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At least 595 records · Page 33Linked to original sources
PubMed2005

Electronic structure of orotic acid III geometric feature and thermal properties of some transition metal orotic acid complexes.

The complexes of orotic acid with Co(II), Ni(II), Fe(III), Cu(II), and Cd(II) were prepared and their stoichiometry were determined by elemental analysis. Co(II) and Ni(II) give complexes with orotic acid of 1:1 ratio whereas that of the remaining transition metals give complexes of 1:2 ratio. The stereochemistry of the studied metal complexes has been established by analyses of their electronic spectra and magnetic susceptibilities. The mode of bonding in the studied series of metal complexes was established via, analysis of their infrared spectra. The present analysis leads to the conclusion that all metal ions studied coordinate to orotic acid via N(1) and the adjacent carboxylate group. Thermal decomposition studies of orotic acid complexes have been carried out as to understand the status of water molecules present in these complexes as well as to know their general decomposition pattern. Theoretical investigation of the electronic structure of the studied metal complexes has been carried out. MO computations at the HF-level were performed. Charge density distribution, extent of distortion from regular geometry, dipole moment, and orientation were computed and discussed.

Cobalt↗
PubMed2004

Spectroscopic and quantum chemical characterization of the electronic structure and bonding in a non-heme FeIV[double bond]O complex.

High valent FeIV=O species are key intermediates in the catalytic cycles of many mononuclear non-heme iron enzymes involving the binding and activation of dioxygen. Using variable temperature magnetic circular dichroism (VT MCD) spectroscopy and experimentally calibrated density functional calculations, we are able to present the first detailed description of the electronic structure of a non-heme FeIV=O S = 1 complex. These studies define the nature of the FeIV=O bond and present the basis for understanding high-valent oxygen intermediates in non-heme iron enzymes.

Circular Dichroism↗
PubMed1996

Single-Crystal Raman Spectroscopy of the Rubidium Alums RbM(III)(SO(4))(2).12H(2)O (M(III) = Al, Ga, In, Ti, V, Cr, Fe) between 275 and 1200 cm(-)(1): Correlation between the Electronic Structure of the Tervalent Cation and Structural Abnormalities.

Low-temperature single-crystal Raman spectra for RbM(III)(SO(4))(2).12H(2)O (M(III) = Al, Ga, In, Ti, V, Cr, Fe) and RbM(III)(SO(4))(2).12D(2)O (M(III) = Al, V) have been collected and assigned in the range 275-1200 cm(-)(1). These results permit classification of the Ti and V rubidium sulfate alums to the beta modification, whereas the remaining tervalent cations give the expected alpha modification. The dimorphism of the rubidium sulfate alums is explained in terms of the electronic structure of the tervalent cation, where the observation of the beta modification is associated with unequal occupancy of the t(2g) (O(h)()) orbitals. For the rubidium vanadium alums the (3)E(g) <-- (3)A(g) electronic Raman (eR) transition permits quantification of the trigonal field splitting of the t(2g) (O(h)()) orbitals (ca. 1940 cm(-)(1)). The profile of the eR band is sensitive both to changes in temperature and to deuteration. Analysis of the eR band profile suggests a reduced spin-orbit splitting of the (3)E(g) manifold, this being ascribed to excited state Jahn-Teller (J-T) effects. The similarity of the Raman spectra of the cesium and rubidium titanium sulfate alums suggest that they exhibit closely related structural chemistry, with both subject to phase transitions below 80 K. The observation that modes of E(g) symmetry are coupled to the structural change is consistent with the interpretation that the trigonal field leaves an orbital doublet ground term for titanium(III), leading to a cooperative J-T effect.

Journal Article↗
PubMed2006

The peculiar electronic structure of PbSe quantum dots.

PbSe is a pseudo-II-VI material distinguished from ordinary II-VI's (e.g., CdSe, ZnSe) by having both its valence band maximum (VBM) and its conduction band minimum (CBM) located at the fourfold-degenerate L-point in the Brillouin zone. It turns out that this feature dramatically affects the properties of the nanosystem. We have calculated the electronic and optical properties of PbSe quantum dots using an atomistic pseudopotential method, finding that the electronic structure is different from that of ordinary II-VI's and, at the same time, is more subtle than what k.p or tight-binding calculations have suggested previously for PbSe. We find the following in PbSe dots: (i) The intraband (valence-to-valence and conduction-to-conduction) as well as interband (valence-to-conduction) excitations involve the massively split L-manifold states. (ii) In contrast to previous suggestions that the spacings between valence band levels will equal those between conduction band levels (because the corresponding effective-masses me approximately mh are similar), we find a densely spaced hole manifold and much sparser electron manifold. This finding reflects the existence of a few valence band maxima in bulk PbSe within approximately 500 meV. This result reverses previous expectations of slow hole cooling in PbSe dots. (iii) The calculated optical absorption spectrum reproduces the measured absorption peak that had previously been attributed to the forbidden 1Sh --> 1Pe or 1Ph --> 1Se transitions on the basis of k.p calculations. However, we find that this transition corresponds to an allowed 1Ph --> 1Pe excitation arising mainly from bulk states near the L valleys on the Gamma-L lines of the Brillouin zone. We discuss this reinterpretation of numerous experimental results.

Journal Article↗
PubMed2005

Electronic structure study of the initiation routes of the dimethyl sulfide oxidation by OH.

In the present work the potential energy surface (PES) corresponding to the different initiation routes of the oxidation mechanism of DMS by hydroxyl radical in the absence of O(2) has been studied, and connections among the different stationary points have been established. Single-point high level electronic structure calculations at lower level optimized geometries have been shown to be necessary to assure convergence of energy barriers and reaction energies. Our results demonstrate that the oxidation of DMS by OH turns out to be initiated via three channels: a hydrogen abstraction channel that through a saddle point structure finally leads to CH(3)SCH(2) + H(2)O, an addition-elimination channel that firstly leads to an adduct complex (AD) and then via an elimination saddle point structure finally gives CH(3)SOH and CH(3) products, and a third channel that through a concerted pathway leads to CH(3)OH and CH(3)S. The H-abstraction and the addition-elimination channels initiate by a common pathway that goes through the same reactant complex (RC). Our theoretical results agree quite well with the branching ratios experimentally assigned to the formation of the different products. Finally, the calculated equilibrium constants of the formation of the complex AD and the hexadeuterated complex AD from the corresponding reactants, as a function of the temperature, are in good accordance with the experimental values.

Journal Article↗
PubMed2005

The influence of nuclear volume and electronic structure on the rotational energy of platinum monoxide, PtO.

The pure rotational spectra of seven isotopic species of platinum monoxide have been measured with a cavity pulsed jet Fourier-transform microwave spectrometer. The molecules were prepared by laser ablation of Pt foil in the presence of O2 and stabilized in a supersonic jet of argon. A multi-isotopomer Dunham-type analysis of the spectra produced values for Y01 and Y11, along with unusually large values for Born-Oppenheimer breakdown (BOB) parameters for both Pt and O atoms. The values of the BOB parameters have been rationalized in terms of the molecular electronic structure and finite nuclear size (field shift) effects. A large negative 195Pt effective nuclear spin-rotation constant has been rationalized in terms of the electron-nucleus dipole-dipole hyperfine constant. Precise internuclear separations have been evaluated.

Journal Article↗
PubMed2003

Intercalation of trioxatriangulenium ion in DNA: binding, electron transfer, x-ray crystallography, and electronic structure.

Trioxatriangulenium ion (TOTA(+)) is a flat, somewhat hydrophobic compound that has a low-energy unoccupied molecular orbital. It binds to duplex DNA by intercalation with a preference for G-C base pairs. Irradiation of intercalated TOTA(+) causes charge (radical cation) injection that results in strand cleavage (after piperidine treatment) primarily at GG steps. The X-ray crystal structure of TOTA(+) intercalated in the hexameric duplex d[CGATCG](2) described here reveals that intercalation of TOTA(+) results in an unusually large extension of the helical rise of the DNA and that the orientation of TOTA(+) is sensitive to hydrogen-bonding interactions with backbone atoms of the DNA. Electronic structure calculations reveal no meaningful charge transfer from DNA to TOTA(+) because the lowest unoccupied molecular orbital of TOTA(+), (LUMO)(T), falls in the gap between the highest occupied molecular orbital, (HOMO)(D), and the (LUMO)(D) of the DNA bases. These calculations reveal the importance of backbone, water, and counterion interactions, which shift the energy levels of the bases and the intercalated TOTA(+) orbitals significantly. The calculations also show that the inserted TOTA(+) strongly polarizes the intercalation cavity where a sheet of excess electron density surrounds the TOTA(+).

Circular Dichroism↗
PubMed2006

Electronic structure origins of polarity-dependent high-TC ferromagnetism in oxide-diluted magnetic semiconductors.

Future spintronics technologies based on diluted magnetic semiconductors (DMSs) will rely heavily on a sound understanding of the microscopic origins of ferromagnetism in such materials. Discoveries of room-temperature ferromagnetism in wide-bandgap DMSs hold great promise, but this ferromagnetism remains poorly understood. Here we demonstrate a close link between the electronic structures and polarity-dependent high-TC ferromagnetism of TM(2+):ZnO DMSs, where TM(2+) denotes 3d transition metal ions. Trends in ferromagnetism across the 3d series of TM(2+):ZnO DMSs predicted from the energies of donor- and acceptor-type excited states reproduce experimental trends well. These results provide a unified basis for understanding both n- and p-type ferromagnetic oxide DMSs.

Electronics↗
PubMed2005

Electronic structural changes between nickel(II)-semiquinonato and nickel(III)-catecholato states driven by chemical and physical perturbation.

The selective synthesis of tetracoordinate square-planar low-spin nickel(II)-semiquinonato (Ni(II)-SQ) and nickel(III)-catecholato (Ni(III)-Cat) complexes, 1 and 2, respectively, was achieved by using bidentate ligands with modulated nitrogen-donor ability to the nickel ion. The electronic structures of 1 and 2 were revealed by XPS and EPR measurements. The absorption spectra of 1 and 2 in a noncoordinating solvent, dichloromethane (CH2Cl2), are completely different from those in tetrahydrofuran (THF), being a coordinating solvent. As expected from this result, the gradual addition of N,N-dimethylformamide (DMF), which is also a coordinating solvent like THF, into a solution of 1 or 2 in CH2Cl2 leads to color changes from blue (for 1) and brown (for 2) to light green, which is the same color observed for solutions of 1 or 2 in THF. Furthermore, the same color changes are induced by varying the temperature. Such spectral changes are attributable to the transformation from square-planar low-spin Ni(II)-SQ and Ni(III)-Cat complexes to octahedral high-spin Ni(II)-SQ ones, caused by the coordination of two solvent molecules to the nickel ion.

Journal Article↗
PubMed2006

Electronic structure and solvatochromism of merocyanines NMR spectroscopic point of view.

(1)H and (13)C NMR spectra of two series of malononitrile-based merocyanines, which possess positive and negative solvatochromism have been in detail investigated in low polar chloroform and polar dimethyl sulfoxide (DMSO). Careful attribution of signals in spectra has been made with the help of two-dimensional NMR experiments (COSY, NOESY, HMBC, and HMQC). Hence, the dependence of merocyanines electronic structure on their chemical structure and solvent nature has been studied by this powerful method. It has been shown that there exists a good correlation between the calculated charges on carbon atoms of a polymethine chain and their chemical shifts in (13)C NMR spectra. The influence of solvent polarity on bond orders for dyes with positive and negative solvatochromism is also observed. The comparison of (13)C NMR spectra of merocyanines and corresponding parent ionic dyes allows to determine their sign of solvatochromism irrespectively of electronic spectra, and also to find the key atoms of chromophore whose signals in (13)C NMR spectra are most informative.

Absorption↗
PubMed2006

Metal-metal bonding in molecular actinide compounds: electronic structure of [M2X8](2-) (M = U, Np, Pu; X = Cl, Br, I) complexes and comparison with d-block analogues.

Density functional and multiconfigurational (ab initio) calculations have been performed on [M(2)X(8)](2-) (X = Cl, Br, I) complexes of 4d (Mo, Tc, Ru), 5d (W, Re, Os), and 5f (U, Np, Pu) metals in order to investigate general trends, similarities and differences in the electronic structure and metal-metal bonding between f-block and d-block elements. Multiple metal-metal bonds consisting of a combination of sigma and pi interactions have been found in all species investigated, with delta-like interactions also occurring in the complexes of Tc, Re, Np, Ru, Os, and Pu. The molecular orbital analysis indicates that these metal-metal interactions possess predominantly d(z2) (sigma), d(xz) and d(yz) (pi), or d(xy) and d(x2-y2) (delta) character in the d-block species, and f(z3) (sigma), f(z2x) and f(z2y) (pi), or f(xyz) and f(z) (delta) character in the actinide systems. In the latter, all three (sigma, pi, delta) types of interaction exhibit bonding character, irrespective of whether the molecular symmetry is D(4h) or D(4d). By contrast, although the nature and properties of the sigma and pi bonds are largely similar for the D(4h) and D(4d) forms of the d-block complexes, the two most relevant metal-metal delta-like orbitals occur as a bonding and antibonding combination in D(4h) symmetry but as a nonbonding level in D(4d) symmetry. Multiconfigurational calculations have been performed on a subset of the actinide complexes, and show that a single electronic configuration plays a dominant role and corresponds to the lowest-energy configuration obtained using density functional theory.

Journal Article↗
PubMed2006

Photoluminescence of europium(III) dithiocarbamate complexes: electronic structure, charge transfer and energy transfer.

For the first time, we observed photoluminescence in Eu(III) dithiocarbamate complexes at room temperature -- more specifically in [Eu(Et(2)NCS(2))(3)phen], [Eu(Et(2)NCS(2))(3)bpy] and the novel [Eu(Ph(2)NCS(2))(3)phen], where phen stands for 1,10-phenanthroline and bpy for 2,2'-bipyridine. Correlations between the electronic structure of the dithiocarbamate ligands on one hand, and covalency, intensity, and ligand field spectroscopic parameters on the other, could be established. Moreover, the relative values of the emission quantum efficiencies obtained for these complexes, as well as their dependence with temperature, could be satisfactorily described by a theoretical methodology recently developed.

Journal Article↗
PubMed2004

Electronic structure and bonding of Au on a SiO2 cluster: a nanobullet for tumors.

The binding of gold atoms to a small silica cluster has been studied by using density functional theory with generalized gradient approximation for exchange and correlation. It is shown that gold atoms bind to silicon atoms with dangling bonds and serve as seeds for the growth of Au islands. The large electron affinity of gold causes a significant change in the electronic structure of silica resulting in a substantial reduction in the highest occupied and the lowest unoccupied molecular orbital and the optical gap, thus allowing it to absorb near infrared radiation. This suggests that a small cluster can have a similar functionality in the treatment of cancer as the large size nanoshell, but for a different mechanism.

Breast Neoplasms↗
PubMed2006

Locking down the electronic structure of (monopyrrolo)tetrathiafulvalene in [2]rotaxanes.

[reaction: see text] The redox potentials of a highly constrained [2]rotaxane have been measured and used to model the energy of the HOMO of tetrathiafulvalene-based bistable [2]rotaxanes in their two co-conformationally isomeric states. Restrained from co-conformational movements, the measured oxidation and reduction potentials provide insights into the orbital energies and electronic structure of a (monopyrrolo)tetrathiafulvalene unit when encircled by a tetracationic cyclobis(paraquat-p-phenylene) ring.

Journal Article↗
PubMedDate not supplied

[The electronic structure and radiorprotective activity of a series of substituted iminodihydrofurans].

Compounds that increase the survival rate of lethally exposed hybrid (CBA x C57B1/6)F1 mice have been revealed within the series of iminodihydrofurans. The quantum-chemical estimates, made by the MNDO method, show that substances which are capable of donor-acceptance interaction with DNA nucleotides and have the energy of donor orbitals, comparable with the electron structure of nucleotides, possess radioprotective efficacy.

Animals↗
PubMed2006

Ionization energy and electronic structure of polycytidine.

Ribonucleic acid (RNA) polycytidine (poly rC) homopolymer thin films were prepared on highly oriented pyrolytic graphite (HOPG) substrates. The films were grown from aqueous solution directly in a vacuum in multiple steps with use of an electrospray (ES) deposition system. Before poly rC deposition and after each deposition step the sample was characterized with X-ray and ultraviolet photoemission spectroscopy (XPS, UPS). Evaluation of the UP-spectra sequence allowed the determination of ionization energy and highest occupied molecular orbital (HOMO) electronic structure, as well as the charge injection barriers between HOPG and poly rC. Comparison with earlier results on polyadenosine (poly rA) indicates significant differences between ionization energies (poly rC: 8.1 eV; poly rA: 6.8 eV) and orbital alignment at the graphite interface. The larger ionization energy of poly rC results in a larger hole injection barrier and a smaller electron injection barrier relative to the HOPG Fermi level.

Electrons↗
PubMed2005

A study of electronic structures of Pt3M (M=Ti,V,Cr,Fe,Co,Ni) polycrystalline alloys with valence-band photoemission spectroscopy.

The surface valence-band densities of states (DOS) of Pt(3)M (M=Ti,V,Cr,Fe,Co,Ni) polycrystalline alloys were investigated with ultraviolet photoemission spectroscopy. Upon annealing the ion-sputter-cleaned alloys at high temperatures, the observed valence-band DOS spectra clearly show the modified electronic structures on the surfaces suggesting the surface segregation of Pt as predicted in thermodynamic models. The measured d-band centers and widths for the annealed alloy surfaces show qualitatively the same trend as predicted by density-functional-theory calculations based on the model of a Pt "skin" on the topmost surface layer and a subsurface layer enriched in the 3d transition metal.

Journal Article↗
PubMed2005

Dioxygen activation by copper, heme and non-heme iron enzymes: comparison of electronic structures and reactivities.

Enzymes containing heme, non-heme iron and copper active sites play important roles in the activation of dioxygen for substrate oxidation. One key reaction step is CH bond cleavage through H-atom abstraction. On the basis of the ligand environment and the redox properties of the metal, these enzymes employ different methods of dioxygen activation. Heme enzymes are able to stabilize the very reactive iron(IV)-oxo porphyrin-radical intermediate. This is generally not accessible for non-heme iron systems, which can instead use low-spin ferric-hydroperoxo and iron(IV)-oxo species as reactive oxidants. Copper enzymes employ still a different strategy and achieve H-atom abstraction potentially through a superoxo intermediate. This review compares and contrasts the electronic structures and reactivities of these various oxygen intermediates.

Binding Sites↗