Benzenoid Isomers Research Articles

Structural Expansion of Cyclohepta[def]fluorene towards Azulene-Embedded Non-Benzenoid Nanographenes.

Non-benzenoid non-alternant nanographenes (NGs) have attracted increasing attention on account of their distinct electronic and structural features in comparison to their isomeric benzenoid counterparts. In this work, we present a series of unprecedented azulene-embedded NGs on Au(111) during the attempted synthesis of cyclohepta[def]fluorene-based high-spin non-Kekulé structure. Comprehensive scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM) evidence the structures and conformations of these unexpected products. The dynamics of the precursor bearing 9-(2,6-dimethylphenyl)anthracene and dihydro-dibenzo-cyclohepta[def]fluorene units and its reaction products on the surface are analyzed by density functional theory (DFT) and molecular dynamics (MD) simulations. Our study sheds light on the fundamental understanding of precursor design for the fabrication of π-extended non-benzenoid NGs on a metal surface.

Dewar benzenoids in cyclophenacene nanobelts

Cyclophenacene molecules are carbon nanobelts and are the smallest possible hydrogen capped armchair carbon nanotubes. In this density functional theory study Dewar benzene derivatives within cyclophenacene isomers are examined for the [n]cyclophenacenes with n = 6–14 belt sizes (for even values of n). Cyclophenacenes containing Dewar benzenoids are found to have higher energy electronic singlet-triplet state transitions compared to the purely Kekulé benzenoid isomers of the same size. The [6]cyclophenacene isomer containing two maximally spaced Dewar benzenoids is found to have a similar energy to the exclusively Kekulé benzenoid isomer and the addition of solvent is predicted to stabilize the Kekulé form.

Revisiting Acepleiadylene: Two-Step Synthesis and π-Extension toward Nonbenzenoid Nanographene.

Acepleiadylene (APD), a nonbenzenoid nonalternant isomer of pyrene, exhibits different electronic properties from pyrene, but has been rarely studied since its first synthesis in 1956, probably due to the difficulties in synthesis and further derivatization. In this work, we revisited this long-known compound and developed a new two-step synthetic route to efficiently access APD on the gram scale. Theoretical and experimental characterizations elucidated the unique properties of APD as compared with its benzenoid isomer pyrene, particularly revealing its dipolar structure with a narrow optical gap. The functionalization of APD was demonstrated for the first time, providing doubly brominated APD as a key precursor for further π-extension. As a proof of concept, a π-extended APD and a cyclotrimer nanographene (C48H24) were constructed, opening up new avenues to nonbenzenoid nanographenes with low HOMO-LUMO gaps.

Comparative study on structural sensitivity of eigenvalue–based molecular descriptors

Structural sensitivity is one of the most important and the least investigated property of the topological molecular descriptors. This paper reports results on the structural sensitivity of graph energy, Estrada index, and resolvent energy on several series of catacondensed and pericondensed isomeric benzenoid hydrocarbons. Recently, a novel method for assessing the structural sensitivity of topological molecular descriptors was put forward, which is applied here. This algorithm consists of several different steps and it is based on Tanimoto index and Morgan circular fingerprints. It was found that graph energy, Estrada index, and resolvent energy exert similar structural sensitivity on catacondensed isomers. The graph energy showed the best performance on pericondensed molecules. Additionally, the sensitivities of these descriptors were tested on the catacondensed isomers with the increasing number of bays, coves, and fjords. It was revealed that these descriptors gradually change with the increasing number of these structural features. The Estrada index and resolvent energy perform similarly and in some cases with the same structural sensitivity. This may be attributed to the high correlation between them. The graph energy showed superiority over the Estrada index and resolvent energy on pericondensed isomers.

RESOLVENT ENERGY AND ESTRADA INDEX OF BENZENOID HYDROCARBONS

The relationship between the resolvent and Estrada indices for the alkanes has been recently demonstrated. That relationship involved the first Zagreb index besides these two eigenvalue-based molecular descriptors. In this paper, the quality of the correlation is tested in the case of isomeric benzenoid hydrocarbons, where the first Zagreb index is constant. Extraordinary linear correlations are identified for all studied groups of isomeric benzenoid hydrocarbons. Additionally, the relationship of these indices with the boiling points of a set of benzenoid hydrocarbons is presented.

Open-Shell and Antiaromatic Character Induced by the Highly Symmetric Geometry of the Planar Heptalene Structure: Synthesis and Characterization of a Nonalternant Isomer of Bisanthene.

A nonbenzenoid hydrocarbon, difluoreno[1,9,8- alkj:1',9',8'- gfed]heptalene 1, is synthesized. Experimental and theoretical investigations demonstrate that the planar and symmetric heptalene core within 1 effectively induces the antiaromatic and open-shell character. These properties are not shared by bisanthene 2, a benzenoid isomer of 1.

Aromaticity of Nonplanar Fully Benzenoid Hydrocarbons.

The Clar aromatic sextet theory can provide a qualitative description of the dominant modes of cyclic π-electron conjugation in benzenoid molecules and of the relative stability among a series of isomeric benzenoid systems. In a series of nonplanar fully benzenoid hydrocarbons, the predictions of the Clar theory were tested by means of several different theoretical approaches: topological resonance energy (TRE), energy effect (ef), harmonic oscillator model of aromaticity (HOMA) index, six center delocalization index (SCI), and nucleus-independent chemical shifts (NICS). To assess deviations from planarity in the examined molecules, four different planarity descriptors were employed. It was shown how the planarity indices can be used to quantify the effect of nonplanarity on the local and global aromaticity of the studied systems.

Identification and quantification of seven fused aromatic rings C26H14peri-condensed benzenoid polycyclic aromatic hydrocarbons in a complex mixture of polycyclic aromatic hydrocarbons from coal tar

A methodology for the characterization of groups of polycyclic aromatic hydrocarbons (PAHs) using a combination of normal phase liquid chromatography with ultraviolet–visible spectroscopy (NPLC/UV–vis) and gas chromatography with mass spectrometry (GC/MS) was used for the identification and quantification of seven fused aromatic rings C26H14peri-condensed benzenoid polycyclic aromatic hydrocarbons, PAHs, in standard reference material (SRM) 1597a, complex mixture of PAHs from coal tar. The NPLC/UV–vis isolated the fractions based on the number of aromatic carbons and the GC/MS allowed the identification and quantification of five of the nine C26H14 PAH isomers; naphtho[1,2,3,4-ghi]perylene, dibenzo[b,ghi]perylene, dibenzo[b,pqr]perylene, naphtho[8,1,2-bcd]perylene, and dibenzo[cd,lm]perylene using a retention time comparison with authentic reference standards. For the other four benzenoid isomers with no available reference standards the following two approaches were used. First, the annellation theory was used to achieve the potential identification of benzo[qr]naphtho[3,2,1,8-defg]chrysene, and second, the elution distribution in the GC fractions was used to support the potential identification of benzo[qr]naphtho[3,2,1,8-defg]chrysene and to reach the tentative identifications of dibenzo[a,ghi]perylene, naphtho[7,8,1,2,3-pqrst]pentaphene, and anthra[2,1,9,8-opqra]naphthacene. It is the first time that naphtho[1,2,3,4-ghi]perylene, dibenzo[b,ghi]perylene, dibenzo[b,pqr]perylene, naphtho[8,1,2-bcd]perylene, and dibenzo[cd,lm]perylene are quantified, and the first time that benzo[qr]naphtho[3,2,1,8-defg]chrysene is potentially identified, in any sample, in any context.

Stability order of isomeric benzenoid hydrocarbons and Kekulé structure count

The commonly accepted opinion that the thermodynamic stability of isomeric benzenoid hydrocarbons (assessed by their total ?-electron energy and various resonance energies) increases with increasing number of Kekul? structures is shown to be violated in numerous cases. The smallest examples of such anomalous behavior are two hexacyclic pericondensed benzenoids of formula C24H14 and several pairs of heptacyclic catacondensed benzenoids of formula C30H18.

Comparing the stability of tribenzo[b,n,pqr]perylene and tribenzo[b,k,pqr]perylene

Two isomeric benzenoid hydrocarbons – tribenzo[b,n,pqr]perylene and tribenzo[b,k,pqr]perylene played a crucial role in the formulation of the Clar aromatic sextet theory. The basic assumption of this theory is that tribenzo[b,n,pqr]perylene is more stable than tribenzo[b,k,pqr]perylene because the former has five, whereas the latter only four aromatic sextets. We now approach this stability problem from a different direction. By means of a recently developed molecular-orbital-based method it is possible to estimate the energy effects of individual cycles, as well as pairs, triplets, etc. of cycles in polycyclic conjugated molecules. From these energy-effects one can better understand which structural details are responsible for the thermodynamic stability of the underlying molecule. In particular, it is possible to rationalize (in a quantitative manner) the causes of differences in the thermodynamic stability of isomers. Our analysis corroborates the conclusion of Clar theory, but points out a number of hitherto overlooked structure-stability connections.

Isomer enumeration of practical benzenoids

Recent development of the leapfrog operation in enumeration and understanding the chemical properties of benzenoid isomer sets having at least one synthesized representative is stressed. Some new enumeration results and correlations of the periodic table for the more stable total resonant sextet (TRS) benzenoid hydrocarbon subset [Table PAH6(sextet)] are presented.

Why is phenanthrene more stable than anthracene

By means of a recently developed molecular-orbital-based method it is possible to estimate the energy effects of individual cycles, as well as pairs, triplets, etc. of cycles in polycyclic conjugated molecules. From these energyeffects one can better understand which structural details are responsible for the thermodynamic stability of the underlying molecule. In particular, it is possible to rationalize (in a quantitative manner) the causes of differences in the thermodynamic stability of isomers. Such an analysis is here performed on the two isomeric benzenoid hydrocarbons with three hexagons – anthracene and phenanthrene.

Estrada Index of Benzenoid Hydrocarbons

A structure-descriptor EE, recently proposed by Estrada, is examined. If λ1, λ2, . . . ,λn are the eigenvalues of the molecular graph, then . In the case of benzenoid hydrocarbons with n carbon atoms and m carbon-carbon bonds, EE is found to be accurately approximated by means of the formula a1 n cosh (√2m/n)+a2, where a1 ≈ 1.098 and a2 = −0.64 are empirically determined fitting constants. Within classes of benzenoid isomers (which all have equal n and m), the Estrada index is linearly proportional to the number of bay regions.

Reaction Mechanisms Governing the Formation of Polycyclic Aromatic Hydrocarbons in the Supercritical Pyrolysis of Toluene: C28H14Isomers

Reaction pathways for the formation of C28H14 polycyclic aromatic hydrocarbons (PAH)of which there are eight benzenoid isomersduring the supercritical pyrolysis of toluene are described in detail. These reaction mechanisms involve the addition of benzyl, methyl, and/or phenyl radicals to smaller PAH products in three specific reactions: (A) addition of methyl and benzyl, usually to a position adjacent to a bay region; (B) addition of phenyl to a bay region; and (C) addition of two methyls to a bay region. Using these three types of reactions, we are able to explain why the five identified C28H14 PAH−benzo[a]coronene, phenanthro[5,4,3,2-efghi]perylene, benzo[cd]naphtho[3,2,1,8-pqra]perylene, benzo[ghi]naphtho[8,1,2-bcd]perylene, and benzo[pqr]naphtho[8,1,2-bcd]perylene−are present in our product mixture and why bisanthene, a C28H14 isomer that we know is not present from UV spectral data, is not formed. We then determine reaction pathways for the remaining two C28H14 benzenoid isomers, naphthaceno[3,4,5,6,7-defghij]naphthacene and tribenzo[cd,ghi,lm]perylene, to deduce that the sixth C28H14 PAH detected in our product mixture by HPLC/MS is most likely tribenzo[cd,ghi,lm]perylene.

Identification of a new C 28H 14 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 C 28H 14 isomers, is also a C 28H 14 PAH. The UV spectrum of compound I has features of a benzenoid PAH, of which there are only eight C 28H 14 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 C 28H 14 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 C 28H 14 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.

Relating Total π-Electron Energy and Resonance Energy of Benzenoid Molecules with Kekulé- and Clar-Structure-Based Parameters

Within classes of isomeric benzenoid hydrocarbons various Kekule- and Clar-structure-based parameters (Kekule structure count, Clar cover count, Herndon number, Zhang–Zhang polynomial) are all mutually correlated. This explains why both the total π-electron energy (E), the Dewar resonance energy (DRE), and the topological resonance energy (TRE) are well correlated with all these parameters. Nevertheless, there exists an optimal value of the variable of the Zhang–Zhang polynomial for which it yields the best results. This optimal value is negative-valued for E, around zero for TRE, and positive-valued for DRE. A somewhat surprising result is that TRE and DRE considerably differ in their dependence on Kekule- and Clar-structure-based parameters.

On the Relationship between π-Electron Energy and Topological Resonance Energy

Within series of isomeric benzenoid hydrocarbons there is a very good linear correlation between the topological resonance energy (TRE) and the total π-electron energy (Eπ ). Furthermore, the slope of the TRE vs. Eπ regression line is almost independent of the benzenoid isomers considered, and (for all sets of isomers) is nearly equal to 0.5. This implies that comparison of benzenoid isomers with regard to their aromaticity can be made, with equal success, by using both TRE and Eπ . However, Eπ is computed significantly simpler than TRE, and thus advantage should be given to the former. Correlations between TRE and Eπ exist also in the case of non-benzenoid isomers (both alternant and non-alternant), but are of much inferior quality.

A simple formula for calculating resonance energy of benzenoid hydrocarbons

The topological resonance energy ( TRE ) of a catacondensed benzenoid hydrocarbon with h six-membered rings and K Kekule structures can be calculated by the (approximate) formula TR E = Ah + B + CK e–Dh , where A = 0 . 136, B = –0 . 223, C = 0 . 281, and D = 0 . 454. Thus, within classes of benzenoid isomers, TR E is an increasing linear function of K . The greater is h , the smaller the effect of the number of Kekule structures on TR E.

Extending and modifying the Hall rule

The Hall rule claims that the HMO total π-electron energy ( E) of isomeric benzenoid hydrocarbons is a linear function of the number of Kekulé structures ( K). We now show that already in the case of benzenoid isomers with a small number of hexagons (4 ⩽ h ⩽ 9), E is better reproduced by a linear function of K λ , where the exponent λ is significantly smaller than unity. This exponent is found to be a decreasing function of h and its limit value (for h → ∞) seems to be equal to, or near to, 1/3. A simple model of such a K-dependence of E is constructed.

What Do We Know about C28H14 and C30H14 Benzenoid Hydrocarbons and Their Evolution to Related Polymer Strips?

While critically reviewing the current status of what is known about C28H14 and C30H14 benzenoid isomers, which are ubiquitous pyrolytic constituents, some new insights will be presented. Representative isomers belonging to these benzenoid hydrocarbons are at the crossroads to homologous series that extend to infinite polymer strips that have been the object of numerous molecular modeling studies. The goal of these studies is to understand their potential electronic properties for material science applications. Along the way, some prior numerical results are supplemented.