David Dixon, David Feller, and Giovanni Sandrone,
Pacific Northwest National Laboratory
Douglas McLemore and Steven Strauss, Colorado State University
Karl O. Christe, Air Force Research Laboratory
Ricky Kendall, Ames Laboratory/Iowa State University
and Pacific Northwest National Laboratory

Research Objectives

We are developing a series of benchmark calculations based on solving the electronic Schrödinger equation in order to reliably predict the energetics and kinetics of chemical processes involved in the combustion of hydrocarbon fuels as well as in nuclear waste remediation. Our goal is to develop a composite theoretical approach that reliably predicts a variety of thermodynamic quantities, including heats of formation, without recourse to empirical parameters, so as to minimize the number of expensive experimental measurements needed to model complex systems and to extend limited experimental results.

Computational Approach

Our approach is to use coupled cluster theory with single and double excitations and a perturbative correction for the triples (CCSD(T)) to treat the n-particle problem in conjunction with extrapolations to the complete basis set limit of correlation- consistent basis sets to treat the 1-particle problem. We include additional corrections to account for core/valence, atomic spin-orbit, and molecular scalar relativistic effects as well as for higher order excitations. We are planning to predict kinetic parameters by using transition state theory (TST) including variational TST. We are completing the development of an interface between our computational chemistry code, NWChem, and the POLYRATE program, which combines variational TST and multidimensional semiclassical calculations of quantum tunneling effects.

Accomplishments

Extensive calculations have been done on the fluoride anion and fluorocation affinities of more than 100 main group compounds. This has led to the development of the first quantitative Lewis acidity scale. Work on developing a generalized acid/base scale is under way.

Local density functional theory optimized structure of the anion tetraphenylborate, (B(C6H5) 4)-, which was used in the In Tank Precipitation process at the Savannah River Site.

Heats of formation were obtained from ab initio calculations for seven small silicon-containing molecules; for fluorinated compounds including CF₃ and its cation and anion, CF₄, C₂F₄, and CFCF₃; for benzene and six other small hydrocarbons; and for a number of phenyl/OH substituted boron compounds (neutrals and anions). We have improved our calculations of the heats of formation of CH, CH₂, CH₃, CH₄, HCO, and CH₂O.

Design and implementation of an interface based on the Extensible Computational Chemistry Environment (Ecce) for calculating reaction energies and thermodynamic properties is continuing.

Significance

Developments in computational chemistry over the next few years are likely to result in a major increase in our ability to compute the thermochemical properties of molecules as well as the kinetics of chemical reactions involved in combustion and atmospheric oxidation processes. In addition, the thermodynamic and kinetic properties of materials play a critical role in the design of most chemical processes, so improved modeling and simulation methodologies will help make U.S. industry more competitive globally and more responsive to environmental concerns.

Publications

D. A. Dixon, D. Feller, and G. Sandrone, "The heats of formation of simple perfluorinated carbon compounds," J. Phys. Chem. A 103, 4744 (1999).

D.A. Dixon and D. Feller, "The heats of formation of CF₂, FCO and CF₂O," J. Phys. Chem. A 102, 8209 (1998).

D. A. Dixon and D. F. Feller, "Computational chemistry and process design," Chem. Engr. Sci. 54, 1929 (1999).