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DFT Analysis (dft + analysis)
Selected AbstractsLewis Acid Induced [2+2] Cycloadditions of Silyl Enol Ethers with ,,,-Unsaturated Esters: A DFT AnalysisEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 18 2005Manuel Arnó Abstract The Lewis acid (LA) induced cycloaddition of trimethysilyl vinyl ether with methyl acrylate has been studied by DFT methods at the B3LYP/6-31G* level. In the absence of an LA, a [4+2] cycloaddition between the silyl enol ether and methyl acrylate in the s-cis conformation takes place through an asynchronous, concerted bond-formation process. This cycloaddition presents a large activation enthalpy of 21.1 kcal,mol,1. Coordination of the LA AlCl3 to the carbonyl oxygen atom of methyl acrylate yields a change of molecular mechanism from a concerted to a two-step mechanism and produces a drastic reduction of the activation energy. This stepwise mechanism is initialized by the nucleophilic attack of the enol ether at the ,-position of methyl acrylate in a Michael-type addition. The very low activation energy (7.1 kcal,mol,1)associated with this nucleophilic attack can be related to the increase of the electrophilicity of the LA-coordinated ,,,-unsaturated ester, which favors the cycloaddition through a polar process. The subsequent ring-closure allows the formation of the corresponding [2+2] and [4+2] cycloadducts. While the [4+2] cycloadduct is formed by kinetic control, the [2+2] cycloadducts are formed by thermodynamic control. The energetic results provide an explanation for the conversion of [4+2] cycloadducts into the thermodynamically more stable [2+2] ones. The cis/trans ratio found for the catalytic [2+2] process is in agreement with the experimental outcome. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] ChemInform Abstract: DFT Analysis of Cubane-Type FeIr3S4 Clusters.CHEMINFORM, Issue 9 2008Activation at the Tetrahedral Fe Site., Dinitrogen Binding 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] New tin(IV) complexes with sterically hindered o -iminobenzoquinone ligand: Synthesis and structureHETEROATOM CHEMISTRY, Issue 6 2009Alexandr V. Piskunov The reduction of 4,6-di-tert-butyl-N-(2, 6-di-iso-propylphenyl)-o-iminobenzoquinone (imQ) by tin amalgam in hexane solution leads to new six-coordinated o-iminoquinonato tin(IV) complex (iSQ)2SnAP (1) (where iSQ and AP are o-iminosemiquinolate and dianion o-amidophenolate, respectively). Variable temperature magnetic susceptibility measurements of 1 have shown that this complex possesses a weak ferromagnetic exchange between o-iminosemiquinonate ligands. The oxidation of 1 with air oxygen produces new o-iminoquinonolate tin(IV) derivatives [(iSQ)Sn(AP)]2O (2) and (iSQ)2Sn(OH)2 (3) containing ,-oxo- and hydroxo-ligands, respectively. The electronic structure of 1 was examined by DFT analysis. Complexes 1,3 have been investigated using single-crystal X-ray diffraction. © 2009 Wiley Periodicals, Inc. Heteroatom Chem 20:332,340, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20555 [source] Structural Effects on the Electronic Absorption Properties of 5,6-Dihydroxyindole Oligomers: The Potential of an Integrated Experimental and DFT Approach to Model Eumelanin Optical Properties,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 3 2008Marco D'Ischia Elucidation of the relationships between structural features and UV,visible absorption properties of 5,6-dihydroxyindole oligomers is an essential step towards an understanding of the unique optical properties of eumelanins. Herein, we report the first combined experimental and density functional theory (DFT) investigation of the 5,6-dihydroxyindole oligomers so far isolated. 2,2,-Biindolyl 2 and the 2,4,-biindolyl 3 absorb at longer wavelengths relative to 2,7,-biindolyl 4 and their spectra were well predicted by DFT analysis. The absorption bands of 2,4,:2,,4,,- and 2,4,:2,,7,,-triindolyls 5 and 6 also fall at different wavelengths and can be interpreted by DFT simulations as being due to a combination of two main separate transitions. Tetramer 7, in which two 2,4,-biindolyl units are linked through a 2,3,-connection, exhibits a broad chromophore extending over the entire UV range without well defined absorption maxima. Within the dimer,tetramer range examined, three key points emerge: (1) an increase in oligomer chain length does not result in any regular and predictable bathochromic shift; (2) a marked broadening of the absorption bands occurs when going from the monomer to the tetramer structure; and (3) the mode of coupling of the indole units is a crucial, hitherto unrecognized, structural parameter affecting the electronic absorption properties of 5,6-dihydroxyindole oligomers. It is concluded that use of experimentally characterized oligomeric scaffolds as a basis for DFT calculations is a most promising approach to building reliable structural models for studies of eumelanins optical properties. [source] BINOL-3,3,-Triflone N,N -Dimethyl Phosphoramidites: Through-Space 19F,31P Spin,Spin Coupling with a Remarkable Dependency on Temperature and Solvent Internal PressureCHEMISTRY - A EUROPEAN JOURNAL, Issue 26 2008Matthias Kruck Abstract A combined computational and experimental study of the effects of solvent, temperature and stereochemistry on the magnitude of the through-space spin,spin coupling between 31P and 19F nuclei which are six-bonds apart is described. The reaction of 3-trifluoromethylsulfonyl-2,,2-dihydroxy-1,1,-binaphthalene (3-SO2CF3 -BINOL) with hexamethylphosphorous triamide (P(NMe2)3) generates a pair of N,N -dimethylphosphoramidites which are diastereomeric due to their differing relative configurations at the stereogenic phosphorous centre and the axially chiral (atropisomeric) BINOL unit. Through-space NMR coupling of the 31P and 19F nuclei of the phosphoramidite and sulfone is detected in one diastereomer only. In the analogous N,N -dimethylphosphoramidite generated from 3,3,-(SO2CF3)2 -BINOL only one of the diastereotopic trifluoromethylsulfone moieties couples with the 31P of the phosphoramidite. In both cases, the magnitude of the coupling is strongly modulated (up to 400,%) by solvent and temperature. A detailed DFT analysis of the response of the coupling to the orientation of the CF3 moiety with respect to the P-lone pair facilitates a confident assignment of the stereochemical identity of the pair of diastereomers. The analysis shows that the intriguing effects of environment on the magnitude of the coupling can be rationalised by a complex interplay of solvent internal pressure, molecular volume and thermal access to a wider conformational space. These phenomena suggest the possibility for the design of sensitive molecular probes for local environment that can be addressed via through-space NMR coupling. [source] A Concerted Approach for the Determination of Molecular Conformation in Ordered and Disordered MaterialsCHEMISTRY - A EUROPEAN JOURNAL, Issue 22 2007Jan Sehnert Abstract We present the successful application of a concerted approach for the investigation of the local environment in ordered and disordered phases in the solid state. In this approach we combined isotope labeling with computational methods and different solid-state NMR techniques. We chose triphenylphosphite (TPP) as an interesting example of our investigations because TPP exhibits two crystalline modifications and two different amorphous phases one of which is highly correlated. In particular we analyzed the conformational distribution in three of these phases. A sample of triply labeled 1-[13C]TPP was prepared and 1D MAS as well as wide-line 13C NMR spectra were measured. Furthermore we acquired 2D 13C wide-line exchange spectra and used this method to derive highly detailed information about the phenyl orientation in the investigated TPP phases. For linkage with a structure model a DFT analysis of the TPP molecule and its immediate environment was carried out. The ab initio calculations of the 13C chemical shift tensor in three- and six-spin systems served as a base for the calculation of 1D and 2D spectra. By comparing these simulations to the experiment an explicit picture of all phases could be drawn on a molecular level. Our results therefore reveal the high potential of the presented approach for detailed studies of the mesoscopic environment even in the challenging case of amorphous materials. [source] Interactions between Oxygen Atoms on Pt(100): Implications for Ordering during Chemisorption and CatalysisCHEMPHYSCHEM, Issue 10 2010Da-Jiang Liu Abstract We present a DFT analysis of the interactions between chemisorbed oxygen on the unreconstructed (1×1)-Pt(100) surface. These interactions control ordering of O not just for single-species adsorption, but also within O domains during coadsorption and reaction with other species such as CO. The calculations indicate that O prefers bridge sites, as deduced previously. In addition, we find a large difference in the interactions between O at different types of bridge site pairs separated by one lattice constant. There is strong repulsion for pairs separated by a Pt atom, but only a weak interaction for pairs separated by a fourfold hollow site. This finding elucidates the tendency for striped (n×1)-O ordering often observed in chemisorption and reaction studies. [source] | ||||||||||