Patterns of pi-electron delocalization in aromatic and antiaromatic organic compounds in the light of Hückel's 4n + 2 rule.
ABSTRACT The total pi-electron delocalization of a series of classical aromatic and antiaromatic organic compounds is separated into ortho (1,2), meta (1,3), para (1,4), and successive contributions (the so-called delocalization crossed terms) and the changes that take place in these crossed terms when two electrons are added or removed are analyzed. Our results show that these changes follow a similar alternation pattern in all cases. The patterns found represent a kind of electronic footprints that makes it possible to discern between aromatic and antiaromatic systems.
Article: Metalloaromaticity[Show abstract] [Hide abstract]
ABSTRACT: In the last decade, the study of aromaticity has experienced enormous progress. The new discoveries, which include species such as the metallabenzenes, heterometallabenzenes, metallabenzynes, metallabenzenoids, metallacyclopentadienes, metallacyclobutadienes, and all-metal and semimetal clusters, have joined the classical organic aromatic molecules such as benzene, benzenoid and nonbenzenoid polycyclic aromatic hydrocarbons, and heteroaromatic species to conform the current aromatic zoo. These new molecules, which are potentially useful for certain purposes as specific and very efficient catalysts, molecular electronic devices, molecular magnets, drugs, and other as yet unimagined applications, have brought a complete revolution in the field. At variance with the classical aromatic organic molecules that possess only π-electron delocalization, aromaticity in these new species is much more complex. These compounds have σ-, π-, δ-, and ϕ-electron delocalization. In addition, they can combine different types of aromaticity thus giving rise to double or triple aromaticity, the so-called multifold aromaticity. The new molecules can also have conflicting aromaticity, i.e., they can be aromatic in one component and antiaromatic in another. Moreover, most of the old indicators are not valid to discuss the complex aromaticity of these novel compounds. The lack of reliable measures of aromaticity for these systems has triggered the development of new and more general and reliable indices that can be applied to both classical organic and inorganic aromatic compounds. Among them, the use of multicenter electronic delocalization indices is advocated because they help to detect the different types of aromaticity and provide reasonable qualitative orderings of aromaticity. © 2012 John Wiley & Sons, Ltd.Wiley interdisciplinary reviews: Computational Molecular Science. 03/2013; 3(2):105-122. · 9.04 Impact Factor
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ABSTRACT: In this paper we present the results of a high-resolution single crystal X-ray diffraction experiment at 15 K on a benzothiazol-substituted phosphane and a subsequent charge density study based on multipole refinement and a topological analysis according to Bader's quantum theory of atoms in molecules. Although two valence shell charge concentrations (VSCCs) in the non-bonding region of each phosphorus and sulfur atom were found, the integration of both heteroatomic basins emphasizes charge depletion. Nevertheless they are attractive for C-HP and C-HS hydrogen bonding in the solid state. The nature of the P-C bonds and the question of aromaticity in the heterocycles were subject to our investigations. The ellipticities along the bonds were analysed to approach delocalization. The source function is employed to visualise atomic contributions to aromaticity. Theoretical calculations have been carried out to compute nuclear chemical shifts, induced ring currents and a variety of delocalization indices. All applied measures for delocalization point in the same direction: while heteroaromaticity is present in the benzothiazolyl substituents, the bridging P-C bonds are only involved marginally, almost preventing total conjugation of the phosphane. The charge density distributions around the phosphorus and the sulfur atoms have very similar features but turn out to be chemically very different from each other. Commonly used simplifying concepts have difficulties in providing a comprehensive view on the electronic situation in the molecule. Our results raise doubts on the validity of the common interpretation of VSCCs as one-to-one representations of Lewis lone pairs.Physical Chemistry Chemical Physics 11/2013; · 4.20 Impact Factor
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ABSTRACT: It is demonstrated that there is a direct connection between aromaticity and the anisotropy of the π-electron density on planes parallel to the molecular ring. The electron density anisotropy on the plane is measured through the ratio of the in-plane Hessian eigenvalues associated with the eigenvectors lying in the plane. Computations on a wide-ranging set of well-characterized monocyclic systems containing heteroatoms validate the correlation between this one-electron density-based descriptor and aromaticity; in aromatic compounds, the in-plane Hessian eigenvalues are degenerate (or near degenerate) and the anisotropy of the π-electron density is undirected, whereas the results for antiaromatic rings are reversed and the degeneracy of the eigenvalues completely disappears. This finding is in line with our very recent study on [n]annulenes and provides further evidence that the anisotropy of the π-electron density should be considered as a new manifestation of aromaticity.Physical Chemistry Chemical Physics 05/2014; · 4.20 Impact Factor