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At least 703 records · Page 39Linked to original sources
PubMed2005

First structural characterization of a delocalized, mixed-valent, triangular Cu37+ species: chemical and electrochemical oxidation of a CuII3(mu3-O) pyrazolate and electronic structure of the oxidation product.

The chemical or electrochemical one-electron oxidation of the all-CuII complex [Cu3(mu3-O)(mu-pz)3X3]2- leads to its formally CuII2CuIII analogue (pz = pyrazolato anion; X = Cl- and PhCOO-). The X-ray single-crystal structure and density functional theory analysis of the latter agree in revealing the delocalized nature of its mixed-valent Cu3(7+) core.

Copper↗
PubMed2002

Structural, electronic, and magnetic consequences of O-carbonyl vs O-alkoxy ester coordination in new dicopper complexes containing the Cu2(mu-Cl)2 core.

The complexes [Cu2(mu-Cl)2(Cl)2(L)2] (L = dialkylpyridine-2,6-dicarboxylate; R = Et, L = depc, 1; R = i-Pr, L = dppc, 2) have been prepared and their magnetic properties studied. The crystal structures of complexes 1 and 2 have been solved. Compound 1 belongs to the P space group with Z = 2, a = 8.3020(10) A, b = 9.2050(10) A, c = 10.065(2) A, alpha = 99.040(10), beta = 100.810(10), and gamma = 106.502(10) whereas 2 belongs to the C2/c space group with Z = 8, a = 11.6360(10) A, b = 25.906(3) A, c = 11.76579(10) A, and beta = 107.900(10). The different alkyl ester substitutes produce substantial structural and electronic differences. The Cu2Cl2 core geometry is planar for 1 whereas it adopts a butterfly shape in the case of 2. Furthermore, in 2 the dppc ligand coordinates only by the carbonyl oxygen atoms whereas in 1 the depc ligand coordinates through carbonyl and alkoxy oxygen atoms. Magnetic susceptibility data show a ferromagnetic coupling between the two Cu(II) centers in both cases (J = 39.9(6) cm(-1) for 1, and J = 51.3(5) cm(-1) for 2) with very weak antiferromagnetic interactions (J ' = -0.59 cm(-1) and -0.57 cm(-1) for 1 and 2, respectively). Theoretical calculations at the extended Hückel level have also been carried out to further understand the electronic nature of complexes 1 and 2.

Journal Article↗
PubMed2003

Electronic structure of ReO3Me by variable photon energy photoelectron spectroscopy, absorption spectroscopy and density functional calculations.

Valence photoelectron (PE) spectra have been measured for ReO(3)Me using a synchrotron source for photon energies ranging between 20 and 110 eV. Derived branching ratios (BR) and relative partial photoionization cross sections (RPPICS) are interpreted in the context of a bonding model calculated using density functional theory (DFT). Agreement between calculated and observed ionization energies (IE) is excellent. The 5d character of the orbitals correlates with the 5p --> 5d resonances of the associated RPPICS; these resonances commence around 47 eV. Bands with 5d character also show a RPPICS maximum at 35 eV. The RPPICS associated with the totally symmetric 4a(1) orbital, which has s-like character, shows an additional shape resonance with an onset of 43 eV. The PE spectrum of the inner valence and core region measured with photon energies of 108 and 210 eV shows ionization associated with C 2s, O 2s, and Re 4f and 5p electrons. Absorption spectra measured in the region of the O1s edge showed structure assignable to excitation to the low lying empty "d" orbitals of this d(0) molecule. The separation of the absorption bands corresponded with the calculated orbital splitting and their intensity with the calculated O 2p character. Broad bands associated with Re 4d absorption were assigned to (2)D(5/2) and (2)D(3/2) hole states. Structure was observed associated with the C1s edge but instrumental factors prevented firm assignment. At the Re 5p edge, structure was observed on the (2)P(3/2) absorption band resulting from excitation to the empty "d" levels. The intensity ratios differed from that of the O 1s edge structure but were in good agreement with the calculated 5d character of these orbitals. An absorption was observed at 45 eV, which, in the light of the resonance in the 4a(1) RPPICS, is assigned to a 4a(1) --> ne, na(2) transition. The electronic structure established for ReO(3)Me differs substantially from that of TiCl(3)Me and accounts for the difference in chemical behavior found for the two complexes.

Journal Article↗
PubMed2006

In-plane anisotropy of the electronic structure for the charge- and orbital-ordered state in half-doped manganite with layered structure.

We report on the in-plane anisotropy of the electronic response in the spin-, charge-, and orbital-ordered phase of a half-doped layered-structure manganite. The optical conductivity spectra for a single domain of Eu1/2Ca3/2MnO4 unambiguously show the anisotropic charge dynamics which well corresponds to the theoretical calculation: the optical conductivity with the polarization along the zigzag ferromagnetic chain direction exhibits a smaller gap and a larger intensity at lower energies than that of the perpendicular polarization mostly due to the charge and orbital ordering and the associated quantum interference effect.

Journal Article↗
PubMed2004

Molecular and electronic structures by design: tuning symmetrical and unsymmetrical linear trichromium chains.

The preparation, properties, and crystal structures of 12 trichromium extended metal atom chain (EMAC) compounds of the type Cr(3)(L)(4)X(2) (L = equatorial ligands dipyridylamide (dpa) or di-4,4'-ethyl-2,2'-pyridylamide (depa), and X = axial ligands, e.g., halide or pseudohalide ions) with large variations in metal-metal distances are reported here. These complexes, which belong to a broad class of fundamentally interesting trinuclear molecules over which the electrons may or may not be delocalized, pose significant theoretical and experimental challenges which are dealt with in this report. Complexes with strongly donating axial or equatorial ligands tend to favor a symmetrical (D(4)) molecular structure, while more weakly donating ligands give rise to unsymmetrical (C(4)) structures; the physical properties of these two classes of compounds are discussed fully, and important comparisons with a reported DFT model of the electronic structures of the compounds are made.

Journal Article↗
PubMed2006

Electronic structures of 3d-metal mononitrides.

Bond distances, vibrational frequencies, electron affinities, ionization potentials, and dissociation energies of the title molecules in neutral, positively, and negatively charged ions were studied by use of density functional methods B3LYP, BLYP, BHLYP, BPW91, and B3PW91. The calculated results are compared with experiments and previous theoretical studies. It was found that the calculated properties are highly dependent on the functionals employed, in particular for the dissociation energy and vibrational frequency. For neutral species, pure density functional methods BLYP and BPW91 have relatively good performance in reproducing the experimental bond distance and vibrational frequency. For cations, hybrid exchange functional methods B3LYP and B3PW91 are good in predicting the dissociation energy. For both neutral and charged species, BHLYP tends to give smaller dissociation energy.

Journal Article↗
PubMed1998

Pulsed ENDOR and ESEEM Study of [Bis(maleonitriledithiolato)nickel](-): An Investigation into the Ligand Electronic Structure.

By a combination of Q-band pulsed ENDOR (electron nuclear double resonance) and X-band ESEEM (electron stimulated echo envelope modulation) techniques, we have determined the hyperfine tensors for ethylene (C1) and cyano (C2) carbons and N, of [Ni(mnt)(2)](-), along with the quadrupole tensor for nitrogen. These measurements give pi electron spin densities of rho(C1) approximately 0.03 in the C1 2p(z)() orbital, rho(C2) < 0.003, rho(N) approximately 0.01, such that in total, approximately 0.15 of the spin resides on the ligand atoms C and N, while the rest resides in the NiS(4) core, giving rho(NiS(4)(-)) = 0.85. These results are compared with extended Hückel and density functional (BLYP) MO calculations, as well as with Xalpha calculations reported earlier.

Journal Article↗
PubMed2006

Electronic structure calculations on the C4 cluster.

The ground and the electronically excited states of the C4 radical are studied using interaction configuration methods and large basis sets. Apart from the known isomers [l-C4(X(3)Sigmag (-)) and r-C4(X(1)Ag)], it is found that the ground singlet surface has two other stationary points: s-C4(X(1)Ag) and d-C4(X(1)A1). The d-C4 form is the third isomer of this cluster. The isomerization pathways from one form to the other show that deep potential wells are separating each minimum. Multireference configuration interaction studies of the electronic excited states reveal a high density of electronic states of these species in the 0-2 eV energy ranges. The high rovibrational levels of l-C4((3)Sigmau (-)) undergo predissociation processes via spin-orbit interactions with the neighboring (5)Sigmag + state.

Journal Article↗
PubMed2000

Molden: a pre- and post-processing program for molecular and electronic structures.

Molden is a software package for pre- and postprocessing of computational chemistry program data. Interfacing to the ab initio programs Games-US/UK and Gaussian and to the semi-empirical package MOPAC is provided. The emphasis is on computation and visualization of electronic and molecular properties but, e.g., reaction pathways can be simulated as well. Some molecular properties of interest are processed directly from the output of the computational chemistry programs, others are calculated in MOLDEN before display. The package features different options to display MOLecular electronic DENsity, each focusing on a different structural aspect: molecular orbitals, electron density, molecular minus atomic density and the Laplacian of the electron density. To display difference density, either the spherically averaged atomic density or the oriented ground state atomic density can be used for a number of standard basis sets. The quantum mechanical electrostatic potential or a distributed multiple expansion derived electrostatic potential can be calculated and atomic charges can be fitted to these potentials calculated on Connolly surface(s). Reaction pathways and molecular vibrations can be visualized. Input structures can be generated with a Z-matrix editor. A variety of graphics languages is supported: XWindows, postscript, VRML and Povray format.

Computer Simulation↗
PubMed2002

A semiempirical study on the electronic structure of 10-deacetylbaccatin-III.

We performed a conformational and electronic analysis for 10-deacetylbaccatin-III (DBAC) using well-known semiempirical methods (parametric method 3 (PM3) and Zerner's intermediate neglect of differential overlap (ZINDO)) coupled to the concepts of total and local density of states (LDOS). Our results indicate that regions presented by paclitaxel (Taxol) as important for the biological activity can be traced out by the electronic features present in DBAC. These molecules differ only by a phenylisoserine side chain. Compared to paclitaxel, DBAC has a simpler structure in terms of molecular size and number of degrees of freedom (d.f.). This makes DBAC a good candidate for a preliminary investigation of the taxoid family. Our results question the importance of the oxetane group, which seems to be consistent with recent experimental data.

Alkaloids↗
PubMed2004

Investigation of the electronic and structural properties of potassium hexaboride, KB6, by transport, magnetic susceptibility, EPR, and NMR measurements, temperature-dependent crystal structure determination, and electronic band structure calculations.

The electronic and structural properties of potassium hexaboride, KB(6), were examined by transport, magnetic susceptibility, EPR, and NMR measurements, temperature-dependent crystal structure determination, and electronic band structure calculations. The valence bands of KB(6) are partially empty, but the electrical resistivity of KB(6) reveals that it is not a normal metal. The magnetic susceptibility as well as EPR and NMR measurements show the presence of localized electrons in KB(6). The EPR spectra of KB(6) have two peaks, a broad ( approximately 320 G) and a narrow (less than approximately 27 G) line width, and the temperature-dependence of the magnetic susceptibility of KB(6) exhibits a strong hysteresis below 70 K. The temperature-dependent crystal structure determination of KB(6) shows the occurrence of an unusual variation in the unit cell parameter hence supporting that the hysteresis of the magnetic susceptibility is a bulk phenomenon. The line width DeltaH(pp) of the broad EPR signal is independent of temperature and EPR frequency. This finding indicates that the line broadening results from the dipole-dipole interaction, and the spins responsible for the broad EPR peak has the average distance of approximately 1.0 nm. To explain these apparently puzzling properties, we examined a probable mechanism of electron localization in KB(6) and its implications.

Journal Article↗
PubMed2005

Electronic structure of 1,3,5-triaminobenzene trication and related triradicals: doublet versus quartet ground state.

Quantum chemical calculations have been carried out to determine the electronic ground state of the parent 1,3,5-triaminobenzene trication triradical (TAB3+,C6H9N3 3+) containing a six-membered benzene ring coupled with three exocyclic amino NH(*+)2 groups, each containing an unpaired electron, as the simplest model for high-spin polyarylamine polycations. Related triradicals, including the 1,3,5-trimethylenebenzene (TMB, C9H9) and its nitrogen derivatives such as the monocation C8H9N+, the dication C7H9N2 2+, and the neutral C8H8N, C7H7N2, and C6H6N3 systems containing NH groups, have also been considered. Results obtained using the CASSCF [multiconfigurational complete active space (SCF--self-consistent field)] method, with active spaces ranging from (9e/9o) to (15e/12o), followed by second-order perturbation theory [CASPT2 and MS-CASPT2 (MS--multistate)] with polarized 6-311G(d,p) and natural orbital (ANO-L) basis sets reveal the following: (i) both TAB3+ and TMB (D3h) have a quartet 4A"1 ground state with doublet-quartet 2B1-4A"1 energy gaps of 8.0+/-2.0 and 12.4+/-2.0 kcal/mol, respectively; (ii) in the neutral N series, the quartet state remains the electronic ground state, irrespective of the number of N atoms, but each with slightly reduced gap, 11 kcal/mol for C8H8N (4A"), 10 kcal/mol for C7H7N2 (4A2), and 9 kcal/mol for C6H6N3 (4A2); and (iii) the ground state of monoamino cation and diamino dication is a low-spin doublet state (2B1 for C8H9N+ and 2A2 for C7H9N2 2+) and lying well below the corresponding quartet state by 10 and 12 kcal/mol, respectively. In the monocationic and dicationic amino systems, a slight preference is found for the low-spin state, apparently violating Hund's rule. This effect is due to the splitting of the orbital energies and the presence of the positive charge whose delocalization strongly modifies the electronic distribution and some structural features. In the latter cations, the positive charge basically pushes unpaired electrons onto the ring forming a kind of distonic radical cations and thus gives a preference for a low-spin state.

Journal Article↗
PubMed2004

Structural and electronic properties of Si n, Si n+, and AlSi n-1 (n=2-13) clusters: theoretical investigation based on ab initio molecular orbital theory.

The geometric and electronic structures of Si(n), Si(n) (+), and AlSi(n-1) clusters (2< or =n< or =13) have been investigated using the ab initio molecular orbital theory under the density functional theory formalism. The hybrid exchange-correlation energy function (B3LYP) and a standard split-valence basis set with polarization functions [6-31G(d)] were employed for this purpose. Relative stabilities of these clusters have been analyzed based on their binding energies, second difference in energy (Delta (2)E) and fragmentation behavior. The equilibrium geometry of the neutral and charged Si(n) clusters show similar structural growth. However, significant differences have been observed in the electronic structure leading to their different stability pattern. While for neutral clusters, the Si(10) is magic, the extra stability of the Si(11) (+) cluster over the Si(10) (+) and Si(12) (+) bears evidence for the magic behavior of the Si(11) (+) cluster, which is in excellent agreement with the recent experimental observations. Similarly for AlSi(n-1) clusters, which is isoelectronic with Si(n) (+) clusters show extra stability of the AlSi(10) cluster suggesting the influence of the electronic structures for different stabilities between neutral and charged clusters. The ground state geometries of the AlSi(n-1) clusters show that the impurity Al atom prefers to substitute for the Si atom, that has the highest coordination number in the host Si(n) cluster. The fragmentation behavior of all these clusters show that while small clusters prefers to evaporate monomer, the larger ones dissociate into two stable clusters of smaller size.

Journal Article↗
PubMed2005

Electronic structure of metal-semiconductor nanojunctions in gold CdSe nanodumbbells.

The electronic properties of metal-semiconductor nanojunctions are investigated by scanning tunneling spectroscopy of gold-tipped CdSe rods. A gap similar to that in bare CdSe nanorods is observed near the nanodumbbell center, while subgap structure emerges near the metal-semiconductor nanocontact. This behavior is attributed to the formation of subgap interface states that vanish rapidly towards the center of the rod, consistent with theoretical predictions. These states lead also to modified Coulomb staircase, and in some cases to negative differential conductance, on the gold tips.

Journal Article↗
PubMed2004

Electronic structure of regular bacterial surface layers.

We report photoemission and near-edge x-ray absorption fine structure measurements of the occupied and unoccupied valence electronic states of the regular surface layer of Bacillus sphaericus, which is widely used as the protein template for the fabrication of metallic nanostructures. The two-dimensional protein crystal shows a semiconductorlike behavior with a gap value of approximately 3.0 eV and the Fermi energy close to the bottom of the lowest unoccupied molecular orbital. We anticipate that these results will open up new possibilities for the electric addressability of biotemplated low-dimensional hybrid structures.

Bacillus↗