Isomers Of Aromatic Compounds Research Articles

A New Reaction Involved in Forming the Composition of Oils: Methylation of Aromatic Cycles

One of the processes that can locally be involved in forming the composition of aromatic hydrocarbons in oils is the reaction of electrophilic methylation of aromatic cycles. Up to 40% of the original oil compounds can be transformed into methylation products. Methylation is demonstrated to most likely proceed during catagenesis, after the generation of the fluid that had produced a given oil accumulation. The methylation of aromatic cycles is a definite indicator that the source rocks are either clays or silicites containing relatively much clay material. The methylation reaction is proved not to be widespread even within a given basin, and hence, analysis for the presence of electrophilic methylation products can be efficiently applied in geochemical reconstructions. One of the consequences of methylation may be changes in the concentrations of isomers of aromatic compounds that are used in calculating maturity coefficients, and this may lead to misinterpretations of the results.

Distinguishing Isomeric Aromatic Radical Cations by Using Energy-Resolved Ion Trap and Medium Energy Collision-Activated Dissociation Mass Spectrometry.

Different collision-activated dissociation (CAD) methods were evaluated for their effectiveness at distinguishing several ionized isomeric aromatic compounds by using a linear quadrupole ion trap/orbitrap mass spectrometer. The compounds were ionized by using atmospheric pressure chemical ionization (APCI) with carbon disulfide solvent in the positive ion mode to generate stable molecular ions with limited fragmentation. They were subjected to CAD in the linear quadrupole ion trap (ITCAD) and in an octupole collision cell (medium-energy collision-activated dissociation, MCAD; also known as HCD). Experiments conducted by attempting tovaryion activation times revealed that MCAD and ITCAD occur in the microsecond and millisecond time regimes, respectively. MCAD was found to impart substantially greater internal energies into the molecular ions compared to ITCAD. Accordingly, molecular ions subjected to MCAD favored dissociation via fast σ-bond cleavages, while molecular ions subjected to ITCAD tended to favor rearrangement reactions. MCAD used in the energy-resolved mode (ER-MCAD) enabled the distinction of six ionized isomeric compounds from each other based on modified crossing-point energies (collision energies where the molecular ions and selected fragment ions have an equal abundance). This was not true for ER-ITCAD. Overall, MCAD was superior over ITCAD at the differentiation of isomeric ions, and it provided more detailed structural information.

High-performance liquid chromatography separation of unsaturated organic compounds by a monolithic silica column embedded with silver nanoparticles

The optimization of a porous structure to ensure good separation performances is always a significant issue in high-performance liquid chromatography column design. Recently we reported the homogeneous embedment of Ag nanoparticles in periodic mesoporous silica monolith and the application of such Ag nanoparticles embedded silica monolith for the high-performance liquid chromatography separation of polyaromatic hydrocarbons. However, the separation performance remains to be improved and the retention mechanism as compared with the Ag ion high-performance liquid chromatography technique still needs to be clarified. In this research, Ag nanoparticles were introduced into a macro/mesoporous silica monolith with optimized pore parameters for high-performance liquid chromatography separations. Baseline separation of benzene, naphthalene, anthracene, and pyrene was achieved with the theoretical plate number for analyte naphthalene as 36,000 m(-1). Its separation function was further extended to cis/trans isomers of aromatic compounds where cis/trans stilbenes were chosen as a benchmark. Good separation of cis/trans-stilbene with separation factor as 7 and theoretical plate number as 76,000 m(-1) for cis-stilbene was obtained. The trans isomer, however, is retained more strongly, which contradicts the long- established retention rule of Ag ion chromatography. Such behavior of Ag nanoparticles embedded in a silica column can be attributed to the differences in the molecular geometric configuration of cis/trans stilbenes.

β-Cyclodextrin-Functionalized Silver Nanoparticles for the Naked Eye Detection of Aromatic Isomers

We report herein the development of a highly robust, quantitative, sensitive, and naked eye colorimetric detection method for different isomers of aromatic compounds using β-CD-modified silver nanoparticle (AgNPs) probes. This assay relies on the distance-dependent optical properties of Ag nanoparticles and the different inclusion binding strength of the aromatic guests to β-CD host. In the presence of different isomers of aromatic compounds, AgNPs could be rapidly induced to aggregate, thereby resulting in apricot-to-red color change. The variety and concentration of different isomers of aromatic compounds could be determined by monitoring with the naked eye or a UV-vis spectrometer. The present detection limit for different isomers of aromatic compounds is 5 × 10(-5) M. We believe that the surface architectures of AgNPs after the introduction of the CD-based host-guest recognition would be applicable for a range of chemical and bioanalytical molecular sensing systems in aqueous media.

SERS as a probe for adsorbate orientation on silver nanoclusters

AbstractSurface‐enhanced Raman scattering (SERS) spectroscopy has been used to characterize multilayers of three isomeric aromatic compounds adsorbed on silver nanoclusters. The three structural isomers, all of which adsorb in the carboxylate form onto the silver nanoclusters, bind in two different geometries to the silver surface. Different molecular configurations correlate to differences in bonding strength of these molecules to the silver surface, which can be probed by SERS. For ortho‐hydroxybenzoic acid (salicylic acid), we observed red shifts of major SERS peaks in comparison to the normal Raman vibrations of nonadsorbed crystalline material. For this molecule the steric hindrance between the adjacent carboxylate and hydroxyl groups causes the carboxylate group to rotate from the common flat geometry of benzene substituents on surfaces and bond directly through one of the oxygen atoms to the surface. In this case, strong coordinative bonding between the carboxylate group and the metal surface causes the red shifts in the SERS peaks. For para‐, and meta‐hydroxybenzoic acid, the steric hindrance is less likely since the two functional groups are not at adjacent positions, and therefore these molecules adsorb on the silver surface in a totally flat geometry. For these molecules, in contrast to the ortho isomer, the CO2 interacts with the surface through an extended π bond, and these molecules are physically adsorbed in the common flat position. Therefore, for the meta and para substituents, we do not observe significant red shifts in the SERS spectrum. Copyright © 2009 John Wiley & Sons, Ltd.

Cation-modified silikalit-2 as a selective adsorbent for gas chromatography columns

A selective adsorbent was proposed on the basis of synthetic zeolite silikalit-2 modified with cadmium, tallium, and silver cations. It is intended for the gas chromatographic separation of some isomeric benzene derivatives. The adsorbent possesses pronounced retention properties to para isomers of aromatic compounds, which is due to the molecular sieve properties of the zeolite and the ability of benzene derivatives to form unstable complexes with cations entering the composition of the zeolite. Low selectivity to ortho and meta isomers is due to only the complexation effect.

Atmospheric-pressure ionization studies and field dependence of ion mobilities of isomeric hydrocarbons using a miniature differential mobility spectrometer

The ionization pathways and ion mobility were determined for sets of structural isomeric and stereoisomeric non-polar hydrocarbons (saturated and unsaturated cyclic hydrocarbons and aromatic hydrocarbons) using a novel miniature differential mobility spectrometer with atmospheric-pressure photoionization (APPI) to assess how structural and stereochemical differences influence ion formation and ion mobility. The analytical results obtained using the differential mobility spectrometry (DMS) were compared with the reduced mobility values measured using conventional time-of-flight ion mobility spectrometry (IMS) with the same ionization technique. The majority of differences in DMS ion mobility spectra observed among isomeric cyclic hydrocarbons can be explained by the formation of different product ions. Comparable differences in ion formation were also observed using conventional IMS and by investigations using the coupling of ion mobility spectrometry with mass spectrometry (APPI-IMS-MS) and APPI-MS. Using DMS, isomeric aromatic hydrocarbons can in the majority of cases be distinguished by the different behavior of product ions in the strong asymmetric radio frequency (rf) electric field of the drift channel. The different peak position of product ions depending on the electric field amplitude permits the differentiation between most of the investigated isomeric aromatics with a different constitution; this stands in contrast to conventional IMS in which comparable reduced mobility values were detected for the isomeric aromatic compounds.

Open‐Tubular Capillary Electrochromatography Using Capillaries Modified with p‐tert‐Butylcalix[4]arene‐1,3‐bis(allyloxyethy)ether

Preparation and characterization of p‐tert‐butylcalix[4]arene‐1,3‐bis(allyloxyethy)ether (CA[4]‐BAE) chemically coated capillaries via free radical reaction with vinyltriethoxysilane (VTES), which was attached onto the inner wall, was reported here previously IR spectra, SEM characterization, and decreased electroosmotic flow (EOF) suggested that the capillary was coated with CA[4]‐BAE successfully. A slight slope of EOF versus pH ranging from 5 to 9 would help to make the separation reproducible. The CA[4]‐BAE‐coated capillary had shown improved separations of isomeric toluidines, isomeric naphthols, biphenyl derivatives, and polycyclic aromatic hydrocarbons (PAHs) in comparison with an uncoated capillary. The special selectivity on isomeric aromatic compounds indicated that there was a certain extent of host‐guest interactions between solutes and the CA[4]‐BAE coating.

Comparison of two liquid crystals as stationary phases in capillary gas chromatography

The comparative gas chromatographic behavior has been investigated for two new liquid-crystal stationary phases, 2-(3-chlorobenzyloxy)-3-hexanoxy-4-(4-chlorobenzoyloxy)-4′-(4-methylbenzoyloxy)azobenzene (denoted 3-Cl) and 2-(3-methylbenzyloxy)-3-hexanoxy-4-(4-chlorobenzoyloxy)-4′-(4-methylbenzoyloxy)azobenzene (denoted 3-CH3). The structures of the main core of the two liquid crystals are the same but they differ by lateral aromatic branch— 3-chlorobenzyloxy and 3-methylbenzoyloxy. Their thermal properties were established by use of differential scanning calorimetry (DSC) and their chromatographic separation properties by use of glass capillary columns. Interesting analytical performance was obtained in the separation of isomeric aromatic compounds, polyaromatic compounds, volatile aroma compounds, andcis andtrans isomers. Separation efficiency was higher for 3-Cl, especially for volatile aroma compounds, light aromatic compounds, and polyaromatic compounds, although the phases were similar in otherways.

Preparation and characteristics of two novel calix[4]arene polysiloxane stationary phases for capillary gas chromatography

Two novel calix[4]arene derivatives, 5-1′,1′-dimethylundecyloxyphenylmethyl-11,17,23-tris-1″, 1″-dimethylethyl- 25,26,27,28-tetrabenzyloxycalix[4]arene polysiloxane (C[4]TB-PSO) and 5-1′,1′-dimethylundecyloxyphenyl- methyl-11,17,23-tris-1″,1″-dimethylethyl- 25,26,27,28-tetraethoxycarbonylmethyloxycalix[4]arene polysiloxane (C[4]TECM-PSO), were synthesized and used as stationary phases in capillary gas chromatography. The new phases demonstrated high column efficiency, a wide operating temperature range, high thermal stability, medium polarity and excellent selectivity towards alcohol isomers and positional isomers of aromatic compounds. The selectivity mechanism for positional isomer compounds was investigated by the measurements of thermodynamic parameters, ΔH, ΔS and ΔG. The inclusion properties of the calix[4]arenes are discussed.

Molecular distribution of monocarboxylic acids in Asuka carbonaceous chondrites from Antarctica.

Molecular distribution of low-molecular-weight monocarboxylic acids was studied in three CM2 Asuka carbonaceous chondrites (A-881280, A-881334 and A-881458), which were recovered from Antarctica by the 29th Japanese Antarctic Research Expedition in 1988. GC and GC/MS analyses identified more than 30 monocarboxylic acids in A-881458, including aliphatic and aromatic acids with various structural isomers. Isomeric phenolic compounds were also identified. The aliphatic carboxylic acids have straight-chain structures having 2 to 12 carbon atoms (C2 to C12), and branched-chain structures (C4 to C9). The quantities of straight-chain acids decrease logarithmically with increasing carbon number. At the same carbon number, a straight-chain isomer is always predominant compared to branched-chain isomers. All of the 14 possible C4, C5 and C6 aliphatic monocarboxylic acids (not including optical isomers) have been identified, although all the isomers were not reported in Murchison and Y-791198 meteorites. Of the 17 possible isomeric C7 acids, at least 14 isomers were tentatively identified by mass spectra (EI and CI mode). At C8 or above, peaks of branched-chain isomers become obscure, probably due to the large number of isomers and small concentrations. Branched-chain monocarboxylic acids over C6 have never been reported in Muchison. Although occurrence of aliphatic acids are similar between A-881458 and Murchison at C4, C5 and C6 acids, a major difference is that A-881458 as well as Y-791198 have straight-chain predominance among isomers in contrast to Murchison being branched-chain predominant. In the case of isomeric aromatic compounds such as toluic acids and cresols, m-toluic acid and p-cresol are more abundant among their isomers, respectively. The molecular distribution may not reflect thermodynamic equilibrium but rather a kinetically controlled process for their formation mechanism. The other two CM2 chondrites (A-881280 and A-881334) were depleted in carboxylic acids in spite of similar carbon contents. The depletion is not due to weathering on ice, because the degrees of weathering are small and similar among the three chondrites. Probably those latter two chondrites may have been subjected to aqueous alteration or metamorphism on their meteorite parent bodies.

Selective isotopic exchange of polyfimctional ions in tandem mass spectrometry: Methodology, applications and mechanism

AbstractIsotopic exchange of mass‐selected odd‐ and even‐electron molecular ions of aromatic compounds upon collision with deuterated gases was investigated as a function of reagent gas, interaction time and collision energy. Use of ND3 as reagent allows exchange of all active hydrogens for the compound types studied, providing a count of the total number of active hydrogens present in the analyte. CH3OD exchanges specific types of active hydrogens, such as phenolic and carboxylic hydrogens, without exchanging amino hydrogens. This selectivity assists in the identification and enumeration of different types of active hydrogens present in polyfunctional compounds. The H–D exchange patterns serve to differentiate isomeric aromatic compounds containing methoxy, amino, hydroxy and carboxylic acid substituents. Trapping of mass‐selected ions in the collision region of a triple quadrupole mass spectrometer greatly enhances the degree of H–D exchange, thereby facilitating determination of the number of active hydrogens in the analyte. Triple stage mass spectrometric experiments, performed in a pentaquadrupole mass spectrometer, help elucidate the exchange process. Isotopic exchange in the collision region of a tandem mass spectrometer also provides insights into the site of protonation in molecules containing several functional groups. The proximity of the functional groups and the proton affinity difference between the analyte and the reagent gas are important factors in site‐specific H–D exchange in polyfunctional compounds. An investigation of the effects of collision energy reveals that cluster ion formation plays a major role in the exchange mechanism operating in the triple quadrupole and that H–D exchange, ion‐molecule adduct formation and endothermic fragmentation are competitive reaction channels.

Liquid crystal stationary phases for gas chromatography and supercritical fluid chromatography

AbstractNovel monomeric and polymeric liquid crystalline compounds were synthesized as stationary phases for gas chromatography (GC) and supercritical fluid chromatography (SFC). Monomeric liquid crystalline compounds were used in packed column gas chromatography for the separation of isomeric aromatic compounds and insect sex pheromones. Liquid crystalline polymers possess long nematic ranges and a uniform coating was easily achieved in glass and fused silica capillaries, which could stand temperatures up to 250°C in GC and pressures of 200 MPa at 160°C in SFC. The columns provide excellent selectivity and resolution for fused ring aromatic compounds such as the isomers anthracene and phenanthrene or triphenylene and chrysene.

Elektronenstossinduzierte fragmentierung von acetylenverbindungen — VII: Ein vergleich der massenspektren von offenkettigen und aromatischen C 9H 12O-alkoholen

The principal difference in the electron impact induced behaviour of primary and secondary acetylenic alcohols and the corresponding isomeric aromatic compounds consist in a very easy isomerisation of the open chain compounds to ketones before fragmentation is involved, while with aromatics this behaviour is much less pronounced as here the hydrogen transfer is combined with a loss of the aromaticity.

芳香族異性体間の殺菌効力差の研究 (II)

芳香族異性体間の殺菌効力差の研究 (II)