Showing papers by "Peter J. Rossky published in 1999"

Computer simulation of the excited state dynamics of betaine-30 in acetonitrile

Abstract: Time-dependent studies of the excited state dynamics of betaine-30 in acetonitrile at room temperature have been carried out using a mixed classical/quantum molecular dynamics simulation methodology. The π-electron system of the solute molecule is treated quantum mechanically using the semiempirical Pariser−Parr−Pople Hamiltonian, including the solvent influence on electronic structure. The remaining interactions are treated via empirical potentials. Transition probabilities between adiabatic electronic states are evaluated using surface hopping methods, including all nuclear degrees of freedom in the coupling. The dynamics treats the (rigid) solvent and the dihedral angles for relative rotation of rings of an otherwise rigid solute classically. The contribution of all remaining solute intramolecular vibrations is included in the nonadiabatic coupling via an approximate, but purely quantum mechanical, treatment. Analysis of the dynamics reveals that, after excitation to the first excited state, the energy...

Evaluation of Functional Group Contributions to Excess Volumetric Properties of Solvated Molecules

Abstract: We develop extensions to a previous methodology for evaluating the excess compressibility of solvation. These extensions make it possible to analyze solution compressibilities in terms of the hydration shell model of solvation. The methodology is applied to three model soluteswater, methane, and methanolin water. We find that the compressibility is accounted for by localized effects of the solute on the solvent. In addition, for the case of methanol, we find that the localized effects of one solute functional group are independent of the effects of the other. This creates the opportunity for estimation of group-additive contributions to the compressibility, and we illustrate a technique for the extraction of such contributions from molecular dynamics simulation data. The technique is easily generalized for examination of large solutes, including biologically important macromolecules.

Nonadiabatic molecular dynamics simulation of photoexcitation experiments for the solvated electron in methanol

Abstract: Nonadiabatic quantum molecular dynamics simulations have been performed to simulate the pump-and-probe photoexcitation experiments of the ground state equilibrium solvated electron in methanol carried out by Barbara et al. [Chem. Phys. Lett. 232, 135 (1995)]. We have characterized both the time evolution of the quantum solute, the solvated electron, and the solvation response of the classical methanol bath. The quantum energy gap provides an excellent tool to gain insight into the underlying microscopic details of the solvation process. The solvent response is characterized for both processes by a fast Gaussian component and a biexponential decay. The present results suggest that the residence time of the solvated electron in the first excited state is substantially longer than inferred from the cited experiments. The experimentally observed fast exponential portion of the relaxation more likely corresponds to the adiabatic solvent response than to the lifetime of the excited state electron. By comparing to photoexcitation simulations in water, it is shown that the simulated excited state lifetime is about three times longer in methanol than in water, predicting a less substantial increase than a recent calculation based on nonadiabatic coupling elements alone. Hydrogen-bonding statistical analysis provides interesting additional details about the dynamics. We find that the hydrogen-bonding network is significantly different in the first solvent shell around the electron in ground and first excited states, the distribution around the latter, larger and more diffuse, ion resembling more that of the pure liquid. Transformation of the corresponding hydrogen bonding structures takes place on a 1 ps time scale.

The effect of vicinal polar and charged groups on hydrophobic hydration.

Abstract: The use of a linear relationship between free energy of hydrophobic hydration and solvent-accessible apolar surface area has been helpful in interpreting the thermodynamics of biological macromolecules. However, a recent study (Y.-K. Cheng, P. J. Rossky, Nature 1998, Vol. 392, pp. 696–699) has established a substantial enthalpic dependence on biomolecular surface topography, originating from solvent hydrogen-bonding loss in a restrictive geometry. In this study, we use molecular dynamics simulations of 2-Zn insulin in water solvent to explore the further effect of vicinal polar or charged groups on hydrophobic hydration at a biomolecular surface. In contrast to the case for solvent more isolated from such polar solute influences, the binding energies of the water that is proximal to the hydrophobic dimeric interface of insulin and vicinal to polar and charged groups are comparable to the bulk solvent value, a result of compensating interaction primarily with the solute counterions. The results suggest a special importance for such polar/charged groups in biological processes involving hydrophobic surface regions of restricted geometry and also suggest a general route for tuning the hydrophobicity of interfaces. © 1999 John Wiley & Sons, Inc. Biopoly 50: 742–750, 1999

Acid-Base Behavior in Hydrothermal Processing of Wastes

Abstract: A new technology, hydrothermal oxidation (also called supercritical water oxidation), is being developed to treat high level nuclear wastes Nitrates are reduced to nitrogen; furthermore, phosphates, alumina sludge, and chromium are solubilized, and the sludge is reconstituted as fine oxide particles A major obstacle to development of this technology has been a lack of scientific knowledge of chemistry in hydrothermal solution above 350 C, particularly acid-base behavior, and transport phenomena, which is needed to understand corrosion, metal-ion complexation, and salt precipitation and recovery Our objective is to provide this knowledge with in-situ UV-vis spectroscopic measurements and fully molecular computer simulation