Session P26: Focus Session: Electron & Ion Solvation in Clusters & the Condensed Phase I

How do solvent structure and counterion distribution control quantum solvation in liquids?

Molecular liquids differ from each other not only in their polarity or their ability to make or accept hydrogen bonds but also in their intrinsic packing. Here, we show that the way a solvent packs can have dramatic effects on the dynamics of electron transfer reactions. Using a combination of nonadiabatic mixed quantum/classical molecular dynamics simulations and ultrafast pump-probe spectroscopy, we show that the presence of intrinsic cavities in liquid THF makes charge transfer dynamics in this solvent different from that in other solvent such as water. For example, we find that photoexcitation can cause solvated electrons in THF to transfer from one cavity to another, providing a mechanism for light-induced electron relocalization. We also find that the way a solvent distributes counterions around a reacting solute can dramatically alter not only the rate but even the products of charge transfer reaction. For example, following excitation of the charge-transfer-to-solvent (CTTS) band of iodide in THF, we find that for soft counterions such as tetrabutylammonium, roughly 10{\%} of the ejected electrons form a loose complex with the counterion within a few ps of excitation. For harder counterions such as sodium, however, we find that there can be photoinduced transfer of the CTTS electron from the of I\={ } anion to the Na$^{+}$. If the sodium cations are complexed into crown ethers, however, electron transfer to Na$^{+}$ is shut off. Finally, we also investigate electron solvation and the CTTS dynamics of I\={ } in THF/water mixtures. We find that CTTS excitation leads to ejection of the electron in an initially THF-rich environment characteristic of the I\={ } solvation structure, but that the electrons subsequently become hydrated on a tens to hundreds of ps time scale.   

Dynamics in the First Hydration Shell of Anions

We will describe our recent efforts to elucidate theoretically the vibrational and reorientation dynamics of water molecules in the first hydration shells of anions in aqueous solution, to assist in the interpretation of recent ultrafast infrared spectroscopic experiments on this issue. In particular, we will discuss (a) OH vibrational frequency dephasing for an iodide ion dilute in a solution of HOD in D2O and (b) the reorientation dynamics for an HOD in the first hydration shell of a chloride ion dilute in a solution of HOD in D2O. This work has been performed in collaboration with Damien Laage, Suyong Re and Bruno Nigro of the Dept. de Chimie, Ecole Normale Superieure, Paris.   

Electronic Excitation in Aqueous Anions

Anions when hydrated in water exhibit new features in their electronic absorption spectrum. In the completely hydrated medium of bulk water, charge transfer bands are fully developed and valence transitions exhibited in vacuum can also lead to production of solvated electrons. Using broadband femtosecond transient absorption spectroscopy, we have recorded two-photon absorption spectra that characterize the spectrum of electronic states of aqueous organic and inorganic anions and explored the electronic relaxation dynamics occurring after excitation of valence and charge-transfer-to-solvent states. The detachment dynamics typically are strongly dependent on the excitation energy. The overall solvated electron yields can be understood in terms of competing non-adiabatic, solvation and vibrational relaxation pathways in the excited state. Understanding these electronic states and pathways provides several critical tests for solution electronic structure theories.   

Infrared spectroscopy of hydrated sulfate dianions

The first infrared spectra of a multiply-charged anion in the gas phase are presented. The spectra of SO$_{4}^{2-}\cdot $(H$_{2}$O)$_{n}$, with $n$=3 to 24, show four main bands assigned to two vibrations of the dianionic core, the water bending mode, and solvent libration. The triply degenerate SO$_{4}^{2-}$ antisymmetric stretch vibration probes the local solvent symmetry, while the solvent librational band is sensitive to the solvent hydrogen bonding network. The spectra and accompanying electronic structure calculations indicate a highly symmetric structure for the $n=$6 cluster and closure of the first solvation shell at $n=$12.   

Charge-transfer (CT) dynamics of iodide salts in tetrahydrofuran (THF) and THF-water mixtures.

We have used the spectral sensitivity of the solvated electron to its local environment to probe counterion and cosolvent effects on ultrafast CT dynamics in THF and THF-water mixtures following 1-photon excitation of the I\={ } CTTS band. We find that dynamics in pure THF are dramatically influenced by the presence of the counterion, such that CTTS-generated electrons associate strongly with nearby cations and recombine negligibly with the geminate iodine radical. Studies in solvent mixtures aim to examine preferential ion solvation according to its effects on CT, focusing on THF-rich mixtures, in which water is thought to preferentially solvate equilibrated electrons. Results demonstrate that electrons are initially introduced into water-\textit{deficient} regions of these solutions, subsequently hydrating over 10's-100's of picoseconds. Trends in the CT and hydration dynamics of electrons generated near various counterions and in solutions of varied water content are used to develop an understanding of the local solvent environments of these ion pairs.   

Density Functional Theory calculations of energies of ions in water and in nanopores

Accurate estimates of ion hydration and electrostatic energies are critical for predicting the permeation or rejection of ions in water-filled nanopores. Ab initio Molecular Dynamics methods (AIMD), based on Density Functional Theory (DFT), accounts for the electronic properties and polarizability of materials, water molecules, and ions, and it may appear to be the method of choice for predicting accurate ion energies in water and in nanopores. In practice, applying DFT coupled with the use of periodic boundary conditions in a charged simulation cell leads to anomalous shifts in the electrostatic potential. Using the projector augmented-wave (PAW) method, Wannier functions, and appropriate corrections, we report energies of ions in several systems that can be referenced to interfaces or unambiguous (``vacuum'') values.   

Hydrogen bonding and coordination in normal and supercritical water from X-ray inelastic scattering

A direct measure of hydrogen bonding in water under conditions ranging from the normal state to the supercritical regime is derived from the Compton scattering of inelastically-scattered X-rays. First, we show that a measure of the number of electrons $n_{e}$ involved in hydrogen bonding at varying thermodynamic conditions can be directly obtained from Compton profile differences. Then, we use first-principles simulations to provide a connection between $n_{e}$ and well-defined structural measures for the number of hydrogen bonds $n_{HB}$. Our study shows that over the broad range studied the relationship between $n_{e}$ and $n_{HB}$ is linear, allowing for a direct experimental measure of bonding and coordination in water. In particular, the transition to supercritical state is characterized by a sharp increase in the number of water monomers, but also by a significant number of residual dimers and trimers.   

Binding Energies between Guest Atoms in Clathrate II

The guest atom displacements in clathrates II have been reported on experimental and theoretical points of views. The recent papers on the displacements are given in the reference [1]. The displacements are found to be about 0.6 \AA \ from the center of the Si$_{28}$ cage to the hexagonal ring between the Si$_{28}$ cages. The binding energies between the guest atoms however have been unknown so far. In the present work we calculate the energies between Na atoms in clathrates II Na$_2$@Si$_{136}$ and Na$_{24}$@Si$_{136}$ with a density functional analysis. We will discuss the cohesion mechanism of the clathrates based on the binding nature between the cations in Zintl phase. [1] H. Takenaka and K. Tsumuraya, Mater. Trans., 47, 63 (2006).   

Solvation of anions by aromatic molecules probed by infrared spectroscopy

We have studied the interaction of chloride ions with partially fluorinated benzenes by gas phase infrared photodissociation spectroscopy. Our studies were motivated by the fact that fluorination changes the charge distribution in a benzene molecule. While C$_{6}$H$_{6}$ has a negatively charged carbon ring and a positively charged hydrogen periphery, C$_{6}$F$_{6}$ has a positively charged carbon ring and a negatively charged fluorine periphery. If the interaction between a closed-shell anion (such as Cl$^{-})$ and the aromatic molecule were based mostly on electrostatic interaction, such an ion would bind to C$_{6}$H$_{n}$F$_{6-n}$ via the $\pi $ system for small $n$ and via H bonds to the periphery for large $n$. We have used IR spectroscopy of Ar solvated Cl$^{-}\cdot $C$_{6}$H$_{n}$F$_{6-n}$ complex anions to investigate if this paradigm holds, using the red shift and intensity increase of CH stretching modes for H-bonded CH oscillators to discern whether Cl$^{-}$ binds to the $\pi $ system rather than to the periphery at different levels of F substitution.