Session L41: Condensed Phase Dynamics and Structure

Temperature and Lengthscale Dependence of Solvophobic Solvation in a Water-like Liquid

Temperature and lengthscale dependence of the solvation of cavity solutes is investigated along the saturation curve of the Jagla liquid, a simple liquid consisting of particles that interact via a spherically symmetric potential combining hard and soft core interactions. The results are compared with an identical calculation for a model of a typical atomic liquid, the Lennard- Jones potential, and with predictions for cavity solubilities in water made by the recently developed cavity equation of state. We find that the Jagla liquid captures the qualitative thermodynamic behavior of hydrophobic hydration as a function of temperature for both small and large lengthscale solutes. The results suggest that a competition between two lengthscales that favors low- density, open structures as temperature is decreased is an essential interaction of a liquid that models hydrophobic hydration.   

Effects of Physical Confinement on the Hysteresis between Melting and Freezing Temperatures of Decanol

There is substantial evidence that physical confinement alters melting and freezing temperatures of materials. These have been qualitatively explained using free energy considerations. However, it is not clear how physical confinement influences melting and freezing when the bulk material itself exhibits substantial supercooling. Bulk 2-decanol, for instance, exhibits substantial hysteresis between its melting (approximately -23$^{\circ}$C) and freezing (-3$^{\circ}$C) temperatures. Evidently, both its melting and freezing temperatures are influenced by physical size. However, the hysteresis between the freezing and melting temperatures seems less sensitive to physical size. This may be the result of differences in homogeneous versus heterogeneous nucleation in physically confined 2-decanol.   

Evaluating the Liquid Liquid Phase Transition Hypothesis of Supercoooled Water

To explain the anomalous behavior of supercooled water it has been conjectured that buried within an experimentally inaccessible region of liquid water's phase diagram there exists a second critical point, which is the terminus of a first order transition line between two distinct liquid phases. The so-called liquid-liquid phase transition (LLPT) has since generated much study, though to date there is no consensus on its existence. In this talk, we will discuss our efforts to systematically study the metastable phase diagram of supercooled water through computer simulation. By employing importance-sampling techniques, we have calculated free energies as a function of the density and long-range order to determine unambiguously if two distinct liquid phases exist. We will argue that, contrary to the LLPT hypothesis, the observed phenomenology can be understood as a consequence of the limit of stability of the liquid far away from coexistence. Our results suggest that homogeneous nucleation is the cause of the increased fluctuations present upon supercooling. Further we will show how this understanding can be extended to explain experimental observations of hysteresis in confined supercooled water systems.   

Anomalous lattice parameter isotope-shift in hexagonal ice Ih from first principle calculations

The lattice parameters of light (H$_2$O) and heavy (D$_2$O) Ih ice differ by 0.09\% [1]. The larger lattice constant is that of the heavier isotope, contrary to normal expectations. This isotope shift of the lattice constant is linked to the zero point energy of phonons in ice. In particular, it can be linked to the anti-correlation of the O-H stretch frequency and the O-O distance in H-bonded materials. In order to determine which phonons give the anomaly, we calculate Gr\"{u}neisen parameters of H$_2$O and D$_2$O ice using first principles density functional theory, within the frozen phonon approximation. Our results show a strong dependence on the density functional chosen. We analyze these differences and make connections to experiment. These results indicate that not only H-bond effects but also van der Waals interactions are necessary to reproduce the correct lattice constant zero-point shifts in ice. \\[4pt] [1] B. K. R\"{o}ttger et. al., Acta Cryst. B {\bf 50}, 644-648 (1994).   

Local effects in the X-ray absorption spectrum of salt water

We have used first principles molecular dynamics and theoretical X-ray absorption spectroscopy (XAS) to investigate the aqueous solvation of cations in MgCl$_2$, CaCl$_2$, and NaCl solutions. We focus our discussion on the species-specific effects that Mg$^{2+}$, Ca$^{2+}$, and Na$^{+}$ have on the X-ray absorption spectrum of the respective solutions. For the divalent cations, we find that the water molecules that form a rigid first solvation shell around Mg$^{2+}$ and a more flexible solvation shell around Ca$^{2+}$ also exhibit differing hydrogen bonding characteristics. Acceptor hydrogen bonds present in the water surrounding Ca$^{2+}$ enhance a post-edge peak near 540 eV in the XAS spectrum, while the absence of such hydrogen bonding features for the first shell surrounding Mg$^{2+}$ corresponds to a diminished intensity at the post-edge peak. For Na$^+$, we find that a broad tilt angle distribution results in broadened post-edge features, despite donor-and-acceptor populations comparable to Ca$^{2+}$. We present re-averaged spectra of the MgCl$_2$, CaCl$_2$, and NaCl solutions that provide an explanation of concentration-dependent features that have been found in corresponding experimental measurements.   

Nuclear momentum distribution and potential energy surface in hexagonal ice

The proton momentum distribution in ice Ih has been recently measured by deep inelastic neutron scattering and calculated from open path integral Car-Parrinello simulation. Here we report a detailed investigation of the relation between momentum distribution and potential energy surface based on both experiment and simulation results. The potential experienced by the proton is largely harmonic and characterized by 3 principal frequencies, which can be associated to weighted averages of phonon frequencies via lattice dynamics calculations. This approach also allows us to examine the importance of quantum effects on the dynamics of the oxygen nuclei close to the melting temperature. Finally we quantify the anharmonicity that is present in the potential acting on the protons.   

The quantum nature of the hydrogen bond: insight from path-integral molecular dynamics

Hydrogen (H) bonds are weak, generally intermolecular bonds, that hold together much of soft matter, the condensed phases of water, network liquids, and many ferroelectric crystals. The small mass of H means H-bonds are inherently quantum mechanical; effects such as zero point motion and tunneling should be considered, although often are not. In particular, a consistent picture of quantum nuclear effects on the strength of H-bonds and consequently the structure of H-bonded systems is still absent. Here, we report \textit{ab initio} path-integral molecular dynamics studies on the quantum nature of the H-bond. Systematic examination of a range of H-bonded systems shows that quantum nuclei weaken weak H-bonds but strengthen relatively strong ones. This correlation arises from a competition between anharmonic intermolecular bond bending and intramolecular bond stretching. A simple rule of thumb enables predictions to be made for H-bonded bonded materials in general with merely classical knowledge (e.g.\ H-bond strength or H-bond length). Our work rationalizes the contrasting influence of quantum nuclear dynamics on a wide variety of materials, including liquid water and HF, and highlights the need for flexible molecules in force-field based studies of quantum nuclear dynamics.   

Hydration phase diagram for BaO terminated BaTiO$_3$

This study reveals geometries H{$_2$}O adopts upon adsorption on BaO terminated BaTiO$_3$(BTO) at low to high saturation. A hydration phase diagram for the aforementioned termination is presented, for moderate temperatures, and moderate to ultra high vacuum H$_2$O pressures. Calculations suggest a very stable H$_2$O adsorption for wide range of pressures, including high vacuum conditions (p$_{H_2O}$ 10$^{-12}$ bar). This opens venues for mechanistic studies and hopefully will serve as a guide to condition that might suppress H$_2$O adsorption on BTO for applications where it is undesired.   

Topological reaction coordinates to explore the structure of atomic clusters and organic molecule isomers from first principles

We introduce a simple reaction coordinate based on spectral graph theory which describes the topology of the network of chemical bonds around a given atom. We employ the reaction coordinate in combination with DFT-based first-principles metadynamics to systematically explore the possible structures of silicon and carbon clusters (including fullerene-like cages) for sizes of tens of atoms. From our extensive exploration we are able to estimate the fractal dimension of the configuration space, which both for silicon and carbon clusters turns out to be quite low. Using the same approach we simulate the interconversion among a large number of chemically relevant organic molecules which are isomers of the C$_4$H$_5$N formula unit, and we demonstrate the possibility of automatically exploring isomerisation, association, and decomposition reactions without prior knowledge of the products involved.   

Transport through a quantum dot with excitonic dot-lead coupling

We study the effect of a Coulombic dot-lead interaction on transport through a quantum dot hybridized to two Luttinger-liquid leads.\footnote{F. Elste, D. R. Reichman, and A. J. Millis, arXiv:1010.2251} A bosonization approach is applied to treat the interaction between charge fluctuations on the dot and the dynamically generated image charge in the leads.\footnote{F. Elste, D. R. Reichman, and A. J. Millis, Phys.\ Rev.\ B \textbf{81}, 205413 (2010)} The nonequilibrium distribution function of the dot and the tunneling current are computed within a master-equation approach. Particular attention is paid to two situations: (i) a quantum dot placed between two leads such that it cuts the Luttinger liquid into two semi-infinite quantum wires; (ii) a quantum dot side-hybridized to two parallel infinite quantum wires. The presence of the excitonic dot-lead coupling is found to enhance transport in the vicinity of the Coulomb-blockade threshold. This behavior is in contrast to the usual power-law suppression of electronic tunneling which is found if this interaction is ignored.   

Spectroscopic and Theoretical Investigations of the Potential Energy Surfaces of Molecules with Intramolecular $\pi $-type Hydrogen Bonding

Spectroscopic methods and theoretical calculations have been utilized to investigate the conformations of several cyclic organic molecules. The laser induced fluorescence (LIF) spectra of 2-indanol show the presence of four conformations. The one with intramolecular hydrogen bonding between the --OH group and the benzene ring is of lowest energy. The potential energy surface (PES) in terms of the ring puckering and internal rotational vibrations, which govern the conformational changes, was determined. 3-Cyclopenten-1-ol possesses a similar PES as established from its infrared and Raman spectra and theoretical calculations. This PES also shows the presence of four conformations. The $\pi $-bonding conformer lies at lowest energy. LIF has been used to study the conformational energies of 2-hydroxytetralin, and 2-cyclohexenol has been investigated by infrared and Raman techniques. The analyses of the hydrogen bonding in these molecules as well as in a dozen others were supported by both \textit{ab initio} and DFT calculations.   

Raman Spectra and the Potential Energy Function for the Internal Rotation of 1,3-Butadiene and its Isotopomers

The gas-phase Raman spectra of 1,3-butadiene-d$_{0}$, 2,3-d$_{2}$, 1,1,4,4-d$_{4}$, and --d$_{6}$ have been collected with CCD detection with numerous scans of ten hours or more. For each isotopomer eight Raman transitions in the 240-330 cm$^{-1}$ region corresponding to double quantum jumps of the A$_{u}$ internal rotation ($\nu _{13})$ were observed for the \textit{trans} conformer. Weaker bands in the 170-260 cm$^{-1}$ region were assigned to the \textit{gauche} conformation, which lies at higher electronic energy. A periodic potential function for the internal rotation, which fits the data for all the isotopomers, was determined. This function shows the \textit{gauche} form to be 966 cm$^{-1}$ higher in energy and the barrier between the \textit{trans} and \textit{gauche} structures to be 2055 cm$^{-1}$. The \textit{cis} structure has an energy 408 cm$^{-1}$ higher than the \textit{gauche}. Fourteen combination band or hot band series involving $\nu _{13}$ for the \textit{trans} conformer were also observed, and these allow the internal rotation levels in various excited vibrational states to be determined.   

Transition and Excited States of 1,1'-azo-bis-1,2,3-triazole

A novel photochromic molecule has been recently synthesized\footnote{Yu-Chuan Li et al. \emph{J. Am. Chem. Soc.}, 2010, 132, 12172}. The photo-isomerization of this nitrogen-rich small molecule is efficiently controlled by a xenon flash lamp suggesting a potential in photonic and molecular mechanics applications. We perform a synergistic quantum molecular dynamics (QMD), real-time time dependent density functional theory (TDDFT) and TDDFT- perturbation theory study to capture and elucidate the transition state, excitation energies and optical properties of the molecule. We also use it to test performance of recently developed real-time TDDFT method\footnote{V. Goncharov, K. Varga, \emph{Phys. Rev. B} 2010, submitted.} to calculate hyperpolarizabilities and compare results with the Sternheimer method.   

Importance of Electronic Relaxation for Inter-Coulombic Decay in Aqueous Systems

Inspired by recent photoelectron spectroscopy experiments on hydroxide solutions, we have examined the conditions necessary for enhanced (and, in the case of solutions, detectable) intermolecular Coulombic decay (ICD) -- Auger emission from an atomic site other than that originally excited. We present general guidelines, based on energetic and spatial overlap of molecular orbitals, for this enhancement of ICD-based energy transfer in solutions. These guidelines indicate that this decay process should be exhibited by broad classes of biomolecules and suggest a design criterion for targeted radiooncology protocols. Our findings indicate that ICD processes in hydroxide solutions are not dependent on hydroxide hydrogen bond donation.