Temperature Dependent Kinetics of the OH/HO2/O3 Chain Reaction by Time-Resolved IR Laser Absorption Spectroscopy

Accurate determination of reaction energetics and kinetics of the HO2˙ + O3 → OH˙ + 2O2 reaction

In the present work, we have studied the HO₂˙ + O₃ → HO˙ + 2O₂ reaction using chemical kinetics and quantum chemical calculations. We have employed the post-CCSD(T) method to estimate the barrier height and reaction energy for the title reaction. In the post-CCSD(T) method, we have included zero point energy corrections, contributions from full triple excitations and partial quadratic excitations at the coupled-cluster level, and core corrections. We have also computed the reaction rate in the temperature range of 197-450 K and found good agreement with all the available experimental results. In addition, we have also fitted the computed rate constants with the Arrhenius expression and obtained an activation energy of 1.0 ± 0.1 kcal mol^-1, almost identical to the value recommended by IUPAC and JPL.

Role of post-CCSD(T) corrections in predicting the energetics and kinetics of the OH • +O 3 reaction

The present work investigates the OH • +O 3 reaction by means of chemical kinetics and quantum chemical calculations. To predict the reaction barrier height and reaction energy, we have...

Effect of NH3 and HCOOH on the H2O2 + HO → HO2 + H2O reaction in the troposphere: competition between the one-step and stepwise mechanisms

The H₂O₂ + HO → HO₂ + H₂O reaction is an important reservoir for both radicals of HO and HO₂ catalyzing the destruction of O₃. Here, this reaction assisted by NH₃ and HCOOH catalysts was explored using the CCSD(T)-F12a/cc-pVDZ-F12//M06-2X/aug-cc-pVTZ method and canonical variational transition state theory with small curvature tunneling. Two possible sets of mechanisms, (i) one-step routes and (ii) stepwise processes, are possible. Our results show that in the presence of both NH₃ and HCOOH catalysts under relevant atmospheric temperature, mechanism (i) is favored both energetically and kinetically than the corresponding mechanism (ii). At 298 K, the relative rate for mechanism (i) in the presence of NH₃ (10, 2900 ppbv) and HCOOH (10 ppbv) is respectively 3–5 and 2–4 orders of magnitude lower than that of the water-catalyzed reaction. This is due to a comparatively lower concentration of NH₃ and HCOOH than H₂O which indicates the positive water effect under atmospheric conditions. Although NH₃ and HCOOH catalysts play a negligible role in the reservoir for both radicals of HO and HO₂ catalyzing the destruction of O₃, the current study provides a comprehensive example of how acidic and basic catalysts assisted the gas-phase reactions.

The H₂O₂ + HO → HO₂ + H₂O reaction is an important reservoir for both radicals of HO and HO₂ catalyzing the destruction of O₃.

Role of the (H2O)n (n = 1–3) cluster in the HO2 + HO → 3O2 + H2O reaction: mechanistic and kinetic studies

Upon incorporation of the catalyst (H₂O)_n (n = 1–3) into the reaction HO₂ + HO → H₂O + ³O₂, the catalytic effects of water, water dimer, and water trimer mainly arise from the contribution of a single molecule of water vapor.

Degradation of VX surrogate profenofos on surfaces via in situ photo-oxidation

Surface degradation of profenofos (PF), a VX nerve gas surrogate, was investigated using in situ photo-oxidation that combines simple instrumentation and ambient gases (O2 and H2O) as a function of exposure conditions ([O3], [OH], UV light λ = 185 and/or 254 nm, relative humidity) and PF film surface density (0.38-3.8 g m(-2)). PF film 0.38 g m(-2) fully degraded after 60 min of exposure to both 254 and 185 nm UV light in humidified air and high ozone. The observed pseudo-first-order surface reaction rate constant (kobs = 0.075 ± 0.004 min(-1)) and calculated hydroxyl concentration near the film surface ([OH]g = (9 ± 2) × 10(7) molecules cm(-3)) were used to determine the second-order rate constant for heterogeneous reaction of PF and OH (k(OH)PF = (5 ± 1) × 10(-12) cm(3) molec(-1) s(-1)). PF degradation in the absence of 185 nm light or without humidity was lower (70% or 90% degradation, respectively). With denser PF films ranging from 2.3 to 3.8 g m(-2), only 80% degradation was achieved until the PF droplet was redissolved in acetonitrile which allowed >95% PF degradation. Surface product analysis indicated limited formation of the nontoxic phosphoric acid ester but the formation of nonvolatile chemicals with increased hydrophilicity and addition of OH.

Theoretical studies on the electron capture properties of the H2SO4...HOO˙ complex and its implications as an alternative source of HOOH

To better understand the potential role of sulfuric acid aerosols in the atmosphere, the electron capture properties of the H(2)SO(4)...HOO˙ complex have been systematically investigated by employing the MP2 and B3LYP methods in combination with the atoms in molecules (AIM) theory, energy decomposition analysis (EDA), and ab initio molecular dynamics. It was found that the electron capture process is a favorable reaction thermodynamically and kinetically. The excess electron can be captured by the HOO˙ fragment initially, and then the proton of the H(2)SO(4) fragment associated with the intermolecular H-bonds is transferred to the HOO˙ fragment without any activation barriers, resulting in the formation of the HOOH species directly. Therefore, the electron capture process of the H(2)SO(4)...HOO˙ complex provides an alternative source of HOOH in the atmosphere. The nature of the coupling interactions in the electron capture products are clarified, and the most stable anionic complex is also determined. Additionally, the influences of the adjacent water molecules on the electron capture properties are investigated, as well as the distinct IR features of the most stable electron capture product.

The Gas-Phase Reaction between O3 and HO Radical: A Theoretical Study

We report a theoretical study on the reaction of ozone with hydroxyl radical, which is important in the chemistry of the atmosphere and in particular participates in stratospheric ozone destruction. The reaction is a complex process that involves, in the first stage, a pre-reactive hydrogen-bonded complex (C1), which is formed previous to two transition states (TS1 and TS2) involving the addition of the hydroxyl radical to ozone, and leads to the formation of HO4 polyoxide radical before the release of the products HO2 and O2. The reaction is computed to be exothermic by 42.72 kcal mol(-1), which compares quite well with the experimental estimate, and the energy barriers of TS1 and TS2 with respect to C1 are computed to be 1.80 and 2.26 kcal mol(-1) at 0 K. A kinetic study based on the variational transition state theory (VTST) predicts a rate constant, at 298 K, of 7.37 x 10(-14) cm3 molecule(-1) s(-1), compared to the experimentally recommended value of 7.25 x 10(-14) cm3 molecule(-1) s(-1).

The Gas-Phase Hydrogen-Bonded Complex between Ozone and Hydroperoxyl Radical. A Theoretical Study

We report a theoretical study on the gas-phase hydrogen-bonded complexes formed between ozone and hydroperoxyl radical, which are of interest in atmospheric chemistry. We have employed CASSCF, CASPT2, QCISD, and CCSD(T) theoretical approaches employing 6-311+G(2df,2p) and aug-cc-pVTZ basis sets, and we have found three complexes whose stabilities are computed to be 2.02, 1.19, and 1.34 kcal/mol, respectively, at 0 K. In addition, we have also found three transition states connecting these complexes that lie below the energy of the separate reactants. To help for possible experimental identification of these hydrogen-bonded complexes, we report also the computed harmonic vibrational frequencies along with the frequency shifts of the complexes, relative to the monomers, and the computed rotational constants.

Theoretical Characterization of the Gas-Phase O3⋅⋅⋅HO Hydrogen-Bonded Complex

We report a theoretical study on two gas-phase hydrogen-bonded complexes formed between ozone and hydroxyl radical that have relevance to atmospheric chemistry. This study was carried out by using CASSCF, CASPT2, QCISD, and CCSD(T) theoretical approaches in conjunction with the 6-311+G(2df,2p) and aug-cc-pVTZ basis sets. Both complexes have a planar structure and differ from each other in the orientation of the electronic density of the unpaired electron associated with the HO radical moiety. Our calculations predict their stabilities to be 0.87 and 0.67 kcal mol(-1), respectively, at 0 K and show the importance of anharmonic effects in computing the red shift of the HO stretch originating from the hydrogen-bonding interaction. We also report two transition states involving the movement of the HO moiety on the potential energy surfaces of these hydrogen-bonded complexes.

Photolysis of ozone in aqueous solutions in the presence of tertiary butanol

The ozone decomposition quantum yield (phi) in millimolar and higher-concentration aqueous tertiary butanol solution is 0.64 +/- 0.05 (observed over a wavelength range from 250 to 280 nm) and rises toward lower tertiary butanol concentrations (phi approximately 1.5 at 10(-5) M at pH 2) on account of the onset of the well-known *OH-radical-induced chain reaction. The destruction of the organic is initiated by hydrogen-atom abstraction through OH radicals which are produced via the reaction of the photolytically generated O(1D) with the solvent water at a quantum yield of phi(*OH) of about 0.1. There is no decomposition of ozone in the dark on the time scale of the photolysis experiment. The efficiency of tertiary butanol destruction with respect to ozone consumption ([O3]0 = 3 x 10(-4) M), defined by the ratio delta[t-BuOH]/delta[O3], termed eta(t-BuOH), is 0.26 at millimolar tertiary butanol concentrations, determined at the stage of essentially complete ozone consumption. It diminishes toward lower tertiary butanol concentrations (delta[t-BuOH]/delta[O3] approximately 0.17 at [t-BuOH]0 = 1 x 10(-4) M). Part of the effect of the ozone, apart from being a source of *OH radicals, rests on the intervention of HO2*/O2*- which is produced in the course of the peroxyl-radical chemistry of the tertiary butanol in this dioxygen-saturated environment and converted into further *OH radical by reaction with ozone. Moreover in this system, organic free radicals and peroxyl radicals react with the ozone. On the basis of the experimental and mechanistic-simulation data, the quantum yield of direct (by hv) ozone cleavage in aqueous solution is estimated at about 0.5.

Recent progress in infrared absorption techniques for elementary gas-phase reaction kinetics

Sensitive and precise measurements of rate coefficients, branching fractions, and energy disposal from gas-phase radical reactions provide information about the mechanism of elementary reactions as well as furnish modelers of complicated chemical systems with rate data. This chapter describes the use of time-resolved infrared laser absorption as a tool for investigating gas-phase radical reactions, emphasizing the exploitation of the particular advantages of the technique. The reaction of Cl atoms with HD illustrates the complementarity of thermal kinetic measurements with molecular beam data. Measurements of second-order reactions, such as the self-reactions of SiH3 and C3H3 radicals, and determinations of product branching fractions in reactions such as CN + O2 rely on the wide applicability of infrared absorption and on the straightforward relationship of absorption to absolute concentration. Finally, investigations of product vibrational distributions, as in the CN + H2 reaction, provide additional insight into the details of reaction mechanisms.