Kinetics of the Homogeneous Exchange Reaction 12C18O+13C16O⇆13C18O+12C16O. Single-Pulse Shock-Tube Studies

The isotopic exchange reaction between two CO molecules was studied under homogeneous conditions in a single-pulse shock tube at high dilution in argon within the 2000–2675°K temperature range. From a total of 100 shocks, run at varying initial pressures and compositions, the following rate law was derived: rate of exchange= kb[CO]1.45[Ar]0.55, where kb = 1014.36±0.42exp{− (76 ± 5) × 103 / RT} cc mole−1·sec−1. The possibility of a chain mechanism was ruled out. A two-step mechanism composed of a vibrational excitation step and an atom-switching step is proposed. It was found that exchange occurs at high probability when one of the CO molecules is elevated to the 7th–10th vibrational level before colliding with another CO molecule. A dipole interaction stabilizing the transition state is proposed in explanation of the relatively low activation energy of this exchange reaction. Two alternative reaction systems are described, and the corresponding rate constants are computed and compared with the experimental results.