Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Q3: Focus Session: Solvation, Dynamics, and Reactivity in Complex Environments II |
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Sponsoring Units: DCP Chair: Francesco Paesani, University of California, San Diego Room: 107 |
Wednesday, March 5, 2014 2:30PM - 3:06PM |
Q3.00001: Water in Room Temperature Ionic Liquids Invited Speaker: Michael Fayer Room temperature ionic liquids (or RTILs, salts with a melting point below 25 $^{\circ}$C) have become a subject of intense study over the last several decades. Currently, RTIL application research includes synthesis, batteries, solar cells, crystallization, drug delivery, and optics. RTILs are often composed of an inorganic anion paired with an asymmetric organic cation which contains one or more pendant alkyl chains. The asymmetry of the cation frustrates crystallization, causing the salt's melting point to drop significantly. In general, RTILs are very hygroscopic, and therefore, it is of interest to examine the influence of water on RTIL structure and dynamics. In addition, in contrast to normal aqueous salt solutions, which crystallize at low water concentration, in an RTIL it is possible to examine isolated water molecules interacting with ions but not with other water molecules. Here, optical heterodyne-detected optical Kerr effect (OHD-OKE) measurements of orientational relaxation on a series of 1-alkyl-3-methylimidazolium tetrafluoroborate RTILs as a function of chain length and water concentration are presented. The addition of water to the longer alkyl chain RTILs causes the emergence of a long time bi-exponential orientational anisotropy decay. Such decays have not been seen previously in OHD-OKE experiments on any type of liquid and are analyzed here using a wobbling-in-a-cone model. The orientational relaxation is not hydrodynamic, with the slowest relaxation component becoming \textit{slower} as the viscosity \textit{decreases} for the longest chain, highest water content samples. The dynamics of isolated D$_{2}$O molecules in 1-butyl-3-methylimidazolium hexafluorophosphate (BmImPF$_{6}$) were examined using two dimensional infrared (2D IR) vibrational echo spectroscopy. Spectral diffusion and incoherent and coherent transfer of excitation between the symmetric and antisymmetric modes are examined. The coherent transfer experiments are used to address the nature of inhomogeneous broadening by observing $\sim$ 100 fs time scale oscillations in the shape of the 2D IR spectra. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q3.00002: Comparative Study of the Intermolecular Dynamics of Benzene/Ionic Liquid Mixtures and Benzyl Functionalized Ionic Liquids: Femtosecond OKE Spectroscopic Measurements Edward Quitevis, Lianjie Xue, George Tamas Ionic liquids (ILs) are salts with melting points below 100 $^{\circ}$C that are comprised of an organic cation and an inorganic or organic anion. There is great interest in obtaining a molecular level understanding of their unique physical and chemical properties, of which one of them is their ability, despite being inherently polar liquids, to dissolve large quantities of nonpolar aromatic compounds. In order to understand further the solvation of aromatic molecules in ILs, we have performed optical effect (OKE) spectroscopic measurements on 1-benzyl-3-methylimidazolum bistriflate, 1,3-dibenzylimidazolum bistriflate and the corresponding 1:1 and 2:1 benzene/1,3-dimethylimazolium bistrifate (C$_{6}$H$_{6}$/C$_{1}$C$_{1})$ mixtures. In contrast to being free in benzene/IL mixtures, the benzene rings are tethered to the imidazolium ring via methylene linkages in the case of first two ILs. The intermolecular Kerr spectra indicate that the motion of the benzene rings becomes increasingly more restricted in going from neat benzene to benzene dissolved in 1,3-dimethylimazolium bistrifate to benzene rings tethered to the imidazolium ring. This restriction causes the Kerr spectra effectively to shift to higher frequency in going from neat liquid benzene to C$_{6}$H$_{6}$/C$_{1}$C$_{1}$ mixtures to benzylimidazolium ILs. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q3.00003: Solvation dynamics in room temperature ionic liquids studied by ultrafast vibrational spectroscopy Sean Garrett-Roe, Zhe Ren, Duane Couchot-Vore, Thomas Brinzer Room temperature ionic liquids are a challenging new area for understanding solvation dynamics. These solvent systems are liquids with a delicate balance of electrostatic, dispersion, and hydrogen-bonding forces which lead to complex structure and dynamics on many time- and length-scales. Here we probe the dynamics of thiocyanate ions in several imidazolium bis(trifluoromethylsulfonyl)amide ionic liquids from femtoseconds to 100 ps using ultrafast vibrational spectroscopy. Two-dimensional infrared (2D-IR) spectroscopy of thiocyanate ions detects both intertial motion (on the hundreds of femtosecond timescale) as well as slower, diffusive motions (on the tens of picosecond timescale). The 2D-IR experiments show that the rate of fluctuation of the electrostatic environment around the thiocyanate is sensitive to hydrogen bonding at the 2-position of the imidazolium ring, depends mildly on water concentration, changes with counter-ion, and is roughly independent of the thiocyanate concentration (up to 30 mM). The results are compared to ab initio simulations which predicted a 10 -- 15 picosecond hydrogen bond lifetime. The implications for topics such as the concept of ionicity, the effect of hydrogen bonding on viscosity, and structural and dynamical heterogeneity will be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q3.00004: Comparative Study of the Intermolecular Dynamics and Physical Properties of Branched and Linear Alkyl Chain Imidazolium Ionic Liquids Lianjie Xue, Fehmi Bardak, George Tamas, Eshan Gurung, Edward Quitevis, Yung Koh, Sindee Simon The optical Kerr effect (OKE) spectra, densities, viscosities, and transition temperatures of 1-alkyl-3-methylimidazolium bis\textbraceleft (trifluoromethane)sulfonyl\textbraceright amide ionic liquids (ILs) with branched alkyl chains, -C$_{\mathrm{n-3}}$CH(CH$_{3})_{2}$ (branched ILs), were measured and compared to those with linear alkyl chains, -C$_{\mathrm{n-1}}$CH$_{3}$ (linear ILs), for $n$ $=$ 3, 4, 5, 6 and 7. The results show that a branched IL has a higher viscosity and transition temperature T$_{\mathrm{g}}$ than the corresponding linear IL with the same n, whereas the densities of each branched/linear IL pair are the same within experimental error. For short alkyl chains (n$=$3 and 4) the intermolecular part of the OKE spectrum of the branched ILs tends to be narrower and lower in frequency than that of the linear ILs. This suggests that branching softens the intermolecular modes. For long alkyl chains (n$=$5-7), the difference between the intermolecular spectrum of the branched IL and that of the linear IL with the same n decreases, which indicates that the branching effect becomes smaller when the alkyl chains get longer. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 4:18PM |
Q3.00005: Structure and Dynamics of Nanoconfined Liquids Invited Speaker: Ward Thompson Dramatic changes in the molecular-level structure and dynamics of liquids occur upon nanoscale confinement in materials ranging from sol-gels to reverse micelles. A number of vibrational and electronic spectroscopic techniques are routinely used to probe the behavior of such nanoconfined liquids. However, in general, what information is contained in the spectra of confined liquids and how the complex molecular-level structural and dynamical properties can be extracted is still an open question. This issue will be discussed in the context of molecular dynamics simulations by examining the affected liquid properties and, when possible, the predicted spectroscopic signals for mesoporous amorphous silica systems. These confining frameworks have been relatively well characterized experimentally, present different surface chemistries, and are sufficiently transparent to permit the study of their contents by a variety of spectroscopies. A particular focus will be the molecular-level origins of the modified liquid behaviour, including those relevant to chemical processes, {\em e.g.}, reorientational and hydrogen-bond dynamics. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q3.00006: Confinement effects on collective water dynamics: Molecular dynamics study of optical Kerr response in silica nanopores Anatoli Milischuk, Branka Ladanyi We report the results of the study of the effects of confinement on collective dynamical properties of water in model nanopores at ambient conditions. The main focus is on approximately cylindrical pores composed of amorphous silica, with diameters ranging from 20 to 40 {\AA}, designed to represent MCM-41 materials. Results for hydrophilic and hydrophobic pores of similar dimensions, but with roughness reduced compared to silica nanopores, are also considered. The main quantity studied is the polarizability anisotropy time correlation function (TCF), which is related to the experimentally-observed optical Kerr effect (OKE) nuclear response. We investigate the effects on this TCF of the reduced molecular translational and rotational water mobility in the layers near the interface. We find that these effects lead to pore diameter dependent slowdown of polarizability anisotropy relaxation, in agreement with OKE experiments. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q3.00007: A Molecularly-Based Theory for Electron Transfer in Polar Solvents Bilin Zhuang, Zhen-Gang Wang Using field-theoretic methods, we develop a mean-field theory for charge solvation in equilibrium and nonequilibrium conditions, and apply it to study electron transfer reactions. The resulting dipolar mean-field theory (DMFT) considers the effects of solvent dipole moments and polarizabilities, and consists of simple sets of equations for the equilibrium and nonequilibrium conditions as well as an analytical expression for the free energy. With no adjustable parameters, the DMFT predicts the activation and the reorganization energies in good agreement with previous data. We have shown that, as DMFT is able to describe the solvent properties in the immediate vicinity of the charges, it is unnecessary to distinguish the inner-sphere and the outer-sphere solvent molecules in the calculation of reorganization energy. Furthermore, we examine the nonequilibrium free energy surfaces of electron transfer, and find that the parabolic approximation is not applicable for reactions with a large free energy change. In the case of solvent mixtures, we show that there are three classes of solvent structures around the ions, and the solvent structure is a key factor determining how the activation energy of electron transfer in a solvent mixture compares to those in the pure solvent components. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q3.00008: Polar nanoregions in water - a study of the dielectric properties of TIP4P/2005, TIP4P2005f and TTM3F Daniel Elton, Marivi Fernandez-Serra Using molecular dynamics simulation we present a critical comparison of the dielectric properties of three models of water - TIP4P/2005, TIP4P/2005f and TTM3F. Dipole spatial correlation is measured using the distance dependent Kirkwood function along with one dimensional and two dimensional dipole correlation functions. We find that the introduction of flexibility alone does not significantly affect dipole correlation and only affects $\varepsilon(\omega)$ at high frequencies. By contrast the introduction of polarizability increases dipole correlation and yields a more accurate $\varepsilon(\omega)$. Additionally the introduction of polarizability creates temperature dependence in the dipole moment even at fixed density, yielding a more accurate value for $d \varepsilon / d T$ compared to non-polarizable models. To understand the physical origin of the dielectric properties of water we make analogies to the physics of polar nanoregions (PNRs) in relaxor ferroelectric materials. We show that $\varepsilon(\omega,T)$ and $\tau_D(T)$ for water have striking similarities with relaxor ferroelectrics, a class of materials characterized by large frequency dispersion in $\varepsilon(\omega,T)$, VFT behaviour in $\tau_D(T)$, and the existence of PNRs. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q3.00009: Systematic investigation of the electronic and structural properties of chloride ion in aqueous solution by advanced density functional Arindam Bankura, Charles Swartz, Michael L. Klein, Xifan Wu In a recent photoelectron spectroscopy (PES) experiment,\footnote{B. Winter et al. J. Am. Chem. Soc., \textbf{128}, 3864 (2006)} electron binding energies have been measured in aqueous chloride ion solutions. The position of the highest occupied molecular orbital (HOMO) of the chloride ion was found to be 1.25-1.50 eV above with respect to the valence band maximum (VBM) of water. Theoretically, we have computed the PES for the aqueous chloride ion solutions, in which the molecular solvation structures are generated from the {\it ab initio} molecular dynamics using gradient-corrected (PBE) and hybrid density functional (PBE0).\footnote{X. Wu et al. Phys. Rev. B, \textbf {79}, 085102 (2009)} Using PBE level of theories we consistently found that HOMO level of the chloride ion on average below the VBM of the water. Whereas the HOMO of the anion was found above the VBM of water when the electronic structure calculations were carried out at the PBE0 level of theories. A substantial improvement in the result was found when the trajectory was generated using the corrections accounting for the effects of dispersion forces into the DFT-GGA scheme)\footnote{A. Tkatchenko et al. Phys. Rev. Lett., \textbf {102}, 073005 (2009)}. and hybrid density functional. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q3.00010: Relaxation Dynamics of the Solvated Electron in Water, Methanol and Ethanol Madeline H. Elkins, Holly L. Williams, Daniel M. Neumark The solvated electron, an isolated electron in solution, is of fundamental interest to the study of solvation. Lacking nuclear degrees of freedom, this highly reactive transient can act as a simple probe of solute-solvent interaction and condensed phase, non-adiabatic dynamics. We present time resolved photoelectron spectra of solvated electrons in water, methanol and ethanol microjets. Unlike prior results from transient absorption (TA) experiments in bulk\footnote{K. Yokoyamma et al., J. Phys. Chem. A, \textbf{102}, 6957 (1998)} or extrapolated from time resolved photoelectron spectroscopy (TRPES) of cluster anions\footnote{A. Bragg et al. Science, \textbf{306}, 669 (2004)}, our technique allows for state specific assignment of the individual features without relying on extrapolation methods. In water, our results reproduce the relaxation timescales found in the TA experiments and provide convincing support of the so called ``non-adiabatic'' mechanism. These results suggest that both binding energies and relaxation dynamics can be extrapolated from water cluster anion experiments; however, results in methanol and ethanol provide a qualitative agreement with TA experiments but a more complex story with regard to anionic cluster TRPES. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q3.00011: Quantum diffusion of atomic hydrogen isotopes in water David Bartels, Jonathan Walker Atomic hydrogen or deuterium radicals can readily be generated in water with a high energy electron beam, and studied using time-resolved electron paramagnetic resonance (EPR). The light isotope muonium, formed from an electron orbiting a positive muon (mass $=$ 0.11 amu), can also be studied by muonium spin resonance (MuSR) at facilities where muon beams are generated. It is to be expected that the diffusion of these isotopes would depend on their mass, and particularly in the case of muonium there should be effects of quantum zero point energy. The ring polymer molecular dynamics (RPMD) simulation method has already been applied to predict a large reduction of diffusion coefficient for muonium relative to hydrogen atoms$^{\mathrm{1}}$. In the present work, we present both EPR and MuSR measurements of spin exchange rate between the atomic hydrogen isotope and (Ni$^{\mathrm{2+}})_{\mathrm{aq}}$ ions. The spin exchange is a diffusion-limited process, and so should directly indicate the relative diffusion rates of the atomic isotopes. Surprisingly, the muonium diffusion appears to be more classical than quantum in character. New RPMD simulations with a quantized water model will be presented to model the experimental result. .(1) Markland, T. E.; Habershon, S.; Manolopoulos, D. E. \textit{J. Chem. Phys.} \textbf{2008}, \textit{128}, 194506. [Preview Abstract] |
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