Local Aromaticity in Benzo- and Benzocyclobutadieno-Annelated Phenanthrenes

The effects of benzo- and benzocyclobutadieno-(BCBD-)annelation on the local aromaticity of the central ring (X) of phenanthrene were examined. The local aromaticity of the ring X was estimated using a graph-theory-based index and two geometry-based indices. DFT calculations of phenanthrene congeners showed that many of these molecules are nonplanar, with significant steric strain. All three indices indicate that the effect of benzo-annelation is opposite to that of BCBD-annelation. The deviation of some phenanthrene derivatives from planarity does not influence the effects of annelation. This finding is in accord with some previously reported studies on the dependence of the aromaticity of the ring(s) on distortion from planarity.     

A Computational Study on Substituted and Unsubstituted Mono and Diazaanthracenes

Substituted (F, Cl, OH, and NO₂ ) diazaanthracene derivatives have been considered theoretically to obtain information about their stabilities and aromaticities. The expected decrease of aromaticity of parent anthracene by mono or diaza substitution has been compensated by substitution of one of the hydrogens of the system by an electronegative atom. The position of the substituent has been proved to be strongly effective on the aromaticity of the structure such that, the aromaticity is enhanced when the susbtituent is closer to the aza points.     

Dynamic nuclear polarization 13C n.m.r. of coal

A novel n.m.r. method for determining the aromaticity of coal has been evaluated. Polarization transfer from the paramagnetic electrons is used to enhance the intensity of the ¹³C n.m.r. spectrum. Analysis of five samples of Gondwana bituminous steam coals shows that the aromaticity of inertinite is far greater than that of vitrinite and that variation of maceral composition is a major factor determining coal aromaticity. This has been confirmed by the study of maceral concentrates.     

The Electronic Nature and Reactivity of the Larger Acenes

The astounding electronic performance of acenes such as tetracene and pentacene (and their heteroaromatic counterparts) stands to revolutionize the field of organic electronics, promising novel consumer electronics for power generation and information display in lightweight, flexible form factors. These results have rekindled interest in acenes larger than pentacene, to determine how electronic properties are further altered by extending conjugation. This research has had to encompass many fundamental studies, as even the simple nature of aromaticity in larger acenes is still a controversial topic. Further, the reactivity of acenes toward oligomerization or oxidation becomes significant in larger acenes, while the diminishing solubility complicates isolation and characterization. This article surveys recent works describing the nature of aromaticity and reactivity in larger acenes, methods used to prepare parent acenes larger than pentacene, and functionalization approaches to soluble, reasonably stable large acenes.     

Nonlocal electronic distribution in metallic clusters: a critical examination of aromatic stabilization

All-metal clusters, such as Al(4)M(4) (M = alkali metal ion), exhibit interesting features of multi-fold aromaticity/antiaromaticity. Such characteristics arise particularly because of the poor sigma-pi separation in this class of systems. This Account presents computational strategies to unambiguously determine the aromaticity/antiaromaticity characteristics of such clusters. Computations of the linear and nonlinear optical responses show that all-metal clusters are orders of magnitude more polarized than the conventional pi-conjugated molecules. We also propose new strategies to stabilize all-metal antiaromatic systems through complexation to transition metals and discuss mechanisms for substitution reactions within the conventional organometallic systems by Al(4)M(4). Additionally, we find that these all-metal clusters form stacked superclusters that are extremely stable and aromatic.     

Dispersion of optical properties of graptolite epiderms with increased maturity in early Paleozoic organic sediments

The dispersion of optical properties (e.g. reflectance, refractive and absorptive indices) of graptolite epiderms in early Paleozoic sediments with low to high maturity levels (conodont alteration INDEX = 1–5) has been determined in the visible spectrum (450–650 nm). The results were compared with those for vitrinite of different rank and asphaltic pyrobitumen. The dispersion follows similar trends to those of coal macerais such as resinite, vitrinite, and also asphalt and pyrobitumen. This indicates similar molecular structural changes, e.g. an increase in aromaticity and ordering of aromatic carbons for graptolite epiderms with increased maturity. The dispersion of optical properties of graptolites in sediments with a low maturity level falls from blue to red in the visible spectrum, indicating low aromatic carbon content. Those in more matured sediments rise from blue to red indicating high aromaticity and molecular ordering for these graptolites. Graptolite epiderms are sensitive to a rise in temperature and are a good indicator of the maturity of early Paleozoic sediments.     

Determination of Algerian Hassi-Messaoud asphaltene aromaticity with different solid-state NMR sequences

Algerian oil well deposit derived asphaltene fraction was characterized by different MAS/NMR sequences to investigate asphaltene aromaticity and the best cross-polarization contact time. The aromaticity was estimated by single pulse sequence (SP), Hahn-echo (HE), cross-polarization (CP) and variable cross-polarization (VACP) sequences. The values found ranging from 0.58 to 0.48 are of the same order of magnitude as ones published in the literature. The discrepancies between the values are thought to be relevant to both the specificity of each sequence and the asphaltene structure. Spectra band de-convolution enables us the determination of the average number of carbon atoms per side chain according to each sequence. The obtained values spanning from 3 to 7 are also sequence nature dependent.     

Aromaticities of various coals measured by reaction with fluorine and TGA-DTA studies of the reaction products

The previously described method for estimating aromaticity by fluorination has been applied to coals of various ranks and used to compare the aromaticity of a solvent-refined coal with that of the feed coal from which it was derived. Coals of various degrees of fluorination were also studied by thermogravimetric analysis and differential thermal analysis.     

Isomeric Structures of Furoxan-Fused [2,2]paracyclophane—A DFT Study

The present study theoretically deals with furoxan-fused [2,2]paracyclophane systems and some structures possibly involved in their isomerization processes. These intermediate structures are certain di- and tetra-nitroso derivatives and systems having dioxadiazatidine ring(s). Computationally, B3LYP/6–31G(d,p) level of density functional theory has been employed. Certain properties of the systems, such as geometries, energies, and aromaticity of the benzenoid rings, are harvested and discussed.     

沙特原油活性组分的结构对油水界面性质的影响

测定了从沙特原油中分离出的胶质和沥青质的红外、紫外光谱和相对分子质量。对含有胶质、沥青质的模拟油和模拟水体系,测定了其界面张力和界面剪切黏度。胶质和沥青质的相对分子质量分别为497和1 786;光谱分析结果表明,沥青质比胶质含有更多的芳香环结构。含胶质和沥青质体系的界面张力分别是33.205 8 mN/m、31.732 5 mN/m,相差不大。对于界面剪切黏度,沥青质体系远大于胶质体系,同在100 mg/L,胶质体系剪切速率在0-0.5 rad/s时,界面剪切黏度最大不超过0.002 5 mN/m,而沥青质体系剪切速率在0.3 rad/s下,界面剪切黏度最大值大于1.000 0 mN/m,并且其曲线具有吸附曲线的特征,沥青质体系具有较强的界面膜,其界面膜的形成经历了从液态扩张膜到液态凝聚膜的变化过程。界面膜强度的大小与界面活性物质的相对分子质量的大小和芳香环结构含量的多少有关。     

Nucleus-Independent Chemical Shift Criterion for Aromaticity in Some of the Corannulene Derivatives as Carbon Nanostructure: Effect of Substituent Groups on Aromaticity

Substitution of the peripheral H atoms in the corannulene molecule as a carbon nanostructure by OH, CH₃, NH₂, and NO₂ groups was done and the obtained structures were optimized employing MP2/6-31G(d) level of theory. Calculations of the nucleus-independent chemical shift (NICS) were performed to analyze the aromaticity of the corannulene rings and its derivatives upon substitution at B3LYP/6-311+G(d) level of theory. Comparison of NICS(1)_zz for concave and convex sides of corannulene derivatives indicated that substitution of H atoms in the corannulene molecule with OH, CH₃, NO₂, and NH₂ groups leads to decreasing of the aromaticity character of corannulene. We discussed about two efficient factors on the aromaticity and antiaromaticity of polycyclic aromatic compounds: electron donating or withdrawing and resonance effects. We showed that NICS values in all structures are strongly dependent on the type, number, and position of the substituted groups.     

Deactivation of HDS catalyst in two-stage RDS process: II. Effect of crude oil and deactivation mechanism

Aging tests of HDS catalyst in two-stage RDS process (HDM/HDS) were performed with Arabian light atmospheric residue as feedstock and the deactivation mechanism was discussed by comparison with previous results using Kuwait residue. It was found that the HDS catalyst in the second stage was deactivated by coke deposition rather than by metals and that asphaltene aromaticity in the feed was a good index for the HDS catalyst deactivation. Furthermore, it was confirmed that fouling rate was strongly affected by the combination of asphaltene aromaticity in HDS feed and reaction temperature in HDS itself.     

A novel semi-empirical relationship between aromaticities measured from 1H and 13C n.m.r. spectra

An empirical correlation between ¹H n.m.r. measured aromaticity (H_ar) and ¹³C n.m.r. measured aromaticity (C_ar) has been found. Over 300 samples which are diverse in origins and structural characteristics have been studied. A semi-empirical functional relationship between H_ar and C_ar has been devised. It is suggested that for distillable samples with, nominally, less than 50% boiling above 320 °C, the derived function can be used to rapidly and conveniently approximate C_ar from H_ar. The difference between observed and calculated C_ar values for 303 such samples shows a standard deviation of just 1.8%.     

Initial solvation of coal with tetralin under liquefaction conditions

Secondary vitroplast, produced by treating a Liddell vitrinite concentrate with tetralin in a continuous flow, packed-bed microreactor under a nitrogen pressure of 13.8 MPa at 360, 410 and 460 °C, has been analysed by i.r. spectroscopy, gel permeation chromatography, solid state ¹³C n.m.r. and reflectance microscopy. Results showed that this intermediate in the liquefaction process had the same carbon aromaticity as the untreated vitrinite concentrate and that both its aromaticity and reflectance were independent of the temperature of formation. It was concluded that the initial solution of the vitrinite concentrate in tetralin involved no significant hydrogenation of the vitrinite.     

芳香性的判断及Gaussian98和GaussView在其教学上的运用

简要介绍和讨论芳香性定义与判据。建议改进芳香性的教学方法,并举例说明Gaussian98和GaussView在确定芳香性教学上的运用。     

Hydrocarbon structural group analysis of Athabasca asphaltene and its g.p.c. fractions by C n.m.r.

Asphaltene from the Athabasca oil sand bitumen separated according to molecular weight by gel permeation chromatography was examined by high resolution solution-state ¹³C and ¹H and solid state ¹³C n.m.r. Integration of the inverse gated decoupled solution state and CP/MAS ¹³C n.m.r. spectra gave the aromaticity of each fraction as well as the relative number of carbon atoms responsible for well resolved signals in the aliphatic portion of the spectra. A two-pulse spin-echo ¹³C n.m.r. sequence in the solution state, and dipolar dephasing in the solid state, permitted the assignment of each aliphatic carbon signal to a methyl, methylene, or methine carbon; no quaternary aliphatic carbons were observed. These signals show that the aromatic core of the asphaltene is surrounded by alkyl chains with a mean length of 7.7 for the lowest molecular weight fraction (MW 1200), increasing to 12 for the highest MW fraction (MW 16900). Aromaticity of the fractions obtained from solution spectra ranged from 34 to 48%, the highest aromaticity being associated with the lowest molecular weight asphaltenes. In the solid state, the aromaticity ranged from 31 to 42 %. The degree of branching ranged from 0.5 per chain for the low MW asphaltene to 1.0 for the high MW fraction. The highest naphthenic carbon content was found in the high MW fractions; the 1200 MW fraction appeared to be nearly devoid of such carbon, this fraction having a much higher aromaticity than the second-lowest MW fraction.     

Synthesis of PET-based Liquid Crystalline Copolyesters: Influence of the Overall Degree of Aromaticity on Phase Segregation

Abstract

Novel PET-based copolyesters have been synthesised by transesterification of PET with equimolar amounts of sebacic acid (S) and 4,4′-diacetoxybiphenyl (B) and with varying amounts of 4-acetoxybenzoic acid (H). The structure, the morphology, and the thermal properties of the PET-S-B-H copolyesters have been studied by scanning electron microscopy (SEM), polarized optical microscopy (POM), differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXS). It has been demonstrated that segregation of a liquid-crystalline phase, during polycondensation, provides the driving force for compositional differentiation, with the result that the final products are “synthetic polymer blends”. The characteristics of PET-S-B-H copolymers have been compared with those of PET-S-Q-H copolyesters having the same molar composition (Q-hydroquinone). It has been shown that the phase segregation is more prominent in PET-S-B-H copolyesters than in PET-S-Q-H polymers of similar molar composition. The influence of the overall degree of aromaticity of the copolyester on the segregation phenomenon has been discussed.     

Carbonization of petroleum feedstocks II: Chemical constitution of feedstock asphaltenes and mesophase development

The characterization of the asphaltene fractions of a range of petroleum feedstocks by FT-IR, ¹H and ¹³C NMR spectroscopy indicated distinct differences in the molecular structure of the asphaltenes. Some of these differences could be related to the variations in the size of the principal optical texture of the semi-cokes produced by carbonization. The principal optical texture size was observed to increase steadily with the increasing hydrogen aromaticity of the asphaltenes over the whole range of the feedstocks used. There was no consistent correlation, however, between the carbon aromaticity of the asphaltenes and the optical texture size. The correlation between the hydrogen aromaticity and the principal optical texture size was attributed to structural differences among the asphaltenes that are critically important for the mesophase development.     

Fundamentals of technology of mechanochemical synthesis of titanium carbide using various carbon components

It was established that the main criteria for achieving mechanochemical synthesis in titanium-carbon system represent the structure of carbon modifications and their degree of aromaticity. The promise of the use of soot + graphite carbon composition for the mechanochemical synthesis of titanium carbide is mentioned. It has been shown that titanium carbide synthesized according to this technology has high sorption activity.     

Functionalized Nano Graphene Platelets as Green Catalyst to Synthesize New and Known Benzoyl-1,4-diazanaphthalene and Study of Their Local Aromaticity

Functionalized graphene has been prepared and employed for the solvent-free synthesis of some novel and known benzoyl-1,4-diazanaphthalene from the condensation of various arylglyoxals with 4-bezoyl-1,2-phenylenediamine. Catalyst loads as low as 0.003 g could be used leading to high yields of pure products. Magnetic, geometry, and electronically based indices have been also employed to investigate the correlation between aromaticity variation in new formed ring (ring A) and yield of reaction.