH_vap) to be 112.7 ± 12.3 kJ/mol at 298 K. A rapid scan Fourier transform infrared spectrometer was used to obtain vibrational information useful in tracking the appearance and disappearance of species in the hypergolic reactions of BMIM⁺DCBH^- and BMIM⁺DCA^- with white fuming nitric acid (WFNA) and in the thermal decomposition of these energetic ionic liquids. Attenuated total reflectance measurements recorded the infrared spectra of the reactant sample (BMIM⁺DCBH^-) and the liquid reaction products after reacting with WFNA. Computational chemistry efforts, aided by the experimental results, were used to propose key reaction pathways leading to the hypergolic ignition of BMIM⁺DCBH^- + WFNA. Experimental results indicate that the hypergolic reaction of BMIM⁺DCBH^- with WFNA generates both common and unique intermediates as compared to previous BMIM⁺DCA^- + WFNA investigations: nitrous oxide was generated during both hypergolic reactions indicating that it may play a crucial role in the hypergolic ignition process, NO₂ was generated in significantly higher concentrations for BMIM⁺DCBH^- than for BMIM⁺DCA^-, CO₂ was only generated for BMIM⁺DCA^-, and HCN was only generated during thermal decomposition and hypergolic ignition of BMIM⁺DCBH^-.

Greener synthesis of dimethyl carbonate from carbon dioxide and methanol using a tunable ionic liquid catalyst

Several types of ionic liquids (ILs) performance towards dimethyl carbonate (DMC) synthesis using cheap reactant (methanol) and waste CO2 which is abundantly available in the environment are discussed. We synthesized ILs with cheap raw materials such as ethylene glycol. The main aim of this study is to synthesize efficient catalysts for the production of profitable fuel additives. ILs show high thermal stability, less viscosity, and low vapor pressure. In addition, some ILs have high CO2 absorption capacity due to moderate acid-base properties. These ILs reversibly capture more CO2 which is more efficient towards mass transport of methanol at optimum reaction conditions which enhance the DMC yield. This catalytic system is easily reusable for several reactions without decreased performance under the same reaction conditions. These reaction conditions had an effect on the synthesis of DMC. Temperature, pressure, IL loading, and IL/DMAP ratio were fine tuned. We propose a mechanism which the reaction may follow. The synthesized ILs required moderate reaction conditions and reduce waste gases (CO2) from the environments as they have high CO2 absorption capacity compared to the metal oxide catalyst. Therefore, this catalytic system helps and gives new direction to synthesize new catalyst for other application.

Computational Study of the Reaction of 1-Methyl-4-amino-1,2,4-triazolium Dicyanamide with NO2: From Reaction Dynamics to Potential Surfaces, Kinetics and Spectroscopy

Direct dynamics trajectories were calculated at the B3LYP/6-31G(d) level of theory in an attempt to understand the reaction of 1-methyl-4-amino-1,2,4-triazolium dicyanamide (MAT⁺DCA^-) with NO₂. The trajectories revealed an extensive intra-ion-pair proton transfer in MAT⁺DCA^-. The reaction pathways of the ensuing HDCA (i.e., HNCNCN) and [MAT⁺ - H_C5⁺] (i.e., deprotonated at C5-H of MAT⁺) molecules as well as DCA^- with NO₂ were identified. The reaction of NO₂ with HDCA and DCA^- produces HNC(-ONO)NCN and NCNC(-ONO)N^- or NCNCN-NO₂^-, respectively, whereas that with [MAT⁺ - H_C5⁺] results in the formation of 5-O-MAT (i.e., 4-amino-2-methyl-2,4-dihydro-3 H-1,2,4-triazo-3-one) + NO and [MAT⁺ - H₂⁺] + HNO₂. Using trajectories for guidance, structures of intermediates, transition states and products, and the corresponding reaction potential surfaces were elucidated at B3LYP/6-311++ G(d,p). Rice-Ramsperger-Kassel-Marcus (RRKM) theory was utilized to calculate the reaction rates and statistical product branching ratios. A comparison of direct dynamics simulations with RRKM modeling results indicate that the reactions of NO₂ with HDCA and DCA^- are nonstatistical. To validate our computational results, infrared and Raman spectra of MAT⁺DCA^- and its reaction products with NO₂ were calculated using an ionic liquid solvation model. The calculated spectra reproduced the vibrational frequencies detected in an earlier spectroscopic study of MAT⁺DCA^- droplets with NO₂ [ Brotton , S. J. ; J. Phys. Chem. Lett. 2017 , 8 , 6053 ].

Oxidation of a Levitated Droplet of 1-Allyl-3-methylimidazolium Dicyanamide by Nitrogen Dioxide

Understanding the reaction mechanisms of ionic liquids and their oxidizers is necessary to develop the next generation of hypergolic, ionic-liquid-based fuels. We studied reactions between a levitated droplet of 1-allyl-3-methylimidazolium dicyanamide ([AMIM][DCA]), with and without hydrogen-capped boron nanoparticles, and nitrogen dioxide (NO₂). The reactions were monitored with Fourier-transform infrared (FTIR) and Raman spectroscopy. The emergence of new structures in the FTIR and Raman spectra is consistent with the formation of functional groups including organic nitrites (RONO), nitroamines (R¹R²NNO₂), and carbonitrates (R¹R²C=NO₂^-). Possible reaction mechanisms based on these new functional groups are discussed. The reaction rates were deduced at various temperatures by heating the levitated droplets with a carbon dioxide laser. We thereby determined an overall activation energy of 38.5 ± 2.3 kJ mol^-1 for the oxidation of [AMIM][DCA] for the first time.

Ignition Delay Reduction with Sodium Addition to Imidazolium-Based Dicyanamide Ionic Liquid

A range of ionic liquids (ILs) have been synthesized and modeled to better understand the role of the cation in the ignition of hypergolic ionic liquids. Vogelhuber et al. have shown by density functional theory methods that the addition of sodium cations to an ionic liquid promotes ignition with white fuming nitric acid (WFNA) by lowering energy barriers. To validate this prediction, solid sodium dicyanamide (Na⁺DCA^-) was added at various weight percents to 1-butyl-3-methylimidazolium dicyanamide (BMIM⁺DCA^-). The ignition delay was measured for each mixture with WFNA. Overall, it was found that the Na⁺DCA^- lowered the ignition delay by 11 ms at 7 wt %. The calculations done by Vogelhuber et al. appear to be consistent with this observation. The sodium cation may play a role by orienting the anion with the WFNA resulting in the favorable reaction energetics observed.

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.

Contrast between the mechanisms for dissociative electron attachment to CH3SCN and CH3NCS

The kinetics of thermal electron attachment to methyl thiocyanate (CH₃SCN), methyl isothiocyanate (CH₃NCS), and ethyl thiocyanate (C₂H₅SCN) were measured using flowing afterglow-Langmuir probe apparatuses at temperatures between 300 and 1000 K. CH₃SCN and C₂H₅SCN undergo inefficient dissociative attachment to yield primarily SCN^- at 300 K (k = 2 × 10^-10 cm³ s^-1), with increasing efficiency as temperature increases. The increase is well described by activation energies of 0.17 eV (CH₃SCN) and 0.14 eV (C₂H₅SCN). CN^- product is formed at <1% branching at 300 K, increasing to ∼30% branching at 1000 K. Attachment to CH₃NCS yields exclusively SCN^- ionic product but at a rate at 300 K that is below our detection threshold (k < 10^-12 cm³ s^-1). The rate coefficient increases rapidly with increasing temperature (k = 6 × 10^-11 cm³ s^-1 at 600 K), in a manner well described by an activation energy of 0.51 eV. Calculations at the B3LYP/def2-TZVPPD level suggest that attachment to CH₃SCN proceeds through a dissociative state of CH₃SCN^-, while attachment to CH₃NCS initially forms a weakly bound transient anion CH₃NCS^-* that isomerizes over an energetic barrier to yield SCN^-. Kinetic modeling of the two systems is performed in an attempt to identify a kinetic signature differentiating the two mechanisms. The kinetic modeling reproduces the CH₃NCS data only if dissociation through the transient anion is considered.

Theoretical Investigation of the Reactivity of Sodium Dicyanamide with Nitric Acid

There is a need to replace current hydrazine fuels with safer propellants, and dicyanamide (DCA^-)-based systems have emerged as promising alternatives because they autoignite when mixed with some oxidizers. Previous studies of the hypergolic reaction mechanism have focused on the reaction between DCA^- and the oxidizer HNO₃; here, we compare the calculated pathway of DCA^- + HNO₃ with the reaction coordinate of the ion pair sodium dicyanamide with nitric acid, Na[DCA] + HNO₃. Enthalpies and free energies are calculated in the gas phase and in solution using a quantum mechanical continuum solvation model, SMD-GIL. The barriers to the Na[DCA] + HNO₃ reaction are dramatically lowered relative to those of the reaction with the bare anion, and an exothermic exit channel to produce NaNO₃ and the reactive intermediate HDCA appears. These results suggest that Na[DCA] may accelerate the ignition reaction.

Effect of Boron Clusters on the Ignition Reaction of HNO3 and Dicynanamide-Based Ionic Liquids

Many ionic liquids containing the dicynamide anion (DCA^-, formula N(CN)₂^-) exhibit hypergolic ignition when exposed to the common oxidizer nitric acid. However, the ignition delay is often about 10 times longer than the desired 5 ms for rocket applications, so that improvements are desired. Experiments in the past decade have suggested both a mechanism for the early reaction steps and also that additives such as decaborane can reduce the ignition delay. The mechanisms for reactions of nitric acid with both DCA^- and protonated DCAH are considered here, using accurate wave function methods. Complexation of DCA^- or DCAH with borane clusters B₁₀H₁₄ or B₉H₁₄^- is found to modify these mechanisms slightly by changing the nature of some of the intermediate saddle points and by small reductions in the reaction barriers.

Catalytic Decomposition of Hydroxylammonium Nitrate Ionic Liquid: Enhancement of NO Formation

Hydroxylammonium nitrate (HAN) is a promising candidate to replace highly toxic hydrazine in monopropellant thruster space applications. The reactivity of HAN aerosols on heated copper and iridium targets was investigated using tunable vacuum ultraviolet photoionization time-of-flight aerosol mass spectrometry. The reaction products were identified by their mass-to-charge ratios and their ionization energies. Products include NH₃, H₂O, NO, hydroxylamine (HA), HNO₃, and a small amount of NO₂ at high temperature. No N₂O was detected under these experimental conditions, despite the fact that N₂O is one of the expected products according to the generally accepted thermal decomposition mechanism of HAN. Upon introduction of iridium catalyst, a significant enhancement of the NO/HA ratio was observed. This observation indicates that the formation of NO via decomposition of HA is an important pathway in the catalytic decomposition of HAN.

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.