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PubMed1997

ESR Studies of Oxochromium(V) Porphyrin Complexes: Electronic Structure of the Cr(V)=O Moiety.

ESR spectra of oxochromium(V) porphyrins were obtained to investigate the electronic structure of the Cr(V)=O moiety. At room temperature, the chlorooxochromium(V) meso-tetramesitylporphyrin complex 1-Cl exhibits an isotropic ESR signal, split into nine signals due to hyperfine coupling with four equivalent( 14)N, at g = 1.982. A small isotropic satellite signal, split into a quartet due to hyperfine coupling by the (53)Cr isotope is also observed. At 77 K, 1-Cl shows an anisotropic ESR signal with g( parallel) = 1.986 and g( perpendicular) = 1.980 and small satellite g( parallel) signals arising from hyperfine coupling with the (53)Cr isotope. The ESR parameters of 1-Cl, obtained from ESR spectral simulation, show that the pi-bonding character of the Cr(V)=O bond is not as strong as that of the Cr(V)&tbd1;N bond. To explore substituent effects on the bond character of the Cr(V)=O moiety, ESR spectra of perchloratooxochromium(V) porphyrins having various electronegative substituents were recorded. With an increase in electronegativity of the meso-substituent, the g( perpendicular) value and the (53)Cr hyperfine coupling constant (|a((53)Cr)|) are increased. These changes suggest an increase in the pi-bonding character of the Cr(V)=O moiety with an increase in the electronegativity of the meso-substituent. Furthermore, the hyperfine coupling constant of the oxo ligand, |a((17)O)|, obtained from ESR spectra of (17)O-labeled oxochromium(V) porphyrin is increased with an increase in electronegativity of the meso-substituent. This indicates that the radical character of the oxo ligand is increased by the electronegative meso-substituent. These data suggest that the pi-antibonding orbital of Cr=O is destabilized with an increase in electronegativity of the meso-substituent, which may facilitate bond breaking of the Cr=O moiety in an epoxidation reaction.

Journal Article↗
PubMed2005

Electronic structure of A- and B-site doped lanthanum manganites: a combined X-ray spectroscopic study.

The electronic properties of a series of colossal magnetoresistance (CMR) compounds, namely LaMnO3, La(1-x)Ba(x)(MnO3 (0.2 < or = x < or = 0.55), La(0.76)Ba(0.24)Mn(0.84)Co(0.16)O3, and La(0.76)Ba(0.24)Mn(0.78)Ni(0.22)O3, have been investigated in a detailed spectroscopic study. A combination of X-ray photoelectron spectroscopy (XPS), X-ray emission spectroscopy (XES), X-ray absorption spectroscopy (XAS), and resonant inelastic X-ray scattering (RIXS) was used to reveal a detailed picture of the electronic structure in the presence of Ba, Co, and Ni doping in different concentrations. The results are compared with available theory. The valence band of La(1-x)()Ba(x)MnO3 (0 < or = x < or = 0.55) is dominated by La 5p, Mn 3d, and O 2p states, and strong hybridization between Mn 3d and O 2p states is present over the whole range of Ba concentrations. Co-doping at the Mn site leads to an increased occupancy of the e(g) states near the Fermi energy and an increase in the XPS valence band intensity between 0.5 and 5 eV, whereas the Ni-doped sample shows a lower density of occupied states near the Fermi energy. The Ni d states are located in a band spanning the energy range of 1.5-5 eV. XAS spectra indicate that the hole doping leads to mixed Mn 3d-O 2p states. Furthermore, RIXS at the Mn L edge has been used to probe d-d transitions and charge-transfer excitations in La(1-x)Ba(x)MnO3.

Journal Article↗
PubMed2001

Evolution of III-V nitride alloy electronic structure: the localized to delocalized transition.

Addition of nitrogen to III-V semiconductor alloys radically changes their electronic properties. We report large-scale electronic structure calculations of GaAsN and GaPN using an approach that allows arbitrary states to emerge, couple, and evolve with composition. We find a novel mechanism of alloy formation where localized cluster states within the gap are gradually overtaken by a downwards moving conduction band edge, composed of both localized and delocalized states. This localized to delocalized transition explains many of the hitherto puzzling experimentally observed anomalies in III-V nitride alloys.

Journal Article↗
PubMed2005

Role of surface electronic structure in thin film molecular ordering.

We show that the orientation of pentacene molecules is controlled by the electronic structure of the surface on which they are deposited. We suggest that the near-Fermi level density of states above the surface controls the interaction of the substrate with the pentacene pi orbitals. A reduction of this density as compared to noble metals, realized in semimetallic Bi(001) and Si(111)(5 x 2)Au surfaces, results in pentacene standing up. Interestingly, pentacene grown on Bi(001) is highly ordered, yielding the first vertically oriented epitaxial pentacene thin films observed to date.

Journal Article↗
PubMed2001

4d Electronic structure analysis of ruthenium in the perovskite oxides by Ru K- and L-edge XAS.

The 4d electronic structure of ruthenium in the perovskite oxides, La2MRuIVO6 (M = Zn, Mg, and Li) and Ba2YRuVO6, has been investigated by the Ru K-and L-edge XANES and EXAFS analyses. Such X-ray absorption spectroscopic results clarify that the RuIV (d4) and RuV (d3) ions are stabilized in nearly regular Oh site. Comparing the Ru L-edge XANES spectra of perovskites containing isovalent ruthenium, it has been found that the t2g state is mainly influenced by A site cation, whereas the eg is mainly affected by neighboring B site cation. The experimental EXAFS spectra in the range of R < or = approximately 4.5 A are well reproduced by ab-initio calculation based on crystallographic data, which supports the long-range structure presented by Rietveld refinement.

Journal Article↗
PubMed2003

Density functional theory investigation of the active site of [Fe]-hydrogenases: effects of redox state and ligand characteristics on structural, electronic, and reactivity properties of complexes related to the [2Fe]H subcluster.

The effects of redox state and ligand characteristics on structural, electronic, and reactivity properties of complexes related to the [2Fe](H) subcluster of [Fe]-hydrogenases have been investigated by DFT calculations and compared with experimental and theoretical data obtained investigating both the enzyme and synthetic model complexes. Our results show that Fe(II)Fe(II) species characterized by OH or H(2)O groups terminally coordinated to the iron atom distal to the terminal sulfur ligand (Fe(d)) are less stable than corresponding mu-OH or mu-H(2)O species, suggesting that the latter are destabilized or kinetically inaccessible in the enzyme. In addition, results obtained investigating Fe(I)Fe(I) and Fe(II)Fe(I) complexes show that structure and relative stability of species characterized by a mu-CO group are significantly affected by the electronic properties of the ligands coordinated to the iron atoms. The investigation of reaction pathways for H(2) activation confirms and extends a previous hypothesis indicating that H(2) can be cleaved on Fe(II)Fe(II) species. In particular, even though [Fe]-hydrogenases are proposed to bind and activate H(2) at a single iron center, the comparison of our data with experimental results obtained studying synthetic complexes (Zhao, X.; Georgakaki, I. P.; Miller, M. L.; Mejia-Rodriguez, R.; Chiang, C.-Y.; Darensbourg, M. Y. Inorg. Chem. 2002, 41, 3917) suggests that activation paths involving both metal ions are also possible. Moreover, mu-H Fe(II)Fe(I) complexes are predicted to correspond to stable species and might be formed in the enzyme catalytic cycle.

Binding Sites↗
PubMed2006

Electronic structure and normal vibrations of CH3(OCH2CH2)nOCH3-M+-CF3SO3- (n = 2-4, M = Li, Na, and K).

Electronic structure and the vibrational frequencies of CH(3)(OCH(2)CH(2))(n)OCH(3)-M(+)-CF(3)SO(3)(-) (n = 2-4, M = Li, Na, and K) complexes have been derived from ab initio Hartree-Fock calculations. The metal ion shows varying coordination from 5 to 7 in these complexes. In tetraglyme-lithium triflate, Li(+) binds to one of the oxygens of CF(3)SO(3)(-) (triflate or Tf(-)) unlike for potassium or sodium ions, which possess bidentate coordination. Structures of glyme-MTf complexes thus derived agree well with those determined from X-ray diffraction experiments. The metal ion binds more strongly to ether oxygens of tetraglyme than its di- or triglyme analogues and engenders contraction of SO (for oxygens binding to metal ion) bonds with consequent frequency upshift for the corresponding vibration in the complex relative to those in the free MTf ion pairs. Complexation of the diglyme with LiTf engenders the largest downshift (91 cm(-1)) for the SO(2) stretching vibration of the free anion, which suggests stronger binding of lithium to the diglyme than the tri- (79 cm(-1)) or tetraglyme (70 cm(-1)). A frequency shift in the opposite direction for the SO (where oxygens do not coordinate to the metal) and CF(3) stretchings, which stems from the ion-polymer and anion-ion interactions, has been noticed. These frequency shifts have been analyzed using natural bond orbital analysis and difference electron density maps coupled with molecular electron density topography.

Journal Article↗
PubMed2000

n-sigma charge-transfer interaction and molecular and electronic structural properties in the hydrogen-bonding systems consisting of p-quinone dianions and methyl alcohol.

Molecular and electronic structural properties of the hydrogen-bonded complexes of p-quinone dianions (PQ(2)(-)) were investigated by electrochemistry and spectroelectrochemistry of PQ in MeCN combined with ab initio MO calculations. Hydrogen bonding between PQ(2)(-) and MeOH was measured as the continuous positive shift of the apparent second half-wave reduction potentials with increasing concentrations of MeOH. Detailed analyses of the behavior reveal that PQ(2)(-) forms the 1:2 hydrogen-bonded complexes at low concentrations of MeOH and the 1:4 complexes at high concentrations, yielding the formation constants. Temperature dependence of the formation constants allows us to yield the formation energy as 76.6 and 118.9 kJ mol(-)(1) for the 1:2 and 1:4 complex formation of the 1,4-benzoquinone dianion (BQ(2)(-)) with MeOH, respectively. These results show that the pi-dianions involving the quinone carbonyl groups exhibit very strong hydrogen-accepting ability. The longest wavelength band of the spectra of BQ(2)(-) and the chloranil dianion (CL(2)(-)) is assigned to the (1)B(3u) <-- (1)A(g) band mainly contributed from an intramolecular charge-transfer (CT) configuration. Hydrogen bonding allows the band of BQ(2)(-) and CL(2)(-) to be blue-shifted, depending on the strength of the hydrogen bonds. CNDO/S-CI calculations reveal that the blue shift is ascribed to stabilization of the ground state by the hydrogen bonding involving strong n-sigma-type CT interaction. The HF/6-31G(d) calculation results show that the structure of PQ(2)(-) is characterized by a lengthening of the C=O bonds and a benzenoid ring. The geometrical properties of the hydrogen-bonded complexes of PQ(2)(-) are a slight lengthening of the C=O bonds and a short distance of the hydrogen bonds. It is demonstrated that this situation is due to the strong n-sigma CT interaction in the hydrogen bonds. The results suggest that the differing functions and properties of biological quinones are conferred by the n-sigma CT interaction through hydrogen bonding of the dianions with their protein environment.

Anions↗
PubMed1989

Prediction of chemical carcinogenicity from molecular and electronic structures: a comparison of MINDO/3 and CNDO/2 molecular orbital methods.

Molecular orbital methods have been used to calculate the electronic structures of a number of chemical carcinogens and non-carcinogens using the MINDO/3 (Modified Intermediate Neglect of Differential Overlap, version 3) and CNDO/2 (Complete Neglect of Differential Overlap) procedures; results from the two methods show good agreement. Spatial conformations of the same series of chemicals have also been determined. High affinity for cytochromes P-448 (P450 I) and high potential for chemical carcinogenicity/toxicity show good correlation with molecular planarity (high values of area/depth2) and low values of the difference between the frontier orbital energies (delta E) of the chemical. This computer graphic procedure identifies substrates of the cytochromes P-448 and is highly suited for the rapid screening of new chemicals for potential mutagenicity, carcinogenicity and certain other forms of toxicity. The implications of these findings to the mechanism of chemical carcinogenicity are discussed.

Carcinogenicity Tests↗
PubMed2000

The electronic structure and spectra of Ru(II) and Ru(III) complexes with imidazole and its derivatives.

The calculations of the electronic structure and spectra of [Ru(NH3)5L]2+ (L = imidazole, histidine) and [Ru(NH3)5L]3+ (L = imidazole, N-imidazolate anion, 4-methylimidazole, 4-methyl-1N-imidazolate anion and 1N-bound histidine) complexes are performed in the framework of the CI method in the INDO/CNDO approximation. The MO diagram is obtained. The assignment of all transitions with energies of 4-5 eV is made and the nature of corresponding excited states is discussed. For the Ru(II) complexes, the lower energy observable transition is assigned to d-->pi* type, whereas the higher energy one is assigned to pi-->pi* type. In the spectra of the Ru(III) complexes with charged ligands both transitions are of pi-->d character, while in the case of uncharged ligands, the higher energy transition mostly incorporates pi-->pi* excitations.

Histidine↗
PubMed2006

Electronic structure and optical properties of charged oligofluorenes studied by VIS/NIR spectroscopy and time-dependent density functional theory.

The electronic structure and optical properties of charged oligofluorenes were studied experimentally and theoretically. Measurements of the optical absorption spectra of charged oligofluorenes in dilute solutions have been performed by using the pulse radiolysis technique. In addition, optical absorption spectra of radical cations and anions in a solid matrix were measured after gamma-irradiation at 77 K. The optical absorption spectra were measured in the range of 440-2100 nm (0.6-2.8 eV) and compared with results from time-dependent density functional theory (TDDFT) calculations. The calculated charge induced deformations and charge distribution do not indicate the occurrence of polaronic effects. The potential energy profiles for rotation around the inter-unit bond show that oligofluorenes are nonplanar in their neutral state, while they tend to more planar structures in their charged state. The optical absorption spectra of charged oligofluorenes are dependent on the angle between neighboring units. TDDFT absorption energies shift to lower values with increasing chain length, which suggests that the charge delocalizes along the oligomer chain.

Journal Article↗
PubMed2002

Electronic structure of multiquantum giant vortex states in mesoscopic superconducting disks.

We report self-consistent calculations of the microscopic electronic structure of the so-called giant vortex states. These multiquantum vortex states, detected by recent magnetization measurements on submicron disks, are qualitatively different from the Abrikosov vortices in the bulk. We find that, in addition to multiple branches of bound states in the core region, the local tunneling density of states exhibits Tomasch oscillations caused by the single-particle interference arising from quantum confinement. These features should be directly observable by scanning tunneling spectroscopy.

Journal Article↗
PubMed2002

Electronic structure of YbXCu(4) (X = In, Cd, Mg) investigated by high-resolution photoemission spectroscopy.

The valence-band electronic structure of YbXCu(4) (X = In, Cd, Mg) has been investigated by means of temperature-dependent high-energy-resolution photoemission spectroscopy using a He I resonance line (hnu = 21.22 eV) and synchrotron radiation (hnu = 800 eV). Intensities of the structure due to the Yb(2+) 4f(7/2) states in the He I spectra of YbInCu(4) and YbCdCu(4) gradually increase with decreasing temperature from 300 to 10 K, and Yb(2+) 4f(7/2) structures are clearly observed as peaks near the Fermi level (E(F)) at 10 K. The enhancement of the Yb(2+) 4f(7/2) peak from 50 to 10 K is much greater for YbInCu(4) than for YbCdCu(4). On the other hand, the Yb(2+) 4f(7/2) states of YbMgCu(4) are observed as a broad structure near E(F). In the synchrotron radiation photoemission spectra of YbInCu(4) and YbCdCu(4), the structures due to the Yb(2+) and Yb(3+) 4f states are recognized at all temperatures. The intensity ratio Yb(2+)/Yb(3+) gradually increases with decreasing temperature. The energy separations between the Yb(2+) and Yb(3+) 4f structures of YbInCu(4) increase from 50 to 20 K. For YbMgCu(4), on the other hand, almost only the Yb(2+) structures are observed and little temperature dependence has been detected.

Journal Article↗
PubMed2001

Electronic structure and catalysis on metal surfaces.

The powerful computational resources available to scientists today, together with recent improvements in electronic structure calculation algorithms, are providing important new tools for researchers in the fields of surface science and catalysis. In this review, we discuss first principles calculations that are now capable of providing qualitative and, in many cases, quantitative insights into surface chemistry. The calculations can aid in the establishment of chemisorption trends across the transition metals, in the characterization of reaction pathways on individual metals, and in the design of novel catalysts. First principles studies provide an excellent fundamental complement to experimental investigations of the above phenomena and can often allow the elucidation of important mechanistic details that would be difficult, if not impossible, to determine from experiments alone.

Journal Article↗
PubMed2003

The electronic structure of Cu+, Ag+, and Au+ zeolites.

A variety of procedures have been used to prepare d10-zeolite materials. The electronic structure of these materials can be regarded to a first approximation as a superposition of the framework, of the charge compensating ions, of solvent molecules and of guest species. Zeolite oxygen to d10-ion charge transfer transitions dominate the electronic spectra if the ions coordinate to the zeolite oxygens. Specific coordination sites can influence the energy and the intensity of these transitions remarkably. Intra guest transitions dominate in quantum dot materials, as discussed in detail for luminescent Ag2S zeolite A. The zeolite is not needed for the photocatalytic water oxidation on Ag+/AgCl photo anodes with visible light. It can, however, be used to increase the active surface area substantially.

Journal Article↗
PubMed2000

Probing the electronic structure of transition metal ion centres in proteins by coherent Raman-detected electron paramagnetic resonance spectroscopy.

The simultaneous excitation of a paramagnetic sample with optical (laser) and microwave radiation can cause an amplitude or phase modulation of the transmitted light at the microwave frequency. The detection of this modulation indicates the presence of coupled optical and electron paramagnetic resonance (EPR) transitions in the sample. Here we report the first application of this technique to a biomolecule: the blue copper centre of Pseudomonas aeruginosa azurin. Using optical excitation at 686 nm, in the thiol to copper(II) charge transfer band, we measure a coherent Raman-detected EPR spectrum of a frozen aqueous solution. Its lineshape is characteristic of the magnetic circular dichroism along each principal g-value axis. This information allows electronic and structural models of transition metal ion centres in proteins to be tested.

Electron Spin Resonance Spectroscopy↗
PubMed2004

Spectroscopic and theoretical study of the molecular and electronic structures of a terthiophene-based quinodimethane.

The UV/Vis, infrared absorption, and Raman scattering spectra of 3',4'-dibutyl-5,5"-bis(dicyanomethylene)-5,5"-dihydro-2,2':5',2"-terthiophene have been analyzed with the aid of density functional theory calculations. The compound exhibits a quinoid structure in its ground electronic state and presents an intramolecular charge transfer from the terthiophene moiety to the C(CN)2 groups. The molecular system therefore consists of an electron-deficient terthiophene backbone end-capped with electron-rich C(CN)2 groups. The molecule is characterized by a strong absorption in the red, due to the HOMO-->LUMO pi-pi* electronic transition of the terthiophene backbone that shifts hypsochromically on passing from the solid state to solution and with the polarity of the solvent. The analysis of the vibrational spectra confirms the structural conclusions and supports the existence of an intramolecular charge transfer. Vibrational spectra in several solvents and as a function of temperature have also been studied. Significant frequency upshifts of the vibrations involved in the pi-electron-conjugated pathway have been noticed upon solution in polar solvents and with the lowering of the temperature. Finally, we propose a quinoid molecule as a reliable structural and electronic model for dication species in doped oligothiophenes or for bipolaron charged defects in doped polythiophene.

Crystallography, X-Ray↗
PubMed2006

Probing actinide electronic structure using fluorescence and multi-photon ionization spectroscopy.

The role of the 5f orbitals in actinide bond formation is one of the central issues of actinide chemistry. This question can be explored using relativistic quantum chemistry calculations, but the theoretical methods must be validated using definitive experimental data. For simple ionic compounds such as actinide oxides and halides, the electronic structure can be examined using electronic and photoelectron spectroscopy of gas phase molecules. The present article surveys recent spectroscopic and theoretical studies of the oxides of thorium and uranium. The results show that the 5f-like states are spectators and that the complex patterns of low-lying electronic states for these molecules can be understood in terms of a semi-empirical ligand field model. Comparisons with recent high-level theoretical calculations are presented and discussed.

Journal Article↗