T. H. Dunning, B. C. Garrett, M. Dupuis, S. S. Xantheas, D. Feller, K. A. Peterson, L. X. Dang, G. K. Schenter, S. M. Kathmann, E. Arcia, B. Park, and C. J. Burnham, Pacific Northwest National Laboratory
Y. Borisov, Russian Academy of Sciences
D. Tzeli and A. Mavridis, University of Athens, Greece
G. E. Froudakis, University of Crete, Greece
J. Dai and Z. Bacic, New York University

Research Objectives
Research in the Molecular Theory Group is designed to provide a fundamental understanding of how molecular processes in condensed phase systems influence the environment. The goals of the research are:

1. to apply available theoretical techniques to the study of fundamental molecular-level processes that govern the chemistry and physics of natural/contaminated systems and remediation technologies

2. to extend current state-of-the-art methods to treat progressively more complex systems and develop new theoretical techniques that allow us to transcend current computational limitations

3. to incorporate fundamental molecular-level information into models that can simulate dynamical processes in large, complex systems

4. to integrate complementary theoretical approaches for examining multispecies, multiphase systems characteristic of natural and polluted environments and remediation methods

5. to link theory and experiment through collaborative studies.

Methods for computing the rates of activated chemical reactions in solution are also being developed, based upon well established gas-phase theories. Because of the importance of reactions involving light atoms that occur in aqueous solutions (e.g., acid and base catalyzed reactions involve proton transfer reactions), the accurate treatment of quantum mechanical effects is a focus of this work.

Equilibrium properties of clusters and solutions, including structural properties (e.g., radial distribution functions and coordination numbers) and average energetics (e.g., enthalpies and free energies) are obtained using classical and quantum statistical mechanics. In addition, time-dependent properties (e.g., correlation functions) are obtained from molecular dynamics (MD) simulations.

Covalently bonded materials such as glasses are being treated using a hybrid of quantum mechanical and classical force fields. Recently, a model potential that treats the bonding and nonbonding interactions separately was developed to specifically address the questions related to the geometric structure of covalently bonded liquids and amorphous materials.

Accomplishments
Research in the area of aqueous clusters is focused on properties (e.g., structure, energetics, and spectra) of aqueous clusters and aqueous solutions containing inorganic and organic species that occur in natural and contaminated groundwater, and on molecular processes at aqueous interfaces (vapor/liquid, liquid/liquid, and liquid/solid). The goal of this research is to understand the correlation between molecular-scale processes — solvation, association and reaction — and the composition and behavior of species in aqueous environments. Recent results have improved our understanding of the properties of aqueous clusters, infrared spectra of ion-water clusters, thermodynamics of aqueous clusters, acid-base chemistry, chemical reactions of CHCs, benzene-water chemistry, chemistry of the water/CHC interface, and ion-ligand binding and ion selectivity of separation agents (e.g., crown ethers and calixarenes).

Transport of Cs+ ion across a CCl4/H2O interface.

One of our most significant accomplishments involves the thermodynamics of aqueous clusters. The fundamental definition of a finite temperature cluster was examined in terms of its relation to the measurement of growth kinetics. To accomplish this, a new theoretical approach to the understanding of vapor-phase nucleation was developed. Previous molecular approaches to nucleation focused on the evaluation of the equilibrium distribution of clusters. Our new approach focuses on the evaluation of rate constants for cluster evaporation and condensation. Using variational transition state theory to determine dynamical bottlenecks, a definition of a “physically consistent cluster” naturally falls out of the theory, a result that has eluded the field for the last 30 years.

Research in the area of chemistry and physics of covalently bonded materials (e.g., networked oxides) is focused on studying the properties of amorphous materials involved in waste processing, waste storage, and nuclear fuels. The goal is to provide insight into the long-term performance of materials that contain radionuclides or are exposed to radiation. Recent accomplishments include discoveries concerning the structure and thermodynamics of glasses, the diffusion and reactivity of water in glasses, and radiation damage in oxides.

In addition to the research efforts in the applications areas, we are also developing new theoretical and modeling methods, including basis set development/methods assessment, models for accurate thermochemistry, theory of single molecule chemical dynamics, and interpretation of electron standing wave experiments.

Significance
Because of the fundamental nature of this research, it supports the mission of the DOE Office of Science to advance basic research that is the foundation for DOE’s applied missions in energy resources, environmental quality, and national security. Our research seeks fundamental understanding of chemical transport and reactivity in condensed phases, thermal and non-thermal (i.e., radiation) chemistry, interfacial molecular and ionic transport, and other processes in complex systems related to energy use, environmental remediation, and waste management. A major focus of this research is on fundamental problems in chemical physics that underlie environmental chemistry.

Publications
P. Ayotte, S. B. Nielsen, G. H. Weddle, M. A. Johnson, and S. S. Xantheas, "Spectroscopic observation of ion-induced water dimmer dissociation in the X^–•(H₂O) ₂ (X = F, Cl, Br, I) clusters," J. Phys. Chem. A 103, 10665 (1999).

E. R. Batista, S. S. Xantheas, and H. Jonsson, “Electric fields in ice and near water clusters,” J. Chem. Phys. 112, 3285 (2000).

L. X. Dang, “Computer simulation studies of ion transport across a liquid/liquid interface,” J. Phys. Chem. B 103, 8195 (1999).