Association and isotopic exchange reactions of CH(CD)[X 2Π]+CO

Abstract

The reaction rates for CH12 and CD12 with normal isotopic abundance CO and CO13 have been studied at 293 K for pressures between 12.5 and 500 Torr and at 100 Torr for temperatures be-tween 293 and 650 K. The pressure and temperature dependence of the addition reaction of CH with CO have been measured. The addition rate coefficient can be fit to the expression 7.2±0.3×10−12(T/293)−2.4±0.2 cm3 molecule−1 s−1 at 100 Torr total pressure (He buffer). A fit of the pressure dependence to a Troe expression with Fc=0.6 yields a low-pressure rate constant (k0) of 2.4±0.3×10−30 cm6 molecule−2 s−1. The rate for carbon atom exchange has been measured by comparison of the C13 labeled and unlabeled reaction rates. The isotopic exchange reaction is 1.0±0.2×10−12 cm3 molecule−1 s−1 at 20 Torr. The deuterium isotope effect on the exchange rate is large, with an inverse kinetic isotope effect (kH/kD)=0.28±0.08 at 20 Torr. This inverse isotope effect reflects the competition between collisional stabilization and isomerization, and is a convolution of isotope effects for the isomerization, unimolecular dissociation, and stabiliza-tion rates. The experimental results are consistent with a mechanism for exchange that in-volves isomerization of an HCCO adduct via an oxiryl intermediate, and indicate that insertion into the C–O bond is not important in this reaction.