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At least 1,081 records · Page 60Linked to original sources
PubMed1995

Carbon-13 NMR studies of the influence of axial ligand orientation on haem electronic structure.

Three-quarters of the carbon-13 resonances of nuclei attached to the four haems of Desulfovibrio vulgaris ferricytochrome c3 are assigned. Preliminary analysis of their Fermi contact interactions shows that the shifts are directly related to the orientation of both of the axial histidine ligands in each case and the approach can therefore be used to obtain structural information in other cytochromes with bis-histidinyl coordination. The implications for the control of redox potential in cytochromes are discussed.

Cytochrome c Group↗
PubMed2005

Electronic structure of cis-Mo(P)(NO)2, where P is a porphyrin: an organometallic perspective of metalloporphyrin-NO complexes.

This study presents a first MO analysis of the stereochemistry of cis-Mo(P)(NO)(2), where the Mo(NO)(2) unit eclipses a pair of opposite Mo-N bonds and also adopts a remarkable horseshoe-like conformation. In addition, we have uncovered a number of analogies--in terms of commonalities of metal-ligand orbital interactions--between the dinitrosylmetalloporphyrins, Fe(P)(NO)(2) and Mo(P)(NO)(2), and the two dialkylmetalloporphyrins, Ru(P)(CH(3))(2), and Zr(P)(CH(3))(2).

Ligands↗
PubMed2006

Trigonal bipyramidal iron(III) and manganese(III) oxo, sulfido, and selenido complexes. An electronic-structural overview.

Using density functional theory calculations, we have carried out a broad survey of trigonal bipyramidal iron(III) and manganese(III) oxo, sulfido, selenido, and hydroxo complexes, with tripodal tetradentate "triureidoamine" supporting ligands. The calculations reproduce the experimentally observed high-spin states of these compounds; a multifunctional analysis suggests that the high-spin nature of these species follows largely from their trigonal bipyramidal geometry. In conjunction with earlier calculations, the present study provides a broad overview of spin density profiles in iron-oxo species in general. Iron-oxo d(pi)-p(pi) interactions invariably result in a substantial spin density on the oxygen, which in turn may be significantly tuned by hydrogen bonding interactions. The oxygen spin densities are smaller in analogous manganese-oxo species, indicating that manganese is less adept at pi-bonding than iron, which parallels earlier findings on porphyrin systems. The Fe(III)-S/Se spin density profiles provide one of the first confirmations in a transition metal context of Schleyer's prediction that the heavier p-block elements are as effective as their second-row congeners in terms of their pi-donating ability.

Ferric Compounds↗
PubMed2006

Pyrolysis mechanism of carbon matrix precursor cyclohexane--the formation of condensed-ring aromatics and the growing process of molecules.

Based on the experiments, the UAM1 method was adopted to investigate benzene condensation of an important intermediate and the molecule growing mechanism during the cyclohexane pyrolysis process. The conclusions were drawn as follows: (1) from the viewpoint of thermodynamics, the condensation of benzene and C4H5* is a spontaneous reaction and the rising temperature will increase the spontaneous tendency of the reaction. (2) From the viewpoint of kinetic, the condensation of benzene and C4H5* is a two-step reaction. The rate-determining step is step 2 of hydrogen removal from intermediate C10H10 (I1) with the activation energy of 350.61 kJ/mol below 1473 K while the rate-determining step is step 1 that free radical C4H5* attacks benzene to form intermediate C10H10 (I1) with the activation energy DeltaE(0)(not equal to theta)=31.74 kJ/mol above 1473 K. (3) The space structure, electronic structure and thermodynamics parameters of molecular reaction of dense-ring aromatizing compounds can be used to replace the resonance energy and free valence to judge the activation of thermodynamic reaction of compounds. And (4) the analysis of the space structure, electronic structure and thermodynamic parameters show that the growing process of molecules with benzene used as initial reactants becomes more easier as the multi-ring aromatizing molecular system increases.

Benzene↗
PubMed1977

Electronic structure of polyenes related to the visual chromophore. A simple model for the observed band shapes.

A model is presented which attributes the widths of absorption bands in carotenoids to conformational disorder induced by the beta-ionylidene moiety. With reasonable parameter choices, the model gives a good quantitative fit to the spectra observed for four carotenoids and at the same time accounts for the lack of structure in the long-wavelength absorption of retinal.

Carotenoids↗
PubMed1982

The electronic structure of Fe2+ in reaction centers from Rhodopseudomonas sphaeroides. II. Extended x-ray fine structure studies.

Extended x-ray absorption fine structure (EXAFS) studies were performed on reaction centers (RC) of the photosynthetic bacterium Rhodopseudomonas sphaeroides R-26. RC containing two, one, and no quinones (2Q, 1Q, 0Q) samples were studied. The average ligand distance of the first coordination shell was determined to be 2.10 +/- 0.02 A with a more distant shell at 4.14 +/- 0.05 A. The Fe2+ site in RC was found to have a very large structural disorder parameter, from which a spread in ligand distance per iron site of approximately +/- 0.1 A was deduced. The most likely coordination number of the first shell is six, with a mixture of oxygens and nitrogens as ligands. The edge absorption results are consistent with the Fe2+ being in distorted octahedral environment. The EXAFS spectra of the 2Q and 1Q samples with and without O-phenanthroline were found to be the same. This indicates that either the secondary quinone and o-phenanthroline do not bind to Fe2+ or that they replace an equivalent ligand. The 0Q sample showed a 12% decrease in the EXAFS amplitude, which was restored upon addition of o-phenanthroline. These results can be explained by either a loss of a ligand or a severe conformational change when the primary quinone was removed.

Chemical Phenomena↗