Accurate Kinetic Studies of OH + HO2 Radical–Radical Reaction through Direct Measurement of Precursor and Radical Concentrations with High-Resolution Time-Resolved Dual-Comb Spectroscopy

The radical–radical reaction between OH and HO2 has been considered for a long time as an important reaction in tropospheric photochemistry and combustion chemistry. However, a significant discrepancy of an order of magnitude for rate coefficients of this reaction is found between two recent experiments. Herein, we investigate the reaction OH + HO2 via direct spectral quantification of both the precursor (H2O2) and free radicals (OH and HO2) upon the 248 nm photolysis of H2O2 using infrared two-color time-resolved dual-comb spectroscopy. With quantitative and kinetic analysis of concentration profiles of both OH and HO2 at varied conditions, the rate coefficient kOH+HO2 is determined to be (1.10 ± 0.12) × 10–10 cm3 molecule–1 s–1 at 296 K. Moreover, we explore the kinetics of this reaction under conditions in the presence of water, but no enhancement in the kOH+HO2 can be observed. This work as an independent experiment plays a crucial role in revisiting this prototypical radical–radical reaction.


Figure S5.
Comparison of the measured and the simulated temporal profiles of OH and HO2 at varied conditions.Here, the OH was measured near 3484 cm −1 and HO2 was measured near 1123 cm −1 .Table S1.Summary of experimental conditions of each experimental set.
Table S2.Summary of experimental conditions, methods and results for determination of the rate coefficient of the reaction OH + HO2.

References
Note S1.Description of previous investigations of the OH + HO2 reaction.
−16 Figure S1 shows the comparison of the rate coefficient kOH+HO2 obtained from previous experimental and theoretical studies at around 300 K.
−9 According to the most recent experiment by Speak et al., a rather small rate coefficient, kOH+HO2 = (1.00±0.32)×10 -1 cm 3 molecule -1 s -1 , was obtained by measuring the temporal profile of OH and HO2 upon 248-nm photolysis of H2O2 (50% v/v with water) in low pressure condition based on laser-induced fluorescence (LIF). 1 This value was smaller by an order of magnitude comparing to kOH+HO2 = (1.02±0.06)×10-10 cm 3 molecule -1 s -1 reported by Assaf et al., 2 which was determined based on time profiles of OH and HO2 generated in the 248-nm photolysis of H2O2/(COCl)2/CH3OH/O2 mixture by using CW cavity ring-down spectroscopy as well as the LIF signal of OH.Other experiments were carried out by employing the microwave discharge, [3][4][5][6]9 or flash photolysis cells, 7,8 coupled with LIF, 5,7 resonance fluorescence (RF), 3,4,9 electron paramagnetic resonance (EPR) spectroscopy, 6 or UV absorption spectroscopy.8 The detailed descriptions of these experiments are summarized in Table S2. Currently, the prefer value from IUPAC is 1.1×10 -10 cm 3 molecule -1 s -1 , which is mainly based on the experimental results reported by Keyser with an uncertainty of approximately 30%. 4 For the theoretical studies, the reaction rate coefficients are scattered in the range of (0.1-1.7)×10 -10 cm 3 molecule -1 s -1 at 300 K, as shown in Figure S1(b).1, 10−16 For the previous theoretical studies before 2018, the reaction rate coefficients are reported in the relatively small range of (0.5-1.0)×10 -10 cm 3 molecule -1 s -1 at 300 K.However, the rate coefficients of this reaction reported from two recent theoretical studies show a large discrepancy by a factor up to 15.In 2020, Liu et al.
studied the kinetics of OH + HO2 using ring polymer molecular dynamics (RPMD) and quantum dynamics (QD) methods.They first calculated the rate coefficient by employing the QD method, and the value was reported to be 3.5×10 -11 cm 3 molecule -1 s -1 .Furthermore, based on the Bannett-Chandler factorization, they obtained the RPMD rate coefficient with a value of 1.66×10 -10 cm 3 molecule -1 s -1 , which is the largest value in the theoretical studies. 10−20 The concentration of H2O2, [H2O2]0, inside the multipass cell can be estimated by using the flow rate of each stream, mixing ratios of the H2O2/N2 pre-mixtures, and the total pressure of the reactor.The [H2O2]0 were also directly measured by probing the H2O2 absorption lines near 1269 cm −1 before photolysis for double checking the initial concentration of H2O2 inside the reactor.b For the experiments, the OH Χ 2 Π3/2 (1←0) P(4.5) doublet transitions near 3407 cm −1 were probed.
d For the experiments, five absorption peaks of HO2 near 1123 cm −1 were probed.
e in unit of mJ cm −2 .
f in unit of molecule cm −3 .
8][19][20] The [H2O2]0 in the reactor was estimated by using the flow rate of each stream, mixing ratios of the H2O2/N2 pre-mixtures, and the total pressure of the reactor.In addition, the H2O2 absorption lines near 1269 cm −1 were probed before photolysis for double checking the concentration of H2O2 inside the reactor.h For the experiments, the H2O absorption lines near 1121 and 3410 cm −1 were probed before photolysis.

Figure S1 .
Comparison of the experimental and theoretical results of the rate coefficient kOH+HO2, at around 300 K.

Figure S2 .
Figure S2.Schematic of the experimental setup.

Figure S3 .
Figure S3.Difference absorbance spectra of OH with fitted Voigt curve.

Figure S1 .
Figure S1.Comparison of the experimental and theoretical results of the rate coefficient kOH+HO2, at around 300 K. (a) Comparison of the experimental results of kOH+HO2 at 295-308 K.A summary of methods, conditions, and results of each experiment are listed in Table S2.(b) Comparison of the theoretical results of kOH+HO2 at 300 K.

Figure S2 .
Figure S2.Schematic of the experimental setup.Here, DCS is the dual-comb source, LWP is the longwave pass filter, PG is the pressure gauge, PD is the photodiode, DAQ is the data acquisition board, and UVS is the ultraviolet spectrometer.To generate water-free H2O2, the urea hydrogen peroxide powder was mixed with an equivalent amount of dry sea sand (SiO2) in a glass container to avoid agglomeration during the heating process in a water bath at ~45°C.Upon heating, a wellcalibrated nitrogen gas flow would pass through the container at a flow rate of 1000 sccm to carry out the generated gaseous H2O2 into a 50-cm UV absorption cell before injection into the multipass cell.A D2 lamp (StellarNet Inc., SL3) and a UV spectrometer (Oceanhood, XS11639) were used to obtain UV absorption spectra of H2O2 inside the 50-cm cell.The mixing ratio of the H2O2/N2

Figure S3 .
Figure S3.Difference absorbance spectra of OH with fitted Voigt curve.The spectrum (black open dots) was obtained by interleaving of four dual-comb spectra recorded with spectral sampling spacings of 279 MHz (0.93 × 10 -3 cm -1 ) or 291 MHz (0.97 × 10 -3 cm -1 ) at 0-40 μs after photolysis of the flowing mixture of H2O2/N2 ([H2O2]0 = 1.92×10 15 molecule cm −3 , PT = 30.3Torr, 296 K) at 248 nm with the photolysis energy of 28.5 mJ cm -2 .The spectrum was curve-fitted with the multipeak Voigt function to derive the integrated absorption area of each absorption transition.The red line represents the fitted curve and the blue open dots represent the fitting residual.The line strength for one of X 2 Π3/2 (1 ← 0) P(4.5) doublet transitions was determined to be (3.38±0.25)×10−20cm molecule −1 in our previous work.21

Figure S5 .
Figure S5.Comparison of the measured and the simulated temporal profiles of OH and HO2 at varied conditions.The concentration temporal profiles of (a) OH and (b) HO2 were recorded upon photolysis of the flowing mixtures of H2O2/N2 ([H2O2]0 = 5.8×10 14 molecule cm −3 , PT = 20.3Torr, 296 K) (red) and H2O2/N2 ([H2O2]0 = 4.1×10 14 molecule cm −3 , PT = 15.3Torr, 296 K) (blue) at 248 nm with the photolysis energy of 38.0 mJ cm -2 .The open symbols represent the measured temporal profiles with a time resolution of 40 μs.The solid lines represent the simulated profiles using the kinetic model, as shown in Table1, with kOH+HO2 = 1.10×10 −10 .Here, the OH time traces were obtained by analyzing the time-resolved spectra of the OH X 2 Π3/2 (1 ← 0) P(2.5) transitions near 3484 cm −1 and the HO2 time traces were obtained by analyzing the time-resolved spectra of five absorption peaks near 1123 cm −1 .

Table of content
Note S1.Description of previous investigations of the OH + HO2 reaction.

Table S1 .
Summary of experimental conditions of each experimental set.
a Each set includes 4~5 individual experimental measurements.

Table S2 .
Summary of experimental conditions, methods and results for determination of the rate coefficient of the reaction OH + HO2.photolysis of the O3/H2O/O2 mixture in He, Ar, or N2 to generate OH and HO2 radicals.Measurement of the OH and HO2 time profiles by UV absorption spectroscopy.OH radicals are formed from the reaction between H and NO.HO2 radicals can then be generated from OH + O3.Measurement of the OH time profiles by resonance fluorescence (RF).The HO2 is first converted to OH then observed by RF.