LASER-INDUCED FLUORESCENCE STUDY OF METHOXY RADICAL FORMATION FROM THE REACTIONS OF F(P-2) ATOMS WITH CH3OH, CD3OH, AND CH3OD

Kinetic aspects of the formation and consumption of methoxy radicals in the reactions of F(2P) atoms with CH3OH, CH3OD, and CD3OH have been studied in a flow reactor at 300 K. Laser-induced fluorescence from the transition (A 2A1 → X 2E) was used to monitor the concentration of methoxy. Reaction of F with CH3OH forms both CH3O (1a) and CH2OH (1b) radicals, but CH2OH reacts quickly with F2 to regenerate fluorine atoms (12): F(2P) + CH3OH → HF + CH3O (1a); F(2P) + CH3OH → HF + CH2OH (1b); F2 + CH2OH → F + FCH2OH (12). The effect of this chain reaction sequence is that, in the presence of excess F2 and CH3OH, each F atom initially present leads to one CH3O radical. The role of excess F2 was independently established as follows: methoxy was not observed from the reaction, Cl + CH3OH → CH2OH + HCl, but appeared when F2 was added downstream of the CH3OH inlet. The fractional yield of methoxy, Γ(CH3OH→CH3O), in the elementary reaction of F with methanol can be obtained from the ratio of CH3O LIF signals in the absence and presence of F2, respectively. We find Γ(CH3OH→CH3O) = 0.4 ± 0.1. From the deuterium-labeled reactants we find the kinetic isotope effect in reaction 1a to be kO-H/kO-D = 1.1 ± 0.6, and that in reaction 1b to be kC-H/kC-D = 1.5 ± 0.6. On the basis of our data and data of others for exothermic H abstraction by F(2P), OH(2H), and CN(2Σ), we propose a general correlation of the magnitude of the primary kinetic isotope effect with the location of the transition state on the potential energy surface. © 1990 American Chemical Society.