Which NICS aromaticity index for planar pi rings is best

Benzene is the emblematic example of an aromatic molecule, and the problem of its structure has given rise to a chemical serial story running over several decades. The epistemological digest of this story written by Stephen G. Brush1,2 shows how this problem has been at the root of important concepts such as those of mesomery and resonance. Before the advent of quantum mechanics, chemists had thought of the benzene structures in terms of two center bonds attempting to preserve the valence of the carbon atom and to explain its chemical properties. Kekulé’s theory of the structure of the benzene molecule3 invokes the oscillatory hypothesis in which “the fourth valence of each carbon oscillates between its neighbors, synchronously with all the other fourth valences, so that the structure switches rapidly between the two structures”,1 whereas Claus proposed a diagonal hypothesis4 in which the fourth valence of each carbon is directed toward the carbon in the para position. The latter hypothesis has been rejected because it enables only two derivatives, and it has been revised to remove this inconsistency: instead of forming a bond, the fourth valence stops near the center of the ring in the Armstrong-Baeyer formula,5 or there is only one bridging bond as in the Dewar’s bridged benzene formula.6 In Thiele’s partial valence model,7 the adjacent carbon-carbon bonds are considered as intermediate between single and double bonds. These formulas were later considered by K. C. Ingold to set up his intra-annular tautomerism,8 which appears to be the generalization of Kekulé’s oscillatory hypothesis. Ingold’s tautomerism hypothesis was later called mesomerism.9 The mesomery is an important concept in chemistry, which implicitly introduces the electron delocalization in the context of the prequantum electronic theory. The first applications of quantum chemistry to the benzene problem led on the molecular orbital (MO) side Erich Hückel to propose his famous 4n + 2 rule10 and on the valence bond (VB) side Pauling and Wheland to identify resonance with Ingold’s mesomerism.11,12 An enlighting contribution of modern VB theory on the benzene structure has been brought by Shaik et al.,13 who have shown that the hexagonal symmetry of benzene is due to the σ-system because the π component is distortive along a Kekulean distortion. * Authors to whom correspondence should be addressed (telephone +34-972-418912; fax +34-972-418356; e-mail miquel.sola@ udg.es or silvi@lct.jussieu.fr). ‡ Universitat de Girona. § Université Pierre et Marie Curie. 3911 Chem. Rev. 2005, 105, 3911−3947

Theoretical evaluation of electron delocalization in aromatic molecules by means of atoms in molecules (AIM) and electron localization function (ELF) topological approaches.

The research on 1,2,3-triazoles has been lively and ever-growing since its stimulation by the advent of click chemistry The attractiveness of 1H-1,2,3-triazoles and their derivatives originates from their unique combination of facile accessibility via click chemistry and truly diverse supramolecular interactions, which enabled myriads of applications in supramolecular and coordination chemistry The nitrogen-rich triazole features a highly polarized carbon atom allowing the complexation of anions by hydrogen and halogen bonding or, in the case of the triazolium salts, via charge-assisted hydrogen and halogen bonds On the other hand, the triazole offers several N-coordination modes including coordination via anionic and cationic nitrogen donors of triazolate and triazolium ions, respectively After CH-deprotonation of the triazole and the triazolium, powerful carbanionic and mesoionic carbene donors, respectively, are available The latter coordination mode even features non-innocent ligand behavior Moreover, these supramolecular interactions can be combined, eg, in ion-pair recognition, preorganization by intramolecular hydrogen bond donation and acceptance, and in bimetallic complexes Ultimately, by clicking two building blocks into place, the triazole emerges as a most versatile functional unit allowing very successful applications, eg, in anion recognition, catalysis, and photochemistry, thus going far beyond the original purpose of click chemistry It is the intention of this review to provide a detailed analysis of the various supramolecular interactions of triazoles in comparison to established functional units, which may serve as guidelines for further applications

Beyond click chemistry - supramolecular interactions of 1,2,3-triazoles.

Aromaticity as a cornerstone of heterocyclic chemistry.

Among many various kinds of molecular interactions, the H-bond has a special position. The term is ubiquitous in the world that surrounds us, but also it is often applied in different ways. The H-bond is of great importance in natural sciences. This relates particularly to biological aspects, such as molecular recognition that could be a basis for the creation of life,1-4 formation of higher order structures of peptides and nucleic acids,5 and biochemical processes, particularly the enzymes catalyzed.6,7 One can say that the H-bond plays a double role in biological systems: on one hand, as a relatively strong directional interaction, it leads to relatively stable supramolecular structures, and on the other hand, because of dynamic features of the proton, it is an active site for initiation of chemical reactions. H-bonds are the source of specific properties of associated liquids, with water being the most popular among them.8 Water as a medium in which life was most probably created is saturated by H-bonds with highly mobile protons in between, even in the solid state.9 In many crystal lattices of organic compounds, the H-bonds are a decisive factor governing packing.10 In designing new interesting crystal structures, which is the subject of fast developing crystal engineer* To whom correspondence should be addressed. E-mail: slagra@ uni.lodz.pl or slagra@ccmsi.us. Fax: +48-42-6790447. 3513 Chem. Rev. 2005, 105, 3513−3560

Interrelation between H-bond and Pi-electron delocalization.

Statistical analyses of quantitative definitions of aromaticity, ASE (aromatic stabilization energies), RE (resonance energies), Λ (magnetic susceptibility exaltation), NICS, HOMA, I5, and AJ, evaluated for a set of 75 five-membered π-electron systems:  aza and phospha derivatives of furan, thiophene, pyrrole, and phosphole (aromatic systems), and a set of 30 ring-monosubstituted compounds (aromatic, nonaromatic, and antiaromatic systems) revealed statistically significant correlations among the various aromaticity criteria, provided the whole set of compounds is involved. Hence, broadly considered, the various manifestations of aromaticity are related and aromaticity can be regarded statistically as a one-dimensional phenomenon. In contrast, when comparisons are restricted to some regions or groups of compounds, e.g., aromatic compounds with ASE > 5 kcal/mol or polyhetero-five-membered rings, the quality of the correlations can deteriorate or even vanish. In practical applications, energetic, geometric, ...

To what extent can aromaticity be defined uniquely

Separation of the Energetic and Geometric Contributions to Aromaticity. Part 2. Analysis of the Aromatic Character of Benzene Rings in Their Various Topological Environments in the Benzenoid Hydrocarbons. Crystal and Molecular Structure of Coronene

Tadeusz M. Krygowski

Papers

To what extent can aromaticity be defined uniquely

Separation of the Energetic and Geometric Contributions to Aromaticity. Part 2. Analysis of the Aromatic Character of Benzene Rings in Their Various Topological Environments in the Benzenoid Hydrocarbons. Crystal and Molecular Structure of Coronene