Bicyclic Radical Research Articles

The atmospheric oxidation mechanism of acetophenone initiated by the hydroxyl radicals

Acetophenone (C8H8O, AP) is a common indoor air component but with no information on its atmospheric oxidation. The atmospheric oxidation mechanism of AP in gas phase, initiated by its reaction with the OH radical, is investigated here using quantum chemistry and chemical kinetics calculations. The reaction starts mainly by OH additions to the aromatic ring, and the predicted rate coefficient suggests a lifetime of ∼8 days in the atmosphere. The additions form adducts AP-i-OH (Ri) (i = 1, 2, 3, 4, the position on the aromatic ring). The adducts Ri would react with O2 or undergo unimolecular decompositions. The adduct R1 reacts with O2 rather slowly with an effective rate coefficients of <10−20 cm3 molecule−1 s−1, and it would alternatively decompose to phenol and acetyl radical. The dominate fate of R2, R3, and R4 in their reaction with O2 proceeds via direct hydrogen-atom abstraction, forming hydroxyacetophenones, and only R3 would have a fraction of bicyclic radical formation as in oxidation of alkyl benzenes. The main products from oxidation of AP include phenol and three isomers of hydroxyacetophenones, suggesting that oxidation of AP by OH would reduce its unpleasant effect to human beings.

Stable Bicyclic Functionalized Nitroxides: The Synthesis of Derivatives of Aza-nortropinone–5-Methyl-3-oxo-6,8-diazabicyclo[3.2.1]-6-octene 8-oxyls

In recent decades, bicyclic nitroxyl radicals have caught chemists’ attention as selective catalysts for the oxidation of alcohols and amines and as additives and mediators in directed C-H oxidative transformations. In this regard, the design and development of synthetic approaches to new functional bicyclic nitroxides is a relevant and important issue. It has been reported that imidazo[1,2-b]isoxazoles formed during the condensation of acetylacetone with 2-hydroxyaminooximes having a secondary hydroxyamino group are recyclized under mild basic catalyzed conditions to 8-hydroxy-5-methyl-3-oxo-6,8-diazabicyclo[3.2.1]-6-octenes. The latter, containing a sterically hindered cyclic N-hydroxy group, upon oxidation with lead dioxide in acetone, virtually quantitatively form stable nitroxyl bicyclic radicals of a new class, which are derivatives of both 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl (TEMPON) and 3-imidazolines.

Atmospheric oxidation mechansim of polychlorinated biphenyls (PCBs) initiated by OH radicals

Long-range atmospheric transport (LRAT) is the main route for circulating polychlorinated biphenyls (PCBs) from sources to sinks. In the atmosphere, PCBs containing six and less chlorine substitutions exist mainly as vapour, which can be oxidized by OH radical. Here, using quantum chemistry and transition state theory, we calculated the rate coefficients for reactions of OH radical with selected PCBs. The predicted rate coefficients agree with the available experimental values within a factor of 3. Calculations show that all PCBs considered here are persistent with their half-lives longer than 24 h. Reactions of PCBs with OH radical start with OH addition to the phenyl rings, forming PCB-n-OH adducts. Fate of biphenyl-n-OH (BP-n-OH, n = 2, 3, 4) adducts in the atmosphere is investigated. Calculations show that these radical adducts react similarly to benzene-OH adducts, forming hydroxybiphenyl (HO-BP) as main product and bicyclic radicals as minor products in their reaction with O2. Effective rates of reaction with O2 in the atmosphere are relatively slow, ∼1400, ∼45000, and ∼800 s−1 for BP-2-OH, BP-3-OH, and BP-4-OH, respectively. This suggests considerable reactions between BP-n-OH adducts and NO2, forming nitrobiphenyls. The bicyclic radicals from BP-n-OH + O2 would further transform to highly oxidized products as observed in a previous study. PCB-OH adducts react similarly as BP-n-OH radicals. For the three PCB-OH radicals considered here, their reactions with O2 also form HO-PCBs and bicyclic radicals.

Kinetics and mechanism of [rad]OH-initiated atmospheric oxidation of organophosphorus plasticizers: A computational study on tri-p-cresyl phosphate

Understanding the atmospheric fate of organophosphorus plasticizers is important for their environmental risk assessment. However, limited information is available at present. In this study, density functional theory (DFT) calculations were performed to investigate the transformation mechanism and kinetics of tri-p-cresyl phosphate (TpCP) initiated by OH. Results show that the initial reactions are dominated by H-abstraction and OH addition to form TpCP-radical, TpCP–OH adducts and aryl phosphodiester. The H-abstraction pathways are more favorable than the OH addition pathways. The TpCP-radical and TpCP–OH adducts can further react with O2 in the atmosphere to finally form benzaldehyde phosphate, hydroxylated TpCP and bicyclic radicals. Based on the transition state theory, the calculated rate constant (kOH) of TpCP with OH at T = 298 K is 1.9 × 10−12 cm3molecule−1s−1 with an atmospheric lifetime of 4.2 days, which demonstrates that gaseous TpCP is atmospherically persistent. This study provides a comprehensive investigation of the OH-initiated oxidation of TpCP, which is useful for understanding its mechanism of transformation and evaluating the risk in atmospheric environment.

Thermodynamic Control of Isomerizations of Bicyclic Radicals: Interplay of Ring Strain and Radical Stabilization.

The rearrangements of 4-substituted bicyclo[2.2.2]oct-5-en-2-yl radicals, generated from the corresponding Diels-Alder adducts with phenylseleno acrylates by radical-induced reductive deselenocarbonylations, give the 2-substituted bicyclo[3.2.1]oct-6-en-2-yl radicals with some substituents, e.g., alkoxy and phenyl, but not for silyloxymethyl or benzyl substituents. Theoretical calculations with DFT give the thermodynamics of these reactions and the origins of these processes.

The Atmospheric Oxidation Mechanism of Benzyl Alcohol Initiated by OH Radicals: The Addition Channels.

Benzyl alcohol (BA) is present in indoor atmospheres, where it reacts with OH radicals and undergoes further oxidation. A theoretical study is carried out to elucidate the reaction mechanism and to identify the main products of the oxidation of BA that is initiated by OH radicals. The reaction is found to proceed by H-abstraction from the CH2 group (25 %) and addition to the ipso (60 %) and ortho (15 %) positions of the aromatic ring. The BA-OH adducts react further with O2 via the bicyclic radical intermediates-the same way as for benzene-forming mainly 3-hydroxy-2-oxopropanal and butenedial. If NOx is low, the bicyclic peroxy radicals undergo intramolecular H-migration, forming products containing OH, OOH, and CH2 OH/CHO functional groups, and contribute to secondary organic aerosol (SOA) formation.

The atmospheric oxidation mechanism of 2-methylnaphthalene.

The atmospheric oxidation mechanism of 2-methylnaphthalene (2-MN) initiated by OH radicals is investigated by using quantum chemistry at BH&HLYP/6-311++G(2df,2p) and ROCBS-QB3 levels and kinetic calculations by transient state theory and unimolecular reaction theory coupled with master equation (RRKM-ME). This reaction is mainly initiated by OH additions, forming adducts Rn (2-MN-n-OH, n = 1-8). The fates of R1 and R3, representing the α- and β-adducts, are examined. The fates of R1 and R3 are found to be drastically different. In the atmosphere, R1 reacts with O2via O2 addition to the C2 position to form R1-2OO-a/s, which will undergo a bimolecular reaction with the atmospheric NO or unimolecular isomerization via intramolecular H-shifts, of which the latter is found to be dominant and accounts for the formation of dicarbonyl compounds observed in experimental studies. The role of the tricyclic radical intermediates formed from the ring-closure of R1-2OO is rather limited because their formation is endothermic and reversible, being contrary to the important role of the analogous bicyclic radical intermediates in the oxidation of benzenes. On the other hand, the fate of R3 is similar to that of the benzene-OH adduct, and the tricyclic intermediates will play an important role. An oxidation mechanism is proposed based on the theoretical predictions, and the routes for the experimentally observed products are suggested and compared.

Atmospheric oxidation mechanism of m-xylene initiated by OH radical.

The atmospheric oxidation mechanism of m-xylene (mX) initiated by the OH radical is investigated at M06-2X and ROCBS-QB3 levels, coupled with reaction kinetics calculations by using transition state theory and unimolecular RRKM-ME theory. The calculations show that the reaction between OH and mX is dominated by OH addition to the C2 and C4 positions, forming adducts mX-2-OH (R2) and mX-4-OH (R4). In the atmosphere, R2 and R4 react with O2 by irreversible H-abstraction to dimethylphenols or by reversible additions to bicyclic radical intermediates, which would recombine again with O2 to form bicyclic peroxy radicals, to bicyclic alkoxyl radicals by reacting with NO or HO2, and eventually to final products such as glyoxal, methylglyoxal, and their coproducts. The effects of reaction pressure and temperature are explored by RRKM-ME calculations. A mechanism at 298 K is proposed on the basis of current predictions and previous experimental and modeling results. The predicted product yields support the values in the SAPRC mechanism, even though the predicted yield of 1.0% for glyoxal is lower than the value of ∼11% from the experimental measurements.

The atmospheric oxidation mechanism of 1,2,4-trimethylbenzene initiated by OH radicals.

The atmospheric oxidation mechanism of 1,2,4-trimethylbenzene (1,2,4-TMB) initiated by OH radicals is investigated using quantum chemistry calculations at M06-2X and ROCBS-QB3 levels. The calculations show that the initiation of the reaction is dominated by OH addition to C1, C3 and C5 to form 1,2,4-TMB-OH adducts R1, R3, and R5 with branching ratios of 0.22, 0.19, and 0.38, respectively, using ROCBS-QB3 energies. In the troposphere, the adducts react with O2 by irreversible H-abstraction to form phenolic compounds and by reversible addition to TMB-OH-O2 peroxy radicals, which will cyclize to bicyclic radicals, similar to those in benzene, toluene, and xylenes. The bicyclic radicals can further recombine with O2 to generate bicyclic peroxy and alkoxyl radicals. The bicyclic alkoxyl radicals would break the ring directly to form 1,2-dicarbonyl products and unsaturated 1,4-dicarbonyl co-products, or undergo another cyclization to form an epoxy group, followed by the ring-breakage to form 1,2-dicarbonyl products and epoxy-1,4-dicarbonyl co-products. The predicted yields of products agree reasonably with the previous experimental measurements, while considerable discrepancies also exist for the yields of nitrates, biacetyl, 4-oxo-2-pentenal, and butenedial, etc. Our mechanism also predicts a new type of epoxy-1,4-dicarbonyl compounds with a total yield of ∼0.32. The epoxy-1,4-dicarbonyl compounds have not been suggested or reported in previous studies.

Atmospheric oxidation mechanism of toluene.

The atmospheric oxidation mechanism of toluene initiated by OH radical addition is investigated by quantum chemistry calculations at M06-2X, G3MP2-RAD, and ROCBS-QB3 levels and by kinetics calculation by using transition state theory and unimolecular reaction theory coupled with master equation (RRKM-ME). The predicted branching ratios are 0.15, 0.59, 0.05, and 0.14 for OH additions to ipso, ortho, meta, and para positions (forming R1-R4 adducts), respectively. The fate of R2, R4, and R1 is investigated in detail. In the atmosphere, R2 reacts with O2 either by irreversible H-abstraction to form o-cresol (36%), or by reversible recombination to R2-1OO-syn and R2-3OO-syn, which subsequently cyclize to bicyclic radical R2-13OO-syn (64%). Similarly, R4 reacts with O2 with branching ratios of 61% for p-cresol and 39% for R4-35OO-syn, while reaction of R1 and O2 leads to R1-26OO-syn. RRKM-ME calculations show that the reactions of R2/R4 with O2 have reached their high-pressure limits at 760 Torr and the formation of R2-16O-3O-s is only important at low pressure, i.e., 5.4% at 100 Torr. The bicyclic radicals (R2-13OO-syn, R4-35OO-syn, and R1-26OO-syn) will recombine with O2 to produce bicyclic alkoxy radicals after reacting with NO. The bicyclic alkoxy radicals would break the ring to form products methylglyoxal/glyoxal (MGLY/GLY) and their corresponding coproducts butenedial/methyl-substituted butenedial as proposed in earlier studies. However, a new reaction pathway is found for the bicyclic alkoxy radicals, leading to products MGLY/GLY and 2,3-epoxybutandial/2-methyl-2,3-epoxybutandial. A new mechanism is proposed for the atmospheric oxidation mechanism of toluene based on current theoretical and previous theoretical and experimental results. The new mechanism predicts much lower yield of GLY and much higher yield of butenedial than other atmospheric models and recent experimental measurements. The new mechanism calls for detection of proposed products 2,3-epoxybutandial and 2-methyl-2,3-epoxybutandial.

Atmospheric oxidation mechanism of chlorobenzene

The atmospheric oxidation mechanism of chlorobenzene (CB) initiated by the OH radicals is investigated at M06-2X/6-311++G(2df, 2p) and ROCBS-QB3 levels. The oxidation is initiated by OH addition to the ortho (∼50%), para (∼33%) and meta (∼17%) positions, forming CB-OH adducts as R2, R3, and R4; while the ipso-addition is negligible (∼0.2%). The reactions of the CB-OH adducts with the atmospheric oxygen are further investigated in detail by coupling the unimolecular reaction rate theory calculations with master-equation (RRKM-ME). The CB-OH adducts react with O2 either by irreversible H-abstraction to form chlorophenol and HO2 or by reversible additions to form CB-OH–O2 radicals, which subsequently cyclize to bicyclic radicals. RRKM-ME calculations show that the addition reactions of CB-OH and O2 at the atmospheric pressure are close to but not yet reach their high-pressure-limits. The RRKM-ME simulations predict the yields of 93%, 38%, and 74% for ortho-, meta- and para-chlorophenols from the reactions of O2 with R2, R3 and R4, being lower than their high-pressure-limit yields of 95%, 48%, an 80%, respectively. Overall, the yield of chlorophenols is determined as 72% at the atmospheric pressure.

Atmospheric Oxidation Mechanism of Benzene. Fates of Alkoxy Radical Intermediates and Revised Mechanism

The fate of alkoxy radicals formed in the atmospheric oxidation of benzene initiated by OH radical is investigated by using quantum chemistry and kinetics calculations. The two alkoxy radicals (R2 and R3), formed from the commonly accepted bicyclic radical intermediates, are found to undergo ring-closure preferentially, in addition to the ring-breakage, as suggested in previous studies. The ratio between the ring-closure and ring-breakage is ∼2:1. The ring-closure route will lead to equal amounts of glyoxal and 2,3-epoxybutandial, while the ring-breakage route leads to glyoxal and butenedial. Overall, the new mechanism suggests the yield of glyoxal to be three times that of butenedial, consistent with the previous experimental measurements. The new mechanism calls for the search of the newly proposed product 2,3-epoxybutandial.

Mechanism and Kinetics of the Atmospheric Oxidative Degradation of Dimethylphenol Isomers Initiated by OH Radical

Dimethylphenols are highly reactive in the atmosphere, and their oxidation plays a vital role in the autoignition and combustion processes. The dominant oxidation process for dimethylphenols is by gas-phase reaction with OH radical. In the present study, the reaction of OH radical with dimethylphenol isomers is studied using density functional theory methods, B3LYP, M06-2X, and MPW1K, and also at the MP2 level of theory using 6-31G(d,p) and 6-31+G(d,p) basis sets. The activation energy values have also been calculated using the CCSD(T) method with 6-31G(d,p) and 6-311+G(d,p) basis sets using the geometries optimized at the M06-2X/6-31G(d,p) level of theory. The reactions subsequent to the principal oxidation steps are studied, and the different reaction pathways are modeled. The positions of the OH and CH3 substituents in the aromatic ring have a great influence on the reactivity of dimethylphenol toward OH radical. Accordingly, the reaction is initiated in four different ways: H-atom abstraction from the phenol group, H-atom abstraction from a methyl group, H-atom abstraction from the aromatic ring by OH radical, or electrophilic addition of OH radical to the aromatic ring. Aromatic peroxy radicals arising from initial H-atom abstraction and subsequent O2 addition lead to the formation of hydroperoxide adducts and alkoxy radicals. The O2 additions to dimethylphenol-OH adduct results in the formation of epoxide and bicyclic radicals. The rate constants for the most favorable reaction pathways are calculated using canonical variational transition state theory with small curvature tunneling corrections. This study provides thermochemical and kinetic data for the oxidation of dimethylphenol in the atmosphere and demonstrates the mechanism for the conversion of peroxy radical into aldehydes, hydroperoxides, epoxides, and bicyclic radicals, and their lifetimes in the atmosphere.

Atmospheric oxidation mechanism of naphthalene initiated by OH radical. A theoretical study

The atmospheric oxidation mechanism of naphthalene (Nap) initiated by the OH radical is investigated using density functional theory at B3LYP and BB1K levels. The initial step is dominated by OH addition to the C(1)-position of Nap, forming radical C(10)H(8)-1-OH (R1), followed by the O(2) additions to the C(2) position to form peroxy radical R1-2OO, or by the hydrogen abstraction by O(2) to form 1-naphthol. In the atmosphere, R1-2OO will react with NO to form R1-2O, undergo intramolecular hydrogen transfer from -OH to -OO to form R1-P2O1 radicals, or possibly undergo ring-closure to R1-29OO bi-cyclic radical; while the formation of other bi-cyclic intermediate radicals is negligible because of the extremely high Gibbs energy barriers of >100 kJ mol(-1) (relative to R1+O(2)). The mechanism is different from the oxidation mechanism of benzene, where the bi-cyclic intermediates play an important role. Radicals R1-P2O1 will dissociate to 2-formylcinnamaldehyde, while R1-2O will be transformed to stable products C(10)H(6)O(3) via epoxide-like intermediates. A few reaction pathways suggested in previous experimental studies are found to be invalid.

ChemInform Abstract: Radical-Based Synthesis of Terpenoids. Stereoselectivity in the Trapping of Radicals from Cyclization of 3-(1-Ethoxy-2-haloethoxy) cyclohexenes.

Stereoselectivity in the intermolecular trapping of the bicyclic radical intermediates from cyclization of 3-(1-ethoxy-2-haloethoxy)cyclohexenes with methyl acrylate was found to be affected remarkably by the conformation of the radical intermediates. The optimized conformers of 8-methoxy-7-oxabicyclo[4.3.0]non-2-yl radicals, models for the radical intermediates, were obtained by PM3 RHF molecular orbital calculations, and the stereoselectivity was analyzed in detail.

Origins of Stereoselectivity in the trans Diels−Alder Paradigm

The regioselectivity and stereoselectivity aspects of the Diels-Alder/radical hydrodenitration reaction sequence leading to trans-fused ring systems have been investigated with density functional calculations. A continuum of transition structures representing Diels-Alder and hetero-Diels-Alder cycloadditions as well as a sigmatropic rearrangement have been located, and they all lie very close in energy on the potential energy surface. All three pathways are found to be important in the formation of the Diels-Alder adduct. Reported regioselectivities are reproduced by the calculations. The stereoselectivity of radical hydrodenitration of the cis-Diels-Alder adduct is found to be related to the relative conformational stabilities of bicyclic radical intermediates. Overall, the computations provide understanding of the regioselectivities and stereoselectivities of the trans-Diels-Alder paradigm.

Density functional theory study on the mechanism of OH-initiated atmospheric photooxidation of ethylbenzene

The reaction mechanism for ethylbenzene with OH radical and O 2 was investigated by density functional theory (DFT) method. The structures, energetics, and relative stability of OH-ethylbenzene reaction intermediate radicals have been determined, and their activation barriers have been analyzed to assess the energetically favorable pathways to propagate the ethylbenzene oxidation. The results of the theoretical study indicate that OH addition is predicted to occur dominantly at the ortho position, with branching ratios of 0.53, 0.31, 0.10, and 0.06 for ortho, para, meta, and ipso additions, respectively, and the calculated overall rate constant is 1.24 × 10 −11 cm 3 molecule −1 s −1, showing a very good agreement with available experimental data. Under atmospheric conditions, oxygen is expected to add to the ethylbenzene-OH adducts forming ethylbenzene peroxy radicals. And subsequent ring closure of the peroxyl radicals to form bicyclic radicals. With relatively low barriers, isomerization of the ethylbenzene bicyclic radicals to more stable epoxide radicals likely occurs, competing with O 2 addition to form bicyclic peroxy radicals. The study provides thermochemical data for assessment of the photochemical production potential of ozone and formation of toxic products and secondary organic aerosol from ethylbenzene photooxidation.

Reply to “Comment on ‘Primary Atmospheric Oxidation Mechanism for Toluene’”

Reply to “Comment on ‘Primary Atmospheric Oxidation Mechanism for Toluene’”

Theoretical Mechanistic Study of the Oxidative Degradation of Benzene in the Troposphere: Reaction of Benzene-HO Radical Adduct with O2.

Competing pathways arising from the reaction of hydroxycyclohexadienyl radical (1) with O2, a key reaction in the oxidative degradation of benzene under tropospheric conditions, have been investigated by means of density functional theory (UB3LYP) and quantum-mechanical (UCCSD(T) and RCCSD(T)) electronic structure calculations. The energetic, structural, and vibrational results furnished by these calculations were subsequently used to perform conventional transition-state computations to predict the rate coefficients and evaluate the product yields. The trans stereoisomer of the peroxyl radical (4) produced by the O2 addition to position 2 of benzene ring in radical 1 is energetically more stable than the cis one, although the rate coefficients at 298 K for the formation of both isomers are predicted to be similar. The cyclization of the cis isomer of 4 to a bicyclic allyl radical (5) involves calculated barrier heights (ΔU(⧧), ΔE(⧧), ΔH(⧧), and ΔG(⧧)) significantly lower than those of the cyclization of the trans isomer of 4. This implies that the formation of the cis isomer of 4 can lead to irreversible loss of radical 1 and that the observed chemical equilibrium 1 + O2 ↔ 4 essentially involves the trans isomer of 4. Although the reaction enthalpies computed for the O2 addition to position 4 of benzene ring in radical 1, affording the cis and trans stereoisomers of a peroxyl radical (6), are similar to those for the addition to position 2, the latter addition mode is clearly preferred because it involves lower barrier heights. The barrier heights computed for the cyclization of either the cis or the trans isomers of 6 to a bicyclic radical bearing a peroxy bridge (7) are about twice those computed for the cyclization of either the cis or the trans isomers of 4. Thus, under tropospheric conditions, it is unlikely that the O2 addition to position 4 of the benzene ring in radical 1 can contribute to the formation of benzene oxidation products.

Density Functional Theory Study on OH-Initiated Atmospheric Oxidation ofm-Xylene

The OH-initiated oxidation reactions of m-xylene are investigated using density functional theory. The structures, energetics, and relative stability of the OH-m-xylene reaction intermediate radicals have been determined, and their activation barriers have been analyzed to assess the energetically favorable pathways to propagate the oxidation. OH addition occurs preferentially at the two ortho positions with the branching ratios of 0.97, 0.02, and 0.01 for ortho, meta, and ipso additions, respectively. The results reveal that the OH-m-xylene-O2 peroxy radicals preferentially cyclize to form bicyclic radicals under atmospheric conditions rather than reacting with NO to lead to ozone formation, and the decomposition to O2 and the hydroxyl m-xylene adduct is competitive with the cyclization process. The bicyclic radicals of m-xylene formed from the major OH-addition pathways (i.e., ortho positions) are more probable to form bicyclic peroxy radicals by reacting with O2. This study provides thermochemical and kinetic data of the OH-initiated reactions of m-xylene for assessment of the role of aromatic hydrocarbons in photochemical production of ozone, toxic products, and secondary organic aerosols.