Special Issue "Aromaticity and Molecular Symmetry"

Guest Editor
Prof. Dr. Paul v. R. Schleyer
Computational Chemistry Annex, The University of Georgia, Athens, GA 30602-2525, USA
Website: http://www.chem.uga.edu/schleyer/
E-Mail:
Interests: adamantane synthesis; cage molecules; hydrogen bonding; reactive intermediates; computational chemistry; aromaticity and planar hypercoordination

Submission Information

All papers should be submitted to symmetry@mdpi.org. To be published continuously until the deadline and papers will be listed together at the special issue website.

Submitted papers should not have been published nor be under consideration for publication elsewhere. All papers are refereed through a peer-review process. A guide for authors is available on the Instructions for Authors page. Symmetry is a new international, peer-reviewed, quarterly open access journal published by Molecular Diversity Preservation International.

Open Access publication is free of charge for manuscripts submitted in 2009 and published in the first few issues od Symmetry. English correction fees and/or formatting fees of 250 CHF will be billed in certain cases (250 CHF per paper for those papers that require extensive additional formatting and/or English corrections).

Leading papers

1. Schleyer, P.v.R. Aromaticity (Editorial). Chemical Reviews, 2001, 101, 1115-1118.
2. Balaban, A.T.; Schleyer, P.v.R.; Rzepa, H.S. Crocker, Not Armit and Robinson, Begat the Six Aromatic Electrons. Chemical Reviews, 2005, 105, 3436-3447.
3. Schleyer, P.v.R. Introduction: Delocalization-π and σ (Editorial). Chemical Reviews, 2005, 105, 3433-3435.

• Stability
• Planar hypercoordination
• Delocalization
• Resonance
• Conjugated ring
• Conjugation
• π Symmetry
• Delocalized conjugated π system
• Antiaromaticity
• Polycyclic aromatic hydrocarbons
• Hückel's rule

Title: Electron Delocalization and Aromaticity of All-Metal Clusters
Author: Miquel Solà; E-mail: miquel.sola@udg.edu
Affiliation: Departament of Chemistry, Institute of Computational Chemistry, Girona University, Campus de Montilivi, 17071 Girona, Catalonia, Spain
Abstract: to be added

Title: Structures of Annulenes in the Ground and Lowest Excited States
Authors: C.Gellini and Piero Salvi; E-mail: piero.salvi@unifi.it
Affiliation: Dipartimento di Chimica, Universitá di Firenze, Via della Lastruccia 3, 50019 Sesto Fiorentino, Firenze, Italy
Abstract: This paper introduces general considerations on structural properties of reference cyclic conjugated systems, such as cyclobutadiene, benzene and cyclooctatetraene, in terms of potential energy surfaces along bond length alternation and bond distortion coordinates. The main interest of the paper is concerned with [n]annulenes, n ranging from 10 to 18. The rich variety of conformational isomers is described as well as bridging and dehydro processes, these latters as a means to reduce the molecular flexibility of annulenes. Specific attention is devoted to the structures of bridged [10]- and [14]-annulenes in the ground and low-lying excited states. Aromaticity along the series [10] to [18] annulenes is examined on the basis of energy and magnetic criteria with emphasis about the transition from bond equalized to bond alternating annulenic structures. Finally, porphyrins are presented from a chemical point of view as a special group of [18]-annulene structures adopting a planar geometry in the unsubstituted macrocycle and planar/non planar geometries for small/large size substituents along the cycle periphery. Results on their excited states and on relaxation processes from upper electronic levels are reported.

Title: Computed NMR Shielding Effects over Fused Aromatic/Antiaromatic Hydrocarbons
Author: Ned H. Martin; E-mail: martinn@uncw.edu
Affiliation: Department of Chemistry and Biochemistry, University of North Carolina Wilmington, 601 S. College Road, Wilmington, NC 28403-5932, USA
Abstract: Through-space isotropic NMR shielding values were computed using GIAO at the HF/6-31G(d,p) level in Gaussian 03 on a diatomic hydrogen probe 2.5 Å above the surface of a series of fused aromatic/antiaromatic ring conjugated hydrocarbons, including several combinations of benzene fused with cyclobutadiene, benzene fused with tropylium ion, and benzene fused with cyclopentadienyl anion. Subtraction of the isotropic shielding value of H₂ alone gave shielding increments, which were graphed against Cartesian coordinates of the probe position over each hydrocarbon resulting in three-dimensional isotropic shielding increment surfaces. The results are discussed in relation to the degree of computed bond length alternation, the amount of pi electron delocalization and (for the ions), the distribution of natural population analysis charge. These data are interpreted in terms of aromaticity and antiaromaticity of the component rings in the fused hydrocarbons.

Title: Aromaticity and Adaptive Natural Density Partitioning
Author: Alex Boldyrev; E-mail: a.i.boldyrev@usu.edu
Abstract: to be added

Type of Paper: Article
Author: Prof. Dr. Ewa D. Raczynska
Affiliation: Department of Chemistry, Warsaw University of Life Sciences (SGGW), ul. Nowoursynowska 159c, 02-776 Warszawa, Poland; E-Mail: ewa_raczynska@sggw.pl
Title: On the reformulated harmonic oscillator model of aromaticity (HOMA) index and its application to heteroaromatic systems
Abstract: Although the HOMA index reformulated in 1993 [T.M. Krygowski, J. Chem. Int. Comput. Sci. 33 (1993) 70] has been very often applied to aromatic systems [T.M. Krygowski, M.K. Cyranski, Chem. Rev. 101 (2001) 1385, and references cited therein] it seems to be inappropriate for heteroaromatic compounds containing C, N, O and other heteroatoms in the same molecule. The reformulated HOMA (rHOMA) values are close to zero, or even, they are negative for furan and its derivatives, whereas they are close to unity for benzene, benzenoid hydrocarbons and their nitrogen derivatives (pyridine, pyrimidine, s-triazine, etc.). According to the general physicochemical and chemical knowledge of aromatic systems containing heteroatoms, their HOMA index should be close to unity. Some negative rHOMA were also observed for tautomeric keto-enol systems [E.R. Raczynska, W. Kosinska, B. Osmialowski, R. Gawinecki, Chem. Rev. 105 (2005) 3561]. The main reason of these discrepancies is an application of different measures of pi-electron delocalization for CC and CX bonds. For example, already oderately delocalized molecules, H₂C=CH*CH=CH₂ * -H₂C*CH=CH*CH₂+ * +H₂C*CH=CH*CH₂- and O=CH*OH * -O*CH=OH+, were employed as reference compounds for the CC and CO bonds, whereas very weakly delocalized molecules, H₃C*NH₂ and H₂C=NH, and (CH₃)₂C=N*N(CH₃)₂ and H₃CN=NCH₃, were used for the CN and NN bonds.
Taking these observations into account, we modified the HOMA index as follows. HOMA can be estimated according to equation proposed for the reformulated index: HOMA = 1 * {**(Ropt * Ri)2}/n, where n is the number of bonds taken into account. For the systems containing the even number of atoms, the * normalization constants for CC and CX bonds can be calculated according to the procedure proposed for the rHOMA index: * = 2*[(Ropt * Rs)2 + (Ropt * Rd)2]-1, where Rs and Rd are the reference single and double CC or CX bonds, and Ropt is the optimum CC or CX bond length for full delocalized system. For the systems containing the odd number of atoms (2i+1), the * constants should be calculated from the following equation: * = (2i+1)*[(i+1)*(Ropt * Rs)2 + i*(Ropt * Rd)2]-1. The Ropt parameter can be taken for the corresponding completely delocalized system, e.g. benzene (CC bond) and s-triazine (CN bond) for six-membered systems. For the reference Rs and Rd bonds, the CC and CX bond lengths of very weakly delocalized molecules can be applied as proposed in the original HOMA procedure. Moderately delocalized molecules can be also used in particular cases. The HOMA index modified in this way we applied to some simple heteroaromatic systems including those which display prototropic tautomerism. For investigations, we used the DFT method.

Type of Paper: Review
Title: Polyanionic Hexagon: X₆^n- (X = Si, Ge)
Author: Masae Takahashi; E-mail: masae@imr.tohoku.ac.jp
Abstract: The equilibrium structure of hexasilabenzene (SiH)₆ is a chair form like non-aromatic hexane (CH₂)₆ and is different from the structure of aromatic D_6h benzene (CH)₆, although silicon belongs to the same group in periodic table as carbon. Common explanation is that the energy difference between valence s and p orbitals is mall only in first-row elements, allowing efficient hybridization and the Energy difference in higher rows is much larger. Despite of the common understanding, we recently designed an aromatic silicon six-membered ring with D_6h symmetry, where electrons are doped forming anion to stabilize the planar ring structure (M. Takahashi and Y. Kawazoe, Organometallics 2005, 24, 2433-2440). In the review, I introduce (1) the chemical background of unsaturated silicon and germanium focusing on the challenge to obtain aromatic hexagon, (2) Hückel arenes found in Zintl phase of crystal, (3) the stability of polyanionic hexagon based on the electron counting rule (Wade rule), (4) the reason to have been able to obtain planar aromatic hexagon for silicon and germanium, (5) the design of synthetic accessible aromatic hexagon, and finally (6) the conclusions and future outlook of polyanionic hexagons.

Title: On the Importance of Clar Structures of Polybenzenoid Hydrocarbons as Revealed by the π-Contribution to the Electron Localization Function
Authors: Jun Zhu¹, Joshua R. Smith^1,2, Christian Dahlstrand¹, Sébastien Villaume¹ and Henrik Ottosson¹ ; E-mail: henrik.ottosson@biorg.uu.se
Affiliations: ¹ Department of Biochemistry and Organic Chemistry, Box 576, Uppsala University, 751 23 Uppsala, Sweden
² Department of Chemistry, Humboldt State University, 1 Harpst Street, Arcata, CA 95521, USA
Abstract: The degree of aromaticity of a series of polybenzenoid hydrocarbons (PBHs) has been analyzed through properties of the π-contribution to the electron localization function (ELF_π), calculated with B3LYP hybrid density functional theory. The local aromaticities of the different six-membered rings in a PBH are analyzed via the ring-closure bifurcation values of the ELF_π (RCBV(ELF_π)) and the spans in the bifurcation values (ΔBV(ELF_π)). The results from the analysis of the ELFπ properties are compared to the qualitative description of local aromaticity of PBHs in terms of Clar’s aromatic π-sextet rule. The PBHs are analyzed both at their optimal geometries and at geometries where the CC bond lengths have been frozen so that the PBH structure corresponds to a structure with less than the maximum number of π-sextet rings.

Title: Correlation Between Electron Delocalization and Ring Current Maps in Inorganic Monocycles
Authors: Stijn Fias and Patrick Bultinck; E-mail: Patrick.Bultinck@UGent.be
Affiliation: Ghent University, Department of Inorganic and Physical Chemistry, Krijgslaan 281, 9000 Gent, Belgium
Abstract: Following up on a previous paper (De Proft, F.; Fowler, P.W.; Havenith, R.W.A.; Schlever, P.v.R.; Van Lier, G.; Geerlings, P. Chem. Eur. J., 2004, 10, 940) orbital resolved ring current maps are computed within the ipsocentric approach as well as orbital resolved multicenter indices characterizing the degree of delocalization of each orbital. Both results are compared to assess whether any link can be established between both approaches and to obtain insight in whether or not aromaticity is in this case multidimensional.

Type of Paper: Review
Title: Synthesis and Reactions of Dibenzo[a,e]pentalenes, and Aromaticity of their Derivatives
Author: Masaichi Saito
Affiliation: Department of Chemistry, Graduate School of Science and Engineering, Saitama University, Shimo-okubo, Sakura-ku, Saitama-city, Saitama 338-8570, Japan; E-Mail: masaichi@chem.saitama-u.ac.jp
Abstract: Pentalene has received recent considerable attention as a ligand of sandwich-type transition metal complexes. In contrast, dibenzopentalene, more π-extended than pentalene, has received less attention in spite of its potential usefulness as a building block of ladder-type π-conjugated molecules of growing recent interest. However, some novel efficient methods for the synthesis of dibenzopentalenes have very recently been reported. This review surveys recent advances in the synthesis of dibenzopentalenes. Aromaticity of their ionic species is also described.