[Show abstract][Hide abstract] ABSTRACT: Interactions of a lithium bis(trifluoromethane sulfonyl)imide (Li+Tf2N-) ion pair with oligoethers are investigated via the density function theory (DFT). As a model for polymer electrolytes polyethyleneoxide (PEO) and perfluoropolyether (PFPE), CR3(OCR2CR2)n=1-5OCR3 (R = H or F) are considered. Topographical analysis of the molecular electrostatic potential (MESP) is performed to determine preferential binding sites of Li+. Our study shows that the MESP value near oxygen sites of the polymer backbone is more negative for PEO than for PFPE. This result indicates that substitution of hydrogen by fluorine in polyethers leads to reduction in Li+–polymer interactions, in concert with the experimental ionic conductivity results. S-O stretching vibrations of Tf2N- are calculated for the lithium salt in the presence and absence of the electrolytes. The blue and red shifts predicted for S-O stretching are further explained by natural bond orbital analysis and molecular electron density topography. The S-O stretching vibrations can be used as a useful tool to understand the ion pair interactions and thus ion transport phenomena in the polymer electrolytes.
Full-text · Article · Feb 2016 · Physical Chemistry Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: Supercapacitors with graphene oxide (GO) electrodes in a parallel plate configuration are studied with molecular dynamics (MD) simulations. The full range of electrode oxidation from 0% (pure graphene) to 100% (fully oxidized GO) is investigated by decorating the graphene surface with hydroxyl groups. The ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI +BF4-) is examined as an electrolyte. Capacitance tends to decrease with increasing electrode oxidation, in agreement with several recent measurements. This trend is attributed to the decreasing reorganization ability of ions near the electrode and a widening gap in the double layer structures as the density of hydroxyl groups on the electrode surface increases.
[Show abstract][Hide abstract] ABSTRACT: Dielectric relaxation, related polarization and conductivity, and solvation dynamics of the ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate BMI+PF6-, are studied via molecular dynamics computer simulations in the temperature range 300K ≤ T ≤ 500K. Two main bands of its dielectric loss spectrum show differing temperature behaviors. As T increases, the absorption band in the microwave region shifts to higher frequencies rapidly, whereas the location of the bi-modal far-IR band remains nearly unchanged. Their respective intensities tend to decrease and increase. The static dielectric constant of BMI+PF6- is found to decrease weakly with T. The ultrafast inertial component of solvation dynamics becomes, in general, slower while their dissipative relaxation component becomes faster. Roles played by ion reorientations and translations in governing dynamic and static dielectric properties of the ionic liquid are examined. A brief comparison with available experimental results is also made.
No preview · Article · Aug 2013 · The Journal of Physical Chemistry B
[Show abstract][Hide abstract] ABSTRACT: Structural properties of a small hexapeptide molecule modeled after metal-binding siderochrome immersed in a room-temperature ionic liquid (RTIL) are studied via molecular dynamics simulations. We consider two different RTILs, each of which is made up of the same cationic species, 1-butyl-3-methylimidazolium (BMI+), but different anions, hexafluorophosphate (PF6-) and chloride (Cl-). We investigate how anionic properties such as hydrophobicity/hydrophilicity or hydrogen bonding capability affect the stabilization of the peptide in RTILs. To examine the effect of peptide-RTIL electrostatic interactions on solvation, we also consider a hypothetical solvent (BMICl0)-Cl-0, a non-ionic counter-part of BMI+Cl-. For reference, we investigate solvation structures in common polar solvents, water and dimethylsulfoxide (DMSO). Comparison of BMI+Cl- and (BMICl0)-Cl-0 shows that electrostatic interactions of the peptide and RTIL play a significant role in the conformational fluctuation of the peptide. For example, strong electrostatic interactions between the two favor an extended conformation of the peptide by reducing its structural fluctuations. The hydrophobicity/hydrophilicity of RTIL anions also exerts a notable influence; specifically, structural fluctuations of the peptide become reduced in more hydrophilic BMI+Cl-, compared with those in more hydrophobic BMI+PF6-. This is ascribed to the good hydrogen-bond accepting power of chloride anions, which enables them to bind strongly to hydroxyl groups of the peptide and to stabilize its structure. Transport properties of the peptide are examined briefly. Translations of the peptide significantly slow down in highly viscous RTILs.
No preview · Article · Nov 2012 · Bulletin- Korean Chemical Society
[Show abstract][Hide abstract] ABSTRACT: Supercapacitors with two single-sheet graphene electrodes in the parallel plate geometry are studied via molecular dynamics (MD) computer simulations. Pure 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI+BF4-) and a 1.1 M solution of EMI+BF4- in acetonitrile are considered as prototypes of room-temperature ionic liquids (RTILs) and organic electrolytes. Electrolyte structure, charge density and associated electric potential are investigated by varying the charges and separation of the two electrodes. Multiple charge layers formed in the electrolytes in the vicinity of the electrodes are found to screen the electrode surface charge almost completely. As a result, the supercapacitors show nearly an ideal electric double layer behavior, i.e., the electric potential exhibits essentially a plateau behavior in the entire electrolyte region except for sharp changes in screening zones very close to the electrodes. Due to its small size and large charge separation, BF4- is considerably more efficient in shielding electrode charges than EMI+. In the case of the acetonitrile solution, acetonitrile also plays an important role by aligning its dipoles near the electrodes; however, the overall screening mainly arises from ions. Because of the disparity of shielding efficiency between cations and anions, the capacitance of the positively-charged anode is significantly larger than that of the negatively-charged cathode. Therefore, the total cell capacitance in the parallel plate configuration is primarily governed by the cathode. Ion conductivity obtained via the Green-Kubo (GK) method is found to be largely independent of the electrode surface charge. Interestingly, EMI+BF4- shows higher GK ion conductivity than the 1.1 M acetonitrile solution between two parallel plate electrodes.
Full-text · Article · Jan 2012 · Faraday Discussions
[Show abstract][Hide abstract] ABSTRACT: Energy density of supercapacitors based on a single-sheet graphene electrode is studied via molecular dynamics (MD) computer simulations. Two electrolytes of different types, pure 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI+BF4–) and an 1.1 M solution of EMI+BF4– in acetonitrile, are considered as a prototypical room-temperature ionic liquid (RTIL) and organic electrolyte, respectively. Structure of ions near the electrode surface varies significantly with its charge density, especially in pure RTIL. Specific capacitance normalized to the electrode surface area is found to be higher in EMI+BF4– than in acetonitrile solution by 55–60%. This is due to strong screening of the electrode charge by RTIL ions in the former. The RTIL screening behavior is found to be rather insensitive to temperature T. As a result, the capacitance of supercapacitors based on pure EMI+BF4– decreases by less than 5% as T increases from 350 to 450 K. The difference in size and shape between cations and anions and the resulting difference in their local charge distribution as counterions near the electrified graphene surface yield cathode–anode asymmetry in the electrode potential in RTIL. As a consequence, specific capacitance of the positively charged electrode is higher than that of the negatively charged electrode by more than 10%. A similar degree of disparity in electrode capacitance is also found in acetonitrile solution because of its nonvanishing potential at zero charge. Despite high viscosity and low ion diffusivity of EMI+BF4–, its overall conductivity is comparable to that of the acetonitrile solution thanks to its large number of charge carriers. The present study thus suggests that as a supercapacitor electrolyte, RTILs are comparable in power density to organic electrolytes, while the former yield considerably better energy density than the latter at a given cell voltage.
Full-text · Article · Nov 2011 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: The structure and dynamics of benzene inside and outside of single-walled carbon nanotubes (SWNTs) in the (n,n) armchair configuration are studied via molecular dynamics computer simulations. Irrespective of the nanotube diameter, benzene molecules form cylindrical solvation shell structures on the outside of the nanotubes. Their molecular planes near the SWNTs in the first external solvation shell are oriented parallel to the nanotube surface, forming a π-stacked structure between the two. By contrast, the benzene distributions in the interior of the SWNTs are found to vary markedly with the nanotube diameter. In the case of the (7,7) and (8,8) nanotubes, internal benzene forms a single-file distribution, either in a vertex-to-vertex (n = 7) or face-to-face (n = 8) orientation between two neighboring molecules. Inside a slightly wider (9,9) nanotube channel, however, a cylindrical single-shell distribution of benzene arises. A secondary solvation structure, which begins to appear inside (10,10), develops into a full structure separate from the first internal solvation shell in (12,12). The ring orientation of internal benzene is generally parallel to the nanotube wall for n = 9-12, while it becomes either slanted with respect to (n = 7), or perpendicular to (n = 8), the nanotube axis. The confinement inside the small nanotube pores exerts a strong influence on the dynamics of benzene. Both translational and rotational dynamics inside SWNTs are slower and more anisotropic than in liquid benzene. It is also found that reorientational dynamics of internal benzene deviate dramatically from the rotational diffusion regime and change substantially with the nanotube diameter.
Full-text · Article · Mar 2011 · Physical Chemistry Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: Solvation structure and dynamics of a saturated solution of carbon dioxide in the room-temperature ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMI+PF6-) at 313 K and 0.15 kbar are investigated via molecular dynamics computer simulations by employing a diatomic probe solute. It is found that the mixture shows preferential solvation, which is mainly controlled by the solute-BMI+PF6- electrostatic interactions and thus dictates differing roles for CO2 as the solute charge distribution varies. The local structure and density of BMI+PF6- and CO2 in the vicinity of the solute become enhanced and reduced, respectively, as its dipole moment increases. As a result, equilibrium solvation dynamics of a nonpolar solute in the mixture have a strong CO2 character, whereas those of a dipolar solute are very similar to, albeit faster than, solvation dynamics in pure BMI+PF6-. Related nonequilibrium solvent response couched in dynamic Stokes shifts and accompanying solvation structure relaxation, in particular, CO2 structure reorganization, shows interesting dependence on the solute charge distribution. Ion transport in the mixture is much faster than in pure BMI+PF6-, indicating that the addition of cosolvent CO2 reduces the viscosity of the ionic liquid, significantly. The effective polarity of the mixture, measured as solvation-induced stabilization of a dipolar solute, is found to be comparable to that of neat BMI+PF6-, consonant with solvatochromic measurements.
No preview · Article · Aug 2010 · The Journal of Physical Chemistry B
[Show abstract][Hide abstract] ABSTRACT: Supercapacitors composed of carbon nanotube (CNT) micropores in the room-temperature ionic liquid (RTIL) 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI+BF4(-)) are studied via molecular dynamics (MD)computer simulations. It is found that the distribution of RTIL ions inside the micropore varies significantly with the pore size. Internal solvation of small (6,6) and (7,7) CNTs with an electrified interior wall is effected almost exclusively via counterions. Surprisingly, these counterions, even though they all have the same charge, lead to a charge density characterized by multiple layers with alternating signs. This intriguing feature is attributed to the extended nature of RTIL ion charge distributions, which result in charge separation through preferential orientation inside the electrified nanotubes. In the case of larger (10,10) and (15,15) CNTs, counterions and coions develop multilayer solvation structures. The specific capacitance normalized to the pore surface area is found to increase as the CNT diameter decreases from (15,15) to (7,7). As the pore size further reduces from (6,6) to(5,5), however, the specific capacitance diminishes rapidly. These findings are in excellent agreement with recent experiments with carbon-based materials. A theoretical model based on multiple charge layers is proposed to understand both the MD and experimental results.
[Show abstract][Hide abstract] ABSTRACT: Adiabatic electron transfer (ET) in the room-temperature ionic liquid 1-butyl-3-methyldicyanamide (BMI(+)DCA(-)) and in aprotic acetonitrile is studied with molecular dynamics (MD) computer simulation techniques using a model diatomic reaction complex. The influence of barrier crossing dynamics on ET kinetics is examined directly via constrained reaction coordinate MD, while the corresponding effect arising from activation and deactivation processes in the reactant and product states is analyzed with the aid of simulation results on solvation dynamics. The departure from the transition state theory (TST) rate constant caused by barrier crossing is found to be moderate and comparable in BMI(+)DCA(-) and acetonitrile despite a huge difference in their viscosity. A theoretical analysis shows that the Grote-Hynes theory yields a reasonable agreement with the MD results on barrier crossing in both solvents, whereas the Kramers theory fails completely in BMI(+)DCA(-). The influence of activation and deactivation dynamics on ET kinetics in BMI(+)DCA(-) varies markedly with reaction free energetics because of the biphasic nature of solvation dynamics, viz., ultrafast subpicosecond relaxation followed by slow subnanosecond decay. This indicates that dynamic factors controlling adiabatic ET in BMI(+)DCA(-) transition from barrier crossing to activation/deactivation as the barrier height for the forward and/or backward reaction decreases. This regime change of ET dynamics is accompanied by the breakdown of TST as the reaction becomes activation-limited in BMI(+)DCA(-). By contrast, activation and deactivation dynamics do not play a major role in acetonitrile.
No preview · Article · Oct 2009 · The Journal of Physical Chemistry B
[Show abstract][Hide abstract] ABSTRACT: Single- and double-walled carbon nanotubes in the armchair configuration solvated in the room-temperature ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI(+)BF(4)(-)) are studied via molecular dynamics (MD) computer simulations. Cations and anions show smeared-out, cylindrical shell-like distributions outside of the nanotubes irrespective of the nanotube diameter. The ion distributions inside the nanotubes vary markedly with their diameter. For example, in the case of (n,n) single-walled nanotubes, EMI(+) and BF(4)(-) ions separately form single-shell zigzag and chiral distributions for (8,8) and (10,10), respectively, while (12,12) develops a second internal solvation structure. The first internal solvation shell of (15,15) nanotubes consists of alternating layers of cations and anions along the nanotube axis. In the azimuthal direction, these cations and anions, respectively, form a pentagonal structure, whereas the corresponding ions for (20,20) show disordered octagonal structures. The smallest nanotube that allows solvent ions inside the tunnel is (7,7) with a diameter of 0.95 nm, which shows a single file distribution of internal ions, Imidazole rings of cations in the first internal and external solvation shells are mainly parallel to the nanotube surface, indicating pi-stacking between the nanotubes and EMI(+) ions there.
[Show abstract][Hide abstract] ABSTRACT: Dielectric susceptibility and related conductivity of the neat ionic liquid 1-ethyl-3-methylimidazolium hexafluorophosphate (EMI+PF6-) are studied via molecular dynamics computer simulations. Both ion translations and reorientations contribute to dielectric relaxation, while their cross-correlation does not play any significant role. Interestingly, ion translational dynamics are found to enhance the static dielectric constant epsilon 0. The increment in epsilon 0 is attributed to rapid development of large anticorrelation in the autocorrelation function of the ionic current, i.e., hindered ion translations of strong librational character. One consequence of hindered translational dynamics is that the real part of conductivity has a maximum in the terahertz region and decreases with diminishing frequency. This in turn yields significant dielectric absorption in the far-IR region, consonant with recent terahertz time-domain spectroscopy measurements. Reorientational dynamics of cations show a marked deviation from diffusion. The well-known relation in the diffusion regime for reorientational correlation times tau R(l) proportional, variant [ l( l + 1)] (-1) fails completely for EMI+PF6-, where l is the order of Legendre polynomials used in the expansion of reorientational time correlation functions. It is found that dielectric continuum theory generally does not provide a reliable framework to describe solvation dynamics in EMI+PF6- even though the inclusion of ion conductivity in dielectric relaxation tends to improve the continuum description. This is ascribed mainly to electrostrictive effects absent in many continuum formulations.
No preview · Article · Oct 2008 · The Journal of Physical Chemistry B
[Show abstract][Hide abstract] ABSTRACT: The S(N)1 ionization reaction RX --> R(+) + X(-) for 2-chloro-2-methylpropane in ionic liquid 1-ethyl-3-methylimidazolium hexafluorophosphate ([emim](+)P(-)(6)) is studied via molecular dynamics computer simulations. By employing a two-state valence-bond description for electronic structure variations of the reaction complex, the free energy curve relevant to its SN1 ionization in [emim](+)P(-)(6)) is computed via the thermodynamic integration method and compared with those in water and in acetonitrile. It is found that the detailed reaction mechanism differs among the three solvents. To be specific, the dissociation of 2-chloro-2-methylpropane in [emim](+)P(-)(6)) is a stepwise process consisting of the formation of a solvent-separated ion pair and ensuing dissociation, while that in acetonitrile appears to proceed without any stable reaction intermediates. The S(N)1 pathway in water on the other hand is characterized by the formation of a contact ion pair, followed by dissociation to free ions. The activation free energy in water is much lower than those in [emim](+)P(-)(6)) and acetonitrile. Between the two latter solvents, the barrier height is lower in [emim](+)P(-)(6)) than in acetonitrile, indicating that the S(N)1 reactivity of 2-chloro-2-methylpropane would be higher in [emin](+)P(-)(6)) than in acetonitrile. Its implication for solvolysis in these solvents is briefly discussed.
No preview · Article · Mar 2008 · The Journal of Physical Chemistry B
[Show abstract][Hide abstract] ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
[Show abstract][Hide abstract] ABSTRACT: Free energies and dynamics of electron-transfer reactions for a diatomic donor-acceptor complex in ambient and supercritical water are studied via molecular dynamics computer simulations using a two-state electronic description. The free energy perturbation method is employed to examine diabatic electronic curves relevant to charge separation and recombination and electron self-exchange. It is found that the diabatic curves are anharmonic and vary with the charge distributions of the donor-acceptor complex, consonant with earlier studies under ambient conditions. Nonetheless, the extent of their anharmonicity and dependence on charge distributions grows with decreasing solvent density so that the Marcus free energy relationship breaks down in low-density supercritical water. The influence of solvent dynamics associated with activation, deactivation, and adiabatic barrier crossing on reaction kinetics is analyzed. Although the transition state theory generally provides a reasonable framework to describe electron-transfer kinetics in a wide range of thermodynamic conditions, the deviation of the reaction rate from the transition state theory predictions increases as the water density decreases.
No preview · Article · Feb 2008 · The Journal of Physical Chemistry B
[Show abstract][Hide abstract] ABSTRACT: A brief account of recent simulation and theoretical model studies of various solution-phase processes in room-temperature ionic liquids is given. These include structure and dynamics of equilibrium and nonequilibrium solvation, solute rotation and vibrational energy relaxation, and free energetics and dynamics of unimolecular electron-transfer reactions. Special attention is paid to both the aspects shared by and the contrasts with polar solvents under normal conditions. A brief comparison with available experiments is also made.
No preview · Article · Dec 2007 · Accounts of Chemical Research
[Show abstract][Hide abstract] ABSTRACT: The effects of solute polarizability on solvation and solute transport in the room-temperature ionic liquid 1-ethyl-3-methylimidazolium hexafluorophosphate (EMI+PF(6)-) are investigated via molecular dynamics simulations. A valence-bond description is employed to account for the instantaneous adjustment of the solute electronic charge distribution to the fluctuating solvent environment. It is found that the ultrafast inertial component of solvation dynamics becomes slower as the solute polarizability grows. Moreover, its contribution to overall solvent relaxation becomes reduced with increasing polarizability, especially in the case of nonequilibrium solvation dynamics. Overall, the inclusion of the solute electronic polarizability in the simulations improves the agreement with time-dependent Stokes shift measurements.
No preview · Article · Jun 2007 · The Journal of Physical Chemistry B
[Show abstract][Hide abstract] ABSTRACT: Reaction free energetics and dynamics of unimolecular electron-transfer processes in ionic liquid 1-ethyl-3-methylimidazolium hexafluorophosphate (EMI+PF6-) are investigated via molecular dynamics computer simulations employing a model diatomic solute and compared with those in aprotic acetonitrile. Using the free energy perturbation method, diabatic free energy curves relevant to charge separation and recombination processes are studied over a wide range of the reaction coordinate. The diabatic curves are found to vary with the solute charge distribution, especially in EMI+PF6-. Nevertheless, if the free energy of reaction is not that substantial, the Marcus free energy relationship holds reasonably well, provided that the reorganization free energy averaged between the reactant and product states is employed. The effective polarity, measured as solvation-induced stabilization of dipolar solutes, is higher for EMI+PF6- than for acetonitrile, consonant with many solvatochromic measurements. Thus, in the normal regime, activation barriers for charge separation and recombination reactions are, respectively, lower and higher in EMI+PF6- than in acetonitrile. The influence of solvent dynamics on reaction kinetics through modulations of activation, deactivation, and barrier crossing is analyzed. Even though overall solvent relaxation dynamics in EMI+PF6- are considerably slower than those in acetonitrile, the deviation of the rate constant from the transition state theory predictions is found to be small for both solvents. Implications of this finding for other reactions in ionic liquids are briefly discussed.
No preview · Article · May 2007 · The Journal of Physical Chemistry B
[Show abstract][Hide abstract] ABSTRACT: Vibrational energy relaxation (VER) dynamics of a diatomic solute in ionic liquid 1-ethyl-3-methylimidazolium hexafluorophosphate (EMI(+)PF(6) (-)) are studied via equilibrium and nonequilibrium molecular dynamics simulations. The time scale for VER is found to decrease markedly with the increasing solute dipole moment, consonant with many previous studies in polar solvents. A detailed analysis of nonequilibrium results shows that for a dipolar solute, dissipation of an excess solute vibrational energy occurs almost exclusively via the Lennard-Jones interactions between the solute and solvent, while an oscillatory energy exchange between the two is mainly controlled by their electrostatic interactions. Regardless of the anharmonicity of the solute vibrational potential, VER becomes accelerated as the initial vibrational energy increases. This is attributed primarily to the enhancement in variations of the solvent force on the solute bond, induced by large-amplitude solute vibrations. One interesting finding is that if a time variable scaled with the initial excitation energy is employed, dissipation dynamics of the excess vibrational energy of the dipolar solute tend to show a universal behavior irrespective of its initial vibrational state. Comparison with water and acetonitrile shows that overall characteristics of VER in EMI(+)PF(6) (-) are similar to those in acetonitrile, while relaxation in water is much faster than the two. It is also found that the Landau-Teller theory predictions for VER time scale obtained via equilibrium simulations of the solvent force autocorrelation function are in reasonable agreement with the nonequilibrium results.
No preview · Article · Aug 2006 · The Journal of Chemical Physics