Experimental determination of the rate constants of the reactions of HO₂ + DO₂ and DO₂ + DO₂

Abstract

The rate constants of the reactions of DO 2 + HO 2 (R1) and DO 2 + DO 2 (R2) have been determined by the simultaneous, selective, and quantitative measurement of HO 2 and DO 2 by continuous wave cavity ring-down spectroscopy (cw-CRDS) in the near infrared, coupled to a radical generation by laser photolysis. HO 2 was generated by photolyzing Cl 2 in the presence of CH 3 OH and O 2 . Low concentrations of DO 2 were generated simultaneously by adding low concentrations of D 2 O to the reaction mixture, leading through isotopic exchange on tubing and reactor walls to formation of low concentrations of CH 3 OD and thus formation of DO 2 . Excess DO 2 was generated by photolyzing Cl 2 in the presence of CD 3 OD and O 2 , small concentrations of HO 2 were always generated simultaneously by isotopic exchange between CD 3 OD and residual H 2 O. The rate constant k 1 at 295 K was found to be pressure independent in the range 25-200 Torr helium, but increased with increasing D 2 O concentration k 1 = (1.67 ± 0.03) × 10 −12 × (1 + (8.2 ± 1.6) × 10 −18 cm 3 × [D 2 O] cm −3 ) cm 3 s −1 . The rate constant for the DO 2 self-reaction k 2 has been measured under excess DO 2 concentration, and the DO 2 concentration has been determined by fitting the HO 2 decays, now governed by their reaction with DO 2 , to the rate constant k 1 . A rate constant with insignificant pressure dependence was found: k 2 = (4.1 ± 0.6) × 10 −13 (1 + (2 ± 2) × 10 −20 cm 3 × [He] cm −3 ) cm 3 s −1 as well as an increase of k 2 with increasing D 2 O concentration was observed: k 2 = (4.14 ± 0.02) × 10 −13 × (1 + (6.5 ± 1.3) × 10 −18 cm 3 × [D 2 O] cm −3 ) cm 3 s −1 . The result for k 2 is in excellent agreement with literature values, whereas this is the first determination of k 1 . K E Y W O R D SDO 2 radicals, HO 2 radicals, kinetics Int J Chem Kinet. 2020;52:197-206. How to cite this article: Assali M, Rakovsky J, Votava O, Fittschen C. Experimental determination of the rate constants of the reactions of HO 2 + DO 2 and DO 2 + DO 2 . Int J Chem Kinet. 2020;52:197-206.

Rate constant and branching ratio of the reaction of ethyl peroxy radicals with methyl peroxy radicals

“…The geometric mean value rule is an empirical approach that allows for the estimation of cross-reaction rate coefficients from the self-recombination rate constants of the reacting partners 68 It has shown to work to better than 20% in the prediction of radical–radical rate coefficients for a series of hydrocarbon radicals 69 and has proven to be valid also for the cross reaction of HO 2 and DO 2 radicals. 70 When applying this rule to the cross reaction of CH 3 O 2 and C 2 H 5 O 2 and using the values for the self-reactions from Table 3, one obtains an excellent agreement ( k 1,geometric rule = 3.74 × 10 −13 cm 3 s −1 ) with the rate constant obtained in this work. However, when using the value for the CH 3 O 2 self-reaction recently obtained by Onel et al , 12 the geometric mean rule predicts a rate constant for the cross reaction of only k 1 = 2.9 × 10 −13 cm 3 s −1 .…”