Atena Necula

High temperature behavior of alternant and nonalternant polycyclic aromatic hydrocarbons

Abstract Three different flash vacuum pyrolysis (FVP) set-ups were employed for the study of the thermal behavior of alternant and nonalternant polyclic aromatic hydrocarbons (PAH). The 1,2-switch of carbon atoms in the thermal automerizations of PAH, previously observed in benzene, pyrene and benz[ a ]anthracene, has been additionally observed for [1- 13 C]anthracene, [6- 13 C]benzo[ c ]phenanthrene and [1- 13 C]- and [4- 13 C]phenanthrenes at temperatures between 900 and 1100°C. Bay region PAH (chrysene, benz[ a ]anthracene, phenanthrene, picene, perylene, benzo[ ghi ]perylene, benzo[ e ]pyrene, and benz[ a,c ]-anthracene) were discovered to undergo an intriguing transformation when heated at 1100°C; the transformation is believed to be triggered by cyclodehydrogenation across the bay region. Thermal formation of five-membered rings was used to synthesize a series of new nonalternant hydrocarbons. When the thermal closure of a five-membered ring is energetically unfavorable, migration of an ethynyl group from one benzene ring to another along the edge of a PAH (e.g. anthracene) can take place.

Identification of Some Novel Cyclopenta-Fused Polycyclic Aromatic Hydrocarbons in Ethylene Flames

Abstract The product suite of polycyclic aromatic hydrocarbons (PAH) produced in typical hightemperature (1300–1700K) fuel-rich flames is characterized by the abundance of peripherally-fused cyclopenta-PAH (CP-PAH), the most prominent of which are acenaphthylene (C12H8) and cyclopenta[cd]pyrene (C18H10). Although many other CP-PAH are thought to be present, their structural elucidation has been contingent on the availability of synthetic reference standards, and this has hindered their positive identification in flames. Previous studies using synthetic standards have resulted in the identification of a limited number of other CP-PAH including cyclopent[hi]acephenanthrylene, cyclopent[hi]aceanthrylene and all three isomeric dicyclopentapyrenes. In this work, we report the identification in ethylene flames of three additional CP-PAH, benzo[ghi]cyclopenta[cd]perylene, cyclopenta[bc]coronene and cyclopenta[cd]fluoranthene. Their positive identification was made possible by the availability of reference quanti...

The Identification of New Ethynyl-Substituted and Cyclopenta-Fused Polycyclic Aromatic Hydrocarbons in the Products of Anthracene Pyrolysis

Abstract The recent synthesis of new reference standards of polycyclic aromatic hydrocarbons (PAH) has enabled us to identify six new PAH species among the products of anthracene, pyrolyzed in argon at temperatures of 1300 to 1500 K. The anthracene product samples are analyzed by high performance liquid chromatography (HPLC) with diode-array ultraviolet-visible (UV) detection, and the identifications are made by matching each product components HPLC elution time and UV absorption spectrum with those of the corresponding reference standard. The newly identified PAH products include 1-ethynylacenaphthylene (C14H8) as well as five cyclopenta-fused PAH (CP-PAH): cyclopenta[cd]fluoranthene (C18H10); dicyclopenta[cd, mn]pyrene, dicyclo-penta[cd,fg]pyrene, dicyclopenta[cd,jk]pyrene (C20H10); and benzo[ghi]cyclopenta[cd]-perylene (C24H12). Undetectable at temperatures < 1300 K, the yields of the newly identified CP-PAH rise quickly with temperature above 1350 K, levelling off somewhat at temperatures approaching...

Thermal migration of an ethynyl group from one benzene ring to another by reversible vinylidene CH insertion

Abstract Evidence is presented for the high temperature opening of a 5-membered ring by extrusion of a carbene (the reverse of a CH bond insertion), which results in the net thermal migration of an ethynyl group from one benzene ring to another.

The Identification of Cyclopenta-Fused and Ethynyl-Substituted Polycyclic Aromatic Hydrocarbons in Benzene Droplet Combustion Products

Abstract In order to investigate new aspects of polycyclic aromatic hydrocarbon (PAH) growth and soot formation, we have synthesized special reference standards of cyclopenta-fused PAH (CP-PAH) and ethynyl-substituted PAH. We have identified several of these CP-PAH and ethynyl-PAH in benzene droplet combustion products, using high pressure liquid chromatography (HPLC) and ultraviolet-visible (UV) absorption spectroscopy. Although one CP-PAH identified in these products - acenaphthylene - has previously been identified as a product of a variety of combustion systems, we have identified six additional CP-PAH and two ethynyl-PAH which have never before been unequivocally identified as the products of benzene pyrolysis or combustion: acephenanthrylene, aceanthrylene, cyclopent[hi]acephenanthrylene, cyclopenta[cd]fluoranthene, cyclopenta[cd] pyrene, dicyclopenta[cd, jk]pyrene, 2-ethynylnaphthalene, and 1-ethynylacenaphthylene. We present the corresponding UV absorption spectra obtained from the HPLC analysis of benzene droplet combustion products, and compare them to the UV absorption

Cyclopenta-fused polycyclic aromatic hydrocarbons from brown coal pyrolysis

To examine certain aspects of coal tar composition, we have pyrolyzed acid-washed Yallourn brown coal under nitrogen at temperatures of 600 to 1000°C in a fluidized-bed reactor. Analysis of the product tar by reverse-phase high-performance liquid chromatography with diode-array ultraviolet-visible absorption detection reveals that the tars are composed of a large number of polycyclic aromatic compounds, many of which are polycyclic aromatic hydrocarbons (PAH) with peripherally fused cyclopenta rings (CP-PAH). Among PAH, CP-PAH are of particular interest because of their proneness to oxidation in the en vironment, their relatively high biological activity, and their postulated role in soot formation. Of the 10 CP-PAH identified in our tar samples, 4 of the most abundant are acenaphthylene (C12H8), acephenanthrylene and aceanthrylene (C16H10), and cyclopental [cd]pyrene (C18H10)—all of which have been detected previously in products of coal pyrolysis and/or combustion. The recent synthesis of several new CP-PAH reference standards, however, has enabled us to also identify, in the brown coal tars, six additional CP-PAH-cyclopent[hi]acephenanthrylene and cyclopenta[cd]fluoran thene (C18H10), dicyclopenta[cd, mn]pyrene and dicyclopental[cd, jk]pyrene (C20H10), benzo[ghi]cyclopenta[cd]perylene (C24H12), and cyclopenta[bc]coronene (C26H12)—none of which has ever before been identified in coal products. The mass fractions of individual CP-PAH span a range of four orders of magnitude—from 0.000062 for cyclopenta[bc]coronene to 0.265 for acenaphthylene in the 1000°C tar smaple. Accounting for approximately one-third of the mass of the tar produced at 1000°C, the CP-PAH yields show a monotonic increase with pyrolysis temperature—confirming that the CP-PAH are not primary products of coal devolatilization but instead result from secondary pyrolytic reactions in the gas phase. Possible reaction mechanisms are explored.

Automerization of 1,2,3,4-tetrahydrophenanthrene

Abstract Treatment of either [1− 13 C)- or [4− 13 C]-1,2,3,4-tetrahydrophenanthrene with AlCl 3 -H 2 O or with AlBr 3 -H 2 O in refluxing benzene yields a 1:1 mixture of the two isotopomers.

CYCLOPENT[bc]ACENAPHTHYLENE: A PUTATIVE INTERMEDIATE ON THE PATHWAY FOR THERMAL CYCLODEHYDROGENATION OF PHENANTHRENE

Phenanthrene, the archetypal “bay region” polycyclic aromatic hydrocarbon (PAH), loses two hydrogen atoms when subjected to flash vacuum pyrolysis (FVP) at 1100 °C and suffers a major skeletal rearrangement to form pyracylene (1), along with 1-, 3-, and 4-ethynylacenaphthylene (2, 3, and 4, respectively). Evidence is presented that implicates cyclopenta[bc]acenaphthylene (7) as a key intermediate on the reaction pathway from phenanthrene to these four C14H8 products.

Classification of Select Aceanthrylenes, Acephenanthrylenes and Dicyclopentapyrenes as Alternant versus Nonalternant Polycyclic Aromatic Hydrocarbons on the Basis of Their Fluorescence Quenching Behavior in the Presence of Nitromethane and Cetylpyridinium Cation Selective Quenching Agents

Abstract Fluorescence behavior of dicyclopenta[cd, fg]pyrene, dicyclopenta[cd, jk]pyrene and dicyclopenta[cd, mn]pyrene is reported in the presence of nitromethane and cetylpyridinium (CPy+) cation quenching agents. Nitromethane and CPy+ are known to selectively quench fluorescence emission of alternant polycyclic aromatic hydrocarbons (PAHs). Emission intensities of nonalternant PAHs, with a few noted exceptions, are unaffected. Experimental measurements show that nitromethane and CPy+ effectively quenched fluorescence emission of the three dicyclopentapyrenes, thus suggesting that the perimeter C = C double bonds in the two cyclopenta-rings are alkenic, rather than aromatic in nature. Also reported are the fluorescence properties of 14 aceanthrylene and acephenanthrylene derivatives, as well as several structurally related compounds, to further document the cetylpyridinium cations quenching selectivity.