Peter H. Berens

A computer simulation method for the calculation of equilibrium constants for the formation of physical clusters of molecules: Application to small water clusters

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Molecular dynamics and spectra. I. Diatomic rotation and vibration

The pure rotational and vibrational–rotational absorption bands for a diatomic are calculated directly from classical molecular dynamics, classical linear response theory, and classical statistical mechanical ensemble averaging, with the use of simple quantum corrections. The experimental spectral band intensities and contours are well reproduced for CO from dilute gas phase through solution in compressed Ar to solution in liquid Ar by these ’’Newtonian’’ classical spectral calculations. The typical evolution seen in vibrational spectra from multiple‐peaked gas phase bands to single‐peaked solution bands is observed. The Newtonian gas phase calculations also match quantum and correspondence principle classical spectral calculations. This molecular dynamic approach may be applied to compute the spectra of complex molecules or of liquids for which a normal model analysis may be impractical, and may also be extended to nonequilibrium systems, for example, to compute transient vibrational spectra during chemi...

Thermodynamics and quantum corrections from molecular dynamics for liquid water

In principle, given the potential energy function, the values of thermodynamic variables can be computed from statistical mechanics for a system of molecules. In practice for the liquid state, however, two barriers must be overcome. This paper treats the first problem, how to quantum correct the classical mechanical thermodynamic values available from molecular dynamics, Monte Carlo, perturbation, or integral methods in order to compare with experimental quantum reality. A subsequent paper will focus on the second difficulty, the effective computation of free energy and entropy. A simple technique, derived from spectral analysis of the atomic velocity time histories, is presented here for the frequency domain quantum correction of classical thermodynamic values. This technique is based on the approximation that potential anharmonicities mainly affect the lower frequencies in the velocity spectrum where the system behaves essentially classically, while the higher spectral frequencies, where the deviation f...

Molecular dynamics and spectra. II. Diatomic Raman

This paper and paper I in this series [P.H. Berens and K.R. Wilison, J. Chem. Phys. 74, 4872 (1981)] indicate that infrared and Raman rotational and fundamental vibrational–rotational spectra of dense systems (high pressure gases, liquids, and solids) are essentially classical, in that they can be computed and understood from a basically classical mechanical viewpoint, with some caveats for features in which anharmonicity is important, such as the detailed shape of Q branches. It is demonstrated here, using the diatomic case as an example, that ordinary, i.e., nonresonant, Raman band contours can be computed from classical mechanics plus simple quantum corrections. Classical versions of molecular dynamics, linear response theory, and ensemble averaging, followed by straightforward quantum corrections, are used to compute the pure rotational and fundamental vibration–rotational Raman band contours of N2 for the gas phase and for solutions of N2 in different densities of gas phase Ar and in liquid Ar. The e...

Diffusion of hydrogen through platinum membranes at high pressures and temperatures

The diffusion of hydrogen through platinum membranes has been measured at 450, 500, 550 and 600°C at 2000 bar pressure, using the hydrogen sensor technique. Ag + AgCl + 3 M HC1 was the starting solution inside the platinum tube. Hydrogen diffuses out of the platinum tube into a system containing Fe2O3 + Fe3O4 + H2O; that is, a solution with a fixed hydrogen fugacity. After quench, the drop in fH2 inside the platinum tube was calculated from measurements of pH and chloride molality. fH2 is initially roughly proportional to t12. Diffusion constants were calculated from these data by numerical integration, and the results can be expressed by logD (cm2/sec) = − 5489.6/T, K - 4.648.

Molecular Dynamics of Chemical Reactions in Solution

We hope to answer one of the most fundamental and important unsolved questions in chemistry: how, from a molecular perspective, do chemical reactions in solution actually occur. The key to solving this long-standing problem is to understand the molecular dynamics, i.e., the motions of the atoms and the forces that drive them. We have already developed theoretical techniques and computational procedures involving specialized computer hardware needed to calculate the molecular dynamics for many chemical reactions in solution. From the dynamics we have derived the interface for experimental verification, namely transient electronic, infrared, and Raman spectra as well as X-ray diffraction, all of which are potentially observable manifestations of the atomic motions during the reaction. We have tested our approach on the simple inorganic I2 photodissociation and solvent caging reaction. The agreement between molecular dynamics based theory and experimental picosecond transient electronic absorption spectrum as a function of solvent, time, and wavelength is sufficiently close as to indicate that for the first time we are discovering at least part of the molecular dynamics by which a real solution chemical reaction takes place.

Picosecond Dynamics of I 2 Photodissociation

While liquid solution reactions are much more important in chemistry, gas phase reactions are much better understood. Given the central importance of solution reactions to inorganic, organic, industrial and biochemistry, it is rather surprising that, as yet, there is not a single such reaction whose molecular dynamics are understood in detail. Theoretical and experimental evidence already makes clear that much of the important molecular dynamic action in solution reactions occurs on the picosecond and subpicosecond time scales. The dihalogen photodissociation and recombination reactions, X 2 + hv→X + X→X 2, involving the simplest possible molecular reactants and products, diatomics, and in rare gas solution involving only two elements, seem excellent candidates for study.

Picosecond Dynamics Of Solution Reactions

There is now a four order of magnitude time range (≈100 fs to 1 ns) over which molecular dynamic calculations can overlap short light pulse experimental measurements to probe the microscopic nature of chemical processes. In this paper we combine theoretical molecular dynamics calculations of picosecond transient spectra with experimental measurements of such spectra to probe the molecular dynamics of a chemical reaction in solution. The example illustrated is the photodissociation of Ι2 followed by solvent caging, radical recombination to form a new highly vibra-tionally excited Ι2 molecule, and the subsequent decay of this vibrational energy to the solvent. We suggest, as Nes-bitt and Hynes do also, that the lifetimes observed in Ι2 picosecond absorption experiments may not be due to gem-inate recombination times, but perhaps to the time necessary for the recombined Ι2 molecules to vibrationally decay to levels with high Franck-Condon factors for absorption.

Spectra from Molecular Dynamics.

Abstract : It has been shown, for systems for which the potential energy and the appropriate connection to the radiation field (dipole moment or polarizability) are known sufficiently accurately, that infrared, electronic, and nonresonance Raman spectra can be computed from classical molecular dynamics followed by simple quantum corrections to the spectra. Note that no adjustable parameters are needed for any of the spectra presented here. These essentially classical spectra can be compared to experimentally measured spectra to check that the underlying computed dynamics are correct, and the agreement illustrated here indicated that a basically classical view of the atomic motions involved in these spectra is a useful one, in harmony with our well calibrated physical intuition.