Youngseon Shim

Seoul National University, Seoul, Seoul, South Korea

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Publications (23)102.45 Total impact

  • Youngseon Shim, Hyung J Kim
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    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.
    The Journal of Physical Chemistry B 08/2013; · 3.61 Impact Factor
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    Youngseon Shim, Hyung J Kim, Younjoon Jung
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    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.
    Faraday Discussions 01/2012; 154:249-63; discussion 313-33, 465-71. · 3.82 Impact Factor
  • Youngseon Shim, YounJoon Jung, Hyung J. Kim
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    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.
    The Journal of Physical Chemistry C. 11/2011; 116(34).
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    Youngseon Shim, Younjoon Jung, Hyung J Kim
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    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.
    Physical Chemistry Chemical Physics 03/2011; 13(9):3969-78. · 3.83 Impact Factor
  • Youngseon Shim, Hyung J Kim
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    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.
    The Journal of Physical Chemistry B 08/2010; 114(31):10160-70. · 3.61 Impact Factor
  • Youngseon Shim, Hyung J Kim
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    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.
    ACS Nano 04/2010; 4(4):2345-55. · 12.03 Impact Factor
  • Youngseon Shim, Hyung J Kim
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    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.
    The Journal of Physical Chemistry B 10/2009; 113(39):12964-72. · 3.61 Impact Factor
  • Youngseon Shim, Hyung J Kim
    ACS Nano 08/2009; 3(7):1693-702. · 12.03 Impact Factor
  • Youngseon Shim, Hyung J Kim
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    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.
    The Journal of Physical Chemistry B 10/2008; 112(35):11028-38. · 3.61 Impact Factor
  • Youngseon Shim, Hyung J Kim
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    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.
    The Journal of Physical Chemistry B 03/2008; 112(9):2637-43. · 3.61 Impact Factor
  • Youngseon Shim, Hyung J Kim
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    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.
    The Journal of Physical Chemistry B 02/2008; 112(2):585-94. · 3.61 Impact Factor
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    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.
    ChemInform 01/2008; 39(7).
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    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.
    Accounts of Chemical Research 12/2007; 40(11):1130-7. · 20.83 Impact Factor
  • Daun Jeong, Youngseon Shim, M Y Choi, Hyung J Kim
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    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.
    The Journal of Physical Chemistry B 06/2007; 111(18):4920-5. · 3.61 Impact Factor
  • Youngseon Shim, Hyung J Kim
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    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.
    The Journal of Physical Chemistry B 05/2007; 111(17):4510-9. · 3.61 Impact Factor
  • Youngseon Shim, Hyung J Kim
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    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.
    The Journal of Chemical Physics 08/2006; 125(2):24507. · 3.12 Impact Factor
  • Youngseon Shim, Daun Jeong, M Y Choi, Hyung J Kim
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    ABSTRACT: Reorientational time correlation functions C(l)(t)( identical withP(l)[cos theta(t)]) for a diatomic solute in 1-ethyl-3-methylimidazolium hexafluorophosphate (EMI(+)PF(6) (-)) are analyzed via molecular dynamics computer simulations, where <...> denotes an equilibrium ensemble average, P(l) the lth order Legendre polynomial and theta(t) the angle between the solute orientation at time t and its initial direction. Overall results are indicative of heterogeneous dynamics in EMI(+)PF(6) (-). For a small nondipolar solute, C(l)(t) are well-described as stretched exponential functions in wide time ranges. One striking feature is that after rapid initial relaxation, C(2)(t) decays more slowly than C(1)(t). As a result, the correlation time associated with the former is considerably longer than that with the latter. This is ascribed to solvent structural fluctuations, which allow large-amplitude solute rotations. As the solute size grows, relaxation of C(l)(t) approaches exponential decay.
    The Journal of Chemical Physics 08/2006; 125(6):61102. · 3.12 Impact Factor
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    Jinsong Duan, Youngseon Shim, Hyung J Kim
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    ABSTRACT: Solvation in supercritical water under equilibrium and nonequilibrium conditions is studied via molecular dynamics simulations. The influence of solute charge distributions and solvent density on the solvation structures and dynamics is examined with a diatomic probe solute molecule. It is found that the solvation structure varies dramatically with the solute dipole moment, especially in low-density water, in accord with many previous studies on ion solvation. This electrostrictive effect has important consequences for solvation dynamics. In the case of a nonequilibrium solvent relaxation, if there are sufficiently many water molecules close to the solute at the outset of the relaxation, the solvent response measured as a dynamic Stokes shift is almost completely governed by inertial rotations of these water molecules. By contrast, in the opposite case of a low local solvent density near the solute, not only rotations but also translations of water molecules play an important role in solvent relaxation dynamics. The applicability of a linear response is found to be significantly restricted at low water densities.
    The Journal of Chemical Physics 06/2006; 124(20):204504. · 3.12 Impact Factor
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    ABSTRACT: The molecular dynamics (MD) simulation study of solvation structure and free energetics in 1-ethyl-3-methylimidazolium chloride and 1-ethyl-3-methylimidazolium hexafluorophosphate using a probe solute in the preceding article [Y. Shim, M. Y. Choi and H. J. Kim, J. Chem. Phys. 122, 044510 (2005)] is extended to investigate dynamic properties of these liquids. Solvent fluctuation dynamics near equilibrium are studied via MD and associated time-dependent friction is analyzed via the generalized Langevin equation. Nonequilibrium solvent relaxation following an instantaneous change in the solute charge distribution and accompanying solvent structure reorganization are also investigated. Both equilibrium and nonequilibrium solvation dynamics are characterized by at least two vastly different time scales--a subpicosecond inertial regime followed by a slow diffusive regime. Solvent regions contributing to the subpicosecond nonequilibrium relaxation are found to vary significantly with initial solvation configurations, especially near the solute. If the solvent density near the solute is sufficiently high at the outset of the relaxation, subpicosecond dynamics are mainly governed by the motions of a few ions close to the solute. By contrast, in the case of a low local density, solvent ions located not only close to but also relatively far from the solute participate in the subpicosecond relaxation. Despite this difference, linear response holds reasonably well in both ionic liquids.
    The Journal of Chemical Physics 02/2005; 122(4):44511. · 3.12 Impact Factor
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    ABSTRACT: Solvation in 1-ethyl-3-methylmidazolium chloride and in 1-ethyl-3-methylimidazolium hexafluorophosphate near equilibrium is investigated via molecular dynamics computer simulations with diatomic and benzenelike molecules employed as probe solutes. It is found that electrostriction plays an important role in both solvation structure and free energetics. The angular and radial distributions of cations and anions become more structured and their densities near the solute become enhanced as the solute charge separation grows. Due to the enhancement in structural rigidity induced by electrostriction, the force constant associated with solvent configuration fluctuations relevant to charge shift and transfer processes is also found to increase. The effective polarity and reorganization free energies of these ionic liquids are analyzed and compared with those of highly polar acetonitrile. Their screening behavior of electric charges is also investigated.
    The Journal of Chemical Physics 02/2005; 122(4):44510. · 3.12 Impact Factor

Publication Stats

255 Citations
102.45 Total Impact Points

Institutions

  • 2001–2012
    • Seoul National University
      • • Department of Chemistry
      • • Department of Physics and Astronomy
      • • Center for Theoretical Physics
      Seoul, Seoul, South Korea
  • 2005–2011
    • Carnegie Mellon University
      • Department of Chemistry
      Pittsburgh, PA, United States