Kinetics of Elementary Processes Relevant to Incipient Soot Formation

M. C. Lin, Emory University

Research Objectives

Rate constants for key elementary reactions relevant to incipient soot formation (e.g., reactions of C₂H₃, C₃H₃, and C₆H₅ with alkanes, alkenes, and alkynes), measured with multiple experimental methods, are elucidated and quantitatively extrapolated to the combustion regime with high-level ab initio MO and statistical theory calculations.

Computational Approach

For reactions containing seven or fewer heavy atoms (e.g., C₆H₅ + CH₄ -> C₆H₅ + CH₃) we employed the modified Gaussian 2 method (G2M) to obtain the energies and molecular parameters of reactants, transition states, and products. For systems greater than seven heavy atoms, the hybrid density functional theory with the 6-31G(d,p) basis set (such as B3LYP/6-31G(d,p)) was used. Rate-constant calculations were carried out with the conventional and/or canonical variational transition-state theory (TST or RRKM).

Accomplishments

Rate constants for several reactions involving C6H5 (phenyl radical) have been calculated, e.g.:

(a) C₆H₅ + CH₄ -> C₆H₆ + CH₄
(b) C₆H₅ + C₆H₆ -> C₁₂H₁₁ -> C₁₂H₁₀ + H
(c) C₆H₅ + C₆H₅CH₃ -> C₆H₆ + C₆H₅CH₂

Reaction (a) is a direct abstraction process; this open-shell system contains seven heavy atoms. We have carried out the ab initio MO/TST calculation up to G2M (rcc, MP2)//B3LYP/6-31G(d,p). The predicted rate constant with tunneling corrections agrees closely with our measured value. The barrier for the reaction was predicted to be 101 kcal/mole.

Both reactions (b) and (c) were studied at the B3LYP/6-31G(d,p) level of theory because of the large molecular size. Reaction (b) takes place by a long-lived C₁₂H₁₁ adduct involving the addition and decomposition transition states. RRKM calculations for the forward and reverse addition (H+C₁₂H₁₀) processes correlate the measured rate constants for the two processes very well.

Reaction (c) is dominated by the direct H-abstraction process, whose barrier was predicted to be 1.4 kcal/mole at the B3LYP/6-31G(d,p) level of theory, agreeing nicely with the experimental value of 1.8 kcal/mole. The results of these three systems are being prepared for publication.

Optimized geometries of the various species involved in the reaction of the C₃H₃ radical with acetylene.

Significance

Both experimental measurements for these phenyl-radical reactions and ab initio MO calculations for reactions such as C₆H₅ + CH₄ have been carried out nearly to the limits of our current technology. The C₆H₅ + CH₄ reaction, calculated at the G2M//B3LYP/6-31G(d,p) level of theory to approximate the restricted open-shell RCCSD(T)/6-311+G(3f,2d) method, requires more than 2000 hrs of NERSC CPU time.

The results of these calculations not only help elucidate the mechanisms of large combustion-reaction systems, but also help extend low-temperature kinetic data reliably to the combustion regime.

Publications

I. V. Tokmakov, J. Park, S. Gheyas and M. C. Lin, "Experimental and theoretical studies of the reaction of phenyl radical with methane," J. Phys. Chem. (submitted, 1998).

L. V. Moskaleva and M. C. Lin, "Theoretical investigation of the unimolecular decomposition of C₅H₅ and C₅H₆ and their reverse processes" (in preparation, 1998).