Ionic Liquid Clusters Research Articles

Study on the control of electrowetting and permeation of ionic liquids on porous substrates via molecular dynamics simulations

The electrowetting and permeation behaviors of ionic liquids, which enable the control of their transport at low flow rates without external mechanical forces, significantly promote the development of microfluidic technology. A micro-valve system developed with the combination of porous substrates and micro-channels allows the control of ionic liquid on–off states and the adjustment of its flow rates solely using electrical energy. However, the nanoscale mechanisms of electrowetting of ionic liquid clusters on the surface of porous materials have not been fully revealed. In this study, molecular dynamics simulations are employed to investigate the impact of substrate wettability, pore characteristics, electric field features, and ionic liquid types on electrowetting. The results indicate that the substrates should be maintained hydrophobic initially to decrease spontaneous permeation and avoid over-wetting. Increasing the electric field strength promotes permeation, but an excessively intense electric field intensifies the ion field evaporation. Furthermore, the electrowetting and permeation mechanisms of various substrate pore sizes and pore spacings can be explained by the motion characteristics of cations and anions. Kinematic analysis of the ionic liquids climbing the capillary sidewalls under an alternative electric field at a controllable velocity is provided. These conclusions can provide theoretical guidance and reference for the electrowetting and permeation of ionic liquids.

A many-body energy decomposition analysis (MB-EDA) scheme based on a target state optimization self-consistent field (TSO-SCF) method.

In this paper, we combine an energy decomposition analysis (EDA) scheme with many-body expansion (MBE) to develop a MB-EDA method to study the cooperative and anti-cooperative effects in molecular cluster systems. Based on the target state optimization self-consistent field (TSO-SCF) method, the intermolecular interaction energy can be decomposed into five chemically meaningful terms, i.e., electrostatic, exchange, polarization, charge transfer and dispersion interaction energies. MB-EDA can decompose each of these terms in MBE. This MB-EDA has been applied to 3 different cluster systems: water clusters, ionic liquid clusters, and acetonitrile-methane clusters. This reveals that electrostatic, exchange, and dispersion interactions are highly pairwise additive in all systems. In water and ionic liquid clusters, the many-body effects are significant in both polarization and charge transfer interactions, but are cooperative and anti-cooperative, respectively. For acetonitrile-methane clusters, which do not involve hydrogen bonds or charge-charge Coulombic interactions, the many-body effects are quite small. The chemical origins of different many-body effects are deeply analyzed. The MB-EDA method has been implemented in Qbics (https://qbics.info) and can be a useful tool for understanding the many-body behavior in molecular aggregates at the quantum chemical level of theory.

Dynamics and thermochemistry of the negatively charged clusters in a 2-hydroxyethylhydrazinium nitrate ionic liquid system.

The formation and fragmentation of negatively charged 2-hydroxyethylhydrazinium nitrate ([HOCH2CH2NH2NH2]+NO3-, HEHN) ionic liquid clusters were examined using a guided-ion beam tandem mass spectrometer furnished with collision-induced dissociation of selected ions with Xe atoms. Measurements included the compositions of cluster ions formed in the ionization source, and the dissociation products, cross sections, and 0 K threshold energies for individually selected cluster ions. To identify the structures of the main cluster ion series [(HEHN)n(HNO3)0-1NO3]- formed, molecular dynamics simulations were employed to create initial geometry guesses, followed by optimization at the ωB97XD/6-31+G(d,p) level of theory, from which global minimum structures were identified for reaction thermodynamics analyses. A comparison was made between the cluster formation and fragmentation in the negatively charged 2-hydroxyethylhydrazinium nitrate with those in the positive mode (reported by W. Zhou et al., Phys. Chem. Chem. Phys., 2023, 25, 17370). In both modes, the cluster ions were predominantly composed of m/z below 350; loss of a neutral 2-hydroxyethylhydrazinium nitrate ion pair represents the most important cluster fragmentation pathway, followed by intra-ion pair proton transfer-mediated 2-hydroxyethylhydrazine and HNO3 elimination; and all clusters started to dissociate at threshold energies less than 1.5 eV. The overwhelming similarities in the formation and fragmentation chemistry of positively vs. negatively charged 2-hydroxyethylhydrazinium nitrate clusters may be attributed to their inherent ionic nature and high electric conductivities.

Inferring electrospray emission characteristics from molecular dynamics and simulated retarding potential analysis

In this work, we present coordinated molecular dynamics, ion cluster acceleration, and retarding potential analysis simulations to determine cluster fragmentation behavior in a realistic emitter geometry for electrosprays operating in the pure ionic regime. Molecular dynamics simulations are used to determine the fragmentation rates of ionic liquid clusters as a function of internal energy, electric field strength, and cluster size. A simplified model of electrospray cluster acceleration is developed from previous electrohydrodynamic emission models and used to simulate retarding potential analysis curves. Fragmentation rates and beam composition are inferred for experimental data based on the molecular dynamics and cluster acceleration simulations. We find that for these experimental data, temperatures of EMI-BF4 dimers likely range between 590 and 687 K while trimer temperatures are larger between 989 and 1092 K. The percentage of monomers, dimers, and trimers in the beam is approximately 45%, 30%–43%, and 13%–25%, respectively. Both ionic liquid cluster temperatures and beam composition agree with previous analysis of this experimental work, supporting the use of coordinated molecular dynamics and retarding potential analysis as a method of inferring electrospray beam parameters. Insights gained from this simulation process are discussed in the context of currently unexplained electrospray emitter behavior and experimental results including the presence of tetramers and trimers in the beam and fragmentation rates in high electric field regions.

Fragmentation modeling of gas-phase ionic liquid clusters in high-voltage electric field

Cluster fragmentation subjected to a high-voltage electric field may play an important role in controlling the novel ionic liquid (IL) electrospray propulsion but has received little attention. In this work, the micro-fragmentation mechanisms of three kinds of IL clusters are numerically investigated using both static and dynamic approaches based on the quantum mechanical principles. Firstly, it reveals that the Coulomb interaction dominates the total inter-fragment energy for all three IL dimers, whereas the hydrogen-bonding interaction is identified as the main cause for the much slower break-up rate of the 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM-BF4]) dimer. In addition to the electrostatic interaction, the van der Waals interaction is a secondary energy component to which only a small fraction of atoms contribute significantly, regardless of IL cluster type. We find, only when the applied electric field is larger than 1 V/nm, the total inter-fragment energies of all three IL clusters start to dramatically reduced, indicating the possible occurrence of massive break-up. Secondly, ab initio molecular dynamics simulation further confirms that the [EMIM-BF4] negatively charged dimer is the most stable of three IL clusters under the identical conditions. As far as the high-voltage electric field is concerned, the one-parameter exponential function of break-up ratio versus time, routinely used in the field-free region, is not valid. Finally, on the basis of simulated results, a compound exponential-decay model is proposed, and the derived differential-sum equations from constant electric fields can be further applied to the non-uniform electric field.

Assessing the propensity toward ionization in nanosized clusters of protic ionic liquids by Ab-initio methods

Nanosized clusters of protic ionic liquids are investigated using ab-initio DFT methods to understand their propensity to remain ionic. Protic ionic liquids are based on acid/base pairs and are ionic because of proton transfer from the acid to the base. Two prototypical ionic liquids based on triethylamine and two acids of different strengths are used here to trace the tendency toward proton transfer when the system size is gradually increased from an isolated pair of molecules to cluster of several pairs. We have found that the compounds based on the weak acid shows the existence of a well-defined minimal size that represents the threshold for the onset of ionization phenomena.

Modification of Thiol-Ene Ionogels with Octakis(methacryloxypropyl) Silsesquioxane.

The effect of polyhedral oligomeric silsesquioxane (POSS) on the synthesis and properties of hybrid organic–inorganic ionogels was investigated using octakis(methacryloxypropyl) silsesquioxane (methacryl-POSS). Ionogels were prepared in situ by thiol-ene photopolymerization of triallyl isocyanurate with pentaerythritol tetrakis(3-mercaptopropionate) in a mixture of imidazolium ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMImNTf2) and propylene carbonate (PC). Investigations included the kinetics of hybrid materials formation and selected physical and mechanical properties. The disadvantage of ionogels without the methacryl-POSS modifier is leakage and insufficient mechanical properties. Modifying the thiol-ene matrix by the addition of methacryl-POSS made it possible to obtain non-leaking ionogels with improved mechanical and conductive properties. The steric hindrance of POSS cages and high-density network formation played important roles in ionogel synthesis: decrease of polymerization rate (with almost no effect on conversion), as well as dimensions of the formed polymer spheres during dispersion polymerization (highly cross-linked polymer has poorer solubility in polymerizing medium at a similar conversion, and nucleation begins at lower conversion), an increase of glass transition temperature and puncture strength. Hybrid ionogels with high ionic conductivity in the range of 4.0–5.1 mS∙cm−1 with the maximum parameter for 1.5 wt.% addition of the methacryl-POSS were obtained, which can be associated with ion-pair dissociations in ionic liquid clusters caused by methacryl-POSS.

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, [2Cnmim: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 [2Cnmim: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 [2Cnmim: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, [Cn-2mim:PF6:Cnmim]+ for n = 4, 6, and 8. The absolute BDEs of these mixed [Cn-2mim:PF6:Cnmim]+ 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 [2Cnmim:PF6]+ clusters are both more accurately and more precisely determined. Comparisons are made to results for the analogous [2Cnmim:BF4]+ and [Cn-2mim:BF4:Cnmim]+ clusters previously examined to elucidate the effects of the [PF6]- and [BF4]- anions on the binding.

Structures and Spectroscopic Properties of Large Molecules and Condensed-Phase Systems Predicted by Generalized Energy-Based Fragmentation Approach.

ConspectusThe structures and spectroscopic properties of molecules and condensed-phase systems are usually experimentally characterized by X-ray, infrared (IR), Raman, nuclear magnetic resonance (NMR), and electronic absorption/emission spectra. Quantum mechanics (QM) calculations are critical in quantitatively understanding the relationship between the structure and physicochemical properties of various chemical systems. However, it is very challenging to apply traditional QM methods to large molecules and condensed-phase systems with large unit cells due to their steep computational scaling with the system size. To overcome this difficulty, theoretical chemists have developed various linear (or low) scaling QM methods, among which energy-based fragmentation methods have achieved great success for large molecules or clusters. One of the most popular energy-based fragmentation methods is the generalized energy-based fragmentation (GEBF) approach developed by us.In this approach, the ground-state energy of a large molecule can be evaluated from the ground-state energies of a series of embedded subsystems. In this Account, we focus on the recent developments and applicability of the GEBF approach for the structures and spectroscopic properties of complicated large molecules and condensed-phase systems. With new fragmentation schemes, the GEBF approach can now describe ionic liquid clusters and metal-containing supramolecular systems accurately and can provide accurate binding energies for host-guest complexes. In addition, the GEBF approach is now available for describing the localized excited states of large systems including a chromophore. More importantly, the GEBF approach under periodic boundary conditions (PBC-GEBF) has been developed to deal with periodic molecular crystals and liquids. Then, the ground-state energy (or property) per unit cell of a periodic condensed phase system can be predicted with QM calculations on nonperiodic embedded subsystems. This feature enables accurate electron correlation calculations on molecular crystals and liquids to be feasible on ordinary workstations. The PBC-GEBF approach has been applied to predict the crystal structures, lattice energies, and spectroscopic properties of some typical molecular crystals and solutions. By combining the GEBF method and machine learning (ML) method, a GEBF-ML force field has been developed for long normal alkanes, and the IR spectra of long alkanes can be obtained from the GEBF-ML molecular dynamics (MD) simulations. The GEBF and its periodic variant are expected to play increasingly important roles in investigating real-life chemical systems of broad interests at the ab initio levels.

Absolute Trends and Accurate and Precise Gas-Phase Binding Energies of 1-Alkyl-3-Methylimidazolium Tetrafluoroborate Ionic Liquid Clusters from Combined Independent and Competitive TCID Measurements.

Ionic liquid (IL) development efforts have focused on achieving desired properties via tuning of the IL through variation of the cations and anions. However, works geared toward a microscopic understanding of the nature and strength of the intrinsic cation-anion interactions of ILs have been rather limited such that the intrinsic strength of the cation-anion interactions in ILs is largely unknown. In previous work, we employed threshold collision-induced dissociation approaches supported and enhanced by electronic structure calculations to characterize the nature of the cation-anion interactions in and determine the bond dissociation energies (BDEs) of a series of four 2:1 clusters of 1-alkyl-3-methylimidazolium cations and tetrafluoroborate anions, [2Cnmim:BF4]+. 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]+, to determine the structural and energetic effects of the size of the 1-alkyl substituent on the binding. The variation in the strength of binding determined for these [2Cnmim:BF4]+ clusters was found to be similar in magnitude to the average experimental uncertainty in these determinations. To definitively establish an absolute order of binding among these [2Cnmim:BF4]+ clusters, we extend this work here to include competitive binding studies of three mixed 2:1 clusters of 1-alkyl-3-methylimidazolium cations and tetrafluoroborate anions, [Cn-2mim:BF4:Cnmim]+ for n = 4, 6, and 8. Importantly, the results of the present work simultaneously provide the absolute BDEs of these mixed [Cn-2mim:BF4:Cnmim]+ clusters and the absolute relative order of the intrinsic binding interactions as a function of the cation with significantly improved precision. Further, by combining the thermochemical results of the previous and present studies, the BDEs of the [2Cnmim:BF4]+ clusters are more accurately and precisely determined.

Hybrid structure of ionic liquid and ZnO nano clusters for potential application in dye-sensitized solar cells

“Green processes” for the production of renewable energy such as the utilization of solar energy would decisively help to avoid depletion of resources and environmental degradation. In this connection, we have investigated hybrid structures of the ionic liquid (IL) 1-ethyl-3-methylimidazolium tetrafluoroborate (C2mim BF4) and (ZnO)n nano clusters as charge-transfer promoters concerning their application in highly efficient dye-sensitized solar cells (DSSCs) using density functional theory (DFT). Structural properties, frontier molecular orbitals, and energy gaps (Eg) were analyzed to understand the interaction mechanism between the IL and ZnO nano clusters. The influence of (ZnO)n nano clusters on electronic properties of the IL was investigated by analyzing electrostatic potential (ESP) mapping, work function (ϕ), and density of states. The highest binding energy between IL and ZnO nano cluster was obtained for the IL/(ZnO)2 hybrid structure, which has enhanced charge-transfer and a smaller energy gap (3.09 eV) compared to the other considered systems. The photo-energy-conversion efficiencies (η) of DSSCs based on IL/(ZnO)n (n = 2–12) hybrid systems were predicted under irradiance with an air mass 1.5 global (AM 1.5 G) spectrum. Solar parameters such as open-circuit voltage Voc, fill factor FF, and short-circuit current density Jsc were determined to evaluate η. The maximum efficiency (1.96%) is found for the IL/(ZnO)2 system due to the high short-circuit-current density, improved electron extraction, and low work function, which allows the injection of more electrons into the TiO2 surface. Our results on the hybrid structures of IL and ZnO nano clusters reveal a possibility for the design of highly efficient next-generation DSSCs.

Predicting Entropic Effects of Water Mixing with Ionic Liquids Containing Anions of Strong Hydrogen Bonding Ability: Role of the Cation.

Ionic liquids (ILs) such as choline dihydrogen phosphate exhibit an extraordinary solubilizing ability for proteins such as cytochrome C when mixed with 20 wt % water. Most widely used imidazolium-based ionic liquids coupled with dihydrogen phosphate do not exhibit the same solubilizing properties, suggesting that a multifunctional cation such as choline might play a key role in enhancing these properties of ionic liquid mixtures with water. In this theoretical work, we compare intermolecular interactions between the water molecule and ionic liquid ions in two ion-paired clusters of choline- and 1-butyl-3-methyl-imidazolium-based ionic liquids coupled with acetate, dihydrogen phosphate, and mesylate. Gibbs free energy (GFE) of solvation of water in these ionic liquids was calculated. Incorporation of a water molecule into ionic liquid clusters was accompanied by negative GFEs of solvation in both types of cations. These results were in good agreement with previously reported experimental GFEs of solvation of water in ILs. Compared to imidazolium-based clusters, strong interionic interactions of choline ionic liquids resulted in more negative GFEs due to their smaller deformation upon the addition of a water molecule, with dihydrogen phosphate and mesylate predicting the lowest GFEs of -30.1 and -43.5 kJ/mol-1, respectively. Lower GFEs of solvation of water in choline-based clusters were also accompanied with smaller entropic penalties, suggesting that water easily incorporates itself into the existing ionic network. Analysis of the intramolecular bonds within the water molecule showed that the choline hydroxyl group donates electron density to the neighboring water molecule, leading to additional polarization. The predicted infrared spectra of clusters of ionic liquids with water showed a pronounced red shift due to strongly polarized O-H bonds, in excellent agreement with the experimentally measured infrared spectra of ionic liquid mixtures with water. Increased polarization of water in choline-based ionic liquids undoubtedly creates more effective solvents for stabilizing biological molecules such as proteins.

Cyclic Octamer of Hydroxyl-functionalized Cations with Net Charge Q=+8e Kinetically Stabilized by a 'Molecular Island' of Cooperative Hydrogen Bonds.

Cyclic octamers are well‐known structural motifs in chemistry, biology and physics. These include covalently bound cyclic octameric sulphur, cylic octa‐alkanes, cyclo‐octameric peptides as well as hydrogen‐bonded ring clusters of alcohols. In this work, we show that even calculated cyclic octamers of hydroxy‐functionalized pyridinium cations with a net charge Q=+8e are kinetically stable. Eight positively charged cations are kept together by hydrogen bonding despite the strong Coulomb repulsive forces. Sufficiently long hydroxy‐octyl chains prevent “Coulomb explosion” by increasing the distance between the positive charges at the pyridinium rings, reducing the Coulomb repulsion and thus strengthen hydrogen bonds between the OH groups. The eightfold positively charged cyclic octamer shows spectroscopic properties similar to those obtained for hydrogen‐bonded neutral cyclic octamers of methanol. Thus, the area of the hydrogen bonded OH ring represents a ‘molecular island’ within an overall cationic environment. Although not observable, the spectroscopic properties and the correlated NBO parameters of the calculated cationic octamer support the detection of smaller cationic clusters in ionic liquids, which we observed despite the competition with ion pairs wherein attractive Coulomb forces enhance hydrogen bonding between cation and anion.

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.

Structure and Stability of the Ionic Liquid Clusters [EMIM]n[BF4]n+1- (n = 1-9): Implications for Electrochemical Separations.

Precise functionalization of electrodes with size-selected ionic liquid (IL) clusters may improve the application of ILs in electrochemical separations. Herein we report our combined experimental and theoretical investigation of the IL clusters 1-ethyl-3-methylimidazolium tetrafluoroborate [EMIM]n[BF4]n+1- (n = 1-9) and demonstrate their selectivity and efficiency toward targeted adsorption of ions from solution. The structures and energies of the IL clusters, predicted with global optimization, agree with and help interpret the ion abundances and stabilities measured by high-mass-resolution electrospray ionization mass spectrometry and collision-induced dissociation experiments. The [EMIM][BF4]2- cluster, which was identified as the most stable IL cluster, was selectively soft-landed onto a working electrode. Electrochemical impedance spectroscopy revealed a lower charge transfer resistance on the soft-landed electrode containing [EMIM][BF4]2- compared with an electrode prepared by drop-casting of an IL solution containing the full range of IL clusters. Our findings indicate that specific IL clusters may be used to increase the efficiency of electrochemical separations by lowering the overpotentials involved.

Counting cations involved in cationic clusters of hydroxy-functionalized ionic liquids by means of infrared and solid-state NMR spectroscopy.

In hydroxy-functionalized ionic liquids, two types of hydrogen bonding coexist: the conventional H-bonds between cation and anion (c-a) and those between cation and cation (c-c), although the interaction between like-charged ions is supposed to be much weaker due to the repulsive Coulomb forces. Counting the cations involved in either (c-a) or (c-c) clusters is a challenge. For that purpose, we recently performed neutron diffraction (ND) measurements and molecular dynamics (MD) simulations at and above room temperature accompanied by NMR solid-state experiments in the glassy state of the ILs. In principle, these methods are suitable for determining the populations of (c-a) and (c-c) cluster species. For different reasons we could only address single temperatures and/or small temperature intervals above 300 K. The by far largest temperature range with reasonable efforts is accessible by simple infrared (IR) spectroscopy. However, counting (c-a) or (c-c) hydrogen bonds is a difficult task due to the different transition dipole moments resulting in varying intensities and broad vibrational bands. Here we present a method for deriving the number of cations involved in (c-a) ion pairs from IR spectra in the OH stretch region. This procedure provides access to the equilibria of (c-a) and (c-c) hydrogen bonds as a function of temperature allowing derivation of the transition enthalpy.

Hybrid Distributed/Shared Memory Model for the RI-MP2 Method in the Fragment Molecular Orbital Framework.

The general distributed data interface (GDDI) that was developed for the fragment molecular orbital (FMO) method is combined with the shared memory OpenMP parallel middleware to support a threading multilevel parallelism. First, GDDI partitions [logical] compute nodes into groups, which are statically or dynamically assigned to different fragments. A small number of processes are created on each compute node. Each process subsequently spawns multiple threads for the actual computation. The performance of the hybrid GDDI/OpenMP approach relative to the pure GDDI model was examined in terms of the FMO/RI-MP2 method; that is, the second-order Moller-Plesset (MP2) correlation energy was evaluated using the resolution-of-the-identity (RI) and the FMO approximations. The GDDI and OpenMP workload balances are handled by an arithmetic progression and a loop fusion, respectively. Other OpenMP properties, such as threadprivate or shared memory, are combined with the low memory demand of the RI two-electron integrals to enhance the performance. Benchmark calculations demonstrate that because the hybrid parallel model can make use of multiprocessor resources more efficiently than the regular distributed memory-based GDDI model, calculations for small to large water clusters containing 139-2165 molecules and an ionic liquid cluster exhibit node linear scaling and speedups of a factor of 10×.

The Fragment Molecular Orbital Method Based on Long-Range Corrected Density-Functional Tight-Binding.

The presently available linear scaling approaches to density-functional tight-binding (DFTB) based on the fragment molecular orbital (FMO) method are severely impacted by the problem of artificial charge transfer due to the self-interaction error (SIE), which hampers the simulation of zwitterionic systems such as biopolymers or ionic liquids. Here we report an extension of FMO-DFTB where we included a long-range corrected (LC) functional designed to mitigate the DFTB SIE, called the FMO-LC-DFTB method, resulting in a robust method which succeeds in simulating zwitterionic systems. Both energy and analytic gradient are developed for the gas phase and the polarizable continuum model of solvation. The scaling of FMO-LC-DFTB with system size N is shown to be almost linear, O( N1.13-1.28), and its numerical accuracy is established for a variety of representative systems including neutral and charged polypeptides. It is shown that pair interaction energies between fragments for two mini-proteins are in excellent agreement with results from long-range corrected density functional theory. The new method was employed in long time scale (1 ns) molecular dynamics simulations of the tryptophan cage protein (PDB: 1L2Y ) in the gas phase for four different protonation states and in stochastic global minimum structure searches for 1-ethyl-3-methylimidazolium nitrate ionic liquid clusters containing up to 2300 atoms.

Dissociation pathways of protic ionic liquid clusters: Alkylammonium nitrates.

Protic ionic liquids are promising candidates for many applications, including as spacecraft propellants. For both fundamental interest and understanding clustering and dissociation during electrospray-based propulsion, it is useful to explore the dissociation pathways of protic ionic liquid clusters, as well as the factors affecting the relative contributions of each pathway to the observed MS/MS spectra. With that said, most of the published reports on ionic liquid cluster dissociation have focused on aprotic ionic liquids. The purpose of the current work is to explore the dissociation pathways (eg, loss of amine, nitric acid, or ion pair) of alkylammonium nitrates using energy-resolved collision-induced dissociation. Here, it was found that, in general, protic ionic liquids have multiple dissociation pathways-namely, protic ionic liquids can lose their neutralized cation (here, an alkylamine) or neutralized anion (here, nitric acid)-in addition to the ion pair dissociation familiar to aprotic salt and aprotic ionic liquid clusters. In general, increasing the basicity of the cation (here, through increasing the degree of alkylation) decreases the propensity to follow these alternative pathways. Interestingly, increasing the cluster size has a similar effect: as cluster size increases, nitric acid loss decreases. These results will help better model and design protic ionic liquids for electrospray-based spacecraft propulsion and help provide a better understanding for the general behavior of protic ionic liquids versus aprotic ionic liquids within mass spectrometers.

Theoretical Study of Ionic Liquid Clusters Catalytic Effect on the Fixation of CO2

Conversion of carbon dioxide (CO2) to valuable materials is important to academia and the chemical industry. Fixation of carbon dioxide catalyzed by ionic liquids (ILs) is an attractive option. Much work has been done in the fixation of carbon dioxide catalyzed by ILs, but most of it focused on reactions catalyzed by one molecule. In this work, we studied the existence of IL clusters in a series of systems containing hydroxyl-functionalized ILs (HFILs) through a series of theoretical calculations. The intermolecular interactions and microstructures of IL clusters were studied by molecular dynamics (MD) simulations. The results indicated that three-ion clusters to six-ion clusters exist in systems, while the number of clusters decreased as the cluster size increased. Instead of studying the single ion or ionic pair catalytic processes, we gained more insight into the catalytic effects of ionic liquid clusters on the fixation of CO2 by using density functional theory (DFT) calculations. Results suggested that the activity of ILs could be enhanced to a certain extent in the presence of clusters.