William A. Lester, Jr. and Michael Frenklach,
University of California, Berkeley, and Lawrence Berkeley National Laboratory
Xenophon Krokidis, Lawrence Berkeley National Laboratory Nigel W. Moriarty, University of California, Berkeley

Research Objectives

This research will accomplish the accurate specification of important reaction pathways in the postulated mechanism of soot formation.

Computational Approach

The version of diffusion Monte Carlo (DMC) that is used employs effective core potentials to minimize computational effort. Variational Monte Carlo computations are carried out to test trial functions for DMC constructed as products of independent particle wave functions, e.g., HF, MCHF, and CI, and correlation functions that depend on interparticle distances.

Accomplishments

We have quantitatively characterized the important pathways leading to the formation of cyclopentadienyl radical from the reaction of propargyl radical with acetylene to order 1 kcal/mol. Further, we have determined the heat of formation of the radical to 1 kcal/mol-essential information in understanding the mechanism of reaction. We have also been able to resolve uncertainty connected with the ground state of cyclopentadienyl radical and to establish it firmly as a doublet A2 state. We have unambiguously characterized bonding in propargyl radical using the electron localization function (ELF) method (see figure). We have determined the atomization energy of propargyl radical to an accuracy of 1 kcal/mol using the DMC method and have obtained a result in excellent agreement with experiment.

Significance

With elementary steps determined to 1 kcal/mol, uncertainties in reaction paths can be resolved, removing ambiguity in mechanisms for the formation of successively larger precursors to soot formation. The ultimate goal is full characterization of the mechanism of soot formation, which will provide valuable insight on how to reduce a major pollution source.

The figure shows domains of localization (or "basins" in topological terms) of pairs of electrons in C3H3 as they are reproduced by ELF. These domains are distinguished as core (C) or valence (V), and correspond to regions in real space where pairs of electrons are likely to be localized. The valence localization domains may correspond to bonds or regions where lone pairs or single electrons (in the case of radicals) may be found. Within this approach multicenter bonds are possible.

Publications

J. C. Grossman, W. A. Lester, Jr., and S. G. Louie, "Quantum Monte Carlo and density functional theory characterization of 2-cyclopentenone and 3-cyclopentenone formation from O(3P) + cyclopentadiene," J. Am. Chem. Soc. (in press).

J. C. Grossman, W. A. Lester, Jr., and S. G. Louie, "Cyclo-pentadiene stability: Quantum Monte Carlo, coupled cluster, and density functional theory determinations," Mol. Phys. 96, 629 (1999).

X. Krokidis, N. W. Moriarty, W. A. Lester, Jr., and M. Frenklach, "Propargyl radical: An electron localization function study," Chem. Phys. Letters (in press).