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.     

Overcrowding motifs in large PAHs. An ab initio study.

Planar and overcrowded LPAHs C(34)H(18) anthra[9,1,2-cde]benzo[rst]penaphene (1), benzo[rst]phenanthro[10,1,2-cde]pentaphene (2), tetrabenzo[a,cd,j,lm]perylene (3), tetrabenzo[a,cd,lm,o]perylene (4), and LPAHs C(38)H(18) anthra[2,1,9,8-klmno]naphtho[3,2,1,8,7-vwxyz]hexaphene (5), dianthra[2,1,9,8-stuva;2',1',9',8'-hijkl]pentacene (6), dibenzo[jk,uv]dinaphtho[2,1,8,7-defg;2',1',8',7'-opqr]perylene (7), diphenanthro[5,4,3-abcd;3',4',5'-lmno]perylene (8), potential products of peri-peri reductive couplings of benzanthrone and of naphthanthrone, respectively, were subjected to an ab initio study with emphasis on overcrowding motifs. The HF and DFT B3LYP methods were employed to calculate energies and geometries of the minima conformations of these LPAHs. The most stable LPAHs in these series were found to be planar C(2)(v)()-1 and C(2)(v)()-5, respectively. Among overcrowded LPAHs, twisted-folded C(2)-3 and C(2)-7 with two cove regions were found to be more stable than their respective isomers twisted-folded C(2)-4 and C(2)-8 with one fjord region each, in contrast to the semiempirical predictions. The energy differences between the most stable planar isomer and the overcrowded isomers were significantly smaller in the C(38)H(18) series, than in the C(34)H(18) series. Overcrowded twisted-folded C(2)-7 with two coves was found to be more stable than planar C(2)(h)()-6 by 2.0 kJ/mol (at B3LYP/6-311G), indicating enhanced role of aromatic stabilization and decreased destabilization due to overcrowding, with increasing the number of aromatic rings. Heats of formation of LPAHs 1-8 were derived from the ab initio total energies (at B3LYP/6-31G). A search of the conformational spaces of 3 and 4 revealed an anti-folded local minimum C(i)()-3 and a syn-folded transition state C(s)()-4, 23.7 and 120.3 kJ/mol higher in energy than the twisted-folded C(2)-3 and C(2)-4, respectively (at B3LYP/6-31G). The cove and fjord torsion angles in the C(38)H(18) series were found to be smaller than in the C(34)H(18) series. The nonbonding distances between carbon atoms at cove and fjord regions of the overcrowded LPAHs were found to be smaller than the sum of the van der Waals radii of two carbon atoms     

The electronic spectrometry of two similar tetrabenzoperylene isomers

The u.v. absorbance and fluorescence excitation spectra of two large polycyclic aromatic hydrocarbons (PAHS) are compared. Tetrabenzo[a,cd,j, lm]perylene, the previously known isomer, behaves as expected, while the new isomer, tetrabenzo[a,cd,f,lm]perylene, has a u.v. absorbance spectrum which is very different from its fluorescence excitation spectrum. This is the only known PAH in which this behavior has been found.     

A comparison of temperature and mobile phase composition effects for bonded liquid crystal and polymeric stationary phases in HPLC

Summary Using two polycyclyic aromatic hydrocarbons as solutes, a comparison is made between a bonded liquid crystal stationary phase and a conventional polymeric C-18 phase. The bonded nematic liquid crystal phase was the silanized form of 4-[4-(allyloxy)benzoyl-oxy]biphenyl and the polymeric phase was Vydac 201TP. Both phases display shape and planarity selectivity as indicated by the results of the variable temperature and mobile phase composition studies. The slot theory of retention can be used to explain these results. However, the liquid crystal phase is more sensitive to molecular geometry, probably due to its more ordered structure on the surface. Variable temperature experiments which compare retention during both heating and cooling provides additional support for this conclusion. With the polymeric bonded C-18 phase, each solute had identical retention at the same temperature during both the heating and cooling cycles. On the bonded liquid crystal phase, measurable differences in retention were observed at identical temperatures depending on whether the column was heated or cooled. This effect is attributed to a degree of partially reversible disordering which occurs as the column temperature was increased. However, conditioning with the appropriate mobile phase can restore the original retention characteristics of the bonded liquid crystal phase.     

Polycyclic aromatic hydrocarbon solute probes. Part II. Effect of solvent polarity on the fluorescence emission fine structures of coronene derivatives

The fluorescence properties of coronene (Co), benzo[a]coronene (BCo), naphtho[2,3-a]coronene (NCo), dibenzo[a,j]coronene (DCo), naphtho[1,2,3,4-ghi]perylene, benzo[pqr]naphtho[8,1,2-bcd]perylene and dibenzo[cd,lm]perylene dissolved in solvents of varying polarity are reported. Measurements indicated that the emission intensities of the four coronene derivatives depended on solvent polarity. The Co, BCo and NCo scales have been defined as the ratio of the fluorescence intensities of bands I and III of the vibronic spectra. Band III of dibenzo[a,j]coronene was not clearly identifiable in all the solvents studied, and the DCo scale was therefore defined as the intensity ratio of band I and IV. Emission intensity ratios of the three perylene derivatives remained nearly constant, irrespective of solvent polarity.     

Donor-acceptor copolymers from cyclopentannulation polymerizations with dicyclopenta[cd,jk]pyrene and dicyclopenta[cd,lm] perylene acceptors

A series of donor-acceptor copolymers with dicyclopenta[cd,jk]pyrene and dicyclopenta[cd,lm]perylene acceptor units was prepared via palladium catalyzed cyclopenta-annulation reactions. The acceptor units were paired with diethynyl containing donor groups based on benzo[1,2-b:4,5-b’]dithiophene, thieno[3,2-b]thiophene, and 4-octyl-4H-dithieno[3,2-b:2′,3′-d]pyrrole to create six polymer variants. The cyclopentannulation polymerization resulted in copolymers with molecular weights (Mn) of 6–14 kDa and broad light absorption in the visible region with band gaps of 1.38–1.85 eV. The synthetic methodology, as well as optoelectronic properties, including thin-film absorption and cyclic voltammetry, of the donor-acceptor copolymers are presented.     

Influence of temperature on the behaviour of small linear peptides in capillary electrochromatography

The influence of temperature, T, on the retention times, peak widths, peak symmetry coefficients and theoretical plate numbers of two small linear peptides, [Met5]enkephalin and [Leu5]enkephalin, has been studied with capillary electrochromatography (CEC) capillary columns of 100 microm I.D. and 250 mm packed length with a total length of 335 mm, containing 3 microm Hypersil n-octadecyl bonded silica. With increasing column temperature from 15 to 60 degrees C, the electroosmotic flow (EOF) and the column efficiencies increased, whereas the retention coefficients (Kcec) of both peptides decreased. A linear relationship was found between the EOF value and the square root of the temperature over this temperature range, with a linear regression correlation of 0.998. Non linear Van 't Hoff plots (In Kcec versus 1/T) were observed for these peptides between 15 and 60 degrees C, suggesting that a phase-transition occurred with the n-octadecyl chains bonded on the silica surface, affecting the CEC retention behaviour of these peptides. In CEC systems, the Kcec values of peptides incorporate contributions from both electrophoretic migration and chromatographic retention. Positive and negative Kcec values can, in principle, thus arise with these charged analytes. However, the Kcec values of the enkephalin peptides under all temperature conditions studied were positive with an eluent composed of water-50 mM NH4OAc/AcOH, pH 5.2-acetonitrile (5:2:3, v/v) and therefore the chromatographic component dominates the retention process with these small peptides under these conditions.     

Liquid chromatography analysis of monosubstituted sulfobutyl ether-beta-cyclodextrin isomers on porous graphitic carbon

The retention behaviour of the three positional isomers of monosubstituted sulfobutyl ether-beta-cyclodextrin was investigated on a porous graphitic carbon (PGC) column. The influence of the mobile phase composition (nature and concentration of organic and electronic modifiers) was studied as well as the effect of column temperature. These hydrophilic and anionic analytes were highly retained on the PGC stationary phase compared to octadecyl bonded phases. The retention is mainly governed by a reversed-phase mechanism with electronic interaction playing a secondary role. An increase in solute retention and efficiency with temperature was observed. Successful isocratic separation with satisfactory baseline resolution of the three isomers of monosubstituted sulfobutyl ether-beta-cyclodextrin was achieved at 75 degrees C on a Hypercarb column by using ammonium acetate as electronic modifier in water-acetonitrile (83:17). The chromatographic methodology developed can be easily used for relative quantification of each isomer within a mixture and can be applied for semi-preparative purification of each one. The evaporative light scattering detector allows the detection of these non UV-visible absorbing molecules.     

Diindeno[cd:1m]perylene dianion. Local vs. peripheral contributions to the aromatic character

Diindeno[cd;lm]perylene ($\text{1}$) is shown to undergo a two electron reduction process to its dianion $\text{2}$. Both neutral and doubly charged systems exhibit an enhanced diamagnetic character which is believed to be acquired via two different mechanisms.     

Molecular shape recognition capability of liquid-crystal bonded phases in reversed-phase high performance liquid chromatography

Summary The chromatographic retention behaviour of two liquidcrystal bonded phases have been evaluated using polycyclic aromatic hydrocarbons (PAHs) as the probe samples in reversed-phase high performance liquid chromatography (RP-HPLC). The results clearly indicate that these phases have better planarity and shape recognition capabilities than commercially-avaialble polymeric octadecylsilica (ODS) phases whose strong planarity and shape selectivities were found earlier. It can also be concluded from the chromatographic observations that the shape recognition capability of these phases is dependent on both mobile phase composition and column temperature, but that the effect of mobile phase and temperature on the shape selectivity work independently. The retention behaviour can be explained by changes in the phase structure with changes of eluent composition and temperature.     

Non-trivial temperature effects on the cation exchange chromatography and chelation ion chromatography of metal ions

The effect of column temperature upon the retention of metal ions on sulfonated and mono-, di-, and amino-carboxylated cation exchange columns has been investigated. The retention of alkali, alkaline earth and transition metal ions on each of the above types of cation exchanger was studied over the temperature range 19-65 degrees C. A major difference between the behaviour of mono- and divalent metal ions was shown on each of the above stationary phases, with the monovalent alkali metals exhibiting clearly exothermic behaviour (a decrease in retention with increased temperature) under acidic eluent conditions and an apparent relationship between retention factor and the magnitude of the temperature effect. The effect of temperature upon alkaline earth metal ions was less defined, although strongly endothermic behaviour (increase in retention with temperature) could be seen on all stationary phases through correct choice of eluent. The transition metal ions studied showed endothermic behaviour on all four stationary phases, with the sulfonated column unexpectedly showing the largest increases in retention. The above behaviour can be partially explained through the dominance of the type of solute-stationary phase interaction governing retention. In several of the above columns, both ion-exchange and surface complexation interactions can occur, with the effects of temperature indicating which process dominates under specific eluent conditions.     

Comparison of column properties in reversed-phase chromatography: monolithic, cholesterolic and mixed bonded stationary phases

A commercial Chromolith C18 column and two new stationary phases with mixed ligands bonded on the Kromasil silica gel support, SG-MIX and SG-Chol, were characterized using simple tests based on the retention of non-polar, basic and acidic compounds. Polar and methylene selectivity tests in acetonitrile-water and methanol-water mobile phases revealed lower hydrophobicities of the SG-MIX and SG-Chol columns in comparison to the Chromolith column. The columns were further characterized using new test criteria - gradient oligomer capacity and isomeric selectivity and peak symmetry of naphthalene di-sulphonic acids in aqueous mobile phases. The cholesterolic column shows greater gradient oligomer selectivity for the separation of oligoethylene glycol samples than the SG-MIX and the Chromolith columns. Increased retention and peak tailing, but decreased isomeric selectivity for naphthalene-di-sulphonic acids was observed with the SG-MIX column, because of interactions with various polar bonded groups.     

Effect of overcrowding in mass spectra for six violanthrenes

The mass spectra of six highly purified violanthrene samples of class A—(benzo[rst]anthro[10,1,2-cde]pentaphen, benzo[rst]phenanthro[10,1,2-cde]pentaphene), class B—(benzo[rst]phenanthro[1,10,9-cde]pentaphene, dibenzo[a,rst]naphtho[8,1,2-cde]pentaphene) and class T—(tetrabenzo[a,cd,j,lm]perylene, tetrabenzo[de,hi,op,st]pentacene) were measured. The dependence of relative ion instensities for monovalent, divalent and trivalent ions [M]ⁱ⁺, [M – 2n]ⁱ⁺ and [M – 15]ⁱ⁺ (i = 1, 2, 3) showed clearly the effect of overcrowding in condensed aromatic rings, i.e. the number of H₂ molecules which are readily eliminated from the molecular ions is zero for class A, one for class B, and two for class T. These numbers are the same as the number of overcrowded hydrogen atom pairs for each class.     

Fullerenes separation with monomeric type C30 stationary phase in high-performance liquid chromatography

The temperature effect on the separation of fullerenes in LC was examined using monomeric type C30, C18 and C8 alkyl bonded stationary phases. It appears that the C30 phase exhibits superior separation ability for fullerenes. It is observed that the maximum retention temperature of fullerenes on the C30 phase is around 20 degrees C. A strong correlation between the changes in NMR spectra and the retention behavior of the solutes was found. The interpretation of the retention behavior of fullerenes on the alkyl bonded stationary phases, including the behavior in subambient temperature, is discussed using the information obtained by CP-MAS solid-state NMR spectroscopy and LC.     

Identification of the factors that influence the reproducibility of chromatographic retention data

Principal component analysis was used to identify the parameters that influence the column-to-column and batch-to-batch reproducibility of retention times and retention factors measured on Symmetry C18, Kromasil C18, Luna C18 (2) and Vydac RP C18, all reversed-phase silica columns. We devised a procedure that allows the determination of the differences in column volume and packing density between two columns, provided that these columns are packed with identical stationary phases (i.e., phases that originate from the same batch). Principal component analysis of the retention times confirmed that the column-to-column variations of the column volume and the total porosity of the bed are the factors that influence the reproducibility of the retention times, the column volume being the major factor. For the fluctuations of the retention factors, the column phase ratios (or the bed porosities) and some specific, secondary retention mechanisms are responsible. All the C18 columns investigated proved to behave in a very similar fashion. Two principal components were always sufficient to characterize the variations of either the retention times or the retention factors.     

Fast methods for screening of trichothecenes in fungal cultures using gas chromatography-tandem mass spectrometry

The excess adsorption isotherms of acetonitrile, methanol and tetrahydrofuran from water on reversed-phase packings were studied, using 10 different columns packed with C1-C6, C8, C10, C12, and C18 monomeric phases, bonded on the same type of silica. The interpretation of isotherms on the basis of the theory of excess adsorption shows significant accumulation of the organic eluent component on the adsorbent surface on the top of "collapsed" bonded layer. The accumulated amount was shown to be practically independent of the length of alkyl chains bonded to the silica surface. A model that describes analyte retention on a reversed-phase column from a binary mobile phase is developed. The retention mechanism involves a combination of analyte distribution between the eluent and organic adsorbed layer, followed by analyte adsorption on the surface of the bonded phase. A general retention equation for the model is derived and methods for independent measurements of the involved parameters are suggested. The theory was tested by direct measurement of analyte retention from the eluents of varied composition and comparison of the values obtained with those theoretically calculated values. Experimental and theoretically calculated values are in good agreement.     

First identification of benzo[ghi]naphtho[8,1,2-bcd]perylene as a product of fuel pyrolysis, using high-performance liquid chromatography with diode-array ultraviolet-visible absorbance detection and mass spectrometry

We present HPLC/UV/MS evidence to support the identification of benzo[ghi]naphtho[8,1,2-bcd]perylene as a product of supercritical toluene pyrolysis. Mass spectral data confirm that compound I-eluting in between co-eluting benzo[a]coronene/phenanthro[5,4,3,2-efghi]perylene and benzo[pqr]naphtho[8,1,2-bcd]perylene, all three of which have been unequivocally identified as C(28)H(14) products of toluene pyrolysis-is also a C(28)H(14) product component. The UV spectrum of compound I is presented, and indicates that it is a benzenoid polycyclic aromatic hydrocarbon (PAH). Five of the eight benzenoid C(28)H(14) PAH isomers have published UV spectra, and characteristics of the remaining three are deduced from annelation theory. Only one of these compounds, benzo[ghi]naphtho[8,1,2-bcd]perylene, is predicted to have a UV spectrum with characteristics that we find in the spectrum of compound I. In addition, benzo[ghi]naphtho[8,1,2-bcd]perylene is the only benzenoid C(28)H(14) isomer whose length-to-breadth ratio is consistent with the HPLC retention time of compound I. The reaction mechanism through which benzo[ghi]naphtho[8,1,2-bcd]perylene is formed in this environment is shown, and is consistent with reaction pathways of other large PAH found in this product mixture.     

Characterization of the properties of stationary phases for liquid chromatography in aqueous mobile phases using aromatic sulphonic acids as the test compounds

We investigated the effects of the concentration of naphthalene sulphonic acids (NSAs) as anionic test compounds in the injected sample and of the salt additives to the mobile phase on ion-exclusion. The retention behaviour of NSAs sensitively reflects even minor changes in the ionic and hydrophobic interactions and can be useful for predicting the effects of the stationary phases in reversed-phase chromatography of polar and ionic compounds, both small ones and biopolymers, e.g., oligonucleotides. We studied chromatographic properties of several stationary phases intended for separations in aqueous mobile phases: a C18 column end-capped with polar hydrophilic groups, a densely bonded C8 column doubly end-capped with short alkyl groups, a short alkyl stationary phase designed to keep full pore accessibility in highly-aqueous mobile phases and a Bidentate column with “bridged” C18 groups attached to the silica hydride support. The chemistry and pore structure of various types of column packing materials and of the salt additives to the mobile phase affect the proportion of the pore volume non-accessible to anions due to ion-exclusion and consequently the peak asymmetry and hydrophobic selectivity in reversed-phase chromatography of organic acids. We also addressed the problems connected with the determination of column hold-up volume in aqueous mobile phases. The accessibility of the stationary phase for anionic compounds in contact with the sample zone is affected by ion-exclusion due to repulsive interactions with the negatively charged surface in the pores of the stationary phase. The accessible part of the stationary phase increases and consequently the migration velocity along the column decreases with increasing concentration of the sample in the zone moving along the column. Because of a limited access to the stationary phase, its capacity can be easily overloaded. The combination of the column overload and ion-exclusion effects may result in fronting or tailing peak asymmetry. To explain this behaviour, we proposed a modified Langmuir model, respecting the variation of the column capacity due to the effects of sample concentration on ion-exclusion.     

Retention properties of novel β-CD bonded stationary phases in reversed-phase HPLC mode

With the given special structures, the CD bonded stationary phases are expected to have complementary retention properties with conventional C18 stationary phase, which will be helpful to enhance the polar selectivity in RP mode separation. In this work, two β-cyclodextrin (β-CD) bonded stationary phases for reversed-phase HPLC, including 1, 12-dodecyldiol linked β-CD stationary phase (CD1) and olio (ethylene glycol) (OEG) linked β-CD stationary phase (CD2), have been synthesized via click chemistry. The resulting materials were characterized with FT-IR and elemental analysis, which proved the successful immobilization of ligands. The similarities and differences in retention characteristics between the CD and C18 stationary phases have been elucidated by using comparative linear solvation energy relationships (LSERs). The force related to solute McGowan volume has no significant difference, while the hydrogen bonding and dipolar interactions between solutes and CD stationary phases are stronger than between solutes and C18, which is attributed to the special structures (CD and triazole groups) of CD stationary phases. Chemical origins are interpreted by comparison between CD1 and CD2. Similar dispersive interactions of CD1 and CD2 are attributed to their similar length of spacer arms. CD2 which contains OEG spacer arm has relative weaker HBD acidity but stronger HBA basicity. CD stationary phases display no serious different methylene selectivity and higher polar selectivity than in the case of C18. Higher acid selectivity and lower basic selectivity are observed on CD2 than on CD1. Distinctive retention properties and good complementary separation selectivity to C18 make the novel CD bonded stationary phases available for more application in RPLC.     

Retention and selectivity effects caused by bonding of a polar urea-type ligand to silica: a study on mixed-mode retention mechanisms and the pivotal role of solute-silanol interactions in the hydrophilic interaction chromatography elution mode

The separation properties of five silica packings bonded with 1-[3-(trimethoxysilyl)propyl]urea in the range of 0–3.67 μmol m⁻² were investigated in the hydrophilic interaction chromatography (HILIC) elution mode. An increase of the ligand surface density promoted retention of non-charged polar compounds and even more so for acids. An opposite trend was observed for bases, while the amphoteric compound tyrosine exhibited a U-shaped response profile. An overall partitioning retention mechanism was incompatible with these observations; rather, the substantial involvement of adsorptive interactions was implicated. Support for the latter was provided by column-specific changes in analyte retention and concomitant selectivity effects due to variations of salt concentration, type of salt, pH value, organic modifier content, and column temperature. Silica was more selective for separating compounds differing in charge state (e.g. tyramine vs. 4-hydroxybenzoic acid), while in cases where structural differences of solutes resided in non-charged polar groups (e.g. tyramine vs. 5-hydroxydopamine, nucleoside vs. nucleobase) more selective separations were obtained on bonded phases. Hierarchical cluster analysis of the home-made urea-type and three commercial amide-type bonded packings evinced considerable differences in separation properties. The present data emphasise that the role of the packing material under HILIC elution conditions is hardly just the polar support for a dynamic coating with a water-enriched layer. Three major retention mechanisms are claimed to be relevant on bare silica and the urea-type bonded packings: (i) HILIC-type partitioning, (ii) HILIC-type weak adsorption such as hydrogen bonding between solutes and ligands or solutes and silanols (potentially influenced by individual degrees of solvation, salt bridging, etc.), (iii) strong electrostatic (ionic) solute–silanol interactions (attractive/repulsive). Even when non-charged polar bonded phases are used, solute–silanol interactions should not be discounted, which makes them a prime parameter to be characterised by HILIC column tests. Multi/mixed-mode type separations seem to be common under HILIC elution conditions, associated with a great deal of selectivity increments. They are accessible and controllable by a careful choice of the type of packing, the mobile phase composition, and the temperature.