Ilan Benjamin

Theoretical study of the water/1,2‐dichloroethane interface: Structure, dynamics, and conformational equilibria at the liquid–liquid interface

Very little is known about the structure and dynamics of the interface between liquid water and liquid 1,2‐dichloroethane (DCE), despite the fact that a molecular level understanding of this and similar interfaces is of fundamental importance for the proper interpretation of many studies of charge transfer at interfaces. Molecular dynamics calculations are used to show that the interface is molecularly sharp with capillary wavelike distortions whose structure and dynamics closely resemble those expected from the capillary wave model. Molecular level structural information such as pair correlation functions and hydrogen bonding statistics is also consistent with this picture. The orientation of water at the interface is similar to what is known at other water interfaces. The dynamics of water and DCE translational and rotational motion are only slightly modified at the interface. The DCE gauche–trans isomerization reaction is investigated at the interface and in the bulk. A continuum electrostatic model fo...

A unified algebraic model description for interacting vibrational modes in ABA molecules

A simple yet realistic model Hamiltonian which describes the essence of many aspects of the interaction of vibrational modes in polyatomics is discussed. The general form of the Hamiltonian is that of an intermediate case between the purely local mode and purely normal mode limits. Resonance interactions of the Fermi and Darling–Dennison types are shown to be special cases. The classical limit of the Hamiltonian is used to provide a geometrical content for the model and to illustrate the ‘‘phase‐like’’ transition between local and collective (i.e., normal) mode behavior. Such transitions are evident as the coupling parameters in the Hamiltonian are changed and also for a given Hamiltonian as the energy is changed. Applications are provided to higher lying vibrational states of specific molecules (H2O, O3, SO2, C2H2, and C2D2).

Theoretical study of ion solvation at the water liquid–vapor interface

Molecular dynamics calculations are reported for several ions at the liquid–vapor interface of water. The transition from the bulk to the interface region is investigated from structural, energetic, and dynamical points of view by calculating ion–water geometries, radial distribution functions, solvent molecular reorientation times, solvent polarization fluctuations, and solvation free energy as a function of distance from the interface. It is shown that ions tend to keep most of the structural and dynamical properties of their first solvation shell intact as they are moved into the interface, and that the tendency for negative adsorption (positive free energy of adsorption) is associated with weaker and fewer long range interactions. A comparison of some of the molecular dynamics results to predictions of simple continuum models is discussed, showing generally poor quantitative agreement.

Vibrational relaxation of I−2 in water and ethanol: molecular dynamics simulation

Abstract The vibrational relaxation of I − 2 in water and ethanol using molecular dynamics simulations. In both solvents, the relaxation rate is ≈0.6–0.7 ps, in qualitative agreement with the experiments of Barbara and co-workers. A Landau-Teller model for the relaxation rate is in good agreement with the full molecular dynamics calculations. Simulations of the neutral I 2 molecule vibrational relaxation in the same solvents are used to sort out the effects of solute charge and vibrational frequency. We show that the fast relaxation of the I − 2 molecule is due to both its low vibrational frequency and the long-range solvent-solute Coulombic interactions.

Proposed experimental probes of chemical reaction molecular dynamics in solution: ICN photodissociation

Knowledge of how translational and rotational motions are influenced by the solvent during the course of a photodissociation ‘‘half‐collision’’ reaction in solution is of interest in itself and can also help our understanding of how thermally activated reactions take place in solution by means of fluctuations in translational and rotational motion. With this goal, the molecular dynamics of the photodissociation of the triatomic molecule ICN are compared in the gas phase and in Xe solution. The time evolution of the trajectories (particularly with respect to interfragment distance and CN orientation) and of the energy partitioning (particularly into fragment translational recoil and into rotation of the CN) are displayed. Two types of solution experiments are proposed and simulated, both closely related to recent gas phase studies by Dantus, Rosker, and Zewail. These experiments are designed to probe the detailed dynamics of chemical reactions in solution during the time period the reaction is in progress,...

Molecular dynamics of adiabatic and nonadiabatic electron transfer at the metal–water interface

A molecular model for an electron transfer reaction at a solution–electrode interface is developed. The solvent diabatic free energy curves for the reaction Fe+3+e−→Fe+2 are calculated using an umbrella sampling procedure. These are used to calculate the rate of electron transfer as a function of the electrode–solution potential difference in the electronically nonadiabatic region. A model Hamiltonian for the adiabatic case is also developed and used to calculate the adiabatic free energy surface. Reactive flux correlation functions are used to determine the solvent dynamical corrections to the rate. A comparison between the molecular dynamics and the Kramers and Grote–Hynes theories is made.

Ultrafast photodissociation of I3− in ethanol: A molecular dynamics study

A detailed molecular model of I3− photodissociation in liquid ethanol is developed. Extensive molecular dynamics trajectory calculations are used to determine product energy distribution, time‐dependent spectra, and reorientation dynamics in semiquantitative agreement with experimental data.

Molecular dynamics computer simulations of solvation dynamics at liquid/liquid interfaces

The solvent dynamic response to electronic transitions at several liquid/liquid interfaces is studied using molecular dynamics computer simulations. The interfaces examined are between water and one of four different organic liquids. The electronic transitions involve a change in the permanent dipole of a dipolar solute located at the interface. Two locations of the solute relative to the interface are studied and are compared with the same process in each of the bulk liquids. The different organic liquids are 1-octanol, 1,2-dichloroethane, n-nonane, and carbon tetrachloride. They are selected to give a range of polarity and of interface structure. The solvent dynamic response at the interface is much more complex than in the bulk. The total relaxation involves multiple time scales corresponding to contributions from both solvents and from the unique structural and dynamic properties of the interface. In particular, interfacial water relaxation may contain a slow component not present in the bulk nor at t...

Adsorption of Na+ and Cl− at the charged water–platinum interface

The adsorption of Na+ and Cl− at the charged water–platinum interface as a function of external voltage is investigated by molecular dynamics computer simulation. Generally, although the water structure is significantly affected by a strong external electric field, the structure of the ion–water complex at the surface is much less affected. At electric field values comparable to those found in experimental systems, Cl− is ‘‘contact adsorbed’’ on the metal and is mainly solvated by the water layer adjacent to the metal. In contrast, the small Na+ is solvated equally well by both adsorbed water molecules and water molecules outside the inner layer. At higher electric field values that are close to the upper end of what is believed to exist in electric double layers, both ions lose part of their hydration shell. The dynamic of the ion motion towards the metal up to about one solvent layer from the surface is in reasonable agreement with experimentally known ion conductivities at low electric fields, but it o...

Solvation of Na+ and Cl− at the water–platinum (100) interface

The structural, energetic, and dynamical aspects of the solvation of Na+ and Cl− at the water–plantinum (100) interface are investigated by molecular‐dynamics computer simulation. Although the structure of interfacial water is significantly different from that of bulk water, the structure of the ion–water solvation complex at the interface closely resembles that in the bulk. The free energy of adsorption is calculated as a function of the distance from the metal. It is nonmonotonic and is qualitatively very different for Na+ and Cl−. The shape of the free‐energy curve can be explained in terms of solvation structure and the local perturbation of the interfacial water structure. The reorientation dynamics of water near the ion show that the structure‐breaking effect of Cl− at the interface is much more significant than in the bulk, but that Na+ slows down water reorientation both in the bulk and at the interface. Collective solvent dynamics, as measured by equilibrium fluctuations of solvent–ion electrosta...