Molecular Structure and Interactions in the Ionic Liquid 1-Ethyl-3-methylimidazolium Trifluoromethanesulfonate

Effect of isopropyl side chain branching and different anions on electronic structure, vibrational spectra, and hydrogen bonding of isopropyl-imidazolium-based ionic liquids: Experimental and theoretical investigations

In the present work, four branched methylated, 1,2-dimethyl-3-isopropyl-imidazolium (i-[C₃Dmim⁺]) and protonated,1-methyl-3-isopropyl-imidazolium (i-[C₃mim⁺])-based ionic liquids (ILs) with varying anion (Br^-, BF₄^-, PF₆^-, and NTf₂^-) were synthesized and investigated by NMR, infrared (IR) and Raman spectroscopy. Based on infrared and Raman spectroscopy, complete vibrational assignments have been performed. The IR and Raman analysis revealed that the vibrational spectra are virtually unaffected upon methylation, while significant frequency changes were observed by changing anion. Furthermore, to determine the electronic structure, energetic stability, and vibrational properties of these i-[C₃Dmim]Y, i-[C₃mim]Y (Y = Br, BF₄, PF₆, and NTf₂) ion pairs, quantum chemical calculations including the dispersion correction method are performed both on single ions and on ionic couples. The calculated electron density was analyzed to expose non-covalent intra- and interionic interactions by the quantum theory of atoms in molecules (AIM) and interpreted in terms of both anion dependence and type of interaction. Computational results suggest that for all ionic couples the most energetically stable configuration is obtained with the anions located close to the C2 position of the imidazolium cation. However, in the case of i-[C₃mim]NTf₂ and i-[C₃Dmim]BF₄, similar energies were obtained in configurations 2 and 3 where the anion is located above the imidazolium ring. For i-[C₃mim]Br a stronger hydrogen bond is predicted than for other studied ILs. Calculations indicate that a red shift of the CH stretching bands should occur due to hydrogen bonding; indeed, such displacement of bands is experimentally observed.

Quantum chemical study of the reaction paths and kinetics of acetaldehyde formation on a methanol-water ice model

Acetaldehyde (CH₃CHO) is ubiquitous in interstellar space and is important for astrochemistry as it can contribute to the formation of amino acids through reaction with nitrogen containing chemical species. Quantum chemical and reaction kinetics studies are reported for acetaldehyde formation from the chemical reaction of C(³P) with a methanol molecule adsorbed at the eighth position of a cubic water cluster. We present extensive quantum chemical calculations for total spin S = 1 and S = 0. The UωB97XD/6-311++G(2d,p) model chemistry is employed to optimize the structures, compute minimum energy paths and zero-point vibrational energies of all reaction steps. For the optimized structures, the calculated energies are refined by CCSD(T) single point computations. We identify four transition states on the triplet potential energy surface (PES), and one on the singlet PES. The reaction mechanism involves the intermediate formation of CH₃OCH adsorbed on the ice cluster. The rate limiting step for forming acetaldehyde is the C–O bond breaking in CH₃OCH to form adsorbed CH₃ and HCO. We find two positions on the reaction path where spin crossing may be possible such that acetaldehyde can form in its singlet spin state. Using variational transition-state theory with multidimensional tunnelling we provide thermal rate constants for the energetically rate limiting step for both spin states and discuss two routes to acetaldehyde formation. As expected, quantum effects are important at low temperatures.

Acetaldehyde (CH₃CHO) is ubiquitous in interstellar space and is important for astrochemistry as it can contribute to the formation of amino acids. The reaction mechanism for its formation on a methanol ice grain may involve intersystem spin crossing.

A comprehensive multidisciplinary investigation on CO2 capture from diesel engine

Climate change and global warming are the visible consequences of the increased amount of carbon dioxide (CO₂) in the atmosphere. Among the various sources of anthropogenic CO₂ emission, the diesel engine has a significant contribution. The development of a reliable system to efficiently minimize CO₂ emissions from diesel engines to the safest level is lacking in the open literature. Therefore, a comprehensive multidisciplinary approach has been applied in this paper to investigate the efficacy of the post-combustion carbon capture (PCC) process for the diesel engine. The experiments have been performed on the exhaust of a direct injection diesel engine at five different brake powers with blends of aqueous ammonia (AQ_NH₃), monoethanolamine (MEA), N,N-dimethylethanolamine (DMEA), and 1-ethyl-3-methylimidazolium tetrafluoroborate (C₂mim BF₄) ionic liquid (IL) as an absorbent for CO₂ capture. The reaction mechanism of these absorbent with CO₂ are also studied by the geometrical, energetical, MESP, frontier molecular orbitals, and NBO analysis using the first-principles density functional theory (DFT) calculations. The maximum CO₂ absorption efficiency of almost 97% was achieved for the blend consisting of 67% of AQ_NH₃ and 33% of MEA. Moreover, AQ_MEA and blend of AQ_NH₃, DMEA, and C₂mim BF₄ ionic liquid showed 96% and 94% CO₂ absorption efficiency, respectively.

Anodic Activity of Hydrated and Anhydrous Iron (II) Oxalate in Li-Ion Batteries

We discuss the applicability of the naturally occurring compound Ferrous Oxalate Dihydrate (FOD) (FeC2O4·2H2O) as an anode material in Li-ion batteries. Using first-principles modeling, we evaluate the electrochemical activity of FOD and demonstrate how its structural water content affects the intercalation reaction and contributes to its performance. We show that both Li0 and Li+ intercalation in FOD yields similar results. Our analysis indicates that fully dehydrated ferrous oxalate is a more promising anodic material with higher electrochemical stability: it carries 20% higher theoretical Li storage capacity and a lower voltage (0.68 V at the PBE/cc-pVDZ level), compared to its hydrated (2.29 V) or partially hydrated (1.43 V) counterparts.

Highly efficient and stable ionic liquid-based gel electrolytes

Gel electrolytes are promising candidates for dye-sensitized solar cells (DSSCs) and other devices, but the ways to obtain stable gels always result in sacrifice of their ionic conductivity. This contradiction seriously limits the practical application of gel electrolytes. Herein, a new design strategy using rich carboxylic group-modified silica nanoparticles (COOH-SiO₂) with a branched, well-organized framework to develop ionic liquid-based gel electrolytes possessing high conductivity is demonstrated. The branched network of COOH-SiO₂ and the strong interaction in electrolytes result in the effective solidification of ionic liquids. Moreover, adding COOH-SiO₂ to ionic liquid electrolytes contributes to salt dissociation, decreases the activation energy, and improves the charge transport and recombination characteristics at the electrolyte/electrode interface. DSSCs fabricated with COOH-SiO₂ nanoparticles deliver a higher short-circuit photocurrent density (J_sc) than the reference cell. A maximum efficiency of 8.02% with the highest J_sc value of 16.60 mA cm^-2 is obtained for solar cells containing 6 wt% COOH-SiO₂.

Heterogeneous Nucleation of Butanol on NaCl: A Computational Study of Temperature, Humidity, Seed Charge, and Seed Size Effects

Using a combination of quantum chemistry and cluster size distribution dynamics, we study the heterogeneous nucleation of n-butanol and water onto sodium chloride (NaCl)₁₀ seeds at different butanol saturation ratios and relative humidities. We also investigate how the heterogeneous nucleation of butanol is affected by the seed size through comparing (NaCl)₅, (NaCl)₁₀, and (NaCl)₂₅ seeds and by seed electrical charge through comparing (Na₁₀Cl₉)⁺, (NaCl)₁₀, and (Na₉Cl₁₀)⁻ seeds. Butanol is a common working fluid for condensation particle counters used in atmospheric aerosol studies, and NaCl seeds are frequently used for calibration purposes and as model systems, for example, sea spray aerosol. In general, our simulations reproduce the experimentally observed trends for the NaCl–BuOH–H₂O system, such as the increase of nucleation rate with relative humidity and with temperature (at constant supersaturation of butanol). Our results also provide molecular-level insights into the vapor–seed interactions driving the first steps of the heterogeneous nucleation process. The main purpose of this work is to show that theoretical studies can provide molecular understanding of initial steps of heterogeneous nucleation and that it is possible to find cost-effective yet accurate-enough combinations of methods for configurational sampling and energy evaluation to successfully model heterogeneous nucleation of multicomponent systems. In the future, we anticipate that such simulations can also be extended to chemically more complex seeds.

Structures and non-covalent interaction behaviours of binary systems containing the ionic liquid 1-(2'-hydroxylethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and chloroform

Mixtures of ionic liquids (ILs) and molecular solvents can overcome the drawbacks (high viscosity, high polarity, and high cost) of pure ILs and extend their practical use. The structural and interaction properties of ILs form the bases for understanding their properties. In this work, the structural properties of the mixtures of an IL, 1-(2'-hydroxylethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2OHMIM][Tf₂N]), with chloroform, a molecular solvent of weak polarity, in various concentrations were analysed using Fourier transform infrared spectroscopy and density functional theory calculations. Excess spectra were used to analyse the infrared spectra. The IL forms a stable ion cluster-CDCl₃ complex with CDCl₃ in the concentration range investigated. In the ion cluster-CDCl₃ complex, the hydrogen atom of CDCl₃ forms hydrogen-bonds with the fluorine atoms of the anion. In addition, the chlorine atom of CDCl₃ forms a halogen-bond with the oxygen atom of the anion. All the hydrogen and halogen-bonds identified between the [C2OHMIM][Tf₂N] ion cluster and CDCl₃ exhibit low strength, closed shells, and electrostatically dominant interactions.

Molecular level insights into the direct health impacts of some organic aerosol components

Quantum chemistry and biomodeling indicate that the studied organic aerosol components cannot directly cause oxidative stress or mutagenicity/carcinogenicity.

A Systematic Study of DFT Performance for Geometry Optimizations of Ionic Liquid Clusters

Clusters of two ion pairs of imidazolium-based ionic liquids were optimized with 43 different levels of theory, including DFT functionals and MP2-based methods combined with varying Dunning's basis sets, and added dispersion corrections. Better preforming DFT functionals were then applied to clusters consisting of four ion pairs. Excellent performance of some DFT functionals for the two ion pair clusters did not always match that of the four ion-paired clusters despite interionic distances remaining constant between the optimized two and four ion-paired clusters of the same ionic liquid. Combinations of DFT functional and basis set such as ωB97X-D/cc-pVDZ, M06-2X/aug-cc-pVDZ, B3LYP-D3/cc-pVTZ, and TPSS-D3/cc-pVTZ gave excellent results for geometry optimization of two ion-paired clusters of imidazolium ionic liquids but gave larger deviations when applied to the four ion-paired clusters of varying ionic liquids. Empirical dispersion corrections were seen to be crucial in correctly capturing correlation effects in the studied ionic liquid clusters, becoming more important in larger clusters. Dunning's double-ζ basis set, cc-pVDZ, is associated with the smallest root mean squared deviations for geometries; however, it also produces the largest deviations in total electronic energies. ωB97X-D and M06-2X produced the best performance with the augmented version of this basis set. The triple-ζ basis set, cc-pVTZ, leads to the best performance of most of the DFT functionals (especially the dispersion-corrected ones) used, whereas its augmented version, aug-cc-pVTZ, was not seen to improve results. The combinations of functional and basis set that gave the best geometry and energetics in both two and four ion-paired clusters were PBE-D3/cc-pVTZ, ωB97X-D/aug-cc-pVDZ, and BLYP-D3/cc-pVTZ. All three combinations are recommended for geometry optimizations of larger clusters of ionic liquids. PBE-D3/cc-pVTZ performed the best with an average deviation of 2.3 kJ mol^-1 and a standard deviation of 3.4 kJ mol^-1 for total electronic energy when applied to four ion-paired clusters. Geometries optimized with FMO2-SRS-MP2/cc-pVTZ produced total energy within 2.0 kJ mol^-1 off the benchmark in two ion-paired clusters, with the cc-pVDZ basis set performing unsurprisingly poorly with the same method. The error increased to 4.8 kJ mol^-1 on average in four ion-paired clusters, with the smallest RMSD deviations in geometries when compared to the benchmark ones. This study is the first report that investigated the performance of DFT functionals for two and four ion-paired clusters of a wide range of ionic liquids consisting of commonly used cations such as pyrrolidinium, imidazolium, pyridinium, and ammonium. It also identified the importance of assessing the performance of quantum chemical methods for ionic liquids on a variety of cation-anion combinations.

Impact of alkyl chain length and water on the structure and properties of 1-alkyl-3-methylimidazolium chloride ionic liquids

Conformational isomerism in C_nmim Cl (n = 2, 4, 6, 8, and 10) is identified by marker IR bands for the first time.

Theoretical study on the structure and electronic properties of alkylimidazolium iodide ionic liquids: the effect of alkyl chain length

Study on the cation–anion interaction energies of 1-alkyl-3-methylimidazolium iodide ionic liquids using electronic structure calculations and investigation of their correlations with thermophysical properties.

The structure and hydrogen-bond behaviours of binary systems containing ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate and methanol/ethanol

Mixing ionic liquids (ILs) with a molecular cosolvent can largely reduce the high viscosities, high polarities, and high costs of ILs. The macroscopic properties of IL-cosolvent mixtures have been studied extensively. However, some fundamental questions regarding the microscopic properties of the binary mixtures still remain to be answered. In this work, the structural and hydrogen-bond features of binary systems containing 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF₄]) and methanol/ethanol were studied by using the combination of Fourier-transform infrared spectroscopy (FTIR) and density functional theory (DFT) calculations. Excess infrared absorption spectroscopy with enhanced spectral resolution was used to analyse the original IR spectra. The alcohol tetramer/larger multimers, alcohol trimer, anion-alcohol, ion pair-alcohol, and ion cluster-alcohol complexes were identified in the excess spectra. With the increasing [BMIM][BF₄], the alcohol multimers gradually broke out from the larger multimers into smaller multimers. The hydrogen-bonded complex related with anion [BF₄]^- and alcohol gradually changes from anion-alcohol complex to ion pair-alcohol complex. The ion cluster-alcohol appears when the x(alcohol) is <0.50. The most stable optimized geometries of anion-alcohol, ion pair-alcohol, and ion cluster-alcohol were carefully analysed, and the hydrogen-bonds were identified. All of the hydrogen-bonds in these studied complexes had weak strength, closed shells and electrostatically dominant interactions.

Computational Study of the Effect of Mineral Dust on Secondary Organic Aerosol Formation by Accretion Reactions of Closed-Shell Organic Compounds

The effect of dust aerosols on accretion reactions of water, formaldehyde, and formic acid was studied in the conditions of earth's troposphere at the DLPNO-CCSD(T)/aug-cc-pVTZ//ωB97X-D/6-31++G** level of theory. A detailed analysis of the reaction mechanisms in the gas phase and on the surface of mineral dust, represented by mono- and trisilicic acid, revealed that mineral dust has the potential of decreasing reaction barrier heights. Specifically, at 0 K, mineral dust can lower the apparent energy barrier of the reaction of formaldehyde with formic acid to zero. However, when the entropic contributions to the reaction free energies were accounted for, mineral dust was found to selectively enhance the reaction of water with formaldehyde, while inhibiting the reaction of formaldehyde and formic acid, in the lower parts of the troposphere (with temperatures around 298 K). In the upper troposphere (with temperatures closer to 198 K), mineral dust catalyzes both reactions and also the reaction of methanol with formic acid. Despite the intrinsic potential of mineral dust, calculation of the catalytic enhancement parameter for a likely range of dust aerosol concentrations suggested that dust aerosols will not contribute to secondary organic aerosol formation via dimerization of closed-shell organic compounds. The main reason for this is the relatively low absolute concentration of tropospheric dust aerosol and its inefficiency in increasing the effective reaction rate coefficients.

Spectroscopic and computational insights into the ion-solvent interactions in hydrated aprotic and protic ionic liquids

Ionic liquids (ILs) and their aqueous solutions are emerging media for solving and manipulating biochemical molecules such as proteins. Unleashing the full potential however requires a detailed mechanistic understanding of how suitable protic and aprotic ILs behave in the presence of water in the first place. The present work aims at making an important step by performing a combined experimental and computational study of two selected ILs and their mixtures with water: the aprotic cholinium propionate ([Chl][Pro]) and the protic N-methyl-2-pyrrolidonium propionate ([NMP][Pro]). IR and Raman spectroscopy reveal stronger ion-solvent interactions in [Chl][Pro]-H₂O systems compared to [NMP][Pro]-H₂O mixtures. This can be explained by the tightly packed ion-pair associations in [NMP][Pro] comprising the protic -N⁺-H counterpart, which allows the establishment of highly directional and strong interionic hydrogen bonds. The spectral decomposition of the O-D stretching band into three sub-peaks showed that the protic [NMP][Pro] favors the self-association of water molecules. On the other hand, the predominant fraction of water-anion/cation aggregates exists in aprotic [Chl][Pro]. These hydrated systems can be envisaged using quantum-chemical calculations in the following way: H₂O[Chl]⁺H₂O[Pro]^-H₂O and H₂O[NMP]⁺[Pro]^-H₂O, which implied preferable solvent-shared ion-pair (SIP) configurations for [Chl][Pro]-H₂O systems, whereas the contact ion-pair (CIP) state prevails for the [NMP][Pro]-H₂O systems. The latter holds even in the water-rich regime. In future work, these findings will be the basis for an understanding of the underlying principles that govern the interactions of ions with bio-molecules in aqueous solutions.

The Nature of Cation-Anion Interactions in Magnetic Ionic Liquids as Revealed Using High-Pressure Fourier Transform Infrared (FT-IR) Spectroscopy

Magnetic ionic liquids are a group of magneto-responsive compounds that typically possess high ionic conductivities and low vapor pressures. In spite of the general interest in these materials, a number of questions concerning the fundamental interactions among the ions remain unanswered. We used vibrational spectroscopy to gain insight into the nature of these interactions. Intramolecular vibrational modes of the ions are quite sensitive to their local potential energy environments, which are ultimately defined by cation-anion coordination schemes present among the ions. Ambient pressure Fourier transform infrared (FT-IR) spectroscopy indicates comparable interaction motifs for 1-ethyl-3-methylimidazolium tetrachloroferrate(III), [emim]FeCl₄, and 1-ethyl-3-methylimidazolium tetrabromoferrate(III), [emim]FeBr₄, magnetic ionic liquids. However, the vibrational modes of [emim]FeCl₄ generally occur at slightly higher frequencies than those of [emim]FeBr₄. These differences reflect different interaction strengths between the [emim]⁺ cations and FeCl4- or FeBr4- anions. This conclusion is supported by gas-phase ab initio calculations of single [emim]FeCl₄ and [emim]FeBr₄ ion pairs that show longer C-H···Br-Fe interaction lengths compared to C-H···Cl-Fe. Although the IR spectra of [emim]FeCl₄ and [emim]FeBr₄ are comparable at ambient pressure, a different series of spectroscopic changes transpire when pressure is applied to these compounds. This suggests [emim]⁺ cations experience different types of interaction with the anions under high-pressure conditions. The pressure-dependent FT-IR spectra highlights the critical role ligands attached to the tetrahalogenoferrate(III) anions play in modulating cation-anion interactions in magnetic ionic liquids.

Sulfate Donor Based Dinuclear Heteroleptic Triple-Stranded Helicates from Sulfite and Ditopic Nitrogen Donor Ligands and Their Transformation to Dinuclear Homoleptic Double-Stranded Mesocates

Sulfate donor based supramolecular coordination complexes [{ fac-Re(CO)₃}(μ-SO₄)(L ⁿ)₂{ fac-Re(CO)₃}] (1-3) were obtained using ditopic N donors (L ⁿ; n = 1-3), NaHSO₃, and Re₂(CO)₁₀ in a one-pot, multicomponent, coordination-driven self-assembly approach, in which SO₃^2- becomes oxidized to SO₄^2- during the reaction and acts as a building framework. Complexes 1-3 were characterized using IR, ESI-TOF-MS, and ¹H NMR spectroscopy. The structures of complexes 1-3 were confirmed using single-crystal X-ray diffraction analysis. The transformation of the dinuclear heteroleptic triple-stranded helicate to the dinuclear homoleptic double-stranded mesocate [{Re(CO)₃Cl}₂(L ⁿ)₂] (L ⁿ = L¹, L², L³; 4a-6a) was achieved by the addition of BaCl₂. The direct treatment of Re(CO)₅X (X = Cl, Br) with L¹/L²/L³ yielded the dinuclear homoleptic double-stranded helicates [{Re(CO)₃X}₂ (L ⁿ)₂] (4b-6b and 7-9).

Evidence of C--F-P and aromatic π--F-P weak interactions in imidazolium ionic liquids and its consequences

A simple change from alkyl group to alkene in side chain of imidazolium cation with same anion resulted in a drastic impact on physical properties (e.g., melting point) from bmimPF₆ IL to cmimPF₆ IL. The underlying reasons have been elucidated by structural and interaction studies with the help of DSC, SCXRD, vibrational and multi-nuclear NMR spectroscopic techniques. Experiments reveal existence of new weak interactions involving the carbon and π cloud of the imidazolium aromatic ring with fluoride of PF₆ anion (i.e., C₂--F-P and π--F-P) in cmimPF₆ but are absent in structurally similar prototype IL, bmimPF₆. Though weak, these interactions helped to form ladder type supramolecular arrangement, resulting in quite high melting point for cmimPF₆ IL compared to bmimPF₆ IL. These findings emphasize that an IL system can behave uniquely because of the existence of uncommon weak interactions.

Temperature- and pressure-dependent infrared spectroscopy of 1-butyl-3-methylimidazolium trifluoromethanesulfonate: A dipolar coupling theory analysis

Continued growth and development of ionic liquids requires a thorough understanding of how cation and anion molecular structure defines the liquid structure of the materials as well as the various properties that make them technologically useful. Infrared spectroscopy is frequently used to assess molecular-level interactions among the cations and anions of ionic liquids because the intramolecular vibrational modes of the ions are sensitive to the local potential energy environments in which they reside. Thus, different interaction modes among the ions may lead to different spectroscopic signatures in the vibrational spectra. Charge organization present in ionic liquids, such as 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([C₄mim]CF₃SO₃), is frequently modeled in terms of a quasicrystalline structure. Highly structured quasilattices enable the dynamic coupling of vibrationally-induced dipole moments to produce optical dispersion and transverse optical-longitudinal optical (TO-LO) splitting of vibrational modes of the ionic liquid. According to dipolar coupling theory, the degree of TO-LO splitting is predicted to have a linear dependence on the number density of the ionic liquid. Both temperature and pressure will affect the number density of the ionic liquid and, therefore, the amount of TO-LO splitting for this mode. Therefore, we test these relationships through temperature- and pressure-dependent FT-IR spectroscopic studies of [C₄mim]CF₃SO₃, focusing on the totally symmetric SO stretching mode for the anion, ν_s(SO₃). Increased temperature decreases the amount of TO-LO splitting for ν_s(SO₃), whereas elevated pressure is found to increase the amount of band splitting. In both cases, the experimental observations follow the general predictions of dipolar coupling theory, thereby supporting the quasilattice model for this ionic liquid.

Effects of imidazolium-based ionic liquids with different anions on wheat seedlings

The effects of five imidazolium ionic liquids with different anions were studied in hydroponically grown wheat seedlings at concentrations of 50, 100, 150, 200, 250, and 300 mg L^-1. The results showed that shoots and roots grew shorter and dry weight decreased with increasing concentrations of ionic liquids. Moreover, the antioxidant enzyme activities decreased and malondialdehyde (MDA) content was greater in the leaves of wheat seedlings subjected to ionic liquid (IL) treatments. The order of influence of ionic liquids on these indexes was [C₄mim][TfO]> [C₄mim][Cl]> [C₄mim][BF₄]> [C₄mim][Lact]> [C₄mim][Ala]. A transmission electron microscope (TEM) was used to observe leaf and root cellular structures, such as chloroplast, nucleus, mitochondria, and rough endoplasmic reticulum, in wheat exposed to ionic liquids at a concentration of 150 mg L^-1. The results showed that the cellular structures of wheat were affected, and the degree of the effect of five ILs was consistent with the general trend of the measured indexes in this study. Ionic liquids influence the growth of plants by impeding growth, disrupting metabolic physiology and changing cellular structures. The degree of toxicity of imidazolium-based ionic liquids with different anions varies.