Initiation Chemistries in Hydrocarbon (Aut)Oxidation

For the (aut)oxidation of toluene to benzyl hydroperoxide, benzyl alcohol, benzaldehyde, and benzoic acid, the thermochemical profiles for various radical‐generating reactions have been compared. A key intermediate in all of these reactions is benzyl hydroperoxide, the heat of formation of which has been estimated by using results from CBS‐QB3, G4, and G3B3 calculations. Homolytic O?O bond cleavage in this hydroperoxide is strongly endothermic and thus unlikely to contribute significantly to initiation processes. In terms of reaction enthalpies the most favorable initiation process involves bimolecular reaction of benzyl hydroperoxide to yield hydroxy and benzyloxy radicals along with water and benzaldehyde. The reaction enthalpy and free energy of this process is significantly more favorable than those for the unimolecular dissociation of known radical initiators, such as dibenzoylperoxide or dibenzylhyponitrite.     

Ab Initio energetics of Si?O bond cleavage

A multilevel approach that combines high‐level ab initio quantum chemical methods applied to a molecular model of a single, strain‐free Si O Si bridge has been used to derive accurate energetics for Si O bond cleavage. The calculated Si O bond dissociation energy and the activation energy for water‐assisted Si O bond cleavage of 624 and 163 kJ mol⁻¹, respectively, are in excellent agreement with values derived recently from experimental data. In addition, the activation energy for H₂O‐assisted Si O bond cleavage is found virtually independent of the amount of water molecules in the vicinity of the reaction site. The estimated reaction energy for this process including zero‐point vibrational contribution is in the range of −5 to 19 kJ mol⁻¹. © 2017 Wiley Periodicals, Inc.     

Gas-Phase Synthesis of 3-Vinylcyclopropene via the Crossed Beam Reaction of the Methylidyne Radical (CH; X2Π) with 1,3-Butadiene (CH2CHCHCH2; X1Ag)

The crossed molecular beam reactions of the methylidyne radical (CH; X²Π) with 1,3-butadiene (CH₂CHCHCH₂; X¹A_g) along with their (partially) deuterated counterparts were performed at collision energies of 20.8 kJ mol⁻¹ under single collision conditions. Combining our laboratory data with ab initio calculations, we reveal that the methylidyne radical may add barrierlessly to the terminal carbon atom and/or carbon−carbon double bond of 1,3-butadiene, leading to doublet C₅H₇ intermediates with life times longer than the rotation periods. These collision complexes undergo non-statistical unimolecular decomposition through hydrogen atom emission yielding the cyclic cis- and trans-3-vinyl-cyclopropene products with reaction exoergicities of 119±42 kJ mol⁻¹. Since this reaction is barrierless, exoergic, and all transition states are located below the energy of the separated reactants, these cyclic C₅H₆ products are predicted to be accessed even in low-temperature environments, such as in hydrocarbon-rich atmospheres of planets and cold molecular clouds such as TMC-1.     

Chemistry of NOx and HNO3 Molecules with Gas-Phase Hydrated O.− and OH− Ions

The gas-phase reactions of O ^{. −}(H₂O)_n and OH⁻(H₂O)_n, n=20–38, with nitrogen-containing atmospherically relevant molecules, namely NO_x and HNO₃, are studied by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry and theoretically with the use of DFT calculations. Hydrated O ^{. −} anions oxidize NO ^. and NO₂ ^. to NO₂⁻ and NO₃⁻ through a strongly exothermic reaction with enthalpy of −263±47 kJ mol⁻¹ and −286±42 kJ mol⁻¹, indicating a covalent bond formation. Comparison of the rate coefficients with collision models shows that the reactions are kinetically slow with 3.3 and 6.5 % collision efficiency. Reactions between hydrated OH⁻ anions and nitric oxides were not observed in the present experiment and are most likely thermodynamically hindered. In contrast, both hydrated anions are reactive toward HNO₃ through proton transfer from nitric acid, yielding hydrated NO₃⁻. Although HNO₃ is efficiently picked-up by the water clusters, forming (HNO₃)_0–2(H₂O)_mNO₃⁻ clusters, the overall kinetics of nitrate formation are slow and correspond to an efficiency below 10 %. Combination of the measured reaction thermochemistry with literature values in thermochemical cycles yields ΔH_f(O⁻(aq.))=48±42 kJ mol⁻¹ and ΔH_f(NO₂⁻(aq.))=−125±63 kJ mol⁻¹.     

Kinetics of the gas‐phase elimination of α‐Bromophenylacetic acid under maximum inhibition

The gas phase elimination kinetics of the title compound was studied over the temperature range of 260.1–315.0°C and pressure range of 20–70 Torr. This elimination, in seasoned static reaction system and in the presence of at least fourfold of the free radical inhibitor toluene, is homogeneous, unimolecular and follows a first‐order rate law. The reaction yielded mainly benzaldehyde, CO, and HBr, and small amounts of benzylbromide and CO₂. The observed rate coefficients are expressed by the following Arrhenius equations: For benzaldehyde formation: log k₁ (s⁻¹) = (12.23 ± 0.26) − (164.9 ± 2.7) kJ mol⁻¹ (2.303 RT)⁻¹ For benzylbromide formation: log k₁ (s⁻¹) = (13.82 ± 0.50) − (192.8 ± 5.5) kJ mol⁻¹ (2.303 RT)⁻¹ The mechanisms are believed to proceed through a semi‐polar five‐membered cyclic transition state for the benzaldehyde formation, while a four‐centered cyclic transition state for benzylbromide formation. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 725–728, 1999     

An Intramolecular Hydrogen-Shift in a Peroxy Radical at Cryogenic Temperatures: The Reaction of 2-Hydroxyphenyl Radical with O2

Peroxy radical hydrogen-shifts are pivotal elementary reaction steps in the oxidation of small hydrocarbons in autoignition and the lower atmosphere. Although these reactions are typically associated with a substantial barrier, we demonstrate that the [1,5]H-shift in the peroxy species derived from the 2-hydroxyphenyl radical 1 is so facile that it even proceeds rapidly in an argon matrix at 35 K through a proton-coupled electron transfer mechanism. Hydrogen-bound complexes of o-benzoquinone are identified as the main reaction products by infrared spectroscopy although their formation through O−O bond scission is hampered by a barrier of 11.9 kcal mol⁻¹ at the ROCCSD(T)/cc-pVTZ/UB3LYP/6–311G(d,p) level of theory.     

The kinetics and mechanisms of gas phase elimination of primary,secondary, and tertiary 2‐hydroxyalkylbenzenes

2‐Phenylethanol, racemic 1‐phenyl‐2‐propanol, and 2‐methyl‐1‐phenyl‐2‐propanol have been pyrolyzed in a static system over the temperature range 449.3–490.6°C and pressure range 65–198 torr. The decomposition reactions of these alcohols in seasoned vessels are homogeneous, unimolecular, and follow a first‐order rate law. The Arrhenius equations for the overall decomposition and partial rates of products formation were found as follows: for 2‐phenylethanol, overall rate log k₁(s⁻¹)=12.43−228.1 kJ mol⁻¹ (2.303 RT)⁻¹, toluene formation log k₁(s⁻¹)=12.97−249.2 kJ mol⁻¹ (2.303 RT)⁻¹, styrene formation log k₁(s⁻¹)=12.40−229.2 kJ mol⁻¹(2.303 RT)⁻¹, ethylbenzene formation log k₁(s⁻¹)=12.96−253.2 kJ mol⁻¹(2.303 RT)⁻¹; for 1‐phenyl‐2‐propanol, overall rate log k₁(s⁻¹)=13.03−233.5 kJ mol⁻¹(2.303 RT)⁻¹, toluene formation log k₁(s⁻¹)=13.04−240.1 kJ mol⁻¹(2.303 RT)⁻¹, unsaturated hydrocarbons+indene formation log k₁(s⁻¹)=12.19−224.3 kJ mol⁻¹(2.303 RT)⁻¹; for 2‐methyl‐1‐phenyl‐2‐propanol, overall rate log k₁(s⁻¹)=12.68−222.1 kJ mol⁻¹(2.303 RT)⁻¹, toluene formation log k₁(s⁻¹)=12.65−222.9 kJ mol⁻¹(2.303 RT)⁻¹, phenylpropenes formation log k₁(s⁻¹)=12.27−226.2 kJ mol⁻¹(2.303 RT)⁻¹. The overall decomposition rates of the 2‐hydroxyalkylbenzenes show a small but significant increase from primary to tertiary alcohol reactant. Two competitive eliminations are shown by each of the substrates: the dehydration process tends to decrease in relative importance from the primary to the tertiary alcohol substrate, while toluene formation increases. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 401–407, 1999     

The gas-phase ozonolysis reaction of methylbutenol: A mechanistic study

The gas-phase ozonolysis reaction of methylbutenol through the Criegee mechanism is investigated. The initial reaction leads to a primary ozonide (POZ) formation with barriers in the range of 10–28 kJ mol⁻¹. The formation of 2-hydroxy-2-methyl-propanal (HMP) and formaldehyde-oxide is more favorable, by 10 kJ mol⁻¹, than the syn-CI and formaldehyde. The unimolecular dissociation of the more stable syn-CI via 1,5-H transfer into an epoxide is more favored than the epoxide and ³O₂ formation. The ester channel led to the formation of the acetone and formic acid favorably from the anti-CI. The hydration of the anti-CI with H₂O and (H₂O)₂ is significantly barrierless with a higher plausibility to the latter, and thus they may lead to the formation of peroxides and ultimately OH radicals, as well as airborne particulate matter. Reaction of anti-CI with water dimers enhances its atmospheric reactivity by a factor of 28 in reference to water monomers.     

Gold Nanoparticles Formation via Au(III) Complex Ions Reduction with l‐Ascorbic Acid

In this paper, the kinetics and mechanism of gold nanoparticles formation during the redox reaction between [AuCl₄]− complex and l ‐ascorbic acid under different conditions were described. It was also shown that reagent concentration, chloride ions, and pH influence kinetics of nucleation and growth. To establish rate constants of these stages, the model of Finke and Watzky was applied. From Arrhenius and Eyring dependencies, the values of activation energy (22.5 kJ mol⁻¹ for the nucleation step and 30.3 kJ mol⁻¹ for the growth step), entropy (about −228 J K⁻¹ mol⁻¹ for the nucleation step and −128 J K⁻¹ mol⁻¹ for the growth step), and enthalpy (19.8 kJ mol⁻¹ for nucleation and 27.8 kJ mol⁻¹ for particles growth) were determined. It was also shown that the disproporationation reaction had influence on the rate of nanoparticles formation and may have impact on final particles morphology.     

Thermochemistry and Kinetics of the Thermal Decomposition of 1‐Chlorohexane

A theoretical kinetic study of the thermal decomposition of 1‐chlorohexane in gas phase between 600 and 1000 K was performed. Transition‐state theory and unimolecular reaction rate theory were combined with molecular information provided by quantum chemical calculations. Particularly, the B3LYP, BMK, M05–2X, and M06–2X formulations of the density functional theory (DFT) and the high‐level ab initio methods G3B3 and G4 were employed. The possible reaction channels for the thermal decomposition of 1‐chlorohexane were investigated, and the reaction takes place through the elimination of HCl with the formation of 1‐hexene. The derived high‐pressure limit rate coefficients are k _∞(600–1000 K) = (8 ± 5) × 10¹³ exp[‐((56.7 ± 0.4) kcal mol⁻¹/RT )] s⁻¹. The pressure effect over the reaction was analyzed from the calculation of the low‐pressure limit rate coefficients and the falloff curves. In addition, the standard enthalpies of formation at 298 K of −46.9 ± 1.5 kcal mol⁻¹ for 1‐chlorohexane and 5.8 ± 1.5 kcal mol⁻¹ for C₆H₁₃ radical were derived from isodesmic and isogiric reactions at high levels of theory.     

Mechanistic and kinetic study on the ozonolysis of 2,4-hexadienedial

The formation and unimolecular reactions of primary ozonides and carbonyl oxides arising from the O₃-initiated reactions of 2,4-hexadienedial (HDE) have been investigated using the density functional theory and ab initio method. The activation energies of O₃ cycloaddition to the >C=C< and >C=O bonds of HDE for the formation primary ozonides (POZ1 and POZ2) are 4.79 and 21.37 kcal mol^?1, respectively, implying that the initial O₃ to the >C=C< bond is favorable pathway. Cleavage of POZ1 to form carbonyl oxides occurs with a barrier of 12.19–21.35 kcal mol^?1, and the decomposition energies range from ?1.09 to ?15.75 kcal mol^?1. The CHOCHOO radical, the hydroxyl radical (OH) formation via H-migration is more favorable than the dioxirane formation via rearrangement. However, the CHOCH=CHCHOO radical, the dioxirane formation via rearrangement is more favorable than OH formation. Using the transition state theory, the rate constants of formation of POZ1 and POZ2 are 1.49 × 10^?19 and 6.03 × 10^?25 cm³ molecule^?1 s^?1 at 300 K, respectively. This study shows that the hyperconjugative effect makes O₃ addition to >C=C< and >C=O bonds of HDE more difficult than to >C=C< bond of ethylene and isoprene and to >C=O bond of formaldehyde. The largest rate constants of OH formation and dioxirane formation in the unimolecular reactions of carbonyl oxides are 6.13 × 10^?4 and 7.93 × 10^?1 s^?1 at 300 K, respectively. The dioxirane is main product in the unimolecular reaction of the carbonyl oxides arising from the O₃-initiated reaction of HDE.     

Mechanistic Insight into Peptidyl-Cysteine Oxidation by the Copper-Dependent Formylglycine-Generating Enzyme

The copper-dependent formylglycine-generating enzyme (FGE) catalyzes the oxygen-dependent oxidation of specific peptidyl-cysteine residues to formylglycine. Our QM/MM calculations provide a very likely mechanism for this transformation. The reaction starts with dioxygen binding to the tris-thiolate Cu^I center to form a triplet Cu^II-superoxide complex. The rate-determining hydrogen atom abstraction involves a triplet-singlet crossing to form a Cu^II−OOH species that couples with the substrate radical, leading to a Cu^I-alkylperoxo intermediate. This is accompanied by proton transfer from the hydroperoxide to the S atom of the substrate via a nearby water molecule. The subsequent O−O bond cleavage is coupled with the C−S bond breaking that generates the formylglycine and a Cu^II-oxyl complex. Moreover, our results suggest that the aldehyde oxygen of the final product originates from O₂, which will be useful for future experimental work.     

2‐(2‐Oxazolin‐2‐yl)benzene‐1,4‐diol: X‐ray and density functional theory studies

In the crystal structure of the title compound, C₉H₉NO₃, there are strong intra­molecular O—H⋯N and inter­molecular O—H⋯O hydrogen bonds which, together with weak inter­molecular C—H⋯O hydrogen bonds, lead to the formation of infinite chains of mol­ecules. The calculated inter­molecular hydrogen‐bond energies are −11.3 and −2.7 kJ mol⁻¹, respectively, showing the dominant role of the O—H⋯O hydrogen bonding. A natural bond orbital analysis revealed the electron contribution of the lone pairs of the oxazoline N and O atoms, and of the two hydr­oxy O atoms, to the order of the relevant bonds.     

Studies on the oxidation mechanism of H2S based on direct examination of the key reactions

By conducting an excimer laser photolysis (193 and 248 nm) behind shock waves, three elementary reactions important in the oxidation of H₂S have been examined, where, H, O, and S atoms have been monitored by the atomic resonance absorption spectrometry. For HS + O₂ → products (1), the rate constants evaluated by numerical simulations are summarized as: k₁ = 3.1 × 10⁻¹¹exp|-75 kJ mol⁻¹/RT| cm³molecule⁻¹s⁻¹ (T = 1400-1850 K) with an uncertainty factor of about 2. Direct measurements of the rate constants for S + O2 → SO + O (2), and SO + O₂ → SO₂ + O (3) yield k₂ = (2.5 ± 0.6) × 10⁻¹¹ exp|-(15.3 ± 2.5) kJ mol⁻¹/RT| cm³molecule⁻¹s⁻¹ (T = 980-1610 K) and, k₃ = (1.7 ± 0.9) × 10⁻¹² exp|-(34 ± 11) kJ mol⁻¹/RT| cm³molecule⁻¹s⁻¹ (T = 1130-1640 K), respectively. By summarizing these data together with the recent experimental results on the H(SINGLE BOND)S(SINGLE BOND)O reaction systems, a new kinetic model for the H₂S oxidation process is constructed. It is found that this simple reaction scheme is consistent with the experimental result on the induction time of SO₂ formation obtained by Bradley and Dobson. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 57–66, 1997.     

Molecular structure,conformational mobility,vibrational spectra,and thermochemistry of peroxyacetic acid: an ab initio and density functional study

The structure of the peroxyacetic acid (PAA) molecule and its conformational mobility under rotation about the peroxide bond was studied by ab initio and density functional methods. The free rotation is hindered by the trans-barrier of height 22.3 kJ mol^–1. The equilibrium molecular structure of AcOOH (C _s symmetry) is a result of intramolecular hydrogen bond. The high energy of hydrogen bonding (46 kJ mol^–1 according to natural bonding orbital analysis) hampers formation of intermolecular associates of AcOOH in the gas and liquid phases. The standard enthalpies of formation for AcOOH (–353.2 kJ mol^–1) and products of radical decomposition of the peroxide — AcO^· (–190.2 kJ mol^–1) and AcOO^· (–153.4 kJ mol^–1) — were determined by the G2 and G2(MP2) composite methods. The O—H and O—O bonds in the PAA molecule (bond energies are 417.8 and 202.3 kJ mol^–1, respectively) are much stronger than in alkyl hydroperoxide molecules. This provides an explanation for substantial contribution of non-radical channels of the decomposition of peroxyacetic acid. The electron density distribution and gas-phase acidity of PAA were determined. The transition states of the ethylene and cyclohexene epoxidation reactions were located (E _a = 71.7 and 50.9 kJ mol^–1 respectively).     

Cleavage of a P=P Double Bond Mediated by N-Heterocyclic Carbenes

The reaction of the bulky diphosphenes (Rind)P=P(Rind) ( 1 ; Rind=1,1,3,3,5,5,7,7-octa-R-substituted s-hydrindacen-4-yl) with two molecules of N-heterocyclic carbene (NHC; 1,3,4,5-tetramethylimidazol-2-ylidene) resulted in the quantitative formation of the NHC-bound phosphinidenes NHC→P(Rind) ( 2 ), along with the cleavage of the P=P double bond. The reaction times are dependent on the steric size of the Rind groups (11 days for 2 a (R=Et) and 2 h for 2 b (R=Et, Me) at room temperature). The mechanism for the double bond-breaking is proposed to proceed via the formation of the NHC-coordinated, highly polarized diphospehenes 3 as an intermediate. Approach of a second NHC to 3 induces P−P bond cleavage and P−C bond formation, which proceeds through a transition state with a large negative Gibbs energy change to afford the two molecules of 2 , thus being the rate-determining step of the overall reaction with the activation barriers of 80.4 for 2 a and 29.1 kJ mol⁻¹ for 2 b .     

Thermodynamic properties of potassium metasilicate (K2SiO3)

The standard enthalpy of formation of potassium metasilicate (K₂SiO₃), determined by hydrofluoric acid solution calorimetry, was found to be ΔH^o_f,298 = −363.866±0.421 kcal mol⁻¹ (−1522.415±1.762 kj mol⁻¹). The standard enthalpy of formation from the oxides was found to beΔH^o₂₉₈ = −64.786±0.559 kcal mol⁻¹ (−271.065±2.339 kJ mol⁻¹).These experimentally determined data were combined with data from the literature to calculate the Gibbs energies of formation and equilibrium constants of formation over the temperature range of the literature data. The standard enthalpies of formation and Gibbs energies of formation are given as functions of temperature. The standard Gibbs energy of formation is ΔG_f,298.15⁰ = −341.705 kcal mol⁻¹ (−1429.694 kJ mol⁻¹).     

Protonation of CO2, COS,CS2: Proton affinities and the structures of the protonated species

The geometric structures of a number of isomers of the ions formed by protonation of CO₂, COS and CS₂, and of the parent molecules themselves, have been fully optimized using ab initio quantum chemical methods. Stable minima have been found both for molecules protonated at the terminal atom and at the central carbon atom; ions of the latter type show strong near-degeneracy effects which have been ignored in previous calculations. Proton affinities of CO₂, COS and CS₂ have been calculated: for CO₂ the theoretical result (565 kJ mol⁻¹) is in excellent agreement with experiment (540 kJ mol⁻¹), given that the experimental proton affinity includes a contribution from zero-point vibration of ≈ −27 kJ mol⁻¹. For COS, for which no experimental value is available, the calculations give almost identical results for both O and S protonated species (619 and 636 kJ mol⁻¹, respectively). It may not therefore be possible to distinguish these two isomers experimentally. The theoretical result for CS₂ (678 kJ mol⁻) suggests that the current experimental value of the proton affinity (699 kJ mol⁻¹) is too high, since this value includes a zero-point vibration contribution of some −19 kJ mol⁻¹).     

Thermochemical properties from ab initio calculations: π- and σ-Free radicals of importance in soot formation: •C3H3 (propargyl), •C4H3, •C13H9 (phenalenyl), •C6H5 (phenyl), •C10H7 (naphthyl), •C14H9 (anthryl), •C14H9 (phenanthryl), •C16H9 (pyrenyl), •C12H7 (acenaphthyl), and •C12H9 (biphenylyl)

The calculated difference in the standard heat of formation Δ Δ_fH°(298.15) of n- and i-C₄H₃^• free radicals is 37.9 kJ mol⁻¹ for G3MP2B3 and 45.0 kJ mol⁻¹ for CCSD(T)-CBS (W1U) calculations, which seems to preclude the direct even-carbon radical pathway to benzene and higher PAH (polycyclic aromatic hydrocarbon) formation including soot in a hydrocarbon flame. For the phenyl-type σ-radicals listed in the title, absolute values of Δ_fH°(298.15) have been calculated using G3MP2B3-computed values of bond dissociation energies D°(298.15) and combined with experimental values of Δ_fH° (298.15) for the parent hydrocarbon because of a slight systematic overprediction of the thermodynamic stability of large PAHs by the applied computational G3MP2B3 method. Standard enthalpies of formation Δ_fH°(298.15) as well as absolute entropies S° and heat capacities C°_p are given for a series of π- and σ-free radicals important to combustion as a function of temperature. A spread of roughly 40 kJ mol⁻¹ in the average C H bond strength of PAH leading to σ-radicals has been calculated, the lowest leading to 4-phenanthryl (463.6 kJ mol⁻¹), the highest leading to 2-biphenylyl radical (502.5 kJ mol⁻¹). © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 395–415, 2008     

Standard enthalpies of formation of bis(β-diketonate)cobalt(II) complexes: The mean (CoO) bond-dissociation enthalpies

The standard enthalpies of formation, at 298 K, of the 1-phenyl-1,3-butanedione (HBZAC) and 1,1,1-trifluoro-2,4-pentanedione (HTFAC) crystalline complexes of cobalt(II) were determined by precise solution—reaction calorimetry: ΔH⁰_f{Co(BZAC)₂,cr} = −632±6.0 kJ mol⁻¹ ΔH⁰_f{Co(TFAC)₂,cr} = −2140±10 kJ mol⁻¹. The average molar bond-dissociation enthalpies, <D>(CoO) were derived.