Small-curvature Tunneling Correction Research Articles

Experimental and Computational Investigations of the Tropospheric Photooxidation Reactions of 1,1,1,3,3,3-Hexafluoro-2-Methyl-2-Propanol Initiated by OH Radicals and Cl Atoms.

The gas-phase kinetics for the reactions of OH radicals and Cl atoms with 1,1,1,3,3,3-hexafluoro-2-methyl-2-propanol (HF2M2P) were measured at temperatures between 268 and 363 K using the relative rate experimental technique. Methane and acetonitrile were used as reference compounds to measure the rate coefficients of the title reactions. For the reactions of HF2M2P with OH radicals and Cl atoms, the rate coefficients were measured to be (7.07 ± 1.21) × 10-15 and (2.85 ± 0.54) × 10-14 cm3 molecule-1 s-1, respectively, at 298 K. The obtained Arrhenius expressions for the reactions of HF2M2P with OH radicals and Cl atoms are kHF2M2P+OHExp-(268-363K) = (7.84 ± 0.75) × 10-14 exp [-(717 ± 59)/T] and kHF2M2P+ClExp-(268-363K) = (3.21 ± 0.45) × 10-12 exp [-(1395 ± 83)/T] cm3 molecule-1 s-1. In addition to the experimental measurements, computational kinetic calculations were also performed for the title reactions at the M06-2X/MG3S//M06-2X/6-31 + G(d,p) level of theory using advanced methods such as the canonical variational transition-state theory coupled with small curvature tunneling corrections at temperatures between 200 and 400 K. Theoretical calculations reveal that the H-abstraction from the CH3 group is a more favorable reaction channel than that from the OH group. Thermochemistry, branching ratios, cumulative atmospheric lifetime, global warming potential, acidification potential, and photochemical ozone creation potential of HF2M2P were calculated in the present investigation.

A theoretical study of the Ḣ- and HOȮ-assisted propen-2-ol tautomerizations: Reactive systems to evaluate collision efficiency definitions on chemically activated reactions using SS-QRRK theory

Abstract In combustion, enols can undergo keto-enol tautomerizations, which are intermediate steps in the formation of pollutant species. In this work, we performed a theoretical kinetic study of the step-wise propen-2-ol tautomerization catalyzed by hydrogen and hydroperoxyl radicals. Ab initio calculations at the CCSD(T)/aug-cc-pVTZ//M06-2X/cc-pVTZ level were run, and rate constants were calculated using the multistructural torsional variational transition state theory with small-curvature tunneling corrections. Hydrogen and hydroperoxyl radicals can induce a step-wise mechanism toward keto formation with a lower barrier than that of unimolecular tautomerization. The potential energy surface comprising these reactions is complex, involving different intermediates that are connected by different types of pathways. The hydrogen-assisted tautomerization consists of two steps where the formation of an intermediate radical takes place as a result of the addition of the hydrogen atom to the double bond of propen-2-ol. The high-pressure limit rate constants of the reactions of this intermediate radical toward propen-2-ol and acetone exhibit an Arrhenius behavior, in agreement with previous works. In the hydroperoxyl-assisted tautomerization, the acetone formation has two routes involving an overall of four steps. The route with the highest energy barrier becomes prominent above 800 K due to multistructural anharmonicity effects, which must be included for an accurate kinetic description of the titled reactions. Calculations of pressure-dependent rate constants showed that the original system-specific quantum Rice-Ramsperger-Kassel theory, together with the modified strong collision model (SS-QRRK/MSC), significantly underpredict the bimolecular stabilization rate constants for the hydrogen-assisted tautomerization above 1200 K by factors of up to three orders of magnitude when compared with the benchmark Rice-Ramsperger-Kassel-Markus/master equation method. To solve this problem, we tested two alternative definitions of the collision efficiency parameter by using an improved implementation of the SS-QRRK/MSC approach developed by us for chemically activated reactions. One of these definitions, provided by Gilbert et al. (1983), corrected the bimolecular stabilization rate constant behavior and yielded a maximum deviation factor of only 4.5 at 2000 K and 100 atm. For the hydroperoxyl-assisted tautomerization, pressure effects are negligible because the stabilization of the energized adduct cannot compete with the reaction leading to the final product for most of the physical conditions studied. Our calculated rate constants can be used to perform more accurate kinetic modeling of alcohols. Besides, the implementation of the SS-QRRK theory with the collision efficiency of Gilbert et al. (1983) proposed in this work is useful for computing pressure-dependent rate constants of chemically activated reactions, including all possible refinements (multi-dimensional tunneling, multistructural anharmonicity, etc.) considered in high-pressure limit calculations.

Kinetic investigation of the reaction of ethylperoxy radicals with ethanol

AbstractThe thermodynamic and kinetic investigation for the reaction of ethylperoxy radical (CH3CH2OO•) with ethanol (CH3CH2OH) was studied computationally with variational transition state theory. The geometry optimization calculations showed that both the reactants have two conformers. The energetics and thermodynamic properties were calculated using the G4 composite method. The results showed that the abstraction of H atom from the ─CH2─ site is most feasible, when compared to the other pathways, since it has the lowest energy barrier. The rate coefficient calculations for the title reaction were carried out using the multistructural canonical variational transition state theory with small curvature tunneling corrections in the temperature range of 400‐1500 K. The results showed a similar trend to that of the energetics in which the abstraction of the H atom from the ─CH2─ site has a larger rate coefficient, when compared to other reaction pathways. The reactivity trend towards the H atom abstraction by ethylperoxy radicals varies as ─CH2 > ─CH3 > ─OH. The contributions from the ground‐state conformers of both the reactants were included into the total kinetics using the Boltzmann probability distribution. The obtained temperature‐dependent expression for the studied reaction is ktotal = 1.51 × 10−32 T6.3 exp(−3691/T) cm3 molecule−1 s−1.

Atmospheric chemistry of CHF2CF2OCH2CF3: Reactions with Cl atoms, fate of CHF2CF2OC•HCF3 radical, formation of OH radical and Criegee Intermediate

Abstract The kinetics and mechanism for hydrogen abstraction reactions of CHF2CF2OCH2CF3 with Cl atoms were studied using the DFT-based M06-2X/6-311++G(d,p) quantum chemical method. The thermal rate coefficients for H-abstraction reactions were evaluated by canonical variational transition state theory along with the small curvature tunneling correction over the temperature range of 200–1000 K. A three-parameter model equation has been proposed to describe the temperature variation of rate coefficient as k(CHF2CF2OCH2CF3+Cl) = 5.24 × 10−22T3.2exp(-580/T)(in cm3 molecule−1s−1). Atmospheric lifetime of CHF2CF2OCH2CF3 is estimated to be 3.10 years and global warming potentials at the 100-year time horizon is calculated as 601. The atmospheric fate of generated CHF2CF2OC•HCF3 radical are reported. The rate coefficient values for the reaction of important degradation products like CHF2CF2OCHO and CHF2CF2OOH with OH/Cl are also reported for the first time. Production of OH radicals from the unimolecular decomposition of CHF2CF2OC(OO•)HCF3 peroxy radical and the possible formation of Fluorinated Criegee Intermediate from self-reaction of RO2 radicals are discussed.

Theoretical investigations on the kinetics of dimethoxymethane with peroxy radicals

The reaction kinetics of dimethoxymethane (DMM) towards H atom abstraction by hydroperoxy and methyl peroxy radicals were investigated using the G4 composite method. All the studied reactions were found to be endothermic and non-spontaneous in nature. Canonical Variational Transition State Theory in combination with Small Curvature Tunneling corrections were adopted to calculate the rate coefficients in the temperature range of 400–1500 K. The Arrhenius expressions for the reaction of DMM with CH3OO and HO2 radicals were obtained as ktotal = 1.50 × 10−12 exp (−8159/T) and ktotal = 4.40 × 10−12 exp (−7251/T) cm3 molecule−1 s−1 respectively.

Oxidation kinetics of n-pentanol: A theoretical study of the reactivity of the 1‑hydroxy‑1-peroxypentyl radical

n-Pentanol has been considered as a promising alternative fuel for compression-ignition engines due to its potential to reduce greenhouse gases and pollutant emissions. Engine performance is strongly dominated by fuel oxidation chemistry, and thus a more accurate determination of the coefficients of the reactions ruling its oxidation is essential for the utilization of n-pentanol in combustion engines. The reactions involving 1‑hydroxy‑1-pentyl and molecular oxygen were found to play an important role in controlling the low temperature oxidation chemistry, but have not been investigated experimentally or theoretically; this is also the case for the reactions of the 1‑hydroxy‑1-peroxypentyl radical, which is formed by the addition of oxygen to the radical center of 1‑hydroxy‑1-pentyl. This work presents a theoretical study with high level ab initio calculations at the CCSD(T)/aug-cc-pVTZ//M06-2X/cc-pVTZ level of theory to shed light on the fate of the 1‑hydroxy‑1-peroxypentyl radical. The rate coefficients of all the possible intra-molecular hydrogen shift reactions of that radical were computed using variational transition state theory with small curvature tunneling corrections. For certain reactions, tunneling and variational effects are very pronounced, proving the need for robust methodologies to account for these effects. The hydrogen shift reaction leading to a concerted HO2 elimination and formation of n-pentanal is the dominant pathway and governs the reactivity of 1‑hydroxy‑1-peroxypentyl radical at any temperature. The reverse of this reaction was thereby investigated as well. For this prominent pathway, the effects of multistructural (multiple conformers) torsional anharmonicity of the stationary points were taken into account in order to refine the forward and reverse rate coefficients. The rate coefficients calculated at room temperature are compared to those calculated using a previously developed cost-effective multi-conformer transition state theory approach. The system-specific quantum Rice-Ramsperger-Kassel (SS-QRRK) theory was used to compute the pressure-dependent rate coefficients, which indicate significant pressure dependence at intermediate and high temperatures. Implementation of the calculated reaction rate coefficients in chemical kinetics models of n-pentanol revealed that our computed rate coefficients enable better insights into the chemistry of n-pentanol, and help to understand how n-pentanal is formed.

Can (H2O) n (n = 1–2) as effective catalysts in the CH2OO + H2S reaction under tropospheric conditions?

The addition reaction of CH2OO + H2S → HSCH2OOH without and with catalyst X (X = H2O and (H2O)2) has been investigated by CCSD(T)-F12a/VTZ-F12//B3LYP/aug-cc-pVTZ method and canonical variational transition state theory with small curvature tunneling correction. When H2O was introduced in the CH2OO + H2S reaction, it not only acts as a catalyst for producing HSCH2OOH, but also plays as a reactant to forming HOCH2OOH. The formation channel of HSCH2OOH is more important than the formation channel of HOCH2OOH with its calculated rate constant larger by 11.0–43.2 times within the temperature 280–320 K. Then, (H2O)2 catalysed CH2OO + H2S → HSCH2OOH reaction has been taken into account with its rate lower 1.9–4.2 times than the reaction of CH2OO + H2S → HSCH2OOH with water. Also, CH2OO + H2S with H2O cannot compete with the CH2OO + H2S reaction without water. This is different from CH2OO + (H2O)2 reaction, which is about 4 orders of magnitude larger than the rate constant for CH2OO + H2O reaction. Such discrepancy is possible because C(CH2OO)···O(H2O) interaction has been enhanced more obviously by H2O as compared to that of C(CH2OO)···O(H2S) interaction.

Cl-Initiated Photo-oxidation Studies of Methyl Valerate and Methyl Isovalerate under Tropospherically Relevant Conditions.

Rate coefficients for the reactions of methyl valerate (MV) and methyl isovalerate (MIV) were investigated using the relative rate (RR) technique in the temperature range of 268-363 K and at 760 Torr of N2. The rate coefficients at 298 ± 2 K were measured to be kMV+Cl = (1.69 ± 0.40) × 10-10 cm3 molecule-1 s-1 and kMIV+Cl = (1.06 ± 0.23) × 10-10 cm3 molecule-1 s-1. To comprehend the experimentally measured rate coefficients, kinetic parameters for the title reactions were computed at the CCSD(T)/cc-pVTZ//BHandHLYP/6-311+G(d,p) level of theory over the temperature range of 200-400 K using canonical variational transition (CVT) state theory in conjunction with small curvature tunneling (SCT) corrections and the interpolated single-point energy (ISPE) method. Additionally, the degradation mechanisms were proposed based on the analyzed products. Moreover, to further understand the atmospheric fate of these molecules in the troposphere, atmospheric lifetimes, radiative forcings (RFs), and global warming potentials (GWPs) were calculated.

Cl Atom-Initiated Photo-Oxidation Reactions of Vinyl Trifluoroacetate and Allyl Trifluoroacetate in Tropospheric Conditions.

Relative rate (RR) technique coupled with gas chromatography (GC) was used to investigate the kinetics of the reactions of two hydrofluoroesters (allyl trifluoroacetate (CF3C(O)OCH2CH═CH2, ATFA) and vinyl trifluoroacetate (CF3C(O)OCH═CH2, VTFA)) with Cl atoms between 268 and 363 K and at 760 Torr of N2 (or air). The temperature-dependent Arrhenius expressions for the title reactions were obtained to be k268-363KVTFA+Cl = [(7.83 ± 2.26) × 10-12 exp((974 ± 89)/T)] cm3 molecule-1 s-1 and k268-363KATFA+Cl = [(9.03 ± 1.92) × 10-12 exp((883 ± 65)/T)] cm3 molecule-1 s-1, respectively. A negative temperature dependency was observed for both the reactions. In addition to this, the kinetics of the studied reactions was evaluated computationally at the CCSD(T)/cc-pVTZ//MP2/6-311++G(d,p) level of theory in the temperature range of 200-400 K using canonical variational transition (CVT) state theory in conjunction with small curvature tunneling (SCT) corrections and interpolated single point energy (ISPE) methods. The product analysis of the reactions of VTFA and ATFA with Cl atoms in the presence of O2 was also investigated using gas chromatography-mass spectrometry (GC-MS) and gas chromatography-infrared spectroscopy (GC-IR). The plausible oxidation mechanism of the title reactions was proposed based on the product analyses. Further, to comprehend the impact of these molecules on the troposphere, atmospheric lifetimes, lifetime-corrected radiative forcing (RF), and global warming potential (GWP) were estimated and are presented in this manuscript.

Effect of water and ammonia on the HO + NH3 → NH2 + H2O reaction in troposphere: Competition between single and double hydrogen atom transfer pathways

A comprehensive investigation of the roles of H2O and NH3 on the HO + NH3 → NH2 + H2O reaction in the troposphere has been carried out by CCSD(T)-F12a/cc-pVDZ-F12//M06-2X/6–311+G(2d,2p) method, and the canonical variational transition state theory with small curvature tunneling correction. The results show that both H2O and NH3 catalyzed reactions prefer the single hydrogen atom transfers (HAT) pathways than the double HAT routes. Meanwhile, the catalytic effect of H2O is more obvious as compared with NH3 with its effective rate constant (k'(WM)) larger by 2–5 orders of magnitude. However, within the temperature range of 213–320 K, the calculated value of k'(WM) is smaller by 5–9 orders of magnitude than the corresponding rate constant of the naked reaction. This result is in agreement with H2O catalyzed OH + CH2CH2, OH + CH2O and OH + CH2NH reactions, where water-assisted reaction cannot accelerate the reaction.

Hydrolysis of HNSO2: A potential route for atmospheric production of H2SO4 and NH3

AbstractThe hydrolysis of sulfonylamine (HNSO2) results in the formation of sulfuric acid along with ammonia, and is of significant interest due to their negative impact on environment and life on Earth. The formation of H2SO4 through the reaction of HNSO2 with (H2O)2‐4 has been studied using high level electronic structure calculations. This hydrolysis reaction is a step‐wise process, in the first step a H‐atom from H2O is transferred to the N‐atom of HNSO2 which results in the formation of NH2, and in the next step, H2SO4, NH3 and water molecule(s) are formed. The results show that the energy barrier associated with the formation of intermediates and product complexes is reduced by 7 to 10 kcal/mol when the number of water molecules is increased from 2 to 4. The rate constant was calculated using canonical variational transition state theory with small curvature tunneling correction over the temperature range of 200 to 1000 K. At 298 K, the calculated rate constant for the formation of intermediate in the first step is 2.24 × 10−16, 1.03 × 10−12, and 2.10 × 10−11 cm3 mol−1 s−1, respectively, for the reaction with water dimer, trimer and tetramer. The calculated enthalpy and free energy show that the reaction corresponding to the formation of H2SO4 is highly exothermic and exoergic in nature.

Mechanism and kinetics of diuron oxidation by hydroxyl radical addition reaction

Diuron is a phenyl urea herbicide used to control weeds in agricultural lands. The degradation of diuron in the atmosphere takes place dominantly via reaction with OH radicals. In this work, the OH addition reaction of diuron has been studied by using density functional theory methods M06-2X, ωB97X-D and MPWB1K with 6-31G(d,p) basis set. The calculated thermochemical parameters show that OH addition reaction occurs favourably at C2 position of diuron. The rate constant is calculated for the favourable initial reaction pathway by using canonical variational transition state theory with small curvature tunnelling (SCT) correction over the temperature range of 200-1000K. The reaction of initially formed diuron-OH adduct intermediate with O2 leads to the formation of peroxy radical intermediate. The reaction of peroxy radical intermediate with HO2 and NOx (x = 1, 2) radicals is studied in detail. The results obtained from time-dependent density functional theory (TDDFT) calculations show that the intermediates and products formed from oxidation of diuron can be easily photolyzed in the sunlight. This study provides thermodynamical and kinetic data for the atmospheric oxidation of diuron by OH radical addition reaction and demonstrates the atmospheric chemistry of diuron and its derivatives.

Reaction mechanism and kinetics of Criegee intermediate CH2OO with CH2 = C(CH3)CHO

The comprehensive mechanism and kinetics for CH2OO with methacrolein (CH2 = C(CH3)CHO, MACR) reaction were investigated at the CCSD(T)/6-311+G(2df,2p)//M06-2X/6-311+G(2df,2p) level of theory. The results show that three kinds of mechanisms including cycloaddition, oxidation and insertion have been identified. Among them, the cycloaddition of CH2OO adding to CO bond of MACR is more favorable, which occurs via a deeply submerged barrier leading to the formation of energized secondary ozonide (SOZ). The nascent SOZ mainly further decomposes into methacrylic acid and formaldehyde or formic acid and methacrolein, in which formic acid generation follows stepwise and concerted mechanism involving a catalysis for rearrangement of CH2OO by methacrolein. The calculated rate constant for the title reaction at 300 K using canonical variational transition state theory with small curvature tunneling correction is 1.00 × 10−12 cm3 molecule−1 s−1, which is close to the experimental value of (4.4 ± 1.0) × 10−13 cm3 molecule−1 s−1. The results reveal that the reaction provides a new pathway to produce organic acids. However, the contribution of acids from this reaction is small as its relative lower rate, compared to atmospheric CH2OO loss occurring through the hydrolysis of CH2OO + (H2O)n (n = 1–2).

Mechanism and Kinetics of Diuron Oxidation Initiated by Hydroxyl Radical: Hydrogen and Chlorine Atom Abstraction Reactions.

Diuron is a herbicide that has been classified as an environmental pollutant because of its harmful effects on living beings and environment. In the present work, the OH-initiated oxidation reaction of diuron is investigated by performing electronic structure calculations based on density functional theory (DFT) methods, M06-2X, ωB97X-D, and MPWB1K using the 6-31G(d,p) basis set. The CBS-QB3 method is used to validate the results obtained from the DFT methods. All possible initial hydrogen and chlorine atom abstraction reaction pathways involved in the oxidation of diuron were studied, and the favorable reaction pathways were found by analyzing the potential energy surface and thermochemistry of the reactions. The results obtained from the present work show that hydrogen atom abstraction from methyl and amine groups of diuron are energetically favorable, which leads to the formation of diuron radical intermediate and water molecule. The rate constant is calculated for most favorable reaction pathways by using canonical variation transition state theory (CVT) with small curvature tunnelling (SCT) correction over the temperature range 298-1000 K. The atmospheric lifetime of diuron is found to be around 15 days. The subsequent reaction of most favorable diuron radical intermediate with other atmospheric reactive species, such as O2, H2O, HO2, and NOx (x = 1, 2) radicals are studied. The time-dependent DFT calculation is performed to study the photolysis of diuron and favorable diuron radical intermediates. This study provides thermochemical and kinetic data for the oxidation of diuron initiated by OH radical through H atom abstraction reaction.

Theoretical study of the formation of dinitro-pyrenes from mononitro-pyrenes initiated by OH radicals

Nitrated polycyclic aromatic hydrocarbons are more mutagenic and carcinogenic than parent PAHs. The formation mechanism of dinitro-pyrenes initiated by OH from mononitro-pyrenes was conducted by quantum chemical calculations. The products are 1,2-, 1,7-, 2,4-, 2,5- and 2,7- dinitropyrenes. Water molecules can decrease the Gibbs activation barriers of water loss step of NO2-OH-nitropyrene. By canonical variational transition state theory with small curvature tunneling correction, the calculated overall rate constants for 1-, 2- and 4-nitropyrene are 8.13 × 10−13 cm3 molecule−1 s−1, 8.90 × 10−13 cm3 molecule−1 s−1 and 1.50 × 10−12 cm3 molecule−1 s−1 at 298 K and 1 atm, respectively.

Photo-Oxidation Reaction Kinetics and Mechanistics of 4-Hydroxy-2-butanone with Cl Atoms and OH Radicals in the Gas Phase.

The temperature-dependent rate coefficients for the gas-phase reaction of 4-hydroxy-2-butanone (4H2BN) with Cl atoms and OH radicals were explored experimentally using relative rate technique and computational methods. The concentrations of the reactants as well as products were followed using gas chromatography (GC) with the flame ionization detector, GC/mass spectrometry, and GC/infrared spectroscopy as analytical techniques. Formaldehyde was obtained as the major product during the title reaction. The kinetics of 4H2BN with Cl atoms and OH radicals were measured over the temperature range of 298-363 K at 760 Torr in the N2 atmosphere using C3H8, C2H4, isopropanol, and n-propanol as reference compounds. The temperature-dependent rate coefficients for the reaction of 4H2BN with Cl atoms and OH radicals were obtained as kExpt( T) = [(1.52 ± 0.86) × 10-26] T5 exp[(2474 ± 450)/ T] cm3 molecule-1 s-1 and kexpt( T) = [(2.09 ± 0.24) × 10-12] exp[-(409 ± 15)/ T] cm3 molecule-1 s-1, respectively. Theoretical calculations were carried out at the M062X/6-31G(d,p) and M06-2X/6-31+G(d,p) level of theories, and the rate coefficients for H abstraction reactions were evaluated using the canonical variational transition state theory with the inclusion of small-curvature tunneling correction over the temperature range of 200-400 K. The rate coefficients obtained over the studied temperature range were used to fit the data, and the Arrhenius expression was obtained to be kCl(Theory) (200-400 K) = (6.10 × 10-25) T4.42 exp(2397/ T) cm3 molecule-1 s-1, kOH(Theory) (200-400 K) = (1.13 × 10-19) T2.27 exp(1505/ T) cm3 molecule-1 s-1, respectively, for the reactions of Cl atoms and OH radicals with 4H2BN. The possible reaction mechanism proposed based on the obtained products for the title reaction, thermochemistry, branching ratios, and atmospheric implications and cumulative lifetime of 4H2BN were also explored in this study.

Can a single water molecule catalyze the OH+CH2CH2 and OH+CH2O reactions?

The reaction of OH radicals with ethylene (CH2CH2) and formaldehyde (CH2O) molecules with and without water have been investigated using ab initio/DFT potential energy surfaces (PESs) at CCSD(T)/aug-cc-pVTZ//BHandHLYP//aug-cc-pVTZ levels of theory. The rate coefficients for the bimolecular reaction pathways OH + CH2X···H2O (X = CH2, O) and CH2X + H2O⋯HO were calculated using canonical variational transition state theory (CVT) with small curvature tunneling (SCT) correction. The kinetic results show that OH radical adds to C=C bond in CH2CH2, and abstract the hydrogen atoms from CH2O, similar to its isoelectronic analogous OH + CH2NH (+H2O) reaction. The catalytic effect of a single water molecule on OH + CH2X (X = CH2, O) reaction system shows that the initial water complexation step is essential in the rate coefficients calculation. The calculated rate coefficient for the OH + CH2CH2(+H2O) reaction at 300K is 6 × 10−16 cm3 molecule−1 s−1 and for OH + CH2O(+H2O) reaction at 300K is 8.1 × 10−14 cm3 molecule−1 s−1. The rate coefficient for OH + CH2CH2(+H2O) reaction is at least two orders of magnitude smaller than OH + CH2NH(+H2O) reaction (at 300K is 5.1 × 10−14 cm3 molecule−1 s−1) and rate coefficient for OH + CH2O reaction is in good agreement with OH + CH2NH reaction. In general, the rate coefficients for OH + CH2X (X = CH2, O)⋯H2O and for CH2X + H2O⋯HO reactions are ∼3–4 orders of magnitude smaller than reaction without water molecule. Our results predict that catalytic effect of single water molecule on OH + CH2CH2 and OH + CH2O reactions can make a negligible contribution to the gas phase removal of CH2CH2 and CH2O by OH radicals because the dominated water-assisted process depends parametrically on water concentration. As a result, the overall reaction rate coefficients are smaller. The present results provide a better understanding of gas phase catalytic effect of a water molecule on the most important atmospheric and combustion reaction prototypes.

Quantum Suppression of Intramolecular Deuterium Kinetic Isotope Effects in a Pericyclic Hydrogen Transfer Reaction.

It is generally accepted that hydrogen tunneling enhances both primary and secondary H/D kinetic isotope effects (KIEs) over what would be expected under the assumptions of classical barrier transition. Previous studies have exclusively shown that the effects of tunneling upon primary H/D KIEs have been much larger than those observed for secondary H/D KIEs. Here we present a series of experimental H/D KIE results associated with the Chugaev elimination of methyl xanthate derived from β-phenylethanol over the temperature range of 180 to 290 °C. Intramolecular H/D KIEs computed according to classical transition state theory (TST) are markedly overestimated relative to experimentally measured values. Experimental intermolecular H/D KIEs and direct dynamic calculations based on canonical variational transition state theory (CVT) with small-curvature tunneling correction (SCT) reveal that this result is largely the consequence of extraordinary tunneling enhancement of the secondary H/D KIE. This unexpected behavior is examined in the context of other similar hydrogen transfer reactions.

Theoretical study on the gas phase reaction of methyl chavicol with hydroxyl radical

Methyl chavicol (MC) is an oxygenated aromatic compound, suspected to be harmful to human health at high concentration. The present study focuses on the gas-phase oxidation mechanism of methyl chavicol initiated by OH radical by employing combined quantum chemical calculations and kinetic modelling. The initiation of the reaction is dominated by the abstraction of methyl chavicol hydrogen atom by OH radical and electrophilic addition of OH radical to the methyl chavicol. Energetically favourable reaction pathways and the stable products were identified by obtaining the reaction heats and potential energy surface. The rate constant for the favourable initial reactions has been calculated over the temperature range of 278–350 K using canonical variational transition state (CVT) theory with small curvature tunnelling (SCT) correction. The calculated thermodynamic and kinetic results reveal that the abstraction of hydrogen atom and addition of OH radical is more likely will occur at allyl group carbon atoms, C7, C8 and C9 of methyl chavicol to form radical intermediate, I4 and MC-OH adducts, I5 and I6. The subsequent secondary reactions for the initially formed intermediates were studied with O2, NO and HO2 radicals. The possible oxidation products identified are 4-methoxybenzaldehyde, (4-Methoxy-phenyl)-acetaldehyde and 4-methoxy toluene. The products identified in the present study are same as that identified experimentally and contribute to secondary organic aerosol loading in the atmosphere.

Atmospheric oxidation mechanism and kinetics of 2-bromo-4,6-dinitroaniline by OH radicals - a theoretical study.

2-Bromo-4,6-dinitroaniline (BNA) is identified as a domestic-dust pollutant in urban environments, with deleterious atmospheric effects. In the present work, we studied the reaction pathways and kinetics for BNA oxidation by the OH radical using quantum-chemical methods and canonical-variational transition-state theory with small-curvature tunneling correction (CVT/SCT). OH-radial-mediated BNA oxidation was studied by considering OH addition to carbon atoms (C1 to C6) of BNA and H-atom abstraction at the -NH2 group and carbon atoms (C3 and C5) of BNA by OH radicals. It is observed that an OH-addition reaction is energetically more favorable. In addition, the rate constant was calculated for the favorable initial OH-addition reactions over the temperature range of 278 to 1000 K. The subsequent reactions for the favorable BNA-OH adduct intermediate with O2, HO2 and NO radicals are studied. We have identified the following possible end products from this BNA-oxidation reaction: (i) 2-amino-3-bromo-6-hydroperoxy-5-methyl-1-nitro-cyclohexa-2,4 dienol, (ii) 2-amino-1-bromo-6-hydroperoxy-5-methyl-3-nitro-cyclohexa-2,4-dienol, (iii) 2-amino-1-bromo-6-hydroperoxy-5-methyl-3-nitro-cyclohexa-2,4-dienol, (iv) 3-amino-4-bromo-4-hydroperoxy-8-methyl-2-nitro-6,7-dioxa-bicyclo oct-2-en-8-ol, (v) 2-amino-1-bromo-6-hydroperoxy-5-methyl-3-nitro-cyclohexa-2,4-dienol, and (vi) 3-amino-2-bromo-8-methyl-4-nitro-6,7-dioxa-bicyclo oct-3-ene-2,8-diol.