Simulation studies of ionic liquids: orientational correlations and static dielectric properties

The ionic liquids BMIM+I-, BMIM+BF4-, and BMIM+PF6- were simulated by means of the molecular dynamics method over a time period of more than 100 ns. Besides the common structural analysis, e.g., radial distribution functions and three dimensional occupancy plots, a more sophisticated orientational analysis was performed. The angular correlation functions g(00)110(r) and g(00)101(r) are the first distance dependent coefficients of the pairwise orientational distribution function g(rij,Omega1,Omega2,Omega12). These functions help to interpret the three dimensional plot and reveal interesting insights into the local structure of the analyzed ionic liquids. Furthermore, the collective network of ionic liquids can be characterized by the Kirkwood factor Gkappa(r) [J. Chem. Phys. 7, 911 (1939)]. The short-range behavior (r<10 A) of this factor may be suitable to predict the water miscibility of the ionic liquid. The long-range limit of Gkinfinity is below 1 which demonstrates the strongly coupled nature of the ionic liquid networks. In addition, this factor relates the orientational structure and the dielectric properties of the ionic liquids. The static dielectric constant epsilon(omega=0) for the simulated system is 8.9-9.5. Since in ionic liquids the very same molecule contributes to the total dipole moment as well as carries a net charge, a small, but significant contribution of the cross term between the total dipole moment and the electric current to epsilon(omega=0) is observed.     

Molecular dynamics simulations of the structure and transport properties of tetra-butylphosphonium amino acid ionic liquids

Systematic molecular dynamics simulations are used to study the structure, dynamics and transport properties of the ionic liquids composed of the tetra-butylphosphonium ([TBP](+), or [P(C(4)H(9))(4)](+)) cation with six amino acid ([AA](-)) anions. The structural features of these ionic liquids were characterized by calculating the partial site-site radial distribution functions, g(r), and computing the dihedral angle distribution of n-butyl side chains in the [TBP](+) cations. The dynamics of the ionic liquids are described by studying the velocity autocorrelation function (VACF) and the mean-square displacement (MSD) for the centers of mass of the ions at different temperatures. The ionic diffusion coefficients and the electrical conductivities were evaluated from both the Einstein and Green-Kubo methods. The cross-correlation terms in the electric-current autocorrelation functions, which are an indication of the ion pair correlations, are investigated. The cationic transference numbers were also estimated to study the contributions of the anions and cations to the transport of charge in these ionic liquids. We determined the role of the amino acid anion structures on the dynamical behavior and the transport coefficients of this family of ionic liquids. In general, the MSD and self-diffusion coefficients of the relatively heavier non-planar [TBP](+) cations are smaller than those of the lighter amino acid anions. Introducing polar functional groups (acid or amide) in the side chain of [AA](-) decreases the diffusion coefficient and electrical conductivity of AAILs. The major factors for determining the magnitude of the transport coefficients are the chemical functionality and the length of the alkyl side chain of the [AA](-) anion of these [TBP][AA] ionic liquids.     

Local hydrogen bonding dynamics and collective reorganization in water: ultrafast infrared spectroscopy of HOD/D(2)O

We present an investigation into hydrogen bonding dynamics and kinetics in water using femtosecond infrared spectroscopy of the OH stretching vibration of HOD in D(2)O. Infrared vibrational echo peak shift and polarization-selective pump-probe experiments were performed with mid-IR pulses short enough to capture all relevant dynamical processes. The experiments are self-consistently analyzed with a nonlinear response function expressed in terms of three dynamical parameters for the OH stretching vibration: the frequency correlation function, the lifetime, and the second Legendre polynomial dipole reorientation correlation function. It also accounts for vibrational-relaxation-induced excitation of intermolecular motion that appears as heating. The long time, picosecond behavior is consistent with previous work, but new dynamics are revealed on the sub-200 fs time scale. The frequency correlation function is characterized by a 50 fs decay and 180 fs beat associated with underdamped intermolecular vibrations of hydrogen bonding partners prior to 1.4 ps exponential relaxation. The reorientational correlation function observes a 50 fs librational decay prior to 3 ps diffusive reorientation. Both of these correlation functions compare favorably with the predictions from classical molecular dynamics simulations. The time-dependent behavior can be separated into short and long time scales by the 340 fs correlation time for OH frequency shifts. The fast time scales arise from dynamics that are mainly local: fluctuations in hydrogen bond distances and angles within relatively fixed intermolecular configurations. On time scales longer than the correlation time, dephasing and reorientations reflect collective reorganization of the liquid structure. Since the OH transition frequency and dipole are only weakly sensitive to these collective coordinates, this is a kinetic regime which gives an effective rate for exchange of intermolecular structures.     

Molecular dynamics study of polarizable point dipole models for molten sodium iodide

The structure, the ionic transport properties, and the dynamics of long-wavelength charge-density fluctuations, for two polarizable point dipole models of molten NaI, have been studied by molecular dynamics simulations. These models are based on a rigid ion potential to which the induced dipole polarization of the anions is added. The polarization is added in such a way that point dipoles are induced on the anions by both local electric field and short-range damping interactions that oppose the electrically induced dipole moments. The two polarizable ion models differ only in the range of the damping polarization interactions. The influence of the induced anion polarization on the different properties of simulated molten NaI is discussed.     

Phase-sensitive sum frequency revealing accommodation of bicarbonate ions, and charge separation of sodium and carbonate ions within the air/water interface

Interfacial water structure plays a key role in many chemical, biological, and environmental processes. Here, in addition to conventional VSFG, we employ phase-sensitive sum frequency generation (PS-SFG) to investigate the average direction of the transition dipole of interfacial water molecules that is intrinsically contained in the sign of the second-order nonlinear susceptibility, χ((2)). The orientation of water at air/aqueous inorganic salt interfaces of Na(2)CO(3) and NaHCO(3) was inferred from the direct measurement of the transition dipole moment of the interfacial water molecules. It is found that bicarbonate and its counterion sodium do not significantly perturb the interfacial water structure, whereas carbonate strongly orients water so that the water hydrogens point down toward the bulk solution. This is consistent with the picture of carbonate anions residing many layers below the water surface with a preference for the sodium cations to be above the anions and thereby closer to the topmost layer of the water surface.     

Physical properties of ionic liquids consisting of the 1-butyl-3-methylimidazolium cation with various anions and the bis(trifluoromethylsulfonyl)imide anion with various cations

Physical properties of 4 room-temperature ionic liquids consisting of the 1-butyl-3-methylimidazolium cation with various perfluorinated anions and the bis(trifluoromethylsulfonyl)imide (Tf2N-) anion with 12 pyrrolidinium-, ammonium-, and hydroxyl-containing cations are reported. Electronic structure methods are used to calculate properties related to the size, shape, and dipole moment of individual ions. Experimental measurements of phase-transition temperatures, densities, refractive indices, surface tensions, solvatochromic polarities based on absorption of Nile Red, 19F chemical shifts of the Tf2N- anion, temperature-dependent viscosities, conductivities, and cation diffusion coefficients are reported. Correlations among the measured quantities as well as the use of surface tension and molar volume for estimating Hildebrand solubility parameters of ionic liquids are also discussed.     

Hydrogen bonding and induced dipole moments in water: predictions from the Gaussian charge polarizable model and Car-Parrinello molecular dynamics

We compare a new classical water model, which features Gaussian charges and polarizability (GCPM) with ab initio Car-Parrinello molecular dynamics (CPMD) simulations. We compare the total dipole moment, the total dipole moment distribution, and degree of hydrogen bonding at ambient to supercritical conditions. We also compared the total dipole moment calculated from both the electron density (partitioning the electron density among molecules based on a zero electron flux condition), and from the center of localized Wannier function centers (WFCs). Compared to CPMD, we found that GCPM overpredicts the dipole moment derived by partitioning the electron density and underpredicts that obtained from the WFCs, but exhibits similar trends and distribution of values. We also found that GCPM predicted similar degrees of hydrogen bonding compared to CPMD and has a similar structure.     

Can octupolar molecules be poled by an external electric field?

Octupolar molecules are generally believed to be of potential use in developing nonlinear optical materials owing to the fact that they do not easily form molecular aggregates. This is often put against the conjectured drawback that electric fields have no poling, or ordering, effect for this class of molecules because of the lack of a permanent ground state dipole moment. In this paper, we analyze this notion in some detail and present results from molecular dynamics computer simulations of an ensemble of a prototypical octupolar molecule, the 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) molecule, dissolved in chloroform. It is found that TATB molecules indeed show rather significant dipole moments in solutions because of the dual action of the thermal motions of the atoms and the strong intermolecular interactions. Applied electric fields accordingly show significant effects on the orientations of the molecular dipole moments. We also find that TATB molecules can aggregate because of the strong hydrogen-bonding interactions between the molecules, though they lack a static permanent dipole moment. Thus, the simulation results for TATB molecules in solution present us with a totally different notion about the collective properties of octupolar molecules. Taking account of quantum chemistry results, we found that the collective molecular nonlinear optical (NLO) properties are enhanced after the onset of the electric field, showing significant anisotropic characteristics.     

Molecular dynamics study of polarization effects on AgI

Three different models of AgI are studied by molecular dynamics simulations. The first one is the rigid ion model (RIM) with the effective pair potential of the Vashishta and Rahman form and the parametrization proposed by Shimojo and Kobayashi. The other two are polarizable ion models in which the induced polarization effects have been added to the RIM effective pair potential. In one of them (PIM1), only the anions are assumed to be polarizable by the local electric field. In the other one (PIM2s), the silver polarization is also included, and a short-range overlap-induced polarization opposes the electrically induced dipole moments. This short-range polarization is proved to be necessary to avoid overpolarization when both species are assumed to be polarizable. The three models reproduce the superionic character of alpha-AgI at 573 K and the liquid behavior of molten AgI at 923 K. The averaged spatial distribution of the cations in the alpha-phase obtained for PIM1 appears to be in better agreement with experimental data analysis. The PIM1 also reproduces the structure factor prepeak at about 1 A(-1) observed from neutron diffraction data of molten AgI. The three models retain in the liquid phase the superionic character of alpha-AgI, as the mobility of the cations is significantly larger than that for the anions. The ionic conductivity for the polarizable ion models is in better agreement with experimental data for alpha-AgI and molten AgI.     

Electrostatic properties of aqueous salt solution interfaces: a comparison of polarizable and nonpolarizable ion models

The effects of ion force field polarizability on the interfacial electrostatic properties of approximately 1 M aqueous solutions of NaCl, CsCl, and NaI are investigated using molecular dynamics simulations employing both nonpolarizable and Drude-polarizable ion sets. Differences in computed depth-dependent orientational distributions, "permanent" and induced dipole and quadrupole moment profiles, and interfacial potentials are obtained for both ion sets to further elucidate how ion polarizability affects interfacial electrostatic properties among the various salts relative to pure water. We observe that the orientations and induced dipoles of water molecules are more strongly perturbed in the presence of polarizable ions via a stronger ionic double layer effect arising from greater charge separation. Both anions and cations exhibit enhanced induced dipole moments and strong z alignment in the vicinity of the Gibbs dividing surface (GDS) with the magnitude of the anion induced dipoles being nearly an order of magnitude larger than those of the cations and directed into the vapor phase. Depth-dependent profiles for the trace and z z components of the water molecular quadrupole moment tensors reveal 40% larger quadrupole moments in the bulk phase relative to the vapor which mimics a similar observed 40% increase in the average water dipole moment. Across the GDS, the water molecular quadrupole moments increase nonmonotonically (in contrast to the water dipoles) and exhibit a locally reduced contribution just below the surface due to both orientational and polarization effects. Computed interfacial potentials for the nonpolarizable salts yield values 20-60 mV more positive than pure water and increase by an additional 30-100 mV when ion polarizability is included. A rigorous decomposition of the total interfacial potential into ion monopole, water and ion dipole, and water quadrupole components reveals that a very strong, positive ion monopole contribution is offset by negative contributions from all other potential sources. Water quadrupole components modulated by the water density contribute significantly to the observed interfacial potential increments and almost entirely explain observed differences in the interfacial potentials for the two chloride salts. By lumping all remaining nonquadrupole interfacial potential contributions into a single "effective" dipole potential, we observe that the ratio of quadrupole to "effective" dipole contributions range from 2:1 in CsCl to 1:1.5 in NaI, suggesting that both contributions are comparably important in determining the interfacial potential increments. We also find that oscillations in the quadrupole potential in the double layer region are opposite in sign and partially cancel those of the "effective" dipole potential.     

Solvent effects on electronic properties from Wannier functions in a dimethyl sulfoxide/water mixture

We present an efficient implementation for the calculation of maximally localized Wannier functions (MLWFs) during parallel Car-Parrinello molecular dynamics simulations. The implementation is based on a block Jacobi method. The calculation of MLWFs results in only a moderate (10%-20%) increase in computer time. Consequently it is possible to calculate MLWFs routinely during Car-Parrinello simulations. The Wannier functions are then applied to derive molecular dipole moments of dimethyl sulfoxide (DMSO) in gas phase and aqueous solution. We observe a large increase of the local dipole moment from 3.97 to 7.39 D. This large solvent effect is caused by strong hydrogen bonding at the DMSO oxygen atom and methyl groups. Decomposing the dipole moment into local contributions from the S-O bond and the methyl groups is used to understand the electrostatic response of DMSO in aqueous solution. A scheme is given to derive charges on individual atoms from the MLWFs using the D-RESP methodology. The charges also display large solvent effects and give insight into the transferability of recent force field models for DMSO.     

Carbon dioxide capture by aminoalkyl imidazolium-based ionic liquid: a computational investigation

Efficient technologies/processes for CO(2) capture are greatly desired, and ionic liquids are recognized as promising materials for this purpose. However, the mechanisms for selectively capturing CO(2) by ionic liquids are unclear. In this study, the interactions between CO(2) and 1-n-amino-alkyl-3-methyl-imidazolium tetrafluoroborate, an amino imidazolium ionic liquid (AIIL), in its CO(2) capturing process, are elucidated with both quantum chemistry and molecular dynamics approaches on the molecular level. The effects of the straight aminoalkyl chain length in imidazolium-based cations on CO(2) capture are explored, and thereby the factors governing CO(2) capture for this ionic liquid family, e.g., ionic liquid structure, charge distribution, intermolecular interactions, thermodynamic properties and absorption kinetics, are analyzed. Molecular dynamics simulations are used to study the diffusion of the involved compounds and liquid structures of the CO(2)-AIIL systems. The results show that the amino-alkyl chain length plays an important role in governing the absorption properties of AIILs, including the free energies of absorption, equilibrium constants, desorption temperature, absorption rate constants, diffusion coefficients, and organization of CO(2) around cations and anions. This study provides useful information about rational design of ionic liquids for efficient CO(2) capture.     

Dynamical Dimension to the Hofmeister Series: Insights from First‐Principles Simulations

A systematic characterization of the competing kosmotropic and chaotropic effects of a series of divalent salts on the aqueous H‐bonding structure by means of first‐principles molecular dynamics simulations is presented. The structural properties are quantified by means of experimental and computed ¹H NMR chemical shifts, whereby the local environments of cations and anions can be discriminated. Complementary to the well‐established structural features, a dynamical aspect is added to the concept of kosmotropes and chaotropes. The H‐bond dynamics, quantified in terms of the H‐bonding autocorrelation functions, shows a good correlation with the structural kosmotropic and chaotropic modifications, which are commonly referred to as the Hofmeister series. The considerably enhanced (reduced) fluctuations of the H‐bonding network in the hydration shells around the anions (cations) are a complementary dynamical dimension to the concept of kosmotropic/chaotropic behavior of solvated ions.     

Core ionization energies of atoms and ions calculated using the generalized Sturmian method

The physical event of the umbrella inversion of ammonia has been studied in detail by application of the formalisms of frontier orbital theory, the density functional theory, the localized molecular orbital method, and the energy partitioning analysis. An intuitive structure for the transition state and dynamics of the physical process of structural reorganization prior to inversion have been suggested. The computation starts with the CNDO/2 equilibrium geometry, and thereafter the cycle proceeds through all the conformations of ammonia obtained by varying the ∠HNH angle in steps of 2° from its equilibrium value up to the transition state. The geometry of each conformation is optimized with respect to the length of the N–H bond. The glimpses of the charge density reorganization during the movement of the molecule from equilibrium conformation toward the transition state is computed in terms of dipole moment and the quantum mechanical hybridizations of bond pair and lone pair of N atom through the localized molecular orbitals (LMOs) of all the conformations. Results demonstrate that as the geometry of the molecule begins to evolve through the reorganization of structure, the N–H bond length and the dipole moment begin to decrease, and the trend continues up to the transition state. The dipole moment of the molecule at the suggested transition state is zero. The computed nature of quantum mechanical hybridization of bond pair and lone pair of the N atom as a function of reaction coordinates of the ∠HNH angles reveals that the percentage of s character of the lone pair hybrid decreases and that of the bond pair hybrid forming the σ(N–H) bond increases during the process of geometry reorganization from the equilibrium shape to the transition state. The rationale of the zero dipole moment of the transition state for inversion is not straightforward from its point‐group symmetry, but is self‐evident from its electronic structure drawn in terms of LMOs. The electronic structure of the transition state, which may be drawn in terms of the LMOs, seems to closely reproduce its suggested intuitive structure. The pattern of variation of dipole moment and nature of the changes of the percentage of the s character in the lone pair hybrid creating dipole with the evolution of geometry during the physical process of structural reorganization for the inversion are found to be nicely correlated according to the suggestion of Coulson. The profiles of the increasing strength of the N–H bond and the increasing percentage of s character of the bond pair hybrid of N atom forming this bond as a function of reaction coordinates are also found to be correlated in accordance with the suggestion of Coulson. The profile of global hardness as a function of reaction coordinate seems to demonstrate that the dynamics of the evolution of the molecular structure from equilibrium shape to the transition state following the course of suggested mode of structural reorganization conforms to the principle of maximum hardness (PMH). The profiles of parameters like the energies of highest occupied and lowest unoccupied molecular orbital (HOMO and LUMO), the gap in energy between HOMO and LUMO, the global hardness, the global softness, and chemical potential as a function of reaction coodinates of a continuous structural evolution from equilibrium shape to the transition state mimic the potential energy diagram of the total energy. Both the frontier orbital parameters and the density functional quantities are found to be equally effective and reliable to monitor the process of necessary structural reorganization prior to the inversion of mofecules. An energy partitioning analysis demonstrates that the origin of barrier has no unique single source rather as many as four mutually exclusive but interacting one‐ and two‐center energy terms within the molecule entail the origin and the height of the barrier. From a close analysis of the results, it seems highly probable that the necessary structural reorganization prior to umbrella inversion of ammonia most realistically occurs following the course of normal modes of vibration of the molecule. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 80: 1–26, 2000     

Molecular dynamics study of polarizable ion models for molten AgBr

Three different polarizable ion models for molten AgBr have been studied by molecular dynamics simulations. The three models are based on a rigid ion model (RIM) with a pair potential of the type proposed by Vashishta and Rahman for alpha-AgI, to which the induced dipole polarization of the ions is added. In the first (PIM1) the dipole moments are only induced by the local electric field, while in the other two (PIM1s and PIM2s) a short-range overlap induced polarization opposes the electrically induced dipole moments. In the PIM1 and the PIM1s only the anions are assumed polarizable, while in the PIM2s both species are polarizable. Long molecular dynamics simulations show that the PIM2s is an unphysical model since, for some improbable but possible critical configurations, the ions become infinitely polarized. The results of using the PIM1, the PIM1s, as well as those of the simple RIM, have been compared for the static structure and ionic transport properties. The PIM1 reproduces the broad main peak of the total structure factor present in the neutron diffraction data, although the smoothed three-peak feature of this broad peak is slightly overestimated. The structural results for the PIM1s are intermediate between those for the RIM and the PIM1, but fail to reproduce the experimental features within the broad principal peak. Concerning the ionic transport properties, the value of the conductivity obtained using PIM1 is in good agreement with experimental values, while the self-diffusion coefficients and the conductivity for the PIM1s are lower than the corresponding values using the PIM1 and the RIM.     

On the performance of molecular polarization methods. I. Water and carbon tetrachloride close to a point charge

The three main methods to implement molecular polarization (point dipoles, fluctuating charges, and shell model) are tested against high level ab initio calculations for a molecule (water, carbon tetrachloride) close to a point charge (at the distance of a lithium or magnesium ion). The goal is to check whether an approximation (linear polarization) strictly valid at large intermolecular distances is sufficiently accurate for liquid state molecular dynamics simulations, where strong polarization effects are to be expected at short separations. The monitored observable is the molecular dipole moment as a function of the charge-molecule distance for selected molecular orientations. Analytic formulas are derived for the components of the molecular polarization tensor, facilitating the optimization of the performance for each polarization method as a function of its underlying parameters. Overall, the methods studied provide a remarkably good representation of the induced dipole, with no divergences appearing even at the shortest distances. For water close to a monovalent point charge the point dipole model, implemented with one or three dipoles, accurately reproduces the water dipole moment at all distances. Deficiencies appear as the molecular polarizability and/or charge increase: basically, the ab initio induced moments grow faster at intermediate distances than the linear increase characteristic of the phenomenological polarization methods, suggesting that nonlinear effects (hyperpolarizability) cannot be neglected in these cases. Regarding the capabilities of each method, the point dipole method is the one that performs best overall, with the shell model achieving acceptable results in most instances. The fluctuating charge method shows some noticeable limitations for implementations of comparable complexity (in terms of the number of sites required).     

Insight into the chemical bonding and electrostatic potential: A charge density study on a quinazoline derivative

2,3-Trimethylene-3,4-dihydroquinazoline shares the heterocyclic core with natural compounds and synthetic drugs. The hydrochloride of the compound forms excellent dihydrate crystals which have allowed us to collect high-resolution X-ray diffraction data and obtain the experimental charge density. The solid may be understood as built up from pairs of heterocyclic cations and chloride anions; a direct hydrogen bond links the halide to the formally cationic pyrimidine NH group. The hydrate water molecules interact with the anions, forming an infinite chain along the crystallographic a axis between the stacks of the heterocyclic cations. Based on the experimental charge density, a dipole moment of 16.1 Debye is calculated for a pair of the hydrogen-bonded quinazolinium cation and the chloride anion in the extended crystal structure.     

Theory of relaxation spectra and dielectric relaxation of rigid rodlike particles incorporated in a polymer network

The theory of molecular mobility and relaxation spectra is developed for rodlike particles embedded in a polymer network with allowance for the involvement of the particles in collective network dynamics through topological entanglements with network fragments. A regular cubic coarse-grained network model is used, where the motion of junctions describes the mobility of large fragments (domains) of the initial network with a size equal to the distance between adjacent rodlike particles. The involvement of the rods in collective network dynamics is taken into account by introducing an effective quasi-elastic potential acting between the rods and junctions of the coarse-grained network and preventing long-distance diffusion of the embedded particles. The viscoelastic parameters of the coarse-grained (“renormalized”) network are functions of the viscoelastic characteristics of the initial network. The relaxation time spectra are calculated as well as the frequency dependences of the dielectric loss factor of the embedded particles that possess a permanent dipole moment directed along the major axis of each rod. Depending on the ratio between the viscoelastic characteristics of the rods and the network, the frequency dependence of the dielectric loss factor may have two maxima. The high-frequency maximum corresponds to local orientational movements of particles at fixed junctions of the coarse-grained network, which correspond to the position of the domain centers in the initial network. The low-frequency maximum corresponds to movements of particles involved in large-scale dynamics of network fragments. The dependence of the dielectric loss factor on the ratio between the viscoelastic parameters of the rods and the network is studied.     

Structural and dynamic properties of concentrated alkali halide solutions: a molecular dynamics simulation study

The physicochemical properties of alkali halide solutions have long been attributed to the collective interactions between ions and water molecules in the solution, yet the structure of water in these systems and its effect on the equilibrium and dynamic properties of these systems are not clearly understood. Here, we present a systematic view of water structure in concentrated alkali halide solutions using molecular dynamics simulations. The results of the simulations show that the size of univalent ions in the solution has a significant effect on the dynamics of ions and other transport properties such as the viscosity that are correlated with the structural properties of water in aqueous ionic solution. Small cations (e.g., Li+) form electrostatically stabilized hydrophilic hydration shells that are different from the hydration shells of large ions (e.g., Cs+) which behave more like neutral hydrophobic particles, encapsulated by hydrogen-bonded hydration cages. The properties of solutions with different types of ion solvation change in different ways as the ion concentration increases. Examples of this are the diffusion coefficients of the ions and the viscosities of solutions. In this paper we use molecular dynamics (MD) simulations to study the changes in the equilibrium and transport properties of LiCl, RbCl, and CsI solutions at concentrations from 0.22 to 3.97 M.