CO + OH → CO2 + H: The relative reaction rate of five CO isotopologues

The reaction of carbon monoxide with the hydroxyl radical (CO + OH) plays a central role in tropospheric chemistry. While the analysis of stable isotope enrichment has been used to refine models of the sources and sinks of atmospheric CO and CO2, less is known about the mechanism behind the enrichment. We have initiated the present work to provide a larger set of experimental observations of the effect. We report the OH radical reaction rate of 12C16O relative to 13C16O, 12C18O and 13C18O and the reaction rate of 12C17O relative to 12C18O at 295 K. At 1013 mbar the relative rates are 0.960 ± 0.014, 0.943 ± 0.019, 0.948 ± 0.024 and 0.996 ± 0.0067 respectively (klight/kheavy, 2σ errors indicated); results at 506 and 66 mbar are also reported. The hydroxyl radical was generated by the UV photolysis of ozone in the presence of water. The concentration of the carbon monoxide isotopologues as a function of photolysis time was determined using a global fit of the rovibrationally resolved FTIR spectrum of the gas mixture in the reaction cell. The observed inverse kinetic isotope effect is best understood in terms of the effect of isotopic substitution on the relative rate of unimolecular dissociation of the HOCO intermediate to reform reagents versus dissociation to products.