G.W. Robinson

Structure and dynamics of liquid water between plates

Using the recently developed SPC‐FP water model (simple point charge model with flexible bonds and polarization) and the molecular dynamics method, we investigate the structure and properties of liquid water between two rigid plates. In one case the plates are neutral and in the other the plates are electrically charged. In both cases substantial differences from bulk state water are found, structurally and dynamically. We observe some anomalies compared with normal liquids and attribute these to the breakage of hydrogen bonds under the influence of the solid–liquid interface. Adding an external torque enhances such breakage through the attempted alignment of the water molecules. A combination of these two contributions determines the resultant dynamical behavior of water between charged plates. The information obtained from this work should be helpful in the understanding of ‘‘hydrophobic effects’’ in aqueous solutions. The behavior of water near large polar or nonpolar molecular solutes is also revealed...

Molecular dynamics simulation of liquid carbon disulphide with a harmonic intramolecular potential

Using a 3-centre Lennard-Jones plus Coulomb potential, we have performed a molecular dynamics simulation of liquid carbon disulphide at room temperature. The intramolecular vibrations have been included for the first time. The calculated liquid structure, the dynamical properties, the Raman spectrum, and the interaction-induced far infrared spectrum are all in good agreement with experimental measurements. Compared with other MD simulations using rigid CS2 molecules and a somewhat different intermolecular potential, the present model provides measurable improvement.

Memory kernel in liquid phase cis–trans isomerization

The memory kernel in liquid phase cis–trans isomerization is examined from the point of view of existing experimental data as well as MD simulations. In agreement with nonlinear system/bath coupling theories, when the generalized Langevin equation is written in the conventional way, we find that the ‘‘effective’’ memory kernel in this equation depends on the properties of the isomerizing particle, including the barrier height. In addition, it is found that the angular velocity distribution for the isomerizing molecule may be strongly influenced by ‘‘shape effects,’’ caused by actual molecular differences between the cis and trans configurations. This distribution may not be Maxwellian when considered over a partial angular range, e.g., the ‘‘barrier region.’’ If this is a general phenomenon, the selection of correct initial conditions in the reactive flux method would create uncertainties in the application of that method to the calculation of barrier crossing rates.

Space‐dependent friction in the theory of activated rate processes: The Hamiltonian approach

The Hamiltonian approach of Pollak–Grabert–Hanggi to the theory of activated rate processes is generalized to allow for different frictions in the well and the barrier regions. Estimates of the escape rate in the case of exponential friction can be determined from the derived equations of motion, which are exact for all values of the coupling in the well and barrier regions. The rate results are plotted in several cases.

Theoretical study of memory kernel and velocity correlation function for condensed phase isomerization. I. Memory kernel

A unified memory kernel function is proposed on the basis of molecular dynamics simulations and comparison with experimental data. A frozen‐solvent‐like friction arises from the near‐neighbor solvent cage effect, while the interaction between the system and the disordered heat bath or ‘‘broken cage’’ gives rise to a white‐noise friction. On short time scales, the apparent friction is a ‘‘series addition’’ of these two limits, each solute molecule experiencing either an intact cage or a broken cage environment. For longer time scale behavior, the solvent is able to evolve, and some system molecules can experience both types of environments during the course of their own dynamics: the effective friction for these molecules then appears as a ‘‘parallel addition’’ of the same two contributions. This leads to a memory kernel having a simple exponential behavior. As the time scale for the system dynamics becomes still longer, the cage becomes indistinguishable from the heat bath, and a purely Markoffian relaxat...

Possible breakdown of the reaction coordinate concept in condensed-phase chemistry

Abstract In liquid-phase isomerization reactions, local departures from Maxwellian distributions appear to occur in different zones of the reaction coordinate chosen as dihedral angle connecting cis and trans forms. These deviations become appreciable for light masses and high barriers. The results partially reflect the fact that the reaction coordinate derived from isolated-molecule considerations is a poor representation of the solution-phase dynamics.

Does the kramers equation provide potential surface information

Abstract Condensed-phase cis—trans isomerization is treated using a periodic potential function. Derived rate expressions allow an analysis of experimental or molecular dynamics data. Such analysis suggests order-of-magnitude inconsistencies among the fitting parameters.

Nonequilibrium computer simulation of a salt solution

A nonequilibrium computer simulation is performed to investigate the relaxation of a realistic polar solvent near a rapidly dissociating ion pair. The time evolution of the reaction coordinate, the ultrashort time scale changes in solvation energy and solvent forces, the local density response, the heating of certain librational degrees of freedom, and the time‐dependent polarization are studied during the first 125 fs of the reaction. It is found that the relaxation behaviors in the anionic and cationic shells are very different. On average, the solvation process under study takes about 30‐40 fs to break the original cage. After another 50 fs, the solvated ion pair reforms a new metastable structure, which feeds energy back into the reacting system to break the cage further. This procedure is apparently repeated many times until dissociation is complete. The results obtained in this work provide a graphic picture of some of the features of ultrashort dynamics of ionic photodissociation reactions in a pol...

Polar molecule in a nonpolar liquid. A molecular dynamics study

Molecular dynamics techniques are used to study the structural changes in liquid carbon disulphide resulting from the introduction of a polar solute. It is found that both translational relaxation and rotational relaxation of the solvent in the shell surrounding the solute are faster than in the bulk liquid. Moreover, shifts in the intramolecular vibrational spectra and variations of the dielectric properties of the cage solvent are in evidence. The dynamical and spectroscopic behavior of the solute is also discussed. Microscopic friction experienced by the solute molecule for both translational and rotational motions is computed using the generalized stochastic theory. Information of this type is needed for constructing valid theories of ultrafast chemical reaction dynamics in condensed matter.

A microscopic form of the extended kramers equation. A simple friction model for cis-trans isomerization reactions

Abstract A friction model for cis-trans isomerization is proposed. The friction is a linear combination of two limits, reflecting relative time scales of system and solvent motions. The resulting extended Kramers equation fits MD results exactly and is applicable to real isomerizing systems as well.