Aromaticity, concept

The most advanced MO and DFT calculations support the idea of an aromatic transition state. The net effect on reaction rate of any substituent is determined by whether it stabilizes the transition state or the ground state more effectively. The aromatic concept of the transition state predicts Aat it would be stabilized by substituents at all positions, and this is true for phenyl substituents, as shown in Table 11.2. 

Finally, we briefly compare the NBO donor-acceptor picture of geminal delocalization with Dewar s sigma-aromaticity concept and the associated pi-complex... 

Heterocyclic chemistry offers a much wider prospect for exploring the possibilities and limitations of the aromaticity concept than hydrocarbon chemistry. As a result, quantitative measures for aromaticity have been devised, and improved molecular calculation methods allow for a better evaluation of the experimental data already obtained and predictions of novel aromatic heterocycles. 

This section is devoted to heterocyclic molecules isoelectronic with dications and dianions of the molecules that are central to the aromaticity concept, i.e., cyclobutadiene and benzene (Scheme 5). 1,3-Diboretene... 

VI. Aromaticity Concept in the Chemistry of Group 14 Elements next to Carbon... 

Thus, with an increase in the number of silicon atoms in a cyclic structure, analysis, in terms of the aromaticity concept, of the structural types and stability loses its prognostic value. A striking example of this is given by the pyramidalization of the Si atoms, typical of disilene, in hexasilaben-... 

Finally, the concept of aromaticity in heterocyclic chemistry is the subject of an overview by Professor V. I. Minkin and Drs. B. Ya. Simkin and M. N. Glukhovtsev of Rostov State University in Russia. This new review shows just how wide the aromaticity concept has become. The present authors have successfully extended its range to the consideration of numerous very diverse heterocyclic systems. 

Aromaticity is the simplest way to explain the stability of unsaturated cyclic hydrocarbons with (4n + 2) electrons delocalized in the rr-orbitals perpendicular to the ring plane.1 Even though the introduction of the aromaticity concept in chemistry is quite old, its definition is still controversial. It is not surprising to find many attempts to define this term depending on different approaches to describe the electronic structure. In view of these problems of subjectivity, it is remarkable that aromaticity is useful to rationalize and understand the structure and reactivity of many organic molecules. As a result, the concept of aromaticity is truly a cornerstone in organic chemistry. In 1971, Wade proposed a similar concept to describe delocalized cr-bonding in closed-shell boron deltahedra.2-4 However, stability based on aromaticity had not been confirmed for any metallic moiety until Li et al. published their seminal paper entitled Observation of all-metal aromatic molecules, 5... 

The resonance energy per n electron (REPE) has been used as a measure for the aromaticity of molecules [24], For a recent and comprehensive overview of the aromaticity concept, see, for example, Refs. [25,26],... 

Although the data for the two series of compounds <90JPR(332)853, 90JPR(332)870) are completely different, there is a significant relationship between the loadings and the scores for both series which indicates good reliability for this aromaticity concept. 

Similar arguments hold for the transition state of the electrocyclic ring closure of hexatriene forming 1,3-cyclohexadiene (Figure 13.22). There is a strong diatropic ring current in the transition structure, which is in agreement with the aromaticity concept of pericyclic transition states. 

Subsequendy, the aromaticity concept built on carbocydes has been applied successfully to heterocyclic organic compounds, i.e., to a family of aromatic compounds which contain one or more heteroatoms as a part of their cyclic TC-electron system. However, some of the measures developed for aromatic hydrocarbons, particularly those based on geometrical parameters, have been modified to be used for heterocycles (1993JCI70, 2001T5715, 2010SY1485). 

More advanced developments of Huckel s theory, such as the Pariser, Parr, Pople Self-Consistent Field Theory, which takes into account the interaction of the electrons, have led to a deeper understanding of the aromatic concept and are widely applied to determine physical properties of aromatics, particularly when describing dyestuffs. 

Although aromaticity concept has been talked about by all chemists for almost two centuries there exists no unambiguous quantitative definition of aromaticity mainly because it is not an experimentally measurable quantity. It is also not possible to theoretically calculate it as the expectation value of a linear hermitian operator in quantum mechanics, mainly due to the same reason stated above. 

Since there exists no unique definition of aromaticity several criteria need to be analyzed in order to get a proper idea of the aromaticity concept. Benzene is the quintessential example of an aromatic molecule which obeys all the aromaticity criteria. Other aromatic systems adhere to only some of these criteria of this multidimensional concept. Sometimes even two sets of criteria contradict each other. 

Although originally aromaticity concept was restricted to benzene and other related molecules it is no longer confined to a limited class of organic systems only. We present here different types of aromatic systems known today. 

The aromaticity concept has been introduced to the chemistry literature through benzene and other cyclic molecules and their extension to inorganic molecules like borazine and silicazine. The seminal work of Boldyrev and coworkers has paved the way to have a recent upsurge of interest in the field of aromaticity and antiaromaticity of metal clusters (Fig. 13-17). Negative ion photoelectron spectroscopy using a laser vaporization source augmented by ab initio calculations have authenticated the aromatic nature... 

Perhaps the aromaticity concept is unnecessary. Why attribute the stability of these species to aromaticity when simple delocalization of electrons may do as well That question is a good one, and needs an answer. It is not necessary to invoke special effects if simpler concepts will suffice. The principle of Ockham s razor says that when faced with a variety of equally attractive explanations, pick the simplest. 

So, the number of resonance forms is not always a predictor of stabUity. No longer are we faced with equally attractive explanations. The aromaticity concept is necessary to make sense of the data. Aromaticity trumps resonance. A look at the molecular orbitals of cycloheptatrienyl anion shows the source of the instability. The cycloheptatrienyl anion must contain unpaired electrons in degenerate antibonding orbitals (Fig. 13.33)1 It is not aromatic, it is a diradical. 

The literature on aromaticity is so vast that we must be content here with maiking comprehensive reference only to a selection of the major books and reviews that have established themselves in the field in the 50 years after Huckel, as well as to a few more-recent assessments of the aromaticity concept from approximately the past decade.(It is perhaps significant to note in passing that two of the most recent references cited in that list—refs 58 and 59, both from 1996—are entitled What Is Aro-... 

We hope that the aromaticity, antiaromaticity, and conflicting aromaticity concepts would stimulate theoretical analysis of chemical bonding in other known and novel chemical compounds containing transition metal atoms in clusters, nanoparticles, solid compounds, and metallobiomolecules. 

Bultinck P (2007) Critical analysis of the local aromaticity concept in poly-aromatic hydrocarbons. Faraday Discuss 135 347-365 and references therein... 

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