Atmospheric chemistry of 2-nitrobenzaldehyde: Initiated by photo-excitation, OH-oxidation, and small TiO2 clusters adsorption catalysis

Theoretical Study of Hydroxyl Radical Initiated Degradation Mechanism, Kinetics and Subsequent Evolution of Methyl and Ethyl Iodides in the Atmosphere

The degradation and transformation of iodinated alkanes are crucial in the iodine chemical cycle in the marine boundary layer. In this study, MP2 and CCSD(T) methods were adopted to study the atmospheric transformation mechanism and degradation kinetic properties of CH3I and CH3CH2I mediated by ·OH radical. The results show that there are three reaction mechanisms including H-abstraction, I-substitution and I-abstraction. The H-abstraction channel producing ·CH2I and CH3C·HI radicals are the main degradation pathways of CH3I and CH3CH2I, respectively. By means of the variational transition state theory and small curvature tunnel correction method, the rate constants and branching ratios of each reaction are calculated in the temperature range of 200―600 K. The results show that the tunneling effect contributes more to the reaction at low temperatures. Theoretical reaction rate constants of CH3I and CH3CH2I with ·OH are calculated to be 1.42×10-13 and 4.44×10-13cm3 molecule−1 s−1 at T=298 K, respectively, which are in good agreement with the experimental values. The atmospheric lifetimes of CH3I and CH3CH2I are evaluated to be 81.51 and 26.07 day, respectively. The subsequent evolution mechanism of ·CH2I and CH3C·HI in the presence of O2, NO and HO2 indicates that HCHO, CH3CHO, and I-atom are the main transformation end-products. This study provides a theoretical basis for insight into the diurnal conversion and environmental implications of iodinated alkanes.

Kinetic and mechanistic study of the atmospheric degradation of C3F7OCHFCF2SCH2CH2OH with OH radical

Metal-free catalysis on the reactions of nitric acid with aliphatic aldehydes: A new potential source of organic nitrates

The photochemical reaction of peroxy radical (RO2·) and NO has been identified by field and forest studies as important source of organic nitrates (RONO2) in the atmosphere. However, this traditional pathway is not sufficient to explain the high concentration of RONO2. Hence, a new source of the tropospheric RONO2 from the dark reactions of nitric acid (HNO3) with aliphatic aldehydes (C1–C5) under catalysis is provided and examined for the first time by high-level quantum chemistry. The findings show that the reaction between HCHO and HNO3, which produces HOCH2ONO2, can be catalyzed by a series of metal-free catalysts (NH3, CH3NH2, CH3NHCH3, H2O, HNO3, H2SO4, HCOOH, HOOCCOOH). At 296 K, the effective rate constant for the bimolecular HNO3–HCHO reaction under the catalysis of CH3NH2 or CH3NHCH3 can be sufficiently accelerated by 5–8 order of magnitudes through this new loss pathway for HNO3 or HCHO to become competitive with the conventional loss pathway for their photochemical reactions with ·OH radical. Significantly, this new HOCH2ONO2 formation pathway from the dark reaction of HCHO with CH3NH3+NO3−/(CH3)2NH + NO3− was more favorable than the recognized source of RO2· with NO. Efficient catalysis performance of CH3NH2 and CH3NHCH3 is mainly attributed to their excellent proton receptivity capacity by activating the O–H bond of HNO3 to form stable organic nitrates (CH3NH3+NO3− and (CH3)2NH + NO3−) in the rate-determining step transition states. In the case of only considering the barrier, H2SO4 is the best catalyst among the investigated inorganic and organic acids, and dicarboxylic acid (HOOCCOOH) is stronger than monocarboxylic acid HCOOH in facilitating the RONO2 formation reaction. These new findings deepen our understanding on the unexpected source of organic nitrate and loss pathway of HNO3 or HCHO under catalysis in highly polluted regions.

Theoretical investigation on the atmospheric degradation mechanism, kinetics, and fate of hydroxymethyl nitrate initiated by ˙OH radicals

New insights into the ˙OH-initiated degradation mechanism, kinetics, and fate of hydroxymethyl nitrate have been investigated for the first time.

Theoretical investigation on atmospheric reaction mechanism, kinetics and SAR estimations of four-carbon ketones and alcohols

Photoinduced Ion-Pair Inner-Sphere Electron Transfer-Reversible Addition–Fragmentation Chain Transfer Polymerization

Theoretical insights into the gaseous and heterogeneous reactions of halogenated phenols with ˙OH radicals: mechanism, kinetics and role of (TiO₂)_<i>n</i> clusters in degradation processes

New insights into the mechanism of ˙OH-initiated degradation and the kinetics of halogenated phenols onto (TiO2)n clusters with controllable dimensions have been provided for the first time.

Prussian-blue-analog derived hollow Co3O4/NiO decorated CeO2 nanoparticles for boosting oxygen evolution reaction

Mechanistic and kinetic insights into the atmospheric degradation of (CH3)3CF and (CH3)3CCl initiated by Cl atom

Atmospheric oxidation of fluoroalcohols initiated by ˙OH radicals in the presence of water and mineral dusts: mechanism, kinetics, and risk assessment

Mechanisms and kinetic investigations of ˙OH-initiated atmospheric oxidation of fluoroalcohols and the subsequent transformation: effects of water and silica particles.

Synergistic effect of glutaric acid and ammonia/amine/amide on their hydrates in the clustering: A theoretical study

Atmospheric chemistry of CF3CHFCF2OCH2CF2CF3: Kinetics and mechanism on the OH-initiated degradation and subsequent reactions in the presence of O2 and NO

Metal-free catalysis for the reaction of nitrogen dioxide dimer with phenol: An unexpected favorable source of nitrate and aerosol precursors in vehicle exhaust

Kinetics and mechanism of OH-mediated degradation of three pentanols in the atmosphere

Pentanols as potential biofuels have attracted considerable interest, and thus it is of great importance to gain insights into their combustion and atmospheric chemistry.

Can nitrous acid contribute to atmospheric new particle formation from nitric acid and water?

The properties of (HNO3)(HONO)(H2O)n (n = 1–6) clusters are reported including thermodynamics, structures, temperature-dependence, intermolecular forces, optical properties, and evaporation rates.

One-dimensional co-crystallized coordination polymers showing reversible mechanochromic luminescence: cation–anion interaction directed rapid self-recovery

A one-dimensional co-crystallized coordination polymer exhibits mechanochromic luminescence, which can be recovered through rapid solvent treatment or a self-recovery process.

Mechanism, kinetics, and environmental assessment of OH‐initiated transformation of CTDE in the atmosphere

The transformation mechanism and kinetics of 2-chloro-1,1,2-trifluoroethyl- difluoromethyl-ether (CTDE, CHF 2 OCF 2 CHFCl) triggered by OH radicals are studied by density-functional theory methods and canonical variational transition state theory. The computational rate constant including small-curvature tunneling correction is found to be in commendable agreement with the experimental data. Two hydrogen abstraction channels to form the alkyl radicals of C?F 2 OCF 2 CHFCl and CHF 2 OCF 2 C?FCl are observed, and the formation of CHF 2 OCF 2 C?FCl is found to be more favorable than C?F 2 OCF 2 CHFCl kinetically and thermodynamically. Subsequent evolution of CHF 2 OCF 2 C?FCl in the presence of NO and O 2 indicates that the organic nitrate (CHF 2 OCF 2 CONO 2 FCl) is the stable product. The dechlorinate of alkoxy radical (CHF 2 OCF 2 C(O?)FCl) is the most favorable degradation channel, and the estimated ozone depletion potential for CTDE relative to chlorofluorocarbon-11 is 0.0204, which could lead to ozone depletion as a consequence. The computed atmospheric lifetime for CTDE is found to be 3.69 years by considering the combined contribu- tions from OH radicals and Cl atoms. The total radiative forcing and global warming potential of CTDE are, respectively, 0.547 W m −2 ppbv and 628.58 (100 years) at 298 K, suggesting that the contribution of CTDE to the greenhouse effect is moderate.

Ciprofloxacin transformation in aqueous environments: Mechanism, kinetics, and toxicity assessment during •OH-mediated oxidation

The initial reactions of organics with •OH are important to understand their transformations and fates in advanced oxidation processes in aqueous phase. Herein, the kinetics and mechanism of •OH-initiated degradation of ciprofloxacin (CIP), an antibiotic of fluoroquinolone class, are obtained using density functional and computational kinetics methods. All feasible mechanisms are considered, including H-abstraction, •OH-addition, and sequential electron proton transfer. Results showed that the H-abstraction is the dominant reaction pathway, and the product radicals P7single bondH, P9single bondH, and P10single bondH are the dominating intermediates. The aqueous phase rate coefficients for the •OH-triggered reaction of ciprofloxacin are calculated from 273 K to 323 K to examine the temperature dependent effect, and the theoretical value of 6.07 × 109 M−1 s−1 at 298 K is close to the corresponding experimental data. Moreover, the intermediates P7single bondH, P9single bondH, and P10single bondH could easily transform to several stable products in the presence of O2, HO2•, and •OH. The peroxy radical, which is generated from the incorporation of H-abstraction product radicals (P7single bondH, P9single bondH, and P10single bondH) with O2, prefers to produce HO2• into the surrounding through direct concerted elimination rather than the indirect mechanism. In addition, the peroxy radical could react with HO2• via triplet and singlet routes, and the former is more favorable due to its smaller barrier compared with the latter. The hydroxyl-substituted CIP has higher activity than its parent compound in their reactions with •OH due to its lower barrier and faster rate. In addition, the -NHC(O)-containing compound IM3-P10-H-4 is harmful to aquatic fish and is the primary product in the •OH-rich environment according to the ecotoxicity assessment computations. This study can improve our comprehension on CIP transformation in complex water environments.

DFT analysis on the removal of dimethylbenzoquinones in atmosphere and water environments: ·OH-initiated oxidation and captured by (TiO2)n clusters (n=1–6)

The elimination mechanisms and the dynamics of 2,5-dimethylbenzoquinone/2,6-dimethylbenzoquinone are performed by DFT under the presence of ·OH radical and TiO2-clusters. The rate coefficients, calculated within the atmospheric and combustion temperature range of 200–2000 K, agree well with the experimental data. The subsequent reactions including the bond cleavage of quinone ring, O2 addition or abstraction, the reactions of peroxy radical with NO yielding the precursor of organic aerosol are studied. Gaseous water molecule plays an important role in the transformation of alkoxy radical and exhibits a catalytic performance in the enol-ketone tautomerism. The lifetimes of 2,5-dimethylbenzoquinone/2,6-dimethylbenzoquinone are about 12.04–12.86 h at 298 K, which are in favor of the medium range transport of them in the atmosphere. Significantly, the water environment plays a negative role on the ·OH-degradation of dimethylbenzoquinone. Compared to the quinone ring, 2,5-dimethylbenzoquinone onto (TiO2)n clusters (n = 1–6) is easier to be absorbed by TiO2-clusters through its oxygen site because of its strong chemisorption, which indicates that TiO2-clusters are capable of trapping dimethylbenzoquinones effectively. The water environment could weaken the adsorption of 2,5-dimethylbenzoquinone onto (TiO2)n clusters (n = 1–6) by increasing the adsorption energy. This work reveals the removal of dimethylbenzoquinones and the formation of organic aerosol under polluted environments.

Atmospheric chemistry of thiourea: nucleation with urea and roles in NO2 hydrolysis

The nucleation with urea and roles in NO2 hydrolysis in the presence of thiourea.

Theoretical investigation of the mechanism, kinetics and subsequent degradation products of the NO3 radical initiated oxidation of 4-hydroxy-3-hexanone

We reported the H-abstraction reactions of 4-hydroxy-3-hexanone with NO3 with respect to thermodynamics, kinetics, temperature dependence and the subsequent mechanism.

New insights into 3M3M1B: the role of water in ˙OH-initiated degradation and aerosol formation in the presence of NOX (X = 1, 2) and an alkali

The oxidation mechanisms and dynamics of 3-methoxy-3-methyl-1-butanol (3M3M1B) initiated by ˙OH radicals were assessed by the density functional theory and canonical variational transition state theory. The effects of ubiquitous water on the title reactions were analyzed by utilizing an implicit solvation model in the present system. The results suggested that aqueous water played a negative role in the ˙OH-initiated degradation of 3M3M1B with an increase in the Gibbs free barriers. Meanwhile, the barriers were almost independent when explicit water molecules were involved in the gaseous phase, which could reduce the rate constant by approximately 3 orders of magnitude. The kinetic calculations showed that the rate constants were smaller by about 15, 9, 8, and 8 orders of magnitude for hydroxyl-, ammonia-, formic acid-, and sulfur acid-participating reactions, respectively, than that from an unassisted reaction. The results indicated that water, hydroxyl, ammonia, formic acid, or sulfur acid could not facilitate the title reaction when performed in the atmosphere. The investigations of the subsequent oxidation processes of the alkyl radical CH3OC(CH3)2CH2C·HOH indicated that CH3OC(CH3)2CH2CHO was the most favorable product by eliminating an HO2˙ radical. Additionally, the HO2˙ radical could serve as a self-catalyst to affect the above reaction through a double proton transfer process. With the introduction of NO, CH3OC(CH3)2CH2COOH and HNO2 were found to be the main products, which may be regarded as the new source of atmospheric nitrous acid. In the NO2-rich environment, the peroxynitrate of CH3OC(CH3)2CH2CH(OONO2)OH could be formed via the reaction of the CH3OC(CH3)2CH2CH(OO˙)OH radical with NO2. The degradation mechanism of CH3OC(CH3)2CH2CH(OONO2)OH in the presence of water, ammonia, and methylamine was demonstrated, and it was shown that water, ammonia, and methylamine could promote the formation of nitric hydrate and nitrate aerosol. The main species detected in the experiment were confirmed by a theoretical study. The atmospheric lifetimes of 3M3M1B in the temperature range of 217–298 K and altitude of 0–12 km were within the range of 6.83–8.64 h. This study provides insights into the transformation of 3M3M1B in a complex environment.

Theoretical study of H-atom abstraction reactions from CH3CH2OCH2CH3, CHF2CF2OCH2CF3 and CF3CH2OCH3 by NO3 radical & subsequent degradation

Atmospheric fate of methyl pivalate: OH/Cl-initiated degradation and the roles of water and formic acid

The atmospheric degradation mechanism and dynamics of methyl pivalate (MP) by OH radicals and Cl atoms are explored. The rate constants, computed using variational transition-state theory over the range of 200–2000 K at the CCSD(T)/6-311++G(d,p)//B3LYP/6-311G(d,p) level, are all in agreement with the experimental data. The alkyl radicals, which are formed from the reactions of OH or Cl with MP, can react with O2 and NO to produce the peroxyacyl nitrates, organic nitrates, and alkoxy radicals. The atmospheric evolution mechanisms for the (CH3)3CCOOCH2O•, •OCH2(CH3)2CCOOCH3, and •O(CH3)2CCOOCH3 radicals are also clarified. The OH- and Cl-determined atmospheric lifetimes and the global warming potentials (GWPs) of MP are shown to be low, suggesting that its environmental impact can be ignored. The Arrhenius expressions of kOH = 3.62 × 10−23T3.80exp(522.66/T) and kCl = 1.76 × 10−15T1.79exp(−55.89/T) cm3 molecule−1 s−1 are fitted within 200–2000 K. Compared with the OH/Cl-initiated degradation of (CH3)3CCOOCH3, the auto-decomposition reaction of (CH3)3CCOOCH3 → (CH3)2C=CH2 + HCOOCH3 may be more important at the high temperature range of 1500–2000 K. Moreover, the results show that the water and formic acid molecules can promote the degradation of MP. This study is helpful for evaluating the atmospheric implications of gaseous MP.

Insights into the degradation of (CF3)2CHOCH3 and its oxidative product (CF3)2CHOCHO & the formation and catalytic degradation of organic nitrates

In this work, a systematic investigation of the atmospheric oxidation mechanism of (CF3)2CXOCH3 and their oxidative products (CF3)2CXOCHO (X = H, F) initiated by OH radical or Cl atom is performed by density functional theory. This study reveals that the introduction of NO and O2 promotes the formation of organic nitrates, which are hygroscopic and are inclined to form secondary organic aerosols (SOA) and can affect the air quality. The rate constants of the individual reactions are found to be in agreement with the experimental results. One of the intriguing findings of this work is that the peroxynitrite of (CF3)2CHOCH2OONO formed from the subsequent reactions of (CF3)2CHOCH3 is more favorable to isomerize to organic nitrate (CF3)2CHOCH2ONO2 than to dissociate into alkoxy radical (CF3)2CHOCH2O and NO2 because of the lower energy barrier of isomerization. The second significant observation is that the organic nitrate can be degraded more favorably with the presence of NH3, CH3NH2, and CH3NHCH3 than its naked decomposition reaction (CF3)2CHOCH2ONO2→(CF3)2CHOCHO + HONO. The ammonium salt, a vital part of haze, is harmful to human health and can be formed in the existence of the NH3, CH3NH2, and CH3NHCH3. In addition, the toxic substance of peroxyalkyl nitrate (CF3)2CHOC(O)ONO2 which can reduce the visibility of the atmosphere is produced as the primary subsequent oxidation product of (CF3)2CHOCHO in a NO-rich environment. The main species detected experimentally are confirmed by this study. The computational results are crucial to risk assessment and pollution prevention of the volatile organic compounds (VOCs).

Theoretical study on the formation mechanisms, dynamics and the effective catalysis of the nitrophenols

In Situ Encapsulating α-MnS into N,S-Codoped Nanotube-Like Carbon as Advanced Anode Material: α → β Phase Transition Promoted Cycling Stability and Superior Li/Na-Storage Performance in Half/Full Cells

Theoretical insight into the role of urea in the hydrolysis reaction of NO2 as a source of HONO and aerosols

Environmental contextUrea is an important component of dissolved organic nitrogen in rainfall and aerosols, but the sources and the mechanisms of its production are not well understood. This computational study explores the effects of urea and water on the hydrolysis of NO2 and urea nitrate production. The results will aid our interpretation of the role of urea in the formation of atmospheric secondary nitrogen contaminants and aerosols. AbstractThe effects of urea on the hydrolysis reaction 2NO2 + mH2O (m = 1–3) have been investigated by theoretical calculations. The energy barrier (−2.67 kcal mol−1) of the urea-promoted reaction is lower than the naked reaction by 14.37 kcal mol−1. Urea also has a better catalytic effect on the reaction than methylamine and ammonia. Urea acts as a catalyst and proton transfer medium in this process, and the produced HONO may serve as a source of atmospheric nitrous acid. In addition, the subsequent reactions include clusters of nitrite, urea, and nitric acid. Then urea nitrate (UN), which is a typical HNO3 aerosol, can be formed in the subsequent reactions. The production of the acid-base complex (UN-2) is more favourable with an energy barrier of 0.10 kcal mol−1, which is 3.88 kcal mol−1 lower than that of the zwitterions NH2CONH3+NO3− (UN-1). The formation of zwitterions and the hydrolysis reaction are affected by humidity. The multi water-promoted hydrolysis reactions exhibit better thermodynamic stability when the humidity is increased. The extra water molecules act as solvent molecules to reduce the energy barrier. The natural bond orbital (NBO) analysis is employed to describe the donor-acceptor interactions of the complexes. The hydrogen bond interaction between the urea carbonyl and nitric acid of UN-2 is the strongest. The potential distribution maps of the urea nitrate and hydrate are examined, and the result shows that they tend to form zwitterions.

Theoretical insight into OH- and Cl-initiated oxidation of CF3OCH(CF3)2 and CF3OCF2CF2H & fate of CF3OC(X•)(CF3)2 and CF3OCF2CF2X• radicals (X=O, O2)

Understanding the insight into the mechanisms and dynamics of the Cl-initiated oxidation of (CH3)3CC(O)X and the subsequent reactions in the presence of NO and O2 (X = F, Cl, and Br)

In this work, the density functional and high-level ab initio theories are adopted to investigate the mechanisms and kinetics of reaction of (CH3)3CC(O)X (X = F, Cl, and Br) with atomic chlorine. Rate coefficients for the reactions of chlorine atom with (CH3)3CC(O)F (k1), (CH3)3CC(O)Cl (k2), and (CH3)3CC(O)Br (k3) are calculated using canonical variational transition state theory coupled with small curvature tunneling method over a wide range of temperatures from 250 to 1000 K. The dynamic calculations are performed by the variational transition state theory with the interpolated single-point energies method at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-311++G(d,p) level of theory. Computed rate constant is in good line with the available experimental value. The rate constants for the title reactions are in this order: k1

Computational exploration of regioselectivity and atmospheric lifetime in NO3-initiated reactions of CH3OCH3 and CH3OCH2CH3

Theoretical study of the hydrolysis of HOSO+NO2 as a source of atmospheric HONO: effects of H2O or NH3

Environmental contextNitrous acid (HONO) has long been recognized as an important atmospheric pollutant, with the reaction of HOSO+NO2 being a source of HONO. We explore the effects of an additional water or ammonia molecule on this reaction. Calculations show that the ammonia molecule has a more effective role than the water molecule in assisting the reaction. AbstractDepending on different ways that NO2 approaches the HOSO radical, the main reactant complexes HOS(O)NO2 and HOS(O)ONO–L (lowest energy structure of the isomer) were revealed by Lesar et al. (J. Phys. Chem. A 2011, 115, 11008), and the reaction of HOSO+NO2 is a source of trans (t)-HONO and SO2. In the present work, the water molecule in the hydrolysis reaction of HOSO+NO2 not only acts as a catalyst giving the products of t-HONO+SO2, but also as a reactant giving the products of t-HONO+H2SO3, c-HONO+H2SO3 and HNO3+t-S(OH)2. For the reaction of HOSO+NO2+H2O, the main reaction paths 2, 7, and 9 are further investigated with an additional water or ammonia molecule. The CBS-QB3 calculation result shows that the process of HOS(O)NO2–H2O → t-HONO–SO2–H2O is favourable with a barrier of 0.1kcal mol–1. Although the following process of t-HONO–SO2–H2O → t-HONO–H2SO3 is unfavourable with a barrier 33.6kcal mol–1, the barrier is reduced by 17.3 or 26.3kcal mol–1 with an additional water or ammonia molecule. Starting with HOS(O)ONO–L–H2O, the energy barriers of path 7 and path 9 are reduced by 8.9 and 8.5kcal mol–1 with an additional water molecule and by 9.9 and 9.2kcal mol–1 with an additional ammonia molecule. Ammonia is more beneficial than water for assisting the HOSO+NO2+H2O reaction. Three t-HONO–H2SO3 isomers which contain double intermolecular hydrogen bonds are studied by frequency and natural bond orbital calculations. Frequency calculations show that all hydrogen bonds exhibit an obvious red shift. The larger second-order stabilisation energies are consistent with the shorter hydrogen bonds. H2SO3 can promote the process of t-HONO → HNO2, and reduce the barrier by 45.2kcal mol–1. The product NH3–H2SO3 can further form a larger cluster (NH3–H2SO3)n (n=2, 4) including NH4+HSO3– ion pairs.

Atmospheric chemistry of ethers, esters, and alcohols on the lifetimes, temperature dependence, and kinetic isotope effect: an example of CF3CX2CX2CX2OX with OX reactions (X = H, D)

The dual-level direct dynamics method is employed to investigate the hydrogen abstraction reaction of CF3CH2CH2CH2OH (CF3CD2CD2CD2OD) with OH (OD) radicals. Four possible reaction channels caused by different positions of hydrogen atom attack are found. All the stationary points are studied with the ab initio and density functional theories. Single points computation is further refined by CCSD(T) and QCISD(T) methods combined with the 6-311++G(d,p) basis set in the minimum energy paths (MEP). Rate constants for each reaction channel, obtained by canonical variational transition state (CVT) coupled with the small curvatures tunneling (SCT) correction, are found to coincide with the available data in experiments. Calculations show that the variational effect was small in 200–2000 K, while the tunneling effect is large for every reaction channel in low-temperature regions. It is shown that the H-abstraction from the –CH2O– group is the primary channel. Standard enthalpies of formation for the species are computed, and the kinetic isotope effects for reactions CF3CH2CH2CH2OH/CF3CD2CD2CD2OD + OH and CF3CH2CH2CH2OH + OH/OD are discussed to provide valuable information for subsequent research. In addition, atmospheric lifetimes of a series of related ethers, esters, and alcohols are estimated. The Arrhenius expression for the title reaction k(T) = 3.43 × 10−21T3.22 exp(741.70/T) cm3 per molecule per s is also provided.

Theoretical study of the gaseous hydrolysis of NO2 in the presence of NH3 as a source of atmospheric HONO

Environmental context Nitrous acid is an important atmospheric trace gas, but the sources and the chemical mechanisms of its production are not well understood. This study explores the effects of ammonia and water on the hydrolysis of nitrogen dioxide and nitrous acid production. The calculated results show that ammonia is more effective than water in promoting the hydrolysis reaction of nitrogen dioxide. Abstract The effects of ammonia and water molecules on the hydrolysis of nitrogen dioxide as well as product accumulation are investigated by theoretical calculations of three series of the molecular clusters 2NO2–mH2O (m=1–3), 2NO2–mH2O–NH3 (m=1, 2) and 2NO2–mH2O–2NH3 (m=1, 2). The gas-phase reaction 2NO2 + H2O → HONO + HNO3 is thermodynamically unfavourable. The additional water or ammonia in the clusters can not only stabilise the products by forming stable complexes, but also reduce the energy barrier for the reaction. There is a considerable energy barrier for the reaction at the reactant cluster 2NO2–H2O: 11.7kcalmol–1 (1kcalmol–1=4.18kJmol–1). With ammonia and an additional water in the cluster, 2NO2–H2O–NH3, the thermodynamically stable products t-HONO + NH4NO3–H2O can be formed without an energy barrier. With two ammonia molecules, as in the cluster 2NO2–mH2O–2NH3 (m=1, 2), the reaction is barrierless and the product complex NH4NO2–NH4NO3 is further stabilised. The present study, including natural bond orbital analysis on a series of species, shows that ammonia is more effective than water in promoting the hydrolysis reaction of NO2. The product cluster NH4NO2–NH4NO3 resembles an alternating layered structure containing the ion units NH4+NO2– and NH4+NO3–. The decomposition processes of NH4NO2–NH4NO3 and its monohydrate are all spontaneous and endothermic.

Computational study of H-abstraction reactions from CH3OCH2CH2Cl/CH3CH2OCH2CH2Cl by Cl atom and OH radical and fate of alkoxy radicals

Atmospheric chemistry of CF3(CX2)2CH2OH: rate coefficients and temperature dependence of reactions with chlorine atoms and the subsequent pathways of alkyl and alkoxy radicals (X = H, F)

The atmospheric and kinetic properties of CF3(CX2)2CH2OH (X = H, F) with chlorine atoms were studied by density functional and canonical variational transition state theories in conjunction with the small-curvature tunneling correction. The minimum energy path was obtained by the CCSD(T)/6-311++G(d,p)//B3LYP/6-311G(d,p) method. The H-abstraction channel from the –CH2O– group was found to be the dominant channel, whereas that from the –OH site of the title reactions may be negligible because of the high barrier. All rate constants computed within 200–1000 K are in reasonable agreement with the available experimental values. The degradation mechanism of CF3(CX2)2CH2OH is discussed. The subsequent pathways of the CF3(CX2)2C˙HOH and CF3(CX2)2C(O˙)HOH radicals were studied. The atmospheric lifetime and global warming potentials (GWPs) of CF3(CX2)2CH2OH were computed, and it is shown that fluorine substitution may increase the lifetime and GWPs. It is also indicated that fluorine substitution may decrease the reactivity. The reaction enthalpies and reaction Gibbs free energies for all relevant reactions were discussed. The rate coefficient expressions for the title reactions obtained are kT1 = 5.75 × 10−17T2.26 exp(428.02/T) and kT2 = 1.30 × 10−17T1.96 exp(67.40/T) per cm3 per molecule per s.

Theoretical Study on the Reactions of (CF3)2CFOCH3 + OH/Cl and Reaction of (CF3)2CFOCHO with Cl Atom

Reactions of (CF3)2CFOCH3 and (CF3)2CFOCHO with hydroxyl radical and chlorine atom are studied at the B3LYP and BHandHLYP/6-311+G(d,p) levels along with the geometries and frequencies of all stationary points. This study is further refined by CCSD(T) and QCISD(T)/6-311+G(d,p) methods in the minimum energy paths. For the reaction (CF3)2CFOCH3 + OH, two hydrogen abstraction channels are found. The total rate constants for the reactions (CF3)2CFOCH3 + OH/Cl and (CF3)2CFOCHO + Cl are followed by means of the canonical variational transition state with the small-curvature tunneling correction. The comparison between the hydrogen abstraction rate constants by hydroxyl and chlorine atom is discussed. Calculated rate constants are in reasonable agreement with the available experiment data. The standard enthalpies of formation for the reactants, (CF3)2CFOCH3 and (CF3)2CFOCHO, and two products, (CF3)2CFOCH2 and (CF3)2CFOCO, are evaluated by a series of isodesmic reactions. The Arrhenius expressions for the title reactions are given as follows: k1= 1.08 × 10–22 T3.38 exp(−213.31/T), k2= 3.55 × 10–22 T3.61 exp(−240.26/T), and k3= 3.00 × 10 –19 T2.58 exp(−1294.34/T) cm3 molecule–1 s–1.

Theoretical Studies of the Reactions CFxH3−xCOOR+Cl and CF3COOCH3+OH

The mechanism and kinetics of the reactions of CF3COOCH2CH3, CF2HCOOCH3, and CF3COOCH3 with Cl and OH radicals are studied using the B3LYP, MP2, BHandHLYP, and M06‐2X methods with the 6‐311G(d,p) basis set. The study is further refined by using the CCSD(T) and QCISD(T)/6‐311++G(d,p) methods. Seven hydrogen‐abstraction channels are found. All the rate constants, computed by a dual‐level direct method with a small‐curvature tunneling correction, are in good agreement with the experimental data. The tunneling effect is found to be important for the calculated rate constants in the low‐temperature range. For the reaction of CF3COOCH2CH3+Cl, H‐abstraction from the CH2 group is found to be the dominant reaction channel. The standard enthalpies of formation for the species are also calculated. The Arrhenius expressions are fitted within 200–1000 K as kT(1)=8.4×10−20T 2.63exp(381.28/T), kT(2)=2.95×10−21T 3.13exp(−103.21/T), kT(3)=1.25×10−23T 3.37exp(791.98/T), and kT(4)=4.53×10−22T 3.07exp(465.00/T).

Atmospheric chemistry of alkyl iodides: theoretical studies on the mechanisms and kinetics of CH3I/C2H5I + NO3 reactions

The gas-phase reactions of CH3I and C2H5I with NO3 radicals have been studied using a dual-level direct kinetics method. The minimum energy paths have been refined by CCSD(T) and QCISD(T) methods. One displacement and two hydrogen abstraction processes were found for the reaction of CH3I + NO3. For the reaction of C2H5I + NO3, three hydrogen abstraction and one displacement channel were found. The hydrogen abstraction from the –CH2– group was found to be the dominant channel. The displacement channel of the title reactions may be negligible because of the high barrier. The rate constants for the individual reaction channels were followed by means of the canonical variational transition state with the small-curvature tunneling correction. The calculated rate constants were in reasonable agreement with the available data from experiments. The Arrhenius expressions for the title reactions are given as follows: ka = 8.62 × 10−32T6.66 exp(1324.23/T), kb = 9.48 × 10−27T5.75 exp(−655.34/T) cm3 per molecule per s. The atmospheric lifetimes of CH3I and C2H5I determined by reaction with the NO3 radical were about 3.07 and 5.86 h, which indicate that they can be degraded in the gas phase within a short time to serve as a source of reactive iodine compounds at night-time.

Theoretical investigation of the mechanisms and dynamics of the reaction CHF2OCF2CHFCl+Cl