Peroxy Adduct Research Articles

Thermochemistry and Reaction Kinetics of Secondary Ethyl Radical of Methyl Ethyl Sulfide, CH<sub>3</sub>SCH•CH<sub>3</sub>, with <sup>3</sup>O<sub>2</sub> to CH<sub>2</sub>SCH(OO•)CH<sub>3</sub>

The quantum Rice-Ramsperger-Kassel (QRRK) theory is used to analyze the reaction between the activated CH₃S CH•CH₃ and molecular oxygen to account for further reaction and collisional and deactivation. hydroxyl radicals initiate the oxidation of Methyl ethyl sulfide (CH₃SCH₂CH₃) and MES (methylthioethane) under combustion conditions. The CBS-QB3 and G3MP2B3 composite and M062X/6-311+G(2d, p) DFT methods was used to study the thermochemical properties of reactants, products and transition states. These thermochemical properties are used for the calculations for kinetic and thermochemical parameters. Under high pressure and low temperature, isomerization and stabilization of the CH₃SCH(OO•)CH₃ adduct is of importance. Under atmospheric pressure and at temperatures between above 600 ~ 800 K reactions of the chemically activated peroxy adduct become important relative to stabilization. The reaction between CH₃SCH•CH₃ and O₂ forms an energized peroxy adduct CH₃SCH(OO•)CH₃ with a calculated well depth of 30.2 kcal/mol at the CBS-QB3 level of theory. Kinetic parameters are calculated using the thermochemical properties of products, reactants and transition states obtained using under CBS-QB3 method of calculation. At temperature below 500 K, Stabilization of CH₃SCH(OO•)CH₃ adduct is of importance. Temperature of 500-900 K, is optimal for intramolecular hydrogen shift and the isomerization of CH₃SCH(OO•)CH₃ adduct. At temperature above 800 K, all of the subsequent reaction paths are of importance. For a reaction to move forward under pressure 1-4 atm, the recommended optimal temperature is between 600-800 K. A new pathway for the CH₃SCH(OO•)CH₃ adduct is observed, the attachment of peroxyl oxygen radical to sulfur followed by carbon-sulfur bond dissociation and formation of oxygen-sulfur and oxygen-carbon double bonds.

Mechanism and kinetics of the oxidation of 1,3-butadien-1-yl (n-C4H5): a theoretical study.

Ab initio CCSD(T)-F12/cc-pVTZ-f12//B3LYP/6-311G(d,p) calculations of the C4H5O2 potential energy surface have been combined with Rice-Ramsperger-Kassel-Marcus Master Equation (RRKM-ME) calculations of temperature- and pressure-dependent rate constants and product branching ratios to unravel the mechanism and kinetics of the n-C4H5 + O2 reaction. The results indicate that the reaction is fast, with the total rate constant being in the range of 3.4-5.6 × 10-11 cm3 molecule-1 s-1. The main products include 1-oxo-n-butadienyl + O and acrolein + HCO, with their cumulative yield exceeding 90% at temperatures above 1500 K. Two conformers of 1-oxo-n-butadienyl + O are formed via a simple mechanism of O2 addition to the radical site of n-C4H5 followed by the cleavage of the O-O bond proceeding via a van der Waals C4H5OO complex. Alternatively, the pathways leading to acrolein + HCO involve significant reorganization of the heavy-atom skeleton either via formal migration of one O atom to the opposite end of the molecule or its insertion into the C1-C2 bond. Not counting thermal stabilization of the initial peroxy adducts, which prevails at low temperatures and high pressures, all other products share a minor yield of under 5%. Rate constants for the significant reaction channels have been fitted to modified Arrhenius expressions and are proposed for kinetic modeling of the oxidation of aromatic molecules and 1,3-butadiene. As a secondary reaction, n-C4H5 + O2 can be a source for the formation of acrolein observed experimentally in oxidation of the phenyl radical at low combustion temperatures, whereas another significant (secondary) product of the C6H5 + O2 reaction, furan, could be formed through unimolecular decomposition of 1-oxo-n-butadienyl. Both the n-C4H5 + O2 reaction and unimolecular decomposition of its 1-oxo-n-butadienyl primary product are shown not to be a substantial source of ketene.

Merging New Organoborane Chemistry with Living Radical Polymerization

In this issue of Cell Reports Physical Science, Pan and co-workers synthesize alkylcatecholboranes that can be oxidized by oxygen to drive the release of carbon-centered radicals for initiating reversible addition-fragmentation chain transfer (RAFT) polymerization. These organoboranes can slowly decompose to radicals that enable RAFT polymerization with exceptional control yielding ultra-high molecular weight (UHMW) polymers.

Reactions of β-hydroxypropyl radicals with O2 on the HOC3H6OO• potential energy surfaces: A theoretical study

Although addition reactions of β-hydroxypropyl radicals with O2 are important steps in the reaction pathways of propylene oxidation at low temperature, there is a lack of available accurate rate constants for these reactions in the literature. To obtain complete reaction channels and relatively accurate rate constants over a wide range of temperatures and pressures, a systematic method, which combines the high level ab initio calculation, the conventional transition state theory (TST), the variational transition state theory (VTST) and Rice-Ramsberger-Kassel-Marcus/Master-Equation theory (RRKM/ME), was used to investigate these reactions. The CCSD(T)/cc-pVTZ method was applied to calculate potential energy surfaces. High-pressure limit rate constants of barrierless reactions and those reactions with tight transition states were obtained using the VTST and the TST, respectively. RRKM/ME theory was used to compute rate constants in the fall-off range for pressure-dependent reactions. Take CC•COH as an example, the peroxy adduct of CC•COH with O2 can further react to form more stable products, and subsequent reactions include H-shift reactions, HO2 elimination reactions, OH elimination reactions and Waddington reaction. In these reactions, Waddington reaction is one of the most competitive advantage paths, as it has the lowest barrier. This conclusion is similar to that of those reactions about O2 with products from OH addition to ethylene or isobutene. Rate constants of relevant reactions were fitted to the form of a modified Arrhenius expression, and were implemented into the existing mechanism to simulate concentration profiles of main species of propylene oxidation in a jet-stirred reactor. The results show that predicted results of the revised mechanism are closer to the experimental data than those of the original mechanism. Our calculated rate constants over a wide range of temperatures and pressures for these reactions are transferable and can be applied to a wider range of oxidation conditions.

Reaction kinetics and thermochemistry of the chemically activated and stabilized primary ethyl radical of methyl ethyl sulfide, CH3SCH2CH2•, with O2 to CH3SCH2CH2OO•

AbstractOxidation of methyl ethyl sulfide (CH3SCH2CH3, methylthioethane, MES) under atmospheric and combustion conditions is initiated by hydroxyl radicals, MES radicals, generated after loss of a H atom via OH abstraction, will further react with O2 to form chemically activated and stabilized peroxyl radical adducts. The kinetics of the chemically activated reaction between the CH3SCH2CH2• radical and molecular oxygen are analyzed using quantum Rice‐Ramsperger‐Kassel theory for k(E) with master equation analysis and a modified strong‐collision approach to account for further reactions and collisional deactivation. Thermodynamic properties of reactants, products, and transition states are determined by the B3LYP/6‐31+G(2d,p), M062X/6‐311+G(2d,p), ωB97XD/6‐311+G(2d,p) density functional theory, and CBS‐QB3, G3MP2B3, and G4 composite methods. The reaction of CH3SCH2CH2• with O2 forms an energized peroxy adduct CH3SCH2CH2OO• with a calculated well depth of 34.1 kcal mol−1 at the CBS‐QB3 level of theory. Thermochemical properties of reactants, transition states, and products obtained under CBS‐QB3 level are used for calculation of kinetic parameters. Reaction enthalpies are compared between the methods. The temperature and pressure‐dependent rate coefficients for both the chemically activated reactions of the energized adduct and the thermally activated reactions of the stabilized adducts are presented. Stabilization and isomerization of the CH3SCH2CH2OO• adduct are important under high pressure and low temperature. At higher temperatures and atmospheric pressure, the chemically activated peroxy adduct reacts to new products before stabilization. Addition of the peroxyl oxygen radical to the sulfur atom followed by sulfur‐oxygen double bond formation and elimination of the methyl radical to form S(= O)CCO• + CH3 (branching) is a potentially important new pathway for other alkyl‐sulfide peroxy radical systems under thermal or combustion conditions.

Reaction pathways, kinetics and thermochemistry of the chemically-activated and stabilized primary methyl radical of methyl ethyl sulfide, CH3CH2SCH2•, with 3O2 to CH2CH3SCH2OO•

Oxidation of Methyl ethyl sulfide (CH3SCH2CH3, methylthioethane, MES) under atmospheric and combustion conditions is initiated by reaction with hydroxyl radicals. Methyl ethyl sulfide (MES) radicals generated after losing an H atom via OH abstraction subsequently reacts with O2 to form chemically activated and stabilized peroxyl radical adducts. The kinetics of the chemically activated reaction between the CH2•SCH2CH3 radical and molecular oxygen are analyzed using quantum Rice–Ramsperger–Kassel (QRRK) theory for k(E) with master equation analysis and a modified strong-collision approach to account for further reactions and collisional deactivation. Thermodynamic properties of reactants, products and transition states are determined by the CBS-QB3 composite and M062X/6-311+G(2d, p) DFT methods. The reaction of CH2•SCH2CH3 with O2 forms an energized peroxy adduct •OOCH2SCH2CH3 with a calculated well depth of 26.4 kcal/mol at the CBS-Q//M062x/6-311+g(2d,p) levels of theory. Thermochemical properties of reactants, transition states and products obtained under CBS-QB3 level are used for calculation of the thermochemical and kinetic parameters. The temperature and pressure dependent rate coefficients for both the chemically activated reactions of the energized adduct and the thermally activated reactions of the stabilized adducts are presented. Stabilization and isomerization of the •OOCH2SCH2CH3 adduct are important under high pressure and low temperature. At temperatures between above 600–800 K reactions of the chemically activated peroxy adduct become important relative to stabilization under the atmospheric pressure. Two new pathways are observed, in addition to conventional hydrogen atom transfer reactions from the three carbons to the peroxy oxygen radical. One of these new paths is formation of a Criegee intermediate plus CH2•OO• plus CCS•. A second new path involves the peroxy oxygen radical addition to the sulfur moiety followed by carbon-sulfur bond cleavage with formation of carbon–oxygen and oxygen–sulfur double bonds: CH2O + S•(O)CC. These are potentially important new pathways other alkyl-sulfide peroxy radical systems under thermal or combustion conditions.

Organic Acid Formation from the Atmospheric Oxidation of Gem Diols: Reaction Mechanism, Energetics, and Rates

Computational chemistry is used to investigate the gas phase reaction of several gem diols in the presence of OH radical and molecular oxygen (3O2) as would occur in the Earth's troposphere. Four gem diols, represented generically as R-HC(OH)2, with R being either -H, -CH3, -HC(O), and -CH3C(O) are investigated. We find that after the abstraction of the hydrogen atom from the C-H moiety of the diol by atmospheric OH, molecular oxygen quickly adds onto the resulting radicals leading to the formation of a geminal diol peroxy adduct (R-C(OO)(OH)2), which is the key intermediate in the oxidation process. Unimolecular reaction of this R-C(OO)(OH)2 radical adduct, occurs via a proton-coupled electron transfer (PCET) mechanism and leads to the formation of an organic acid and a HO2 radical. Further, the barrier for the unimolecular reaction step decreases along the R substitution series: -H, -CH3, -HC(O), -CH3C(O); this trend most likely arises from increased internal hydrogen bonding along the series. The reaction where the R group is CH3C(O), associated with methylglyoxal diol, has the lowest barrier with its transition state being ∼4.3 kcal/mol above the potential energy well of the corresponding CH3C(O)-C(OO)(OH)2 peroxy adduct. The rate constants for the four diol oxidation reactions were investigated using the MESMER master equation solver kinetics code over the temperature range between 200 and 300 K. The calculations suggest that once formed, gem diol radicals react rapidly with O2 in the atmosphere to produce organic acids and HO2 with an effective gas phase bimolecular rate constant of ∼1 × 10-11 cm3/molecule s at 300 K.

Mechanism of Oxidative Amidation of Nitroalkanes with Oxygen and Amine Nucleophiles by Using Electrophilic Iodine.

Recently, we developed a direct method to oxidatively convert primary nitroalkanes into amides that entailed mixing an iodonium source with an amine, base, and oxygen. Herein, we systematically investigated the mechanism and likely intermediates of such methods. We conclude that an amine-iodonium complex first forms through N-halogen bonding. This complex reacts with aci-nitronates to give both α-iodo- and α,α-diiodonitroalkanes, which can act as alternative sources of electrophilic iodine and also generate an extra equimolar amount of I(+) under O2. In particular, evidence supports α,α-diiodonitroalkane intermediates reacting with molecular oxygen to form a peroxy adduct; alternatively, these tetrahedral intermediates rearrange anaerobically to form a cleavable nitrite ester. In either case, activated esters are proposed to form that eventually reacts with nucleophilic amines in a traditional fashion.

Thermochemistry and Kinetics for 2-Butanone-1-yl Radical (CH2·C(═O)CH2CH3) Reactions with O2

Thermochemistry of reactants, intermediates, transition state structures, and products along with kinetics on the association of CH2·C(═O)CH2CH3 (2-butanone-1-yl) with O2 and dissociation of the peroxy adduct isomers are studied. Thermochemical properties are determined using ab initio (G3MP2B3 and G3) composite methods along with density functional theory (B3LYP/6-311g(d,p)). Entropy and heat capacity contributions versus temperature are determined from structures, vibration frequencies, and internal rotor potentials. The CH2·C(═O)CH2CH3 radical + O2 association results in a chemically activated peroxy radical with 27 kcal mol(-1) excess of energy. The chemically activated adduct can react to stabilized peroxy or hydroperoxide alkyl radical adducts, further react to lactones plus hydroxyl radical, or form olefinic ketones and a hydroperoxy radical. Kinetic parameters are determined from the G3 composite methods derived thermochemical parameters, and quantum Rice-Ramsperger-Kassel (QRRK) analysis to calculate k(E) with master equation analysis to evaluate falloff in the chemically activated and dissociation reactions. One new, not previously reported, peroxy chemistry reaction is presented. It has a low barrier path and involves a concerted reaction resulting in olefin formation, H2O elimination, and an alkoxy radical.

Theoretical investigation into the low-temperature oxidation of ethylbenzene

The classical topic on the oxidation of alkylbenzene has been revisited via performing accurate theoretical calculations to address the salient features for the initial oxidation of ethylbenzene. Potential energy surfaces are mapped out for all possible reactions in the systems of (1-phenylethyl+O2 and 2-phenylethyl+O2). Reaction rate constants at the high-pressure limit are calculated for all possible reactions in these two systems. Direct H abstraction from 1-phenylethyl radical by oxygen molecule appears to be an important route for the formation of styrene from the oxidation of ethylbenzene. Concerted elimination of HO2 is predicted to contribute significantly the production of styrene from system of 2-phenylethyl+O2; especially at the atmospheric pressure and intermediate temperatures. Formation of the other major experimental product, benzaldehyde, is attributed to the unimolecular decomposition of C6H5CH2(O)CH3 rather than to unimolecular isomerisation of the two initial peroxy adducts. Kinetic and mechanistic data presented herein are instrumental for better understanding of the oxidative decomposition of ethylbenzene, i.e., major constituents of commonly formulated fuel surrogates.

Thermodynamic and kinetic analysis on the reaction of dimethyl sulfide radical with oxygen

AbstractThermochemical properties of methylthiol‐methylhydroperoxide, CH3SCH2OOH and its two terminal radicals resulting from the loss of hydrogen atoms, viz. CH3SCH2OO· and C·H2SCH2OOH, are important to understand the stability, reaction paths, and kinetics of the reactions of dimethyl sulfide and related sulfur‐containing hydrocarbons in the atmosphere and the combustion processes. Thermochemical properties ΔHof, S(T) and Cp(T) and internal rotation potentials for reactants, intermediates, and transition states in the methylthiomethyl (CH3SC·H2) radical + O2 reaction system are analyzed to evaluate reaction paths and kinetics versus temperature and pressure. Isodesmic reaction analysis is used to determine the enthalpies of formation (ΔHof298) using density functional B3LYP/6‐311G(d,p) and complete basis set extrapolation CBS‐QB3 computational methods. Entropy and heat capacities Cp(T) are determined using geometric parameters and vibration frequencies obtained at B3LYP/6‐311G(d,p) level of calculation. ΔHfo298 values estimated at the CBS‐QB3 level (units: kcal mol−1) are CH3SC·H2 (32.7), CH3SCH2OOH (–29.1), CH3SCH2OO· (5.2), C·H2SCH2OOH (11.5). Quantum Rice–Ramsperger–Kassel analysis is used to calculate energy‐dependent rate constants, k(E), and master equation is used to account for collisional stabilization of peroxy adduct and the hydroperoxide isomer. The methylthiomethyl radical associates with O2 to form a methylthiomethyl peroxy radical with a well depth of 27.49 kcal mol−1. The peroxy radical can dissociate back to reactants, isomerize via hydrogen shift to form a hydroperoxide methyl radical C·H2SCH2OOH, decompose via hydrogen transfer, and undergo subsequent low‐energy beta scission to form CH3SC(O)H and OH radical. The peroxy radical can also attack the sulfur atom via TS3 to form CH3S(O) + CH2O product set. The C·H2SCH2OOH isomer can decompose via an elimination transition state to CH2S + CH2O + OH, or through a four‐member ring transition state to 1,3‐Oxathietane + OH products. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

Density functional study of the reaction of O 2 with a single site on the zigzag edge of graphene

The reaction between molecular oxygen and an isolated zigzag graphene edge has been studied using density functional theory at the B3LYP/6-31G( d) level of theory. The initial reaction forms a peroxide, Δ H = −135 kJ mol −1. If the graphene edge is pre-oxidised, the dangling peroxy atom can ( E a = 91 kJ mol −1) migrate across contiguous ketone groups until finding another vacant site and stabilizing as a ketone. However, if no further vacant sites are available, the peroxy oxygen has a number of other possibilities open to it, including desorption of an oxygen atom ( E a = 140 kJ mol −1), migration via the basal plane to form a lactone ( E a = 147 kJ mol −1), and direct interaction with an adjacent oxide to form the lactone or a carbonate ( E a = 146 kJ mol −1). The combination of thermal energy and the heat released in the initial formation of the peroxy adduct is likely to be sufficient to overcome these secondary barriers at modest temperatures. Transfer of the dangling peroxy O to the basal plane produces an epoxide that is mobile on the basal surface ( E a = 40–80 kJ mol −1) but that is transferred back to the edge upon coming into proximity of either a vacant edge site or ketone. The instability of the edge epoxide structure implies that it cannot play a significant role in carbon gasification through promoting the reactivity of ketones, contrary to earlier suggestions. The desorption of an oxygen atom creates a very active species capable of reacting with basal or edge sites as well as with oxygen complexes. The reaction of ketone + O has been reported to yield a five-membered ring + CO 2, leading to an overall stoichiometry which is consistent with the observed oxyreactivity of carbon surface oxides identified in isotopic labelling studies in which one O atom is gasified and the other forms a new surface oxide.

Thermochemistry and kinetics of acetonylperoxy radical isomerisation and decomposition: a quantum chemistry and CVT/SCT approach

CBS-QB3 calculations have been used to determine thermochemical and kinetic parameters of the isomerisation and decomposition reactions of the acetonylperoxy radical, CH3C(O)CH2OO* , which has been formed via the reaction of acetonyl radical with O2 leading to the formation of an energised peroxy adduct with a calculated well depth of near 111 kJ mol(-1). This species can undergo subsequent 1,5 and 1,3 H-shifts to give the primary and secondary radicals: C*H2C(O)CH2OOH and CH3C(O)C*HOOH, respectively, or rearrange to give a 3-methyl-1,2-dioxetan-3-yloxy radical. Rate constants for isomerisation and subsequent decomposition have been estimated using canonical variational transition state theory with small curvature tunneling cvt/sct. The variational effect for the isomerisation channels is only moderate but the tunneling correction is significant at temperatures up to 1000 K; the formation of a primary radical by a 1,5-shift is the main reaction channel and the competition with the secondary one starts only at around 1500 K. The fate of the primary acetonylhydroperoxy radical is predominantly to form oxetan-3-one while the ketene and 1-oxy-3-hydroxyacetonyl radical channels only compete with the formation of oxetan-3-one at temperatures >1200 K. In addition, consistent and reliable enthalpies of formation have been computed for the molecules acetonylhydroperoxide, 1,3-dihydroxyacetone, methylglyoxal and cyclobutanone, and for some related radicals.

Identification of the Peroxy Adduct in Multicopper Oxidases by a Combination of Computational Chemistry and Extended X-ray Absorption Fine-Structure Measurements

We have developed a computational method that combines extended X-ray absorption fine structure (EXAFS) refinements with the integrated quantum mechanical and molecular mechanics (QM/MM) method. This method allows us to obtain a structure of a metal site inside a protein that is compatible with both EXAFS data and QM calculations (i.e., that is chemically reasonable). Thereby, the QM/MM calculations play the same role as MM in nearly all NMR and crystallographic refinementsEXAFS ensures that the metal−ligand distances are accurate and QM/MM fills in all the other structural data. We have used this method to show that a structure with a peroxide ion in the center of the trinuclear cluster fits experimental EXAFS data better than a structure with the peroxide ion on the side of the cluster for the peroxide adduct of multicopper oxidases.

Multireference Ab Initio Calculations on Reaction Intermediates of the Multicopper Oxidases

The multicopper oxidases (MCOs) couple the four-electron reduction of dioxygen to water with four one-electron oxidations of various substrates. Extensive spectroscopic studies have identified several intermediates in the MCO catalytic cycle, but they have not been able to settle the structures of three of the intermediates, viz. the native intermediate (NI), the peroxy intermediate (PI), and the peroxy adduct (PA). The suggested structures have been further refined and characterized by quantum mechanical/molecular mechanical (QM/MM) calculations. In this paper, we try to establish a direct link between theory and experiment, by calculating spectroscopic parameters for these intermediates using multireference wave functions from the multistate CASPT2 and MRDDCI2 methods. Thereby, we have been able to reproduce low-spin ground states (S = 0 or S = 1/2) for all the MCO intermediates, as well as a low-lying (approximately 150 cm-1) doublet state and a doublet-quartet energy gap of approximately 780 cm-1 for the NI. Moreover, we reproduce the zero-field splitting (approximately 70 cm-1) of the ground 2E state in a D3 symmetric hydroxy-bridged trinuclear Cu(II) model of the NI and obtain a quantitatively correct quartet-doublet splitting (164 cm-1) for a mu3-oxo-bridged trinuclear Cu(II) cluster. All results support the suggestion that the NI has an O2- atom in the center of the trinuclear cluster, whereas both the PI and PA have an O22- ion in the center of the cluster, in agreement with the QM/MM results and spectroscopic measurements.

Thermochemical and Kinetic Analyses on Oxidation of Isobutenyl Radical and 2-Hydroperoxymethyl-2-propenyl Radical

In recognition of the importance of the isobutene oxidation reaction in the preignition chemistry associated with engine knock, the thermochemistry, chemical reaction pathways, and reaction kinetics of the isobutenyl radical oxidation at low to intermediate temperature range were computationally studied, focusing on both the first and the second O2 addition to the isobutenyl radical. The geometries of reactants, important intermediates, transition states, and products in the isobutenyl radical oxidation system were optimized at the B3LYP/6-311G(d,p) and MP2(full)/6-31G(d) levels, and the thermochemical properties were determined on the basis of ab initio, density functional theory, and statistical mechanics. Enthalpies of formation for several important intermediates were calculated using isodesmic reactions at the DFT and the CBS-QB3 levels. The kinetic analysis of the first O2 addition to the isobutenyl radical was performed using enthalpies at the CBS-QB3 and G3(MP2) levels. The reaction forms a chemically activated isobutenyl peroxy adduct which can be stabilized, dissociate back to reactants, cyclize to cyclic peroxide-alkyl radicals, and isomerize to the 2-hydroperoxymethyl-2-propenyl radical that further undergoes another O2 addition. The reaction channels for isomerization and cyclization and further dissociation on this second O2 addition were analyzed using enthalpies at the DFT level with energy corrections based on similar reaction channels for the first O2 addition. The high-pressure limit rate constants for each reaction channel were determined as functions of temperature by the canonical transition state theory for further kinetic model development.

Thermochemical and kinetic analysis of the allyl radical with O 2 reaction system

Thermochemical properties and kinetics for the allyl radical (CH 2 CHCH 2 ) + O 2 reaction system are analyzed to evaluate reaction paths and kinetics in thermal oxidation. ( Δ H f 298 0 ) are determined using isodesmic reaction analysis at the CBSQ//B3LYP/6-31G(d,p) composite and at density functional levels. Entropies ( S 298 0 ) and heat capacities ( C p 0 ( T ) ) are determined using structures and vibrational frequencies obtained at the B3LYP/6-31G(d,p) level of theory. Internal rotor contributions are included in S and C p( T) values. The allyl radical adds to O 2 to form an energized peroxy adduct [CH 2 CHCH 2OO ]* with a shallow well, only 20 kcal/mol barrier to dissociation. Reaction channels of the peroxy adduct [CH 2 CHCH 2OO ]* include reverse reaction, stabilization, isomerizations via hydrogen shift with subsequent β-scission or R O–OH bond cleavage. The peroxy adduct can also cyclize to four- or five-member cyclic peroxide-alkyl radicals, C H 2Y CCOO and Y CC COO (Y = cyclic). All pathways in this allyl plus O 2 system are found to involve barriers that are higher than reverse reaction. The most accessible product (only 5 kcal/mol above reverse reaction) is the cyclization to a five-member ring, that reacts further with O 2, resulting in carbonyl and chain propagation products. Other important products are: allene + HO 2 via molecular elimination and C H CHCH 2OOH (via hydrogen shift), which undergoes β-scission leading to C 2H 2 + CH 2O + OH above 600 K. Rate constants are estimated as a function of pressure and temperature using quantum Rice–Ramsperger–Kassel analysis for k(E) and master equation for falloff.

Detailed Kinetics and Thermochemistry of C2H5 + O2: Reaction Kinetics of the Chemically-Activated and Stabilized CH3CH2OO• Adduct

The kinetics of the chemically activated reaction between the ethyl radical and molecular oxygen are analyzed using quantum Rice−Ramsperger−Kassel (QRRK) theory for k(E) with both a master equation analysis and a modified strong-collision approach to account for collisional deactivation. Thermodynamic properties of species and transition states are determined by ab initio methods at the G2 and CBS-Q//B3LYP/6-31G(d,p) levels of theory and isodesmic reaction analysis. Rate coefficients for reactions of the energized adducts are obtained from canonical transition state theory. The reaction of C2H5 with O2 forms an energized peroxy adduct with a calculated well depth of 35.3 kcal mol-1 at the CBS-Q//B3LYP/6-31G(d,p) level of theory. The calculated (VTST) high-pressure limit bimolecular addition reaction rate constant for C2H5 + O2 is 2.94 × 1013T-0.44. Predictions of the chemically activated branching ratios using both collisional deactivation models are similar. All of the product formation pathways of ethyl...

Analysis of Tertiary Butyl Radical + O2, Isobutene + HO2, Isobutene + OH, and Isobutene−OH Adducts + O2: A Detailed Tertiary Butyl Oxidation Mechanism

The reaction systems tert-butyl radical + O2, isobutene + HO2, isobutene + OH, and isobutene−OH adducts + O2, which are important to understanding the oxidation chemistry of tertiary butyl radical (C3C•), are analyzed. Thermochemical parameters are determined by ab initio−Mφller−Plesset (MP2(full)/6-31g(d)), complete basis set model chemistry (CBS-4 and CBS-q with MP2(full)/6-31g(d) and B3LYP/6-31g(d) optimized geometries), density functional (B3LYP/6-31g(d)), semiempirical MOPAC (PM3) molecular orbital calculations, and by group additivity estimation. Thermochemical kinetic parameters are developed for each elementary reaction path in these complex systems, and a chemical activation kinetic analysis using quantum Rice−Ramsperger−Kassel (QRRK) theory for k(E) and master equation analysis for falloff is used to calculate rate constants as a function of pressure and temperature. An elementary reaction mechanism is constructed to model experimental data for oxidation of tert-butyl radical. Calculations for loss of tert-butyl precursor, 2,2,3,3-tetramethylbutane (C3CCC3), and production of isobutene and 2,2-dimethyloxirane from the mechanism are compared with experimental data reported in the literature. Reaction of tert-butyl radical (C3C•) with O2 forms an energized tert-butyl peroxy adduct C3COO•* which can dissociate back to reactants, dissociate to isobutene + HO2, or isomerize to tert-butyl hydroperoxide (C3•COOH). This isomer can dissociate to either isobutene + HO2 or 2,2-dimethyloxirane + OH, before it is stabilized. In the tert-butyl radical + O2 reaction system, dissociation of the [C3COO•]* adduct to isobutene + HO2 via HO2 molecular elimination is faster than the hydrogen shift to C3•COOH by a factor of 86:1 at 773 K and 60 Torr. The reaction barrier (reaction enthalpy difference between TS4 and C3•COOH) for the C3•COOH reaction to 2,2-dimethyloxirane + OH is calculated as 17.98 (19.06) kcal/mol at the CBS-q//MP2(full)/6-31g(d) level but is evaluated as 15.58 (18.06) kcal/mol by fitting experimental data. Data in parentheses are thermodynamic properties based on CBS-q//B3LYP/6-31g(d) calculation. Barriers for reactions of HO2 + isobutene → C3•COOH (HO2 addition at CD/C2 carbon atom of isobutene, CD = carbon double bond) and HO2 + isobutene → C2C•COOH (HO2 addition at CD/H2 carbon atom of isobutene) are respectively determined as 7.74 (7.38) and 10.69 (10.82) kcal/mol. 2,2-Dimethyloxirane is formed primarily by HO2 addition to isobutene. OH addition to isobutene results in adducts which further react with O2 to form acetone, formaldehyde, and the OH radical (Waddington mechanism) with these pathways also analyzed.

Oxygen activation catalyzed by methane monooxygenase hydroxylase component: proton delivery during the O-O bond cleavage steps.

The effects of solvent pH and deuteration on the transient kinetics of the key intermediates of the dioxygen activation process catalyzed by the soluble form of methane monooxygenase (MMO) isolated from Methylosinus trichosporium OB3b have been studied. MMO consists of hydroxylase (MMOH), reductase, and "B" (MMOB) components. MMOH contains a carboxylate- and oxygen-bridged binuclear iron cluster that catalyzes O2 activation and insertion chemistry. The diferrous MMOH-MMOB complex reacts with O2 to form a diferrous intermediate compound O (O) and subsequently a diferric intermediate compound P (P), presumed to be a peroxy adduct. The O decay reaction was found to be pH-independent within error at 4 degrees C (kobs = 22 +/- 2 s-1 at pH 7.7; kobs = 26 +/- 2 s-1 at pH 7.0). In contrast, the P formation rate was found to decrease sharply with increasing pH to near zero at pH 8.6; the observed rate constants fit to a single deprotonation event with a pKa = 7.6 and a maximal formation rate at 4 degrees C of kP = 9.1 +/- 0.9 s-1 achieved near pH 6.5. The formation of P was slower than the disappearance of O, indicating that at least one other undetected intermediate (P) must form in between. P decays spontaneously to the highly chromophoric intermediate, compound Q (Q). The decay rate of P matched the formation rate of Q, and both rates decreased sharply with increasing pH to near zero at pH 8.6; the observed rate constants fit to a single deprotonation event with a pKa = 7.6 and a maximal formation rate at 4 degrees C of kQ = 2.6 +/- 0.1 s-1 achieved near pH 6.5. No pH dependence was observed for the decay of Q. The formation and decay rates of P and the formation rate of Q decreased linearly with mole fraction of D2O in the reaction mixture. Kinetic solvent isotope effect values of kH/kD = 1.3 +/- 0.1 (P formation) and kH/kD = 1.4 +/- 0.1 (P decay and Q formation) were observed at 5 degrees C. The linearity of the proton inventory plots suggests that only a single proton is transferred in the transition state of the formation reaction for each intermediate. If these protons are transferred to the bound oxygen molecule, as formally required by the reaction stoichiometry, the data are consistent with a model in which water is formed concurrently with the formation of the reactive bis mu-oxo-binuclear Fe(IV) species, Q.