Two reaction channels are identified for hydrogen abstraction from methanol by chlorine atom. The R1 channel had been found previously, but the R2 channel was found in this group by O. Couronne with the involvement of F. Gilardoni. The R2 channel is consistent with a direct mechanism implied by molecular beam scattering experiments of M. Ahmed, D. S. Peterka, and A. G. Suits at the Berkeley Lab Advanced Light Source.

William Lester and Michael Frenklach, University of California, Berkeley, and Lawrence Berkeley National Laboratory
Alan Aspuru-Guzik, Ivan Ovcharenko, and Nigel Moriarty, University of California, Berkeley
Olivier Couronne, John Harkless, and Alexander Kollias, Lawrence Berkeley National Laboratory
Xenophon Krokidis, Institut Français du Pétrole (IFP), Rueil-Malmaison, France
Zhiwei Sun and Ruzeng Zhu, Institute of Mechanics, Academica Sinica, Beijing, China

Computational Approach
Our dominant computational technique is the quantum Monte Carlo (QMC) method in the diffusion Monte Carlo (DMC) variant. Our version of DMC 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 and correlation functions that depend on interparticle distances.

Accomplishments
The addition reaction of acetylene and propargyl was investigated using several density functional methods. DMC calculations were performed at optimized geometries for many of the equilibrium structures and transition states. This data serves as input into the determination of RRKM rate constants leading to the formation of the cyclo-C₅H₅ radical. A detailed analysis of the reaction pathways is under way using the bonding evolution theory (BET) concepts applied to electron localization function (ELF).

Hydrogen abstraction from methanol by atoms and radicals yields as products either hydroxymethyl or methoxy radicals in competing reaction channels, depending on which nonequivalent hydrogen of the methyl or hydroxyl group of CH₃OH is abstracted:

R1: CH₃OH + Cl --> CH₃O + HCl

R2: CH₃OH + Cl --> CH₂OH + HCl

Computations of these reaction channels to date tend to support experimental results showing the importance of a direct reaction mechanism, but QMC calculations, in progress, are needed to resolve the matter.

QMC was used to compute the atomization energy and the heat of formation of the propargyl radical, C₃H₃, and the computed results compare favorably with experimental measurements. The effective core potential and fixed-node approximations were used in the DMC variant. Two generalized gradient approximation density functionals were applied for comparison.

Significance
(1) Full characterization of the mechanism of soot formation will provide valuable insight on how to reduce a major pollution source. (2) Methanol is an attractive alternative fuel because its combustion generates fewer air pollutants than gasoline. The reaction channels for the formation of CH₃O and CH₂OH are of fundamental importance for combustion and atmospheric chemistry.

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. 122, 705 (2000).

X. Krokidis, N. W. Moriarty, W. A. Lester, Jr., and M. Frenklach, “Propargyl radical: An electron localization function study,” Chem. Phys. Letters 314, 534 (1999).

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