Mechanism Of OH-initiated Oxidation Research Articles

OH initiated oxidation mechanism of monoterpene (linalool) – A first comprehensive theoretical study

This study concerns the oxidations of linalool, a monoterpene alcohol, via both addition of OH radical to unsaturated carbon atoms and H atom abstraction reactions. The molecular entities were optimized at M06-2X/6-311++G(d,p) level of theory. To validate the results, the structures were re-optimized at different functional of DFT and ab initio methods like MPW1K and MP2, CCSD(T), G3B3 with same level of theory respectively. On comparing both reaction channels, the OH addition to C1 carbon atom is the favorable reaction pathway, and competes with O2 oxidation from early literature survey. Expectedly, all level of theories came up with comparable results and predicted that all proposed reactions were exothermic and exergonic. The rate constant calculations show that the addition and abstraction channels are positive and negative temperature dependent respectively. The predicted Wiberg bond indices shows that all the bonds are concerted but not fully synchronized thus; abstraction and addition reactions of LIN are feasible. Further, this study also calculates the life time of LIN with respect to addition reaction is 13.29 min whereas abstraction reaction found a higher life of 23.70 h, which could compete with real time calculations. Overall, OH addition reaction dominates over H abstraction reactions of LIN.

OH-initiated oxidation mechanism and kinetics of organic sunscreen benzophenone-3: A theoretical study

AbstractBenzophenone-3 (BP-3), as an important organic UV filter, is widely used in the sunscreen, cosmetic, and personal care products. The chemical reaction mechanism and kinetics of BP-3 degradation initiated by hydroxyl (OH) radical was investigated in the atmosphere based on the density functional theory (DFT). The results showed that the OH radical is more easily added to the C3 position of the aromatic ring (pathway 3), while the H atom abstraction from the OH group on the aromatic ring (pathway 23) is an energetically favorable reaction pathway. At ambient temperature, 298 K, the overall rate constant for the primary reaction is about 1.50 × 10

Theoretical study on the OH-initiated oxidation mechanism of polyfluorinated dibenzo-p-dioxins under the atmospheric conditions.

Density functional theory (DFT) calculations at the B3LYP/6-311++G(2df,p) level of theory have been carried out to investigate the atmospheric oxidation mechanisms of some polyfluorinated dibenzo-p-dioxins (PFDDs), initiated by OH radical. The computed results show that all OH addition reactions of PFDDs are thermodynamically spontaneous processes and the branch ratio of the PFDD-OH adducts is determined by the magnitude of the Gibbs free energies of activation (Δ(r)G(≠)) and hence rate constants (k) for addition reactions. The OH reactions with all studied PFDDs are dominated by Cγ-addition and the total rate constants for OH addition decrease with increasing the number of fluorine atom substituting at α positions. Under the atmospheric conditions, the subsequent O2 addition reactions of PFDD-OH adducts occur hardly thermodynamically and are slow kinetically. For PFDD-α(β)-OH adducts without F atom at same positions the main reaction pathway is H abstraction by O2, while PFDD-γ-OH adducts will undergo fused-ring C-O bond cleavage, affording the substituted phenoxy radicals.

OH Radical Initiated Oxidation of 1,3-Butadiene: Isomeric Selective Study of the Dominant Addition Channel

We report the first isomeric selective kinetic study of the dominant isomeric pathway in the OH initiated oxidation of 1,3-butadiene in the presence of O(2) and NO using the laser photolysis-laser induced fluorescence (LP-LIF) technique. The photodissociation of the precursor 2-iodo-but-3-en-1-ol results exclusively in the dominant OH-butadiene addition product, permitting important insight into the OH initiated oxidation mechanism. On the basis of analysis of the time dependent OH/OD signals, we have determined a rate constant for O2 addition to the hydroxyalkyl radical of 7.0(-3.0)(+7.0) x 10(-13) cm(3) s(-1), and we find a value of 1.5(-0.6)(+1.0) x 10(-11) cm(3) s(-1) for the overall reaction rate constant of the hydroxy peroxy radical with NO. We also report the first clear experimental evidence of the (E) form of the delta-hydroxyalkoxy channel through isotopic labeling experiments and provide an upper bound of 13 +/- 5% to its branching ratio. This species provides a mechanistic pathway for the formation of 4-hydroxy-2-butenal, which has been identified as a first generation end product. This isomeric selective kinetic study, together with a previous study on the minor channel of the 1,3-butadiene oxidation, yields a comprehensive picture of butadiene oxidation under high NOx conditions relevant to most regions in the continental US.

Mechanism of the OH-Initiated Oxidation of Glycolaldehyde over the Temperature Range 233−296 K

The mechanism of the gas-phase OH-initiated oxidation of glycolaldehyde (HOCH(2)CHO) was studied in the 233-296 K temperature range using a turbulent flow reactor coupled with a chemical ionization mass spectrometer. In the presence of O2, formaldehyde, CO2, formic acid, and glyoxal were observed at room temperature with the yields of 80, 34, 18, and 14%, respectively. Decrease of temperature to 233 K led to significant changes in the yields of the stable products: those of formaldehyde and glyoxal decreased to 50 and 4%, respectively, whereas that of formic acid increased to 52%. It was also found that the OH + glycolaldehyde + O2 reaction proceeds with considerable reformation of OH radicals (by 25% at 296 K). The observed product yields are explained by a mechanism including formation of short-lived intermediate adducts of the primary radicals with O2. The implication of the obtained results for the HOx budget in the upper troposphere is discussed.

Kinetics and Mechanistic Studies of the Atmospheric Oxidation of Alkynes

Kinetics studies of the OH-initiated oxidation of 2-butyne, propyne, and acetylene were conducted at 100 Torr and 298 K using turbulent flow chemical ionization mass spectrometry. The major oxidation products were identified, and with the aid of supporting electronic structure thermodynamics calculations, a general OH-initiated oxidation mechanism for the alkynes is proposed. The major product branching ratio and the product-forming rate constants for the 2-butyne-OH adduct + O(2) reaction were experimentally determined as well. The atmospheric implications of the chemical oxidation mechanism and kinetics results are discussed.

Laboratory and Theoretical Study of the Oxy Radicals in the OH- and Cl-Initiated Oxidation of Ethene

The products of the OH-initiated oxidation mechanism of ethene have been studied as a function of temperature (between 250 and 325 K) in an environmental chamber, using Fourier transform infrared spectroscopy for end product analysis. The oxidation proceeds via formation of a peroxy radical, HOCH2CH2O2. Reaction of this peroxy radical with NO is exothermic and produces chemically activated HOCH2CH2O radicals, of which about 25% decompose to CH2OH and CH2O on a time scale that is rapid compared to collisions, independent of temperature. The remainder of the HOCH2CH2O radicals are thermalized and undergo competition between decomposition, HOCH2CH2O → CH2OH + CH2O (6), and reaction with O2, HOCH2CH2O + O2 → HOCH2CHO + HO2 (7). The rate constant ratio, k6/k7, for the thermalized radicals was found to be (2.0 ± 0.2) × 1025 exp[−(4200 ± 600)/T] molecule cm-3 over the temperature range 250−325 K. With the assumption of an activation energy of 1−2 kcal mol-1 for reaction 7, the barrier to decomposition of the HOCH2CH2O radical is found to be 10−11 kcal mol-1. A study of the Cl-atom-initiated oxidation of ethene was also carried out; the main product observed under conditions relevant to the atmosphere was chloroacetaldehyde, ClCH2CHO. Theoretical studies of the thermal and “prompt” decomposition of the oxy radicals were based on a recent ab initio characterization that highlighted the role of intramolecular H bonding in HOCH2CH2O. Thermal decomposition is described by transition state and the Troe theories. To quantify the prompt decomposition of chemically activated nascent oxy radicals, the energy partitioning in the initially formed radicals was described by separate statistical ensemble theory, and the fraction of activated radicals dissociating before collisional stabilization was obtained by master equation analysis using RRKM theory. The barrier to HOCH2CH2O decomposition is inferred independently as being 10−11 kcal mol-1, by matching both of the theoretical HOCH2CH2O decomposition rates at 298 K with the experimental results. The data are discussed in terms of the atmospheric fate of ethene.

Photooxidation of CS2 in the near‐ultraviolet and its atmospheric implications

Sequential two photon photolysis of CS2 is discussed. Experiments in our lab and others have shown that photolysis of CS2 and O2 mixtures with light of energies below the SC‐S bond dissociation energy produces OCS and SO2. We explain these results by the two photon sequential absorption and subsequent dissociation of CS2 to CS+S. The implications of this mechanism for the oxidation of CS2 in the atmosphere are discussed. Comparison to the currently accepted OH‐initiated oxidation mechanism suggests that the sequential two‐photon mechanism is too slow to be important in the atmosphere.