Direct Measurement of the Equilibrium Constants of the Reaction of Formaldehyde and Acetaldehyde with HO2 Radicals

Rate and equilibrium constants for the reaction of HO2 with formaldehyde R1 CH 2O+ HO 2 <-> HO CH 2O2(R1)and acetaldehyde R2 CH 3 CHO + HO 2 <-> CH 3 CH ( OH )O2(R2)have been directly measured. The concentration of HO2 radicals was followed in a time-resolved method by coupling cw-CRDS (cavity ringdown spectroscopy) to laser photolysis. The reaction of HO2 with CH2O ( R1 ) was measured at 50 Torr helium over the temperature range 292-306K, whereas the reaction of CH3CHO with HO2 ( R2 ) was measured in 50 Torr He but at only 294K. The observed HO2 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 297K were found to be k(1)= (3.3 +/- 0.6)x 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)x 10(-16) molecule(-1)cm(3) at 297K, in good agreement with earlier, more indirect determinations. The equilibrium constant of the reaction with CH3CHO was found to be K-2= (1.7 +/- 0. 5)x 10(-17) molecule(-1)cm(3) at 294K, with the forward rate constant k(2)= (1.5 +/- 0.75)x 10(-14)cm(3) molecule(-1) s(-1) and the rate constant for the back reaction k(-)(2)= (900 +/- 450) s(-1).