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PubMed2005

Tetra-t-butyl magnesium phthalocyanine on gold: electronic structure and molecular orientation.

In this work we have investigated the electronic structure and the molecular orientation of (t-Bu)(4)PcMg (tetra-t-butyl magnesium phthalocyanine) on polycrystalline and single crystalline gold substrates using photoemission spectroscopy and x-ray absorption spectroscopy, and we compare the results to the unsubstituted PcCu (copper phthalocyanine). The C 1s photoemission spectrum is described similar to unsubstituted relatives with an additional component for the aliphatic substituents. The variation of the excitation energy causes distinct differences in the shape of the C 1s spectrum, which is very useful for the analysis of the molecular orientation in the uppermost layer. It is shown that despite of the sterically demanding substituents, ordered sublimed films of (t-Bu)(4)PcMg are accessible, the orientation of the molecules, however, is different from the orientation of the unsubstituted relatives.

Journal Article↗
PubMed2006

Sources of error in electronic structure calculations on small chemical systems.

The sources of error in electronic structure calculations arising from the truncation of the one-particle and n-particle expansions are examined with very large correlation consistent basis sets, in some cases up through valence 10-zeta quality, and coupled cluster methods, up through connected quadruple excitations. A limited number of full configuration interaction corrections are also considered. For cases where full configuration interaction calculations were unavailable or prohibitively expensive, a continued fraction approximation was used. In addition, errors arising from corevalence and relativistic corrections are also probed for a number of small chemical systems. The accuracies of several formulas for estimating total energies and atomization energies in the complete basis set limit are compared in light of the present large basis set findings. In agreement with previous work, the CCSD(T) method is found to provide results that are closer to the CCSDTQ and full configuration-interaction results than the less approximate CCSDT method.

Journal Article↗
PubMed2006

Electronic structure of strongly correlated d-wave superconductors.

We study the electronic structure of a strongly correlated d-wave superconducting state. Combining a renormalized mean field theory with direct calculation of matrix elements, we obtain explicit analytical results for the nodal Fermi velocity upsilon(F), the Fermi wave vector k(F), and the momentum distribution n(k) as a function of hole doping in a Gutzwiller projected d-wave superconductor. We calculate the energy dispersion E(k) and spectral weight of the Gutzwiller-Bogoliubov quasiparticles and find that the spectral weight associated with the quasiparticle excitation at the antinodal point shows a nonmonotonic behavior as a function of doping. Results are compared to angle resolved photoemission spectroscopy of the high-temperature superconductors.

Journal Article↗
PubMed1993

[The studies on electronic structure and structure-activity relationships of [Leu5]-enkephalin].

The quantum chemical (INDO) calculations have been undertaken for [Leu5]-enkephalin. The electronic structure was investigated, the active site, the way of action and structure-activity relationship were discussed. It was found that the region of N(1) is the main active site for acceptance of electron, the region of O(51) and O(52) is the main active site for providing electron when interacting with the opiate receptor. [Leu5]-enkephalin was compared with morphine and R31833 in electronic and spatial structure of active site. Study of the results showed that the N(1) in [Leu5]-enkephalin corresponds to the N in morphine and the N(9) in R31833, phenyl of Tyr1 in [Leu5]-enkephalin corresponds to the phenyl in morphine and the phenyl of phenylethyl in R31833, the O(51) and O(52) in [Leu5]-enkephalin corresponds to the O(3) in morphine and the O(25) in R31833, the O(4) in [Leu5]-enkephalin corresponds to the O(29) or O(27) in R31833. On the basis of these studies, it was inferred that these compounds have common feature in pharmacophore. They not only have the same way of action but also have common site of action in the receptor when they interact with the opiate receptor.

Analgesics, Opioid↗
PubMed1978

Mössbauer effect: studies of the electronic structures of the heme.

On the basis of experimental Mössbauer evidence, the electronic structures of heme iron are discussed for Mb and Hb. In the deoxygenated state, it is concluded from the electric field gradient tensor and the temperature dependence of the quadrupole splitting that the ferrous iron is not in the tetragonal ligand field with the symmetry axis perpendicular to the heme. The experimental results are well explained solely by rhombic perturbation. The electronic ground state is a component of the 5E term, and the principal axis of the anisotrophy is parallel to the heme plane. In the oxygenated state, there is no decisive conclusion on the bonding structure of the O2 molecule to the heme. Still, the two models of Pauling and Griffith are consistent with experimental results. Considering a relaxation between two possible conformational states, the peculiar temperature dependence of the quadrupole splitting is resolved in Fe-O2 model compounds.

Chemical Phenomena↗
PubMed2001

Electronic structure of bis(o-iminobenzosemiquinonato)metal complexes (Cu, Ni, Pd). The art of establishing physical oxidation states in transition-metal complexes containing radical ligands.

The ligand 2-anilino-4,6-di-tert-butylphenol and its 2-(3,5-dichloroanilino)-4,6-di-tert-butylphenol analogue react in CH(3)CN or CH(3)OH solutions with divalent transition metal ions in the presence of air and triethylamine. Depending on the metal:ligand ratio (1:1, 1:2, or 1:3) and the presence (or absence) of the cyclic amine 1,4-dimethyl-1,4,7-triazacyclononane (dmtacn), the following complexes have been isolated as crystalline solids: [Co(III)(L(ISQ))(3)] (1); [Cu(II)(dmtacn)(L(ISQ))]PF(6) (2); [Cu(II)(L(ISQ))(2)] (3); [Ni(II)(L(ISQ))(2)] (4a); [Ni(II)((Cl)L(ISQ))(2)] (4b); [Pd(II)(L(ISQ))(2)] (5). (L(ISQ))(-) represents the monoanionic o-iminobenzosemiquinonate radical (S(rad) = (1)/(2)). Compounds 1-5 have been characterized by single-crystal X-ray crystallography at 100(2) K. For all complexes it is unambiguously established that the O,N-coordinated o-iminobenzosemiquinonato(1-) ligand is present. Complexes 3, 4b, and 5 are square planar molecules which possess an S(t) = (1)/(2), 0, and 0 ground state, respectively, as was established by (1)H NMR and EPR spectroscopies and variable-temperature magnetic susceptibility measurements. Complex 2 possesses an S(t) = 1 ground state which is attained via strong intramolecular ferromagnetic coupling (J = +195 cm(-1)) between the d(x)2-(y)2 magnetic orbital of the Cu(II) ion and the pi-orbital of the ligand radical. Complex 1 contains three mutually orthogonal (L(ISQ))(-*) ligands and has an S(t) = (3)/(2) ground state. It is shown that the electronic structure of 4a and 5 is adequately described as singlet diradical containing a divalent, diamagnetic d(8) configurated central metal ion and two strongly antiferromagnetically coupled (L(ISQ))(-) radical ligands. It is concluded that the same electronic structure prevails in the classic bis(o-diiminobenzosemiquinonato)- and bis(o-benzosemiquinonato)metal complexes of Ni(II), Pd(II), and Pt(II). The electrochemistry of all complexes has been investigated in detail. For 3, 4a, and 5 a series of reversible one-electron-transfer waves leads to the formation of the anions and cations [M(L)(2)](2-),(1-),(1+),(2+) which have been characterized spectroelectrochemically. All redox processes are shown to be ligand-based.

Journal Article↗
PubMed1996

Compounds Containing Copper-Sulfur Layers: Electronic Structure, Conductivity, and Stability.

Compounds of the general formula MCu(2n)X(n)(+1), where M is a monovalent metal and X is a chalcogen, exhibit relatively high conductivity and an interesting structural pattern of copper-chalcogen layers. The electronic structure of a series of copper-sulfur layers with the Cu(2n)S(n)(+1) stoichiometry was studied using the extended Hückel method. Attention was focused on the unoccupied states at the top of the valence band. These states are Cu-S and Cu-Cu antibonding, which accounts for the observed contraction in the plane of the layers. The same states turn out to be strongly delocalized in the plane of the layers, with both copper and sulfur contribution; high mobility of holes in these states is responsible for the substantial conductivity observed in the corresponding materials. The idea of isodesmic reactions, borrowed from computational organic chemistry, was developed to address the relative stabilities of the copper-sulfur layers. We found the Cu(2)S(2)(-) layer to be less stable than the Cu(4)S(3)(-) layer, in accord with experiment.

Journal Article↗
PubMed2006

Many-body electronic structure of americium metal.

We report computer based simulations of energetics, spectroscopy, and electron-phonon interaction of americium using a novel spectral density functional method. This approach gives rise to a new concept of a many-body electronic structure and reveals the unexpected mixed valence regime of Am 5f6 electrons which under pressure acquire the 5f7 valence state. This explains the unique properties of Am and addresses the fundamental issue of how the localization delocalization edge is approached from the localized side in a closed shell system.

Journal Article↗
PubMed1976

Electronic structure and bonding of the amino acids containing first row atoms.

The electronic structures of the amino acids containing first row atoms have been determined for the zwitterionic form using an approximate self-consistent field method, partial retention of diatomic differential overlap. Various energetic quantities including certain proton affinities are present as are eigenvalues for the highest occupied and lowest unoccupied molecular orbitals. It is found that our method, in common with all methods employing minimum basis sets, yields eigenvalues for the highest occupied molecular orbital that are too high. The method does predict the location of this orbital correctly when compared to calculations employing larger basis sets. It is predicted that electron loss due to ionizing radiation should occur from the carboxylate group for the nonaromatic amino acids, while for tyrosine and tryptophan, electron loss should occur from the ring system. No choice between these two sites can be made for phenylalanine. Charge distributions have been obtained which show that only partial zwitterionic character is found in the backbone and that little delocalization of charge from the backbone to the side chain occurs. Localized molecular orbitals have been obtained using the Boys criteria and the bonding in the amino acids is disscussed in terms of these orbitals. Hybridization of various bonds and bond polarities are discussed as is the phenomenon of fractional bonding to carbon.

Amino Acids↗
PubMed2003

Reduction pathway of end-on coordinated dinitrogen. 3. Electronic structure and spectroscopic properties of molybdenum/tungsten hydrazidium complexes.

The spectroscopic properties and electronic structure of the hydrazidium complexes [MF(NNH(3))(depe)(2)](BF(4))(2), M = Mo and W, are investigated (depe = 1,2-bis(diethylphosphino)ethane). Vibrational spectroscopic data for both compounds are evaluated with a quantum-chemistry-assisted normal coordinate analysis, giving an N-N force constant of 6.03 mdyn/A and metal-N force constants of 8.01 (Mo-N) and 7.31 mdyn/A (W-N), respectively. On the basis of these results and DFT calculations on a [MoF(NNH(3))(PH(3))(4)](2+) model system, the N-N bond order in these systems is 1 (single sigma bond) and metal-N bonding corresponds to a triple bond. The metal centers are assigned a +IV oxidation state (d(2) configuration) and the NNH(3) ligand is assigned a -1 formal charge which by sigma- and pi-donation to the metal is reduced to +0.48. The two metal-d electrons are located in the nonbonding (n) d(xy)() orbital. This bonding description is supported by the results of optical absorption spectroscopy showing the n --> (metal-ligand)pi transition at 536 nm (not observed in the tungsten compound) and the (metal-ligand)pi --> (metal-ligand)pi transition at 251 nm for the MoNNH(3) and at 237 nm for the WNNH(3) complex. The activation enthalpy for splitting of the N-N bond in these systems to generate NH(4)(+) is estimated to be larger than 40 kcal/mol. Hydrazidium complexes with diphosphine coligands are therefore inert with respect to N-N cleavage and thus represent the ultimate stage of N(2) reduction at six-coordinate d(6) metal centers in the absence of external reductants.

Journal Article↗
PubMed2003

Determination of ligand-field parameters and f-electronic structures of double-decker bis(phthalocyaninato)lanthanide complexes.

The f-electronic structures of the ground states of anionic bis(phthalocyaninato)lanthanides, [Pc(2)Ln](-) (Pc = dianion of phthalocyanine, Ln = Tb(3+), Dy(3+), Ho(3+), Er(3+), Tm(3+), or Yb(3+)), are determined. Magnetic susceptibilities of the powder samples of [Pc(2)Ln]TBA (TBA = tetra-n-butylammonium cation) in the range 1.8-300 K showed characteristic temperature dependences which resulted from splittings of the ground-state multiplets. NMR signals for the two kinds of protons on the Pc rings at room temperature were shifted to lower frequency with respect to the diamagnetic Y complex in Ln = Tb, Dy, and Ho cases, and to higher frequency in Er, Tm, and Yb cases. The ratios of the paramagnetic shifts of the two positions were near constant in the six cases. This indicates that the shifts are predominantly caused by the magnetic dipolar term, which is determined by the anisotropy of the magnetic susceptibility of the lanthanide ion. Using a multidimensional nonlinear minimization algorithm, we determined a set of ligand-field parameters that reproduces both the NMR and the magnetic susceptibility data of the six complexes simultaneously. Each ligand-field parameter was assumed to be a linear function of atomic number of the lanthanide. The energies and wave functions of the sublevels of the multiplets are presented. Temperature dependences of anisotropies in the magnetic susceptibilities are theoretically predicted for the six complexes.

Journal Article↗
PubMed2006

Sulfur K-edge XAS and DFT calculations on nitrile hydratase: geometric and electronic structure of the non-heme iron active site.

The geometric and electronic structure of the active site of the non-heme iron enzyme nitrile hydratase (NHase) is studied using sulfur K-edge XAS and DFT calculations. Using thiolate (RS(-))-, sulfenate (RSO(-))-, and sulfinate (RSO(2)(-))-ligated model complexes to provide benchmark spectral parameters, the results show that the S K-edge XAS is sensitive to the oxidation state of S-containing ligands and that the spectrum of the RSO(-) species changes upon protonation as the S-O bond is elongated (by approximately 0.1 A). These signature features are used to identify the three cysteine residues coordinated to the low-spin Fe(III) in the active site of NHase as CysS(-), CysSOH, and CysSO(2)(-) both in the NO-bound inactive form and in the photolyzed active form. These results are correlated to geometry-optimized DFT calculations. The pre-edge region of the X-ray absorption spectrum is sensitive to the Z(eff) of the Fe and reveals that the Fe in [FeNO](6) NHase species has a Z(eff) very similar to that of its photolyzed Fe(III) counterpart. DFT calculations reveal that this results from the strong pi back-bonding into the pi antibonding orbital of NO, which shifts significant charge from the formally t(2)(6) low-spin metal to the coordinated NO.

Binding Sites↗
PubMed2005

Electronic structure of Cob(I)alamin: the story of an unusual nucleophile.

The electronic structure and the ligand-field spectrum of cobalt(I) corrin is reported using complete active space multiconfigurational perturbation theory (CASPT2) to address some inconsistencies and the nature of the cobalt(I) "supernucleophile", cob(I)alamin. An assignment of six of the seven intense lines in the experimental spectrum is obtained at a root-mean-square accuracy of 0.14 eV and largest error of 0.21 eV. Agreement is significantly better for CASPT2 than density functional theory (DFT), but DFT does surprisingly well. The correlated wave function implies that the ground state of Co(I) corrin is severely multiconfigurational, with only 67% of the d(8) reference configuration and prominent contributions of 20% from open-shell metal-to-ligand charge-transfer configurations. The ground state exhibits a fascinating degree of covalency between cobalt and the nitrogen orbitals, described by the bonding and antibonding orbital pair of a cobalt d-orbital and a delta-orbital linearly combined from nitrogen orbitals. Thus, the standard description of the d(8) supernucleophile is not completely valid. From a biological perspective, the mixing in of Co(II) configurations in cob(I)alamin may be an important reason for the redox accessibility of the formal Co(I) state of the cofactor, which again provides the catalytic power for one half-reaction of enzymes such as cobalamin-dependent methionine synthase.

Models, Molecular↗
PubMed2005

Characterization of the electronic structure of C50Cl10 by means of soft x-ray spectroscopies.

The electronic structure of the last synthesized fullerene molecule, the C50Cl10, has been characterized by theoretical simulation of x-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and near-edge x-ray-absorption fine structure. All the calculations were performed at the gradient-corrected and hybrid density-functional theory levels. The combination of these techniques provides detailed information about the valence band and the unoccupied molecular orbitals, as well as about the carbon core orbitals.

Journal Article↗
PubMed2006

Electronic structure of C60 on Au(887).

We present an analysis of the electronic structure of C60 adsorbed on a vicinal Au(111) surface at different fullerene coverages using photoemission, x-ray absorption, and scanning tunneling microscopy/spectroscopy (STS). STS provides a straightforward determination of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels with respect to the Fermi energy. At C60 coverages of 0.5 and 1 ML a 2.7 eV wide HOMO-LUMO gap is found. The near-edge x-ray absorption fine structure (NEXAFS) spectrum for the 0.5 ML C60 nanomesh structure displays a significant intensity at the low energy side of the LUMO exciton peak, which is explained as due to absorption into HOMO-LUMO gap states localized at individual C60 cluster edges. From 0.5 to 1 ML we observe a rigid shift of the HOMO-LUMO peaks in the STS spectra and an almost complete quenching of the gap states feature in NEXAFS.

Journal Article↗
PubMed2002

Electronic structure of the muonium center as a shallow donor in ZnO.

The electronic structure and the location of muonium centers (Mu) in single-crystalline ZnO were determined for the first time. Two species of Mu centers with extremely small hyperfine parameters have been observed below 40 K. Both Mu centers have an axial-symmetric hyperfine structure along with a <0001> axis, indicating that they are located at the antibonding (AB(O, parallel )) and bond-center (BC( parallel )) sites. It is inferred from their small ionization energy ( approximately 6 and 50 meV) and hyperfine parameters ( approximately 10(-4) times the vacuum value) that these centers behave as shallow donors, strongly suggesting that hydrogen is one of the primary origins of n type conductivity in as-grown ZnO.

Journal Article↗
PubMed2004

Temperature dependent magnetic anisotropy in metallic magnets from an ab initio electronic structure theory: L1(0)-ordered FePt.

Using a first-principles, relativistic electronic structure theory of finite temperature metallic magnetism, we investigate the variation of magnetic anisotropy K with magnetization M in metallic ferromagnets. We apply the theory to the high uniaxial K material, L1(0)-ordered FePt, and find its magnetic easy axis perpendicular to the Fe/Pt layers for all M and K to be proportional to M2 for a broad range of values of M. For small M, near the Curie temperature, the calculations pick out the easy axis for the onset of magnetic order. Our ab initio results for this important magnetic material agree well with recent experimental measurements, whereas the single-ion anisotropy model fails to give the correct qualitative behavior.

Journal Article↗
PubMed2000

Electronic structure of mott insulators studied by inelastic X-ray scattering

The electronic structure of Mott insulators continues to be a major unsolved problem in physics despite more than 50 years of research. Well-developed momentum-resolved spectroscopies such as photoemission or neutron scattering cannot probe the full Mott gap. High-resolution resonant inelastic x-ray scattering revealed dispersive charge excitations across the Mott gap in a high-critical temperature parent cuprate (Ca(2)CuO(2)Cl(2)), shedding light on the anisotropy of the Mott gap. These charge excitations across the Mott gap can be described within the framework of the Hubbard model.

Journal Article↗