Structural Aspects of Aromaticity

Anisotropy of the Induced Current Density (ACID), a General Method To Quantify and Visualize Electronic 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

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.

Energetic Aspects of Cyclic Pi-Electron Delocalization: Evaluation of the Methods of Estimating Aromatic Stabilization Energies

Two pyrrole rings participate in the aromatic structure of porphyrin. Hence, a 22 π-electron description is better than the usual [18]annulene representation. The dianion and the metal complex system favor different electronic structures.

Global and Local Aromaticity in Porphyrins: An Analysis Based on Molecular Geometries and Nucleus‐Independent Chemical Shifts

Zwei Pyrrolringe sind Bestandteil der aromatischen Struktur von Porphyrin. Daher sollte es eher als 22π-Elektronensystem (siehe rechts) denn als [18]Annulen beschrieben werden. Dagegen bevorzugen sowohl das Dianion als auch die Metallkomplexe grundlegend andere elektronische Strukturen.

Globale und lokale Aromatizität in Porphyrinen: eine Analyse anhand von Molekülgeometrien und kernunabhängigen chemischen Verschiebungen

Angular-group-induced bond alternation. I. Origin of the effect from Ab Initio calculations

Angular group-induced bond alternation II. The magnitude and the nature of the effect and its application to polynuclear benzenoid systems

Analysis of the molecular geometry of the ring in 18 exocyclically substituted benzylic cation derivatives shows that an increase of delocalization of the positive charge over the ring is associated with a considerable decrease of its aromatic character. Separation of the aromatic character into energetic and geometric terms allows one to conclude that the effect of dearomatization due to delocalization of the positive charge in the ring is mostly reflected in the geometric term, i.e. by an increase of the alternation of bond lengths in the ring.

Changes of the Aromatic Character of the Ring in Exocyclically Substituted Derivatives of Benzylic Cation as a Result of Varying Charge at the exo-Carbon Atom.

Marcin Wisiorowski

Papers

Global and Local Aromaticity in Porphyrins: An Analysis Based on Molecular Geometries and Nucleus‐Independent Chemical Shifts

Globale und lokale Aromatizität in Porphyrinen: eine Analyse anhand von Molekülgeometrien und kernunabhängigen chemischen Verschiebungen

Angular-group-induced bond alternation. I. Origin of the effect from Ab Initio calculations

Angular group-induced bond alternation II. The magnitude and the nature of the effect and its application to polynuclear benzenoid systems

Changes of the Aromatic Character of the Ring in Exocyclically Substituted Derivatives of Benzylic Cation as a Result of Varying Charge at the exo-Carbon Atom.