Showing papers on "Hydrogen atom abstraction published in 2019"

Reactivity control of a photocatalytic system by changing the light intensity

Abstract: We report a novel light-intensity dependent reactivity approach allowing us to selectively switch between triplet energy transfer and electron transfer reactions, or to regulate the redox potential available for challenging reductions. Simply by adjusting the light power density with an inexpensive lens while keeping all other parameters constant, we achieved control over one- and two-photon mechanisms, and successfully exploited our approach for lab-scale photoreactions using three substrate classes with excellent selectivities and good product yields. Specifically, our proof-of-concept study demonstrates that the irradiation intensity can be used to control (i) the available photoredox reactivity for reductive dehalogenations to selectively target either bromo- or chloro-substituted arenes, (ii) the photochemical cis–trans isomerization of olefins versus their photoreduction, and (iii) the competition between hydrogen atom abstraction and radical dimerization processes.

Selective Hydrogen Atom Abstraction from Dihydroflavonol by a Nonheme Iron Center Is the Key Step in the Enzymatic Flavonol Synthesis and Avoids Byproducts.

Abstract: The plant non-heme iron dioxygenase flavonol synthase performs a regioselective desaturation reaction as part of the biosynthesis of the signaling molecule flavonol that triggers the growing of leaves and flowers. These compounds also have health benefits for humans. Desaturation of aliphatic compounds generally proceeds through two consecutive hydrogen atom abstraction steps from two adjacent carbon atoms and in nature often is performed by a high-valent iron(IV)-oxo species. We show that the order of the hydrogen atom abstraction steps, however, is opposite of those expected from the C–H bond strengths in the substrate and determines the product distributions. As such, flavonol synthase follows a negative catalysis mechanism. Using density functional theory methods on large active-site model complexes, we investigated pathways for desaturation and hydroxylation by an iron(IV)-oxo active-site model. Contrary to thermochemical predictions, we find that the oxidant abstracts the hydrogen atom from the stro...

Hydrogen Abstraction/Addition Tunneling Reactions Elucidate the Interstellar H2NCHO/HNCO Ratio and H2 Formation

Abstract: Formamide (H2NCHO) is the smallest molecule possessing the biologically important amide bond. Recent interstellar observations have shown a strong correlation between the abundance of formamide and isocyanic acid (HNCO), indicating that they are likely to be chemically related, but no experiment or theory explains this correlation satisfactorily. We performed H + H2NCHO reactions in a para-hydrogen quantum-solid matrix host and identified production of H2NCO and HNCO from hydrogen-abstraction reactions. We identified also D2NCO, DNCO, HDNCO, and HDNCHO from the reaction H + D2NCHO, indicating the presence of hydrogen-addition reactions of DNCO and HDNCO. From the observed temporal profiles of H2NCHO, H2NCO, HNCO, and their deuterium isotopologues, we showed that a dual-cycle consisting of hydrogen abstraction and hydrogen addition can satisfactorily explain the quasi-equilibrium between H2NCHO and HNCO and explain other previous experimental results. Furthermore, this mechanism also indicates that the catalytic formation of H2 from H atoms might occur in interstellar ice grains.

Mechanisms for Hydrogen-Atom Abstraction by Mononuclear Copper(III) Cores: Hydrogen-Atom Transfer or Concerted Proton-Coupled Electron Transfer?

Abstract: In a possibly biomimetic fashion, formally copper(III)–oxygen complexes LCu(III)–OH (1) and LCu(III)–OOCm (2) (L2– = N,N′-bis(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamide, Cm = α,α-dimethylbenzyl) have been shown to activate X–H bonds (X = C, O). Herein, we demonstrate similar X–H bond activation by a formally Cu(III) complex supported by the same dicarboxamido ligand, LCu(III)–O2CAr1 (3, Ar1 = meta-chlorophenyl), and we compare its reactivity to that of 1 and 2. Kinetic measurements revealed a second order reaction with distinct differences in the rates: 1 reacts the fastest in the presence of O–H or C–H based substrates, followed by 3, which is followed by (unreactive) 2. The difference in reactivity is attributed to both a varying oxidizing ability of the studied complexes and to a variation in X–H bond functionalization mechanisms, which in these cases are characterized as either a hydrogen-atom transfer (HAT) or a concerted proton-coupled electron transfer (cPCET). Select theoretical tools have b...

Catalytic Ammonia Oxidation to Dinitrogen by Hydrogen Atom Abstraction

Abstract: Catalysts for the oxidation of NH3 are critical for the utilization of NH3 as a large-scale energy carrier. Molecular catalysts capable of oxidizing NH3 to N2 are rare. This report describes the use of [Cp*Ru(PtBu2 NPh2 )(15 NH3 )][BArF4 ], (PtBu2 NPh2 =1,5-di(phenylaza)-3,7-di(tert-butylphospha)cyclooctane; ArF =3,5-(CF3 )2 C6 H3 ), to catalytically oxidize NH3 to dinitrogen under ambient conditions. The cleavage of six N-H bonds and the formation of an N≡N bond was achieved by coupling H+ and e- transfers as net hydrogen atom abstraction (HAA) steps using the 2,4,6-tri-tert-butylphenoxyl radical (t Bu3 ArO. ) as the H atom acceptor. Employing an excess of t Bu3 ArO. under 1 atm of NH3 gas at 23 °C resulted in up to ten turnovers. Nitrogen isotopic (15 N) labeling studies provide initial mechanistic information suggesting a monometallic pathway during the N⋅⋅⋅N bond-forming step in the catalytic cycle.

A Stable Lithium-Oxygen Battery Electrolyte Based on Fully Methylated Cyclic Ether.

Abstract: Ether-based electrolytes are commonly used in Li-O2 batteries (LOBs) because of their relatively high stability. But they are still prone to be attacked by superoxides or singlet oxygen via hydrogen abstract reactions, which leads to performance decaying during long-term operation. Herein we propose a methylated cyclic ether, 2,2,4,4,5,5-hexamethyl-1,3-dioxolane (HMD), as a stable electrolyte solvent for LOBs. Such a compound does not contain any hydrogen atoms on the alpha-carbon of the ether, and thus avoids hydrogen abstraction reactions. As the result, this solvent exhibits excellent stability with the presence of superoxide or singlet oxygen. In addition the CO2 evolution during charge process is prohibited. The LOB with HMD-based electrolyte was able to run up to 157 cycles, 4 times more than with 1,3-dioxolane (DOL) or 1,2-dimethoxyethane (DME) based electrolytes.

A Nonheme Thiolate-Ligated Cobalt Superoxo Complex: Synthesis and Spectroscopic Characterization, Computational Studies, and Hydrogen Atom Abstraction Reactivity.

Abstract: The synthesis and characterization of a Co(II) dithiolato complex Co(Me3TACN)(S2SiMe2) (1) are reported. Reaction of 1 with O2 generates a rare thiolate-ligated cobalt–superoxo species Co(O2)(Me3TA...

Metal-Free Regioselective Radical Cyclization of 1,6-Enynes with Carbonyl Compounds

Abstract: A regioselective cyclization of 1,6-enynes is reported by a tert-butyl hydroperoxide (TBHP)-mediated, one-pot, cascade reaction with commercially available carbonyl compounds under metal- and additive-free conditions. The reaction scope includes both 1,6-enynes and carbonyl compounds. A possible cascade reaction mechanism, consisting of acyl radical addition, intramolecular cyclization, and hydrogen abstraction, is proposed.

Esters as Radical Acceptors: β‐NHC‐Borylalkenyl Radicals Induce Lactonization by C−C Bond Formation/Cleavage on Esters

Abstract: Substituted propargyl acetates are converted into 4-boryl-2(5H)-furanones upon thermolysis in the presence of an N-heterocyclic carbene borane (NHC-borane) and di-tert-butyl peroxide. The acetyl methyl group is lost during the reaction as methane. Evidence suggests that the reaction proceeds by a sequence of radical events including: 1) addition of an NHC-boryl radical to the triple bond; 2) cyclization of the resultant β-borylalkenyl radical to the ester carbonyl group; 3) β-scission of the so-formed alkoxy radical to provide the 4-boryl-2(5H)-furanone and a methyl radical; and 4) hydrogen abstraction from the NHC-borane to return the initial NHC-boryl radical and methane.

Hydrogen by Deuterium Substitution in an Aldehyde Tunes the Regioselectivity by a Nonheme Manganese(III)–Peroxo Complex

Abstract: Mononuclear nonheme MnIII -peroxo complexes are important intermediates in biology, and take part in oxygen activation by photosystem II. Herein, we present work on two isomeric biomimetic side-on MnIII -peroxo intermediates with bispidine ligand system and reactivity patterns with aldehydes. The complexes are characterized with UV/Vis and mass spectrometric techniques and reaction rates with cyclohexane carboxaldehyde (CCA) are measured. The reaction gives an unusual regioselectivity switch from aliphatic to aldehyde hydrogen atom abstraction upon deuteration of the substrate, leading to the corresponding carboxylic acid product for the latter, while the former gives a deformylation reaction. Mechanistic details are established from kinetic isotope effect studies and density functional theory calculations. Thus, replacement of C-H by C-D raises the hydrogen atom abstraction barriers and enables a regioselectivity switch to a competitive pathway that is slightly higher in energy.

Modeling of aromatics formation in fuel-rich methane oxy-combustion with an automatically generated pressure-dependent mechanism.

Abstract: With the rise in production of natural gas, there is increased interest in homogeneous partial oxidation (POX) to convert methane to syngas (CO + H2), ethene (C2H4) and acetylene (C2H2). In POX, polycyclic aromatic hydrocarbons (PAH) are important undesired byproducts. To improve the productivity of such POX processes, it is necessary to have an accurate chemical mechanism for methane-rich combustion including PAH. A new mechanism was created to capture the chemistry from C0 to C12, incorporating new information derived from recent quantum chemistry calculations, with help from the Reaction Mechanism Generator (RMG) software. For better estimation of kinetics and thermochemistry of aromatic species, including reactions through carbene intermediates, new reaction families and additional data from quantum chemistry calculations were added to RMG-database. Many of the rate coefficients in the new mechanism are significantly pressure-dependent at POX conditions. The new mechanism was validated against electron-ionization molecular beam mass spectrometry (EI-MBMS) data from a high-temperature flow reactor reported by Kohler et al. In this work quantification of additional species from those experiments is reported including phenylacetylene (C8H6), indene (C9H8), naphthalene (C10H8) and acenaphthylene (C12H8) at many temperatures for several feed compositions. Comparison of the experimental species concentration data and the new kinetic model is satisfactory; the new mechanism is generally more accurate than other published mechanisms. Moreover, because the new mechanism is composed of elementary chemical reaction steps instead of global fitted kinetics, pathway analysis of species could be investigated step-by-step to understand PAH formation. For methane-rich combustion, the most important routes to key aromatics are propargyl recombination for benzene, reactions of the propargyl radical with the phenyl radical for indene, and hydrogen abstraction acetylene addition (HACA) for naphthalene.

A Terminal Iridium Oxo Complex with a Triplet Ground State.

Abstract: A terminal iridium oxo complex with an open-shell (S=1) ground state was isolated upon hydrogen atom transfer (HAT) from the respective iridium(II) hydroxide. Electronic structure examinations support large spin delocalization to the oxygen atom. Selected oxo transfer reactions indicate the ambiphilic reactivity of the iridium oxo moiety. Calorimetric and computational examinations of the HAT revealed a bond dissociation free energy for the IrO-H bond that is sufficient for hydrogen atom abstraction towards C-H bonds and small contributions from entropy and spin-orbit coupling to the HAT thermochemistry.

Tunneling Controls the Reaction Pathway in the Deformylation of Aldehydes by a Nonheme Iron(III)-Hydroperoxo Complex: Hydrogen Atom Abstraction versus Nucleophilic Addition.

Abstract: Mononuclear nonheme iron(III)-hydroperoxo intermediates play key roles in biological oxidation reactions. In the present study, we report the highly intriguing reactivity of a nonheme iron(III)-hydroperoxo complex, [(TMC)FeIII(OOH)]2+ (1), in the deformylation of aldehydes, such as 2-phenylpropionaldehyde (2-PPA) and its derivatives; that is, the reaction pathway of the aldehyde deformylation by 1 varies depending on reaction conditions, such as temperature and substrate. At temperature above 248 K, the aldehyde deformylation occurs predominantly via a nucleophilic addition (NA) pathway. However, as the reaction temperature is lowered, the reaction pathway changes to a hydrogen atom transfer (HAT) pathway. Interestingly, the reaction rate becomes independent of temperature below 233 K with a huge kinetic isotope effect (KIE) value of 93 at 203 K, suggesting that the HAT reaction results from tunneling. In contrast, reactions with a deuterated 2-PPA at the α-position and 2-methyl-2-phenylpropionaldehyde proceed exclusively via a NA pathway irrespective of the reaction temperature. We conclude that the bifurcation pathways between NA and HAT result from the tunneling effect in the HAT reaction by 1. To the best of our knowledge, this study reports the first example showing that tunneling plays a significant role in the activation of substrate C-H bonds by a mononuclear nonheme iron(III)-hydroperoxo complex.

Stereodynamical control of product branching in multi-channel barrierless hydrogen abstraction of CH3OH by F.

Abstract: Hydrogen abstraction from methanol (CH3OH) by F atoms presents an ideal proving ground to investigate dynamics of multi-channel reactions, because two types of hydrogen can be abstracted from the methanol molecule leading to the HF + CH3O and HF + CH2OH products. Using the quasi-classical trajectory approach on a globally accurate potential energy surface based on high-level ab initio calculations, this work reports a comprehensive dynamical investigation of this multi-channel reaction, yielding measurable attributes including integral and differential cross sections, as well as branching ratios. It is shown that while complex-forming and direct mechanisms coexist at low collision energies, these barrierless reaction channels are dominated at high energies by the direct mechanism, in which the reaction is only possible for trajectories entering into the respective dynamical cones of acceptance. Perhaps more importantly, the non-statistical product branching is found to be dictated by unique stereodynamics in the entrance channels.

Experimental and Theoretical Investigation of the Pyrolysis of Furfural.

Abstract: The thermal decomposition of furfural is investigated in a flow tube reactor at 30 Torr by synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) at temperatures from 1023 to 1273 K. Over 20 kinds of pyrolysis products, including short-lived radicals, stable oxygen-containing compounds, and hydrocarbons, are identified from the scanning photoionization efficiency (PIE) spectra. Vinylketene (CH2═CH-CH═C═O), which has been shown to be an important primary product, is also directly observed. The possible steps of hydrogen atom addition and hydrogen atom abstraction in the thermal decomposition of furfural are studied by theoretical calculations at the CBS-QB3 level. In addition to unimolecular decomposition, hydrogen atom addition followed by ring opening can lead to the production of vinylketene.

Theoretical study of the F(2P) + NH3 → HF + NH2 reaction on an accurate potential energy surface: dynamics and kinetics

Abstract: The highly exothermic hydrogen abstraction reaction of the F atom with NH3 is investigated using the quasi-classical trajectory method on a newly developed potential energy surface (PES) for the ground electronic state. The full-dimensional PES is constructed by fitting 41 282 ab initio energy points at the level of UCCSD(T)-F12/aug-cc-pVTZ. The flexible fundamental invariant-neural network method is applied in the fitting, resulting in a total root mean square error of 0.13 kcal mol−1. On one hand, the calculated differential cross sections agree reasonably well with the experimental results and indicate that the reaction is dominated by the direct abstraction and stripping mechanisms while a considerable amount of reaction takes place by the indirect “yo–yo” mechanism. The product energy partition also reproduces well the experimental result, which can be understood according to the geometry change along the minimum energy path. On the other hand, the obtained vibrational state distribution of the product HF follows PνHF=2 ≈ PνHF=1 > PνHF=0 > PνHF=3, less consistent with the scattered experimental results. In addition, the calculated thermal rate coefficients have practically no temperature dependence within the statistical errors.

Reactivity of the Indenyl Radical (C9H7) with Acetylene (C2H2) and Vinylacetylene (C4H4)

Abstract: Author(s): Zhao, Long; Prendergast, Matthew B; Kaiser, Ralf I; Xu, Bo; Lu, Wenchao; Ablikim, Utuq; Ahmed, Musahid; Oleinikov, Artem D; Azyazov, Valeriy N; Mebel, Alexander M; Howlader, A Hasan; Wnuk, Stanislaw F | Abstract: The reactions of the indenyl radicals with acetylene (C2 H2 ) and vinylacetylene (C4 H4 ) is studied in a hot chemical reactor coupled to synchrotron based vacuum ultraviolet ionization mass spectrometry. These experimental results are combined with theory to reveal that the resonantly stabilized and thermodynamically most stable 1-indenyl radical (C9 H7 . ) is always formed in the pyrolysis of 1-, 2-, 6-, and 7-bromoindenes at 1500 K. The 1-indenyl radical reacts with acetylene yielding 1-ethynylindene plus atomic hydrogen, rather than adding a second acetylene molecule and leading to ring closure and formation of fluorene as observed in other reaction mechanisms such as the hydrogen abstraction acetylene addition or hydrogen abstraction vinylacetylene addition pathways. While this reaction mechanism is analogous to the bimolecular reaction between the phenyl radical (C6 H5 . ) and acetylene forming phenylacetylene (C6 H5 CCH), the 1-indenyl+acetylene→1-ethynylindene+hydrogen reaction is highly endoergic (114 kJ mol-1 ) and slow, contrary to the exoergic (-38 kJ mol-1 ) and faster phenyl+acetylene→phenylacetylene+hydrogen reaction. In a similar manner, no ring closure leading to fluorene formation was observed in the reaction of 1-indenyl radical with vinylacetylene. These experimental results are explained through rate constant calculations based on theoretically derived potential energy surfaces.

Hydrogen Atom Abstraction by High-Valent Fe(OH) versus Mn(OH) Porphyrinoid Complexes: Mechanistic Insights from Experimental and Computational Studies.

Abstract: High-valent metal-hydroxide species have been implicated as key intermediates in hydroxylation chemistry catalyzed by heme monooxygenases such as the cytochrome P450s. However, in some classes of P450s, a bifurcation from the typical oxygen rebound pathway is observed, wherein the FeIV(OH)(porphyrin) species carries out a net hydrogen atom transfer reaction to form alkene metabolites. In this work, we examine the hydrogen atom transfer (HAT) reactivity of FeIV(OH)(ttppc) (1), ttppc = 5,10,15-tris(2,4,6-triphenyl)-phenyl corrole, toward substituted phenol derivatives. The iron hydroxide complex 1 reacts with a series of para-substituted 2,6-di-tert-butylphenol derivatives (4-X-2,6-DTBP; X = OMe, Me, Et, H, Ac), with second-order rate constants k2 = 3.6(1)–1.21(3) × 104 M–1 s–1 and yielding linear Hammett and Marcus plot correlations. It is concluded that the rate-determining step for O–H cleavage occurs through a concerted HAT mechanism, based on mechanistic analyses that include a KIE = 2.9(1) and DFT cal...

Insights into the reaction mechanism of n-hexane dehydroaromatization to benzene over gallium embedded HZSM-5: effect of H2 incorporated on active sites

Abstract: The catalytic dehydroaromatization of alkanes to aromatics has attracted considerable attention from the scientific community, because it can be used for the upgrading of low-cost alkanes into high added-value aromatics, such as benzene, toluene, and xylene (BTX). In this context, we report the reaction mechanism of n-hexane dehydroaromatization to benzene over two different reduced gallium species embedded in HZSM-5, including univalent Ga+ embedded in HZSM-5 (Ga/HZSM-5) and dihydrido gallium complex (GaH2+) embedded in HZSM-5 (GaH2/HZSM-5) by using the M06-2X/6-31G(d,p) level of calculation. The reaction proceeds by following two main steps: (i) the dehydrogenation of hexane to haxa-1,3,5-triene; (ii) the dehydroaromatization of haxa-1,3,5-triene to benzene. For the univalent Ga+ embedded in HZSM-5, the first step of the hexane dehydrogenation is considered to be the rate-determining step, which requires a high activation energy of 76.6 kcal mol−1. In strong contrast to this, in the case of the GaH2/HZSM-5 catalyst the rate determining step is found to be the second hydrogen abstraction from n-hexane with a lower activation barrier of 11.1 kcal mol−1. The reaction is therefore preferentially taking place over the GaH2/HZSM-5 catalyst. These observations clearly confirm the existence of a dihydrido gallium complex (GaH2+) as one of the most active species for the dehydroaromatization of alkanes and it is obtained in the presence of hydrogen in the catalytic system. This example opens up perspectives for a better understanding of the effect of active species on the catalytic reaction.

N-Hydroxyphthalimide/benzoquinone-catalyzed chlorination of hydrocarbon C-H bond using N-chlorosuccinimide.

Abstract: The direct chlorination of C-H bonds has received considerable attention in recent years. In this work, a metal-free protocol for hydrocarbon C-H bond chlorination with commercially available N-chlorosuccinimide (NCS) catalyzed by N-hydroxyphthalimide (NHPI) with 2,3-dicyano-5,6-dichlorobenzoquinone (DDQ) functioning as an external radical initiator is presented. Aliphatic and benzylic substituents and also heteroaromatic ones were found to be well tolerated. Both the experiments and theoretical analysis indicate that the reaction goes through a process wherein NHPI functions as a catalyst rather than as an initiator. On the other hand, the hydrogen abstraction of the C-H bond conducted by a PINO species rather than the highly reactive N-centered radicals rationalizes the high chemoselectivity of the monochlorination obtained by this protocol as the latter is reactive towards the C(sp3)-H bonds of the monochlorides. The present results could hold promise for further development of a nitroxy-radical system for the highly selective functionalization of the aliphatic and benzylic hydrocarbon C-H.

Hydrogen Bond-Enabled Heterolytic and Homolytic Peroxide Activation within Nonheme Copper(II)-Alkylperoxo Complexes.

Abstract: To explore the reactivity of copper-alkylperoxo species enabled by the heterolytic peroxide activation, room-temperature stable mononuclear nonheme copper(II)-alkylperoxo complexes bearing a N-(2-ethoxyethanol)-bis(2-picolyl)amine ligand (HN3O2), [CuII(OOR)(HN3O2)]+ (R = cumyl or tBu), were synthesized and spectroscopically characterized. A combined experimental and computational investigation on the reactivity and reaction mechanisms in the phosphorus oxidation, C-H bond activation, and aldehyde deformylation reactions by the copper(II)-alkylperoxo complexes has been conducted. DFT-optimized structures suggested that a hydrogen bonding interaction exists between the ethoxyethanol backbone of the HN3O2 ligand and either the proximal or distal oxygen atom of the alkylperoxide moiety, and this interaction consequently results in the enhanced stability of the copper(II)-alkylperoxo species. In the phosphorus oxidation reaction, both experimental and computational results indicated that a phosphine-triggered heterolytic O-O bond cleavage occurred to yield phosphine oxide and alcohol products. DFT calculations suggested that (i) the H-bonding between the ethoxyethanol backbone and distal oxygen of the alkylperoxide moiety and (ii) the phosphine binding to the proximal oxygen of the alkylperoxide moiety engendered the heterolytic peroxide activation. In the C-H bond activation reactions, temperature-dependent reactivity of the copper(II)-alkylperoxo complexes was observed, and a relatively strong activation energy of 95 kcal mol-1 was required to promote the homolytic peroxide activation. A rate-limiting hydrogen atom abstraction reaction of xanthene by the putative copper(II)-oxyl radical resulted in the formation of the dimeric copper product and the substrate radical that further underwent autocatalytic oxidation reactions to form an oxygen incorporated product. Finally, amphoteric reactivity of copper(II)-alkylperoxo complexes has been assessed by conducting kinetic studies and product analysis of the aldehyde deformylation reaction.

Low-Temperature Kinetic Isotope Effects in CH3OH + H → CH2OH + H2 Shed Light on the Deuteration of Methanol in Space.

Abstract: We calculated reaction rate constants including atom tunneling for the hydrogen abstraction reaction CH3OH + H → CH2OH + H2 with the instanton method. The potential energy was fitted by a neural network that was trained to UCCSD(T)-F12/VTZ-F12 data. Bimolecular gas-phase rate constants were calculated using microcanonic instanton theory. All H/D isotope patterns on the CH3 group and the incoming H atom are studied. Unimolecular reaction rate constants, representing the reaction on a surface, down to 30 K, are presented for all isotope patterns. At 30 K, they range from 4100 for the replacement of the abstracted H by D to ∼8 for the replacement of the abstracting H to ∼2 to 6 for secondary KIEs. The 12C/13C kinetic isotope effect is 1.08 at 30 K, while the 16O/18O kinetic isotope effect is extremely small. A simple kinetic surface model using these data predicts high abundances of the deuterated forms of methanol.

Reaction Mechanisms and Kinetics of the Hydrogen Abstraction Reactions of C4–C6 Alkenes with Hydroxyl Radical: A Theoretical Exploration

Abstract: The reaction of alkenes with hydroxyl (OH) radical is of great importance to atmospheric and combustion chemistry. This work used a combined ab initio/transition state theory (TST) method to study the reaction mechanisms and kinetics for hydrogen abstraction reactions by OH radical on C4–C6 alkenes. The elementary abstraction reactions involved were divided into 10 reaction classes depending upon the type of carbon atoms in the reaction center. Geometry optimization was performed by using DFT M06-2X functional with the 6-311+G(d,p) basis set. The energies were computed at the high-level CCSD(T)/CBS level of theory. Linear correlation for the computed reaction barriers and enthalpies between M06-2X/6-311+G(d,p) and CCSD(T)/CBS methods were found. It was shown that the C=C double bond in long alkenes not only affected the related allylic reaction site, but also exhibited a large influence on the reaction sites nearby the allylic site due to steric effects. TST in conjunction with tunneling effects were employed to determine high-pressure limit rate constants of these abstraction reactions and the computed overall rate constants were compared with the available literature data.