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Magnetic Criteria (magnetic + criterion)
Selected AbstractsAromaticity 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] Planar mono-, di- aza- and phospha-naphthalene: Structure and aromaticityINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 9 2007Lei Wang Abstract In this article, we used magnetic criteria, magnetic susceptibility isotropic and nucleus-independent chemical shifts (NICS), calculated with (density functional theory) B3LYP levels at the 6-31G** basis set, to evaluate aromaticity of a set of 29 planar bicyclic ,-electron systems: naphthalene and its mono- and di- aza- and phospha-derivatives. The result showed significant collinearity of the above two magnetic criteria when evaluating N-containing naphthalenes, but did not speak with the same voice when evaluating P-containing naphthalenes. Thus, we could conclude that magnetic susceptibility is a reliable criterion and NICS is not in our case. © 2007 Wiley Periodicals, Inc. Int J Quantum 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] Topological resonance energy, bond resonance energy, and circuit resonance energyJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 1 2008Jun-ichi Aihara Abstract Circuit resonance energy (CRE) is a key quantity that links energetic and magnetic criteria of aromaticity for a polycyclic ,-system. Topological resonance energy (TRE) correlates well with the sum of CREs for an entire ,-system. We found that bond resonance energy (BRE), so far defined to estimate the degree of kinetic stability for a polycyclic ,-system, also correlates very well with the corresponding quantity defined with CREs. These findings justify the view that graph-theoretical quantities, such as TRE and BRE, can be used safely even though they rely on unrealistic reference structures. Copyright © 2007 John Wiley & Sons, Ltd. [source] Functional [6]Pericyclynes: Aromatization to Substituted carbo -Benzenes,CHEMISTRY - A EUROPEAN JOURNAL, Issue 17 2007Catherine Saccavini Dr. Abstract Reductive treatment of stereoisomeric mixtures of variously substituted hexaoxy[6]pericyclynes with SnCl2/HCl led to the corresponding substituted carbo -benzenes. Tetramethoxyhexaphenyl[6]pericylynediol and dimethoxyhexaphenyl[6]pericyclynetetrol thus proved to be alternative precursors of hexaphenyl- carbo -benzene, previously described. Another hexaaryl- carbo -benzenic chromophore with 4-pyridyl and 4-anisyl substituents was targeted for its second-order nonlinear optical properties and was obtained by aromatization of a dimethoxy[6]pericyclynetetrol. Two alkynyl substituents in para positions were also found to be compatible with the C18carbo -benzene ring, provided that the four remaining vertices are substituted by phenyl groups. In the protected series, bis(trimethylsilylethynyl)hexaphenyl- carbo -benzene (C18Ph4(CCTMS)2) could be isolated and fully characterized, even by X-ray crystallography. In the bis-terminal series, the diethynylhexaphenyl- carbo -benzene C18Ph4(CCH)2 could not be isolated in the pure form. It could, however, be generated by two different methods and identified by the corresponding 1H NMR spectra. Unsubstituted carbo -benzene C18H6 remains unknown, but tetraphenyl- carbo -benzenes C18Ph4H2 with two unsubstituted vertices proved to be viable molecules. Whereas the "para" isomer could be characterized by MS and 1H and 13C NMR spectroscopy only in a mixture with polymeric materials, the "ortho" isomer (with adjacent CH vertices) could be isolated, and its structure was determined by using X-ray crystallography. The structure calculated at the B3PW91/6-31G** level of theory turned out to be in excellent agreement with the experimental structure. The 1H and 13C NMR chemical shifts of hexa- and tetraphenyl- carbo -benzenes were also calculated at the B3LYP/6-31+G** level of theory and were found to correlate with experimental spectra. The remote NMR deshielding of peripheral protons (through up to five bonds) revealed a very strong diatropic circulation around the C18 ring, regardless of the substitution pattern. In full agreement with theoretical investigations, it has been demonstrated experimentally that the carbo -benzene ring is "independently" aromatic, in accord with structural,energetic and ,magnetic criteria. [source] Electronic Structure of the Ground and Excited States of ,-CarbolineCHEMPHYSCHEM, Issue 6 2008Vicente Pérez Mondéjar Abstract Coupled-cluster calculations are used to compute the energy of conversion between the neutral and the zwitterionic forms of ,-carboline. The stability of the different species is discussed in terms of charge separation and aromatic character, which is related to magnetic criteria. By means of a linear response formalism the vertical excitation energies and oscillator strengths of the lowest singlet states of both structures as well as of the cationic species are determined. General agreement of the relative position and intensity of the different peaks with experimental data is achieved, but the overall spectra are slightly displaced because of solvent effects. [source] Ab initio and density functional theory studies of the structure, gas-phase acidity and aromaticity of tetraselenosquaric acidCHINESE JOURNAL OF CHEMISTRY, Issue 6 2000Li-Xin Zhou Abstract Results of ab initio self-consistent-field (SCF) and density functional theory (DFT) calculations of the gas-phase structure, acidity (free energy of deprotonation, ,G*) and aromaticity of tetraselenosquaric acid (3, 4-diselenyl-3-cyclobutene-1,2-diselenone, H2 C4 Se4)are reported. The global minimum found on the potential energy surface of tetraselenosquaric acid presents a planar conformation. The ZZ isomer was found to have the lowest energy among the three planar conformers and the ZZ and ZE isomers are very dose in energy. The optimized geometric parameters exhibit a bond length equalization relative to reference compounds, cyclobutanediselenone, and cyclobutanediselenol. The computed aromatic stabilization energy (ASE) by homodesmotic reaction is -77.4 (MP2(fu)/6,311+G** /RHF/6 - 311 + G** ) and - 54.8 kJ/mol (B3LYP/6 - 311 + G** //B3LYP/6 -311 + G**). The aromaticity of tetraselenosquaric acid is indicated by the calculated diamagnetic susceptibility exaltation (A) -19.13 (CSGT(IGAIM)-RHF/6,311 + G**// RHF/6,311 + G** and -32.91 (4,·10,6 m,3/mol)(CSGT(I-GAIM)-B3LYP/6 - 311 + G* * //B3LYP/6 - 311 + G**). Thus, tetraselenosquaric acid fulfils the geometric, energetic and magnetic criteria of aromaticity. The calculated gas-phase acidity is ,G1*(298k) = 1257.7 and ,G*2 (298k) = 1617.1 kJ/mol. Hence, tetraselenosquaric acid is the strongest acid among the three squaric acids (3, 4-dihydroxy-3-cyclobutene-1, 2-dione, H2 C4 3,4-dithiohydroxy-3-cyclobutene-1,2-dithione, H2C4 S4, 3, 4-diselenyl-3-cyclobutene-1,2-diselenone, H2C4Se4). [source] | ||||||||||||||||||||||