Benzene Isomers Research Articles

Isomer-Specific Spectroscopy of Benzene–(H2O)n, n = 6,7: Benzene’s Role in Reshaping Water’s Three-Dimensional Networks

The water hexamer and heptamer are the smallest sized water clusters that support three-dimensional hydrogen-bonded networks, with several competing structures that could be altered by interactions with a solute. Using infrared-ultraviolet double resonance spectroscopy, we record isomer-specific OH stretch infrared spectra of gas-phase benzene-(H2O)(6,7) clusters that demonstrate benzene's surprising role in reshaping (H2O)(6,7). The single observed isomer of benzene-(H2O)6 incorporates an inverted book structure rather than the cage or prism. The main conformer of benzene-(H2O)7 is an inserted-cubic structure in which benzene replaces one water molecule in the S4-symmetry cube of the water octamer, inserting itself into the water cluster by engaging as a π H-bond acceptor with one water and via C-H···O donor interactions with two others. The corresponding D(2d)-symmetry inserted-cube structure is not observed, consistent with the calculated energetic preference for the S4 over the D(2d) inserted cube. A reduced-dimension model that incorporates stretch-bend Fermi resonance accounts for the spectra in detail and sheds light on the hydrogen-bonding networks themselves and on the perturbations imposed on them by benzene.

Synthesis and characterization of three 2D Cd (II) coordination polymers built with terphenyl-2, 2′, 4, 4′-tetracarboxylate and flexible bis(imidazole)

Three new CdII coordination polymers, namely, {[Cd2 (L) (1, 2-bix)2(H2O)2]⋅2H2O}n(1), [Cd2 (L) (1, 3-bix)2]n (2), and {[Cd (H2L) (1, 4-bix) (H2O)2]⋅2H2O}n (3) [H4L=terphenyl-2, 2′, 4, 4′-tetracarboxylic acid, 1, n-bix=1, n-bis(imidazol-l-ylmethyl) benzene (n=2, 3, 4)], have been synthesized under hydrothermal conditions and characterized by single-crystal X-ray diffraction, IR, thermogravimetric analysis, and fluorescent analysis. Complex 1 is a 2D wavelike network structure incorporating the dinuclear Cd2 (1, 2-bix)2 units, whereas complex 2 possesses a 2D stair-like network structure consisting of the dinuclear Cd2O2 units. Complex 3 features a 2D puckered network constructed by 1D [Cd (H2L)]n and 1D [Cd (1, 4-bix)]n chains. The results show that the structural diversity of the polymers is mainly attributed to the usage of diverse ancillary bis (imidazol-1-ylmethyl)-benzene isomers ligands. The thermal stabilities and photoluminescence behaviors were also investigated.

Purification and separation of durene by static melt crystallization

The purification and separation of durene from the mixture containing durene isomers were studied. Since the boiling points of tetramethyl benzene isomers are very close but their melting points are of great differences, static melt crystallization was applied to separate and purify durene from its isomers. Crystallization experiments were carried out under various operating conditions. The effects of cooling rate, crystallization temperature, sweating temperature and sweating time on the yield and purity of crystal were investigated. Orthogonal experimental design method was adopted to analyze the factors that may affect the yield of durene. Under the optimal crystallization conditions, the purity of durene could reach as high as 99.06% with the yield of 75.3% through one crystallization process. By fitting purification data based on sweating time in isothermal operations, the purification rate coefficient was obtained.

Approach to Loschmidt's tetracyclic benzene isomer and related tetra‐ and tricyclic isomers by intramolecular carbene additions

Potential carbene precursors have been considered for three tetracyclic benzene isomers: carbene 13.. for tetracyclo[3.1.0.01,3.03,5]hexane, 11; carbene 19.. for tetracyclo[3.1.0.01,4.02,3]hexane, 10, considered by Loschmidt as a structure for benzene; and carbene 20.. for Ladenburg's prismane, 4. The bicyclic (CH)6 isomer, 20.., avoids intramolecular cycloaddition; it is converted instead to Dewar benzene, 2; the system 13.. – 11.. is a “foiled” carbene system; finally, we were unable to characterize the putative precursor, 19.., for Loschmidt's hydrocarbon; an attempt to generate it by adding an electron to the corresponding carbene cation, 19.+, resulted in conversion to anti‐10. The trajectory of this conversion indicates that “19..” lies at or below the inflection point of the trajectory connecting a putative transition state with anti‐10. The free energies of all six species lie in the range 120–165 kcal/mol, well above any species that may be involved in the topomerization of benzene. Copyright © 2014 John Wiley & Sons, Ltd.

Liquid chromatographic behavior of two alanine-substituted calix[4]arene-bonded silica gel stationary phases.

Two new kinds of alanine-substituted calix[4]arene stationary phases of 5,11,17,23-p-tert-butyl-25,27-bis(l-alanine-methylester-N-carbonyl-methoxy)-26,28-dihyroxycalix[4]arene-bonded silica gel stationary phase (BABS4) and 5, 11, 17, 23-p-tert-butyl-25,26,27,28-tetra(l-alanine-methylester-N-carbonyl-methoxy)-calix[4]arene-bonded silica gel stationary phase (TABS4) were prepared and characterized in the present study. They were compared with each other and investigated in terms of their chromatographic performance by using polycyclic aromatic hydrocarbons, disubstituted benzene isomers, and mono-substituted benzenes as solute probes. The results indicated that both BABS4 and TABS4 exhibited multiple interactions with analytes. In addition, the commonly used Tanaka characterization protocol for the evaluation of commercially available stationary phases was applied to evaluate the properties of these two new functionalized calixarene stationary phases. The Tanaka test results were compared with Zorbax Eclipse XDB C18 and Kromasil phenyl columns, respectively. BABS4 has stronger hydrogen-bonding capacity and ion-exchange capacity than TABS4, and features weaker hydrophobicity and hydrophobic selectivity. Both of them behave similarly in stereoselectivity. Both BABS4 and TABS4 are weaker than C18 and phenyl stationary phases in hydrophobicity and hydrophobic selectivity.

Probing solute–solvent interactions using difluorobenzene isomers in water–monohydric-alcohol mixtures

Isomers, particularly those with similar geometries, provide an interesting mechanism for studying solute–solvent interactions via their diffusion properties. Here nuclear magnetic resonance (NMR) was used to measure the self-diffusion of difluorobenzene isomers in water–monohydric-alcohol systems (i.e., methanol, ethanol, and tert-butanol), at a range of mole fractions for each solvent system. The diffusion behaviour of the difluorobenzene isomers was different to the previously studied dihydroxybenzene and diaminobenzene isomers. While there is immediate usefulness to mixture analysis by the separation of similar molecules such as isomers using solvent modulation, the comparable structure of the three sets of disubstituted benzene isomers allows a comparison of solvent effects on diffusion. It was found that the different positions and functional groups had a significant effect on the diffusion of the isomers. The effect of substituent position, and the type of functional group on the interaction with the mixed solvent has been examined here.

Exploration of Isomers of Benzene by GRRM/SCC-DFTB

Huge numbers of structures, 7000 isomers and 26229 transition structures, have been searched for C6H6 in quantum chemical calculations by using a GRRM/SCC-DFTB program. The stability of benzene is ...

Coordination assemblies of the CdII–BDC/bpt mixed-ligand system: positional isomeric effect, structural diversification and luminescent properties

To further systematically investigate the influence of the positional isomeric ligands on the structures and properties of transition metal complexes, we synthesized six Cd(II) complexes with three positional isomeric dipyridyl ligands (4,4′-bpt, 3,4′-bpt and 3,3′-bpt) and three positional isomeric phenyl dicarboxylate anions (p-BDC, m-BDC and o-BDC), namely, [Cd3(p-BDC)3(4,4′-bpt)2(H2O)5]·H2O (1), [Cd(p-HBDC)(p-BDC)0.5(3,4′-bpt)] (2), [Cd4(p-BDC)4(3,3′-bpt)4]·9H2O (3), [Cd(m-BDC)(3,3′-bpt)(H2O)]·2H2O (4), [Cd(o-BDC)(3,3′-bpt)(H2O)]·(3,3′-bpt)·4H2O (5), [Cd(o-BDC)(3,4′-bpt)(H2O)]·H2O (6). Structural analysis reveals that 1 is composed of a three dimensional (3D) 4-connected net giving an interesting 3-fold interpenetrating architecture. 2 and 3 possess a similar two dimensional (2D) layer structures. 4 presents an infinite one dimensional (1D) tubular-like chain. 5 displays a 2D honeycomb structure consisting of a 1D metal–organic helical chain. 6 exhibits a 2D wave-like layer structure with a (44)-sql network. Structural diversities indicate that the nature of isomeric benzene–dicarboxylates and bpt ligands plays crucial roles in modulating structures of these complexes. Intermolecular forces have important effects on the formation and strengthening of the supra-molecular architecture in these complexes. Moreover, the luminescent properties of them have been briefly investigated.

Rapid simultaneous electrochemical sensing of tea polyphenols

Simultaneous detection of 1,4-, 1,2- and 1,3-dihydroxy benzene (DHB) isomers on a glassy carbon electrode modified with graphitized mesoporous carbon (GCE/GMC) in pH 7 phosphate buffer solution was demonstrated. GCE/GMC showed absence of any strong adsorption of the DHB isomers on the working electrode surface. The electrochemical response of 1,4-DHB resembled that of 1,2,3-trihydroxybenzene (gallo group) on the GCE/GMC. Differential pulse voltammetry was used as a tool for quantification of the DHB isomers. Constructed calibration plots were linear in the window 4 × 10−5 to 2.5 × 10−4, 2.5 × 10−5 to 2.0 × 10−4 and 2.5 × 10−5 to 2.0 × 10−4 mol L−1 for the 1,4-, 1,2- and 1,3-DHB, respectively. Corresponding detection limits (S/N = 3) were 0.91, 1.31 and 0.67 μM. Polyphenols extracted from three different tea samples using hot water were successfully analyzed with recovery values ∼100%.

New oxo-bridged calix[2]arene[2]triazine stationary phase for high performance liquid chromatography

A new oxo-bridged calix[2]arene[2]triazine bonded stationary phase (OCATS) for high performance liquid chromatography (HPLC) was prepared using 3-aminopropyltriethoxysilane as coupling reagent. The structure of new material was characterized by infrared spectroscopy, elemental analysis and thermogravimetric analysis. The chromatographic performance and retention mechanism of the new stationary phase were evaluated in reversed-phase mode compared with ODS using different solute probes including polycyclic aromatic hydrocarbons (PAHs), mono-substituted benzenes, disubstituted benzene isomers. The new OCATS stationary phase could provide various interactions for different solutes, such as hydrophobic, hydrogen bonding, π π and inclusion interactions. The synergistic effects resulting from aromatic rings, bridging oxygen atoms and triazine nitrogen atoms and alkyl linkers in the new material improved the separation selectivity by multiple retention mechanisms. The retention behaviors of the analytes on OCATS column were explained with the assistance of quantum chemistry calculation results using DFT-B3LYP/STO-3G* base group. The OCATS column was successfully employed for the analysis of melamine in infant formula.

Hyperaromatic Stabilization of Arenium Ions: A Remarkable Cis Stereoselectivity of Nucleophilic Trapping of β-Hydroxyarenium Ions by Water

Cis- and trans-1,2-dihydrodiol isomers of benzene undergo acid-catalyzed dehydration to form phenol. In principle the isomeric substrates react through a common β-hydroxybenzenium (cyclohexadienyl) carbocation. Notwithstanding, the isomers show a large difference in reactivity, k(cis)/k(trans) = 4500. This difference is reduced to k(cis)/k(trans) = 440 and 50 for the 1,2-dihydrodiols of naphthalene and 9,10-dihydrodiols of phenanthrene, respectively, and to 6.9 for the dihydrodiols of the nonaromatic 7,8-double bond of acenaphthylene. Because the difference in stabilities of cis- and trans-dihydrodiols should be no more than 2-3-fold, these results imply a high cis stereoselectivity for nucleophilic trapping of a β-hydroxyarenium cation by water in the reverse of the carbocation-forming reaction. This is confirmed by studies of the 10-hydroxy-9-phenanthrenium ion generated from aqueous solvolyses of the trans-9,10-bromohydrin derivative of phenanthrene and the monotrichloroacetate ester of the phenanthrene cis-9,10-dihydrodiol. The cis stereoselectivity of forward and reverse reactions is explained by the formation (in the "forward" reaction) of different conformations of carbocation from cis- and trans-dihydrodiol reactants with respectively β-C-H and β-C-OH bonds in pseudoaxial positions with respect to the charge center of the carbocation optimal for hyperconjugation. Formation of different conformations is constrained by departure of the (protonated) OH leaving group from a pseudoaxial position. The difference in stability of the carbocations is suggested to stem (a) from the greater hyperconjugative ability of a C-H than a C-OH bond and (b) from enhanced conjugation arising from the stabilizing influence of an aromatic ring in the no-bond resonance structures representing the hyperconjugation (C(6)H(6)OH(+) ↔ C(6)H(5)OH H(+)). This is consistent with an earlier suggestion by Mulliken and a demonstration by Schleyer that the benzenium ion is subject to hyperconjugative aromatic stabilization. It is proposed that, in analogy with the terms homoconjugation and homoaromaticity, arenium ions should be considered as "hyperaromatic".

Enzymes involved in the biodegradation of hexachlorocyclohexane: A mini review

The scope of this paper encompasses the following subjects: (i) aerobic and anaerobic degradation pathways of γ-hexachlorocyclohexane (HCH); (ii) important genes and enzymes involved in the metabolic pathways of γ-HCH degradation; (iii) the instrumental methods for identifying and quantifying intermediate metabolites, such as gas chromatography coupled to mass spectrometry (GC-MS) and other techniques. It can be concluded that typical anaerobic and aerobic pathways of γ-HCH are well known for a few selected microbial strains, although less is known for anaerobic consortia where the possibility of synergism, antagonism, and mutualism can lead to more particular routes and more effective degradation of γ-HCH. Conversion and removals in the range 39%-100% and 47%-100% have been reported for aerobic and anaerobic cultures, respectively. Most common metabolites reported for aerobic degradation of lindane are γ-pentachlorocyclohexene (γ-PCCH), 2,5-dichlorobenzoquinone (DCBQ), Chlorohydroquinone (CHQ), chlorophenol, and phenol, whereas PCCH, isomers of trichlorobenzene (TCB), chlorobenzene, and benzene are the most typical metabolites found in anaerobic pathways. Enzyme and genetic characterization of the involved molecular mechanisms are in their early infancy; more work is needed to elucidate them in the future. Advances have been made on identification of enzymes of Sphingomonas paucimobilis where the gene LinB codifies for the enzyme haloalkane dehalogenase that acts on 1,3,4,6-tetrachloro 1,4-cyclohexadiene, thus debottlenecking the pathway. Other more common enzymes such as phenol hydroxylase, catechol 1,2-dioxygenase, catechol 2,3-dioxygenase are also involved since they attack intermediate metabolites of lindane such as catechol and less substituted chlorophenols. Chromatography coupled to mass spectrometric detector, especially GC-MS, is the most used technique for resolving for γ-HCH metabolites, although there is an increased participation of HPLC-MS methods. Scintillation methods are very useful to assess final degradation of γ-HCH.

Preparation of Molecularly Imprinted Polymers for the Detection of Aromatic Hydrocarbons

In this study, the molecularly imprinted polymers (MIPs) are designed to improve their sensitivity and selectivity for specific aromatic hydrocarbons such as benzene, toluene, and xylene isomers. The MIPs based on methyl acrylate (MA) monomer are prepared using toluene and ethylene glycol dimetacrylate (EGDMA) as a template and a cross linking agent, respectively. The binding sites on the MIPs are characterized by using Fourier transform infrared spectrometry (FT-IR), nitrogen adsorption isotherms, and transmission electron microscopy (TEM). The selective behaviors of the MIPs are evaluated by their adsorption properties on a gravimetric apparatus. It is found that the performance is strongly influenced by the composition ratios of cross-linker, functional monomer, and template molecule. The molecular recognition ability can be assessed on the basis of an imprinting effect. The results indicate that the prepared MIPs can be used for the aromatic hydrocarbon sensor materials with high sensitivity and selectivity.

Comparison of SPE-TD-GC-FID with UPLC-PDA and GC-MS Methods for Analysis of Benzene, Toluene and Xylene Isomers in Solid-Liquid Mixing Paints

Three methods were developed for analysis of benzene, toluene, p-xylene, m-xylene and o-xylene (BTX) in a solid-liquid mixing paint. These methods were based on solid phase extraction-thermal desorption-gas chromatography with a flame ionization detector (SPE-TD-GC-FID), ultra performance liquid chromatography-photodiode array detector (UPLC-PDA) and gas chromatography-mass spectrometry (GC-MS). At their optimum conditions of operation, the developed methods were compared in terms of recovery, sensitivity, selectivity and universality. Although the time required for GC-MS analysis was shorter than for GC-FID and UPLC-PDA, it offered relatively poorer recoveries and suffered from matrix interferences. All of the mentioned methods were proven to be ideal for the analysis of targeted analytes; SPE-TD-GC-FID was particularly fit for the determination of trace level BTX residues present in the complex matrix. As one of the sample pretreatment techniques, the novel SPE also showed some selectivity towards BTX and was found to be superior to the conventional SPE.

Incorporating BODIPY Fluorophores into Tetrakis(arylethynyl)benzenes

Six structural isomers of a tetrakis(arylethynyl)benzene (TAEB) chromophore functionalized with dibutylamine and BODIPY moieties as the corresponding donor and acceptor units were prepared. To evaluate the effectiveness of the donor group, two TAEB molecules and three structurally related bis(arylethynyl)benzene (BAEB) isomers containing only acceptors were also synthesized. The electronic absorption and emission spectra of each series were examined. Additionally, computational studies were employed to corroborate the relative energy levels and gaps present in each series.

Solubilities of resorcinol and pyrocatechol and their mixture in supercritical carbon dioxide

The equilibrium solubilities of dihydroxy benzene isomers (resorcinol and pyrocatechol) and its mixture were experimentally determined at different temperatures (308, 318, 328, and 338 K) in the pressure range of 9.8–16.2 MPa. In the ternary system, the solubilities of pyrocatechol increased while the solubilities of resorcinol decreased relative to their binary solubilities. A new association model was developed based on the concept of formation of solvate complex molecules to correlate the solubility of the solid for mixed solids in supercritical carbon dioxide (SCCO 2). The model equation relates the solubility of solute in terms of the cosolute composition, temperature, pressure and density of SCCO 2. The proposed model correlated the solubilities of sixteen solid systems taken from the literature and current experimental data with an average absolute relative deviation (AARD) of around 4%.

Graphane sheets and crystals under pressure

Eight isomeric two-dimensional graphane sheets are found in a theoretical study. Four of these nets—two built on chair cyclohexanes, two on boat—are more stable thermodynamically than the isomeric benzene, or polyacetylene. Three-dimensional crystals are built up from the two-dimensional sheets, and their hypothetical behavior under pressure (up to 300 GPa) is explored. While the three-dimensional graphanes remain, as expected, insulating or semiconducting in this pressure range, there is a remarkable inversion in stability of the five crystals studied. Two stacking polytypes that are not the most stable at ambient pressure (one based on an unusual chair cyclohexane net, the other on a boat) are significantly stabilized with increasing pressure relative to stackings of simple chair sheets. The explanation may lie in the balance on intra and intersheet contacts in the extended arrays.

Heats of formation and hydrogenation of fused bicyclic benzene isomers: The use of Ring Conserving Isodesmic Reactions

Strain has been defined as a source of errors in the determination of heats of formations using bond separation reactions. In this work, we define Ring Conserving Isodesmic Reactions to conserve the strain. First a set of reference systems is defined to reproduce the different cyclic patterns. Those references are then used on bicyclohexadienes to evaluate the effect on the resulting heats of formation. In a last section, the successive heats of hydrogenation of the different systems are analyzed.

Synthesis of heptakis(2,3,6-tri-<I>O</I>-methoxymethyl)-β-cyclodextrin and its application in gas chromatography

A new chiral stationary phase for gas chromatography, heptakis (2, 3, 6-tri-O-methoxymethyl)-beta-cyclodextrin, was synthesized by substituting the 2,3,6-OH groups of beta-cyclodextrin with methoxymethyl groups, and a coated capillary column for gas chromatography was made by coating this new stationary phase with static method. The chromatographic properties and separation abilities of the prepared stationary phase were studied. The test results showed that it possessed good separation abilities to Grob test mixture, disubstituted benzene isomers (e. g. nitrotoluenes, bromotoluenes, dichlorobenzenes, dimethylbenzenes) and some chiral compounds such as methyl 2-hydroxypropionates, ethyl 2-hydroxypropionates, methyl 2-methylsulfonylpropionates. The comparison of chiral separation results between 2,3,6-tri-O-methoxymethyl-beta-cyclodextrin and 2,3,6-tri-O-methyl-beta-cyclodextrin showed that the separation abilities of the two stationary phases were different and complementary to each other for some chiral ester enantiomers.

Polytrifluoromethylation versus Polyfluorination of the Isomers of Kekulé Benzene and Phenol: A Theoretical Study

The study of the polyfluorination and polytrifluoromethylation effects on electronic structure and intrinsic acidities has been performed using DFT B3LYP and NBO calculations for the valence isomers of benzene and phenol, i.e., the structures of prismane, benzvalene, Kekulé, and Dewar systems. Also the isodesmic reaction analysis approach to estimate the effects of the substituents on the acidity of the compounds has been used. Although in some systems the additivity of fluorine substituents was more than 100%, the acidifying effects of the fluorine substituents were comparable to that of the CF(3) in only one case. Isodesmic reaction analysis of substituent effects shows that steric effects of poly-CF(3) substitution in the alicyclic cage compounds are significantly smaller. The relative thermodynamic stabilities of the valence isomers of Kekulé benzene were significantly lower than that of the aromatic cycle. The introduction of fluorine substituents often destabilized the compounds even further. Out of the fluorinated hydroxy derivatives of prismane, Dewar benzene, and benzvalene, not all are predicted to be stable enough to be able to undergo a reversible protonation-deprotonation process. In the case of several hydroxy derivatives deprotonation is accompanied by the rupture of a C(α)-C(β) bond and in some cases by the rearrangement to the corresponding phenolate anion. The isomerization of benzene derivatives resulted in more cases where the acidities increased compared with the respective phenol derivatives. The only hydroxy compound with significantly higher acidity than the respective phenol was pentafluorinated 2-OH-Dewar benzene, which was expected to rival the gas-phase acidity of perfluoro-1-adamantanol, currently the most acidic experimentally measured perfluorinated alcohol.