Thermal cyclodehydrogenations to form 6-membered rings: cyclizations of [5]helicenes

Flash vacuum pyrolysis (FVP) of [5]helicene induces a thermal cyclodehydrogenation to form benzo[ghi]perylene. Evidence is presented that supports an electrocyclization-rearomatization mechanism and is inconsistent with mechanistic alternatives involving the intermediacy of aryl radicals or carbenes in the helicene fjord region.     

Nonclassical Aryl Radicals: Intermediates or Transition States for the Hydrogen Shift Reactions?

Electronic properties of aryl radicals obtained by removing single hydrogen atoms from the sterically congested regions of benzo[c]phenanthrene, biphenyl, triphenylene, phenanthrene, and perylene are studied at the UBLYP/6-311G level of theory. Two structures are considered by each radical, the classical one involving a C-H.C arrangement of atoms and the nonclassical one possessing a three-center C-H-C linkage. The five nonclassical radicals under study are found to be transition states for degenerate 1,4- and 1,5-hydrogen shift reactions that interconvert the classical species. However, the results of the present calculations indicate that the nonclassical structures with the C-H distances in the C-H-C linkages shorter than 1.34 ? should be energy minima representing potentially observable chemical systems. The predicted energy barrier to the 1,5-hydrogen shift in the 1-benzo[c]phenanthrenyl radical is only 9.3 kcal/mol (6.1 kcal/mol with the zero-point energies included), making the hydrogen migration in this system facile at relatively low temperatures. Rigorous analysis of the computed electronic wave functions provides a clear-cut picture of bonding in both the classical and nonclassical aryl radicals.     

Resolution of 13 polycyclic aromatic hydrocarbons by constant-wavelength synchronous spectrofluorometry.

A method capable of determining 13 PAHs (acenaphthene, anthracene, benzo[a]anthracene, benzo[a]pyrene, benzo[b]fluoranthene, benzo[k]fluoranthene, chrysene, dibenzo[ah]anthracene, fluoranthene, fluorene, indene[1,2,3-cd]pyrene, phenanthrene and pyrene) in a mixture of 16 EPA PAHs by second derivative synchronous spectrofluorometry in the constant wavelength mode was developed. It has not been possible to determine the following PAHs in the mixture: acenaphthylene, benzo[ghi]perylene and naphthalene. The approach studied allows the sensitive, rapid and inexpensive identification and quantitation of 13 PAHs in a solution of hexane. The detection limits are <1 microg L(-1) (except for chrysene and phenanthrene).     

Aryl-aryl bond formation by flash vacuum pyrolysis of benzannulated thiopyrans

In contrast to fully unsaturated 7-membered ring sulfur heterocycles (thiepines), some of which extrude sulfur and give the ring-contracted hydrocarbon even at room temperature in solution, benzannulated thiopyrans (6-membered sulfur heterocycles) require flash vacuum pyrolysis (FVP) conditions in the gas phase at temperatures in the range of 1000-1200 degrees C to promote the corresponding reaction. Thus, FVP of benzo[kl]thioxanthene (1) gives fluoranthene, and naphtho[2,1,8,7-klmn]thioxanthene (6) gives benzo[ghi]fluoranthene (7). FVP of thioxanthone (9) gives fluorenone (10), together with lesser amounts of dibenzo[b,d]thiophene (11), from competing decarbonylation.     

Cyclodehydrogenation reactions to cyclopentafused polycyclic aromatic hydrocarbons

B3LYP/6-31G(d,p) electronic structure calculations are employed to elucidate the reaction mechanisms for the conversion of the alternant C(18)H(12) polycyclic aromatic hydrocarbon benzo[c]phenanthrene into the nonalternant C(18)H(10) PAHs cyclopenta[cd]pyrene and benzo[ghi]fluoranthene. Isomerization reactions such as 5/6-ring switching and hydrogen atom scrambling are analyzed. Bay region chemistry, involving the rupture of one benzene ring followed by the formation of a new five-membered ring, is also studied, together with the mechanism for the formation of an aryne. The rearrangement of the latter yields annelated cyclopentadienylidenecarbene, which is then trapped intramolecularly.     

Supercritical fluid extraction of polycyclic aromatic hydrocarbons from liver samples and determination by HPLC-FL

An extraction/clean-up procedure by SFE was developed for isolating PAHs from liver samples for subsequent HPLC-FL determination of ten PAHs in the enriched extract. Recoveries (90-115%) and RSD % (< or =7.7) were satisfactory. When applied to 11 samples of bird of prey (Tyto alba) protected species and classified of special interest, from the Galicia (Northwest to Spain), benzo[ghi]perylene and indeno[1,2,3-cd]pyrene were undetectable; chrysene and benzo[a]pyrene are only detected in one sample; benzo[a]anthracene and benzo[k]fluoranthene are only quantified in one sample and benzo[b]fluoranthene in two samples. The other PAHs, anthracene, fluoranthene and pyrene are present in almost all the samples.     

Second-derivative constant-wavelength synchronous scan spectrofluorimetry for determination of benzo[b]fluoranthene, benzo[a]pyrene and indeno[1,2,3-cd]pyrene in drinking water

The use of derivative constant-wavelength synchronous scan fluorimetry is reported for the determination of three polycyclic aromatic hydrocarbon pollutants in drinking water (linearity range 0.4-4 mug 1(-1)). The limits of detection (LD) and quantification (LQ) (mug 1(-1)) are 0.01 and 0.07 for benzo[b]fluoranthene, 0.03 and 0.12 for benzo[a]pyrene and 0.19 and 0.57 for indeno[1,2,3-cd]pyrene in the presence of three other pollutants, benzo[k]fluoranthene, benzo[ghi]perylene and fluoranthene. The precision (RSD /= 85%) were satisfactory.     

焦炉降尘中多环芳烃的毛细管超临界流体色谱分析

研究了９个多环芳烃混合样品的超临界流体色谱分析条件，并与毛细管气相色谱法做了比较。超临界流体色谱的条件是：柱温１１０℃；程序升压９．０ＭＰａ（５ｍｉｎ）１．４ＭＰａ／ｍｉｎ２８．０ ＭＰａ。各组分保留时间的相对标准偏差为 １．４％～３．０％，定量分析的相对误差为１．４％～６．０％，比毛细管气相色谱法具有明显的优越性。试验了焦炉降尘样品，发现该样品主要由双环和三环的多环芳烃类物质组成。其中萘含量约占８０％。     

Determination of polycyclic aromatic hydrocarbons from buzzards (Buteo buteo) and tawny owl (Strix aluco) by liquid chromatography with fluorescence detection

Supercritical fluid extraction was applied to the determination of naturally contaminated polycyclic aromatic hydrocarbons (PAHs) in bird tissue by liquid chromatography with fluorescence detection (LC-FL). Recoveries (> 90%) and relative standard deviations (< or = 7.7%) were satisfactory. The levels of 10 PAHs were analyzed in 6 classes of tissues (heart, liver, intestine, muscle, lung, and kidney) of 10 buzzards and 2 tawny owls, predatory birds from the Galicia (northwest Spain). The PAHs found most abundantly were pyrene, fluoranthene, benzo[a]anthracene, and anthracene. Chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[ghi]perylene, and indeno[1,2,3-cd]pyrene were not detected. Intestine, kidney, and lung were more polluted than other tissues.     

Determination of Polycyclic Aromatic Hydrocarbons in Airborne Particulate by High Performance Liquid Chromatography

This paper presents a trisolvent ultrasonic extraction and HPLC analysis method for the determination of 11 polycyclic aromatic hydrocarbons in air particulate collected on an air filter by a commercial high volume air sampler. A reverse phase column, Vydac 201 TP, and a gradient mobile phase, acetonitrile/water, were used. The 11 PAHs, fluoranthene, pyrene, benz[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, dibenz[a, h]anthracene, benzo[ghi]perylene, indeno[1,2,3-cd]pyrene, and coronene were completely resolved under experimental conditions. All the PAHs except coronene were monitored by fluorescence with λ_ex=270 nm, λ_em>389 nm. Coronene was monitored by UV with λ=300 nm. The methodology was evaluated by spiking SRM 1649 with a PAH standard and then going through different extraction procedures and analyzing the PAH concentrations without clean-up. An external standard method was used for quantitation. The recovery yields for fluoranthene, benz[a]anthracene, benzo[a]pyrene, benzo[ghi]perylene and indeno[l,2,3-cd]pyrene were above 90%. The detection limits of PAH with fluorescence at λ_ex=270 nm, λ_em>389 nm ranged from 5.7 pg to 69.5 pg.     

Photoreaction of 2-halo-N-pyridinylbenzamide: intramolecular cyclization mechanism of phenyl radical assisted with n-complexation of chlorine radical

The photochemical behavior of 2-halo-N-pyridinylbenzamide (1-4 in Chart 1) was studied. The photoreaction of 2-chloro-N-pyridinylbenzamides 1a, 2a, 3a, and 4 afforded photocyclized products, benzo[c]naphthyridinones (6-9 and 16), in high yield, whereas the bromo analogues 1b, 2b, and 3b produced extensively photoreduced products, N-pyridinylbenzamides (1c, 10, and 11), with minor photocyclized product. Since the photocyclization reaction of 2-chloro-N-pyridinylbenzamide is retarded by the presence of oxygen and sensitized by the presence of a triplet sensitizer, acetone or acetophenone, a triplet state of the chloro analogue is involved in the reaction. Since several radical intermediates, particularly n-complexes of chlorine radical, are identified in the laser flash photolysis of 2-chloro-N-pyridinylbenzamide, an intramolecular cyclization mechanism of phenyl radical assisted with n-complexation of chlorine radical for the cyclization reaction is proposed: the triplet state (78 kcal/mol) of the chloro analogue (1a), which is populated by the excitation of 1a undergoes a homolytic cleavage of the C-Cl bond to give phenyl and chlorine radicals; while chlorine radical holds the neighbor pyridinyl ring with its n-complexation, the intramolecular arylation of the phenyl radical with the pyridinyl ring proceeds to produce a conjugated 2,3-dihydropyridinyl radical and then the conjugated radical aromatizes to afford a cyclized product, benzo[c]naphthyridinone by ejecting a hydrogen. The photoreduction product can be formed by hydrogen atom abstraction of the phenyl sigma radical from the environment.     

Product ion studies of diastereomeric benzo[ghi]fluoranthene tetraols by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and post-source decay

The product ion formation characteristics of the four diastereomeric tetrahydroxy benzo[ghi]fluoranthene compounds formed by hydrolysis of the syn and anti diastereomers of trans-3,4-dihydroxy-5,5a-epoxy-3,4,5,5a-tetrahydrobenzo[ghi]fluoranthene are studied using matrix-assisted laser desorption/ionization and post-source decay (PSD) to determine a correlation between the fragmentation characteristics of these tetraols and the structures of the diol-epoxide diastereomers from which they are hydrolyzed. The tetraols formed by the trans ring opening of the diol epoxides during hydrolysis yield product ion spectra specific for the syn and anti configurations of their precursor diol epoxides. All four diastereomeric tetraols form product ions by the losses of one and/or two water molecules in varying proportions when lithium-cationized molecule ions (m/z 301) are selected for PSD product ion analysis. The differences in the PSD spectra of these four Li+-cationized molecules are rationalized in terms of a water loss mechanism that involves the 1,2 elimination of a hydrogen atom and hydroxyl group that are cis with respect to each other on adjacent carbons.     

The development of solid-surface fluorescence characterization of polycyclic aromatic hydrocarbons for potential screening tests in environmental samples

This paper presents the characterization of polycyclic aromatic hydrocarbons (PAHs) in solid-surface fluorescence as the first step for obtaining new optical sensors for PAHs screening. The fluorescence properties of the EPA-PAHs (naphthalene, acenaphthene, acenaphthylene, fluorene, phenanthrene, anthracene, fluoranthene, pyrene, chrysene, benzo[a]anthracene, benzo[k]fluoranthene, benzo[b]fluoranthene, benzo[a]pyrene, indeno [1,2,3-cd]pyrene, benzo[g,h,i]perylene and dibenzo[a,h]anthracene) on five types of solid-surfaces were evaluated. The experimental variables (pH and percentage of organic solvent in samples) were studied, obtaining different possibilities for making individual sensors for some of these PAHs and the best conditions for developing sensors for PAH screening were also studied.     

FLASH PHOTOLYTIC INVESTIGATIONS OF THE ELECTRON TRANSFER BETWEEN AROMATIC HYDROCARBON RADICALS AND TRYPTOPHAN IN MICELLAR SOLUTIONS

Abstract— Electron transfer reactions between tryptophan and cation radicals of various polycyclic hydrocarbons which are known to differ with respect to their carcinogenic and photodynamic activity have been investigated by flash spectroscopy. The hydrocarbons were solubilized in sodium dodecyl sulfate micelles. Their cation radicals were produced by photoionization probably via the excited singlet states. In the case of benz[a]anthracene, benzo[ghi]perylene, pyrene, benzo[rst]pentaphene, dibenz[a, h]-anthracene and anthracene, the electron transfer takes place in the Stern layer of the micelles and leads to neutral tryptophan radicals. It is assumed that tryptophan forms charge transfer complexes with the cation radicals of 3-methylcholanthrene and 7,12-dimethylbenz[a]anthracene. With perylene tryptophan only reduces the radical yield. New species could not be detected in this case. No correlation exists between the carcinogenic or the photodynamic activity of the hydrocarbons and the electron transfer behaviour of their cation radicals.     

Gas chromatographic-mass spectrometric determination of polycyclic aromatic hydrocarbons formed during the pyrolysis of phenylalanine

The formation of polycyclic aromatic hydrocarbons (PAHs) during pyrolysis process of phenylalanine had been studied. Ten PAHs, including fluorene, phenanthrene, anthracene, fluoranthene, pyrene, benzo[a]anthracene, chrysene, benzo[k]fluoranthene, benzo[e]pyrene, and benzo[a]pyrene were analyzed by gas chromatography-mass spectrometry using selective ion monitoring mode. This technique offers the capability to analyze trace amounts of PAHs in phenylalanine pyrolyzates. The pyrolysis was carried out in a micro-furnace with quartz furnace liner. The injection was conducted with glass pelletizer syringe to avoid metal contamination. Qualitative results were obtained at 900 degrees C and quantitative analysis of 10 PAHs was done for 700 and 900 degrees C.     

Intramolecular dehydro Diels-Alder reactions of diarylacetylenes: switching between benzo[b]- and benzo[c]fluorenones as products by controlling the rearrangement of cyclic allene intermediates

Thermal cyclization of 1-[2-(arylethynyl)phenyl]-3-trimethylsilylpropynones affords a mixture of benzo[b]fluorenones and benzo[c]fluorenones. The ratio of the two isomers can be efficiently varied between 100:0 and 0:100 by introducing substituents with appropriate electronic and steric properties on the aryl rings and using an appropriate solvent.     

Resolution of benzo[a]pyrene in complex mixtures of other polycyclic aromatic hydrocarbons. Comparison of two spectrofluorimetric methods applied to water samples

Two spectrofluorimetric methods, second-derivative constant-energy synchronous luminescence (SDCESL) and constant-wavelength synchronous luminescence (CWSL) in combination with multiple linear regression (MLR), for the quantification of benzo[a]pyrene (BaP) at sub-ng mL-1 levels, in the presence of benzo[b]fluoranthene (BbFt), benzo[k]fluoranthene (BkFt), benzo[ghi]perylene (BghiP) and indeno[1,2,3-cd]pyrene (IP), were developed and compared in detail. SDCESL presents lower limits of detection and quantification than CWSL/MLR and also gives more exact and precise results for levels close to the quantification limit. For BaP, SDCESL achieved quantification limits of 0.019 ng mL-1 in river waters and 0.007 ng mL-1 in drinking waters. This work offers a sensitive, precise, accurate, rapid, simple and economic methodology for monitoring BaP in waters for public consumption, meeting all the requirements of the EC Directive 98/83/CE that fixes the maximum admissible limit for this polycyclic aromatic hydrocarbon in drinking waters at of 0.010 ng mL-1.     

Penetration of mercury-adsorbed phospholipid monolayers by polynuclear aromatic hydrocarbons

The penetration of phospholipid monolayers (dioleoyl lecithin) adsorbed on mercury by polynuclear aromatic hydrocarbons (PAH) is described. The PAH studied were anthracene, phenanthrene, pyrene, benzo[a]anthracene, fluoranthene and perylene. The penetration is monitored by measuring the differential capacitance of the monolayer; the uptake of PAH causes a potential shift (up to ?0.25 V) in the cathodic capacitance peaks. This occurs without displacement of the lipid from the mercury. The differential capacitance is measured by out-of-phase (90°) a.c. voltammetry and rapid cyclic voltammetry. The PAH permeate the mercury-adsorbed lipid layers from dilute aqueous solution; the order of affinity is benzo[a]anthracene > fluoranthrene = pyrene > anthracene = phenanthrene. The rates of penetration vary for the different compounds and depend on their water solubility.     

Quantification of polycyclic aromatic hydrocarbons (PAHs) in human hair by HPLC with fluorescence detection: a biological monitoring method to evaluate the exposure to PAHs

A high-performance liquid chromatographic (HPLC) method with fluorescence detection was developed for the quantification of polycyclic aromatic hydrocarbons (PAHs) in human hair. Fifteen kinds of PAHs classified as priority pollutants by the US EPA were quantified with four perdeuterated PAHs as internal standards. After 50 mg hair samples were washed with n-hexane to remove external contamination of PAHs, the samples were digested in 2.5 M sodium hydroxide. The digests were extracted with n-hexane and then analyzed by HPLC. Eleven kinds of PAHs were identified in hair samples of 20 subjects, and 10 kinds of PAHs were eventually quantified using the internal standards. For anthracene, chrysene and benzo[k]fluoranthene, significant differences were observed between smokers and non-smokers. Although benzo[b]fluoranthene, dibenz[a,h]anthracene, benzo[ghi]perylene and indeno[1,2,3-cd]pyrene were observed in the particulates of indoor and outdoor air, they were not detected in all hair samples. The analysis of PAHs in human hair should be useful as a new biomarker to evaluate the exposure to PAHs.     

Error propagation as a factor in selection of measurement intervals for the determination of polycyclic aromatic hydrocarbons by second-derivative spectrofluorimetry

The most suitable wavelength intervals were selected for the determination of 4 polycyclic aromatic hydrocarbons (PAHs; benzo[g,h,i]perylene, dibenzo[a,h]anthracene, pyrene, and triphenylene) in very complex mixtures of 11 PAHs: anthracene, benz[a]anthracene, benzo[a]pyrene, benzo[b]fluoranthene, benzo[g,h,i]perylene, benzo[k]fluoranthene, chrysene, dibenz[a,h]anthracene, phenanthrene, pyrene, and triphenylene. The multiple linear regression algorithm was applied to measurements made in several wavelength intervals previously selected on the basis of sensitivity and minimum number of interfering compounds. Of the different models obtained, those displaying minimum error propagation in the analytical result were selected. By applying the models proposed in this study, we precisely and accurately determined benzo[g,h,i]perylene, dibenz[a,h]anthracene, pyrene, and triphenylene in complex mixtures--a feat that could not be achieved by the use of constant-wavelength spectrofluorimetry in combination with second-derivative techniques.