Mechanisms for the Reactions of Hydroxyl Radicals with Acrolein: A Theoretical Study

Assessing the role of mineral particles in the atmospheric photooxidation of typical carbonyl compound

Mineral particles are ubiquitous in the atmosphere and exhibit an important effect on the photooxidation of volatile organic compounds (VOCs). However, the role of mineral particles in the photochemical oxidation mechanism of VOCs remains unclear. Hence, the photooxidation reactions of acrolein (ARL) with OH radical (OH) in the presence and absence of SiO₂ were investigated by theoretical approach. The gas-phase reaction without SiO₂ has two distinct pathways (H-abstraction and OH-addition pathways), and carbonyl-H-abstraction is the dominant pathway. In the presence of SiO₂, the reaction mechanism is changed, i.e., the dominant pathway from carbonyl-H-abstraction to OH-addition to carbonyl C-atom. The energy barrier of OH-addition to carbonyl C-atom deceases 21.33 kcal/mol when SiO₂ is added. Carbonyl H-atom of ARL is occupied by SiO₂ via hydrogen bond, and carbonyl C-atom is activated by SiO₂. Hence, the main product changes from H-abstraction product to OH-adduct in the presence of SiO₂. The OH-adduct exhibits a thermodynamic feasibility to yield HO₂ radical and carboxylic acid via the subsequent reactions with O₂, with implications for O₃ formation and surface acidity of mineral particles.

Theoretical Study of Radical-Molecule Reactions with Negative Activation Energies in Combustion: Hydroxyl Radical Addition to Alkenes

Many of the radical-molecule reactions are nonelementary reactions with negative activation energies, which usually proceed through two steps. They exist extensively in the atmospheric chemistry and hydrocarbon fuel combustion, so they are extensively studied both theoretically and experimentally. At the same time, various models, such as a two transition state model, a steady-state model, an equilibrium-state model, and a direct elementary dynamics model are proposed to get the kinetic parameters for the overall reaction. In this paper, a conversion temperature T _C1 is defined as the temperature at which the standard molar Gibbs free energy change of the formation of the reaction complex is equal to zero, and it is found that when T ≫ T _C1, the direct elementary dynamics model with an inclusion of the tunneling correction of the second step reaction is applicable to calculate the overall reaction rate constants for this kind of reaction system. The reaction class of hydroxyl radical addition to alkenes is chosen as the objects of this study, five reactions are chosen as the representative for the reaction class, and their single-point energies are calculated using the method of CCSD(T)/CBS, and it is shown that the highest conversion temperature for the five reactions is 139.89 K, far below the usual initial low-temperature (550 K) oxidation chemistry of hydrocarbon fuels; therefore, the steady-state approximation method is applicable. All geometry optimizations are performed at the BH&HLYP/6-311+G(d,p) level, and the result shows that the geometric parameters in the reaction centers are conserved; hence, the isodesmic reaction method is applicable to this reaction class. To validate the accuracy of this scheme, a comparison of electronic energy difference at the BH&HLYP/6-311+G(d,p) level and the corrected electronic energy difference with the electronic energy difference at the CCSD(T)/CBS level is performed for the five representative reactions, and it is shown that the maximum absolute deviation of electronic energy difference can be reduced from 2.54 kcal·mol^-1 before correction to 0.58 kcal·mol^-1 after correction, indicating that the isodesmic reaction method is applicable for the accurate calculation of the kinetic parameters for large-size molecular systems with a negative activation energy reaction. The overall rate constants for 44 reactions of the reaction class of hydroxyl radical addition to alkenes are calculated using the transition-state theory in combination with the isodesmic correction scheme, and high-pressure limit rate rules for the reaction class are developed. In addition, the thermodynamic parameter is calculated and the results indicate that our dynamics model is applicable for our studied reaction class. A chemical kinetic modeling and sensitivity analysis using the calculated kinetic data is performed for the combustion of ethene, and the results indicate the studied reaction is important for the low-to-medium temperature combustion modeling of ethene.

Atmospherically relevant acrolein–water complexes: spectroscopic evidence of aldehyde hydration and oxygen atom exchange

Rotational spectroscopy and isotopic studies evidence oxygen exchange in water complexes of atmospherically important acrolein.

OH-initiated tropospheric photooxidation of allyl acetate: a theoretical study

The mechanisms of OH-initiated oxidation of allyl acetate in the presence of O2/NO have been investigated by performing density functional theory calculations. Two patterns (OH-addition and H-abstraction) of the initial reaction and the subsequent reactions of the primarily produced intermediates (IM1, IM2, and IM4) have been proposed. The OH-addition reactions are more favorable than the H-abstraction reactions, but H-abstraction from the CH2 group cannot be ignored. The major degradation products have been identified. The rate coefficients and the branching ratios of the primary reactions are obtained over the temperature of 200–500 K and the pressure range of 0.001–1000 atm. The total rate coefficient is 3.17 × 10−11 cm3 molecule−1 s−1 at 298 K and 1 atm. With respect to the typical concentration of OH radical (2.0 × 106 molecule cm−3), the atmospheric lifetime of AAC is estimated to be 4.40 h.

Why don't plants have diabetes? Systems for scavenging reactive carbonyls in photosynthetic organisms

In the present paper, we review the toxicity of sugar- and lipid-derived RCs (reactive carbonyls) and the RC-scavenging systems observed in photosynthetic organisms. Similar to heterotrophs, photosynthetic organisms are exposed to the danger of RCs produced in sugar metabolism during both respiration and photosynthesis. RCs such as methylglyoxal and acrolein have toxic effects on the photosynthetic activity of higher plants and cyanobacteria. These toxic effects are assumed to occur uniquely in photosynthetic organisms, suggesting that RC-scavenging systems are essential for their survival. The aldo–keto reductase and the glyoxalase systems mainly scavenge sugar-derived RCs in higher plants and cyanobacteria. 2-Alkenal reductase and alkenal/alkenone reductase catalyse the reduction of lipid-derived RCs in higher plants. In cyanobacteria, medium-chain dehydrogenases/reductases are the main scavengers of lipid-derived RCs.

Mechanism and kinetic study of 3-fluoropropene with hydroxyl radical reaction

Potential energy surface for the reaction of hydroxyl radical (OH) with 3-fluoropropene (CH₂CHCH₂F) has been studied to evaluate the reaction mechanisms, possible products and rate constants. It has been shown that the CH₂CHCH₂F with OH reaction takes place via a barrierless addition/elimination and hydrogen abstraction mechanism. It is revealed for the first time that the initial step for the barrierless additional process involves a pre-reactive loosely bound complex (CR1) that is 1.60 kcal/mol below the energy of the reactants. Subsequently, the reaction bifurcates into two different pathways to form IM1 (CH₂CHOHCH₂F) and IM2 (CH₂OHCHCH₂F), which can decompose or isomerize to various products via complicated mechanisms. Variational transition state model and multichannel RRKM theory are employed to calculate the temperature-, pressure-dependent rate constants and branching ratios. At atmospheric pressure with He as bath gas, IM1 formed by collisional stabilization is dominated at T≤600 K; whereas the direct hydrogen abstraction leading to CH₂CHCHF and H₂O are the major products at temperatures between 600 and 3000 K, with estimated contribution of 72.9% at 1000 K. Furthermore, the predicted rate constants are in good agreement with the available experimental values.

Acrolein, an α,β-unsaturated carbonyl, inhibits both growth and PSII activity in the cyanobacterium Synechocystis sp. PCC 6803

In this study, we sought to determine whether and how an α,β-unsaturated carbonyl, acrolein, can inhibit the growth of the cyanobacterium Synechocystis sp. PCC6803 (S. 6803). Treatment of S. 6803 with 200 µM acrolein for 3 d significantly and irreversibly inhibited its growth. To elucidate the inhibitory mechanism, we examined the effects of acrolein on photosynthesis. In contrast to dark conditions, the addition of acrolein to S. 6803 under conditions of illumination lowered the CO₂-dependent O₂ evolution rate (photosynthetic activity). Furthermore, treatment with acrolein lowered the activity reducing dimethyl benzoquinone in photosystem II (PSII). Acrolein also suppressed the reduction rate for the oxidized form of the reaction center chlorophyll of photosystem I (PSI), P700. These results indicate that acrolein inhibited PSII activity in thylakoid membranes. The addition of 200 µM acrolein to the illuminated S. 6803 cells gradually increased the steady-state level (Fs) of Chl fluorescence and decreased the quantum yield of PSII. These results suggested that acrolein damaged the acceptor side of PSII. On the other hand, acrolein did not inhibit respiration. From the above results, we gained insight into the metabolism of acrolein and its physiological effects in S. 6803.

Determination of acrolein and other carbonyls in cigarette smoke using coupled silica cartridges impregnated with hydroquinone and 2,4-dinitrophenylhydrazine

A new method for the determination of acrolein and other carbonyls in cigarette smoke using a dual cartridge system has been developed. Each cartridge consists of reagent-impregnated silica particles. The first contains hydroquinone (HQ) for the inhibition of acrolein polymerization, while the second contains 2,4-dinitrophenylhydrazine (DNPH) for the derivatization of carbonyls. Smoke samples were firstly drawn through the cartridge containing HQ-impregnated silica (HQ-silica) and then through the DNPH-impregnated silica (DNPH-silica). Acrolein in the sample was completely trapped in the first HQ-silica cartridge. Some other airborne carbonyls were also trapped by the HQ-silica, and those that pass through were trapped in the second DNPH-silica cartridge. Extraction was performed in the reverse direction to air sampling. When solvent was eluted through the dual-cartridges, excess DNPH was washed into the HQ bed where it reacted with acrolein and other trapped carbonyls to form the corresponding hydrazone derivatives. All of the hydrazones derived from airborne carbonyls were completely separated and measured using high-performance liquid chromatography. This HQ-DNPH-method can be applied for the determination of acrolein and other alpha,beta-unsaturated aldehydes, such as crotonaldehyde, in cigarette smoke.

Quantum chemistry and TST study of the mechanisms and branching ratios for the reactions of OH with unsaturated aldehydes

A theoretical study is presented on the mechanism of OH reactions with three unsaturated aldehydes, relevant to atmospheric chemistry. Using acrolein as test molecule, several methods were tested in conjunction with the 6-311 ++ G(d,p) basis set. Based on the results from this study, the MPWB1K and M05-2X functionals were selected for the further study of acrolein, methacrolein and crotonaldehyde. All possible reaction channels have been modeled. Calculated overall rate coefficients at M05-2X/6-311 ++ G(d,p) are in excellent agreement with experimental data, supporting the proposed mechanisms. The previously proposed global mechanisms were confirmed, and specific mechanisms were identified. The causes of the mechanism for crotonaldehyde being different from the one of acrolein and methacrolein were clarified. The agreement between experiment and calculations validates the use of the chosen DFT methods for kinetic calculations, especially for large systems and cases in which spin contamination is an important issue.

Theoretical investigation of the oxidation pathways of HO•-initiated reactions of acrolein, methacrolein, and trans-crotonaldehyde

Ab initio molecular-orbital calculations have been performed to investigate the reaction mechanisms of the HO•-initiated reactions of the α,β-unsaturated aldehydes: acrolein (ACR), methacrolein (MACR), and trans-crotonaldehyde (CROT). All geometries were fully optimized at the MP2(Full)/6–31G(d,p) level. The correlation energy corrections were introduced by carrying out single-point calculations using both spin-projected second-order Møller–Plesset perturbation theory (PMP2) and coupled-cluster theory (CCSD(T)) using basis sets of different sizes. All reaction pathways studied proceed via a barrierless formation of a loosely bound pre-reactive complex in the entrance channel. The transition-state structures of the HO• additions to the terminal (β) and to the central (α) carbon atoms of the C=C double bond are found to be reactant-like structures. The lowest-energy barrier pathways are found to be the aldehydic H-atom abstraction. The β-addition pathways are found to be energetically more favored than the α-addition pathways. The HO• addition and aldehydic H-atom abstraction pathways are found to be highly exoergic, with the H-atom abstraction pathway being more exoergic than the addition pathways. Although the methyl substitution at the C=C double bond of methacrolein and crotonaldehyde lowers the energies of the transition-state structures of both α- and β-addition pathways, it destabilizes the energies of the transition-state structures of the corresponding aldehydic H-atom abstraction pathways, compared with that of acrolein.