Radical Decomposition Research Articles

Identification of a Radical Decomposition Pathway(s) of Polycyclic Aromatic Hydrocarbon by the Vibrational Frequency Calculations with DFT Method

Identification of a Radical Decomposition Pathway(s) of Polycyclic Aromatic Hydrocarbon by the Vibrational Frequency Calculations with DFT Method

Seasonality, sources and sinks of C1–C5 alkyl nitrates in the Colorado Front Range

We describe observations of C1–C5 alkyl nitrates made at the Boulder Atmospheric Observatory in Northern Colorado in winter 2011, spring 2015, and summer 2015. Average mixing ratios of the alkyl nitrates are similar across the seasons, but increased diel variability in summer suggests increased production balanced by increased loss relative to winter and spring. We use a sequential production-destruction model based on ratios of alkyl nitrates to their parent alkanes to investigate seasonal sources and sinks of C1–C5 alkyl nitrates. We explore the role of uncertainties in the production and loss kinetic parameters on the interpretation of local atmospheric photochemical aging through the use of a photochemical clock based on the evolution of the ratios of alkyl nitrates to their parent alkanes over time. Photochemical age is typically consistent with hours from sunrise, suggesting that the site experiences well-mixed air masses dominated by daily photochemistry with little carry-over from the previous day or from other locations. Contrary to studies in other locations, we obtain good model-measurement agreement using a newer upper-bound ethyl nitrate branching ratios. This suggests that the efficiency of ethyl nitrate production from ethane oxidation has previously been underestimated, and decreases the relative importance of alkoxy radical decomposition versus ethane photochemistry on ethyl nitrate production. We estimate the dry deposition velocity of methyl nitrates is small and consistent with previous estimates, and that deposition velocities increase with carbon number for the C2–C5 RONO2. Dry deposition is a small daytime sink relative to photolysis and reaction with OH for the alkyl nitrates, but improves the model-measurement comparison for methyl nitrate.

Early stages of p-phenylenediamine antiozonants reaction with ozone: Radical cation and nitroxyl radical formation

Physical-chemical considerations suggest that the diene rubber olefinic double bonds protection by N,N′-substituted p-phenylenediamines (PPDs) antiozonants (AOZs) is due to their very low ionization potential in comparison to the ionization potential of the olefinic double bonds. Thus, the ozone scavenging action exerted by PPDs AOZs is due to one electron transfer from the PPDs to the ozone molecule with the formation of the PPDs radical cation. Furthermore, Hammett-type plot of the reaction rate with ozone of a series of aromatic amines including also N,N′-substituted PPDs suggests that the result of ozone oxidation is the formation of PPDs radical cations. In the present study, a series of common N,N′-substituted PPDs used as AOZs in rubber compounds were oxidized with ozone in trichloromethane and/or o-dichlorobenzene solutions and studied with Electron Spin Resonance (ESR). Experimental evidences about the radical cation formation and the subsequent almost quantitative PPDs nitroxyl radical formation as a result of the reaction of PPDs radical cation with ozone radical anion were collected and support experimentally the physical-chemical premises. Upon addition of trichloroacetic acid to the PPDs nitroxyl radical solutions, the immediate decomposition of the nitroxyl radical and the formation of stabilized PPDs radical cation was observed.

Kinetics and mechanism of the reactions of chloromethyl radical with acetylene and decomposition of 1-chloroallyl and 2-chloromethyl vinyl radicals

Three reactions occurring on the C3H4Cl potential energy surface (PES) were studied computationally using quantum chemistry and master equation/RRKM: CH2Cl + C2H2 → Products (1), CH2CHCHCl → Products (2), and CH2ClCHCH → Products (3). The results indicate low importance of the 1,3-Cl shift channel linking the PESs of reactions 1 and 2 despite a relatively low energy barrier. Addition and chemically activated H elimination are the two major products of reaction 1, with branching dependent on pressure and temperature. Cl and cyclopropene are the major products of reaction 2, and CH2Cl + C2H2 – of reaction 3.

Products from the Oxidation of n-Butane from 298 to 735 K Using Either Cl Atom or Thermal Initiation: Formation of Acetone and Acetic Acid-Possible Roaming Reactions?

The oxidation of 2-butyl radicals (and to a lesser extent 1-butyl radicals) has been studied over the temperature range of 298-735 K. The reaction of Cl atoms (formed by 360 nm irradiation of Cl2) with n-butane generated the 2-butyl radicals in mixtures of n-C4H10, O2, and Cl2 at temperatures below 600 K. Above 600 K, 2-butyl radicals were produced by thermal combustion reactions in the absence of chlorine. The yields of the products were measured by gas chromatography using a flame ionization detector. Major products quantified include acetone, acetic acid, acetaldehyde, butanone, 2-butanol, butanal, 1- and 2- chlorobutane, 1-butene, trans-2-butene, and cis-2-butene. At 298 K, the major oxygenated products are those expected from bimolecular reactions of 2-butylperoxy radicals (butanone, 2-butanol, and acetaldehyde). As the temperature rises to 390 K, the butanone decreases while acetaldehyde increases because of the increased rate of 2-butoxy radical decomposition. Acetone and acetic acid first appear in significant yield near 400 K, and these species rise slowly at first and then sharply, peaking near 525 K at yields of ∼25 and ∼20 mol %, respectively. In the same temperature range (400-525 K), butanone, acetaldehyde, and 2-butanol decrease rapidly. This suggests that acetone and acetic acid may be formed by previously unknown reaction channels of the 2-butylperoxy radical, which are in competition with those that lead to butanone, acetaldehyde, and 2-butanol. Above 570 K, the yields of acetone and acetic acid fall rapidly as the yields of the butenes rise. Experiments varying the Cl atom density, which in turn controls the entire radical pool density, were performed in the temperature range of 410-440 K. Decreasing the Cl atom density increased the yields of acetone and acetic acid while the yields of butanone, acetaldehyde, and 2-butanol decreased. This is consistent with the formation of acetone and acetic acid by unimolecular decomposition channels of the 2-butylperoxy radical, which are in competition with the bimolecular channels that form butanone, acetaldehyde, and 2-butanol. Such unimolecular decomposition channels would be unlikely to proceed through conventional transition states because those states would be very constrained. Therefore, the possibility that these decomposition channels proceed via roaming should be considered. In addition, we investigated and were unable to fit our data trends by a simplified ketohydroperoxide mechanism.

A chemical kinetic mechanism for the low- and intermediate-temperature combustion of Polyoxymethylene Dimethyl Ether 3 (PODE3)

Polyoxymethylene Dimethyl Ether (PODEn, n = 3–5) is a potential alternative fuel or a green fuel additive for diesel engines, owing to the unique C–O alternating chain structure (CH3O[CH2O]nCH3) and high cetane number. In this paper, the rate constants for five reaction classes about the model molecule PODE1, including hydrogen abstraction, beta-scission of corresponding R radicals, isomerization of RO2 radicals, and decomposition of QOOH radicals, were calculated with canonical transition state theory (CTST) at the CCSD(T)-F12a/aug-cc-pVTZ//M06-2X/def2-TZVPP or CBS-QB3//B3LYP/CBSB7 level of theory. Based on the quantum chemistry and chemical kinetic study of the model molecule PODE1, the first detailed reaction mechanism (225 species, 1082 reactions) which can describe the ignition characteristics of PODE3 at low and intermediate temperature was developed. To validate this mechanism, rapid compression machine (RCM) was used to conduct the quasi-homogeneous experiments to measure the total ignition delay time at various effective temperatures (640–865 K) for three different PODE3/O2/N2 mixtures (ϕ = 0.5, O2:N2 = 1:8; ϕ = 1.0, O2:N2 = 1:15; ϕ = 1.5, O2:N2 = 1:20) and two different effective pressures (10 bar, 15 bar). Homogeneous Charge Compression Ignition (HCCI) experiments fueled with PODEn (n = 1–4) mixture, in which the mass fraction of PODE3 is 88.9% were also conducted in a naturally aspirated single-cylinder HCCI research engine at 1600 r/min, two exhaust gas recirculation (EGR) rates (0%, 42%), and three different charge-mass equivalence ratios for each EGR rate to take the real engine working condition into consideration. Good agreement was achieved in the comparison of the experimental data and the simulation results utilizing our newly developed mechanism for PODE3. This surrogate model will contribute to the design of fuels blended with PODEn, and to the prediction of the combustion and emission characteristics of engines using PODEn.

Helium Nanodroplet Isolation of the Cyclobutyl, 1-Methylallyl, and Allylcarbinyl Radicals: Infrared Spectroscopy and Ab Initio Computations.

Gas-phase cyclobutyl radical (•C4H7) is produced via pyrolysis of cyclobutylmethyl nitrite (C4H7(CH2)ONO). Other •C4H7 radicals, such as 1-methylallyl and allylcarbinyl, are similarly produced from nitrite precursors. Nascent radicals are promptly solvated in liquid He droplets, allowing for the acquisition of infrared spectra in the CH stretching region. For the cyclobutyl and 1-methylallyl radicals, anharmonic frequencies are predicted by VPT2+K simulations based upon a hybrid CCSD(T) force field with quadratic (cubic and quartic) force constants computed using the ANO1 (ANO0) basis set. A density functional theoretical method is used to compute the force field for the allylcarbinyl radical. For all •C4H7 radicals, resonance polyads in the 2800-3000 cm-1 region appear as a result of anharmonic coupling between the CH stretching fundamentals and CH2 bend overtones and combinations. Upon pyrolysis of the cyclobutylmethyl nitrite precursor to produce the cyclobutyl radical, an approximately 2-fold increase in the source temperature leads to the appearance of spectral signatures that can be assigned to 1-methylallyl and 1,3-butadiene. On the basis of a previously reported •C4H7 potential energy surface, this result is interpreted as evidence for the unimolecular decomposition of the cyclobutyl radical via ring opening, prior to it being captured by helium droplets. On the •C4H7 potential surface, 1,3-butadiene is formed from cyclobutyl ring opening and H atom loss, and the 1-methylallyl radical is the most energetically stable intermediate along the decomposition pathway. The allylcarbinyl radical is a higher-energy •C4H7 intermediate along the ring-opening path, and the spectral signatures of this radical are not observed under the same conditions that produce 1-methylallyl and 1,3-butadiene from the unimolecular decomposition of cyclobutyl.

Removal of Tin from Extreme Ultraviolet Collector Optics by In-Situ Hydrogen Plasma Etching

Extreme ultraviolet (EUV) lithography produces 13.5 nm light by irradiating a droplet of molten Sn with a laser, creating a dense, hot laser-produced plasma and ionizing the Sn to the + 8 through + 12 states. An unwanted by-product is deposition of Sn debris on the collector optic, which focuses the EUV light emitting from the plasma. Consequently, collector reflectivity is degraded. Reflectivity restoration can be accomplished by means of Sn etching by hydrogen radicals, which can be produced by an H2 plasma and etch the Sn as SnH4. It has previously been shown that plasma cleaning can successfully create radicals and restore EUV reflectivity but that the Sn removal rate is not necessarily limited by the radical density. Additionally, while Sn etching by hydrogen radicals has been shown by multiple investigators, quantification of the mechanisms behind Sn removal has never been undertaken. This paper explores the processes behind Sn removal. Experiments and modeling show that, within the parameter space explored, the limiting factor in Sn etching is not radical flux or SnH4 decomposition, but ion energy flux. Thus the removal is akin to reactive ion etching.

Molecular insights for the HFO-1345fz +X (X = Cl, O3 or NO3[rad]) reaction and fate of alkoxy radicals initiated by Cl: DFT investigations

In the present work we investigate the reactions of HFO-1345fz (CF3CF2CH=CH2) with atomic Cl, O3 molecule and NO3 radical employing the CCSD(T)/6-311G(d,p)//M06-2x/6-311++G(d,p) theory. Addition and abstraction reaction channels are considered. The rate constants for the abstraction channel are derived from the transition state theory whereas the variational transition state model was applied to estimate the rate constants for the addition channel over the temperature range 200–1000K. It has been shown that the reaction occurs predominantly via addition to unsaturated carbon of HFO-1345fz. Calculated rate constants accompanying the reaction of HFO-1345fz with the Cl atom turns out to be 1.07×10−10cm3molecule−1sec−1 compared to 1.01×10−19cm3molecule−1sec−1 for O3 and 2.32×10−18cm3molecule−1sec−1 in case of the NO3. Calculated rate constants compare well with the experiments reported earlier. The atmospheric implications and secondary reactions (atmospheric decomposition of alkoxy radical) of the primary product initiated by Cl atom have further been analyzed. The product branching ratio and atmospheric lifetimes are given.

Catalysis of radical reactions in mixed micelles of surfactants with hydroperoxides

The peculiarities of the catalytic action of cationic surfactants (CSurf) in combination with hydroperoxides on the generation of radicals and the influence of various factors on this process (transition metal compounds, oxygen, and external magnetic field) were considered. In the oxidized hydrocarbons (RH), hydroperoxides (ROOH), which are the primary amphiphilic products of oxidation, form mixed micelles {mROOH…nCSurf} with CSurf, in which fast decomposition of ROOH into radicals occurs and other polar components (metal compounds, inhibitors, etc.) can concentrate, which significantly affects the rate and mechanism of oxidation. The cationic surfactants immobilized on a solid support retain the ability to catalyze the decomposition of hydroperoxides, forming radicals, and to initiate radical oxidation and polymerization. It was found that acetylcholine, which is the most important neurotransmitter that plays an important role in the neuromuscular and cognitive activity of living beings, like cationic surfactants, catalyzes the radical decomposition of hydroperoxides in organic media, and the yield of radicals in this process decreases in a magnetic field and in the presence of oxygen.

Aerosol Fragmentation Driven by Coupling of Acid-Base and Free-Radical Chemistry in the Heterogeneous Oxidation of Aqueous Citric Acid by OH Radicals.

A key uncertainty in the heterogeneous oxidation of carboxylic acids by hydroxyl radicals (OH) in aqueous-phase aerosol is how the free-radical reaction pathways might be altered by acid-base chemistry. In particular, if acid-base reactions occur concurrently with acyloxy radical formation and unimolecular decomposition of alkoxy radicals, there is a possibility that differences in reaction pathways impact the partitioning of organic carbon between the gas and aqueous phases. To examine these questions, a kinetic model is developed for the OH-initiated oxidation of citric acid aerosol at high relative humidity. The reaction scheme, containing both free-radical and acid-base elementary reaction steps with physically validated rate coefficients, accurately predicts the experimentally observed molecular composition, particle size, and average elemental composition of the aerosol upon oxidation. The difference between the two reaction channels centers on the reactivity of carboxylic acid groups. Free-radical reactions mainly add functional groups to the carbon skeleton of neutral citric acid, because carboxylic acid moieties deactivate the unimolecular fragmentation of alkoxy radicals. In contrast, the conjugate carboxylate groups originating from acid-base equilibria activate both acyloxy radical formation and carbon-carbon bond scission of alkoxy radicals, leading to the formation of low molecular weight, highly oxidized products such as oxalic and mesoxalic acid. Subsequent hydration of carbonyl groups in the oxidized products increases the aerosol hygroscopicity and accelerates the substantial water uptake and volume growth that accompany oxidation. These results frame the oxidative lifecycle of atmospheric aerosol: it is governed by feedbacks between reactions that first increase the particle oxidation state, then eventually promote water uptake and acid-base chemistry. When coupled to free-radical reactions, acid-base channels lead to formation of low molecular weight gas-phase reaction products and decreasing particle size.

Tailored Polarizing Hybrid Solids with Nitroxide Radicals Localized in Mesostructured Silica Walls

Hyperpolarization by dynamic nuclear polarization relies on the microwave irradiation of paramagnetic radicals dispersed in molecular glasses to enhance the nuclear magnetic resonance (NMR) signals of target molecules. However, magnetic or chemical interactions between the radicals and the target molecules can lead to attenuation of the NMR signal through paramagnetic quenching and/or radical decomposition. Here we describe polarizing materials incorporating nitroxide radicals within the walls of the solids to minimize interactions between the radicals and the solute. These materials can hyperpolarize pure pyruvic acid, a particularly important substrate of clinical interest, while nitroxide radicals cannot be used, even when incorporated in the pores of silica, because of reactions between pyruvic acid and the radicals. The properties of these materials can be engineered by tuning the composition of the wall by introducing organic functionalities.

Assessment of catalytic activities of selected iron hydroxysulphates biosynthesized using Acidithiobacillus ferrooxidans for the degradation of phenol in heterogeneous Fenton-like reactions

Schwertmannite and jarosite, two secondary Fe(III)-hydroxysulfate minerals abundant in acid-mine drainage, can be biosynthesized by an Fe-oxidizing bacterium Acidithiobacillus ferrooxidans, but the differences of their catalytic activities in heterogeneous Fenton-like reactions are still unclear to date. In the present study, the catalytic activities of biosynthesized schwertmannite and jarosite were compared through investigating the heterogeneous Fenton-like degradation of a target organic pollutant phenol. Results showed that these two catalysts efficiently catalyzed heterogeneous Fenton-like degradation of phenol, and the degradation in both systems consisted of an induction period (first-stage) and a followed rapid degradation stage (second-stage). Schwertmannite exhibited a much higher catalytic capacity than jarosite, because of its shorter induction period and higher kinetic rate (k) of the second-stage, but jarosite catalytically degraded phenol in a much wider pH range (pH 3.0–7.5) over schwertmannite (pH 3.0–4.5). Based on the results of radical scavenger effect, H2O2 decomposition, Fe leaching and X-ray photoelectron spectroscopic (XPS) analysis, similar catalytic mechanisms were proposed in schwertmannite/H2O2 and jarosite/H2O2 systems, in which the induction period can be ascribed to the activation of catalysts by H2O2 and the second-stage was mainly attributed to the homogeneous Fenton reaction in bulk solution. During the catalytic degradation of phenol, catechol was firstly accumulated and was then further decomposed into smaller molecular organic acids. The repeated uses of schwertmannite and jarosite for phenol degradation manifested their stability and reusability as Fenton-like catalysts for the degradation of organic pollutants.

Molecular Products and Fundamentally Based Reaction Pathways in the Gas-Phase Pyrolysis of the Lignin Model Compound p-Coumaryl Alcohol.

The fractional pyrolysis of lignin model compound para-coumaryl alcohol (p-CMA) containing a propanoid side chain and a phenolic OH group was studied using the System for Thermal Diagnostic Studies at temperatures from 200 to 900 °C, in order to gain mechanistic insight into the role of large substituents in high-lignin feedstocks pyrolysis. Phenol and its simple derivatives p-cresol, ethyl-, propenyl-, and propyl-phenols were found to be the major products predominantly formed at low pyrolysis temperatures (<500 °C). A cryogenic trapping technique was employed combined with EPR spectroscopy to identify the open-shell intermediates registered at pyrolysis temperatures above 500 °C. These were characterized as radical mixtures primarily consisting of oxygen-linked conjugated radicals. A comprehensive potential energy surface analysis of p-CMA and p-CMA + H atom systems was performed using various DFT protocols to examine the possible role of concerted molecular eliminations and free-radical mechanisms in the formation of major products. Other significant unimolecular concerted reactions along with formation and decomposition of primary radicals are also described and evaluated. The calculations suggest that a set of the chemically activated secondary radical channels is relevant to the low temperature product formation under fractional pyrolysis conditions.

(Keynote) Electroreduction of Nitrocyclopropanes and Nitrophenyl Cyclopropanes: Mechanism of Cleavage of the Radical Anion

There are three possible mechanisms of cleavage of a nitrocyclopropane radical anion 1•‾, mechanisms A (homolytic cleavage to a cyclopropyl radical 2• and a nitrite anion), B (homolytic cleavage to a distonic radical anion 3•‾ ), and C (heterolytic cleavage to a distonic radical anion 4•‾ [1]. With the models synthesized so far (Y = Z= alkyl, H = COOCH3), only products of mechanism A have been observed [1]. On the other hand, only two cleavage mechanisms are possible for a nitrophenyl cyclopropane radical anion 5•‾, mechanisms D (homolytic cleavage to the distonic radical anion 6•‾ ) and E (heterolytic cleavage to the distonic radical anion 7•‾ ) [1]. The isolation and characterization of the products resulting from the cleavage of the nitrophenyl radical anion 5•‾ do not allow to distinguish between the two cleavage mechanisms D (homolytic cleavage) and E (heterolytic cleavage) leading to the distonic radical anions 6•‾ and 7•‾ respectively. To shed light on the mechanisms of cleavage of those radical anions, density functional (DFT) quantum chemical calculations were carried out on species 1-3 (B3LYP 6-31+G(d), including solvation energies in acetonitrile computed using the polarizable continuum method. The results clearly favor mechanism B and species 3 over mechanisms A and C for the decomposition of anion radicals 1•‾ whenZ = Y = alkyl and X = H. [1] F. Couture-Martin, A. Sardashti, C. Cristea, J.M. Chapuzet, J. Lessard, ECS Transactions, 2008, 13, 13-19. Figure 1

Plasma upgrading of 4methylanisole: A novel approach for hydrodeoxygenation of bio oil without using a hydrogen source

Characteristics of pulsed corona discharge were investigated experimentally to combine methyl radical decomposition and hydrodeoxygenation reaction for bio oil upgrading. In contrast to conventional upgrading methods, no hydrogen was used as a feed. The required hydrogen was generated in situ considering the ability of plasma to decompose methyl radicals to hydrogen. Pin to plate corona discharge and 4methylanisole as a model of bio oil were used. Effect of operating and design parameters namely, carrier gas flow rate, pulse repetition frequency, plate electrode diameter, pin number and gap distance were studied on the conversion and selectivity of products. Variation of mentioned parameters can alter the electron and energetic species density and energy which results in improving reaction conversion and product selectivity. The upgrading mechanism of 4ethylanisole was described by a reaction pathway. Demethylation, transalkylation, hydrogenolysis, demethoxylation and methane decomposition are the main chemical reactions. It is worth noting that the optimum condition for methyl decomposition and hydrodeoxygenation reaction are similar for most of design and operating parameters. It was found that the most valuable and abundant products were BTX and phenol with maximum selectivity of about 15% and 46%, respectively. Results indicated that corona discharge plasma can overcome the disadvantages of conventional upgrading process which caused by high pressure hydrogen.

Bimolecular Reactions between Dimethylnitramine and Its Radical Decomposition Products.

Bimolecular reactions between intact nitramines and their radical decomposition products can accelerate thermal decomposition, yet the detailed mechanisms of such reactions are not well understood. We have used density functional theory at the M06/6-311++G(3df,3pd) level to locate transition structures and compute 0 K activation barriers for various gas-phase reactions that may contribute to radical-assisted decomposition of dimethylnitramine (DMNA, (CH3)2NNO2). Our calculations indicate that H abstraction from DMNA is the lowest-barrier mechanism for most radicals and a subsequent N-N β-scission in the alkyl radical 3 leads to an imine intermediate and NO2. H abstraction is thus responsible for conversion of most radicals to NO2. Also, among the nine radicals considered, NO is found to be least reactive and its reactions with DMNA yield dimethylnitrosoamine (DMNSA, (CH3)2NNO), a known product of DMNA decomposition.

Kinetics of the reactions of OH radicals with n-butyl, isobutyl, n-pentyl and 3-methyl-1-butyl nitrates

The kinetics of the reactions of n-butyl (BTN), isobutyl (IBN), n-pentyl (PTN) and 3-methyl-1-butyl (3M1BN) nitrates with OH radicals has been studied using a low pressure flow tube reactor combined with a quadrupole mass spectrometer. The rate constants of the title reactions were determined under pseudo-first order conditions from kinetics of OH consumption in high excess of the nitrates. The overall rate coefficients, kBTN = 1.0 × 10−13 (T/298)3.36 exp(838/T) (T = 288–500 K), kIBN = 2.8 × 10−14 (T/298)4.09 exp(1127/T) (T = 283–500 K), kPTN = 1.26 × 10−12 (T/298)4.56 exp(45/T) (T = 298–496 K) and k3M1BN = 8.47×10−14 (T/298)3.52 exp(1069/T) cm3molecule−1s−1 (T = 288–538 K) (with conservative 15% uncertainty), were determined at a total pressure of 1 Torr of helium. The yields of the carbonyl compounds, n-butanal (n-C3H7CHO) and isobutanal ((CH3)2CHCHO), resulting from the abstraction by OH of an α-hydrogen atom in n-butyl and isobutyl nitrates, followed by α-substituted alkyl radical decomposition, were determined at T = 300 K to be 0.10 ± 0.02 and 0.15 ± 0.03, respectively. The calculated tropospheric lifetimes of BTN, IBN, PTN and 3M1BN indicate that reaction of these nitrates with OH represents an important sink of these compounds in the atmosphere.

Formation Mechanisms of Naphthalene and Indene: From the Interstellar Medium to Combustion Flames.

The article addresses the formation mechanisms of naphthalene and indene, which represent prototype polycyclic aromatic hydrocarbons (PAH) carrying two six-membered and one five- plus a six-membered ring. Theoretical studies of the relevant chemical reactions are overviewed in terms of their potential energy surfaces, rate constants, and product branching ratios; these data are compared with experimental measurements in crossed molecular beams and the pyrolytic chemical reactor emulating the extreme conditions in the interstellar medium (ISM) and the combustion-like environment, respectively. The outcome of the reactions potentially producing naphthalene and indene is shown to critically depend on temperature and pressure or collision energy and hence the reaction mechanisms and their contributions to the PAH growth can be rather different in the ISM, planetary atmospheres, and in combustion flames at different temperatures and pressures. Specifically, this paradigm is illustrated with new theoretical results for rate constants and product branching ratios for the reaction of phenyl radical with vinylacetylene. The analysis of the formation mechanisms of naphthalene and its derivatives shows that in combustion they can be produced via hydrogen-abstraction-acetylene-addition (HACA) routes, recombination of cyclopentadienyl radical with itself and with cyclopentadiene, the reaction of benzyl radical with propargyl, methylation of indenyl radical, and the reactions of phenyl radical with vinylacetylene and 1,3-butadiene. In extreme astrochemical conditions, naphthalene and dihydronaphthalene can be formed in the C6H5 + vinylacetylene and C6H5 + 1,3-butadiene reactions, respectively. Ethynyl-substituted naphthalenes can be produced via the ethynyl addition mechanism beginning with benzene (in dehydrogenated forms) or with styrene. The formation mechanisms of indene in combustion include the reactions of the phenyl radical with C3H4 isomers allene and propyne, reaction of the benzyl radical with acetylene, and unimolecular decomposition of the 1-phenylallyl radical originating from 3-phenylpropene, a product of the C6H5 + propene reaction, or from C6H5 + C3H5.

Radical Decomposition of Ether-Based Electrolytes for Li-S Batteries

In this work, the stability of ether-based electrolytes for Li-S batteries is investigated with particular regard to the effect of dissolved oxygen. Specifically, the performance of two different electrolyte solvents, i.e., 1,2-dimethoxyethane and its mixture with 1,3-dioxolane (DME:DOL, 1:1 v/v), is characterized in cells assembled in dry air environment, which would substantially lower production costs with respect to inert atmosphere (Ar). Although stability of all the components would suggest that Li-S batteries built in both the environments should behave similarly, it is found that cells containing the DME:DOL-based electrolyte are rather unstable in the presence of O2 in contrast to those employing DME-based electrolyte, which show a relatively good performance. The different sensitivity toward O2 of these electrolytes is associated to the ring-opening reaction of DOL, which happens to a greater extent when O2 is present, but occurs also in its absence. Based on these results a mechanism for electrolyte degradation in Li-S cells, and its reaction with dissolved polysulfides is proposed, which rationally explain for the first time the behavior already reported in literature for these kind of batteries. These findings are also relevant to the field of Li-O2 batteries, where these ether-based electrolytes are also used.