G3X-K theory: A composite theoretical method for thermochemical kinetics

Experimental and Theoretical Study of the OH-Initiated Degradation of Piperidine under Simulated Atmospheric Conditions

The OH-initiated photo-oxidation of piperidine and the photolysis of 1-nitrosopiperidine were investigated in a large atmospheric simulation chamber and in theoretical calculations based on CCSD(T*)-F12a/aug-cc-pVTZ//M062X/aug-cc-pVTZ quantum chemistry results and master equation modeling of the pivotal reaction steps. The rate coefficient for the reaction of piperidine with OH radicals was determined by the relative rate method to be kOH-piperidine = (1.19 ± 0.27) × 10-10 cm3 molecule-1 s-1 at 304 ± 2 K and 1014 ± 2 hPa. Product studies show the piperidine + OH reaction to proceed via H-abstraction from both CH2 and NH groups, resulting in the formation of the corresponding imine (2,3,4,5-tetrahydropyridine) as the major product and in the nitramine (1-nitropiperidine) and nitrosamine (1-nitrosopiperidine) as minor products. Analysis of 1-nitrosopiperidine photolysis experiments under natural sunlight conditions gave the relative rates jrel = j1-nitrosoperidine/jNO2 = 0.342 ± 0.007, k3/k4a = 0.53 ± 0.05 and k2/k4a = (7.66 ± 0.18) × 10-8 that were subsequently employed in modeling the piperidine photo-oxidation experiments, from which the initial branchings between H-abstraction from the NH and CH2 groups, kN-H/ktot = 0.38 ± 0.08 and kC2-H/ktot = 0.49 ± 0.19, were derived. All photo-oxidation experiments were accompanied by particle formation that was initiated by the acid-base reaction of piperidine with nitric acid. Primary photo-oxidation products including both 1-nitrosopiperidine and 1-nitropiperidine were detected in the particles formed. Quantum chemistry calculations on the OH initiated atmospheric photo-oxidation of piperidine suggest the branching in the initial H-abstraction routes to be ∼35% N1, ∼50% C2, ∼13% C3, and ∼2% C4. The theoretical study produced an atmospheric photo-oxidation mechanism, according to which H-abstraction from the C2 position predominantly leads to 2,3,4,5-tetrahydropyridine and H-abstraction from the C3 position results in ring opening followed by a complex autoxidation, of which the first few steps are mapped in detail. H-abstraction from the C4 position is shown to result mainly in the formation of piperidin-4-one and 2,3,4,5-tetrahydropyridin-4-ol, whereas H-abstraction from N1 under atmospheric conditions primarily leads to 2,3,4,5-tetrahydropyridine and in minor amounts of 1-nitrosopiperidine and 1-nitropiperidine. The calculated rate coefficient for the piperidine + OH reaction agrees with the experimental value within 35%, and aligning the theoretical numbers to the experimental value results in k(T) = 2.46 × 10-12 × exp(486 K/T) cm3 molecule-1 s-1 (200-400 K).

Protonation Isomer Specific Ion-Molecule Radical Reactions

Through a combination of ion-mobility filtering and laser-equipped quadrupole ion-trap mass spectrometry, the gas-phase reaction kinetics of two protonation isomers of the distonic-radical quinazoline cation are independently measured with ethylene. For these radical addition reactions, protonation site variations drive significant changes in nearby radical reactivity, and this is primarily due to through-space electrostatic effects. Furthermore, quantum chemical methods specifically designed for calculating long-range interactions, such as double-hybrid density functional theory, are required to rationalize the experimentally measured difference in reactivity.

Reactions of Acetonitrile with Trapped, Translationally Cold Acetylene Cations

The reaction of the acetylene cation (C₂H₂⁺) with acetonitrile (CH₃CN) is measured in a linear Paul ion trap coupled to a time-of-flight mass spectrometer. C₂H₂⁺ and CH₃CN are both noted for their astrochemical abundance and predicted relevance for understanding prebiotic chemistry. The observed primary products are c-C₃H₃⁺, C₃H₄⁺, and C₂NH₃⁺. The latter two products react with excess CH₃CN to form the secondary product C₂NH₄⁺, protonated acetonitrile. The molecular formula of these ionic products can be verified with the aid of isotope substitution via deuteration of the reactants. Primary product reaction pathways and thermodynamics are investigated with quantum chemical calculations and demonstrate exothermic pathways to two isomers of C₂NH₃⁺, two isomers of C₃H₄⁺, and the cyclopropenyl cation c-C₃H₃⁺. This study deepens our understanding of the dynamics and products of a pertinent ion-molecule reaction between two astrochemically abundant molecules in conditions that mimic those of the interstellar medium.

Investigating the Association Reactions of HOCH2CO and HOCHCHO with O2: A Quantum Computational and Master Equation Study

Glycolaldehyde, HOCH₂CHO, is an important multifunctional atmospheric trace gas formed in the oxidation of ethylene and isoprene and emitted directly from burning biomass. The initial step in the atmospheric photooxidation of HOCH₂CHO yields HOCH₂CO and HOCHCHO radicals; both of these radicals react rapidly with O₂ in the troposphere. This study presents a comprehensive theoretical investigation of the HOCH₂CO + O₂ and HOCHCHO + O₂ reactions using high-level quantum chemical calculations and energy-grained master equation simulations. The HOCH₂CO + O₂ reaction results in the formation of a HOCH₂C(O)O₂ radical, while the HOCHCHO + O₂ reaction yields (HCO)₂ + HO₂. Density functional theory calculations have identified two open unimolecular pathways associated with the HOCH₂C(O)O₂ radical that yield HCOCOOH + OH or HCHO + CO₂ + OH products; the former novel bimolecular product pathway has not been previously reported in the literature. Master equation simulations based on the potential energy surface calculated here for the HOCH₂CO + O₂ recombination reaction support experimental product yield data from the literature and indicate that, even at total pressures of 1 atm, the HOCH₂CO + O₂ reaction yields ∼11% OH at 298 K.

Kinetics, mechanism, and application of sodium persulfate activated by sodium hydroxide for removing 1,2-dichloroethane from groundwater

1,2-Dichloroethane (1,2-DCA) is a common compound found in groundwater contaminated with organics. This compound is difficult to remove from groundwater and has the potential to inflict significant harm on human health and the environment. This study used sodium persulfate (Na₂S₂O₈) activated by sodium hydroxide (NaOH) to remove 1,2-DCA from aqueous solutions. Density functional theory was employed to calculate the potential energy surface of the reactants, intermediates, transient states, and products to thoroughly analyze the degradation pathways. The computations were performed in combination with in situ remediation of a 1,2-DCA plume from a point source to verify the industrial applicability of the technology. The results showed the 1,2-DCA removal efficiency was impacted considerably by the Na₂S₂O₈ dosage and the dosing sequence of Na₂S₂O₈ and NaOH, with the mean removal ratio reaching 96.24%. A free radical reaction was the main pathway of 1,2-DCA degradation; superoxide radical (O₂^•-) existed stably and played a key role in the reaction, and the main transformation proceeded via a vinyl chloride intermediate. The maximum removal of 1,2-DCA reached 91.79% in the in situ remediation. The developed technology exhibits important advantages in enabling flexible control over chemical dosages, long durations of effective activity, and rapid full-cycle remediation.

Thermal decomposition mechanism and kinetics of perfluorooctanoic acid (PFOA) and other perfluorinated carboxylic acids: a theoretical study

Perfluorinated carboxylic acids (PFCAs), particularly perfluorooctanoic acid (PFOA), are broadly used for chemical synthesis and as surfactants, but they pose a serious threat to humans and wildlife because of toxicity concerns, environmental stability, and tendency to bioaccumulate. PFCA waste is commercially treated in incinerators, however, their exact degradation mechanisms are still unknown. In the present work, we report the decomposition mechanism and kinetics of straight-chain PFCAs using quantum chemistry and reaction rate theory calculations. Degradation mechanisms and associated kinetic parameters are determined for the complete series of straight-chain PFCAs from perfluorononanoic acid (C₈F₁₇COOH, C9) to fluoroformic acid (FCOOH, C1). Our results show that PFCA decomposition follows an analogous mechanism to perfluorinated sulfonic acids, where HF elimination from the acid head group produces a three membered ring intermediate, in this case a perfluorinated α-lactone. These perfluorinated α-lactones are short-lived intermediates that readily degrade into perfluorinated acyl fluorides and CO, thus shortening the perfluorinated chain by one C atom. Because perfluorinated acyl fluorides are known to hydrolyse to PFCAs, repeated cycles of carboxylic acid decomposition followed by acyl fluoride hydrolysis provides a mechanism for the complete mineralization of PFCAs to HF, CO, CO₂, COF₂, and CF₂ during thermal decomposition in the presence of water vapor. These results provide a theoretical basis for future detailed chemical kinetic studies of incineration reactors and will assist in their design and optimisation so as to more efficiently decompose PFCAs and related waste.

PAH Growth in Flames and Space: Formation of the Phenalenyl Radical

Polycyclic aromatic hydrocarbons (PAHs) are intermediates in the formation of soot particles and interstellar grains. However, their formation mechanisms in combustion and interstellar environments are not fully understood. The production of tricyclic PAHs and, in particular, the conversion of a PAH containing a five-membered ring to one with a six-membered ring are of interest to explain PAH abundances in combustion processes. In the present work, resonant ionization mass spectrometry in conjunction with isotopic labeling is used to investigate the formation of the phenalenyl radical from acenaphthylene and methane in an electrical discharge. We show that in this environment the CH cycloaddition mechanism converts a five-membered ring to a six-membered ring. This mechanism can occur in tandem with other PAH formation mechanisms such as hydrogen abstraction/acetylene addition (HACA) to produce larger PAHs in flames and the interstellar medium.

Low-temperature oxidation of monobromobenzene: Bromine transformation and yields of phenolic species

Brominated benzenes and phenols constitute direct precursors in the formation of bromine-bearing pollutants; most notably PBDD/Fs and other dioxin-type compounds. Elucidating accurate mechanisms and constructing robust kinetic models for the oxidative transformation of bromobenzenes and bromophenols into notorious Br-toxicants entail a comprehensive understanding of their initial oxidation steps. However, pertinent mechanistic studies, based on quantum chemical calculations, have only focused on secondary condensation reactions into PBDD/Fs and PBDEs. Literature provide kinetic parameters for these significant reactions, nonetheless, without attempting to compile the acquired Arrhenius coefficients into kinetic models. To fill in this gap, this study sets out to illustrate primary chemical phenomena underpinning the low-temperature combustion of a monobromobenzene molecule (MBZ) based on a detail chemical kinetic model. The main aim is to map out temperature-dependent profiles for major intermediates and products. The constructed kinetic model encompasses several sub-mechanisms (i.e, HBr and benzene oxidation, bromination of phenoxy radicals, and initial reaction of oxygen molecules with MBZ). In light of germane experimental observations, the formulated kinetic model herein offers an insight into bromine speciation, conversion profile of MBZ, and formation of higher brominated congeners of benzene and phenol. For instance, the model satisfactorily accounts for the yields of dibromophenols from oxidation of a 2-bromophenol (2-MBP) molecule, in reference to analogous experimental measurements. From an environmental perspective, the model reflects the accumulation of appreciable loads of 2-bromophenoxy radicals at intermediate temperatures (i.e., a bromine-containing environmental persistent free radical, EPFR) from combustion of MBZ and 2-MBP molecules. Acquired mechanistic/kinetic parameters shall be useful in comprehending the complex bromine transformation chemistry in real scenarios, most notably those prevailing in thermal recycling of brominated flame retardants (BFRs).

Thermal Decomposition Kinetics of the Indenyl Radical: A Theoretical Study

Quantum chemistry and statistical reaction rate theory calculations have been performed to investigate the products and kinetics of indenyl radical decomposition. Three competitive product sets are identified, including formation of a cyclopentadienyl radical (c-C₅H₅) and diacetylene (C₄H₂), which has not been included in prior theoretical kinetics investigations. Rate coefficients for indenyl decomposition are determined from master equation simulations at 1800-2400 K and 0.01-100 atm, and temperature- and pressure-dependent rate coefficient expressions are incorporated into a detailed chemical kinetic model for indene pyrolysis. Indenyl is found to predominantly decompose to o-benzyne (o-C₆H₄) + propargyl (C₃H₃), with lesser amounts of fulvenallenyl (C₇H₅) + C₂H₂ and c-C₅H₅ + C₄H₂.

Reactivity Trends in the Gas-Phase Addition of Acetylene to the N-Protonated Aryl Radical Cations of Pyridine, Aniline, and Benzonitrile

A key step in gas-phase polycyclic aromatic hydrocarbon (PAH) formation involves the addition of acetylene (or other alkyne) to σ-type aromatic radicals, with successive additions yielding more complex PAHs. A similar process can happen for N-containing aromatics. In cold diffuse environments, such as the interstellar medium, rates of radical addition may be enhanced when the σ-type radical is charged. This paper investigates the gas-phase ion-molecule reactions of acetylene with nine aromatic distonic σ-type radical cations derived from pyridinium (Pyr), anilinium (Anl), and benzonitrilium (Bzn) ions. Three isomers are studied in each case (radical sites at the ortho, meta, and para positions). Using a room temperature ion trap, second-order rate coefficients, product branching ratios, and reaction efficiencies are measured. The rate coefficients increase from para to ortho positions. The second-order rate coefficients can be sorted into three groups: low, between 1 and 3 × 10^-12 cm³ molecule^-1 s^-1 (3Anl and 4Anl); intermediate, between 5 and 15 × 10^-12 cm³ molecule^-1 s^-1 (2Bzn, 3Bzn, and 4Bzn); and high, between 8 and 31 × 10^-11 cm³ molecule^-1 s^-1 (2Anl, 2Pyr, 3Pyr, and 4Pyr); and 2Anl is the only radical cation with a rate coefficient distinctly different from its isomers. Quantum chemical calculations, using M06-2X-D3(0)/6-31++G(2df,p) geometries and DSD-PBEP86-NL/aug-cc-pVQZ energies, are deployed to rationalize reactivity trends based on the stability of prereactive complexes. The G3X-K method guides the assignment of product ions following adduct formation. The rate coefficient trend can be rationalized by a simple model based on the prereactive complex forward barrier height.

Five vs. six membered-ring PAH products from reaction of o-methylphenyl radical and two C3H4 isomers

Gas-phase reactions of the o-methylphenyl (o-CH3C6H4) radical with the C3H4 isomers allene (H2C[double bond, length as m-dash]C[double bond, length as m-dash]CH2) and propyne (HC[triple bond, length as m-dash]C-CH3) are studied at 600 K and 4 Torr (533 Pa) using VUV synchrotron photoionisation mass spectrometry, quantum chemical calculations and RRKM modelling. Two major dissociation product ions arise following C3H4 addition: m/z 116 (CH3 loss) and 130 (H loss). These products correspond to small polycyclic aromatic hydrocarbons (PAHs). The m/z 116 signal for both reactions is conclusively assigned to indene (C9H8) and is the dominant product for the propyne reaction. Signal at m/z 130 for the propyne case is attributed to isomers of bicyclic methylindene (C10H10) + H, which contains a newly-formed methylated five-membered ring. The m/z 130 signal for allene, however, is dominated by the 1,2-dihydronaphthalene isomer arising from a newly created six-membered ring. Our results show that new ring formation from C3H4 addition to the methylphenyl radical requires an ortho-CH3 group - similar to o-methylphenyl radical oxidation. These reactions characteristically lead to bicyclic aromatic products, but the structure of the C3H4 co-reactant dictates the structure of the PAH product, with allene preferentially leading to the formation of two six-membered ring bicyclics and propyne resulting in the formation of six and five-membered bicyclic structures.

Pyrolysis of Triclosan and Its Chlorinated Derivatives

Triclosan (TCS) is a commonly used antimicrobial agent which persists in the environment and may undergo chlorination and/or photodegradation to produce toxic polychlorinated dibenzo-p-dioxins and polychlorinated benzenes. TCS accumulates in wastewater treatment biosolids, which may be used to fuel waste-to-energy plants, although little is known about the fate of TCS at high temperatures. Here, we have studied the thermal decomposition of TCS and chlorinated TCS derivatives in the gas phase using computational chemistry coupled with reaction rate theory calculations to predict rate coefficients and develop a chemical kinetic model to simulate TCS pyrolysis in a plug flow reactor. TCS is shown to interconvert with 4-chloro-2-(2,4-dichlorophenoxy)phenol (TCSi) with a relatively low barrier, achieving equilibrium at temperatures of around 900 K and above. Dissociation of TCS and TCSi proceeds in parallel with barriers of ca. 60-65 kcal/mol to produce dichlorodibenzo-p-dioxin chlorobenzoquinone isomers. Reactor simulations demonstrate that TCS incineration at a temperature of 1100 K or higher leads to the formation of toxic chlorinated aromatics.

Decomposition kinetics of perfluorinated sulfonic acids

Perfluorooctanesulfonic acid (PFOS) is a widespread and persistent pollutant of concern to human health and the environment. Although incineration is often used to treat material contaminated with PFOS and related per- and polyfluoroalkyl substances (PFAS), little is known about the precise chemical mechanism for the thermal decomposition of these substances of concern. Here, we present the first study of the thermal decomposition kinetics of PFOS and related perfluorinated acids, using computational chemistry and reaction rate theory methods. We discover that the preferred channel for PFOS decomposition is via an α-sultone that spontaneously decomposes to form perfluorooctanal and SO₂. At 1000 K the halflife for PFOS is predicted to be 0.2 s, decreasing sharply as temperature increases further. These results show that the acid headgroup in PFOS can be efficiently destroyed in incinerators operating at relatively modest temperatures. The new insights provided into the exact decomposition mechanism and kinetics of PFOS will help to improve remediation technologies actively under development.

Thermal decomposition kinetics of glyphosate (GP) and its metabolite aminomethylphosphonic acid (AMPA)

Glyphosate (GP) is a widely used herbicide worldwide, yet accumulation of GP and its main byproduct, aminomethylphosphonic acid (AMPA), in soil and water has raised concerns about its potential effects on human health. Thermal treatment, in which contaminants are vaporised and decomposed in the gas-phase, is one option for decontaminating material containing GP and AMPA, yet the thermal decomposition chemistry of these compounds remains poorly understood. Here, we have revealed the thermal decomposition mechanism of GP and AMPA in the gas phase by applying computational chemistry and reaction rate theory methods. The preferred decomposition channel for both substances involves the elimination of P(OH)3 to yield the imine N-methylene-glycine (from GP) or methanimine (from AMPA), with relatively low barrier heights (ca. 45 kcal mol-1). The half-life of GP and AMPA at 1000 K are predicted to be 0.1 and 4 ms respectively, and they should be readily destroyed via conventional incineration processes. The further decomposition of N-methylene-glycine is expected to also take place at similar temperatures, leading to N-methyl-methanimine + CO2, with a barrier height of ca. 48 kcal mol-1. The imine decomposition products of GP and AMPA are expected to react with water vapour to form simple amines and carbonyl compounds.

Mechanistic study of the reaction of CH2F2 with Cl atoms in the absence and presence of CH4 or C2H6: decomposition of CHF2OH and fate of the CHF2O radical

To assess the atmospheric fate of fluorinated compounds, chamber experiments were performed with Fourier transform infrared spectroscopy investigating the products of difluoromethane, CH2F2, at 296 ± 2 K. The reactions were initiated by reaction of CH2F2 with Cl atoms in the absence and presence of CH4 or C2H6 in air or O2. No evidence of formation of the fluorinated alcohol, CHF2OH, from the reactions of the CHF2O2 radical with either CH3O2 or CH3CH2O2 was observed. However, evidence of an alkoxy radical pathway was observed to form CHF2OH. The alkoxy radical, CHF2O, abstracts a hydrogen atom from CH2F2 (with reaction mixtures of high initial CH2F2 concentrations) to give the alcohol CHF2OH that in turn decomposes with a rate coefficient of k(CHF2OH) = (1.68 × 10-3 ± 0.19 × 10-3) s-1, giving a half-life of the alcohol of (412 ± 48) s. Theoretical calculations indicate that the CHF2OH decomposition is unlikely to be a unimolecular process, and we instead propose that it is catalyzed by -OH groups present in molecules, or on particles or surfaces. HC(O)F is formed in a yield indistinguishable from 100% from the decomposition of CHF2OH. The competition between the reaction of CHF2O radicals with O2 and with CH2F2 was investigated and an experimental rate coefficient ratio of 0.57 ± 0.08 of reaction with O2 over reaction with CH2F2 was determined. Ab initio calculations support a larger reaction barrier for the O2 reaction by 0.5 kcal mol-1, with transition state theory predicting a rate coefficient ratio of 0.35, in reasonable agreement with experiment. The primary product of the atmospheric degradation of CH2F2 is expected to be C(O)F2 formed by the reaction of CHF2O with O2.

Unveiling New Isomers and Rearrangement Routes on the C7H8+ Potential Energy Surface

The unimolecular reactions of C₇H₈⁺ radical cations are among those most studied by mass spectrometry, especially the rearrangement of toluene and cycloheptatriene molecular ions, which are directly connected to the formation of benzylium and tropylium cations. This study reveals important new isomers and isomerization pathways on the C₇H₈⁺ potential energy surface, through the application of gas-phase electronic photodissociation spectroscopy in conjunction with ab initio calculations. Presented are the first gas-phase vibrationally resolved electronic spectra of the o-isotoluene, norcaradiene, bicyclo[3.2.0]hepta-2,6-diene radical cations, and ring-opened products from cyclic C₇H₈⁺ species. The isomerization route from the norbornadiene radical cation to the toluene radical cation, which competes with isomerization to the bicyclo[2.2.1]hepta-2-ene-5-yl-7-ylium radical cation, is identified. Further, this work expands understanding of the C₇H₈⁺ potential energy surface by connecting spiro[2.4]hepta-4,6-diene and acyclic 1,2,4,6-heptatetraene radical cations, and confirms the important role of the o-isotoluene radical cation in the interconversion pathways of C₇H₈⁺ species.

Product detection study of the gas-phase oxidation of methylphenyl radicals using synchrotron photoionisation mass spectrometry

Reactions of ortho and meta-methylphenyl radicals with oxygen form products that depend acutely on the position of the methyl group.

Gas-Phase Oxidation of the Protonated Uracil-5-yl Radical Cation

This study targets the kinetics and product detection of the gas-phase oxidation reaction of the protonated 5-dehydrouracil (uracil-5-yl) distonic radical cation using ion-trap mass spectrometry. Protonated 5-dehydrouracil radical ions (5-dehydrouracilH⁺ radical ion, m/z 112) are produced within an ion trap by laser photolysis of protonated 5-iodouracil. Storage of the 5-dehydrouracilH⁺ radical ion in the presence of controlled concentration of O₂ reveals two main products. The major reaction product pathway is assigned as the formation of protonated 2-hydroxypyrimidine-4,5-dione (m/z 127) + ^•OH. A second product ion (m/z 99), putatively assigned as a five-member-ring ketone structure, is tentatively explained as arising from the decarbonylation (-CO) of protonated 2-hydroxypyrimidine-4,5-dione. Because protonation of the 5-dehydrouracil radical likely forms a dienol structure, the O₂ reaction at the 5 position is ortho to an -OH group. Following this addition of O₂, the peroxyl-radical intermediate isomerizes by H atom transfer from the -OH group. The ensuing hydroperoxide then decomposes to eliminate ^•OH radical. It is shown that this elimination of ^•OH radical (-17 Da) is evidence for the presence of an -OH group ortho to the initial phenyl radical site, in good accord with calculations. The subsequent CO loss mechanism, to form the aforementioned five-member-ring structure, is unclear, but some pathways are discussed. By following the kinetics of the reaction, the room temperature second-order rate coefficient of the 5-dehydrouracilH⁺ distonic radical cation with molecular oxygen is measured at 7.2 × 10^-11 cm³ molecule^-1 s^-1, Φ = 12% (with ±50% total accuracy). For aryl radical reactions with O₂, the presence of the ^•OH elimination product pathway, following the peroxyl-radical formation, is an indicator of an -OH group ortho to the radical site.

G4CEP: A G4 theory modification by including pseudopotential for molecules containing first-, second- and third-row representative elements

The G4CEP composite method was developed from the respective G4 all-electron version by considering the implementation of compact effective pseudopotential (CEP). The G3/05 test set was used as reference to benchmark the adaptation by treating in this work atoms and compounds from the first and second periods of the periodic table, as well as representative elements of the third period, comprising 440 thermochemical data. G4CEP has not reached a so high level of accuracy as the G4 all-electron theory. G4CEP presented a mean absolute error around 1.09 kcal mol(-1), while the original method presents a deviation corresponding to 0.83 kcal mol(-1). The similarity of the optimized molecular geometries between G4 and G4CEP indicates that the core-electron effects and basis set adjustments may be pointed out as a significant factor responsible for the large discrepancies between the pseudopotential results and the experimental data, or even that the all-electron calculations are more efficient either in its formulation or in the cancellation of errors. When the G4CEP mean absolute error (1.09 kcal mol(-1)) is compared to 1.29 kcal mol(-1) from G3CEP, it does not seem so efficient. However, while the G3CEP uncertainty is ±4.06 kcal mol(-1), the G4CEP deviation is ±2.72 kcal mol(-1). Therefore, the G4CEP theory is considerably more reliable than any previous combination of composite theory and pseudopotential, particularly for enthalpies of formation and electron affinities.

Gas-Phase Mechanisms of the Reactions of Reduced Organic Nitrogen Compounds with OH Radicals

Research on the fate of reduced organic nitrogen compounds in the atmosphere has gained momentum since the identification of their crucial role in particle nucleation and the scale up of carbon capture and storage technology which employs amine-based solvents. Reduced organic nitrogen compounds have strikingly different lifetimes against OH radicals, from hours for amines to days for amides to years for isocyanates, highlighting unique functional group reactivity. In this work, we use ab initio methods to investigate the gas-phase mechanisms governing the reactions of amines, amides, isocyanates and carbamates with OH radicals. We determine that N-H abstraction is only a viable mechanistic pathway for amines and we identify a reactive pathway in amides, the formyl C-H abstraction, not currently considered in structure-activity relationship (SAR) models. We then use our acquired mechanistic knowledge and tabulated literature experimental rate coefficients to calculate SAR factors for reduced organic nitrogen compounds. These proposed SAR factors are an improvement over existing SAR models because they predict the experimental rate coefficients of amines, amides, isocyanates, isothiocyanates, carbamates and thiocarbamates with OH radicals within a factor of 2, but more importantly because they are based on a sound fundamental mechanistic understanding of their reactivity.

Formation and stability of gas-phase o-benzoquinone from oxidation of ortho-hydroxyphenyl: a combined neutral and distonic radical study

The o-hydroxyphenyl radical reacts with O₂ to form o-benzoquinone + OH and cyclopentadienone is assigned as a secondary product.

Are the three hydroxyphenyl radical isomers created equal?--The role of the phenoxy radical

We have investigated the thermal decomposition of the three hydroxyphenyl radicals (˙C6H4OH) in a heated microtubular reactor. Intermediates and products were identified isomer-selectively applying photoion mass-selected threshold photoelectron spectroscopy with vacuum ultraviolet synchrotron radiation. Similarly to the phenoxy radical (C6H5-O˙), hydroxyphenyl decomposition yields cyclopentadienyl (c-C5H5) radicals in a decarbonylation reaction at elevated temperatures. This finding suggests that all hydroxyphenyl isomers first rearrange to form phenoxy species, which subsequently decarbonylate, a mechanism which we also investigate computationally. Meta- and para-radicals were selectively produced and spectroscopically detectable, whereas the ortho isomer could not be traced due to its fast rethermalization and rapid decomposition in the reactor. A smaller barrier to isomerization to phenoxy was found to be the reason for this observation. Since hydroxyphenyl species may be present under typical sooting conditions in flames, the resonantly stabilized cyclopentadienyl radical adds to the hydrocarbon pool and can contribute to the formation of polycyclic aromatic hydrocarbons, which are precursors in soot formation.

A Theoretical Study of the Photoisomerization of Glycolaldehyde and Subsequent OH Radical-Initiated Oxidation of 1,2-Ethenediol

It has recently been discovered that carbonyl compounds can undergo UV-induced isomerization to their enol counterparts under atmospheric conditions. This study investigates the photoisomerization of glycolaldehyde (HOCH2CHO) to 1,2-ethenediol (HOCH═CHOH) and the subsequent (•)OH-initiated oxidation chemistry of the latter using quantum chemical calculations and stochastic master equation simulations. The keto-enol tautomerization of glycolaldehyde to 1,2-ethenediol is associated with a barrier of 66 kcal mol(-1) and involves a double-hydrogen shift mechanism to give the lower-energy Z isomer. This barrier lies below the energy of the UV/vis absorption band of glycolaldehyde and is also considerably below the energy of the products resulting from photolytic decomposition. The subsequent atmospheric oxidation of 1,2-ethenediol by (•)OH is initiated by addition of the radical to the π system to give the (•)CH(OH)CH(OH)2 radical, which is subsequently trapped by O2 to form the peroxyl radical (•)O2CH(OH)CH(OH)2. According to kinetic simulations, collisional deactivation of the latter is negligible and cannot compete with rapid fragmentation reactions, which lead to (i) formation of glyoxal hydrate [CH(OH)2CHO] and HO2(•) through an α-hydroxyl mechanism (96%) and (ii) two molecules of formic acid with release of (•)OH through a β-hydroxyl pathway (4%). Phenomenological rate coefficients for these two reaction channels were obtained for use in atmospheric chemical modeling. At tropospheric (•)OH concentrations, the lifetime of 1,2-ethenediol toward reaction with (•)OH is calculated to be 68 h.

Benchmarking Compound Methods (CBS-QB3, CBS-APNO, G3, G4, W1BD) against the Active Thermochemical Tables: A Litmus Test for Cost-Effective Molecular Formation Enthalpies

The theoretical atomization energies of some 45 CxHyOz molecules present in the Active Thermochemical Tables compilation and of particular interest to the combustion chemistry community have been computed using five composite model chemistries as titled. The species contain between 1-8 "heavy" atoms, and a few are conformationally diverse with up to nine conformers. The enthalpies of formation at 0 and 298.15 K are then derived via the atomization method and compared against the recommended values. In general, there is very good agreement between our averaged computed values and those in the ATcT; those for 1,3-cyclopentadiene exceptionally differ considerably, and we show from isodesmic reactions that the true value for 1,3-cyclopentadiene is closer to 134 kJ mol(-1) than the reported 101 kJ mol(-1). If one is restricted to using a single method, statistical measures indicate that the best methods are in the rank order G3 ≈ G4 > W1BD > CBS-APNO > CBS-QB3. The CBS-x methods do on average predict ΔfH(⊖)(298.15 K) within ≈5 kJ mol(-1) but are prone to occasional lapses. There are statistical advantages to be gained from using a number of methods in tandem, and all possible combinations have been tested. We find that the average formation enthalpy coming from using CBS-APNO/G4, CBS-APNO/G3, and G3/G4 show lower mean signed and mean unsigned errors, and lower standard and root-mean-squared deviations, than any of these methods in isolation. Combining these methods also leads to the added benefit of providing an uncertainty rooted in the chemical species under investigation. In general, CBS-APNO and W1BD tend to underestimate the formation enthalpies of target species, whereas CBS-QB3, G3, and G4 have a tendency to overestimate the same. Thus, combining CBS-APNO with a G3/G4 combination leads to an improvement in all statistical measures of accuracy and precision, predicting the ATcT values to within 0.14 ± 4.21 kJ mol(-1), thus rivalling "chemical accuracy" (±4.184 kJ mol(-1)) without the excessive cost associated with higher-level methods such as W1BD.

The reactions of N-methylformamide and N,N-dimethylformamide with OH and their photo-oxidation under atmospheric conditions: experimental and theoretical studies

The atmospheric oxidation of amides is studied with a combination of laser photolysis and smog chamber experiments along with quantum chemical and statistical rate theory calculations.

Molecular salt effects in the gas phase: tuning the kinetic basicity of [HCCLiCl]⁻ and [HCCMgCl₂]⁻ by LiCl and MgCl₂

A combination of gas-phase ion-molecule reaction experiments and theoretical kinetic modeling is used to examine how a salt can influence the kinetic basicity of organometallates reacting with water. [HC≡CLiCl](-) reacts with water more rapidly than [HC≡CMgCl2](-), consistent with the higher reactivity of organolithium versus organomagnesium reagents. Addition of LiCl to [HC≡CLiCl](-) or [HC≡CMgCl2](-) enhances their reactivity towards water by a factor of about 2, while addition of MgCl2 to [HC≡CMgCl2](-) enhances its reactivity by a factor of about 4. Ab initio calculations coupled with master equation/RRKM theory kinetic modeling show that these reactions proceed via a mechanism involving formation of a water adduct followed by rearrangement, proton transfer, and acetylene elimination as either discrete or concerted steps. Both the energy and entropy requirements for these elementary steps need to be considered in order to explain the observed kinetics.

Experimental and theoretical understanding of the gas phase oxidation of atmospheric amides with OH radicals: kinetics, products, and mechanisms

Atmospheric amides have primary and secondary sources and are present in ambient air at low pptv levels. To better assess the fate of amides in the atmosphere, the room temperature (298 ± 3 K) rate coefficients of five different amides with OH radicals were determined in a 1 m(3) smog chamber using online proton-transfer-reaction mass spectrometry (PTR-MS). Formamide, the simplest amide, has a rate coefficient of (4.44 ± 0.46) × 10(-12) cm(3) molec(-1) s(-1) against OH, translating to an atmospheric lifetime of ∼1 day. N-methylformamide, N-methylacetamide and propanamide, alkyl versions of formamide, have rate coefficients of (10.1 ± 0.6) × 10(-12), (5.42 ± 0.19) × 10(-12), and (1.78 ± 0.43) × 10(-12) cm(3) molec(-1) s(-1), respectively. Acetamide was also investigated, but due to its slow oxidation kinetics, we report a range of (0.4-1.1) × 10(-12) cm(3) molec(-1) s(-1) for its rate coefficient with OH radicals. Oxidation products were monitored and quantified and their time traces were fitted using a simple kinetic box model. To further probe the mechanism, ab initio calculations are used to identify the initial radical products of the amide reactions with OH. Our results indicate that N-H abstractions are negligible in all cases, in contrast to what is predicted by structure-activity relationships. Instead, the reactions proceed via C-H abstraction from alkyl groups and from formyl C(O)-H bonds when available. The latter process leads to radicals that can readily react with O2 to form isocyanates, explaining the detection of toxic compounds such as isocyanic acid (HNCO) and methyl isocyanate (CH3NCO). These contaminants of significant interest are primary oxidation products in the photochemical oxidation of formamide and N-methylformamide, respectively.

Atmospheric chemistry of enols: a theoretical study of the vinyl alcohol + OH + O(2) reaction mechanism

Enols are emerging as trace atmospheric components that may play a significant role in the formation of organic acids in the atmosphere. We have investigated the hydroxyl radical ((•)OH) initiated oxidation chemistry of the simplest enol, vinyl alcohol (ethenol, CH2═CHOH), using quantum chemical calculations and energy-grained master equation simulations. A lifetime of around 4 h was determined for vinyl alcohol in the presence of tropospheric levels of (•)OH. The reaction proceeds by (•)OH addition at both the α (66%) and β (33%) carbons of the π-system, yielding the C-centered radicals (•)CH2CH(OH)2, and HOCH2C(•)HOH, respectively. Subsequent trapping by O2 leads to the respective peroxyl radicals. About 90% of the chemically activated population of the major peroxyl radical adduct (•)O2CH2CH(OH)2 is predicted to undergo fragmentation to produce formic acid and formaldehyde, with regeneration of (•)OH. The minor peroxyl radical HOCH2C(OO(•))HOH is even less stable and undergoes almost exclusive HO2(•) elimination to form glycolaldehyde (HOCH2CHO). Formation of the latter has not been proposed before in the oxidation of vinyl alcohol. A kinetic mechanism for use in atmospheric modeling is provided, featuring phenomenological rate coefficients for formation of the three main product channels ((•)O2CH2CH(OH)2 [8%]; HC(O)OH + HCHO + (•)OH [56%]; HOCH2CHO + HO2(•) [37%]). Our study supports previous findings that vinyl alcohol should be rapidly removed from the atmosphere by reaction with (•)OH and O2 with glycolaldehyde being identified as a previously unconsidered product. Most importantly, it is shown that direct chemically activated reactions can lead to (•)OH and HO2(•) (HOx) recycling.

Formation of nitrosamines and alkyldiazohydroxides in the gas phase: the CH3NH + NO reaction revisited

Aminyl free radicals of the form RN(•)H are formed in the photochemical oxidation of primary amines, and their reaction with (•)NO is an important tropospheric sink. Reaction of the parent methylamidogen radical (CH3N(•)H) with (•)NO in the gas phase has been studied using quantum chemical techniques and RRKM theory/master equation based kinetic modeling. Calculations with the G3X-K composite theoretical method indicate that reaction proceeds via exothermic formation of a primary nitrosamine intermediate, CH3NHNO, which can isomerize to an alkyldiazohydroxide, CH3NNOH, and further eliminate water to form diazomethane, CH2NN. Master equation simulations conducted at tropospheric conditions identify that the collisionally stabilized CH3NHNO and CH3NNOH isomers are the major reaction products, with smaller yields of CH2NN + H2O. A previously proposed mechanism in which the primary nitrosamine is destroyed via isomerization to CH2NHNOH, followed by reaction with O2 to produce CH2NH + HO2(•) + (•)NO, is disproved. In the atmosphere, CH2NN may be formed with sufficient vibrational energy to directly dissociate to singlet methylene ((1)CH2) and N2, whereas under combustion conditions this is expected to be the dominant pathway. This study suggests that stabilized primary nitrosamines can indeed form in the photochemical oxidation of amines, along with alkyldiazohydroxides and diazoalkanes. Both classes of compound are potent alkylating agents that may need to be considered in future atmospheric studies.