Local Aromaticity Research Articles

Benzo[b]thiophene Fusion Enhances Local Borepin Aromaticity in Polycyclic Heteroaromatic Compounds.

This report documents the synthesis, characterization, and computational evaluation of two isomeric borepin-containing polycyclic aromatics. The syntheses of these two isomers involved symmetrical disubstituted alkynes that were reduced to Z-olefins followed by borepin formation either through an isolable stannocycle intermediate or directly from the alkene via the trapping of a transient dilithio intermediate. Comparisons of their magnetic, crystallographic, and computational characterization to literature compounds gave valuable insights about the aromaticity of these symmetrically fused [b,f]borepins. The fusion of benzo[b]thiophene units to the central borepin cores forced a high degree of local aromaticity within the borepin moieties relative to other known borepin-based polycyclic aromatics. Each isomer had unique electronic responses in the presence of fluoride anions. The experimental data demonstrate that the local borepin rings in these two compounds have a relatively high amount of aromatic character. Results from quantum chemical calculations provide a more comprehensive understanding of local and global aromatic characters of various rings in fused ring systems built upon boron heterocycles.

Critical evaluation of HOMA and MBL as local aromaticity indices

AbstractThe harmonic oscillator model of aromaticity (HOMA) index is an excellent structural indicator of aromaticity. We found that HOMA values for internal benzene rings in large pericondensed polycyclic aromatic hydrocarbons are often overestimated because of the local aromaticity of adjacent benzene rings. The occurrence of this phenomenon was confirmed by comparing the HOMA with the corresponding SSE(LA) values; SSE(LA) is a graph‐theoretical index of local aromaticity not disturbed by the aromaticity of the adjacent benzene rings. Mean bond length values were likewise assessed by comparing them with the corresponding HOMA and SSE(LA) values.

Regioselective Synthesis of [3]Naphthylenes and Tuning of Their Antiaromaticity.

Polycyclic conjugated hydrocarbons containing four-membered cyclobutadienoids (CBDs) are of great fundamental and technical interest due to the antiaromaticity brought by CBD circuits. However, their synthesis has been challenging, hampering the exploration and understanding of such systems. We report efficient synthesis of a series of unprecedented [3]naphthylene regioisomers in high yields, where three naphthalenoids are fused through two CBDs in linear, angular, and bent regioconnectivity. Their synthesis was enabled by exclusively regioselective catalytic arene-norbornene annulation (CANAL) between dibromonaphthalenes and benzooxanorbornadienes, followed by aromatization. [3]Naphthylene regioisomers exhibited distinct optoelectronic properties. Nucleus-independent chemical shift calculations, NMR spectroscopy, and X-ray crystallography revealed the strong effect of the fusion pattern on the local antiaromaticity and aromaticity in fused CBDs and naphthalenoids, respectively. Thus, our synthetic strategy allows facile access to extended CBD-fused π-systems with tunable local antiaromaticity and aromaticity.

The Influence of Benzo‐Fusion on the Aromatic Pathways and Ring Currents of Cycl[3.2.2]azine

The effect of benzene ring fusion on the aromaticity of cycl[3.2.2]azine was studied by calculating topological resonance energy (TRE), the percentage topological resonance energy (%TRE), and magnetic resonance energy (MRE). The bond resonance energy (BRE) and circuit resonance energy (CRE) indices were used to estimate the local aromaticity. Our BRE and CRE results show that the central nitrogen atom plays a significant role both in the global and local aromaticity of the compounds in our study, and contrary to what has been reported in the literature, none of these compounds are peripheral π‐electronic systems. In the case of benzene ring‐fused derivatives, benzene ring(s) exhibit relatively larger local aromaticity and reduce the local aromaticity of the central portion of cycl[3.2.2]azine to a level comparable to a corresponding non‐benzene fused parent system. Ring current results predict that a strong diamagnetic current flows around the whole molecular perimeter. The diatropic bond current results, as computed here, are in good agreement with the observed 1H‐NMR chemical shifts of these compounds.

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.

The role of the long-range exchange corrections in the description of electron delocalization in aromatic species.

In this article, we address the role of the long-range exchange corrections in description of the cyclic delocalization of electrons in aromatic systems at the density functional theory level. A test set of diversified monocyclic and polycyclic aromatics is used in benchmark calculations involving various exchange-correlation functionals. A special emphasis is given to the problem of local aromaticity in acenes, which has been a subject of long-standing debate in the literature. The presented results indicate that the noncorrected exchange-correlation functionals significantly overestimate cyclic delocalization of electrons in heteroaromatics and aromatic systems with fused rings, which in the case of acenes leads to conflicting local aromaticity predictions from different criteria. © 2017 Wiley Periodicals, Inc.

Is coronene better described by Clar's aromatic π-sextet model or by the AdNDP representation?

The bonding patterns in coronene are complicated and controversial as denoted by the lack of consensus of how its electronic structure should be described. Among the different proposed descriptions, the two most representative are those generated by Clar's aromatic π-sextet and adaptative natural density partitioning (AdNDP) models. Quantum-chemical calculations at the density functional theory level are performed to evaluate the model that gives a better representation of coronene. To this end, we analyse the molecular structure of coronene, we estimate the aromaticity of its inner and outer rings using various local aromaticity descriptors, and we assess its chemical reactivity from the study of the Diels-Alder reaction with cyclopentadiene. Results obtained are compared with those computed for naphthalene and phenanthrene. Our conclusion is that Clar's π-sextet model provides the representation of coronene that better describes the physicochemical behavior of this molecule. © 2017 Wiley Periodicals, Inc.

π-electron content of rings in polycyclic conjugated compounds – A valence bond based measure of local aromaticity

In 2004 Randić and Balaban employed Kekulé structures to assess the π-electron content (EC) of rings in benzenoid hydrocarbons. In this paper we showed how ab initio Valence Bond (VB) calculations based on the set of Kekulé structures of a given polycyclic conjugated molecule can be used to obtain more accurate π-electron contents of rings (VB-EC) as the true computed weights of the Kekulé structures are used, instead of postulating that all Kekulé structures have equal weights as in the EC formula. In the case of benzenoid hydrocarbons, the values of VB-EC and EC are very close. The main difference is found for linear polyacenes, for which, contrary to the EC, the VB-EC values predict that the inner rings are more aromatic than the terminal rings. The original EC method has also the disadvantage that it cannot be applied to heterocyclic conjugated molecules. It was shown that the VB-EC method can be used to assess the local aromaticity in aza derivatives of naphthalene. The calculated VB-EC values were compared with several other aromaticity indices, namely: energy effect (ef), harmonic oscillator model of aromaticity (HOMA) index, six center delocalization index (SCI) and nucleus independent chemical shifts (NICS). The best correlation was found between the VB-EC and SCI, implying that these two indices carry similar information on the local aromaticity.

Rationalizing the relative abundances of trimetallic nitride template-based endohedral metallofullerenes from aromaticity measures.

The synthesis of endohedral metallofullerenes (EMFs) from a carbon soot sample of an arc discharge leads to a variety of EMFs that are obtained in different relative abundances. In the present work, we show that these abundances can be predicted from aromaticity calculations. In particular, we use the normalized Additive Local Aromaticity (ALAN) index. Our results show that the most abundant Sc3N-based and Y3N-based EMFs in fullerene soot are the most aromatic. This study reinforces the idea that aromaticity plays a key role in determining the stability of EMFs.

A Comparative Study of the Two Isomers of Bicalicene

ABSTRACTTwo isomers of bicalicene are possible. The trans-isomer is a stable aromatic compound, known since the 1980s. The cis-isomer has never been experimentally prepared and has so far attracted little attention of theoretical chemists. The properties of cis-bicalicene are now studied theoretically, using the B3LYP/6-311G(d,p) quantum-chemical method. All data obtained (geometric, energetic, as well as those based on harmonic oscillator model of aromaticity and nucleus-independent chemical shift indices of local aromaticity) unequivocally indicate that the “missing” isomer of bicalicene is an unstable antiaromatic species, and that the lack of its existence is no surprise whatsoever.

Systematic investigation of the stability of all dihydro isomers of the more stable C38fullerene isomer with C2 symmetry

The local aromaticity of fullerene C[Formula: see text] ([Formula: see text] was studied using the bond resonance energy (BRE) method. The global aromaticity of all 349 possible isomers of C[Formula: see text]H2based on [Formula: see text] symmetry was investigated using the topological resonance energy (TRE) method. The TRE results show that most of the C[Formula: see text]H2isomers have greater stability than C[Formula: see text] ([Formula: see text]. Based on the BRE results, the preferred addition sites of hydrogen atoms are discussed. The relationship between the addition sites and BREs is analyzed and discussed. We found that the addition sites of hydrogen atoms are strongly dependent on the magnitude of the parent cage BRE values. The most stable isomers of C[Formula: see text]H2are often produced by diminishing of the [Formula: see text]-bonds from those sites in the parent cage at which are located the two carbon atoms with the lowest BRE values. Based on this rule, the preferred addition patterns for non-IPR fullerene cages can be easily predicted.

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

ABSTRACTFunctionalized 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.

Quantitative Assessment of Aromaticity and Antiaromaticity Utilizing Vibrational Spectroscopy.

Vibrational frequencies can be measured and calculated with high precision. Therefore, they are excellent tools for analyzing the electronic structure of a molecule. In this connection, the properties of the local vibrational modes of a molecule are best suited. A new procedure is described, which utilizes local CC stretching force constants to derive an aromaticity index (AI) that quantitatively determines the degree of π-delocalization in a cyclic conjugated system. Using Kekulé benzene as a suitable reference, the AIs of 30 mono- and polycyclic conjugated hydrocarbons are calculated. The AI turns out to describe π-delocalization in a balanced way by correctly describing local aromatic units, peripheral, and all-bond delocalization. When comparing the AI with the harmonic oscillator model of AI, the latter is found to exaggerate the antiaromaticity of true and potential 4n π-systems or to wrongly describe local aromaticity. This is a result of a failure of the Badger relationship (the shorter bond is always the stronger bond), which is only a rule and therefore cannot be expected to lead to an accurate description of the bond strength via the bond length. The AI confirms Clar's rule of disjoint benzene units in many cases, but corrects it in those cases where peripheral π-delocalization leads to higher stability. [5]-, [6]-, [7]-Circulene and Kekulene are found to be aromatic systems with varying degree of delocalization. Properties of the local vibrational modes provide an accurate description of π-delocalization and an accurate AI.

Metallic and Magnetic 2D Materials Containing Planar Tetracoordinated C and N

The top monolayers of surface carbides and nitrides of Co and Ni are predicted to yield new stable 2D materials upon exfoliation. These 2D phases are p4g clock reconstructed, and contain planar tetracoordinated C or N. The stability of these flat carbides and nitrides is high, and ab initio molecular dynamics at a simulation temperature of 1800 K suggest that the materials are thermally stable at elevated temperatures. The materials owe their stability to local triple aromaticity (π, σ-radial, and σ-peripheral) associated with binding of the main group element to the metal. All predicted 2D phases are conductors, and the two alloys of Co are also ferromagnetic, a property especially rare among 2D materials. The preparation of 2D carbides and nitrides is envisioned to be done through surface deposition and peeling, possibly on a metal with a larger lattice constant for reduced affinity.

Theoretical investigation of curved π‐conjugated fullerene flakes: open‐shell character, aromaticity, and third‐order nonlinear optical property

Open‐shell singlet nature, aromaticity, and second hyperpolarizabilities of two kinds of curved π‐conjugated fragments of C60 fullerene, called C60 flakes, together with their corresponding planar analogues are theoretically investigated. It is found that one curved system, A-1, exhibits intermediate open‐shell singlet nature, while the other structure, B-1, and the two corresponding planar systems possess a closed‐shell ground state. The variation of the open‐shell singlet nature is explained by the number of Clar's sextets in the resonance structures, which is associated with the local aromaticity, as quantified by the NICS0 values. The calculated orientationally averaged second hyperpolarizabilities show that the intermediate open‐shell singlet system A‐1 exhibits the best performance, and that the other curved system B‐1 is the worst among the four C60 flakes, whereas all systems exhibit better performance than the original spherical C60 system. This tendency suggests that a lower dimensionality in the π‐conjugated structure is beneficial to the construction of efficient nonlinear optical molecules. The present results not only show that the open‐shell singlet system A‐1 is a performant nonlinear optical molecule, but they also contribute to a deeper understanding of the electronic structure of curved π‐conjugated molecules. Copyright © 2016 John Wiley & Sons, Ltd.

Aromatic Character of Irregular-Shaped Nanographenes.

We found that the Clar sextet formula with the maximum number of sextet rings cannot always be defined meaningfully for large irregular-shaped PAHs. It is true that edge structure is always a primary determinant of the PAH aromaticity pattern. In large PAH molecules, every edge structure modifies the aromaticity pattern near the edge, but its influence fades on going away from the edge. It follows that different textures of the aromaticity pattern appear near different edges. As a result, the entire aromaticity pattern does not always match with a single Clar formula or a single weighted superposed Clar formula. Such an unusual feature of aromaticity patterns could not have been observed distinctly if we had not explored the aromaticity patterns of large irregular-shaped PAH molecules systematically. We used the superaromatic stabilization energy (SSE) as a local aromaticity index, which is the only index of this kind not disturbed by the aromaticity of adjacent benzene rings.

(Invited) The Regioselectivity of Bingel-Hirsch Cycloadditions on IPR Endohedral Metallofullerenes

Given the large number of possible non-equivalent bonds of Endohedral MetalloFullerenes (EMFs), the elucidation of the preferred addition sites for a target reaction is tremendously challenging.1 The development of a set of simple rules able to rationalize the preferred regioselectivities by solely inspecting the fullerene structure would be extremely appealing for the whole fullerene community. This communication describes a set of predictive criteria called Predictive Aromaticity Criteria (PAC), to a priori identify the most reactive bonds of a given EMF towards the Bingel-Hirsch (BH) cycloaddition based on a simple evaluation of the cage structure, and without need to perform any expensive theoretical calculation or experimental measure.2 We take as starting point the investigation of the BH addition of diethylbromomalonate on all non-equivalent bonds of Sc3N@D3h-C78 using density functional theory calculations. From the Sc3N@D3h-C78 analysis, a set of rules that we called Predictive Aromaticity Criteria (PAC) is derived, which are further applied to predict the most favorable BH adducts for the challenging case of Sc3N@D5h-C80. The predictions based on PAC are afterwards fully confirmed by both the computational and experimental results of the complete study of the regioselectivity of BH reaction to Sc3N@D5h-C80. These results provide conclusive evidence on the reliability of the PAC, indicating that these rules are general and applicable to any BH reaction on IPR EMFs. Moreover, this work provides clear evidence of the key role played by the local aromaticity of the cage in EMF functionalization.2-5

The impact of increasing number of nitrogens and their sequences on the aromaticity of azacarbazole derivatives: A theoretical study

The influence of increasing number and changing the sequence of nitrogen atoms on the local aromaticity of azacarbazole derivatives are investigated. The local aromatic characters of the considered compounds are probed using quantitative descriptors including the geometry-based (harmonic oscillator model of aromaticity (HOMA)), magnetic index (nucleus independent chemical shift (NICSs)) and electronic based index (fluctuation delocalization index (FLU)). The aromaticity of pyrrole-like ring of the studied compounds is also analyzed through AIM theory. Furthermore, the global aromaticity of the studied azacarbazole derivatives is scrutinized using the magnetic susceptibility and HOMA together with its EN and GEO components. The obtained results indicate that the aromatic characters of these compounds are not greatly sensitive to the increasing numbers of nitrogen atoms and their sequences.

2-(Naphthalen-1-yl)thiophene as a New Motif for Porphyrinoids: Meso-Fused Carbaporphyrin.

The first synthesis of meso-fused carbaporphyrin via a premodification method was accomplished by substituting two pyrrole moieties and one meso-carbon with 2-(naphthalen-1-yl)thiophene. The obtained global π-conjugation pathway of the macrocycle noticeably disturbs the 10π local aromaticity of naphthalene, and its aromatic nature was supported by NMR spectroscopy together with nucleus-independent chemical shift, anisotropy of the induced current density, and harmonic oscillator stabilization energy calculations. In addition, the meso-fused carbaporphyrin also allowed the formation of a square planar Pd(II) complex.

Establishing the pivotal role of local aromaticity in the electronic properties of boron-nitride graphene lateral hybrids.

Within an attempt to unravel the conundrum of irregular bandgap variations in hybrids of white-graphene (hBN) and graphene (G) observed in both experiment and theory, strong proofs about the decisive role of aromaticity in their electronic properties are brought to light. Sound numerical experiments conducted on zero-, one- and two-dimensional hBNG hybrids demonstrate that upon structural and/or electronic perturbation caused by foreign doping agents, the uniformity in local cyclic electron delocalization of ideal graphene restructures locally creating carbon hexagons of contrasting cyclic electron delocalization (c.c. local aromatic patterns) which may dominate the bandgap size of the resulting systems. In addition, relying on the quantum chemical aspect of aromaticity in terms of quantitative computations of cyclic electron delocalization together with pictorial intrinsic polarizability density representations, this work provides a solid and handy rule-of-thumb to be used in qualitative and intuitive predictions. According to this empirical rule, the origin of any nonmonotonic bandgap variation observed in stoichiometric 0D (BN)n/graphene hybrids with increasing hBN segment lies in instabilities caused by partially substituted benzenoid rings formed locally at the hBNG interfaces. This relationship, established in 0D graphene flakes and extended to 1D periodic ribbons, can be used to understand and qualitatively predict conflicting bandgap variations of vacancy-free 2D periodic lattices, pointing at the property of aromaticity as the missing link needed to solve the puzzle of conflicting bandgap variations in hBNG hybrids observed in experiment.