Gabriel da Silva

Unimolecular β-Hydroxyperoxy Radical Decomposition with OH Recycling in the Photochemical Oxidation of Isoprene

A novel process in the photochemical oxidation of isoprene that recycles hydroxyl (OH) radicals has been identified using first-principles computational chemistry. Isoprene is the dominant biogenic volatile organic compound (VOC), and its oxidation controls chemistry in the forest boundary layer and is also thought to contribute to cloud formation in marine environments. The mechanism described here involves rapid unimolecular decomposition of the two major peroxy radicals (beta-hydroxyperoxy radicals) produced by OH-initiated isoprene oxidation. Peroxy radicals are well-known as key intermediates in VOC oxidation, but up to now were only thought to be destroyed in bimolecular reactions. The process described here leads to OH recycling with up to around 60% efficiency in environments with low levels of peroxy radicals and NO(x). In forested environments reaction of the beta-hydroxyperoxy radicals with HO2 is expected to dominate, with a small contribution from the mechanism described here. Peroxy radical decomposition will be more important in the unpolluted marine boundary layer, where lower levels of NO and HO2 are encountered.

Ethanol Oxidation: Kinetics of the α-Hydroxyethyl Radical + O2 Reaction

Bioethanol is currently a significant gasoline additive and the major blend component of flex-fuel formulations. Ethanol is a high-octane fuel component, and vehicles designed to take advantage of higher octane fuel blends could operate at higher compression ratios than traditional gasoline engines, leading to improved performance and tank-to-wheel efficiency. There are significant uncertainties, however, regarding the mechanism for ethanol autoignition, especially at lower temperatures such as in the negative temperature coefficient (NTC) regime. We have studied an important chemical process in the autoignition and oxidation of ethanol, reaction of the alpha-hydroxyethyl radical with O2(3P), using first principles computational chemistry, variational transition state theory, and Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation simulations. The alpha-hydroxyethyl + O2 association reaction is found to produce an activated alpha-hydroxy-ethylperoxy adduct with ca. 37 kcal mol(-1) of excess vibrational energy. This activated adduct predominantly proceeds to acetaldehyde + HO(2), with smaller quantities of the enol vinyl alcohol (ethenol), particularly at higher temperatures. The reaction to acetaldehyde + HO2 proceeds with such a low barrier that collision stabilization of C2O3H5 isomers is unimportant, even for high-pressure/low-temperature conditions. The short lifetimes of these radicals precludes the chain-branching addition of a second O2 molecule, responsible for NTC behavior in alkane autoignition. This result helps to explain why ignition delays for ethanol are longer than those for ethane, despite ethanol having a weaker C-C bond energy. Given its relative instability, it is also unlikely that the alpha-hydroxy-ethylperoxy radical acts as a major acetaldehyde sink in the atmosphere, as has been suggested.

Oxidation of the Benzyl Radical: Mechanism, Thermochemistry, and Kinetics for the Reactions of Benzyl Hydroperoxide.

Oxidation of the benzyl radical plays a key role in the autoignition, combustion, and atmospheric degradation of toluene and other alkylated aromatic hydrocarbons. Under relevant autoignition conditions of moderate temperature and high pressure, and in the atmosphere, benzyl reacts with O2 to form the benzylperoxy radical, and the further oxidation reactions of this radical are not yet fully characterized. In this contribution, we further develop the reaction chemistry, thermodynamics, and kinetics of benzyl radical oxidation, highlighting the important role of benzyl hydroperoxide and the benzoxyl (benzyloxyl) radical. The benzylperoxy + H reaction mechanism is studied using computational chemistry and statistical reaction rate theory. High-pressure limit rate constants in the barrierless benzylperoxy + H association are obtained from variational transition state theory calculations, with internal rotor contributions. The benzylperoxy + H reaction is seen to produce an activated benzyl hydroperoxide adduct that has 87 kcal mol(-1) excess energy over the ground state. We show that this activated adduct proceeds almost exclusively to the benzoxyl radical + OH across a wide range of temperature and pressure conditions. Minor reaction paths include benzyl + HO2, α-hydroxylbenzyl + OH, and benzaldehyde + H2O, each constituting around 1% of the total reaction rate at higher temperatures. Thermal decomposition of benzyl hydroperoxide, formed by hydrogen abstraction reactions in the benzylperoxy radical and at low temperatures in the benzylperoxy + H and benzyl + HO2 reactions, is also investigated. Decomposition to benzoxyl + OH is fast at temperatures of 900 K and above. The contribution of benzyl hydroperoxide chemistry to the ignition and oxidation of alkylated aromatics is discussed. Benzyl radical oxidation chemistry achieves the conversion of toluene to benzaldehyde, aiding autoignition via processes that either release large amounts of energy or form reactive free radicals through chain-branching.

Thermal Decomposition of the Benzyl Radical to Fulvenallene (C7H6) + H

We show that the benzyl radical decomposes to the C7H6 fragment fulvenallene (+H), by first principles/RRKM study. Calculations using G3X heats of formation and B3LYP/6-31G(2df,p) structural and vibrational parameters reveal that the reaction proceeds predominantly via a cyclopentenyl-allene radical intermediate, with an overall activation enthalpy of ca. 85 kcal mol(-1). Elementary rate constants are evaluated using Eckart tunneling corrections, with variational transition state theory for barrierless C-H bond dissociation in the cyclopentenyl-allene radical. Apparent rate constants are obtained as a function of temperature and pressure from a time-dependent RRKM study of the multichannel multiwell reaction mechanism. At atmospheric pressure we calculate the decomposition rate constant to be k [s(-1)] = 5.93 x 10(35)T(-6.099) exp(-49,180/T); this is in good agreement with experiment, supporting the assertion that fulvenallene is the C7H6 product of benzyl decomposition. The benzyl heat of formation is evaluated as 50.4 to 52.2 kcal mol(-1), using isodesmic work reactions with the G3X theoretical method. Some novel pathways are presented to the cyclopentadienyl radical (C5H5) + acetylene (C2H2), which may constitute a minor product channel in benzyl decomposition.

Gold‐Mediated CI Bond Activation of Iodobenzene

Controversy resolved! A combination of gas-phase ion-molecule reactions and theoretical studies confirm bisligated mononuclear Au(I) complexes are unable to undergo oxidative addition of iodobenzene for Sonogashira coupling, but that the ligated gold clusters [Au(3)L(n)](+) (L=Ph(2)P(CH(2))(n)PPh(2); n=3-6) activate the C-I bond. DFT calculations on the transition states show that the linker size n tunes the cluster reactivity.

Borate-Catalyzed Carbon Dioxide Hydration via the Carbonic Anhydrase Mechanism

The hydration of CO(2) plays a critical role in carbon capture and geoengineering technologies currently under development to mitigate anthropogenic global warming and in environmental processes such as ocean acidification. Here we reveal that borate catalyzes the conversion of CO(2) to HCO(3)(-) via the same fundamental mechanism as the enzyme carbonic anhydrase, which is responsible for CO(2) hydration in the human body. In this mechanism the tetrahydroxyborate ion, B(OH)(4)(-), is the active form of boron that undergoes direct reaction with CO(2). In addition to being able to accelerate CO(2) hydration in alkaline solvents used for carbon capture, we hypothesize that this mechanism controls CO(2) uptake by certain saline bodies of water, such as Mono Lake (California), where previously inexplicable influx rates of inorganic carbon have created unique chemistry. The new understanding of CO(2) hydration provided here should lead to improved models for the carbon cycle in highly saline bodies of water and to advances in carbon capture and geoengineering technology.

First principles pKa calculations on carboxylic acids using the SMD solvation model: effect of thermodynamic cycle, model chemistry, and explicit solvent molecules.

Aqueous pK(a) values are calculated from first principles for a set of carboxylic acids using the SMD solvation model with various model chemistries, thermodynamic cycles, and treatments of explicit solvation. In all, 108 unique theoretical protocols are examined. The direct (D) and water proton exchange (PX) cycles are trialled along with a new approach, termed the semidirect (SD) cycle. The SD thermodynamic cycle offers some improvements over the D and PX schemes, as it bypasses the gas-phase heterolytic bond dissociation calculation required in the conventional D approach while also avoiding an aqueous OH(-) calculation required by the PX method when using water as the reference acid. With all three cycles, the recommended model chemistry employs M05-2X/cc-pVTZ Gibbs energies of solvation with a single discrete water molecule and a high-level composite method for the gas-phase reaction energies. With the SD cycle, these calculations result in a mean unsigned error of less than 1 pK(a) units, with respective mean signed error and maximum unsigned error of less than 0.5 and 2 pK(a) units. Similar results are obtained with the D and PX cycles, and further improvement is required in both the gas and aqueous phase ab initio energy calculations before we can truly discriminate between the thermodynamic cycles investigated here.

Reaction of Methacrolein with the Hydroxyl Radical in Air: Incorporation of Secondary O2 Addition into the MACR + OH Master Equation

Methacrolein (MACR) plays an important role in atmospheric chemistry within the planetary boundary layer, as it is one of the major oxidation products of isoprene and has a short lifetime toward the hydroxyl radical (OH). In this study, quantum chemical techniques and statistical reaction rate theory have been used to simulate the addition of OH to MACR at conditions representative of the troposphere. In this chemically activated reaction, the time scales for product formation versus collisional deactivation of the vibrationally excited adduct are explicitly considered. Furthermore, the subsequent addition of O(2) is also incorporated within a single master equation, so as to investigate doubly activated peroxyl radical formation. The major reaction product of OH addition to MACR is the HOCH(2)C(•)(CH(3))CHO radical formed via addition to the outer (β) carbon. This radical is predominantly in the Z isomer although around a third of the population is quenched as the higher-energy E isomer. Calculated rate constants agree well with experiment when using M06-2X/aug-cc-pVTZ barrier heights, but are somewhat overpredicted using G3SX energies. The overall rate constant is controlled by competition between dissociation of the MACR···OH van der Waals complex back to reactants and isomerization on to MACR-OH adducts, which takes place on a time scale of several nanoseconds, but collisional deactivation of the MACR-OH adducts occurs on a time scale that is around an order of magnitude longer. When O(2) addition is included in the master equation, we observe that the MACR-OH adducts are removed by reaction with O(2) on a similar time scale to collisional deactivation. Around 50% of the subsequent peroxyl radical population is formed with some identifiable excess vibrational energy above singly activated [MACR-OH-O(2)]*, with around 20% provided with an additional 20 kcal mol(-1) (>40 kcal mol(-1) relative to quenched MACR-OH-O(2)) that can go into further unimolecular reaction. This double activation process is expected to lead to some prompt unimolecular decomposition of excited [MACR-OH-O(2)]** peroxyl radicals to yield products including hydroxyacetone and methylglyoxal, regenerating the initiating OH radical in the process.

Quantum chemical study of the acrolein (CH2CHCHO) + OH + O2 reactions.

Acrolein, a beta-unsaturated (acrylic) aldehyde, is one of the simplest multifunctional molecules, containing both alkene and aldehyde groups. Acrolein is an atmospheric pollutant formed in the photochemical oxidation of the anthropogenic VOC 1,3-butadiene, and serves as a model compound for methacrolein (MACR) and methyl vinyl ketone (MVK), the major oxidation products of the biogenic VOC isoprene. In addition, acrolein is involved in combustion and biological oxidation processes. This study presents a comprehensive theoretical analysis of the acrolein + OH + O(2) addition reactions, which is a key photochemical oxidation sequence, using the G3SX and CBS-QB3 theoretical methods. Both ab initio protocols provide relatively similar results, although the CBS-QB3 method systematically under-predicts literature heats of formation using atomization enthalpies, and also provides lower transition state barrier heights. Several new low-energy pathways for unimolecular reaction of the acrolein-OH-O(2) radicals are identified, with energy at around or below that of the acrolein-OH isomers + O(2). In each case these novel reactions have the potential to reform the hydroxyl radical (OH) and form coproducts that include glyoxal, glycolaldehyde (HOCH(2)CHO), formaldehyde (HCHO), CO, and substituted epoxides. Analogous reaction schemes are developed for the photochemical oxidation of MACR and MVK, producing a number of observed oxidation products. The reaction MACR + OH + O(2) --> hydroxyacetone + OH + CO is expected to be of particular importance. This study also proposes that O(2) addition to chemically activated acrolein-OH adducts can provide prompt regeneration of OH in the atmospheric oxidation of acrolein, via a double activation mechanism. This mechanism can also be extended to isoprene, MVK, and MACR. The importance of the novel chemistry revealed here in the atmospheric oxidation of acrolein and other structurally related OVOCs and VOCs requires further investigation. Additionally, a critical evaluation of the acrolein heat of formation is presented, and a new value of -16.7 +/- 1.0 kcal mol(-1) is recommended along with other thermochemical properties, from a W1 level calculation.

The C7H5 fulvenallenyl radical as a combustion intermediate: potential new pathways to two- and three-ring PAHs.

This study proposes the existence of a new, highly resonantly stabilized free radical, fulvenallenyl (C7H5). Fulvenallenyl forms from dissociation or abstraction of the weak H atoms in the C7H6 compounds fulvenallene and 1-ethynylcyclopentadiene, which were recently identified as intermediates in the combustion of aromatic fuels and in sooting flames. The fulvenallenyl radical shares properties of the propargyl and cyclopentadienyl radicals, and like these species, we propose that fulvenallenyl has a significant resonance stabilization energy. This resonance energy is lost upon further reaction, making the fulvenallenyl radical resistant to oxidation and thermal decomposition. As with the resonantly stabilized radicals propargyl and cyclopentadienyl, fulvenallenyl is expected to react with other radicals in molecular weight growth reactions. Several pathways are proposed for self-reaction and cross-reactions of fulvenallenyl that directly produce two- and three-ring PAHs like naphthalene, diphenyl, and phenanthrene. Rate constants are calculated for H atom abstraction from fulvenallene and 1-ethynylcyclopentadiene by the common radicals H, OH, and CH3, to facilitate the inclusion of fulvenallenyl in kinetic models.