Benzenoid Structures Research Articles

Structure of [18]Annulene Revisited: Challenges for Computing Benzenoid Systems.

For cyclic conjugated structures, erratic computational results have been obtained with Hartree-Fock (HF) molecular orbital (MO) methods as well as density functional theory (DFT) with large HF-exchange contributions. In this work, the reasons for this unreliability are explored. Extensive computations on [18]annulene and related compounds highlight the pitfalls to be avoided and the due diligence required for such computational investigations. In particular, a careful examination of the MO singlet-stability eigenvalues is recommended. The appearance of negative eigenvalues is not (necessarily) problematic, but near-zero (positive or negative) eigenvalues can lead to dramatic errors in vibrational frequencies and related properties. DFT approaches with a lower HF admixture generally appear more robust in this regard for the description of benzenoid structures, although they may exaggerate the tendency toward planarity and C-C bond-equalization. For the iconic [18]annulene, the results support a nonplanar equilibrium structure. The density-fitted frozen natural orbital coupled-cluster singles and doubles with perturbative triples [DF-FNO CCSD(T)] method of electron correlation with an aug-pVQZ/aug-pVTZ basis set places the C2 global minimum 1.1 kcal mol-1 below the D6h stationary point.

A Perspective Approach to Study the Valency-Based Irregular Indices for Benzenoid Planar Octahedron Structures

The molecular topology and the chemical structures which are not regular perform a dominant character in designing the compound in connection with their physical and chemical properties. The study of topological descriptors of molecular structures is widely used in the field of cheminformatics, information technology, biomedical science, and many more. Numerous topological indices have been evolved in the theory of chemistry. Graphs which are not regular can be specified using irregular measures. In this article, we find some irregularity indices, which are useful in Quantitative structure activity relationship for benzenoid structures. Based on the exact analytic expressions, the numerical and graphical comparison for benzenoid structures is also provided.

Second leap hyper-Zagreb coindex of certain benzenoid structures and their polynomials

Topological indices are well known for their importance in determining the characteristics of chemical compounds. On the other side, benzenoid hydrocarbons are essential to the food and chemical industries and our environment. In this paper, the second leap hyper-Zagreb coindex and ψ2-polynomial for certain classes of benzenoid structures such as zigzag benzenoid Zp, rhombic benzenoid Rp, multiple segment linear hexagon chains L[p,q], and hammer-like benzenoid Hp have been computed. Furthermore, numerical comparisons between the second leap hyper-Zagreb index and its coindex have been performed using Matlab. Also, correlation between the second leap hyper-Zagreb index and its coindex with physico-chemical properties of some polycyclic aromatic hydrocarbons have been predicted.

Detailed Study of the Formation of Soot Precursors and Soot in Highly Controlled Ethylene(/Toluene) Counterflow Diffusion Flames

We perform spatially resolved measurements of temperature, gaseous species up to three-ring Polycyclic Aromatic Hydrocarbons (PAHs), and soot in atmospheric pressure counterflow diffusion flames. First, we characterize fully a baseline ethylene flame and then a toluene-seeded flame in which an aliquot of ethylene in the feed stream is replaced with 3500 ppm of prevaporized toluene. The goal is twofold: to investigate the impact of a common reference fuel component of surrogates of transportation fuels and bypass the main bottleneck to soot formation from aliphatic fuels, that is, the formation of the first aromatic ring. The composition of the fuel and oxidizer streams are adjusted to maintain a constant stoichiometric mixture fraction and global strain rate, thereby ensuring invariance of the temperature-time history in the comparison between the two flames and decoupling the chemical effects of the fuel substitution from other factors. Major combustion products and critical radicals are fixed by the baseline flame, and profiles of critical C2-C5 species precursors to aromatic formation are invariant in both flames. On the other hand, doping with toluene boosts the aromatic content and soot volume fraction, increasing the mole fraction of benzenoid structures and soot volume fraction by a factor of 2 or 3, relative to the baseline ethylene flame. This finding is consistent with the expectation that the formation of the first aromatic ring is no longer a bottleneck to soot formation in the doped flame. In addition, toluene bypasses completely benzene formation, opening a radical recombination pathway to soot precursors through the production of C14H14 (via dimerization of benzyl radical) and pyrene (through dimerization of indenyl radical).

Metal sandwich and ion complexes in cyclacene nanobelts

Cyclacene carbon nanobelts represent the last unsynthesized building blocks of carbon nanotubes and are the shortest possible hydrogen capped nanotubes of zigzag type. Recent theoretical predictions indicate that stable cyclacene isomers containing Dewar benzene derived structures may exist. The properties of the Hückel-[n]cyclacene nanobelt complexes containing alkali metal and halide ions (n = 5-8), with zero to two Dewar benzenoid structures present, and transition metal complexes (n = 6, 8) with zero and two Dewar structures present, have been calculated using density functional theory. The halides, alkali metals, and transition metals were initially placed centrally within the belt cavities, and binding interactions have been observed for both positive and negative ions. The most stable isomeric forms are found to change for several of the belt sizes, depending on which ions are present. Internal transition metal sandwich complexes with η6-haptic bonds are also indicated to form within the cyclacene belts containing two Dewar benzenoids.

How constraint programming can help chemists to generate Benzenoid structures and assess the local Aromaticity of Benzenoids

Benzenoids are a subfamily of hydrocarbons (molecules that are only made of hydrogen and carbon atoms) whose carbon atoms form hexagons. These molecules are widely studied in theoretical chemistry and have a lot of concrete applications. Then, there is a lot of problems relative to this subject, like the enumeration of all its Kekulé structures (i.e. all valid configurations of double bonds). In this article, we focus our attention on two issues: the generation of benzenoid structures and the assessment of the local aromaticity. On the one hand, generating benzenoids that have certain structural and/or chemical properties (e.g. having a given number of hexagons or a particular structure from a graph viewpoint) is an interesting and important problem. It constitutes a preliminary step for studying their chemical properties. In this paper, we show that modeling this problem in Choco Solver and just letting its search engine generate the solutions is a fast enough and very flexible approach. It can allow to generate many different kinds of benzenoids with predefined structural properties by posting new constraints, saving the efforts of developing bespoke algorithmic methods for each kind of benzenoids. On the other hand, we want to assess the local aromaticity of a given benzenoid. This is a central issue in theoretical chemistry since aromaticity cannot be measured. Nowadays, computing aromaticity requires quantum chemistry calculations that are too expensive to be used on medium to large-sized molecules. In this article, we describe how constraint programming can be useful in order to assess the aromaticity of benzenoids. Moreover, we show that our method is much faster than the reference one, namely NICS.

Molecular Structural Descriptors of Donut Benzenoid Systems

Circumcised donut benzenoid systems are a class of polycyclic aromatic hydrocarbons (PAHs) derived from planar benzenoid networks by strategically eliminating a few hexagons. The study of donut benzenoid systems has been gaining ground because of their neoteric and fascinating characteristics like superaromaticity, ascribed to their unique structure. The distinct nature and properties of donut benzenoids open the door for applications in several industries such as biotechnology, electronics, pharmaceuticals and so on. The investigation of the properties of circumcised donut benzenoids and their relationship to the underlying molecular structures using graph-theoretical techniques is advantageous in the design and synthesis of new donut benzenoid compounds. Studies on topological indices have conclusively proved that they have a strong correlation to various properties of chemical compounds. They also provide a mathematical representation of the molecular structure for QSPR/QSAR techniques. In this paper, the degree-based molecular structural descriptors are calculated for two circumcised donut benzenoid structures using M-polynomial and edge partition techniques, effectively doubling the number of known topological indices for these structures. Also, the numerical values of these indices are computed and compared pictorially.

Degree- and irregularity-based molecular descriptors for benzenoid systems

The study of benzenoid systems has been steadily gaining momentum due to their extensive applications in many emerging fields including nanosciences. Topological descriptors provide a mathematical expression of the molecular structure of chemical compounds and their properties. They serve as efficient and cost-effective tools to theoretically predict the properties of compounds using quantitative structure–activity (QSAR) and structure–property relationship (QSPR) studies. This paper demonstrates the computation of degree-based and irregularity-based topological descriptors using edge-partition techniques for two benzenoid structures. This analysis of degree-based descriptors for these structures can lay the basis for further exploration into benzenoids and their properties.

M-polynomial and topological indices of some transformed networks

<abstract><p>In the chemical industry, topological indices play an important role in defining the properties of chemical compounds. They are numerical parameters and structure invariant. It is a proven fact by scientists that topological properties are influential tools for interconnection networks. In this paper, we will use stellation, medial and bounded dual operations to build transformed networks from zigzag and triangular benzenoid structures. Using M-polynomial, we compute the first and second Zagreb indices, second modified Zagreb indices, symmetric division index, general Randic index, reciprocal general Randic index. We also calculate atomic bond connectivity index, geometric arithmetic index, harmonic index, first and second Gourava indices, first and second hyper Gourava indices.</p></abstract>

M-Polynomial and Degree-Based Molecular Descriptors of Certain Classes of Benzenoid Systems

Benzenoid systems are a notable class of polycyclic aromatic compounds which have applications ranging from household materials to advanced fields like nanotechnology. Hence, the study of their fundamental properties, like observed bioactivities, carcinogenicity, toxicity, and other significant characteristics, have fascinated theoretical chemists for quite a long time. From this perspective, the topological descriptors of these networks, facilitate the prediction of their molecular properties through quantitative structure-activity (QSAR) and structure-property relationships (QSPR) methods. This article focusses on the computation of analytical expressions for nine degree-based topological descriptors using the M-polynomial, for three benzenoid structures: convex benzenoid system, hexabenzocoronene (HBC), and hexa-cata-hexabenzocoronene (cHBC). Besides, the expressions for a few recently developed indices are also calculated using edge-partition techniques. These derived indices are then compared graphically based on their numerical values. This analysis of degree-based descriptors for these structures can lay the groundwork for future researchers to discover and explore new benzenoids and their properties.

Application of chemical graph theory to PAH isomer enumeration and structure in laser desorption/ionization mass spectrometry studies of particulate from an ethylene diffusion flame

Our laboratory recently published data that showed that the PAH composition of soot can be exactly determined and spatially resolved by low-fluence laser desorption ionization, coupled with high-resolution mass spectrometry imaging [1]. This analysis revealed that PAHs of 239–838 Da, containing few oxygenated species, comprise the soot observed in an ethylene diffusion flame. In this paper, we demonstrate that the empirical formula of observed species can aid in the enumeration of isomers and places limits on their structures and thermodynamic stability. Specifically, chemical graph theory (CGT) shows that the vast majority of species observed in the sampled particulate matter may be described as benzenoid, consisting of only fused 6-membered rings. We apply CGT to determine the Dias Parameter, dS, for observed, individual PAH peaks and demonstrate that observed PAH species cluster near low dS, indicative of highly condensed structures, with relatively low populations of edge concavity (armchairs, bays, and fjords). Finally, we quantitatively explore the relative stability of PAH isomers using group-additivity estimates (for benzenoid structures) and those containing a single 5-membered rings using density functional theory. For the latter, we show that highly-condensed, benzenoid structures have lower free energy than those containing five-membered rings, with buried 5-membered rings showing the highest free energies.

The Molecular Composition of Soot

AbstractSoot (sometimes referred to as black carbon) is produced when hydrocarbon fuels are burned. Our hypothesis is that polynuclear aromatic hydrocarbon (PAH) molecules are the dominant component of soot, with individual PAH molecules forming ordered stacks that agglomerate into primary particles (PP). Here we show that the PAH composition of soot can be exactly determined and spatially resolved by low‐fluence laser desorption ionization, coupled with high‐resolution mass spectrometry imaging. This analysis revealed that PAHs of 239–838 Da, containing few oxygenated species, comprise the soot observed in an ethylene diffusion flame. As informed by chemical graph theory (CGT), the vast majority of species observed in the sampled particulate matter may be described as benzenoids, consisting of only fused 6‐membered rings. Within that limit, there is clear evidence for the presence of radical PAH in the particulate samples. Further, for benzenoid structures the observed empirical formulae limit the observed isomers to those which are nearly circular with high aromatic conjugation lengths for a given aromatic ring count. These results stand in contrast to recent reports that suggest higher aliphatic composition of primary particles.

Topological Ring-Currents and Clar Sextets in Fully Benzenoid Hydrocarbons. II. Large Structures Containing More than 18 Rings.

In previous work, π-electron ring currents in several fully benzenoid hydrocarbons were calculated by means of the quasi-graph-theoretical, parameter-free Hückel-London-Pople-McWeeny (HLPM) method. It was found that for fully benzenoid hydrocarbons formed from fewer than about 17 rings all ring currents in full rings are larger in intensity than the ring current in benzene, while ring currents in the empty rings are smaller than the benzene value. There was an indication that this distinction might break down when larger systems are considered. Accordingly, these studies are here extended to a selection of fully benzenoid hydrocarbons containing up to 43 rings. It is still found that all full rings in all structures bear ring currents that are greater than the ring current in benzene but, in several of the larger systems, empty rings are found to bear ring currents greater than the benzene value. In the very largest structures examined, the ring currents in some of the empty rings are greater than the currents in as many as half of the symmetrically nonequivalent full rings. Despite this, because every full ring bears a ring current of intensity greater than that of any of the empty rings that are immediately adjacent to it, all of the full rings are regions of diatropic circulation; there is also strong diamagnetic circulation around the perimeters of each of the fully benzenoid systems studied. Both of these observations, based on the topological HLPM approach, are qualitatively in agreement with the predictions of sophisticated ab initio calculations, earlier performed on some of these fully benzenoid structures by Steiner et al.

Topological Ring-Currents and Clar Sextets in Fully Benzenoid Hydrocarbons I. Structures with Fewer than 18 Rings.

It is well-known that the very special family of conjugated systems described as "fully benzenoid hydrocarbons" have a unique Clar structure that causes all of their rings to be classified as either "full" or "empty". In this work, π-electron ring-currents in more than two dozen such structures are calculated by means of the quasi-graph-theoretical, parameter-free Hückel-London-Pople-McWeeny (HLPM) method. In the case of benzenoid hydrocarbons, the calculated values of such "topological" ring-currents depend on only the carbon-carbon connectivity of the system in question. It is found that, until a certain fully benzenoid hydrocarbon comprising 17 rings is examined, all ring currents in full rings are seen to be larger in intensity than the ring current in benzene, while such currents in empty rings are smaller than the benzene value. There is an indication that this distinction might break down when "giant" fully benzenoid structures are considered. These conclusions are consistent with chemical intuition, with earlier studies examining Clar aromatic sextets by means of NICS calculations, and with previous qualitative ab initio predictions of the patterns of π-electron current-flow in the fully benzenoid hydrocarbons.

Equivalence between Clar covering polynomials of single zigzag chains and tiling polynomials of [formula omitted] rectangles

This paper offers a formal explanation of a rather puzzling and surprising equivalence between the Clar covering polynomials of single zigzag chains and the tiling polynomials of 2×n rectangles for tilings using 1 × 2, 2 × 1 and 2 × 2 tiles. It is demonstrated that the set of Clar covers of single zigzag chains N(n−1) is isomorphic to the set of tilings of a 2×n rectangle. In particular, this isomorphism maps Clar covers of N(n−1) with k aromatic sextets to tilings of a 2×n rectangle using k square 2 × 2 tiles. The proof of this fact is an application of the recently introduced interface theory of Clar covers. The existence of a similar relationship between the Clar covers of more general benzenoid structures and more general tilings of rectangles remains an interesting open problem in chemical graph theory.

Optical and Morphology Changes upon Self-Assembly of Tetrasulphonated Copper Phthalocyanine and Polyaniline Particles on Zinc Oxide Nanorods

Zinc oxide (ZnO) nanorods were grown on the pre-seeded substrate using a facile, solution route technique. The multilayer self-assembly of polyaniline (PAni) and tetrasulphonated phthalocyanine (TSCuPc) on ZnO nanorods was monitored by FTIR, UV-Vis SEM and XRD. Infrared spectrum of the assembled layer showed peaks centered at 1589cm-1and 1504 cm-1, confirmed the presence of quinoid and benzenoid structures of PAni, while peaks centered at ~1165cm-1and ~775cm-1 provided evidence for the presence of TSCuPc species. Absorption spectra of the assembled layer show broadened peaks at ~600nm and ~700nm affirmed the presence of PAni and TSCuPc molecules. A linear increase in the absorbance level at these wavelengths was also observed upon increasing the number of bilayers. The surface modification of the ZnO nanorods after self-assembly was confirmed through SEM. The edges of the ZnO nanorods was observed to change from sharp to dull upon employing layer by layer deposition of PAni and TSCuPc. XRD pattern of the assembled layer showed broadened peaks at low 2Θvalues associated to the amorphous nature of the emeraldine base of PAni. Even so, the diffraction peaks in XRD patterns of ZnO nanorods and ZnO/(TSCuPc-PAni) multilayer have been indexed as hexagonally wurtzite structure. The multilayer deposition of PAni and TSCuPc on the surface of ZnO as monitored by UV-Vis spectroscopy and confirmed by FTIR, XRD, and SEM provides a new way of anchoring dye molecules without carboxylic moieties on the surface of metal oxides

Algorithm for generating generalized resonance structures of single zigzag chains based on interface theory

An alternative formulation of the theory of generalized resonance structures (Clar covers) of single zigzag chains N(n), based on the novel concept of interfaces, is presented. The global structure of every Clar cover can be conveniently expressed as a unique sequence of n interfaces. The complete set of Clar covers is then robustly constructed as the complete set of walks of length \(n+1\) on the associated interface connectivity graph. The presented algorithm is readily generalizable to an arbitrary class of benzenoid structures.

On-Surface Route for Producing Planar Nanographenes with Azulene Moieties.

Large aromatic carbon nanostructures are cornerstone materials due to their increasingly active role in functional devices, but their synthesis in solution encounters size and shape limitations. New on-surface strategies facilitate the synthesis of large and insoluble planar systems with atomic-scale precision. While dehydrogenation is usually the chemical zipping reaction building up large aromatic carbon structures, mostly benzenoid structures are being produced. Here, we report on a new cyclodehydrogenation reaction transforming a sterically stressed precursor with conjoined cove regions into a planar carbon platform by incorporating azulene moieties in their interior. Submolecular resolution STM is used to characterize this exotic large polycyclic aromatic compound on Au(111) yielding unprecedented insight into a dehydrogenative intramolecular aryl-aryl coupling reaction. The resulting polycyclic aromatic carbon structure shows a [18]annulene core hosting peculiar pore states confined at the carbon cavity.

New organic semiconductor thin film derived from p-toluidine monomer

p-Toluidine was used as a precursor to synthesize new organic compound [(E)-4-methyl-N1-((E)-4-methyl-6-(p-tolylimino) cyclohex-3-en-1-ylidene)-N2-(p-tolyl) benzene-1,2-diamine] (MBD) by oxidative reaction via potassium dichromate as oxidizing agent at room temperature. Spin coater was used to fabricate nano-size crystalline thin film of the MBD with thickness 73 nm. The characterizations of the MBD powder and thin film have been described by various techniques including Fourier Transform Infrared (FT-IR), Mass Spectra, X-ray Diffraction (XRD), Scanning Electron Microscope (SEM), UV–Visible measurements and Atomic Force Microscope (AFM). The results revealed that the MBD as an organic material is semi-crystalline containing benzenoid (BenN Ben) and quinonoid (Quin = N = Quin) structures. Various optical constants such as refractive index (n), and the absorption index, (k) of the MBD thin film were determined. The effect of temperature on the electrical resistivity of MBD film was studied by a Keithley 6517B electrometer. The energy band gap value of the MBD thin film was found to be 2.24 eV. Thus, MBD is located in the semiconductor materials range. In addition, structural and optical mechanisms of MBD nanostructured thin film were investigated. The obtained results illustrate the possibility of controlling the organic semiconductor MBD thin film for the optoelectronic applications.

On molecular topological properties of benzenoid structures

The degree-based topological indices correlate certain physicochemical properties such as boiling point, strain energy, and stability, etc., of certain chemical compounds. Among the major classes of topological indices are the distance-based topological indices, degree-based topological indices, and counting-related polynomials and corresponding indices of graphs. Among all of the degree-based indices, namely the first general Zagreb index, general Rndić connectivity index, general sum-connectivity index, atom–bond connectivity index (ABC), and geometric–arithmetic index (GA), are most important due to their chemical significance. In this paper, we compute the first general Zagreb index, general Randić connectivity index, general sum-connectivity index, ABC, GA, ABC4, and GA5 indices of hexagonal parallelogram P(m,n) nanotubes, triangular benzenoid Gn, and zigzag-edge coronoid fused with starphene ZCS(k,l,m) nanotubes by using the line graphs of the subdivision of these chemical graphs.