Reactions Of Polycyclic Aromatic Hydrocarbons Research Articles

Rapid Access to Nanographenes and Fused Heteroaromatics by Palladium‐Catalyzed Annulative π‐Extension Reaction of Unfunctionalized Aromatics with Diiodobiaryls

AbstractEfficient and rapid access to nanographenes and π‐extended fused heteroaromatics is important in materials science. Herein, we report a palladium‐catalyzed efficient one‐step annulative π‐extension (APEX) reaction of polycyclic aromatic hydrocarbons (PAHs) and heteroaromatics, producing various π‐extended aromatics. In the presence of a cationic Pd complex, triflic acid, silver pivalate, and diiodobiaryls, diverse unfunctionalized PAHs and heteroaromatics were directly transformed into larger PAHs, nanographenes, and π‐extended fused heteroaromatics in a single step. In the reactions that afford [5]helicene substructures, simultaneous dehydrogenative ring closures occur at the fjord regions to form unprecedented larger nanographenes. This successive APEX reaction is notable as it stiches five aryl–aryl bonds by C−H functionalization in a single operation. Moreover, the unique molecular structures, crystal‐packing structures, photophysical properties, and frontier molecular orbitals of the thus‐formed nanographenes were elucidated.

Hydrogenation of polycyclic aromatic hydrocarbons over a solid superacid

Hydrogenation reactions of some polycyclic aromatic hydrocarbons (PAHs), including anthracene, 9-phenylanthracene (9-PA) and 9,10-diphenylanthracene (9,10-DPA), were carried out under moderate reaction conditions (with the initial hydrogen pressure of 5MPa at room temperature, reaction temperature was 300°C) over a solid superacid (SSA). The results showed that the reactivity of the PAHs decreased in the order of anthracene >9-PA>9,10-DPA. It was found that not only the hydrogenation and the reversible dehydrogenation through hydrogen atom transfer, but also disproportionation reaction by intermolecular hydrogen transfer took place in the process of PAHs reactions. The catalyst proved to be versatile for the hydrogenation of anthracene and 9-PA through biatomic hydrogen transfer (BAHT) and for the hydrocraking of 9,10-DPA by monatomic hydrogen transfer (MAHT). The SSA is able to catalyze the cleavage of the CarCar bridge bond in 9,10-DPA to produce 9-PA and benzene, in which the key step is the generation of the mobile H+ as well as the H+ transferring to the ipso-position of anthracene ring in 9,10-DPA. The SSA has the selectivity to catalyze the hydrogenation or hydrocracking of different PAHs.

Electrochemistry coupled to (LC-)MS for the simulation of oxidative biotransformation reactions of PAHs

Electrochemistry coupled to liquid chromatography and mass spectrometry was used for simulating the biological and environmental fate of polycyclic aromatic hydrocarbons (PAHs) as well as for studying the PAH degradation behavior during electrochemical remediation. Pyrene and benzo[a]pyrene were selected as model compounds and oxidized within an electrochemical thin-layer cell equipped with boron-doped diamond electrode. At potentials of 1.2 and 1.6 V vs. Pd/H2, quinones were found to be the major oxidation products for both investigated PAHs. These quinones belong to a large group of PAH derivatives referred to as oxygenated PAHs, which have gained increasing attention in recent years due to their high abundance in the environment and their significant toxicity. Separation of oxidation products allowed the identification of two pyrene quinone and three benzo[a]pyrene quinone isomers, all of which are known to be formed via photooxidation and during mammalian metabolism. The good correlation between electrochemically generated PAH quinones and those formed in natural processes was also confirmed by UV irradiation experiments and microsomal incubations. At potentials higher than 2.0 V, further degradation of the initial oxidation products was observed which highlights the capability of electrochemistry to be used as remediation technique.

Identification of Nitration Products during Heterogeneous Reaction of NO2 on Soot in the Dark and under Simulated Sunlight.

Author: The present work, involving the formation of NO and nitrous acid (HONO) and the nitration of polycyclic aromatic hydrocarbon (PAHs) to nitro-PAHs as well as the uptake coefficients of NO2, has been performed on a normal-pressure flow reactor to identify the nitration products during the heterogeneous reaction of soot toward NO2 in the dark and under simulated sunlight. Two types of soot particles, namely the commercial black carbon (BC) and the diesel engine soot (ES), were selected as the studied soot to compare the impacts of soot properties on heterogeneous nitration. During the whole reaction on either of the two studied soots in the dark, a fast reversible physical adsorption is observed at the very beginning, followed by a slow irreversible chemical conversion from NO2 to HONO and NO, in good agreement with the "reduction-oxidation" mechanism. HONO is the most abundant product during the nitration reaction on the two studied soots, contributing to 70-90% of consumed NO2 after 50 min exposure. Reaction orders of NO2 for HONO are determined as 1.20 ± 0.07 and 1.31 ± 0.04 for BC and ES, respectively, which are both close to first-order. Moreover, four sorts of PAHs compounds and their five nitro-derivatives have been identified and quantified during the reaction. About 40% and 20% of the total four measured PAHs are consumed on BC and ES, respectively, resulting in an increase in the total five measured nitro-PAHs by 21-fold on BC and 2.8-fold on ES. Finally, the impacts of light on gaseous and organics products have been investigated and the results confirm that simulated sunlight can enhance the reactivity of PAHs toward NO2 and cause the photolysis of newly formed nitro-compounds with more HONO formation, strongly suggesting that photochemistry of soot in the presence of NO2 is of great importance to be a photochemical source of HONO and would also influence the fates of PAHs and nitro-PAHs on soot.

Reaction kinetics of hydrogen abstraction from polycyclic aromatic hydrocarbons by H atoms.

Hydrogen abstraction reactions of polycyclic aromatic hydrocarbons (PAH) by H atoms play a very important role in both PAH and soot formation processes. However, large discrepancies up to a few orders of magnitude exist among the literature rate constant values. To increase the reliability of the computed rate constants, it is critical to obtain highly accurate potential energy surfaces. For this purpose, we have investigated the energetics of hydrogen abstraction from benzene and naphthalene using both high level-of-theory quantum chemistry methods and a series of density functional theory (DFT) methods, among which M06-2X/6-311g(d,p) has the best performance with a mean unsigned deviation from the CCSD(T)/CBS calculations of 1.0 kcal mol-1 for barrier heights and reaction energies. Thus, M06-2X/6-311g(d,p) has then been applied to compute the potential energy surfaces of the hydrogen abstraction reactions of a series of larger PAH. Based on the quantum chemistry calculations, rate constants are computed using the canonical transition state theory. The effects of the PAH size, structure, and reaction site on the energetics and rate constants are examined systematically. Finally, the hydrogen abstraction rate constants for application in PAH and soot surface chemistry models are recommended.

Estimating population exposure to ambient polycyclic aromatic hydrocarbon in the United States – Part I: Model development and evaluation

PAHs (polycyclic aromatic hydrocarbons) in the environment are of significant concern due to their negative impact on human health. PAH measurements at the air toxics monitoring network stations alone are not sufficient to provide a complete picture of ambient PAH levels or to allow accurate assessment of public exposure in the United States. In this study, speciation profiles for PAHs were prepared using data assembled from existing emission profile data bases, and the Sparse Matrix Operator Kernel Emissions (SMOKE) model was used to generate the gridded national emissions of 16 priority PAHs in the US. The estimated emissions were applied to simulate ambient concentration of PAHs for January, April, July and October 2011, using a modified Community Multiscale Air Quality (CMAQ) model (v5.0.1) that treats the gas and particle phase partitioning of PAHs and their reactions in the gas phase and on particle surface. Predicted daily PAH concentrations at 61 air toxics monitoring sites generally agreed with observations, and averaging the predictions over a month reduced the overall error. The best model performance was obtained at rural sites, with an average mean fractional bias (MFB) of −0.03 and mean fractional error (MFE) of 0.70. Concentrations at suburban and urban sites were underestimated with overall MFB=−0.57 and MFE=0.89. Predicted PAH concentrations were highest in January with better model performance (MFB=0.12, MFE=0.69; including all sites), and lowest in July with worse model performance (MFB=−0.90, MFE=1.08). Including heterogeneous reactions of several PAHs with O3 on particle surface reduced the over-prediction bias in winter, although significant uncertainties were expected due to relative simple treatment of the heterogeneous reactions in the current model.

Aqueous secondary formation of brominated, chlorinated, and mixed halogenated pyrene in presence of halide ions

We examined the secondary production of halogenated derivatives of polycyclic aromatic hydrocarbons (PAHs) in surface seawater. Pyrene was selected as the model compound and exposed to UV irradiation in synthetic seawater for various irradiation times. Pyrene underwent rapid photochemical reactions that produced various halogenated derivatives including 1-chloropyrene, 1-bromopyrene, three unidentified dichloropyrenes, and three unidentified bromochloropyrenes. The production of 1-chloropyrene (220–360 nM) was higher than that of 1-bromopyrene (7.3–12 nM), reflecting the high chlorine content of seawater. A pilot field survey was conducted to test the environmental implications of these results, and fresh, brackish, and seawater samples were collected in southwestern Japan. The variation in the concentration ratios between 1-chloropyrene and pyrene implied the presence of a specific source of 1-chloropyrene in coastal water, which can be partly explained by the secondary production observed in our photolysis experiments. In sum, the photochemical reactions of PAHs are a potential secondary source of halogenated PAHs, especially in marine environments heavily contaminated with PAHs.

Aliphatic and polycyclic aromatic hydrocarbons (PAHs) in soils of the northwest Qinling Mountains: Patterns, potential risk and an appraisal of the PAH ratios to infer their source

ABSTRACTSurface soils from the tourist areas of the northwest Qinling Mountains were analyzed to determine the concentrations, probable sources and potential risks of hydrocarbons. Concentrations of aliphatic and aromatic hydrocarbons ranged from 4.18 to 3240 ng g−1 and 0.0462 to 101 ng g−1 dry weight, respectively. The extent of soil contamination by hydrocarbons was generally typified by unpolluted to slightly polluted levels. The incremental lifetime cancer risks (ILCRs) for exposure to soil-borne PAHs indicated complete safety for tourists. Early diagenesis of natural products, bacteria activities and petroleum were the three main sources of aliphatic hydrocarbons, while the transport of air pollutants from pyrolytic processes was the main origin of PAHs. Because the photochemical reaction of PAHs in the atmosphere would produce lower ratios for Ant/(Ant + Phe), BaA/(BaA + Chr) and IcdP/(IcdP + BghiP), but a higher ratio for Fla/(Fla + Pyr), the source classification highly depended on the diagnostic ratios chosen. The plot of ΣCOM/Σ13PAH vs. ΣLMW/ΣHMWPAH provide additional information to distinguish the origins of PAHs, and it showed a cluster of pyrogenic sources except for sample JFS-8. Four sources were resolved by principal component analysis: (1) a low temperature pyrogenic process related to the use of fossil fuel and biomass, such as charcoal, straw and wood, which contributes 63.1% of the measured PAHs; (2) the potential contribution of diagenetic processes, contributing 18.4%; (3) traffic emissions, contributing 9.27%; and (4) bioconversion/bacterial action, contributing 5.82%. Additionally, there was a good exponential relationship (r2 = 0.969) between the natural n-alkanes ratio (NAR) and carbon preference index for C23–C35 (CPI23–35) for all samples, which is of great use for the determination of the origins of aliphatic hydrocarbon.

Hydrogen atom addition to the surface of graphene nanoflakes: A density functional theory study

Polycyclic aromatic hydrocarbons (PAHs) provide a 2-dimensional (2D) reaction surface in 3-dimensional (3D) interstellar space and have been utilized as a model of graphene surfaces. In the present study, the reaction of PAHs with atomic hydrogen was investigated by means of density functional theory (DFT) to systematically elucidate the binding nature of atomic hydrogen to graphene nanoflakes. PAHs with n=4–37 were chosen, where n indicates the number of benzene rings. Activation energies of hydrogen addition to the graphene surface were calculated to be 5.2–7.0kcal/mol at the CAM-B3LYP/6-311G(d,p) level, which is almost constant for all PAHs. The binding energies of hydrogen atom were slightly dependent on the size (n): 14.8–28.5kcal/mol. The absorption spectra showed that a long tail is generated at the low-energy region after hydrogen addition to the graphene surface. The electronic states of hydrogenated graphenes were discussed on the basis of theoretical results.

Growth reactions of polycyclic aromatic hydrocarbons during the supercritical pyrolysis of n-decane, as determined from doping experiments with 1-methylphenanthrene and fluorene

To better understand mechanisms for the growth of polycyclic aromatic hydrocarbons (PAH) under conditions relevant to the pre-combustion environments of future-high-speed-aircraft fuels, we have performed supercritical pyrolysis experiments with the model fuel n-decane, to which 1-methylphenanthrene and fluorene, two major three-ring PAH products of n-decane pyrolysis, have each been added as a dopant, at concentrations of 0.73mg/g n-decane and 2.3mg/g n-decane, respectively. The experiments are conducted in an isothermal, silica-lined stainless-steel flow reactor at 570°C, 94.6atm, and 133s, conditions of rapid PAH growth. Analyses of the PAH products by high-pressure liquid chromatography with diode-array ultraviolet–visible absorption and mass-spectrometric detection have exposed the important roles of four resonance-stabilized aromatic radicals—9-fluorenyl, 1-phenanthrylmethyl, 9-phenanthrylmethyl, and benz[de]anthracenyl—and revealed the channels through which these radicals react with the principal aliphatic (methyl, ethylene, propene, and 1-butene) and aromatic (benzyl, the two naphthylmethyls, and phenalenyl) reactants in the supercritical n-decane pyrolysis environment to effect PAH growth. 9-Fluorenyl's reactions with these principal reactants—in some cases involving “ring expansion” of fluorene's five-membered ring—result in the formation of phenanthrene, 9-methylphenanthrene, 9-ethylphenanthrene, fluoranthene, and eleven fluoranthene benzologues of five to seven rings. 1-phenanthrylmethyl, stemming from the 1-methylphenanthrene dopant, and 9-phenanthrylmethyl, stemming from the fluorene dopant's production of 9-methylphenanthrene and 9-ethylphenanthrene, react with n-decane's principal reactant species to form chiefly four- to six-ring cata-condensed benzenoid PAH. Their reaction with ethylene leads to formation of the four-ring phenalenyl-type radical benz[de]anthracenyl, which reacts with the principal alkenes and arylmethyl radicals in the system to produce several five- to seven-ring peri-condensed benzenoid PAH. Since the three-ring structures of 1-methylphenanthrene and fluorene are embedded in the structures of many of n-decane's higher-ring-number PAH products, we would expect the PAH-growth pathways outlined here to have widespread applicability to larger PAH in the reaction environment as well.

The impact of drying on structure of sedimentary organic matter in wetlands: Probing with native and amended polycyclic aromatic hydrocarbons

Wetland sediments undergo dry-wet cycles that may change their structural properties and affect geochemical behavior of associated organic compounds. In this study, we examined the effect of drying on particle size distributions and the rapid (24h) sorption reactions of polycyclic aromatic hydrocarbons (PAHs) with salt marsh sediments in Nueces Delta, South Texas. Drying reduced the fraction of fine particles in organically richer sediments, indicating structural rearrangement of organic matter and mineral aggregates. Among the 16 EPA priority PAHs examined, dried sediment preferentially released 1.0–7.5% of phenanthrene, fluoranthene and pyrene to added seawater (solid: water mass ratio of 1/100) - significantly greater than release from sediments maintained in the wet state. On the other hand, drying also increased the affinity of sedimentary organic matter (SOM) for experimentally amended (deuterated) phenanthrene relative to continually wet sediments. Further, deuterated phenanthrene was even more effectively retained when it was added to wet sediment that was subsequently dried and rewetted. These apparently contradictory results can be reconciled and explained by SOM having a heterogeneous distribution of hydrophobic and hydrophilic zones – e.g., a zonal model. We propose that drying changed the orientation of amphiphilic SOM, exposing hydrophobic zones and promoting the release of some of their native PAHs to water. Freshly amended PAHs were only able to penetrate into the surface hydrophobic zone and/or deeper but rapidly accessible (“kinetic”) zone in wet sediments due to the brief adsorption contact time. Subsequent drying presumably then induced structural changes in SOM that isolated these amended PAHs in sites inaccessible to water exchange in the next rewetting. These results provide insights into structural changes of SOM upon drying, and help predict the fate of compounds such as organic contaminants during drought/flood oscillations.

Treatability assessment of polycyclic aromatic hydrocarbons contaminated marine sediments using permanganate, persulfate and Fenton oxidation processes

Various chemical oxidation techniques, such as potassium permanganate (KMnO4), sodium persulfate (Na2S2O8), Fenton (H2O2/Fe2+), and the modified persulfate and Fenton reagents (activated by ferrous complexes), were carried out to treat marine sediments that were contaminated with polycyclic aromatic hydrocarbons (PAHs) and dredged from Kaohsiung Harbor in Taiwan. Experimental results revealed that KMnO4 was the most effective of the tested oxidants in PAH degradation. Owing to the high organic matter content in the sediment that reduced the efficiencies of Na2S2O8 and regular Fenton reactions, a large excess of oxidant was required. Nevertheless, KH2PO4, Na4P2O7 and four chelating agents (EDTA, sodium citrate, oxalic acid, and sodium oxalate) were utilized to stabilize Fe(II) in activating the Na2S2O8 and Fenton oxidations, while Fe(II)-citrate remarkably promoted the PAH degradation. Increasing the molecular weight and number of rings of PAH did not affect the overall removal efficiencies. The correlation between the effectiveness of the oxidation processes and the physicochemical properties of individual PAH was statistically analyzed. The data implied that the reactivity of PAH (electron affinity and ionization potential) affected its treatability more than did its hydrophobicity (Kow, Koc and Sw), particularly using experimental conditions under which PAHs could be effectively oxidized.

Kinetic modeling of particle size distribution of soot in a premixed burner-stabilized stagnation ethylene flame

A detailed model of soot formation is proposed, which consists of a gas-phase kinetic model for the pyrolysis and oxidation of selected hydrocarbon fuels and a kinetic mechanism of soot nucleation and mass/size growth through coagulation and surface reactions. The gas-phase model (Ranzi et al., 2012) was expanded to include the chemistry of Polycyclic Aromatic Hydrocarbons (PAHs) up to four-to-five ring PAHs, with a modular and hierarchical approach. The discrete sectional method was employed to solve the size evolution of the particle size distribution function (PSDF). Analogy and similarity rules were employed to describe heterogeneous reaction kinetics of soot surface reactions. A variable collision efficiency was assumed for the coalescence of small soot particles. Larger particles were assumed to undergo aggregation. The predicted PSDFs are found to be in reasonably good agreement with the experimental data for nascent soot measured in an atmospheric-pressure premixed ethylene–oxygen–argon flame in the burner-stabilized stagnation flame configuration. Sensitivity analyses of the PSDF, number density, and volume fraction were carried out with respect to the rate parameters of addition reactions of acetylene, PAHs, resonantly stabilized radical reactions, and coalescence and aggregation. The results show that the reaction of PAHs and acetylene with soot surfaces and the kinetics of coalescence and aggregation exhibit dominant effects on the detailed and global soot properties for the flame studied, in agreement with conclusions of a large range of previous modeling studies.

Variations of concentration and composition of polycyclic aromatic hydrocarbons in coals in response to dike intrusion in the Huainan coalfield in eastern China

Although extensive studies have been conducted on coals influenced by igneous intrusion, the changes of concentration and composition of polycyclic aromatic hydrocarbons (PAHs) in coals responding to this kind of geological rapid heating are largely unknown. In the present study, thirteen coal samples, including unaltered coal, thermally altered coal and a dike-coal contact sample were collected along a transect approaching an igneous dike intrusion from the No. 3 coal seam of the Zhuji coal mine in Huainan coalfield, China. These samples represent a broad range of coal ranks ranging from high volatile A bituminous to anthracite. Vitrinite reflectance (%Ro) profiles indicate a progressive increase in %Ro value from ambient levels of 0.87–1.02% to 5.16% and include an alteration occurring out to ∼3.6m from the dike. Bulk chemical composition from proximate and ultimate analyses shows the typical trend with distance from the dike. The total amount of extractable PAHs (ΣPAH35), US-EPA priority pollutant PAHs (ΣPAH16) and carcinogenic PAHs (ΣPAH8) range from 3.4–81.1μg/g, 2.5–28.7μg/g and 0.7–16.9μg/g, respectively. PAH concentrations vary regularly with coal rank, reaching a maximum in low volatile bituminous coal (1.62%Ro), located 3.5m from the dike. This rank is higher than reported for coals not affected by contact metamorphism. Contact metamorphism has a dramatic effect on the coalification process and results in a different relationship between the PAH maximum concentration and coal rank. The igneous dike intrusion also has a significant effect on PAH distributions, where the proportions of 4–6 ring PAHs decrease toward the coal-dike contact and PAHs are dominated by those with 2 and 3 rings. The thermally altered coals show a decrease in alkylation compared with that of the unaltered coals, and the dealkylation process is apparently accompanied by the cross linking reaction of PAHs to form a more compact structure of organic macromolecule.

One-shot K-region-selective annulative π-extension for nanographene synthesis and functionalization.

The optoelectronic nature of two-dimensional sheets of sp2-hydridized carbons (for example, graphenes and nanographenes) can be dramatically altered and tuned by altering the degree of π-extension, shape, width and edge topology. Among various approaches to synthesize nanographenes with atom-by-atom precision, one-shot annulative π-extension (APEX) reactions of polycyclic aromatic hydrocarbons hold significant potential not only to achieve a ‘growth from template’ synthesis of nanographenes, but also to fine-tune the properties of nanographenes. Here we describe one-shot APEX reactions that occur at the K-region (convex armchair edge) of polycyclic aromatic hydrocarbons by the Pd(CH3CN)4(SbF6)2/o-chloranil catalytic system with silicon-bridged aromatics as π-extending agents. Density functional theory calculations suggest that the complete K-region selectivity stems from the olefinic (decreased aromatic) character of the K-region. The protocol is applicable to multiple APEX and sequential APEX reactions, to construct various nanographene structures in a rapid and programmable manner.

Formation of nitroanthracene and anthraquinone from the heterogeneous reaction between NO2 and anthracene adsorbed on NaCl particles.

Oxidative derivatives of polycyclic aromatic hydrocarbons (PAHs), that is, nitro-PAHs and quinones, are classed as hazardous semivolatile organic compounds but their formation mechanism from the heterogeneous reactions of PAHs adsorbed on atmospheric particles is not well understood. The heterogeneous reaction of NO2 with anthracene adsorbed on NaCl particles under different relative humidity (RH 0-60%) was investigated under dark conditions at 298 K. The formation of the major products, 9,10-anthraquinone (9,10-AQ) and 9-nitroanthracene (9-NANT), were determined to be second-order reactions with respect to NO2 concentration. The rate of formation of 9,10-AQ under low RH (0-20%) increased as the RH increased but decreased when the RH was further increased in high RH (40-60%). In contrast, the rate of formation of 9-NANT across the whole RH range (0-60%) decreased significantly with increasing RH. Two different reaction pathways are discussed for the formation of 9,10-AQ and 9-NANT, respectively, and both are considered to be coupled to the predominant reaction of NO2 with the NaCl substrate. These results suggest that relative humidity, which controls the amount of surface adsorbed water on NaCl particles, plays an important role in the heterogeneous reaction of NO2 with adsorbed PAHs.

Verification of Heme Catalytic Cycle with 5-Aminosalicylic Acid and Its Application to Soil Remediation of Polycyclic Aromatic Hydrocarbons

Catalytic degradation of pentachlorophenol in soil by heme and hydrogen peroxide has been hypothesized to occur through nonspecific catalytic reactions similar to those involving ligninase. The present study examines the evidence for a heme catalytic mechanism for the oxidation of organic compounds. In the presence of hydrogen peroxide, heme is converted to the ferryl heme radical (Hm-Fe +4• ), which can oxidize organic compounds, such as 5-aminosalicylic acid (5-ASA). A second 5-ASA may later be oxidized by ferryl heme (Hm-Fe +4 ), which reverts to the ferric heme state (Hm-Fe +3 ) to complete the cycle. We believe that this catalytic cycle is involved in the degradation of hazardous pollutants, such as polycyclic aromatic hydrocarbons (PAHs). Remediation via heme catalytic reactions of PAHs in soil from a pole yard was evaluated, and about 96% of PAHs was found to disappear within 42 days after treatment with heme and hydrogen peroxide. In addition, benzo[a]pyrene and six other PAHs were undetectable among a total of 16 PAH compounds examined. Therefore, we propose heme catalysis as a novel technology for the remediation of hazardous compounds in contaminated soil.

Kinetics of the gas-phase reactions of chlorine atoms with naphthalene, acenaphthene, and acenaphthylene.

Reactions of polycyclic aromatic hydrocarbons (PAHs) with chlorine atoms may occur in specific areas such as coastal regions and the marine boundary layer. In this work, rate constants for the gas-phase reactions of naphthalene, acenaphthene, and acenaphthylene with chlorine atoms have been measured using the relative rate technique. Experiments were performed at room temperature (293 ± 2 K) and atmospheric pressure in an atmospheric simulation chamber using a proton-transfer reaction mass spectrometer (PTR-MS) to monitor the concentrations of PAHs and the reference compounds (acetone, methanol, 1,3,5-trimethylbenzene, and isoprene) as a function of time. The rate constants obtained in this work were (in units of cm(3) molecule(-1) s(-1)) (4.22 ± 0.46) × 10(-12), (3.01 ± 0.82) × 10(-10), and (4.69 ± 0.82) × 10(-10) for naphthalene, acenaphthene, and acenaphthylene, respectively. These are the first measurements of the rate constants for gas-phase reactions of Cl atoms with acenaphthene and acenaphthylene. The rate constant determined in this study for the reaction of naphthalene with Cl atoms is not in agreement with the only other previously reported value in the literature. The results are used to assess the potential role of chlorine atom reactions in the atmospheric oxidation of PAHs.

Novel two-step laser ablation and ionization mass spectrometry (2S-LAIMS) of actor-spectator ice layers: Probing chemical composition of D2O ice beneath a H2O ice layer

In this work, we report for the first time successful analysis of organic aromatic analytes imbedded in D2O ices by novel infrared (IR) laser ablation of a layered non-absorbing D2O ice (spectator) containing the analytes and an ablation-active IR-absorbing H2O ice layer (actor) without the analyte. With these studies we have opened up a new method for the in situ analysis of solids containing analytes when covered with an IR laser-absorbing layer that can be resonantly ablated. This soft ejection method takes advantage of the tenability of two-step infrared laser ablation and ultraviolet laser ionization mass spectrometry, previously demonstrated in this lab to study chemical reactions of polycyclic aromatic hydrocarbons (PAHs) in cryogenic ices. The IR laser pulse tuned to resonantly excite only the upper H2O ice layer (actor) generates a shockwave upon impact. This shockwave penetrates the lower analyte-containing D2O ice layer (spectator, a non-absorbing ice that cannot be ablated directly with the wavelength of the IR laser employed) and is reflected back, ejecting the contents of the D2O layer into the vacuum where they are intersected by a UV laser for ionization and detection by a time-of-flight mass spectrometer. Thus, energy is transmitted from the laser-absorbing actor layer into the non-absorbing spectator layer resulting its ablation. We found that isotope cross-contamination between layers was negligible. We also did not see any evidence for thermal or collisional chemistry of PAH molecules with H2O molecules in the shockwave. We call this "shockwave mediated surface resonance enhanced subsurface ablation" technique as "two-step laser ablation and ionization mass spectrometry of actor-spectator ice layers." This method has its roots in the well-established MALDI (matrix assisted laser desorption and ionization) method. Our method offers more flexibility to optimize both the processes--ablation and ionization. This new technique can thus be potentially employed to undertake in situ analysis of materials imbedded in diverse media, such as cryogenic ices, biological samples, tissues, minerals, etc., by covered with an IR-absorbing laser ablation medium and study the chemical composition and reaction pathways of the analyte in its natural surroundings.

Transformation of polycyclic aromatic hydrocarbons (PAHs) on Fe(III)-modified clay minerals: Role of molecular chemistry and clay surface properties

Clay-driven transformation of polycyclic aromatic hydrocarbons (PAHs) is critically influenced by their molecular structure and clay surface properties. In the present study, several PAHs were selected as model molecules to investigate their potential transformation on Fe(III)-saturated clay minerals under various mineralogical and environmental conditions. Results suggest that the reactivity of PAHs is highly correlated with their ionization potential (IP) values. PAHs with IP lower than a threshold between 7.5 and 7.6 prefer undergoing a one-electron transfer reaction. Otherwise, Fe(III)-smectite is unable to degrade PAHs with IP above it. The electron-transfer process leads to the reduction of Fe(III) to Fe(II) and formation of organic radical cations, which are more stable in clay interlayers than at other clay sites. Subsequent reactions of radical cations with oxygenic species (such as H2O) result in formation of oxygenated products. The surface chemical properties, i.e., the hydration of cations, fraction of Fe(III), layer charge location, and type of ligands, strongly affect the interaction between PAHs and Fe(III), thus modulate the reactivity of surface Fe(III)-species on the clay minerals. This study provides the first direct evidence for clay-catalyzed transformation of PAHs supporting the plausibility of their in situ degradation in soils, and demonstrates that abiotic reactions with surface-bound Fe(III) may affect or even dominate the long-term behavior of PAHs in soils, particularly in the presence of swelling clay minerals.