L20. Deuterium Kinetic Isotope Effects in the Reactions of CH [2?] with Methane and Acetylene and C₂H [²S⁺] with O2

Holger S. Thiesemann and Craig A. Taatjes

Combustion Research Facility, Mail Stop 9055, Sandia National Laboratories, Livermore, CA 94551-0969 USA

E-mail: cataatj@ca.sandia.gov

The rate coefficients of the reactions of CH and CD with CH4, CD4, C2H2, and C2D2 have been measured in the temperature range 290 < T < 700 K. All the rate constants show a slightly negative temperature dependence. The observed temperature and pressure dependences are in accordance with a barrierless addition of the methylidyne radical to methane or acetylene with rapid decomposition of the adduct, consistent with an interpretation of these reactions as capture-limited. The CH/CD kinetic isotope effect for all reactions is ~ 1.2, similar to that observed for CH/CD + O2. Such an isotope effect, although relatively small, is larger than would be predicted by a simple transition-state theory which assumes that the conserved modes are unchanged from reactants to transition state. While a negligible effect is observed for deuterium substitution of the acetylene, a large kinetic isotope effect is seen for deuterium substitution of methane; the reaction with CH proceeds 60% faster at 293 K for CH4 than for CD4. Since motion of the methane hydrogens is required to reach a stable adduct in the CH + CH4 system, this suggests that the C-H bonds in the methane molecule are already significantly weakened in the transition state for initial adduct formation.

In a study of another capture-limited reaction, the rate coefficients for the reactions of C2H and C2D with O2 have been measured in the temperature range 295 K £ T £ 700 K. The C2H + O2 reaction has been postulated to proceed through isomerization, via a three- or four-membered ring structure, of an initial HCCOO adduct. These reactions also show a slightly negative temperature dependence in this temperature range, with kC₂H+O₂ = (3.15 ± 0.04) ´ 10-11 (T / 295 K)-(0.16 ± 0.02) cm3 molecule-1 s-1. A temperature-independent kinetic isotope effect of kC₂H / kC₂D = 1.04 ± 0.03 is observed, which can be modeled by simple flexible-transition-state approximations. The C2H + O2 kinetic isotope effect suggests that any rearrangement of the intermediate complex is rapid and facile, and that the total reaction rate is governed by the initial complex formation.

This work is supported by the Division of Chemical Sciences, the Office of Basic Energy Sciences, the U. S. Department of Energy. H.T. thanks the Deutsche Forschungsgemeinschaft (DFG) for a postdoctoral fellowship.