Experimental and ab Initio Study of the HO2·CH3OH Complex: Thermodynamics and Kinetics of Formation

Acetonyl Peroxy and Hydroperoxy Self- and Cross-Reactions: Temperature-Dependent Kinetic Parameters, Branching Fractions, and Chaperone Effects

The temperature-dependent kinetic parameters, branching fractions, and chaperone effects of the self- and cross-reactions between acetonyl peroxy (CH3C(O)CH2O2) and hydro peroxy (HO2) have been studied using pulsed laser photolysis coupled with infrared (IR) wavelength-modulation spectroscopy and ultraviolet absorption (UVA) spectroscopy. Two IR lasers simultaneously monitored HO2 and hydroxyl (OH), while UVA measurements monitored CH3C(O)CH2O2. For the CH3C(O)CH2O2 self-reaction (T = 270-330 K), the rate parameters were determined to be A = (1.5-0.3+0.4) × 10-13 and Ea/R = -996 ± 334 K and the branching fraction to the alkoxy channel, k2b/k2, showed an inverse temperature dependence following the expression, k2b/k2 = (2.27 ± 0.62) - [(6.35 ± 2.06) × 10-3] T(K). For the reaction between CH3C(O)CH2O2 and HO2 (T = 270-330 K), the rate parameters were determined to be A = (3.4-1.5+2.5) × 10-13 and Ea/R = -547 ± 415 K for the hydroperoxide product channel and A = (6.23-4.4+15.3) × 10-17 and Ea/R = -3100 ± 870 K for the OH product channel. The branching fraction for the OH channel, k1b /k1, follows the temperature-dependent expression, k1b/k1 = (3.27 ± 0.51) - [(9.6 ± 1.7) × 10-3] T(K). Determination of these parameters required an extensive reaction kinetics model which included a re-evaluation of the temperature dependence of the HO2 self-reaction chaperone enhancement parameters due to the methanol-hydroperoxy complex. The second-law thermodynamic parameters for KP,M for the formation of the complex were found to be ΔrH250K° = -38.6 ± 3.3 kJ mol-1 and ΔrS250K° = -110.5 ± 13.2 J mol-1 K-1, with the third-law analysis yielding ΔrH250K° = -37.5 ± 0.25 kJ mol-1. The HO2 self-reaction rate coefficient was determined to be k4 = (3.34-0.80+1.04) × 10-13 exp [(507 ± 76)/T]cm3 molecule-1 s-1 with the enhancement term k4,M″ = (2.7-1.7+4.7) × 10-36 exp [(4700 ± 255)/T]cm6 molecule-2 s-1, proportional to [CH3OH], over T = 220-280 K. The equivalent chaperone enhancement parameter for the acetone-hydroperoxy complex was also required and determined to be k4,A″ = (5.0 × 10-38 - 1.4 × 10-41) exp[(7396 ± 1172)/T] cm6 molecule-2 s-1, proportional to [CH3C(O)CH3], over T = 270-296 K. From these parameters, the rate coefficients for the reactions between HO2 and the respective complexes over the given temperature ranges can be estimated: for HO2·CH3OH, k12 = [(1.72 ± 0.050) × 10-11] exp [(314 ± 7.2)/T] cm3 molecule-1 s-1 and for HO2·CH3C(O)CH3, k15 = [(7.9 ± 0.72) × 10-17] exp [(3881 ± 25)/T] cm3 molecule-1 s-1. Lastly, an estimate of the rate coefficient for the HO2·CH3OH self-reaction was also determined to be k13 = (1.3 ± 0.45) × 10-10 cm3 molecule-1 s-1.

Yields of HONO2 and HOONO Products from the Reaction of HO2 and NO Using Pulsed Laser Photolysis and Mid-Infrared Cavity-Ringdown Spectroscopy

The reaction of HO₂ with NO is one of the most important steps in radical cycling throughout the stratosphere and troposphere. Previous literature experimental work revealed a small yield of nitric acid (HONO₂) directly from HO₂ + NO. Atmospheric models previously treated HO₂ + NO as radical recycling, but inclusion of this terminating step had large effects on atmospheric oxidative capacity and the concentrations of HONO₂ and ozone (O₃), among others. Here, the yield of HONO₂, φ_HONO2, from the reaction of HO₂ + NO was investigated in a flow tube reactor using mid-IR pulsed-cavity ringdown spectroscopy. HO₂, produced by pulsed laser photolysis of Cl₂ in the presence of methanol, reacted with NO in a buffer gas mixture of N₂ and CO between 300 and 700 Torr at 278 and 300 K. HONO₂ and its weakly bound isomer HOONO were directly detected by their v₁ absorption bands in the mid-IR region. CO was used to suppress HONO₂ produced from OH + NO₂ and exploit a chemical amplification scheme, converting OH back to HO₂. Under the experimental conditions described here, no evidence for the formation of either HONO₂ or HOONO was observed from HO₂ + NO. Using a comprehensive chemical model, constrained by observed secondary reaction products, all HONO₂ detected in the system could be accounted for by OH + NO₂. At 700 ± 14 Torr and 300 ± 3 K, φ_HONO2 = 0.00 ± 0.11% (2σ) with an upper limit of 0.11%. If all of the observed HONO₂ was attributed to the HO₂ + NO reaction, φ_HONO2 = 0.13 ± 0.07% with an upper limit of 0.20%. At 278 ± 2 K and 718 ± 14 Torr, we determine an upper limit, φ_HONO2 ≤ 0.37%. Our measurements are significantly lower than those previously reported, lying outside of the uncertainty of the current experimental and recommended literature values.

Acetonyl Peroxy and Hydro Peroxy Self- and Cross-Reactions: Kinetics, Mechanism, and Chaperone Enhancement from the Perspective of the Hydroxyl Radical Product

Pulsed laser photolysis coupled with infrared (IR) wavelength modulation spectroscopy and ultraviolet (UV) absorption spectroscopy was used to study the kinetics and branching fractions for the acetonyl peroxy (CH₃C(O)CH₂O₂) self-reaction and its reaction with hydro peroxy (HO₂) at a temperature of 298 K and pressure of 100 Torr. Near-IR and mid-IR lasers simultaneously monitored HO₂ and hydroxyl, OH, respectively, while UV absorption measurements monitored the CH₃C(O)CH₂O₂ concentrations. The overall rate constant for the reaction between CH₃C(O)CH₂O₂ and HO₂ was found to be (5.5 ± 0.5) × 10^-12 cm³ molecule^-1 s^-1, and the branching fraction for OH yield from this reaction was directly measured as 0.30 ± 0.04. The CH₃C(O)CH₂O₂ self-reaction rate constant was measured to be (4.8 ± 0.8) × 10^-12 cm³ molecule^-1 s^-1, and the branching fraction for alkoxy formation was inferred from secondary chemistry as 0.33 ± 0.13. An increase in the rate of the HO₂ self-reaction was also observed as a function of acetone (CH₃C(O)CH₃) concentration which is interpreted as a chaperone effect, resulting from hydrogen-bond complexation between HO₂ and CH₃C(O)CH₃. The chaperone enhancement coefficient for CH₃C(O)CH₃ was determined to be k_A^″ = (4.0 ± 0.2) × 10^-29 cm⁶ molecule^-2 s^-1, and the equilibrium constant for HO₂·CH₃C(O)CH₃ complex formation was found to be K_c(R14) = (2.0 ± 0.89) × 10^-18 cm³ molecule^-1; from these values, the rate constant for the HO₂ + HO₂·CH₃C(O)CH₃ reaction was estimated to be (2 ± 1) × 10^-11 cm³ molecule^-1 s^-1. Results from UV absorption cross-section measurements of CH₃C(O)CH₂O₂ and prompt OH radical yields arising from possible oxidation of the CH₃C(O)CH₃-derived alkyl radical are also discussed. Using theoretical methods, no likely pathways for the observed prompt OH radical formation have been found and the prompt OH radical yields thus remain unexplained.

Temperature Dependence of the Reaction of Chlorine Atoms with CH3OH and CH3CHO

Rate constants of the reactions Cl + CH₃OH → CH₂OH + HCl ( k₁) and Cl + CH₃CHO → CH₃C(O) + HCl ( k₃) were measured at 100 Torr over the temperature range 230.3-297.1 K. Radical chemistry was initiated by pulsed laser photolysis of Cl₂ in mixtures of CH₃OH and CH₃CHO in a flow reactor. Heterodyne near-IR wavelength modulation spectroscopy was used to directly detect HO₂ produced from the subsequent reaction of CH₂OH with O₂ in real time to determine the rate of reaction of Cl with CH₃OH. The rate of Cl + CH₃CHO was measured relative to that of the Cl + CH₃OH reaction. Secondary chemistry, including that of the adducts HO₂·CH₃OH and HO₂·CH₃CHO, was taken into account. The Arrhenius expressions were found to be k₁( T) = 5.02_-1.5^+1.8 × 10^-11 exp[(20 ± 88)/ T] cm³ molecule^-1 s^-1 and k₃( T) = 6.38_-2.0^+2.4 × 10^-11 exp[(56 ± 90)/ T] cm³ molecule^-1 s^-1 (2σ uncertainties). The average values of the rate constants over this temperature range were k₁ = (5.45 ± 0.37) × 10^-11 cm³ molecule^-1 s^-1 and k₃ = (8.00 ± 1.27) × 10^-11 cm³ molecule^-1 s^-1 (2σ uncertainties), consistent with current literature values.

Temperature Dependence Study of the Kinetics and Product Yields of the HO2 + CH3C(O)O2 Reaction by Direct Detection of OH and HO2 Radicals Using 2f-IR Wavelength Modulation Spectroscopy

The HO₂ + CH₃C(O)O₂ reaction consists of three product channels: CH₃C(O)OOH + O₂ (R1a), CH₃C(O)OH + O₃ (R1b), and OH + CH₃C(O)O + O₂ (R1c). The overall rate constant ( k₁) and product yields (α_1a, α_1b, and α_1c) were determined over the atmospherically relevant temperature range of 230-294 K at 100 Torr in N₂. Time-resolved kinetics measurements were performed in a pulsed laser photolysis experiment in a slow flow cell by employing simultaneous infrared (IR) and ultraviolet (UV) absorption spectroscopy. HO₂ and CH₃C(O)O₂ were formed by Cl-atom reactions with CH₃OH and CH₃CHO, respectively. Heterodyne near- and mid-infrared (NIR and MIR) wavelength modulation spectroscopy (WMS) was employed to selectively detect HO₂ and OH radicals. Ultraviolet absorption at 225 and 250 nm was used to detect various peroxy radicals as well as ozone (O₃). These experimental techniques enabled direct measurements of α_1c and α_1b via time-resolved spectroscopic detection in the MIR and the UV, respectively. At each temperature, experiments were performed at various ratios of initial HO₂ and CH₃C(O)O₂ concentrations to quantify the secondary chemistry. The Arrhenius expression was found to be k₁( T) = 1.38_-0.63^+1.17 × 10^-12 exp[(730 ± 170)/ T] cm³ molecule^-1 s^-1. α_1a was temperature-independent while α_1b and α_1c decreased and increased, respectively, with increasing temperatures. These trends are consistent with the current recommendation by the IUPAC data evaluation. Hydrogen-bonded adducts of HO₂ with the precursors, HO₂·CH₃OH and HO₂·CH₃CHO, played a role at lower temperatures; as part of this work, rate enhancements of the HO₂ self-reaction due to reactions of the adducts with HO₂ were also measured.

Kinetics of the ClO + HO2 reaction over the temperature range T = 210–298 K

Temporal traces showing the effect of increasing added methanol (red → orange) to a Cl/Cl₂O photolysis system, showing the progressive reduction of [ClO]₀ with increased [CH₃OH] but an increased rate of ClO loss at lower [ClO]₀, indicative of the ClO + HO₂ reaction.

VUV photoionization cross sections of HO2, H2O2, and H2CO

The absolute vacuum ultraviolet (VUV) photoionization spectra of the hydroperoxyl radical (HO2), hydrogen peroxide (H2O2), and formaldehyde (H2CO) have been measured from their first ionization thresholds to 12.008 eV. HO2, H2O2, and H2CO were generated from the oxidation of methanol initiated by pulsed-laser-photolysis of Cl2 in a low-pressure slow flow reactor. Reactants, intermediates, and products were detected by time-resolved multiplexed synchrotron photoionization mass spectrometry. Absolute concentrations were obtained from the time-dependent photoion signals by modeling the kinetics of the methanol oxidation chemistry. Photoionization cross sections were determined at several photon energies relative to the cross section of methanol, which was in turn determined relative to that of propene. These measurements were used to place relative photoionization spectra of HO2, H2O2, and H2CO on an absolute scale, resulting in absolute photoionization spectra.

First Cavity Ring-Down Spectroscopy HO2 Measurements in a Large Photoreactor

The HO2 radical is one of the most important intermediate species in atmospheric chemistry. We report on the development of a new photoreactor with first in-situ measurement of HO2 radical photostationary concentrations using continuous wave cavity ring-down spectrometry (cw-CRDS). Characterization of the actinic photon flux was carried out by NO2 actinometry. Photolysis of Cl2/methanol mixtures in air under UV light allowed the measurement of HO2 photostationary concentrations of a few 1010 molecules cm-3 with an HO2 detection limit of 1.5 × 1010 molecules cm-3 at 6638.207 cm-1. The feasibility of HO2 direct measurement in a reaction chamber is demonstrated through the measurement of the HO2 overall loss at different pressures showing the importance of HO2 diffusion and wall loss in such low pressure quartz reactor. The rate coefficient for the HO2+HO2 reaction has been measured at 6.6, 24 and 118 mbar and found to be in good agreement with the recommended value.

Developments in Laboratory Studies of Gas-Phase Reactions for Atmospheric Chemistry with Applications to Isoprene Oxidation and Carbonyl Chemistry

Laboratory studies of gas-phase chemical processes are a key tool in understanding the chemistry of our atmosphere and hence tackling issues such as climate change and air quality. Laboratory techniques have improved considerably with greater emphasis on product detection, allowing the measurement of site-specific rate coefficients. Radical chemistry lies at the heart of atmospheric chemistry. In this review we consider issues around radical generation and recycling from the oxidation of isoprene and from the chemical reactions and photolysis of carbonyl species. Isoprene is the most globally significant hydrocarbon, but uncertainties exist about its oxidation in unpolluted environments. Recent experiments and calculations that cast light on radical generation are reviewed. Carbonyl compounds are the dominant first-generation products from hydrocarbon oxidation. Chemical oxidation can recycle radicals, or photolysis can be a net radical source. Studies have demonstrated that high-resolution and temperature-dependent studies are important for some significant species.

Near-infrared kinetic spectroscopy of the HO2 and C2H5O2 self-reactions and cross reactions

The self-reactions and cross reactions of the peroxy radicals C2H5O2 and HO2 were monitored using simultaneous independent spectroscopic probes to observe each radical species. Wavelength modulation (WM) near-infrared (NIR) spectroscopy was used to detect HO2, and UV absorption monitored C2H5O2. The temperature dependences of these reactions were investigated over a range of interest to tropospheric chemistry, 221-296 K. The Arrhenius expression determined for the cross reaction, k2(T) = (6.01(-1.47)(+1.95)) x 10(-13) exp((638 +/- 73)/T) cm3 molecules(-1) s(-1) is in agreement with other work from the literature. The measurements of the HO2 self-reaction agreed with previous work from this lab and were not further refined. The C2H5O2 self-reaction is complicated by secondary production of HO2. This experiment performed the first direct measurement of the self-reaction rate constant, as well as the branching fraction to the radical channel, in part by measurement of the secondary HO2. The Arrhenius expression for the self-reaction rate constant is k3(T) = (1.29(-0.27)(+0.34)) x 10(-13)exp((-23 +/- 61)/T) cm3 molecules(-1) s(-1), and the branching fraction value is alpha = 0.28 +/- 0.06, independent of temperature. These values are in disagreement with previous measurements based on end product studies of the branching fraction. The results suggest that better characterization of the products from RO2 self-reactions are required.

Barrier heights for H-atom abstraction by H*O2 from n-butanol--a simple yet exacting test for model chemistries?

The barrier heights involved in the abstraction of a hydrogen atom from n-butanol by the hydroperoxyl radical have been computed with both compound (CBS-QB3, CBS-APNO, G3) and coupled cluster methods. In particular, the benchmark computations CCSD(T)/cc-pVTZ//MP2/6-311G(d,p) were used to determine that the barrier heights increase in the order alpha < gamma < beta < delta < OH. Two prereaction hydrogen-bonded complexes are formed, one of which connects the TGt conformer of n-butanol to the alpha and beta transition states and the other connects to the gamma and OH channels from the TGg conformer. Four postreaction complexes were also found which link the transition states to the products, hydrogen peroxide + C(4)H(9)O radical. Abstraction from the terminal delta carbon atom does not involve either a pre or postreaction complex. A number of DFT functionals-B3LYP, BMK, MPWB1K, BB1K, MPW1K, and M05-2X-were tested to see whether the correct ranking could be obtained with computationally less expensive methods. Only the later functional predicts the correct order but requires a basis set of 6-311++G(df,pd) to achieve this. However, the absolute values obtained do not agree that well with the benchmarks; the composite G3 method predicts the correct order and comes closest (< or = 2 kJ, mol (-1)) in absolute numerical terms for H-abstraction from carbon.

Measurements of Line Strengths in the 2ν1Band of the HO2Radical Using Laser Photolysis/Continuous Wave Cavity Ring-Down Spectroscopy (cw-CRDS)

Absolute absorption cross sections of the absorption spectrum of the 2nu1 band of the HO2 radical in the near-IR region were measured by continuous wave cavity ring-down spectroscopy (cw-CRDS) coupled to laser photolysis in the wavelength range 6604-6696 cm(-1) with a resolution better than 0.003 cm(-1). Absolute absorption cross sections were obtained by measuring the decay of the HO2 self-reaction, and they are given for the 100 most intense lines. The most important absorption feature in this wavelength range was found at 6638.20 cm(-1), exhibiting an absorption cross section of sigma = 2.72 x 10(-19) cm2 at 50 Torr He. Using this absorption line, we obtain a detection limit for the HO2 radical at 50 Torr of 6.5 x 10(10) cm(-3).