The "unsaturated hydrocarbons" in the gases from the carbonisation of coal,

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Ab initio quantum chemical studies on neutral-radical reactions of ethynyl (C2H) and cyano (CN) with unsaturated hydrocarbons

An Ab Initio/RRKM study of the reaction mechanism and product branching ratios of neutral-radical ethynyl (C2H) and cyano (CN) radical species with unsaturated hydrocarbons is performed. The reactions studied apply to cold conditions such as planetary atmospheres including Titan, the Interstellar Medium (ISM), icy bodies and molecular clouds. The reactions of C2H and CN additions to gaseous unsaturated hydrocarbons are an active area of study. NASA's Cassini/Huygens mission found a high concentration of C2H and CN from photolysis of ethyne (C2H2) and hydrogen cyanide (HCN), respectively, in the organic haze layers of the atmosphere of Titan. The reactions involved in the atmospheric chemistry of Titan lead to a vast array of larger, more complex intermediates and products and may also serve as a chemical model of Earth's primordial atmospheric conditions. The C2H and CN additions are rapid and exothermic, and often occur barrierlessly to various carbon sites of unsaturated hydrocarbons. The reaction mechanism is proposed on the basis of the resulting potential energy surface (PES) that includes all the possible intermediates and transition states that can occur, and all the products that lie on the surface. The B3LYP/6-311g(d,p) level of theory is employed to determine optimized electronic structures, moments of inertia, vibrational frequencies, and zero-point energy. They are followed by single point higher-level CCSD(T)/cc-vtz calculations, including extrapolations to complete basis sets (CBS) of the reactants and products. A microcanonical RRKM study predicts single-collision (zero-pressure limit) rate constants of all reaction paths on the potential energy surface, which is then used to compute the branching ratios of the products that result. These theoretical calculations are conducted either jointly or in parallel to experimental work to elucidate the chemical composition of Titan's atmosphere, the ISM, and cold celestial bodies.<.

Ab Initio Quantum Chemical Studies on Neutral-Radical Reactions of Ethynyl (C2H) and Cyano (CN) with Unsaturated Hydrocarbons

An Ab Initio/RRKM study of the reaction mechanism and product branching ratios of neutral-radical ethynyl (C2H) and cyano (CN) radical species with unsaturated hydrocarbons is performed. The reactions studied apply to cold conditions such as planetary atmospheres including Titan, the Interstellar Medium (ISM), icy bodies and molecular clouds. The reactions of C2H and CN additions to gaseous unsaturated hydrocarbons are an active area of study. NASA’s Cassini/Huygens mission found a high concentration of C2H and CN from photolysis of ethyne (C2H2) and hydrogen cyanide (HCN), respectively, in the organic haze layers of the atmosphere of Titan. The reactions involved in the atmospheric chemistry of Titan lead to a vast array of larger, more complex intermediates and products and may also serve as a chemical model of Earth’s primordial atmospheric conditions. The C2H and CN additions are rapid and exothermic, and often occur barrierlessly to various carbon sites of unsaturated hydrocarbons. The reaction mechanism is proposed on the basis of the resulting potential energy surface (PES) that includes all the possible intermediates and transition states that can occur, and all the products that lie on the surface. The B3LYP/6-311g(d,p) level of theory is employed to determine optimized electronic structures, moments of inertia, vibrational frequencies, and zero-point energy. They are followed by single point higher-level CCSD(T)/cc-vtz calculations, including extrapolations to complete basis sets (CBS) of the reactants and products. A microcanonical RRKM study predicts single-collision (zero-pressure limit) rate constants of all reaction paths on the potential energy surface, which is then used to compute the branching ratios of the products that result. These theoretical calculations are conducted either jointly or in parallel to experimental work to elucidate the chemical composition of Titan’s atmosphere, the ISM, and cold celestial bodies.

The gas-phase ozonolysis of unsaturated volatile organic compounds in the troposphere

The gas-phase reactions of ozone with unsaturated hydrocarbons are significant sources of free radical species (including (OH)-O-center dot) and particulate material in the Earth's atmosphere. In this tutorial review, the kinetics, products and mechanisms of these reactions are examined, starting with a discussion of the original mechanism proposed by Criegee and following with a summary presentation of the complex, free radical-mediated reactions of carbonyl oxide (Criegee) intermediates. The contribution of ozone-terpene reactions to the atmospheric burden of secondary organic aerosol material is also discussed from the viewpoint of the formation of non-volatile organic acid products from the complex chemistry of ozone with alpha-pinene. Throughout the article, currently accepted understanding is supported through the presentation of key experimental results, and areas of persistent or new uncertainty are highlighted.

Chemical Bonding of Hydrocarbons to Metal Surfaces

Using x-ray absorption spectroscopy (XAS), x-ray emission spectroscopy (XES) and x-ray photoelectron spectroscopy (XPS) in combination with density functional theory (DFT) the changes in electronic and geometric structure of hydrocarbons upon adsorption are determined. The chemical bonding is analyzed and the results provide new insights in the mechanisms responsible for dehydrogenation in heterogeneous catalysis. In the case of alkanes, n-octane and methane are studied. XAS and XES show significant changes in the electronic structure upon adsorption. XES shows new adsorption induced occupied states and XAS shows quenching of CH*/Rydberg states in n-octane. In methane the symmetry forbidden gas phase lowest unoccupied molecular orbital becomes allowed due to broken symmetry. New adsorption induced unoccupied features with mainly metal character appear just above the Fermi level in XA spectra of both adsorbed methane and n-octane. These changes are not observed in DFT total energy geometry optimizations. Comparison between experimental and computed spectra for different adsorbate geometries reveals that the molecular structures are significantly changed in both molecules. The C-C bonds in n-octane are shortened upon adsorption and the C-H bonds are elongated in both n-octane and methane. In addition ethylene and acetylene are studied as model systems for unsaturated hydrocarbons. The validity of both the Dewar-Chatt-Duncanson chemisorption model and the alternative spin-uncoupling picture is confirmed, as well as C-C bond elongation and upward bending of the C-H bonds. The bonding of ethylene to Cu(110) and Ni(110) are compared and the results show that the main difference is the amount of back-donation into the molecular π* orbital, which allows the molecule to desorb molecularly from the Cu(110) surface, whereas it is dehydrogenated upon heating on the Ni(110) surface. Acetylene is found to adsorb in two different adsorption sites on the Cu(110) surface at liquid nitrogen temperature. Upon heating the molecules move into one of these sites due to attractive adsorbate-adsorbate interaction and only one adsorbed species is present at room temperature, at which point the molecules start reacting to form benzene. The bonding of the two species is very similar in both sites and the carbon atoms are rehybridized essentially to sp2.

Caste-specific cuticular lipids in the stingless bee Friesella schrottkyi

While a queen control pheromone complex that inhibits worker ovary development has been described for honey bees, no comparable control pheromones have been identified for their sister group, the stingless bees. The aim of the present work was to search for possible control pheromones in the stingless bee Friesella schrottkyi. No volatile substances were found in the heads of queens that might serve as queen control pheromones. On the other hand, distinct differences were found between the cuticular substances of queens and workers. The major hydrocarbons were different between the two castes, and while queens contained methyl-branched alkanes and no unsaturated hydrocarbons, workers contained alkenes and alka-dienes but no methyl branched hydrocarbons. Colonies deprived of a queen produced laying workers. Differences were observed in the cuticular patterns of laying workers and workers from a queen controlled colony.

Gas-phase rate coefficients for the reactions of nitrate radicals with (Z)-pent-2-ene, (E)-pent-2-ene, (Z)-hex-2-ene, (E)-hex-2-ene, (Z)-hex-3-ene, (E)-hex-3-ene and (E)-3-methylpent-2-ene at room temperature

Rate coefficients for reactions of nitrate radicals (NO3) with (Z)-pent-2-ene, (E)-pent-2-ene, (Z)-hex-2-ene, (E)-hex-2-ene, (Z)-hex-3-ene, (E)-hex-3-ene and (E)-3-methylpent-2-ene were determined to be (6.55 +/- 0.78) x 10(-13) cm(3) molecule(-1) s(-1), (3.78 +/- 0.45) x 10(-13) cm(3) molecule(-1) s(-1), (5.30 +/- 0.73) x 10(-13) cm(3) molecule(-1) s(-1), (3.83 +/- 0.47) x 10(-13) cm(3) molecule(-1) s(-1), (4.37 +/- 0.49) x 10(-13) cm(3) molecule(-1) s(-1), (3.61 +/- 0.40) x 10(-13) cm(3) molecule(-1) s(-1) and (8.9 +/- 1.5) x 10(-12) cm(3) molecule(-1) s(-1), respectively. We performed kinetic experiments at room temperature and atmospheric pressure using a relative-rate technique with GC-FID analysis. The experimental results demonstrate a surprisingly large cis-trans (Z-E) effect, particularly in the case of the pent-2-enes, where the ratio of rate coefficients is ca. 1.7. Rate coefficients are discussed in terms of electronic and steric influences, and our results give some insight into the effects of chain length and position of the double bond on the reaction of NO3 with unsaturated hydrocarbons. Atmospheric lifetimes were calculated with respect to important oxidants in the troposphere for the alkenes studied, and NO3-initiated oxidation is found to be the dominant degradation route for (Z)-pent-2-ene, (Z)-hex-3-ene and (E)-3-methylpent-2-ene.

Entwicklung von chiralen- sowie RP-HPLC-Methoden in Verbindung mit hochauflösender MS und deren Anwendung zur Analyse sekundärer organischer Aerosole in der Atmosphäre

Die Erdatmosphäre besteht hauptsächlich aus Stickstoff (78%), Sauerstoff (21%) und Edelga¬sen. Obwohl Partikel weniger als 0,1% ausmachen, spielen sie eine entscheidende Rolle in der Chemie und Physik der Atmosphäre, da sie das Klima der Erde sowohl direkt als auch indirekt beeinflussen. Je nach Art der Bildung unterscheidet man zwischen primären und sekundären Partikeln, wobei primäre Partikel direkt in die Atmosphäre eingetragen werden. Sekundäre Partikel hingegen entstehen durch Kondensation von schwerflüchtigen Verbindungen aus der Gasphase, welche durch Reaktionen von gasförmigen Vorläufersubstanzen (volatile organic compounds, VOCs) mit atmosphärischen Oxidantien wie Ozon oder OH-Radikalen gebildet werden. Da die meisten Vorläufersubstanzen organischer Natur sind, wird das daraus gebil¬dete Aerosol als sekundäres organisches Aerosol (SOA) bezeichnet. Anders als die meisten primären Partikel stammen die VOCs überwiegend aus biogenen Quellen. Es handelt sich da¬bei um ungesättigte Kohlenwasserstoffe, die bei intensiver Sonneneinstrahlung und hohen Temperaturen von Pflanzen emittiert werden. Viele der leichtflüchtigen Vorläufersubstanzen sind chiral, sowohl die Vorläufer als auch die daraus gebildeten Partikel werden aber in den meisten Studien als eine Verbindung betrachtet und gemeinsam analysiert. Die mit Modellen berechneten SOA-Konzentrationen, welche auf dieser traditionellen Vorstellung der SOA-Bil¬dung beruhen, liegen deutlich unterhalb der in der Atmosphäre gefundenen, so dass neben diesem Bildungsweg auch noch andere SOA-Bildungsarten existieren müssen. Aus diesem Grund wird der Fokus der heutigen Forschung vermehrt auf die heterogene Chemie in der Partikelphase gerichtet. Glyoxal als Modellsubstanz kommt hierbei eine wichtige Rolle zu. Es handelt sich bei dieser Verbindung um ein Molekül mit einem hohen Dampfdruck, das auf Grund dieser Eigenschaft nur in der Gasphase zu finden sein sollte. Da es aber über zwei Alde¬hydgruppen verfügt, ist es sehr gut wasserlöslich und kann dadurch in die Partikelphase über¬gehen, wo es heterogenen chemischen Prozessen unterliegt. Unter anderem werden in An¬wesenheit von Ammoniumionen Imidazole gebildet, welche wegen der beiden Stickstoff-He¬teroatome lichtabsorbierende Eigenschaften besitzen. Die Verteilung von Glyoxal zwischen der Gas- und der Partikelphase wird durch das Henrysche Gesetz beschrieben, wobei die Gleichgewichtskonstante die sogenannte Henry-Konstante ist. Diese ist abhängig von der un¬tersuchten organischen Verbindung und den im Partikel vorhandenen anorganischen Salzen. Für die Untersuchung chiraler Verbindungen im SOA wurde zunächst eine Filterextraktions¬methode entwickelt und die erhaltenen Proben anschließend mittels chiraler Hochleistungs-Flüssigchromatographie, welche an ein Elektrospray-Massenspektrometer gekoppelt war, analysiert. Der Fokus lag hierbei auf dem am häufigsten emittierten Monoterpen α-Pinen und seinem Hauptprodukt, der Pinsäure. Da bei der Ozonolyse des α-Pinens das cyclische Grund¬gerüst erhalten bleibt, können trotz der beiden im Molekül vorhanden chiralen Zentren nur zwei Pinsäure Enantiomere gebildet werden. Als Extraktionsmittel wurde eine Mischung aus Methanol/Wasser 9/1 gewählt, mit welcher Extraktionseffizienzen von 65% für Pinsäure Enan¬tiomer 1 und 68% für Pinsäure Enantiomer 2 erreicht werden konnten. Des Weiteren wurden Experimente in einer Atmosphärensimulationskammer durchgeführt, um die Produkte der α-Pinen Ozonolyse eindeutig zu charakterisieren. Enantiomer 1 wurde demnach aus (+)-α-Pinen gebildet und Enantiomer 2 entstand aus (-)-α-Pinen. Auf Filterproben aus dem brasilianischen Regenwald konnte ausschließlich Pinsäure Enantiomer 2 gefunden werden. Enantiomer 1 lag dauerhaft unterhalb der Nachweisgrenze von 18,27 ng/mL. Im borealen Nadelwald war das Verhältnis umgekehrt und Pinsäure Enantiomer 1 überwog vor Pinsäure Enantiomer 2. Das Verhältnis betrug 56% Enantiomer 1 zu 44% Enantiomer 2. Saisonale Verläufe im tropischen Regenwald zeigten, dass die Konzentrationen zur Trockenzeit im August höher waren als wäh¬rend der Regenzeit im Februar. Auch im borealen Nadelwald wurden im Sommer höhere Kon¬zentrationen gemessen als im Winter. Die Verhältnisse der Enantiomere änderten sich nicht im jahreszeitlichen Verlauf. Die Bestimmung der Henry-Konstanten von Glyoxal bei verschiedenen Saataerosolen, nämlich Ammoniumsulfat, Natriumnitrat, Kaliumsulfat, Natriumchlorid und Ammoniumnitrat sowie die irreversible Produktbildung aus Glyoxal in Anwesenheit von Ammoniak waren Forschungs¬gegenstand einer Atmosphärensimulationskammer-Kampagne am Paul-Scherrer-Institut in Villigen, Schweiz. Hierzu wurde zunächst das zu untersuchende Saataerosol in der Kammer vorgelegt und dann aus photochemisch erzeugten OH-Radikalen und Acetylen Glyoxal er¬zeugt. Für die Bestimmung der Glyoxalkonzentration im Kammeraerosol wurde zunächst eine beste¬hende Filterextraktionsmethode modifiziert und die Analyse mittels hochauflösender Mas¬senspektrometrie realisiert. Als Extraktionsmittel kam 100% Acetonitril, ACN zum Einsatz wo¬bei die Extraktionseffizienz bei 85% lag. Für die anschließende Derivatisierung wurde 2,4-Di¬nitrophenylhydrazin, DNPH verwendet. Dieses musste zuvor drei Mal mittels Festphasenex¬traktion gereinigt werden um störende Blindwerte ausreichend zu minimieren. Die gefunde¬nen Henry-Konstanten für Ammoniumsulfat als Saataerosol stimmten gut mit in der Literatur gefundenen Werten überein. Die Werte für Natriumnitrat und Natriumchlorid als Saataerosol waren kleiner als die von Ammoniumsulfat aber größer als der Wert von reinem Wasser. Für Ammoniumnitrat und Kaliumsulfat konnten keine Konstanten berechnet werden. Alle drei Saataerosole führten zu einem „Salting-in“. Das bedeutet, dass bei Erhöhung der Salzmolalität auch die Glyoxalkonzentration im Partikel stieg. Diese Beobachtungen sind auch in der Litera¬tur beschrieben, wobei die Ergebnisse dort nicht auf der Durchführung von Kammerexperi¬menten beruhen, sondern mittels bulk-Experimenten generiert wurden. Für die Trennung der Imidazole wurde eine neue Filterextraktionsmethode entwickelt, wobei sich ein Gemisch aus mit HCl angesäuertem ACN/H2O im Verhältnis 9/1 als optimales Extrak¬tionsmittel herausstellte. Drei verschiedenen Imidazole konnten mit dieser Methode quanti¬fiziert werden, nämlich 1-H-Imidazol-4-carbaldehyd (IC), Imidazol (IM) und 2,2‘-Biimidazol (BI). Die Effizienzen lagen für BI bei 95%, für IC bei 58% und für IM bei 75%. Kammerexperimente unter Zugabe von Ammoniak zeigten höhere Imidazolkonzentrationen als solche ohne. Wurden die Experimente ohne Ammoniak in Anwesenheit von Ammoni¬umsulfat durchgeführt, wurden höhere Imidazol-Konzentrationen gefunden als ohne Ammo¬niumionen. Auch die relative Luftfeuchtigkeit spielte eine wichtige Rolle, da sowohl eine zu hohe als auch eine zu niedrige relative Luftfeuchtigkeit zu einer verminderten Imidazolbildung führte. Durch mit 13C-markiertem Kohlenstoff durchgeführte Experimente konnte eindeutig gezeigt werden, dass es sich bei den gebildeten Imidazolen und Glyoxalprodukte handelte. Außerdem konnte der in der Literatur beschriebene Bildungsmechanismus erfolgreich weiter¬entwickelt werden. Während der CYPHEX Kampagne in Zypern konnten erstmalig Imidazole in Feldproben nach¬gewiesen werden. Das Hauptprodukt IC zeigte einen tageszeitlichen Verlauf mit höheren Kon¬zentrationen während der Nacht und korrelierte signifikant aber schwach mit der Acidität und Ammoniumionenkonzentration des gefundenen Aerosols.

Estudo da seletividade de catalisadores a base de cobre e paládio em transformações de hidrocarbonetos insaturados

Compreender a correlação entre as características de um catalisador particular e seu desempenho catalítico tem sido um dos principais objetos da pesquisa em catálise heterogênea a fim de usar esse conhecimento para o desenho racional de catalisadores mais ativos, seletivos e estáveis. A seletividade é um dos fatores mais importantes a ser controlado pelo desenho de catalisadores, podendo ser alcançada de diversas maneiras, levando-se em consideração mudanças do tipo estrutural, química, eletrônica, de composição, de cinética e de energia. O trabalho descrito nessa tese de doutorado compreende a síntese e caracterização de catalisadores compostos de nanopartículas de óxido de cobre, paládio e cobre-paládio e seu estudo em reações de hidrogenação e oxidação seletivas de hidrocarbonetos insaturados. Os catalisadores foram preparados através da deposição de nanopartículas dos metais cataliticamente ativos sobre suportes magneticamente recuperáveis compostos de nanopartículas de magnetita revestidas por sílica com superfícies funcionalizada com diferentes grupos orgânicos. A natureza magnética do suporte permitiu a fácil separação do catalisador do meio reacional pela simples aproximação de um ímã na parede do reator. O catalisador pôde ser completamente separado da fase líquida, fazendo com que a utilização de outros métodos de separação como filtração e centrifugação, comumente utilizados em sistemas heterogêneos líquidos, fossem completamente dispensados. Os catalisadores foram inicialmente testados em reações de hidrogenação de alquenos e alquinos. As reações de hidrogenação foram realizadas utilizando hidrogênio molecular como agente redutor, dispensando a utilização de agentes redutores mais agressivos. Os catalisadores compostos de NPs de Pd mostram excelente atividade e capacidade de reutilização na hidrogenação de cicloexeno, podendo ser utilizados em até 15 ciclos sem perda de atividade. Nas reações de hidrogenação de alquinos, os catalisadores que contêm cobre mostraram maior seletividade para a obtenção dos produtos de semi-hidrogenação, com destaque para o catalisador composto de NPs de CuPd, que não apresenta nem traços do produto de hidrogenação completa na amostra final. Esse catalisador bimetálico alia as características do paládio (elevada atividade) e do cobre (elevada seletividade) para fornecer um catalisador ativo e seletivo para a transformação desejada. Além disso, os grupos funcionais presentes na superfície do suporte catalítico mostraram influência na atividade e seletividade para a hidrogenação de alquenos e alquinos. Os catalisadores sintetizados também foram testados na reação de oxidação de cicloexeno e mostraram seletividade para a produção do composto carbonílico α,β-insaturado, cicloex-2-en-1-ona, que é um reagente de partida de grande interesse para a síntese de diversos materiais na indústria química. As reações de oxidação foram realizadas utilizando-se apenas O2 como oxidante primário, dispensando o uso de oxidantes tóxicos como cromatos, permanganatos ou compostos halogenados, que não são recomendados do ponto de vista ambiental. Os catalisadores sintetizados puderam ser reutilizados em sucessivos ciclos de oxidação, mostrando seletividade para a formação dos produtos alílicos em todos os ciclos. Os catalisadores foram estáveis sob as condições reacionais e não apresentaram problemas de lixiviação da espécie ativa para o meio reacional, que é comum na catálise heterogênea. Um estudo cinético mostrou que, mesmo no início da reação, o catalisador tem seletividade para a ocorrência de oxidação alílica em detrimento da reação de oxidação direta que dá origem ao epóxidos correspondente, e se mostrou condizente com o mecanismo proposto na literatura para a reação de oxidação de alquenos via radicalar.

Transport of volatile chlorinated hydrocarbons in unsaturated aggregated media

Transport of volatile hydrocarbons in soils is largely controlled by interactions of vapours with the liquid and solid phase. Sorption on solids of gaseous or dissolved comPounds may be important. Since the contact time between a chemical and a specific sorption site can be rather short, kinetic or mass-transfer resistance effects may be relevant. An existing mathematical model describing advection and diffusion in the gas phase and diffusional transport from the gaseous phase into an intra-aggregate water phase is modified to include linear kinetic sorption on ps-solid and water-solid interfaces. The model accounts for kinetic mass transfer between all three phases in a soil. The solution of the Laplace-transformed equations is inverted numerically. We performed transient column experiments with 1,1,2-Trichloroethane, Trichloroethylene, and Tetrachloroethylene using air-dry solid and water-saturated porous glass beads. The breakthrough curves were calculated based on independently estimated parameters. The model calculations agree well with experimental data. The different transport behaviour of the three compounds in our system primarily depends on Henry's constants.

The determination of aromatic, unsaturated and naphthene hydrocarbons in light oils and motor spirits,

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Simulating the formation of secondary organic aerosol from the photooxidation of aromatic hydrocarbons

The formation and composition of secondary organic aerosol (SOA) from the photooxidation of benzene, p-xylene, and 1,3,5-trimethylbenzene has been simulated using the Master Chemical Mechanism version 3.1 (MCM v3.1) coupled to a representation of the transfer of organic material from the gas to particle phase. The combined mechanism was tested against data obtained from a series of experiments conducted at the European Photoreactor (EUPHORE) outdoor smog chamber in Valencia, Spain. Simulated aerosol mass concentrations compared reasonably well with the measured SOA data only after absorptive partitioning coefficients were increased by a factor of between 5 and 30. The requirement of such scaling was interpreted in terms of the occurrence of unaccounted-for association reactions in the condensed organic phase leading to the production of relatively more nonvolatile species. Comparisons were made between the relative aerosol forming efficiencies of benzene, toluene, p-xylene, and 1,3,5-trimethylbenzene, and differences in the OH-initiated degradation mechanisms of these aromatic hydrocarbons. A strong, nonlinear relationship was observed between measured (reference) yields of SOA and (proportional) yields of unsaturated dicarbonyl aldehyde species resulting from ring-fragmenting pathways. This observation, and the results of the simulations, is strongly suggestive of the involvement of reactive aldehyde species in association reactions occurring in the aerosol phase, thus promoting SOA formation and growth. The effect of NO, concentrations on SOA formation efficiencies (and formation mechanisms) is discussed.

On the catalytic hydrogenation of polycyclic aromatic hydrocarbons into less toxic compounds by a facile recoverable catalyst

Here we present the catalytic hydrogenation of polycyclic aromatic hydrocarbons (PAHs) to less toxic mixtures of saturated and partial unsaturated polycyclic hydrocarbons under mild reaction conditions using a magnetically recoverable rhodium catalyst and molecular hydrogen as the exclusive H source. The catalyst is easily recovered after each reaction by placing a permanent magnet on the reactor wall and it can be reused in successive runs without any significant loss of catalytic activity. As an example, anthracene was totally converted into the saturated polycyclic hydrocarbon form (ca. 60%) and the partially hydrogenated form, 1,2,3,4,5,6,7,8-octahydroanthracene (ca. 40%). The catalyst operates in a broad range of temperature and H(2) pressure in both organic and aqueous/organic solutions of anthracene and it also exhibits significant activity at low substrate concentrations (20 ppm). This can be an efficient recycling process for hydrogenation of PAHs present in contaminated fluid waste streams. (C) 2009 Elsevier B.V. All rights reserved.

Polarized–unpolarized ground state of small polycyclic aromatic hydrocarbons

Do polyacenes, circumacenes, periacenes, nanographenes, and graphene nanoribbons show a spin polarized ground state? In this work, we present monodeterminantal (Hartree–Fock (HF) and density functional theory (DFT) types), and multideterminantal calculations (Møller–Plesset and Coupled Cluster), for several families of unsaturated organic molecules (n-Acenes, n-Periacenes and n-Circumacenes). All HF calculations and many DFT show a spin-polarized (antiferromagnetic) ground state, in agreement with previous calculations. Nevertheless, the multideterminantal calculations, carried out with perturbative and variational wavefunctions, show that the more stable state is obtained starting from the unpolarized HF wavefunction. The trend of the stabilization of wavefunctions (polarized or unpolarized) with respect to exchange and correlation potentials, and to the number of benzene rings, has been analyzed. A study of the spin (〈Ŝ2〉) and the spin density on the carbon atoms has also been carried out.