NEWLY IDENTIFIED PRODUCTS OF BENZENE DROPLET COMBUSTION: POLYCYCLIC AROMATIC HYDROCARBONS OF THREE TO TEN RINGS

Gas phase synthesis of [4]-helicene

A synthetic route to racemic helicenes via a vinylacetylene mediated gas phase chemistry involving elementary reactions with aryl radicals is presented. In contrast to traditional synthetic routes involving solution chemistry and ionic reaction intermediates, the gas phase synthesis involves a targeted ring annulation involving free radical intermediates. Exploiting the simplest helicene as a benchmark, we show that the gas phase reaction of the 4-phenanthrenyl radical ([C₁₄H₉]^•) with vinylacetylene (C₄H₄) yields [4]-helicene (C₁₈H₁₂) along with atomic hydrogen via a low-barrier mechanism through a resonance-stabilized free radical intermediate (C₁₈H₁₃). This pathway may represent a versatile mechanism to build up even more complex polycyclic aromatic hydrocarbons such as [5]- and [6]-helicene via stepwise ring annulation through bimolecular gas phase reactions in circumstellar envelopes of carbon-rich stars, whereas secondary reactions involving hydrogen atom assisted isomerization of thermodynamically less stable isomers of [4]-helicene might be important in combustion flames as well.

Helicenes represent key building blocks leading eventually to carbonaceous nanostructures. Here, exploiting [4]-helicene as a benchmark, the authors present a synthetic route to racemic helicenes via a vinylacetylene mediated gas phase chemistry with aryl radicals involving ring annulation.

Investigation of the solvent extracts of humic organic matter (HOM) isolated from the Ravenna Lagoon to study environmental pollution and microbial communities

Solvent extracts of HOM isolated from highly polluted sediments from the Ravenna Lagoon were studied. Diagnostic indicators included polycyclic aromatic hydrocarbons (PAHs) and nonylphenols as hazardous organic pollutants to characterize anthropogenic pollution, as well as fatty acids (FA, analysed as methyl esters, FAME) to characterize microbial communities responsible for natural attenuation processes. The distribution of PAHs including cyclopentafused surrogates pointed to a significant pyrogenic origin, characteristic for methane combustion. The PAH distribution was characterized by high concentrations of highly carcinogenic analytes with molecular weights of 276Da (benzo[ghi]perylene prevailing) and 300Da (coronene prevailing). The PAH pattern as obtained by solvent extraction was very different from that obtained from pyrolysis/thermochemolysis of the HOM polymeric matrix. The FA pattern indicated strong bacterial input, with a significant contribution from methanotrophic bacteria as revealed by monounsaturated members with n:1omega8 and n:1omega5 double bonds in the alkyl chain. Terrestrial inputs as revealed by FAME analysis beyond C(20) with pronounced even-over-odd discrimination were of minor significance. This was confirmed by the pattern of nC(24)-nC(30) alcohols in strong even-over-odd prevalence occurring in relatively low concentrations. The hopane hydrocarbon distribution reflected a distinctive impact from industrial processes utilizing heavy fractions of petroleum as feedstock. Hopanols along with the 17beta(H),21beta(H)-bishomohopanoic acid pointed to hopane producers, including methanotrophic and sulfate-reducing bacteria. Nonylphenols, which could not be detected in the pyrograms of solvent-extracted HOM matrix, had a total concentration of about 70microg g(-1) referred to the HOM in the solvent extract. In addition to common phytosterols including beta-sitosterol, coprostanol could be detected in the solvent extracts pointing to human fecal matter contamination. Concentration of resin acids turned out to be very low, thus no harmful environmental effects are to be expected from these compounds.

In Situ Direct Sampling Mass Spectrometric Study on Formation of Polycyclic Aromatic Hydrocarbons in Toluene Pyrolysis

The gas-phase reaction products of toluene pyrolysis with and without acetylene addition produced in a flow tube reactor at pressures of 8.15-15.11 Torr and temperatures of 1136-1507 K with constant residence time (0.56 s) have been detected in an in situ direct sampling mass spectrometric study by using a vacuum ultraviolet single-photon ionization time-of-flight mass spectrometry technique. Those products range from methyl radical to large polycyclic aromatic hydrocarbons (PAHs) of mass 522 amu (C(42)H(18)) including smaller species, radicals, polyynes, and PAHs, together with ethynyl, methyl, and phenyl PAHs. On the basis of observed mass spectra, the chemical kinetic mechanisms of the formation of products are discussed. Especially, acetylene is mixed with toluene to understand the effect of the hydrogen abstraction and acetylene addition (HACA) mechanism on the formation pathways of products in toluene pyrolysis. The most prominent outputs of this work are the direct detection of large PAHs and new reaction pathways for the formation of PAHs with the major role of cyclopenta-fused radicals. The basis of this new reaction route is the appearance of different sequences of mass spectra that well explain the major role of aromatic radicals mainly cyclopenta fused radicals of PAHs resulting from their corresponding methyl PAHs, with active participation of c-C(5)H(5), C(6)H(5), C(6)H(5)CH(2) ,and C(9)H(7) in the formation of large PAHs. The role of the HACA only seemed important for the formation of stable condensed PAHs from unstable primary PAHs with zigzag structure (having triple fusing sites) in one step by ring growth with two carbon atoms.

Identification of a new C28H14 polycyclic aromatic hydrocarbon as a product of supercritical fuel pyrolysis: Tribenzo[cd,ghi,lm]perylene

Tribenzo[cd,ghi,lm]perylene has been identified as a product of the supercritical pyrolysis of both toluene and Fischer-Tropsch synthetic jet fuel. This identification is based on HPLC/UV/MS data, which show that compound I, eluting immediately after five other C28H14 isomers, is also a C28H14 PAH. The UV spectrum of compound I has features of a benzenoid PAH, of which there are only eight C28H14 isomers. Four of these isomers--benzo[a]coronene, phenanthro[5,4,3,2-efghi]perylene, benzo[cd]naphtho[3,2,1,8-pqra]perylene, and benzo[pqr]naphtho[8,1,2-bcd]perylene--have already been identified as supercritical pyrolysis products by matching their UV spectra with those of respective reference standards. A fifth C28H14 PAH--benzo[ghi]naphtho[8,1,2-bcd]perylene, which does not have a reference standard--has also been recently identified through MS and UV data, use of annellation theory to predict UV spectral characteristics, and length-to-breadth ratio/retention time data. Of the remaining three isomers, bisanthene (IUPAC name phenanthro[1,10,9,8-opqra]perylene) has been determined not to be present in our product mixture, as its UV spectrum does not match that of any of our product PAH. Using annellation theory, we predict the UV spectral characteristics of the two remaining C28H14 benzenoid isomers, for which there are no reference standards (tribenzo[cd,ghi,lm]perylene and naphthaceno[3,4,5,6,7-defghij]naphthacene). Results from this analysis show that the predicted UV spectral features of tribenzo[cd,ghi,lm]perylene match those of compound I--and that those of naphthaceno[3,4,5,6,7-defghij]naphthacene are inconsistent with those of compound I. The length-to-breadth ratio of tribenzo[cd,ghi,lm]perylene also agrees with compound I's HPLC elution behavior. This is the first time that tribenzo[cd,ghi,lm]perylene (IUPAC name phenanthro[2,1,10,9,8,7-pqrstuv]pentaphene) has been identified as a product of fuel pyrolysis or combustion.