Thermochemistry and reaction paths in the oxidation reaction of benzoyl radical: C6H5C•(═O)

Alkyl substituted aromatics are present in fuels and in the environment because they are major intermediates in the oxidation or combustion of gasoline, jet, and other engine fuels. The major reaction pathways for oxidation of this class of molecules is through loss of a benzyl hydrogen atom on the alkyl group via abstraction reactions. One of the major intermediates in the combustion and atmospheric oxidation of the benzyl radicals is benzaldehyde, which rapidly loses the weakly bound aldehydic hydrogen to form a resonance stabilized benzoyl radical (C6H5C(?)═O). A detailed study of the thermochemistry of intermediates and the oxidation reaction paths of the benzoyl radical with dioxygen is presented in this study. Structures and enthalpies of formation for important stable species, intermediate radicals, and transition state structures resulting from the benzoyl radical +O2 association reaction are reported along with reaction paths and barriers. Enthalpies, ΔfH298(0), are calculated using ab initio (G3MP2B3) and density functional (DFT at B3LYP/6-311G(d,p)) calculations, group additivity (GA), and literature data. Bond energies on the benzoyl and benzoyl-peroxy systems are also reported and compared to hydrocarbon systems. The reaction of benzoyl with O2 has a number of low energy reaction channels that are not currently considered in either atmospheric chemistry or combustion models. The reaction paths include exothermic, chain branching reactions to a number of unsaturated oxygenated hydrocarbon intermediates along with formation of CO2. The initial reaction of the C6H5C(?)═O radical with O2 forms a chemically activated benzoyl peroxy radical with 37 kcal mol(-1) internal energy; this is significantly more energy than the 21 kcal mol(-1) involved in the benzyl or allyl + O2 systems. This deeper well results in a number of chemical activation reaction paths, leading to highly exothermic reactions to phenoxy radical + CO2 products.     

Theoretical study of mechanism and kinetics for the reaction of hydroxyl radical with 2′-deoxycytidine

The reaction mechanism and kinetics for the abstraction of hydrogen and addition of hydroxyl radical (OH) to 2′-deoxycytidine have been studied using density functional theory at MX06-2X/6-311+G(d,p) level in aqueous solution. The optimized geometries, energies, and thermodynamic properties of all stationary points along the hydrogen abstraction reaction and the addition reaction pathways are calculated. The single-point energy calculations of the main pathways at CCSD(T)/6-31+G(d,p)//MX06-2X/6-311+G(d,p) level are performed. The rate constants and the branching ratios of different channels are evaluated using the canonical variational transition (CVT) state theory with small-curvature tunneling (SCT) correction in aqueous solution to simulate the biological system. The branching ratios of hydrogen abstraction from the C1′ site and the C5′ site and OH radical addition to the C5 site and the C6 site are 57.27% and 12.26% and 23.85% and 5.69%, respectively. The overall calculated rate constant is 4.47?×?10⁹ dm³ mol^?1 s^?1 at 298 K which is in good agreement with experiments. The study could help better understand reactive oxygen species causing DNA oxidative damage.     

Thermochemistry and kinetics of the 2-butanone-4-yl CH3C(=O)CH2CH2• + O2 reaction system

Thermochemistry and kinetic pathways on the 2-butanone-4-yl (CH₃C(=O)CH₂CH₂•) + O₂ reaction system are determined. Standard enthalpies, entropies, and heat capacities are evaluated using the G3MP2B3, G3, G3MP3, CBS-QB3 ab initio methods, and the B3LYP/6-311g(d,p) density functional calculation method. The CH₃C(=O)CH₂CH₂• radical + O₂ association reaction forms a chemically activated peroxy radical with 35 kcal mol⁻¹ excess of energy. The chemically activated adduct can undergo RO−O bond dissociation, rearrangement via intramolecular hydrogen transfer reactions to form hydroperoxide-alkyl radicals, or eliminate HO₂ and OH. The hydroperoxide-alkyl radical intermediates can undergo further reactions forming ketones, cyclic ethers, OH radicals, ketene, formaldehyde, or oxiranes. A relatively new path showing a low barrier and resulting in reactive product sets involves peroxy radical attack on a carbonyl carbon atom in a cyclic transition state structure. It is shown to be important in ketones when the cyclic transition state has five or more central atoms.     

Theoretical studies of the radical reaction H2CSiH2 + H

Ab initio molecular orbital calculations on the transition states and barrier heights for the addition of atomic hydrogen to silaethylene are carried out. The activation energy for the addition to the silicon site is lower than that to the carbon site, while the exothermicity is smaller.     

Photoinduced electron-transfer reaction of α-bromomethyl-substituted benzocyclic β-keto esters with amines: selective reaction pathways depending on the nature of the amine radical cations

Photoinduced electron-transfer reaction of α-bromomethyl-substituted benzocyclic β-keto esters with tertiary amines was investigated. Debrominated β-keto esters and ring-expanded γ-keto esters were obtained as major products. On the basis of mechanistic experiments it was concluded that these products are formed via a reaction sequence of selective carbon–bromine bond cleavage and subsequent competitive hydrogen abstraction and Dowd–Beckwith ring-expansion of the resulting primary alkyl radicals. The characteristic product distribution observed for the type of amine used is rationalized on the basis of selective reaction pathways of generated radical intermediates that depend on the nature of the amine radical cations.     

An alternate method of treating competition kinetics: reaction of OH radical with IrBr3−6

Carbonate competition is often used to indirectly measure rates of OH reactions by pulse radiolysis. The product CO⁻₃ ion has an optical absorption maximum near 600 nm with a conveniently large extinction coefficient. The standard data treatment, which involves a reciprocal concentration plot, works well provided the reaction is complete at the time of measurement, and the pulse size is reproducible. An alternate method, referred to as ‘elimination of time as independent variable’, is based on a treatment given in Benson’s Foundations of Chemical Kinetics. This method, which involves a log–log plot of the reactant concentrations rather than a reciprocal plot, will be described and compared to the standard method. We are currently using a Febetron 706 to study several reactions of iridium halide complexes, in particular a reaction in which OH oxidizes IrBr³⁻₆ to the −2 ion, for which k=1.4×10⁹ M⁻¹ s⁻¹ was found using either the standard or the alternate competition method.     

Phenoxy radical (C6H5O) formation under single collision conditions from reaction of the phenyl radical (C6H5, X2A1) with molecular oxygen (O2, X3Σg(-)): the final chapter?

The combustion relevant elementary reaction of photolytically generated phenyl radicals (C(6)H(5), X(2)A(1)) with molecular oxygen to form the phenoxy radical (C(6)H(5)O) plus a ground state oxygen atom was investigated under single collision conditions at a collision energy of 21.2 ± 0.9 kJ mol(-1). The reaction was found to proceed indirectly via the involvement of a long-lived phenylperoxy radical (C(6)H(5)O(2)) intermediate that decomposed via a rather loose exit transition state. In comparison with crossed beams data obtained previously at elevated collision energies, we suggest that, as the collision energy rises from 21 to 107 kJ mol(-1), the lifetime of the C(6)H(5)O(2) reaction intermediate decreases, that is, a classical behavior within the osculating complex model.     

Measurement of the rate constants for the reactions of the IO• radical with sulfur-containing compounds H2S, (CH3)2S, and SO2

The reactions of iodine monoxide (IO^•) with sulfur-containing compounds, which are important for the atmospheric chemistry, are studied. An attempt is made to distinguish between the heterogeneous and homogeneous reaction pathways. It is shown that, under the experimental conditions, the reactions proceed on the wall and generate iodine atoms into the gas phase. It is found that, at room temperature, the rate constants for the gas-phase reactions of IO^• with (CH₃)₂S and H₂S are lower than 2.5 × 10⁻¹⁴ and 8.0 × 10⁻¹⁴ cm³ molecule⁻¹ s⁻¹, respectively; the rate constant for the gas-phase reaction of iodine monoxide with SO₂ ≤ 5.6 × 10⁻¹⁵ cm³ molecule⁻¹ s⁻¹.     

Study on the reaction of C60 with DL-valine and cyclohexanone

Reaction of C60 with DL-valine and cyclohexanone in refluxing chlorobenzene under N2 yields three [60]fulleropyrrolidine derivatives (mono-, bis- and triadduct) which are characterized on the basis of spectroscopies and cyclic voltammetry. It is found that the ratio of the three products greatly depends on the reaction conditions.     

Oxidation of phenol in aqueous acid: characterization and reactions of radical cations vis-à-vis the phenoxyl radical

Aqueous sulfuric acid containing up to approximately 14 M acid (H0 > or = -7.0) was used as solvent in pulse radiolytic redox studies to characterize cationic transients of phenol (C6H5OH) and map their reactions. The primary radical yields were first measured to correlate the variation in various radical concentrations as a function of increasing acid fraction in the solvent. Compared to their respective values at pH 2, the G(Ox*) increased with almost a linear slope of approximately 0.024 micromol J(-1) for H0(-1) (or pH(-1)) up to H0 -6.0 (Ox* = *OH + SO4*-), whereas G(H*) increased with a slope of approximately 0.033 micromol J(-1) for H0(-1) (or pH(-1)) up to H0 -5.0. In the presence of > 10 M acid (H0 < -5.0), phenol was oxidized to its radical cation, C6H5OH*+, which further reacted with phenol and generated the secondary, dimeric radical cation, (C6H5OH)2*+, following an equilibrium reaction C6H5OH*+ + C6H5OH <==> (C6H5OH)2*+, with K(eq) = 315 +/- 15 M(-1). The two cationic radicals were characterized from their individual UV-vis absorption spectra and acidity. The C6H5OH*+ absorption peaks are centered at 276 and 419 nm, and it was found to be more acidic (pKa = -2.75 +/- 0.05) than (C6H5OH)2*+ (pKa = -1.98 +/- 0.02), having its major peak at 410 nm. On the other hand, in the presence of < 6.5 M acid the C6H5O* reactions followed the radical dimerization route, independent of the parent phenol concentration.     

Reaction products and kinetics of the reaction of NO2 with γ-Fe2O3

The reaction of NO(2) with Fe(2)O(3) has relevance for both atmospheric chemistry and catalysis. Most studies have focused on hematite, α-Fe(2)O(3), as it is the thermodynamic stable state of iron oxide; however, other forms of Fe(2)O(3) naturally occur and may have different chemistries. In this study, we have investigated the reaction products and kinetics for NO(2) reacting with γ-Fe(2)O(3) powder using diffuse reflectance infrared Fourier transform spectroscopy and compared the results to those of previous studies of NO(2) reacting with α-Fe(2)O(3). Both α- and γ-Fe(2)O(3) produce surface-bound nitrate at the pressures examined in this study (24-212 mTorr); surface-bound nitrite products are observed at all pressures for γ-Fe(2)O(3) whereas nitrite was only observed on α-Fe(2)O(3) at lower pressures. Surface-bound NO(+) and Fe-NO products are observed on γ-Fe(2)O(3), which have not been observed with α-Fe(2)O(3). The reaction kinetics show a first-order dependence on NO(2) pressure and this is used to support the hypothesis of unimolecular reaction of adsorbed NO(2) with the γ-Fe(2)O(3) surface as the slow step in the reaction mechanism. The difference in product formation between NO(2) reacting with γ-Fe(2)O(3) and previous studies of α-Fe(2)O(3) illustrate the fact that care must be taken in generalizing reactivity of different polymorphs.     

Synthesis of water-soluble cystine C60 derivative with catalyst and its active oxygen radical scavenging ability

A novel water-soluble cystine C_(60)derivative was synthesized in the presence of the catalyst,tetrabutylammonium hydroxide (TBAH).The product was characterized by FT-IR,UV,~1H NMR,~(13)C NMR,MS and elemental analysis.Furthermore,the scavenging ability to superoxygen anion radical O_2~(·-)and hydroxyl radical ~·OH was studied by chemiluminescence.It was found that cystine C_(60)derivative showed an excellent efficiency in eliminating superoxygen anion radical and hydroxyl radical.The 50% inhibition concentration(IC_(50))for superoxygen anion radical and hydroxyl radical were 0.167 and 0.008 mg/mL,respectively.     

A kinetic study of the reaction of N,N-dimethylanilines with 2,2-diphenyl-1-picrylhydrazyl radical: a concerted proton-electron transfer?

The reactivity of the 2,2-diphenyl-1-picrylhydrazyl radical (dpph*) toward the N-methyl C-H bond of a number of 4-X-substituted- N, N-dimethylanilines (X = OMe, OPh, CH 3, H) has been investigated in MeCN, in the absence and in the presence of Mg(ClO 4) 2, by product, and kinetic analysis. The reaction was found to lead to the N-demethylation of the N, N-dimethylaniline with a rate quite sensitive to the electron donating power of the substituent (rho (+) = -2.03). With appropriately deuterated N, N-dimethylanilines, the intermolecular and intramolecular deuterium kinetic isotope effects (DKIEs) were measured with the following results. Intramolecular DKIE [( k H/ k D) intra] was found to always be similar to intermolecular DKIE [( k H/ k D) inter]. These results suggest a single-step hydrogen transfer mechanism from the N-C-H bond to dpph* which might take the form of a concerted proton-electron transfer (CPET). An electron transfer (ET) step from the aniline to dpph* leading to an anilinium radical cation, followed by a proton transfer step that produces an alpha-amino carbon radical, appears very unlikely. Accordingly, a rate-determining ET step would require no DKIE or at least different inter and intramolecular isotope effects. On the other hand, an equilibrium-controlled ET is not compatible with the small slope value (-0.22 kcal (-1) K (-1)) of the log k H/Delta G degrees plot. Furthermore, the reactivity increases by changing the solvent to the less polar toluene whereas the reverse would be expected for an ET mechanism. In the presence of Mg (2+), a strong rate acceleration was observed, but the pattern of the results remained substantially unchanged: inter and intramolecular DKIEs were again very similar as well as the substituent effects. This suggests that the same mechanism (CPET) is operating in the presence and in the absence of Mg (2+). The significant rate accelerating effect by Mg (2+) is likely due to a favorable interaction of the Mg (2+) ion with the partial negatively charged alpha-methyl carbon in the polar transition state for the hydrogen transfer process.     

Infrared spectra of Rh atom reaction products with C2H2: the HRhCCH, RhCCH, RhCCH2, and Rh-η2-C2H2 molecules

Laser-ablated Rh atoms react with C(2)H(2) upon co-condensation in excess argon and neon to form the insertion product HRhCCH, the alkyne RhCCH, the vinylidene RhCCH(2), and the metallacycle complex Rh-η(2)-(C(2)H(2)). These species are identified through (13)C(2)H(2), C(2)D(2), and mixed C(2)HD isotopic substitutions and density functional theory isotopic frequency calculations. The HRhCCH molecule is characterized by the CH stretching mode at 3306.2 cm(-1) (Ar) and 3310.9 cm(-1) (Ne), the Rh-H stretching mode at 2090.8 cm(-1) (Ar) and 2111.0 cm(-1) (Ne), and two CCH deformation modes at 584.3 and 573.3 cm(-1) (Ar) and 587.1 and 580.3 cm(-1) (Ne). The absorptions for the vinylidene RhCCH(2) complex are observed at 3150.9 (Ar), 3147.2 (Ne) (CH stretching), 1690.1 (Ar), 1694.3 (Ne) (CC stretching), and 804.9 (Ar), 810.5 cm(-1) (Ne) (CCH deformation). The metallacycle Rh-η(2)-(C(2)H(2)) complex is also identified through CC stretching and CCH deformation modes. The insertion reaction of ground Rh atom to the C-H bond is spontaneous on the basis of the growth of HRhCCH absorptions upon annealing in both solid neon and argon. Here, we show that atomic Rh can convert acetylene to the simple Rh vinylidene complex, analogous to that found for ligand-supported Rh complexes.     

The reaction of β-lactam carbenes with 3,6-dipyridyltetrazines: switch of reaction pathways by 2-pyridyl and 4-pyridyl substituents of tetrazines

The reactions of β-lactam carbenes with both 3,6-di(2-pyridyl)tetrazine and 3,6-di(4-pyridyl)tetrazine were studied. It was found that β-lactam carbenes reacted with 3,6-di(2-pyridyl)tetrazine to produce 5-triazolo[1,5-a]pyridylpyrrol-2-ones in good yields, while with 3,6-di(4-pyridyl)tetrazine, they afforded pyrido[c]cyclopenta[b]pyrrol-2-ones in moderate yields. Both reactions were proposed to follow cascade mechanisms containing a 3,6a-dipyridylpyrrolo[3,2-c]pyrazol-5-one intermediate. The different pathways of the transformation of pyrrolo[3,2-c]pyrazol-5-ones were switched by the 2- and 4-pyridyl substituents. This work not only provided a simple and efficient strategy for the construction of novel triazolo[1,5-a]pyridine and pyrido[c]cyclopenta[b]pyrrole derivatives, respectively, but also revealed two different thermal transformation patterns of 3H-pyrazole compounds.     

Theoretical investigation of the atmospheric chemistry of methyl difluoroacetate: reaction with Cl atoms and fate of alkoxy radical at 298 K

A theoretical study on the mechanism of the reactions of methyl difluoroacetate (MDFA) CF₂HC(O)OCH₃ with Cl atoms is presented. Two conformers relatively close in energy have been identified for MDFA. Geometry optimization and frequency calculations have been performed at the MPWB1K/6-31+G(d,p) level of theory, and energetic information is further refined by calculating the energy of the species using G2(MP2) theory. Transition states (TSs) are searched on the potential energy surface involved during the reaction channels, and each of the TSs is characterized by the presence of only one imaginary frequency. The existence of TSs on the corresponding potential energy surface is ascertained by performing intrinsic reaction coordinate calculation. Our calculations reveal that hydrogen abstraction from the –CH₃ group is thermodynamically and kinetically more facile than that from the –CF₂H group. Theoretically calculated rate constants at 298 K using the canonical transition state theory are found to be in good agreement with the experimentally measured ones. The atmospheric lifetime of CF₂HC(O)OCH₃ was estimated to be 16 years. The atmospheric fate and the main degradation process of alkoxy radical CF₂HC(O)OCH₂O are also discussed for the first time. Our calculation indicates that the fluorine atoms substitution has deactivating effect for the α-ester rearrangement.     

The kinetics,catalysis and inhibition of the Baeyer—Villiger reaction in the processes of liquid-phase oxidation of organic compounds

Literary data on kinetics, catalysis and inhibition of the oxidation reaction of carbonyl compounds with peroxy acids according to the Baeyer—Villiger reaction under aerobic liquidphase oxidation conditions have been considered and discussed. The main reaction channel involves a reversible formation of α-hydroxyperoxy ester and its rearrangement to an ester or a lactone. In the case of homolytic decomposition of α-hydroxyperoxy ester no esters are formed. At all steps the formation and transformation of α-hydroxyperoxy ester are catalyzed by carboxylic acids. The possibility of formation of the second intermediate, presumably dioxirane, is shown. Catalysts of the oxidation processes such as variable-valency metal salts influence the kinetics at all steps in the Baeyer—Villiger reaction. Inhibition of ester formation in the presence of cobalt and manganese salts is associated with catalysis of homolytic decomposition of peroxy acid and α-hydroxyperoxy ester.