http://www.mdpi.com/journal/symmetry/special_issues/aromaticity/ {snippet name="submission_info"} 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. http://www.mdpi.com/2073-8994/2/4/1846/ The paper introduces general considerations on structural properties of aromatic, antiaromatic and non-aromatic conjugated systems in terms of potential energy along bond length alternation and distortion coordinates, taking as examples benzene, cyclobutadiene and cyclooctatetraene. Pentalene, formally derived from cyclooctatetraene by cross linking, is also considered as a typical antiaromatic system. The main interest is concerned with [n]annulenes and model [n]annulene molecular systems, n ranging from 10 to 18. The rich variety of conformational and  configurational isomers and of dynamical processes among them is described. Specific attention is devoted to bridged [10]- and [14]annulenes in the ground and lowest excited states as well as to s-indacene and biphenylene. Experimental data obtained from vibrational and electronic spectroscopies are discussed and compared with ab initio calculation results. Finally, porphyrin, tetraoxaporphyrin dication and diprotonated porphyrin are presented as annulene structures adopting planar/non-planar geometries depending on the steric hindrance in the inner macrocycle ring. Radiative and non-radiative relaxation processes from excited state levels have been observed by means of time-resolved fluorescence and femtosecond transient absorption spectroscopy. A short account is also given of porphycene, the structural isomer of porphyrin, and of porphycene properties. http://www.mdpi.com/2073-8994/2/4/1846/ Fri, 05 Nov 2010 00:00:00 CET Symmetry 2010-11-05 2 4 Article 1846 1924 2073-8994 Structures of Annulenes and Model Annulene Systems in the Ground and Lowest Excited States 2010-11-05 doi: 10.3390/sym2041846 Cristina Gellini Pier Remigio Salvi http://www.mdpi.com/2073-8994/2/4/1745/ The paper reviews the polyanionic hexagons of silicon and germanium, focusing on aromaticity. The chair-like structures of hexasila- and hexagermabenzene are similar to a nonaromatic cyclohexane (CH2)6 and dissimilar to aromatic D6h-symmetric benzene (CH)6, although silicon and germanium are in the same group of the periodic table as carbon. Recently, six-membered silicon and germanium rings with extra electrons instead of conventional substituents, such as alkyl, aryl, etc., were calculated by us to have D6h symmetry and to be aromatic. We summarize here our main findings and the background needed to reach them, and propose a synthetically accessible molecule. http://www.mdpi.com/2073-8994/2/4/1745/ Thu, 30 Sep 2010 00:00:00 CEST Symmetry 2010-09-30 2 4 Review 1745 1762 2073-8994 Polyanionic Hexagons: X6n– (X = Si, Ge) 2010-09-30 doi: 10.3390/sym2041745 Masae Takahashi http://www.mdpi.com/2073-8994/2/3/1653/ The degree of p-electron (de)localization and aromaticity of a series of polybenzenoid hydrocarbons (PBHs) has been analyzed through the π-contribution to the electron localization function (ELFπ), calculated at the B3LYP/6-311G(d,p) hybrid density functional theory level. The extent of p-electron delocalization in the various hexagons of a PBH was determined through analysis of the bifurcation values of the ELFp basins (BV(ELFp)), the spans in the bifurcation values in each hexagon (ΔBV(ELFπ)), and the ring-closure bifurcation values of the ELFπ (RCBV(ELFπ)). These computed results were compared to the qualitative description of local aromaticities of the different hexagons in terms of Clar structures with p-sextets. Benzene, [18]annulene, and thirty two PBHs were analyzed at their equilibrium geometries, and benzene and triphenylene were also analyzed at bond length distorted structures. In general, the description of PBHs in terms of Clar valence structures is supported by the ELFp properties, although there are exceptions. For PBHs at their equilibrium geometries there is a clear sigmoidal relationship between the CC bond lengths and the amount of p-electron (de)localization at these bonds, however, this relationship is lost for bond distorted geometries. In the latter cases, we specifically examined benzene in D3h symmetric “1,3,5-cyclohexatriene” structures and triphenylene in eight different structures. From the distorted benzenes and triphenylenes it becomes clear that there is a distinct tendency for the p-electron network to retain delocalization (aromaticity). The ELFp analysis thus reveals an antidistortive rather than a distortive behavior of the p-electrons in these investigated compounds. http://www.mdpi.com/2073-8994/2/3/1653/ Fri, 20 Aug 2010 00:00:00 CEST Symmetry 2010-08-20 2 3 Article 1653 1682 2073-8994 On the Importance of Clar Structures of Polybenzenoid Hydrocarbons as Revealed by the π-Contribution to the Electron Localization Function 2010-08-20 doi: 10.3390/sym2031653 Jun Zhu Christian Dahlstrand Joshua R. Smith Sébastien Villaume Henrik Ottosson http://www.mdpi.com/2073-8994/2/3/1485/ The HOMA (Harmonic Oscillator Model of Aromaticity) index, reformulated in 1993, has been very often applied to describe π-electron delocalization for mono- and polycyclic π-electron systems. However, different measures of π-electron delocalization were employed for the CC, CX, and XY bonds, and this index seems to be inappropriate for compounds containing heteroatoms. In order to describe properly various resonance effects (σ-π hyperconjugation, n-π conjugation, π-π conjugation, and aromaticity) possible for heteroatomic π-electron systems, some modifications, based on the original HOMA idea, were proposed and tested for simple DFT structures containing C, N, and O atoms. An abbreviation HOMED was used for the modified index. http://www.mdpi.com/2073-8994/2/3/1485/ Mon, 12 Jul 2010 00:00:00 CEST Symmetry 2010-07-12 2 3 Article 1485 1509 2073-8994 On the Harmonic Oscillator Model of Electron Delocalization (HOMED) Index and its Application to Heteroatomic π-Electron Systems 2010-07-12 doi: 10.3390/sym2031485 Raczyńska Hallman Kolczyńska Stępniewski http://www.mdpi.com/2073-8994/2/3/1390/ An efficient algorithm leading to the Fries canonical structure is presented for benzenoid hydrocarbons. This is a purely topological approach, which is based on adjacency matrices and the Hadamard procedure of matrix multiplication. The idea is presented for naphthalene, as an example. The Fries canonical-structures are also derived for anthracene, coronene, triphenylene, phenanthrene, benz[a]pyrene, and one large benzenoid system. The Fries concept can be convenient for obtaining Clar structures with the maximum number of sextets, which in turn effectively represent π-electron (de)localization in benzenoid hydrocarbons. http://www.mdpi.com/2073-8994/2/3/1390/ Tue, 06 Jul 2010 00:00:00 CEST Symmetry 2010-07-06 2 3 Article 1390 1400 2073-8994 How to Find the Fries Structures for Benzenoid Hydrocarbons 2010-07-06 doi: 10.3390/sym2031390 Ciesielski Krygowski Cyrański http://www.mdpi.com/2073-8994/2/2/1156/ The lack of reference aromatic systems in the realm of inorganic aromatic compounds makes the evaluation of aromaticity in all-metal and semimetal clusters a difficult task. To date, calculation of nucleus-independent chemical shifts (NICS) has been the most widely used method to discuss aromaticity in these systems. In the first part of this work, we briefly review our previous studies, showing some pitfalls of the NICS indicator of aromaticity in organic molecules. Then, we refer to our study on the performance of some aromaticity indices in a series of 15 aromaticity tests, which can be used to analyze the advantages and drawbacks of aromaticity descriptors. It is shown that indices based on the study of electron delocalization are the most accurate among those analyzed in the series of proposed tests, while NICS(1)zz and NICS(0)πzz present the best behavior among NICS indices. In the second part, we discuss the use of NICS and electronic multicenter indices (MCI) in inorganic clusters. In particular, we evaluate the aromaticity of two series of all-metal and semimetal clusters with predictable aromaticity trends by means of NICS and MCI. Results show that the expected trends are generally better reproduced by MCI than NICS. It is concluded that NICS(0)π and NICS(0)πzz are the kind of NICS that perform the best among the different NICS indices analyzed for the studied series of inorganic compounds. http://www.mdpi.com/2073-8994/2/2/1156/ Mon, 14 Jun 2010 00:00:00 CEST Symmetry 2010-06-14 2 2 Review 1156 1179 2073-8994 A Critical Assessment of the Performance of Magnetic and Electronic Indices of Aromaticity 2010-06-14 doi: 10.3390/sym2021156 Solà Feixas Jiménez-Halla Matito Poater http://www.mdpi.com/2073-8994/2/1/418/ Through-space isotropic NMR shielding values of a perpendicular diatomic hydrogen probe moved in a 0.5 Å grid 2.5 Å above several polycyclic aromatic/antiaromatic ring and aromatic/aromatic hydrocarbons were computed with Gaussian 03 at the GIAO HF/6-31G(d,p) level. Combinations of benzene fused with cyclobutadiene, with the tropylium ion, and with the cyclopentadienyl anion were investigated. Subtraction of the isolated H2 isotropic value gave shielding increments(Δσ), which, when plotted against Cartesian coordinates of the probe over each hydrocarbon, gave representations of three-dimensional isotropic shielding increment surfaces. The results are related to the degree of bond length alternation, the extent of electron delocalization, and (for the ions) the NPA charge distribution. The shielding increment data are compared to NICS(1) values computed at the same level; both indicate the degree of aromaticity or antiaromaticity of the component rings. http://www.mdpi.com/2073-8994/2/1/418/ Mon, 22 Mar 2010 00:00:00 CET Symmetry 2010-03-22 2 1 Article 418 436 2073-8994 Computed NMR Shielding Effects over Fused Aromatic / Antiaromatic Hydrocarbons 2010-03-22 doi: 10.3390/sym2010418 Martin Teague Mills http://www.mdpi.com/2073-8994/2/1/284/ The NICSzz-scan curves of aromatic organic, inorganic and “all-metal” molecules in conjunction with symmetry-based selection rules provide efficient diagnostic tools of the σ-, π- and/or double (σ + π)-aromaticity. The NICSzz-scan curves of σ-aromatic molecules are symmetric around the z-axis, having half-band widths approximately less than 3 Å with the induced diatropic ring current arising from Tx,y-allowed transitions involving exclusively σ-type molecular orbitals. Broad NICSzz-scan curves (half-band width approximately higher than 3 Å) characterize double (σ + π)-aromaticity, the chief contribution to the induced diatropic ring current arising from Tx,y-allowed transitions involving both σ- and π-type molecular orbitals. NICSzz-scan curves exhibiting two maxima at a certain distance above and below the molecular plane are typical for (σ + π)-aromatics where the π-diatropic ring current overwhelms the σ-type one. In the absence of any contribution from the σ-diatropic ring current, the NICSzz(0) value is close to zero and the molecule exhibits pure π-aromaticity. http://www.mdpi.com/2073-8994/2/1/284/ Mon, 15 Mar 2010 00:00:00 CET Symmetry 2010-03-15 2 1 Article 284 319 2073-8994 Diagnosis of the σ-, π- and (σ+π)-Aromaticity by the Shape of the NICSzz-Scan Curves and Symmetry-Based Selection Rules 2010-03-15 doi: 10.3390/sym2010284 Tsipis Depastas Tsipis http://www.mdpi.com/2073-8994/2/1/76/ Cyclooctatetraene (COT), the first 4nπ-electron system to be studied, adopts an inherently nonplanar tub-shaped geometry of D2d symmetry with alternating single and double bonds, and hence behaves as a nonaromatic polyene rather than an antiaromatic compound. Recently, however, considerable 8π-antiaromatic paratropicity has been shown to be generated in planar COT rings even with the bond alternated D4h structure. In this review, we highlight recent theoretical and experimental studies on the antiaromaticity of hypothetical and actual planar COT. In addition, theoretically predicted triplet aromaticity and stacked aromaticity of planar COT are also briefly described. http://www.mdpi.com/2073-8994/2/1/76/ Fri, 05 Feb 2010 00:00:00 CET Symmetry 2010-02-05 2 1 Review 76 97 2073-8994 Recent Studies on the Aromaticity and Antiaromaticity of Planar Cyclooctatetraene 2010-02-05 doi: 10.3390/sym2010076 Tohru Nishinaga Takeshi Ohmae Masahiko Iyoda http://www.mdpi.com/2073-8994/1/2/226/ We have studied a series of bridged phenylacetylene macrocycles with topologies based on Platonic and Archimedean polyhedra, using density functional calculations to determine both their molecular structure and their electronic response to external magnetic fields (NICS maps). We are able to elucidate the interplay of aromaticity and anti-aromaticity as a function of structural parameters, in particular the symmetry properties of the intramolecular bond connectivities, in these compounds. http://www.mdpi.com/2073-8994/1/2/226/ Fri, 11 Dec 2009 00:00:00 CET Symmetry 2009-12-11 1 2 Article 226 239 2073-8994 Polyhedral Phenylacetylenes: The Interplay of Aromaticity and Antiaromaticity in Convex Graphyne Substructures 2009-12-11 doi: 10.3390/sym1020226 Daniel Sebastiani Matt A. Parker