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Aromaticity
Kinds of Aromaticity Selected AbstractsOn the performance of some aromaticity indices: A critical assessment using a test setJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 10 2008Ferran Feixas Abstract Aromaticity is a central chemical concept widely used in modern chemistry for the interpretation of molecular structure, stability, reactivity, and magnetic properties of many compounds. As such, its reliable prediction is an important task of computational chemistry. In recent years, many methods to quantify aromaticity based on different physicochemical properties of molecules have been proposed. However, the nonobservable nature of aromaticity makes difficult to assess the performance of the numerous existing indices. In the present work, we introduce a series of fifteen aromaticity tests that can be used to analyze the advantages and drawbacks of a group of aromaticity descriptors. On the basis of the results obtained for a set of ten indicators of aromaticity, we conclude that indices based on the study of electron delocalization in aromatic species are the most accurate among those examined in this work. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2008 [source] Strain and Hückel Aromaticity: Driving Forces for a Promising New Generation of Electron Acceptors in Organic Electronics,ANGEWANDTE CHEMIE, Issue 3 2010G. Brunetti Dr. Die Hauptmerkmale elektronenakzeptierender Materialien mit einem 9,9,-Bifluorenyliden-Gerüst sind verringerte Ringspannungen und ein Zugewinn an Aromatizität. Die Dimere (siehe Bild) zeigen eine Absorption nahe dem roten Spektralbereich (ca. 600,nm) sowie HOMO- (5.58,5.06,eV) und LUMO-Niveaus (3.37,3.09,eV), die die Materialien zusammen mit ihrer hohen Löslichkeit und thermischen Stabilität zu vielversprechenden Akzeptoren für Bulk-Heterojunction(BHJ)-Solarzellen machen. [source] ChemInform Abstract: Ab initio Study of the Geometry, Stability, and Aromaticity of the Cyclic S2N+3 Cation Isomers and Their Isoelectronic Analogues.CHEMINFORM, Issue 38 2009Guo-Hua Zhang Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] ChemInform Abstract: Structure and Aromaticity of B6H5+ Cation: A Novel Borhydride System Containing Planar Pentacoordinated BoronCHEMINFORM, Issue 23 2009Hong-Lang Yu Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] ChemInform Abstract: Reactivity, Selectivity, and Aromaticity of Be32- and Its ComplexesCHEMINFORM, Issue 19 2008Debesh Ranjan Roy Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] Evidence for d-Orbital Aromaticity in Sn- and Pb-Based Clusters: Is Sn2- 12 Aromatic?CHEMINFORM, Issue 46 2007De-Li Chen Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source] Observation of d-Orbital Aromaticity.CHEMINFORM, Issue 3 2006Xin Huang Abstract For Abstract see ChemInform Abstract in Full Text. [source] Theoretical Studies on the Structure and Aromaticity of Ti2P6+CHEMINFORM, Issue 48 2005Wei-Qi Li Abstract For Abstract see ChemInform Abstract in Full Text. [source] Novel Pyramidal MB7 (M: Li, Na, K, Rb, or Cs) Species: Structure and AromaticityCHEMINFORM, Issue 31 2004Qian-Shu Li Abstract For Abstract see ChemInform Abstract in Full Text. [source] Aromaticity of Planar B5 - Anion in the MB5 (M: Li, Na, K, Rb, and Cs) and MB5+ (M: Be, Mg, Ca, and Sr) ClustersCHEMINFORM, Issue 17 2004Qian Shu Li Abstract For Abstract see ChemInform Abstract in Full Text. [source] Stability and Aromaticity of BiNi Rings and FullerenesCHEMINFORM, Issue 4 2004Jon M. Matxain Abstract For Abstract see ChemInform Abstract in Full Text. [source] Aromaticity and Electron Affinity of Carbok -[3]radialenes, k = 0, 1, 2.CHEMINFORM, Issue 2 2004Christine Lepetit No abstract is available for this article. [source] Theoretical Study on the Aromaticity of the Pyramidal MB6 (M: Be, Mg, Ca, and Sr) Clusters.CHEMINFORM, Issue 51 2003Qian Shu Li No abstract is available for this article. [source] , Aromaticity of the Bimetallic Au5Zn+ Cluster.CHEMINFORM, Issue 21 2003Hiromasa Tanaka Abstract For Abstract see ChemInform Abstract in Full Text. [source] ChemInform Abstract: An Insight into the Aromaticity of Fullerene Anions: Experimental Evidence for Diamagnetic Ring Currents in the Five-Membered Rings of C606- and C706- .CHEMINFORM, Issue 37 2002Tamar Sternfeld Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] ChemInform Abstract: Probing the Electronic Structure and Aromaticity of Pentapnictogen Cluster Anions Pn5 - (Pn: P, As, Sb, and Bi) Using Photoelectron Spectroscopy and ab initio Calculations.CHEMINFORM, Issue 33 2002Hua-Jin Zhai Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] ChemInform Abstract: On the Aromaticity of Square Planar Ga42- and In42- in Gaseous NaGa4 - and NaIn4 - Clusters.CHEMINFORM, Issue 51 2001Aleksey E. Kuznetsov Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] ChemInform Abstract: Experimental and Theoretical Observations of Aromaticity in Heterocyclic XAl3 - (X: Si, Ge, Sn, Pb) Systems.CHEMINFORM, Issue 33 2001Xi Li Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] ChemInform Abstract: Aromaticity of [60]Fullerene Derivatives (C60Xn, X: H, F; n = 18, 36) Constrained to Have Planar Hexagonal Rings.CHEMINFORM, Issue 12 2001Samantha Jenkins Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] Demonstration of "Möbius" Aromaticity in Planar MetallacyclesCHEMISTRY - A EUROPEAN JOURNAL, Issue 26 2010Michael Mauksch Dr. Abstract Möbius aromaticity, predicted by Edgar Heilbronner in 1964, is a stabilizing effect exhibited by 4,n electron fully conjugated cyclic molecules (or transition states) with an odd number of orbital phase inversions. Although it has previously been suggested that this effect might also apply to planar metallacycles in which a transition metal employs a d orbital in ,-type binding mode, only very few examples of stable twisted molecules composed of main group elements are known. We report herein, the first computationally confirmed 4,n,, aromatic planar metallacyclic examples and their building principles. Aromatic stabilization energy (ASE) of a 8,, metalla-cycloheptatriene [Fe(CH)6H2], with four doubly occupied , orbitals and a HOMA value of +0.80 (cf. benzene=+1.0), an NICS(0) value of ,8.5 (benzene=,9.8, NICS=nucleus independent chemical shift), and with one phase inversion, is +27.5,kcal,mol,1 (about two-thirds of the value for benzene). In contrast, an unknown non-Möbius 1,4-dimetallabenzene [Fe2(CH)4H4], also with 8,, electrons, and without phase inversions, has an ASE of ,4.1,kcal,mol,1 and a NICS(0)=+15.6, indicative of antiaromaticity. Aromaticity of the proposed Möbius aromatic metallacycles is confirmed by using magnetic (NICS(0), NICS(1)zz, ,1H) and geometric (HOMA) aromaticity criteria, planarity, and near equalized CC bond lengths, bonding analysis (Wiberg bond indices, NBO, and NLMO analysis). The role of wave function boundary conditions (periodic vs. antiperiodic) in chemistry is further stressed, being equivalent to Zimmerman's concept of nodal parity for Möbius/Hückel systems. [source] Aromaticity of Tri- and Tetranuclear Metal,Carbonyl Clusters Based on Magnetic CriteriaCHEMISTRY - A EUROPEAN JOURNAL, Issue 3 2007Clémence Corminboeuf Dr. Abstract Recently, the ,-aromaticity model proposed for cyclopropane by Dewar was employed to account for the stability of Group,8 trinuclear metal,carbonyl compounds [M3(CO)12] (M=Fe, Ru, Os). This paper further examines this hypothesis and provides the first quantitative evidence for the ,-aromatic/antiaromatic nature of the [M3(CO)12]/[M4(CO)16] species based on structural and nucleus-independent chemical-shift analysis. In addition, the extent of electron delocalization in tetrahedral [M4(CO)14] and butterfly [M4(CO)15] is analyzed and compared to prototype cycloalkanes. While remarkable analogies exist between metal,carbonyls and cycloalkanes, transition metals provide additional overlap possibilities that affect both the ring strain and the magnetic properties of metal,carbonyl rings and cages. [source] The Interplay of Inverted Redox Potentials and Aromaticity in the Oxidized States of New ,-Electron Donors: 9-(1,3-Dithiol-2-ylidene)fluorene and 9-(1,3-Dithiol-2-ylidene)thioxanthene DerivativesCHEMISTRY - A EUROPEAN JOURNAL, Issue 12 2006Samia Amriou Dr. Abstract Derivatives of 9-(1,3-dithiol-2-ylidene)fluorene (9) and 9-(1,3-dithiol-2-ylidene)thioxanthene (10) have been synthesised using Horner,Wadsworth,Emmons reactions of (1,3-dithiol-2-yl)phosphonate reagents with fluorenone and thioxanthen-9-one. X-ray crystallography, solution electrochemistry, optical spectroscopy, spectroelectrochemistry and simultaneous electrochemistry and electron paramagnetic resonance (SEEPR), combined with theoretical calculations performed at the B3P86/6-31G** level, elucidate the interplay of the electronic and structural properties in these molecules. These compounds are strong two-electron donors, and the oxidation potentials depend on the electronic structure of the oxidised state. Two, single-electron oxidations (<) were observed for 9-(1,3-dithiol-2-ylidene)fluorene systems (9). In contrast, derivatives of 9-(1,3-dithiol-2-ylidene)thioxanthene (10) display the unusual phenomenon of inverted potentials (>) resulting in a single, two-electron oxidation process. The latter is due to the aromatic structure of the thioxanthenium cation (formed on the loss of a second electron), which stabilises the dication state (102+) compared with the radical cation. This contrasts with the nonaromatic structure of the fluorenium cation of system 9. The two-electron oxidation wave in the thioxanthene derivatives is split into two separate one-electron waves in the corresponding sulfoxide and sulfone derivatives 27,29 owing to destabilisation of the dication state. [source] Ring Currents as Probes of the Aromaticity of Inorganic Monocycles : P5,, As5,, S2N2, S3N3,, S4N3+, S4N42+, S5N5+, S42+ and Se42+CHEMISTRY - A EUROPEAN JOURNAL, Issue 4 2004Frank De Proft Prof. Abstract Current-density maps were calculated by the ipsocentric CTOCD-DZ/6-311G** (CTOCD-DZ=continuous transformation of origin of current density-diamagnetic zero) approach for three sets of inorganic monocycles: S42+, Se42+, S2N2, P5, and As5, with 6 , electrons; S3N3,, S4N3+ and S4N42+ with 10 , electrons; and S5N5+ with 14 , electrons. Ipsocentric orbital analysis was used to partition the currents into contributions from small groups of active electrons and to interpret the contributions in terms of symmetry- and energy-based selection rules. All nine systems were found to support diatropic , currents, reinforced by , circulations in P5,, As5,, S3N3,, S4N3+, S4N42+ and S5N5+, but opposed by them in S42+, Se42+ and S2N2. The opposition of , and , effects in the four-membered rings is compatible with height profiles of calculated NICS (nucleus-independent chemical shifts). [source] Computational Investigation of Hydrogen Adsorption by Alkali-Metal-Doped Organic Molecules: Role of AromaticityCHEMPHYSCHEM, Issue 2 2009Kancharlapally Srinivasu Abstract Hydrogen storage: Simple organic molecular systems (CnHn, n=4, 5, 6, 8) are proposed for hydrogen storage purposes based on the concept of aromaticity. The adsorption of hydrogen is attributed to pronounced charge transfer from the sodium atom (green, see picture) to the organic systems and the electrostatic interaction between the ion and hydrogen molecules. Theoretical studies on hydrogen adsorption in small organic molecular systems, such as cyclobutadiene (C4H4), the cyclopentadienyl radical (C5H5), benzene (C6H6), and cyclooctatetraene (C8H8) and their metal-doped modifications, are carried out. Our results reveal that the simple van der Waals surfaces of pure organic molecules are not good enough for hydrogen adsorption due to the weak interaction between hydrogen molecules and the organic molecular surface. However, doping of alkali-metal atoms in the above organic molecular systems increases their hydrogen adsorption ability significantly, mainly due to electron transfer from the metal atom to the carbon surface. This charged surface created around the metal atom is found to enhance the hydrogen adsorption capacity of the complex considerably, both in terms of interaction energy and the number of adsorbed hydrogen molecules, with a hydrogen adsorption capacity ranging from 10 to 12 wt,%. The role of aromaticity in such molecular systems is important in stabilizing these ionized organo-alkali-metal complexes. [source] Novel Superalkali Superhalogen Compounds (Li3)+(SH), (SH=LiF2, BeF3, and BF4) with Aromaticity: New Electrides and AlkalidesCHEMPHYSCHEM, Issue 5 2006Fang-Fang Wang Dr. Abstract Optimized structures, with all real frequencies, of superalkali superhalides (Li3)+(SH), (SH=LiF2, BeF3, and BF4), are obtained, for the first time, at the B3LYP/aug-cc-pVDZ and MP2/aug-cc-pVDZ computational levels. These superalkali superhalides possess three characteristics that are significantly different from normal alkali halides. 1) They have a variety of structures, which come from five bonding mode types: edge,face, edge,edge, face,face, face,edge, and staggered face,edge. We find that the bonding mode type closely correlates with the Li3SH bond energy. 2) The valence electrons on the Li3 ring are pushed out by the (SH), anion, and become excess electrons, conferring alkalide or electride characteristics on these Li3SH species, depending on the bonding mode type. 3) The highest occupied molecular orbital of each Li3SH species is a doubly occupied delocalized , bonding orbital on the Li3 ring, which indicates its aromaticity. It is noticeable that the maximum negative nucleus-independent chemical shift value (about ,10 ppm) moves out from the center of the Li3 ring, owing to repulsion by the SH, anion. We find that these superalkali superhalides are not only complicated "supermolecules", but are also a new type of alkalide or electride, with aromaticity. [source] The Origin of Aromaticity: Important Role of the Sigma Framework in BenzeneCHEMPHYSCHEM, Issue 9 2004Borislav Kova Abstract The physical nature of aromaticity is addressed at a high ab initio level. It is conclusively shown that the extrinsic aromatic stabilization energy of benzene E(ease)B, estimated relative to its linear polyene counterpart(s), is very well-reproduced at the Hartree,Fock (HF) level. This is a consequence of the fact that the contributions arising from the zero-point vibrational energy (ZPVE) and electron correlation are rather small. More specifically, they yield together 2.0 kcal,mol,1to the destabilization of benzene. A careful scrutiny of the HF energies by virial theorem shows further that the kinetic energies of the , and , electrons E(T) and E(T) are strictly additive in the gauge linear zig-zag polyenes, which also holds for their sum Et(T)HF. This finding has the important corollary that E(ease)Bis little dependent on the choice of the homodesmic reactions involving zig-zag polyenes. A detailed physical analysis of the ,- and ,-electron contributions to extrinsic aromaticity requires explicit introduction of the potential energy terms Vne, Vee, and Vnn, which signify Coulomb interactions between the electrons and the nuclei. The Veeterm involves repulsive interaction V between the , and , electrons, which cannot be unequivocally resolved into , and , contributions. The same holds for the Vnnenergy, which implicitly depends on the electron density distribution via the Born,Oppenheimer (BO) potential energy surface. Several possibilities for partitioning V and Vnnterms into , and , components are examined. It is argued that the stockholder principle is the most realistic, which strongly indicates that E(ease)Bis a result of favorable , -framework interactions. In contrast, the ,-electron framework prefers the open-chain linear polyenes. [source] Impact of Kerogen Heterogeneity on Sorption of Organic Pollutants.ENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 8 2009Abstract The overall goal of the present study was to establish correlations between organic pollutant sorption and physicochemical properties of kerogen materials. Three coal samples, each representing a typical kerogen type, were used as the starting materials. A thermal technique was employed to treat the kerogen materials under seven different temperatures ranging from 200 to 500C to simulate different diagenetic history. These samples were systematically characterized for their chemical compositions, functionalities, physical rigidity, and optical properties. The results showed that the chemical, spectroscopic, and optical microscopic properties of each kerogen series changed consistently as a function of treatment temperature or kerogen maturation. The oxygen-to-carbon atomic ratio decreased from 0.29, 0.12, and 0.07 for the original lignite (XF0), fusinite (HZ0), and lopinite (LP0) samples, respectively, to 0.07, 0.06, and 0.04 for XF7, HZ7, and LP7, respectively, that underwent the highest temperature treatment. The hydrogen-to-carbon atomic ratio exhibited similar reducing trend, which is consistent with the aromaticity increasing from 45 to 58% of the original samples to 76 to 81% of highly mature samples. Under the fluorescence microscope, the organic matrix changed from yellow (original lignite sample) and red-brown (original lopinite sample) to colorless for the samples of higher maturation. The measured reflecting index increased from the original samples to the highly mature samples. Moreover, the original and the slightly matured samples exhibited very different chemical compositions and structural units among the three types due to the difference in their source materials. As the kerogen maturation increased, such differences decreased, indicating highly mature kerogen became homogenized regardless of the source material. [source] Availability of polycyclic aromatic hydrocarbons from lampblack-impacted soils at former oil-gas plant sites in California, USAENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 3 2007Lei Hong Abstract Lampblack-impacted soils at former oil-gas plant sites in California, USA, were characterized to assess the sorption of polycyclic aromatic hydrocarbons (PAHs) and the concentration-dependent effects of a residual oil tar phase on sorption mechanism and availability of PAHs. Nuclear magnetic resonance spectroscopy demonstrated similar aromaticity for both lampblack carbon and the oil tar phase, with pronounced resonance signals in the range of 100 to 150 ppm. Scanning-electron microscopic images revealed a physically distinct oil tar phase, especially at high concentrations in lampblack, which resulted in an organic-like film structure when lampblack particles became saturated with the oil tar. Sorption experiments were conducted on a series of laboratory-prepared lampblack samples to systematically evaluate influences of an oil tar phase on PAH sorption to lampblack. Results indicate that the sorption of PAHs to lampblack exhibits a competition among sorption phases at low oil tar contents when micro- and mesopores are accessible. When the oil tar content increases to more than 5 to 10% by weight, this tar phase fills small pores, reduces surface area, and dominates PAH sorption on lampblack surface. A new PAH partitioning model, Kd = KLB-C(1- ftar)° + ftarKtar (, = empirical exponent), incorporates these effects in which the control of PAH partitioning transits from being dominated by sorption in lampblack (KLB-C) to absorption in oil tar (Ktar), depending on the fraction of tar (ftar). This study illustrates the importance of understanding interactions among PAHs, oil tar, and lampblack for explaining the differences in availability of PAHs among site soils and, consequently, for refining site-specific risk assessment and establishing soil cleanup levels. [source] Cubane-Like Bismuth-Iron Cluster: Synthesis, X-ray Crystal Structure and Theoretical Characterization of the [Bi4Fe8(CO)28]4, AnionEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 20 2010Kirill Yu. Abstract The reaction of cyclo -Bi4[Si(SiMe3)3]4 (1) with Na2[Fe(CO)4] in the presence of nBu4NCl leads to the formation of the cage compound [nBu4N]4[Bi4Fe8(CO)28] (2). According to X-ray single-crystal structure analysis, the faces of the tetrahedral Bi4 core are capped by Fe(CO)3 moieties in a ,3 fashion to give a cubanoid Bi4Fe4 framework. The four Fe(CO)4 fragments are ,1 -coordinated to bismuth, each. With 12 skeletal electron pairs the [Bi4Fe8(CO)28]4, anion (2a) is a Bi4Fe4 cubane. The negative charge is localized within cluster 2a according to the NBO analysis of its derivatives. The strength of metal,ligand interactions Bi,,3 -Fe(CO)3 is responsible for the size of the cluster's cubic core. NICS computations at the cage centers of considered molecules show that 2a has paratropic character, whereas removal of four ,1 -Fe(CO)4 fragments from latter causes spherical aromaticity of the modified clusters [Bi4Fe4(CO)12]4, (2aa) and [Bi4Fe4(CO)12]2+ (2ab), mediated by a Bi4 cluster , orbital. [source] Dual Supermesityl Stabilization: A Room-Temperature-Stable 1,2,4-Triphosphole Radical, Sigmatropic Hydrogen Rearrangements, and Tetraphospholide Anion,EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 15 2008Alex S. Ionkin Abstract Cesium 3,5-bis(2,4,6-tri- tert -butylphenyl)-1,2,4-triphospholide (12) and cesium 5-(2,4,6-tri- tert -butylphenyl)tetraphospholide (13) were synthesized and isolated with flat five-membered rings, which are an indication of the aromaticity in these anions. Compound 13 is the first example of a stable tetraphospholide anion, which is structurally characterized. Kinetic stabilization of the 1,2,4-triphospholide system by two supermesityl groups resulted in the detection of the room-temperature-stable radical 17 and the observation of a series of successive sigmatropic hydrogen shifts in the first stable 1H -1,2,4-triphosphole 14 with a P,H bond.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] | |||||||||||||||||||||||||||||||