A systematic computational study on the reactions of HO2 with RO2: The HO2 + CH3O2(CD3O2) and HO2 + CH2FO2 reactions

Combined Experimental and Computational Kinetics Studies for the Atmospherically Important BrHg Radical Reacting with NO and O2

We report on the first experimental determination of the absolute rate constant of the reaction of BrHg + NO in N₂ bath gas using a laser photolysis-laser-induced fluorescence (LP-LIF) system. The rate constant of the reaction of BrHg + NO is determined to be 7.0_-0.9^+1.3 × 10^-12 cm³ molecule^-1 s^-1 over 50-700 Torr and 315-353 K. The absence of a pressure or temperature dependence suggests that this reaction leads mainly to mercury reduction (Hg + BrNO) rather than mercury oxidation (BrHgNO). Our theoretical calculations using NEVPT2 energies on density functional theory (DFT) geometries are consistent with a barrierless reaction to form Hg + BrNO. The equilibrium constants and the rate constants of the reaction BrHg + O₂ ⇌ BrHgOO are computed theoretically because they are too low to be measured in the LP-LIF system. Molecular oxygen quenches the LIF signal of BrHg with a large rate constant of (1.7 ± 0.1) × 10^-10 cm³ molecule^-1 s^-1. Thus, different techniques that capture the absolute [BrHg(X̃)] would be advantageous for kinetics studies of BrHg reactions in the presence of O₂. The computed equilibrium constant suggests a non-negligible upper limit of the fraction of BrHg stored as BrHgOO (up to 0.5) at low-temperature conditions, e.g., in the upper troposphere and in polar winters at ground level. Preliminary results indicate that BrHgOO behaves like HOO or organic peroxy radicals in reactions with atmospheric radicals.

Computational study on mechanisms and kinetics of the atmospheric CFCl2CH2O2 with Cl reaction

Ab initio BMC-CCSD//B3LYP calculations of the potential energy surfaces (PESs) are associated with the rate constants and branch ratio of products by means of RRKM theories to research the mechanism and product distribution of the CFCl₂CH₂O₂ with Cl reaction. The singlet and triplet PESs of this reaction have been calculated. Addition/elimination and S_N2 displacement mechanisms are located on the singlet PES, and S_N2 displacement and H-abstraction are located on the triplet PES. P1 (CFCl₂CHO + HClO) are expected to the primary products at T ≤ 2400 K, which is by original barrierless Cl addition to the terminal-O atom in CFCl₂CH₂O₂ and then eliminate HClO molecule, and the branch ratio of products rely on collision energy. The H-abstraction products on the triplet PES are the dominant products at higher temperatures. At 298 and 500 K, the total rate constants are not subject to pressure, conversely, the total rate constants presented typical falloff behavior at 1000 and 3000 K. The atmospheric lifetime of CFCl₂CH₂O₂ in Cl is around one day. TD-DFT computations imply that IM1 (CFCl₂CH₂OOCl) and IM2 (CFCl₂CH₂OClO) will photolyze under the sunlight.

Computational investigations on the HO2 + CHBr2O2 reaction: mechanisms, products, and atmospheric implications

Using quantum chemistry methods, mechanisms and products of the CHBr₂O₂ + HO₂ reaction in the atmosphere were investigated theoretically. Computational result indicates that the dominant product is CHBr₂OOH + O₂ formed on the triplet potential energy surface (PES). While CBr₂O + OH + HO₂ produced on the singlet PES is subdominant to the overall reaction under the typical atmospheric condition below 300 K. Due to higher energy barriers surmounted, other products including CBr₂O₂ + H₂O₂, CBr₂O + HO₃H, CH₂O + HO₃Br, CHBrO + HO₃ + Br, and CHBr₂OH + O₃ make minor contributions to the overall reaction. In the presence of OH radical, CHBr₂OOH generates CHBr₂O₂ and CBr₂O₂ + H₂O subsequently, which enters into new Br-cycle in the atmosphere. The substitution effect of alkyl group and halogens plays negligible roles to the dominant products in the RO₂ + HO₂ (X = H, CH₃, CH₂OH, CH₂F, CH₂Cl, CH₂Br, CH₂Cl, and CH₂Br) reactions in the atmosphere.

Gas-Phase Reactions of Isoprene and Its Major Oxidation Products

Isoprene carries approximately half of the flux of non-methane volatile organic carbon emitted to the atmosphere by the biosphere. Accurate representation of its oxidation rate and products is essential for quantifying its influence on the abundance of the hydroxyl radical (OH), nitrogen oxide free radicals (NO _x), ozone (O₃), and, via the formation of highly oxygenated compounds, aerosol. We present a review of recent laboratory and theoretical studies of the oxidation pathways of isoprene initiated by addition of OH, O₃, the nitrate radical (NO₃), and the chlorine atom. From this review, a recommendation for a nearly complete gas-phase oxidation mechanism of isoprene and its major products is developed. The mechanism is compiled with the aims of providing an accurate representation of the flow of carbon while allowing quantification of the impact of isoprene emissions on HO _x and NO _x free radical concentrations and of the yields of products known to be involved in condensed-phase processes. Finally, a simplified (reduced) mechanism is developed for use in chemical transport models that retains the essential chemistry required to accurately simulate isoprene oxidation under conditions where it occurs in the atmosphere-above forested regions remote from large NO _x emissions.

Study of low temperature chlorine atom initiated oxidation of methyl and ethyl butyrate using synchrotron photoionization TOF-mass spectrometry

The initial oxidation products of methyl butyrate (MB) and ethyl butyrate (EB) are studied using a time- and energy-resolved photoionization mass spectrometer.

Nitrate radicals and biogenic volatile organic compounds: oxidation, mechanisms, and organic aerosol

Oxidation of biogenic volatile organic compounds (BVOC) by the nitrate radical (NO₃) represents one of the important interactions between anthropogenic emissions related to combustion and natural emissions from the biosphere. This interaction has been recognized for more than 3 decades, during which time a large body of research has emerged from laboratory, field, and modeling studies. NO₃-BVOC reactions influence air quality, climate and visibility through regional and global budgets for reactive nitrogen (particularly organic nitrates), ozone, and organic aerosol. Despite its long history of research and the significance of this topic in atmospheric chemistry, a number of important uncertainties remain. These include an incomplete understanding of the rates, mechanisms, and organic aerosol yields for NO₃-BVOC reactions, lack of constraints on the role of heterogeneous oxidative processes associated with the NO₃ radical, the difficulty of characterizing the spatial distributions of BVOC and NO₃ within the poorly mixed nocturnal atmosphere, and the challenge of constructing appropriate boundary layer schemes and non-photochemical mechanisms for use in state-of-the-art chemical transport and chemistry-climate models. This review is the result of a workshop of the same title held at the Georgia Institute of Technology in June 2015. The first half of the review summarizes the current literature on NO₃-BVOC chemistry, with a particular focus on recent advances in instrumentation and models, and in organic nitrate and secondary organic aerosol (SOA) formation chemistry. Building on this current understanding, the second half of the review outlines impacts of NO₃-BVOC chemistry on air quality and climate, and suggests critical research needs to better constrain this interaction to improve the predictive capabilities of atmospheric models.

Theoretical study on the unimolecular decomposition of 2-chlorinated ethyl hydroperoxide

Chlorine-containing organic compounds have been of major interest since such compounds would serve as temporary reservoirs for HOX, ROX and ClOX radicals. Moreover, it would transport chlorine species to the atmosphere and stratosphere. However, limited studies have been performed on the 2-chlorinated ethyl hydroperoxide. In this work, the mechanism of unimolecular dissociation of 2-chlorinated ethyl hydroperoxide is theoretically studied. The equilibrium structures are optimized at the Boese–Martin for kinetics (BMK) level. And the energies are further refined at the Balanced multi-coefficient correlation-coupled cluster theory with single and double excitations (BMC-CCSD) level on the basis of the optimized geometries. Fifteen reaction channels are finally confirmed including the direct C–O, O–O, O–H, and C–C bond cleavage or the H2-, H2O-, H2O2-, and CH3Cl-elimination.

Isoprene NO3 Oxidation Products from the RO2 + HO2 Pathway

We describe the products of the reaction of the hydroperoxy radical (HO(2)) with the alkylperoxy radical formed following addition of the nitrate radical (NO(3)) and O(2) to isoprene. NO(3) adds preferentially to the C(1) position of isoprene (>6 times more favorably than addition to C(4)), followed by the addition of O(2) to produce a suite of nitrooxy alkylperoxy radicals (RO(2)). At an RO(2) lifetime of ∼30 s, δ-nitrooxy and β-nitrooxy alkylperoxy radicals are present in similar amounts. Gas-phase product yields from the RO(2) + HO(2) pathway are identified as 0.75-0.78 isoprene nitrooxy hydroperoxide (INP), 0.22 methyl vinyl ketone (MVK) + formaldehyde (CH(2)O) + hydroxyl radical (OH) + nitrogen dioxide (NO(2)), and 0-0.03 methacrolein (MACR) + CH(2)O + OH + NO(2). We further examined the photochemistry of INP and identified propanone nitrate (PROPNN) and isoprene nitrooxy hydroxyepoxide (INHE) as the main products. INHE undergoes similar heterogeneous chemistry as isoprene dihydroxy epoxide (IEPOX), likely contributing to atmospheric secondary organic aerosol formation.

OH radical-initiated oxidation degradation and atmospheric lifetime of N-ethylperfluorobutyramide in the presence of O₂/NOx

The OH radical-initiated oxidation degradation of N-ethylperfluorobutyramide (EtFBA) in the presence of O2/NOx was investigated theoretically by using density functional theory (DFT). All possible pathways involved in the oxidation process were presented and discussed. The study shows that the H abstraction from the C(2)H(2) group in EtFBA is the most energetically favorable because of the lowest barrier and highest exothermicity. Canonical variational transition-state (CVT) theory with small curvature tunneling (SCT) contribution was used to predict the rate constants over the temperature range of 180-370 K. At 296 K, the calculated overall rate constant of EtFBA with OH radicals is 2.50 × 10(-12)cm(3)molecule(-1)s(-1). The atmospheric lifetime of EtFBA determined by OH radicals is short, about 4.6 days at 296K. However, the atmospheric lifetimes of its primary oxidation products, C3F7C(O)N(H)C(O)CH3, C3F7C(O)N(H)CH2CHO and C3F7C(O)NH2, are much longer, about 30-50 days. It demonstrates the possibility that the atmospheric oxidation degradation of polyfluorinated amides (PFAMs) contributes to the burden of observed perfluorinated pollutants in the Arctic region. This study reveals for the first time that the water molecule plays an important catalytic effect on several key elementary steps and promotes the degradation potential of EtFBA.

Catalytic mechanisms of Au₁₁ and Au₁₁-nPt n (n=1-2) clusters: a DFT investigation on the oxidation of CO by O₂

The oxidation of CO catalyzed by clusters of Au11, Au10Pt and Au9Pt2 was investigated using the M06 functional suite of the density functional theory. Au and Pt atoms were described with the double-ζ valence basis set Los Alamos National Laboratory 2-double-z (LanL2DZ), whereas the standard 6-311++G(d,p) basis set was employed for the C and O atoms. Our theoretical model showed that (1) after coordination to Au and Au-Pt cluster, O2 and CO are apparently activated, and Mulliken charges show that the gold atoms in the active sites of Au11 are negatively charged; (2) Au-Pt clusters with 11 atoms can effectively catalyze the oxidation of CO by O2; (3) Au11 exhibits good catalytic performance for the oxidation of CO; (4) oxidation of CO occurs preferably on the Au-Pt active sites in Pt-doped clusters, and the single-center mechanisms are more favorable energetically than the two-center mechanisms; (5) after adsorption, an O2 molecule oxidates two CO molecules via stepwise mechanisms; and (6) the catalytic processes are highly exothermic.

Atmospheric fate of methyl vinyl ketone: peroxy radical reactions with NO and HO2

First generation product yields from the OH-initiated oxidation of methyl vinyl ketone (3-buten-2-one, MVK) under both low and high NO conditions are reported. In the low NO chemistry, three distinct reaction channels are identified leading to the formation of (1) OH, glycolaldehyde, and acetyl peroxy R2a , (2) a hydroperoxide R2b , and (3) an α-diketone R2c . The α-diketone likely results from HOx-neutral chemistry previously only known to occur in reactions of HO2 with halogenated peroxy radicals. Quantum chemical calculations demonstrate that all channels are kinetically accessible at 298 K. In the high NO chemistry, glycolaldehyde is produced with a yield of 74 ± 6.0%. Two alkyl nitrates are formed with a combined yield of 4.0 ± 0.6%. We revise a three-dimensional chemical transport model to assess what impact these modifications in the MVK mechanism have on simulations of atmospheric oxidative chemistry. The calculated OH mixing ratio over the Amazon increases by 6%, suggesting that the low NO chemistry makes a non-negligible contribution toward sustaining the atmospheric radical pool.

The atmospheric degradation pathways of BrCH2O2: computational calculation on mechanisms of the reaction with HO2

Mechanisms for the atmospheric degradation reaction of BrCH2O2+HO2 were investigated using quantum chemistry methods. The result indicates that the dominant product is BrCH2OOH+O2((3)Σ). While CH2O+HBr+O3, BrCHO+OH+HO2 and CH2O+Br+HO3 will be competitive to a certain extent in the atmosphere. Meanwhile, the nascent product - BrCH2OOH reacts easily with OH radicals leading to BrCH2O2 again under the atmospheric conditions. Moreover, OH radicals could act as a catalyst in the net reaction of BrCH2OOH→BrCHO+H2O. Thus the proposed product BrCHO+H2O+O2 in the experiment might be generated from the subsequent reaction of BrCH2OOH with extra OH radicals. Comparisons indicate that halogen substitution effect makes minor contributions to the XCH2O2 (X=H, F, Cl and Br)+HO2 reactions in the atmosphere.

Theoretical study of the mechanisms and kinetics of the reactions of hydroperoxy (HO2) radicals with hydroxymethylperoxy (HOCH2O2) and methoxymethylperoxy (CH3OCH2O2) radicals

The peroxy–peroxy radical reactions show spin, conformation and temperature dependence, forming formic acid and hydroxyl radicals.

Oxygen radical-mediated oxidation reactions of an alanine peptide motif - density functional theory and transition state theory study

Background

Oxygen-base (O-base) oxidation in protein backbone is important in the protein backbone fragmentation due to the attack from reactive oxygen species (ROS). In this study, an alanine peptide was used model system to investigate this O-base oxidation by employing density functional theory (DFT) calculations combining with continuum solvent model. Detailed reaction steps were analyzed along with their reaction rate constants.

Results

Most of the O-base oxidation reactions for this alanine peptide are exothermic except for the bond-breakage of the C_α-N bond to form hydroperoxy alanine radical. Among the reactions investigated in this study, the activated energy of OH α-H abstraction is the lowest one, while the generation of alkylperoxy peptide radical must overcome the highest energy barrier. The aqueous situation facilitates the oxidation reactions to generate hydroxyl alanine peptide derivatives except for the fragmentations of alkoxyl alanine peptide radical. The C_α-C_β bond of the alkoxyl alanine peptide radical is more labile than the peptide bond.

Conclusion

the rate-determining step of oxidation in protein backbone is the generation of hydroperoxy peptide radical via the reaction of alkylperoxy peptide radical with HO₂. The stabilities of alkylperoxy peptide radical and complex of alkylperoxy peptide radical with HO₂ are crucial in this O-base oxidation reaction.

Mechanisms of silicon alkoxide hydrolysis-oligomerization reactions: a DFT investigation

Silica aerogels possess a variety of unique and remarkable properties, but the mechanisms of silicon alkoxide, Si(OR)(4), hydrolyses and oligomerization in the initial stage of sol-gel processes are still not well understood. On the basis of density functional theory calculations at the B3LYP/6-31G(d,p)//B3LYP/6-311++G(d,p) basis set level, the hydrolysis and oligomerization reactions of Si(OR)(4) in neutral, acidic, and alkaline solutions were systematically investigated and we found that in acidic solutions the precursor Si(OCH(3))(4) was inclined to hydrolyze rather than to condense and the hydrolysis processes were energetically more favorable than the neutral ones. In alkaline solutions, the hydrolysis products oligomerize through an S(N)1 dimerization mechanism and the condensation rates are fast to form denser colloidal aerogels. Our calculations also testify that the subsequent cyclization reactions are energetically unfavorable.

THEORETICAL STUDY ON THE SELF-REACTION MECHANISM OF CH2ClO2 RADICALS

The complex potential energy surface for the self-reaction of CH 2 ClO 2 radicals, including 12 intermediates, 33 interconversion transition states, and 21 major dissociation products, was theoretically probed at the CCSD(T)/cc-pVDZ//B3LYP/6-311G(2d,2p) level of theory. The geometries and relative energies for various stationary points were determined. Based on the calculated CCSD(T)/cc-pVDZ potential energy surface, the possible mechanism for the studied system was proposed. It is shown that the most feasible channels are those leading to 22 CH 2 ClO + 3 O 2, 2 CH 2 ClO + 2 HO 2 + CHClO , 2 CH 2 ClO + HCl + 2 CH(O)O 2, 2 CH 2 ClO + 3 O 2 + 2 Cl + CH 2 O , and p,s,o- CH 2 ClOOOCl + CH 2 O with the energy barriers of 5.6, 11.8, 12.4, 12.4, and 13.5 kcal/mol, respectively. Their mechanisms are that CH 2 ClO 2 and CH 2 ClO 2 form a tetroxide intermediate first, then the intermediate dissociates to yield the productions or through multi-steps reactions to produce the final products.

Computational study of the reaction mechanism of the methylperoxy self-reaction

To provide insight on the reaction mechanism of the methyperoxy (CH(3)O(2)•) self-reaction, stationary points on both the spin-singlet and the spin-triplet potential energy surfaces of 2(CH(3)O(2)•) have been searched at the B3LYP/6-311++G(2df,2p) level. The relative energies, enthalpies, and free energies of these stationary points are calculated using CCSD(T)/cc-pVTZ. Our theoretical results indicate that reactions on a spin-triplet potential energy surface are kinetically unfavorable due to high free energy barriers, while they are more complicated on the spin-singlet surface. CH(3)OOCH(3) + O(2)(1) can be produced directly from 2(CH(3)O(2)•), while in other channels, three spin-singlet chain-structure intermediates are first formed and subsequently dissociated to produce different products. Besides the dominant channels producing 2CH(3)O• + O(2) and CH(3)OH + CH(2)O + O(2) as determined before, the channels leading to CH(3)OOOH + CH(2)O and CH(3)O• + CH(2)O + HO(2)• are also energetically favorable in the self-reaction of CH(3)O(2)• especially at low temperature according to our results.

Unimolecular decomposition of ethyl hydroperoxide: ab initio/Rice-Ramsperger-Kassel-Marcus theoretical prediction of rate constants

Alkyl hydroperoxides are found to be important intermediates in the combustion and oxidation processes of hydrocarbons. However, studies of ethyl hydroperoxide (CH(3)CH(2)OOH) are limited. In this work, kinetics and mechanisms for unimolecular decomposition of CH(3)CH(2)OOH have been investigated. The potential energy surface of decomposition reactions have first been predicted at the CCSD(T)/6-311+G(3df,2p)//B3LYP/6-311G(d,p) level. The results show that the formation of CH(3)CH(2)O + OH via O-O direct bond dissociation is dominant, the branching ratio of which is over 99% in the whole temperature range from 300 to 1000 K, and its rate constant can be expressed as k1 = 9.26 × 10(52)T(-11.91)exp(-26879/T) s(-1) at 1 atm. The rate constants of the reaction CH(3)CH(2)OOH → CH(3)CH(2)O + OH at different temperatures and pressures have been calculated, which can help us to comprehend the reactions of CH(3)CH(2)OOH at experimental conditions.

Theoretical Study of the HOCH2OO• + HO2 • Reaction: Detailed Molecular Mechanisms of the Three Reaction Channels

The HO2 • + HOCH2OO• reaction was theoretically investigated, using various high-level, single-reference Complete Basis Set methods including CBS-QB3, CBS-QCI/APNO and CBS-Q(MPW1B95) and a new multi-reference CI-PT2 approach. Three major product channels under atmospheric conditions were identified and their molecular mechanisms elucidated in great detail by Intrinsic Reaction Coordinate Analyses (IRC) at the B3LYP/6–311G(d,p) level: (i) Direct head-to-tail H-atom abstraction from the hydroperoxy radical by the alkylperoxy, occurring on the triplet Potential Energy Surface (PES) leading to HOCH2OOH + O2; (ii) A two-step rearrangement of the initial singlet HOCH2OOOOH tetroxide complex to form HC(O)OH + •OH + HO2 •; (iii) A multi-step rearrangement of the initial HOCH2OOOOH tetroxide to yield HC(O)OH + O2(1Δ) + H2O, about twice as fast as the former channel on the singlet-surface. The findings provide an explanation for the observed hydroxyl radical formation in this reaction (Jenkin et al., Phys. Chem. Chem. Phys. 9 (2007) 3149) and rationalize the high overall rate and its pronounced negative temperature dependence (Veyret et al., J. Phys. Chem. 93 (1989) 2368).

Kinetics and branching ratio studies of the reaction of C2H5O2 + HO2 using chemical ionisation mass spectrometry

The overall rate coefficient for the reaction of C(2)H(5)O(2) with HO(2) was determined using a turbulent flow chemical ionization mass spectrometer (TF-CIMS) system over the pressure range of 75 to 200 Torr and temperatures between 195 and 298 K. The temperature dependence of the overall rate coefficient for the reaction between C(2)H(5)O(2) and HO(2) was fitted using the following Arrhenius expression: k(T) = (2.08) x 10(-13) exp [(864 +/- 79)/T] cm(-3) molecule(-1) s(-1). The upper limits for the branching ratios for reactive channels leading to O(3) and OH production were quantified for the first time. A tropospheric model has been used to assess the impact of the experimental error of the rate coefficients determined in this study on predicted concentrations of a number of key species, including O(3), OH, HO(2), NO and NO(2). In all cases it is found that the propagated error is very small and will not in itself be a major cause of uncertainty in modelled concentrations. However, at low temperatures, where there is a wide discrepancy between existing kinetic studies, modelling using the range of kinetic data in the literature shows a small but significant variation for [C(2)H(5)O(2)], [C(2)H(5)OOH], [NO(x)] and the HO(2) : OH ratio. Furthermore, a structure-activity relationship (SAR) was developed to rationalise the reactivity of the reaction between RO(2) and HO(2).

A systematic computational study of the reactions of HO2 with RO2: the HO2 + C2H5O2 reaction

The reaction of HO2 with C2H5O2 has been studied using the density functional theory (B3LYP) and the coupled-cluster theory [CCSD(T)]. The reaction proceeds on the triplet potential energy surface via hydrogen abstraction to form ethyl hydroperoxide and oxygen. On the singlet potential energy surface, the addition-elimination mechanism is revealed. Variational transition state theory is used to calculate the temperature-dependent rate constants in the range 200-1000 K. At low temperatures (e.g., below 300 K), the reaction takes place predominantly on the triplet surface. The calculated low-temperature rate constants are in good agreement with the experimental data. As the temperature increases, the singlet reaction mechanism plays more and more important role, with the formation of OH radical predominantly. The isotope effect of the reaction (DO2 + C2D5O2 vs HO2 + C2H5O2) is negligible. In addition, the triplet abstraction energetic routes for the reactions of HO2 with 11 alkylperoxy radicals (CnHmO2) are studied. It is shown that the room-temperature rate constants have good linear correlation with the activation energies for the hydrogen abstraction.

A Systematic Computational Study of the Reactions of HO2 with RO2: The HO2 + CH2ClO2, CHCl2O2, and CCl3O2 Reactions

Potential energy surfaces for the reactions of HO(2) with CH(2)ClO(2), CHCl(2)O(2), and CCl(3)O(2) have been calculated using coupled cluster theory and density functional theory (B3LYP). It is revealed that all the reactions take place on both singlet and triplet surfaces. Potential wells exist in the entrance channels for both surfaces. The reaction mechanism on the triplet surface is simple, including hydrogen abstraction and S(N)2-type displacement. The reaction mechanism on the singlet surface is more complicated. Interestingly, the corresponding transition states prefer to be 4-, 5-, or 7-member-ring structures. For the HO(2) + CH(2)ClO(2) reaction, there are two major product channels, viz., the formation of CH(2)ClOOH + O(2) via hydrogen abstraction on the triplet surface and the formation of CHClO + OH + HO(2) via a 5-member-ring transition state. Meanwhile, two O(3)-forming channels, namely, CH(2)O + HCl + O(3) and CH(2)ClOH + O(3) might be competitive at elevated temperatures. The HO(2) + CHCl(2)O(2) reaction has a mechanism similar to that of the HO(2) + CH(2)ClO(2) reaction. For the HO(2) + CCl(3)O(2) reaction, the formation of CCl(3)O(2)H + O(2) is the dominant channel. The Cl-substitution effect on the geometries, barriers, and heats of reaction is discussed. In addition, the unimolecular decomposition of the excited ROOH (e.g., CH(2)ClOOH, CHCl(2)OOH, and CCl(3)OOH) molecules has been investigated. The implication of the present mechanisms in atmospheric chemistry is discussed in comparison with the experimental measurements.