Competing Mechanistic Channels in the Oxidation of Aldehydes by Ozone

Ozonation of Gabapentin in Water─Investigating Reaction Kinetics and Transformation Mechanisms of a Primary Amine Using Isotopically Labeled Ozone

Aliphatic amines are abundant micropollutants in wastewater treatment plant effluents. In order to mitigate such micropollutants, ozonation is one of the most commonly employed advanced treatment processes. Current research regarding ozone efficiency is heavily focusing on reaction mechanisms of different contaminant groups, including structures with amine moieties as reactive sites. This study analyzes pH-dependent reaction kinetics and pathways of gabapentin (GBP), an aliphatic primary amine with an additional carboxylic acid group. The transformation pathway was elucidated applying a novel approach using isotopically labeled ozone (18O) and quantum chemistry calculations. While the direct reaction of GBP with ozone is highly pH-dependent and slow at pH 7 (13.7 M-1 s-1), the rate constant of the deprotonated species (1.76 × 105 M-1 s-1) is comparable to those of other amine compounds. Pathway analysis based on LC-MS/MS measurements revealed that ozonation of GBP leads to the formation of a carboxylic acid group and simultaneous nitrate formation, which was also observed in the case of the aliphatic amino acid glycine. Nitrate was formed with a yield of approximately 100%. Experiments with 18O-labeled ozone demonstrated that the intermediate aldehyde does most likely not include any oxygen originating from ozone. Furthermore, quantum chemistry calculations did not provide an explanation for the C-N scission during GBP ozonation without ozone involvement, although this reaction was slightly more favorable than for respective glycine and ethylamine reactions. Overall, this study contributes to a deeper understanding of reaction mechanisms of aliphatic primary amines during wastewater ozonation.

Theoretical study on the formation of Criegee intermediates from ozonolysis of pentenal: An example of trans-2-pentenal

In this study, we investigated the reaction mechanism and kinetics of ozone with trans-2-pentenal using density functional theory (DFT) and conventional transition state theory (CTST). At 298 K and 1 atm, the gas-phase reaction mechanisms and kinetic parameters were calculated at the level of CCSD(T)/6-311+G(d,p)//M06-2X/6-311+G(d,p). Both CC and CO bond cycloaddition as well as hydrogen abstraction were found. The calculations indicated that the main reaction path is 1,3-dipole cycloaddition reactions of ozone with CC bond with the relatively lower syn-energy-barrier of 3.35 kcal mol^-1 to form primary ozonide which decomposed to produce a carbonyl oxide called a Criegee intermediate (CI) and an aldehyde. The subsequent reactions of CIs were analysed in detail. It is found that the reaction pathways of the novelty CIs containing an aldehyde group are extremely similar with general CIs when they react with NO, NO₂, SO₂, H₂O, CH₂O and O₂. The condensed Fukui function were calculated to identify the active site of the chosen molecules. At 298 K and 1 atm, the reaction rate coefficient was 9.13 × 10^-18 cm³ molecule^-1 s^-1 with atmospheric lifetime of 1.3 days. The calculated rate constant is in general agreement with the available experimental data. The branching ratios indicated that syn-addition pathways are prior to anti-addition. The atmospheric ratios for CIs formation and the bimolecular reaction rate constants for the Criegee intermediates with the variety of partners were calculated. Our theoretical results are of importance in atmospheric chemistry of unsaturated aldehyde oxidation by ozone.

Surfactant-Assisted Ozonolysis of Alkenes in Water: Mitigation of Frothing Using Coolade as a Low-Foaming Surfactant

Aqueous-phase ozonolysis in the atmosphere is an important process during cloud and fog formation. Water in the atmosphere acts as both a reaction medium and a reductant during the ozonolysis. Inspired by the atmospheric aqueous-phase ozonolysis, we herein report the ozonolysis of alkenes in water assisted by surfactants. Several types of surfactants, including anionic, cationic, and nonionic surfactants, were investigated. Although most surfactants enhanced the solubility of alkenes in water, they also generated excessive foaming during the ozone bubbling, which led to the loss of products. Mitigation of the frothing was accomplished by using Coolade as a nonionic and low-foaming surfactant. Coolade-assisted ozonolysis of alkenes in water provided the desired carbonyl products in good yields and comparable to those achieved in organic solvents. During the ozonolysis reaction, water molecules trapped within the polyethylene glycol region of Coolade were proposed to intercept the Criegee intermediate to provide a hydroxy hydroperoxide intermediate. Decomposition of the hydroxy hydroperoxide led to formation of the carbonyl product without the need for a reductant typically required for the conventional ozonolysis using organic solvents. This study presents Coolade as an effective surfactant to improve the solubility of alkenes while mitigating frothing during the ozonolysis in water.

Comparison on Reduction of VOCs Emissions from Radiata Pine (Pinus Radiata D. Don) between Sodium Bicarbonate and Ozone Treatments

Volatile organic compounds (VOCs) in wood furniture are an important factor that affects indoor air quality. In this study, radiata pine (Pinus radiata D. Don) was treated with sodium bicarbonate and ozone aqueous solution to reduce the VOC contents without sacrificing mechanical properties. The VOCs of radiata pine were identified by gas chromatography-mass spectrometry (GC-MS), and the functional group changes of wood samples were characterized by Fourier-transform infrared spectroscopy (FTIR). The results showed that the main VOCs of radiata pine include alkenes, aldehydes, and esters. The sodium bicarbonate and ozone treatments almost eliminated the VOC contents of radiata pine. The two treatments mentioned above had little effect on compressive strength and surface color of radiata pine.

Aldehydes gas ozonation monitoring: Interest of SIFT/MS versus GC/FID

Two analytical techniques - online Gas Chromatography coupled with Flame Ionization Detector (often used method for VOCs monitoring) versus Selected Ion Flow Tube coupled with Mass Spectrometry (a more recent technique based on direct mass spectrometry) - were compared in association to an ozone-based gas treatment. Selecting aldehydes as the representative VOCs, their concentrations were monitored during ozonation experiments by both techniques in parallel. Contradictory results were obtained in the presence of ozone. Aldehydes were up to 90% removed due to a reaction with ozone according to GC/FID analysis, whereas with SIFT/MS, aldehydes concentration remained at the same level during the experiments regardless of the ozone presence. In addition, it was demonstrated that the apparent aldehydes removal was affected by GC injector temperature, varying from 90% (when it was at 250 °C) to 60% (at 100 °C). Meanwhile, even when the ozonation reactor was heated to 100 °C, no aldehydes conversion was evidenced by SIFT/MS, suggesting that the GC injector temperature was not the only interference-causing parameter. The ozone-aldehyde reaction is probably catalyzed by some material of GC injector and/or column. An ozone-GC interference was therefore confirmed, making unsuitable the use of GC/FID with silicone stationary phase to monitor aldehydes in presence of high concentrations of ozone (at least 50 ppmv). On the other hand, SIFT/MS was validated as a reliable technique, which can be employed in order to measure VOCs concentrations in ozonation processes.

Barriometry – an enhanced database of accurate barrier heights for gas-phase reactions

The kinetics of many reactions are critically dependent upon the barrier heights for which accurate determination can be difficult. More than 100 accurate barriers are obtained with the high-level W3X-L composite procedure.

Oxidative Rearrangement Coupling Reaction for the Functionalization of Tetrahydro-β-carbolines with Aromatic Amines

The observation of an unexpected oxidative rearrangement coupling reaction led to the development of a novel method for the efficient functionalization of tetrahydro-β-carbolines (THβCs). The treatment of THβCs with photogenerated singlet oxygen (¹ O₂ ) afforded unstable dioxetanes, which underwent further transformation to form new bonds in the presence of trifluoroacetic acid. This operationally simple protocol exhibits broad functional-group tolerance and is suitable for the late-stage functionalization of complex druglike molecules.

Heterogeneous Chemiluminescence from Gas-Solid Phase Interactions of Ozone with Alcohols, Phenols, and Saccharides

Gas-solid phase reactions between ozone (O₃) and three representative solids (alcohols, phenols, and saccharides) were investigated through a heterogeneous chemiluminescence (CL) strategy. When interactions between these two species occurred at the surface of the solid powder, an obvious CL effect was obtained. This performance could be attributed to the evolution of a ROOOH intermediate, which subsequently released emissive ¹O₂ species. This is the first report analyzing the gas-solid phase CL performance of O₃ with alcohols, phenols, and saccharides. It is believed that this strategy can be extended to applications in other gas-solid phase CL analyses utilizing the O₃ system. This has also created a novel area of gas-solid CL performance; thus, relevant processes and mechanisms can be deduced and identified.

An optimized two-step derivatization method for analyzing diethylene glycol ozonation products using gas chromatography and mass spectrometry

The ozonation of hydroxyl compounds (e.g., sugars and alcohols) gives a broad range of products such as alcohols, aldehydes, ketones, and carboxylic acids. This study developed and optimized a two-step derivatization procedure for analyzing polar products of aldehydes and carboxylic acids from the ozonation of diethylene glycol (DEG) in a non-aqueous environment using gas chromatography-mass spectrometry. Experiments based on Central Composite Design with response surface methodology were carried out to evaluate the effects of derivatization variables and their interactions on the analysis. The most desirable derivatization conditions were reported, i.e., oximation was performed at room temperature overnight with the o-(2,3,4,5,6-pentafluorobenzyl) hydroxyl amine to analyte molar ratio of 6, silylation reaction temperature of 70°C, reaction duration of 70min, and N,O-bis(trimethylsilyl)-trifluoroacetamide volume of 12.5μL. The applicability of this optimized procedure was verified by analyzing DEG ozonation products in an ultrafine condensation particle counter simulation system.

Theoretical study on the atmospheric transformation mechanism of pirimiphos-methyl initiated by O3

Pirimiphos-methyl (PMM) is a widely used organophosphorus pesticide that can be released into the atmosphere in gas and condensed phases. It possesses a PS bond and an N,N-dialkyl group adjacent to pyrimidine, which are common functional groups for pesticides. Currently, the reaction mechanisms of O3 with these functional groups are poorly understood. In this study, the mechanisms and possible degradation products for O3-initiated atmospheric oxidation of PMM were investigated using the Density Functional Theory (DFT) method. The results show that H abstraction from the alpha carbon of the N,N-diethyl group and its subsequent reactions (hydroxylation, N-dealkylation, and carbonylation reactions), as well as the transformation of the PS bond to the PO oxone form, are the most favorable reaction pathways for PMM and O3. The Gibbs free energy (ΔG) indicates that the subsequent reactions tend to take place more spontaneously once the initial reaction occurs. In addition, theoretical calculations indicate that water can serve as an effective catalyst in the N-dealkylation reaction process. Water-assisted reactions lead to the activation energy decreasing by 20.2 kcal mol(-1) compared with direct reactions, and thus may represent a dominant reaction pathway for the N-dealkylation process in the atmosphere. These theoretical results provide new insights into O3-initiated degradation of PMM and its analogues.

A novel optical ozone sensor based on purely organic phosphor

An optical ozone sensor was developed based on the finding that a purely organic phosphor linearly loses its phosphorescence emission intensity in the presence of varying concentration of ozone gas and ozonated water. Compared to conventional conductance-based inorganic sensors, our novel sensory film has many advantages such as easy fabrication, low-cost, and portability. NMR data confirmed that phosphorescence drop is attributed to oxidation of the core triplet generating aldehyde group of the phosphor. We observed that linear correlation between phosphorescence and ozone concentration and it can detect ozone concentrations of 0.1 ppm that is the threshold concentration harmful to human tissue and respiratory organs. Like a litmus paper, this ozone sensor can be fabricated as a free-standing and disposable film.

Sequential catalytic role of bifunctional bicyclic guanidine in asymmetric phospha-Michael reaction

The catalytic mechanism and origin of enantioselectivity of bicyclic guanidine-catalyzed phospha-Michael reaction between diphenyl phosphine oxide and β-nitrostyrene were investigated by DFT calculations at M06-2X/cc-pVTZ//M06-2X/cc-pVDZ level in conjunction with the implicit SMD solvation method. The catalyst is found to be involved in all 3 steps of the proposed catalytic cycle, namely (1) tautomerization of phosphine oxide, (2) C-P bond formation and (3) concerted hydrogen transfer. The bifunctional role of the guanidine catalyst is clearly demonstrated in all 3 key steps. Due to the geometry of the bicyclic guanidine catalyst, the preferred orientation of the reactants in the transition state of enantioselective C-P bond forming step favours the R enantiomer, in excellent accord with the observed enantioselectivity. Analysis of various transition states suggests that the asymmetric C-P bond formation is controlled by the hydrogen bonding interaction and steric effect between the catalyst and substrate. Various weaker C-H···X (X = N, O and π) interactions also play a role in stabilizing the key transition states.

Mechanistic and kinetic study of CH2O+O3 reaction

Both singlet and triplet potential energy surfaces for the reaction of ground-state formaldehyde (CH(2)O) and ozone (O(3)) are theoretically investigated at the BMC-CCSD//BHandHLYP/6-311+G(d,p) level. Various possible isomerization and dissociation pathways are probed. Hydrogen abstraction, oxygen abstraction, and C-addition/elimination are found on both the singlet and the triplet surfaces. The major products for the total reaction are HCO and HOOO, which are generated via hydrogen abstraction. The transition state theory (TST) and multichannel RRKM calculations have been carried out for the total and individual rate constants for determinant channels over a wide range of temperatures and pressures.