A directly-dissociative stepwise reaction mechanism for gas-phase peroxyacetic acid

Abstract

The dissociation of peroxyacetic acid (PAA, CH 3C(O)OOH) was studied by UV laser photolysis in the gas phase under collision-free conditions. Three pathways are proposed for PAA dissociation that ultimately produce CH 3 + CO 2 + OH. Two pathways are stepwise with sequential bond dissociation and the other pathway is concerted with simultaneous fission of the same bonds. The laser-induced fluorescence (LIF) spectrum of OH was analyzed after 266 and 240 nm photolysis of PAA. OH is produced in the 2Π ground electronic state with no vibrational energy ( v ″ = 0 ) and rotational distributions that peak at N″ ≈ 3–4 and extend to N″ = 11 at both photolysis wavelengths. Spin-orbit population ratios of the ground electronic state were slightly above unity for both studies. The average OH Λ-doublet ratio was 1.07 ± 0.14 after 266 nm photolysis, which indicates that OH is produced by torsional forces. At 240 nm PAA photolysis, the Λ-doublet ratio increased from unity at N″ = 4 to a statistical value of ∼2 at N″ = 11. Average OH translational energies determined from Doppler profiles were 34 ± 2 and 43 ± 2 kcal mol −1 at 266 and 240 nm photolysis, respectively. The average speed of OH is used to determine the total translational energy of products for the proposed stepwise and concerted pathways. The calculated total translational energies are compared to thermochemical available energy calculations for each pathway to determine the viability of the reactions. Experimental results indicate that dissociation occurs from an excited state, so a comparison of 266 and 240 nm photolysis data is made to discover the direct versus indirect nature of the mechanism. The UV spectra of PAA, acetic acid, hydrogen peroxide, and methylhydroperoxide are reviewed in light of the results. This study concludes that PAA photolysis occurs through initial stepwise O O bond dissociation by a directly-dissociative mechanism from a highly-repulsive electronic state like the hydroperoxides and an indirect mechanism from an exit channel barrier similar to acetic acid is not accessed at these wavelengths.