ESR spectra of free radicals formed during the gas-phase photo-oxidation of formaldehyde: thermal stability of the HOCH 2OO radical

Abstract

Formaldehyde/oxygen/nitrogen mixtures were photolyzed with UV light in a gas flow system. The reaction mixtures were analyzed by pumping a small fraction of the flowing gas through a microprobe, trapping the condensable compounds on a cold finger ( T = 96 K), and measuring the ESR spectrum of the solid. The ESR spectra consisted of partly overlapping components from HO 2 and HOCH 2OO radicals, the latter having g-tensor values g 1 = 2.0025, g 2 = 2.0090 and g 3 = 2.0320. By simulating the experimental spectra with appropriate amplitude factors for their HO 2 and HOCH 2OO components, the concentration ratio [HO 2]/[HOCH 2OO] was determined. From these values and the H 2CO concentrations, measured by UV absorption, equilibrium constants k 1, k −1 for the reaction HO 2+H 2CO ⇌ HOCH 2OO were calculated. The expression k 1/ k −1 = 2.4 × 10 −26 exp(15.2 kcal mol −1/ RT) cm 3 was obtained from measurements in the temperature range 30–100° C.