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Electron Delocalization (electron + delocalization)
Selected AbstractsOrigin of the Paramagnetic Properties of the Mixed-Valence Polyoxometalate [GeV14O40]8, Reduced by Two Electrons: Wave Function Theory and Model Hamiltonian CalculationsEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 34 2009N. Suaud Abstract The aim of the work is to give an explanation of the magnetic properties of a mixed-valence [GeV14O40]8, polyoxometalate reduced by two electrons, which, in contrast to what happens in other two-electron-reduced polyoxometalates, does not show any magnetic coupling between the two unpaired electrons. For this purpose, a quantitative evaluation of the microscopic electronic parameters (electron transfer, magnetic coupling, magnetic orbital energy, and Coulomb repulsion) of the mixed-valence polyoxometalate cluster is performed. The parameters are extracted from valence-spectroscopy large configuration interaction (CI) calculations on embedded fragments. Then, these parameters are used in an extended t - J model Hamiltonian suited to model the properties of the whole anion. The analysis of the wave functions of the lowest singlet and triplet states and of the microscopic parameters emphasizes that the electron delocalization in this mixed-valence cluster is such that each unpaired electron is almost trapped in a different half of the polyoxovanadate, thus disabling any exchange interaction between them.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] Proximate, "Parallel-In-Plane" Preoriented Bis(diazenes) , In-Plane Delocalized Bis(homoconjugated) 4N/5(6)e AnionsEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 21 2003Eberhard Beckmann Abstract Synthetic details are presented for a series of more or less rigid, "parallel-in-plane" preoriented bis(diazenes), with N=N/N=N distances (d) of 3.3,2.9 Å and interorbital angles (,) of 142,164° (X-ray crystal structures). DFT calculations (B3LYP/6,31G*) and one-/two-electron reduction experiments with the two least preoriented, most "distant" bis(diazenes) (dN=N/N=N ca. 3.3 Å; , 142,146°) provide more insight into the structural prerequisites for bis(homoconjugative) in-plane electron delocalization in 4N/5e radical anions and 4N/6e dianions. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] Asymmetric Activation/Deactivation of Racemic Ru Catalysts for Highly Enantioselective Hydrogenation Irrespective of Ketonic Substrates: Molecular Design of Dimethylbinaphthylamine for Enantiomeric Catalysts DiscriminationADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 1-2 2003Koichi Mikami Abstract Asymmetric activation and deactivation of racemic catalysts are two extremes in asymmetric catalysis. In a combination of these two protocols, higher enantioselectivity can be achieved by maximizing the difference in catalytic activity between the enantiomers of racemic catalysts through selective activation and deactivation of enantiomeric catalysts. 3,3,-Dimethyl-2,2,-diamino-1,1,-binaphthyl (DM-DABN) is thus designed as a chiral poison (deactivator) for complete enantiomer resolution of racemic BINAP-Ru(II) catalysts. The catalyst system of DM-DABN, 1,2-diphenylethylenediamine (DPEN), and racemic BINAP-Ru(II) led to great success in highly enantioselective hydrogenation irrespective of the ketonic substrates. The lower catalytic activity of the BINAP-Ru(II)/DM-DABN complex stems from the electron delocalization from the Ru center to the diamine moiety in contrast to the BINAP-Ru(II)/DPEN complex where the highest electron densities are localized on the Ru-N region. The present ,asymmetric activation/deactivation protocol' can provide a guiding principle for the rational design of a molecule for enantiomeric discrimination between racemic catalysts. [source] Perfect planar tetracoordinate carbon in neutral unsaturated hydrocarbon cages: A new strategy utilizing three-dimensional electron delocalizationJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 13 2009Yang Wang Abstract A new series of unsaturated pure and boron-substituted hydrocarbons containing a perfect planar tetracoordinate carbon (ptC) have been proposed by performing density functional computations. The ptC is effectively stabilized through three-dimensional delocalization of ptC's lone pair into ,-conjugated systems, by utilizing a new strategy opening a brand new way of designing ptC structures. Compared to previously proposed ptC-containing hydrocarbon cage compound, a neutral hydrocarbon designed here might be a more viable target for synthetic attempts. © 2009 Wiley Periodicals, Inc. J Comput Chem 2009 [source] On 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] Chemical graph theory and n -center electron delocalization indices: A study on polycyclic aromatic hydrocarbonsJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 10 2007Marcos Mandado Abstract Relations between aromaticity indices derived from chemical graph theory and those based on 6-center electron delocalization are investigated for a series of polybenzenoid hydrocarbons. Aromatic stabilization obtained by means of the effective scaled electron delocalization is highly correlated to the resonance energy, RE, obtained both from SCF MO calculations and conjugated ring circuits model. Local aromaticity of benzene rings is discussed using two different criteria, in one of them aromaticity is just given by the cyclic , -electron conjugation of the ring, whereas terms involving more than one ring are also considered in the other one. Indices derived from chemical graph theory and those obtained from the 6-center electron delocalization give rise to the same local aromaticity. Moreover, 6-center electron delocalization provides more quantitative information. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2007 [source] New insights on the bridge carbon,carbon bond in propellanes: A theoretical study based on the analysis of the electron localization functionJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 5 2007Victor Polo Abstract The nature of the bonding between bridgehead carbon atoms (Ca, Ca,) as well as the ring strain in a family of 10 propellanes formed by three-, four-, or five-member rings: [1.1.1] (I), [2.1.1] (II), [3.1.1] (III), [2.2.1] (IV), [3.2.1] (V), [2.2.2] (VI), [3.3.1] (VII), [3.2.2] (VIII), [3.3.2] (IX), and [3.3.3] (X) are studied by means of the electron localization function (ELF) at the DFT level (B3LYP/cc-pVTZ). The ELF analysis of smaller propellanes (I, II, and III) reveals the coexistence of two resonance forms: one with a nonbonding electron pair partially delocalized between Ca and Ca, atoms outside the cage (ionic) and the other with a bridge bond between the same atoms (covalent). The weights of each form are calculated according to the ELF-basin populations, yielding 94, 88, and 53% for the ionic structure of I, II, and III, respectively, while larger propellanes (IV,X) present only the covalent form. The question of the s-character of the bridge bond is addressed by dissecting the bridge-bond ELF basin into the molecular orbital contributions. Finally, ,-aromaticity associated to surface electron delocalization has been analyzed by means of nucleus-independent chemical shift (NICS) calculations. The results point out that the stability of the fused ring structure of propellanes I, II, and III, can be assigned to the remarkable ,-aromaticity of the involved three-member rings. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2007 [source] The existence of secondary orbital interactionsJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 1 2007Chaitanya S. Wannere Abstract B3LYP/6-311+G** (and MP2/6-311+G**) computations, performed for a series of Diels-Alder (DA) reactions, confirm that the endo transition states (TS) and the related Cope-TSs are favored energetically over the respective exo -TSs. Likewise, the computed magnetic properties (nucleus-independent chemical shifts and magnetic susceptibililties) of the endo - (as well as the Cope) TS's reveal their greater electron delocalization and greater aromaticity than the exo -TS's. However, Woodward and Hoffmann's original example is an exception: their endo -TS model, involving the DA reaction of a syn - with an anti -butadiene (BD), actually is disfavored energetically over the corresponding exo -TS; magnetic criteria also do not indicate the existence of SOI delocalization in either case. Instead, a strong energetic preference for endo -TSs due to SOI is found when both BDs are in the syn conformations. This is in accord with Alder and Stein's rule of "maximum accumulation of double bonds:" both the dienophile and the diene should have syn conformations. Plots along the IRC's show that the magnetic properties typically are most strongly exalted close to the energetic TS. Because of SOI, all the points along the endo reaction coordinates are more diatropic than along the corresponding exo pathways. We find weak SOI effects to be operative in the endo -TSs involved in the cycloadditions of cyclic alkenes, cyclopropene, aziridine, cyclobutene, and cyclopentene, with cyclopentadiene. While the endo -TSs are only slightly lower in energy than the respective exo -TSs, the magnetic properties of the endo -TS's are significantly exalted over those for the exo -TS's and the Natural Bond Orbitals indicate small stabilizing interactions between the methylene cycloalkene hydrogen orbitals (and lone pairs in case of aziridine) with ,-character and the diene , MOs. © 2006 Wiley Periodicals, Inc. J Comput Chem 2007 [source] Theoretical studies on the role of ,-electron delocalization in determining the conformation of N-benzylideneaniline with three types of LMO basis setsJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 7 2006Peng Bao Abstract To understand the role of ,-electron delocalization in determining the conformation of the NBA (Ph,NCH,Ph) molecule, the following three LMO (localized molecular orbital) basis sets are constructed: a LFMO (highly localized fragment molecular orbital), an NBO (natural bond orbital), and a special NBO (NBO-II) basis sets, and their localization degrees are evaluated with our suggesting index DL. Afterward, the vertical resonance energy ,EV is obtained from the Morokuma's energy partition over each of three LMO basis sets. ,EV = ,EH (one electron energy) + ,Etwo (two electron energy), and ,Etwo = ,ECou (Coulomb) + ,Eex (exchange) + ,Eec (or ,,En) (electron correction). ,EH is always stabilizing, and ,ECou is destabilizing for all time. In the case of the LFMO basis set, ,ECou is so great that ,Etwo > |,EH|. Therefore, ,EV is always destabilizing, and is least destabilizing at about the , = 90° geometry. Of the three calculation methods such as HF, DFT, and MPn (n = 2, 3, and 4), the MPn method provides ,EV with the greatest value. In the case of the NBO basis set, on the contrary, ,EV is stabilizing due to ,ECou being less destabilizing, and it is most stabilizing at a planar geometry. The LFMO basis set has the highest localization degree, and it is most appropriate for the energy partition. In the NBA molecule, ,-electron delocalization is destabilization, and it has a tendency to distort the NBA molecular away from its planar geometry as far as possible. © 2006 Wiley Periodicals, Inc. J Comput Chem 27: 809,824, 2006 [source] Substituent effect on local aromaticity in mono and di-substituted heterocyclic analogs of naphthaleneJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 5 2010Afshan Mohajeri Abstract A quantitative study on local aromaticity has been performed on a series of mono- and di-substituted biheterocycles (quinoline, isoquinoline, quinoxaline, quinazoline). Three electronically based indices (PDI, ATI, and FLU) have been employed to investigate the substituent effect on the , -electron delocalization in both heterocycle and benzenoid rings. Three typical substituents (Cl, OCH3, and CN) with different inductive and resonance power have been selected. Generally, substituent causes a reduction in aromaticity irrespective of whether it is electron attracting or electron donating. It is shown that the maximum aromaticity exhibits a similar trend of Cl,>,CN,>,OCH3 for all the studied rings. Moreover, it is found that the substituent situation with respect to the heteroatom has a significant influence on the aromaticity. It results from our study that in di-substituted derivatives, irrespective of whether the two substituents form a meta or para isomer, they preferably choose the position which leads to the maximum aromaticity character. Copyright © 2009 John Wiley & Sons, Ltd. [source] Hydrogen bonds assisted by ,-electron delocalization , the influence of external intermolecular interactions on dimer of formic acidJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 7-8 2008awomir J. Grabowski Abstract MP2 and DFT calculations with the use of 6-311++G(d,p) basis set were carried out to study formic acid dimer as well as this species interacting with additional Lewis acids such as HF, Li+ and Na+. These Lewis acids were positioned near carbonyl or hydroxyl oxygen atoms and their influence on geometrical and other parameters of formic acid dimer was analysed. Additionally the ,quantum theory of atoms in molecules' (QTAIM) was applied as well as the ,natural bond orbitals' (NBO) method. Numerous correlations between geometrical, QTAIM and energetic parameters were found. It was found that ,-electron delocalization is not always connected with the enhancement of H-bond strength. Copyright © 2008 John Wiley & Sons, Ltd. [source] Ab initio study on N,N,,N, -triaminoguanidineJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 12 2007Pansy Iqbal Abstract Electronic structure calculations and second-order delocalizations in N,N,,N,,-triaminoguanidine (TAG) have been studied by employing ab initio MO and density functional methods. There are total 10 rotational isomers on the potential energy (PE) surface of TAG. The effect of three amino groups substitution on guanidine (Gu) has been studied in terms of the primary and the secondary electron delocalizations in TAG by employing Natural Population Analysis (NPA). An increased electron delocalization is observed in protonated triaminoguanidine (TAGP) due to the three strong intramolecular hydrogen bonds and hence accounts for its extra stability. The increase in the electron delocalization upon protonation in TAG can be compared to that in guanidine. The absolute proton affinity (APA) of TAG is less than that of Gu. HOMA and NICS studies have been carried out to understand electron delocalization in TAGP. Copyright © 2007 John Wiley & Sons, Ltd. [source] Stilbene-containing polyactylenes: Molecular design, synthesis, and relationship between molecular structure and NLO propertiesJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 13 2008Xinyan Su Abstract Polyacetylenes (P1,P4) containing different stilbene groups, [(CHC) PhCHCHPhR]n(ROCmH2m+1 (m = 4 (P1), 10 (P2), 16 (P3)), or NO2 (P4)) were designed and synthesized, respectively, using [Rh(nbd)Cl]2 as a catalyst. Their structures and properties were characterized and evaluated by FTIR, 1H-NMR, 13C-NMR, GPC, and UV, PL, respectively. The optical limiting and nonlinear optical properties were investigated by using a frequency doubled, Q-switched, mode-locked Continuum ns/ps Nd:YAG laser system and their optical limiting mechanism was discussed. It is surprising to see that the stilbene pendants endow the polyacetylenes with a high thermal stability (Td , 270 °C), novel optical limiting properties and large third-order nonlinear optical susceptibilities (up to 4.61 × 10,10 esu). The optical limiting mechanism is mainly originated from reverse saturable absorption of molecules. In addition, it is found that the polymer with electron accepted NO2 moiety exhibits better optical properties than that with electron donated alkoxy group because of larger , electron delocalization and dipolar effect. The strong interaction between stilbene pendants and the polyene main chain significantly results in red-shift of fluorescence emitting peak. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4529,4541, 2008 [source] An ENDOR study of oxomolybdenum(V) tris(pyrazolyl)borate complexes; identification of couplings to boron and other heteroatomsMAGNETIC RESONANCE IN CHEMISTRY, Issue 11 2002Robert D. Farley Abstract 11B hyperfine and quadrupolar couplings have been observed by the electron magnetic resonance techniques electron nuclear double resonance and hyperfine sublevel correlation spectroscopy for some tris(pyrazolyl)borato-oxomolybdenum(V) and related nitrosyl complexes including a dinuclear B,B linked complex. The spectra are interpreted according to the electron delocalization onto the ligands, especially the pyrazolylborate. Copyright © 2002 John Wiley & Sons, Ltd. [source] Structural characterization of p -benzosemiquinone radical in a solid state: the radical stabilization by a low-barrier hydrogen bondACTA CRYSTALLOGRAPHICA SECTION B, Issue 6 2006imir Mol Semiquinone (p -benzosemiquinone), a transient organic radical, was detected in the solid state by EPR spectroscopy revealing four symmetrically equivalent protons. A variable-temperature X-ray diffraction analysis (293 and 90,K) and EPR data support a dynamical disorder of the proton. A low-barrier O,H···O hydrogen bond stabilizes the radical. The C,O bond length is 1.297,(4),Å, corresponding to a bond order of ca 1.5. The geometry of the radical implies an electron delocalization throughout the benzenoid ring. Two polymorphs of semiquinone, monoclinic and triclinic, were observed and their structures determined. Their crystal packings were compared with those of quinhydrone polymorphs. [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] Ab initio study on N,N,,N, -triaminoguanidineJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 12 2007Pansy Iqbal Abstract Electronic structure calculations and second-order delocalizations in N,N,,N,,-triaminoguanidine (TAG) have been studied by employing ab initio MO and density functional methods. There are total 10 rotational isomers on the potential energy (PE) surface of TAG. The effect of three amino groups substitution on guanidine (Gu) has been studied in terms of the primary and the secondary electron delocalizations in TAG by employing Natural Population Analysis (NPA). An increased electron delocalization is observed in protonated triaminoguanidine (TAGP) due to the three strong intramolecular hydrogen bonds and hence accounts for its extra stability. The increase in the electron delocalization upon protonation in TAG can be compared to that in guanidine. The absolute proton affinity (APA) of TAG is less than that of Gu. HOMA and NICS studies have been carried out to understand electron delocalization in TAGP. Copyright © 2007 John Wiley & Sons, Ltd. [source] | |||||||||||||||||||||||||