Direct Measurement of the Equilibrium Constants of the Reaction of Formaldehyde and Acetaldehyde with HO_2 Radicals

Rate and equilibrium constants for the reaction of HO_2 with formaldehyde CH_2O + HO_2 ↔ HOCH_2O_2 and acetaldehyde CH_3CHO + HO_2 ↔ CH_3CH(OH)O_2 have been directly measured. The concentration of HO_2 radicals was followed in a time-resolved method by coupling cw-CRDS (cavity ringdown spectroscopy) to laser photolysis. The reaction of HO_2 with CH_2O (R1) was measured at 50 Torr helium over the temperature range 292–306 K, whereas the reaction of CH_3CHO with HO_2 (R2) was measured in 50 Torr He but at only 294 K. The observed HO_2 decay profiles were modeled to take into account secondary chemistry, especially that of the reaction products, hydroxyl-peroxy radical adducts. The rate constants for forward and back reactions of (R1) at 297 K were found to be k_1 = (3.3 ± 0.6) × 10^(−14) cm^3 molecule^(−1)s^(−1) and k_(−1) = (55 ± 5) s^(−1), respectively, both roughly a factor of two slower than earlier measurements (possibly due to falloff effects), while the equilibrium constant was found to be K_1 = (6.0 ± 1.8) × 10^(−16) molecule^(−1) cm^3 at 297 K, in good agreement with earlier, more indirect determinations. The equilibrium constant of the reaction with CH_3CHO was found to be K_2 = (1.7 ± 0.5) × 10^(−17) molecule^(−1) cm^3 at 294 K, with the forward rate constant k_2 = (1.5 ± 0.75) × 10^(−14) cm^3 molecule^(−1)s^(−1) and the rate constant for the back reaction k_(−2) = (900 ± 450)s^(−1).