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At least 775 records · Page 43Linked to original sources
PubMed2005

A systematic density functional theory study of the electronic structure of bulk and (001) surface of transition-metals carbides.

A systematic study of the bulk and surface geometrical and electronic properties of a series of transition-metal carbides (TMC with TM = Ti, V, Zr, Nb, Mo, Hf, Ta, and W) by first-principles methods is presented. It is shown that in these materials the chemical bonding is strongly covalent, the cohesive energies being directly related to the bonding-antibonding gap although the shift of the center of the C(2s) band related peak in the density of states with respect to diamond indicates that some metal to carbon charge transfer does also take place. The (001) face of these metal carbides exhibits a noticeable surface rumpling which grows along the series. It is shown that neglecting surface relaxation results in very large errors on the surface energy and work function. The surface formation induces a significant shift of electronic energy levels with respect to the corresponding values in the bulk. The extent and nature of the shift can be understood from simple bonding-antibonding arguments and is enhanced by the structural rippling of this surface.

Journal Article↗
PubMed1998

Electronic Structure of Luminescent d(0) Niobium and Tantalum Imido Compounds cis,mer-M(NR)Cl(3)L(2).

Electronic absorption and emission spectra are reported for luminescent d(0) monoimido group 5 compounds M(NR)Cl(3)L(2) (M = Nb, Ta; R = alkyl, aryl; L = dme, Cl(-), py). These compounds display weak (epsilon < 200 M(-)(1) cm(-)(1)), well-resolved lowest-energy transitions in the high-energy visible and near-UV regions (20 000 < E(abs) < 29 000 cm(-)(1)). The energy of this absorption band depends strongly on the nature of the imido substituent, with a significant decrease observed when aryl groups are present. Excitation into this transition results in long-lived luminescent excited states. Long emission lifetimes (50 ns to 17 &mgr;s) and high quantum yields (0.001-0.24) are observed, decreasing primarily as a function of the alkyl substituent, being lowest in the aryl imidos. Good overlap is observed with absorption, excitation, and emission mirror spectra, indicating absorption into and emission from the same excited state. The data are consistent with absorption into and emission from a (3)(nb, pi) state, or d(xy)() <-- Ta-N pi. Semiempirical molecular orbital calculations are presented which suggest that the imido compounds may be considered as having highly mixed but localized Ta=N pi-bonding. A significant difference is noted in [Ta(NPh)Cl(5)](2)(-), in which there is appreciable aryl character in Ta=N pi-type orbitals. This accounts for the difference in electronic properties of the aryl imidos compared to the alkyl imidos. An analysis of radiative and nonradiative excited-state deactivation pathways is presented. Significantly, an energy gap law correlation is observed for nonradiative decay in the imido compounds as a group, but a corresponding correlation of radiative rates with emission energy is not observed when aryl and alkyl imidos are compared, evidence of electronic perturbation by the aryl substituent.

Journal Article↗
PubMed2006

Electronic structure and prediction of atomization energy of naked homoleptic uranium hexacarbonyl U(CO)6.

Our ab initio all-electron fully relativistic Dirac-Fock and nonrelativistic Hartree-Fock self-consistent field (SCF) calculations predict the octahedral (Oh) uranium hexacarbonyl U(CO)6 to be bound with the calculated atomization energy of 49.84 and 48.76 eV at the predicted U-C bond lengths (assuming the C-O bond distance fixed at 1.17 A) of 2.53 and 2.63 A, respectively. Moreover, our all-electron fully relativistic Dirac-Fock SCF calculations predict U(CO)6 to be lower in energy by 3.90 eV with respect to dissociation into U plus six CO ligands. We predict U(CO)6 (Oh) to be very stable in view of our predicted large atomization energy (approximately 49 eV) and stability (approximately 4 eV) with respect to dissociation into U plus six CO molecules. Innovative techniques should be devised for the synthesis of uranium hexacarbonyl since the usual synthetic methods have failed so far for this naked actinide hexacarbonyl.

Journal Article↗
PubMedDate not supplied

[Dynamic properties, electronic structure and functional activity of radioprotectors].

The methods of molecular dynamics and quantum chemistry were used to study the correlation between the functional activity of radioprotectors and their dynamic properties and atomic charges. It is shown that electrostatic interactions determine the correlation between the dynamics and functional activity of radioprotectors. The effects of conformation dynamics of aminothiols on the functions of radioprotectors is discussed.

Electrons↗
PubMed1992

Molecular connectivity: treatment of the electronic structure of amino acids.

A molecular connectivity model of atomic charges on the second-row atoms in alpha-amino acids is discussed. A correlation was found between the valence delta (delta v) index from molecular connectivity theory and the charges on atoms that belong to the same class, whereas different classes of carbon atoms had the same slopes on the Q versus sigma v matrix, where Q is the calculated charge distribution. A similar correlation was found for nucleotide bases.

Amino Acids↗
PubMed2002

Electronic structures and energetics of [5,5] and [9,0] single-walled carbon nanotubes.

Standard enthalpies of formation, ionization potentials, electron affinities, and band gaps of finite-length [5,5] armchair and [9,0] zigzag single-walled carbon nanotubes (SWNTs) capped with C(30) hemispheres obtained by halving the C(60) fullerene have been computed at the B3LYP/6-311G* level of theory. Properties of SWNTs are found to depend strongly on the tube length and, in the case of the [9,0] zigzag species, on the relative orientation of the caps. The metallic character of an uncapped infinite-length [5,5] armchair SWNT manifests itself in the oscillatory dependence of the properties of capped finite-length tubes on their size. An infinite-length [9,0] zigzag SWNT is predicted to be a semiconductor rather than a metal irrespective of the presence of caps. The present results underscore the slow convergence of SWNT properties with respect to the tube length and uncover small but significant radial distortions along the long axes of SWNTs.

Journal Article↗
PubMed2005

Electronic structure, magnetic ordering, and formation pathway of the transition metal oxide hydride LaSrCoO(3)H(0.7).

The role of the hydride anion in controlling the electronic properties of the transition metal oxide hydride LaSrCoO(3)H(0.7) is investigated theoretically by full potential DFT band structure calculation and experimentally by determination of the Neel temperature for three-dimensional magnetic ordering. The mechanism by which hydrogen is introduced into the solid is addressed by in situ X-ray diffraction studies of the formation of the oxide hydride, which reveal both a relationship between the microscopic growth of the observed oxide hydride order and the anisotropic broadening of the diffraction profile, and the existence of a range of intermediate compositions.

Journal Article↗
PubMed1979

Effect of removal of a salt-bridge on the oxygen binding properties and the electronic structure of heme in cobalt-iron hybrid hemoglobin.

In order to clarify the role of salt-bridges in hemoglobin, the oxygen equilibrium curves and electron paramagnetic resonance (EPR) spectra of cobalt-iron hybrid hemoglobins were determined. The EPR spectra of deoxy alpha(Co)2 beta(Fe)2 could be interpreted as a mixture of two distinct paramagnetic species: one showed a maximum of the first derivative spectrum at g = 2.39 and the other at g = 2.33. The oxygen equilibrium curves of the hybrid indicated that the former is assignable to the T structure and the latter to the R structure. The cooperativity of oxygen binding of alpha(Co)2 beta(Fe)2 exhibited a maximum at g = 2.33, which is characteristic of the R structure, regardless of the pH. Addition of inositol hexaphosphate (IHP) to des-Arg alpha(Co)2 beta(Fe)2 restored the cooperativity of oxygen binding, which implies that the deoxygenated form of des-Arg alpha(Co)2 beta(Fe)2 is converted to the T structure upon addition of IHP. However, the EPR signal at g = 2.39 was not restored upon conversion to the T structure by addition of IHP. It is therefore concluded that the EPR spectrum of the deoxy alpha(Co) subunit depends both on the quaternary structure and on the localized strain at the heme.

Binding Sites↗
PubMed1975

Electronic structures of cephalosporins and penicillins. 4. Modeling acylation by the beta-lactam ring.

Molecular orbital calculations by the CNDO/2 method are used to study the molecular and electronic details involved in the initial phases of the opening of the beta-lactam ring of a model cephalosporin structure, 7-amino-3-acetoxymethyl-3-cephem. The effect of a simple nucleophile, OH-, approaching the carbonyl carbon center of the beta-lactam ring is monitored by following the charge redistributions that occur in the bicyclic system and in the 3 side chain. A migration of electron density to the ester oxygen of the CH2OAc group is observed with concomitant weakening of the CH2-OAc bond. The results are discussed in relation to the mechanism of acylation of bacterial cell wall enzymes by beta-lactam antibiotics and in relation to the hydrolysis of these molecules. The results indicate that the ability of the 3' substituent of cephalosporins to stabilize electron density transferred to it, i.e., the leavability of the 3' moiety, can be an important factor in activating the beta-lactam toward nucleophilic attack.

Acylation↗
PubMed1998

Evidence for an unusual electronic structure of wheat germ peroxidase compound I.

Oxidized states of wheat germ peroxidase isozyme C2 (WGP C2) were investigated by means of electronic absorption spectroscopy. Addition of one molar equivalent of H2O2 to ferric WGP C2 led to the formation of an oxidized species with an absorption spectrum very similar to that of peroxidase compound II, with a Soret maximum at 411 nm and visible maxima at 523 and 553 nm. The transformation took place with an isosbestic point at 409 nm. Stopped flow spectroscopy showed no inflection points for the formation of this species when it was registered at 420 nm, and we could verify the persistence of the isosbestic point from 20 ms to 10 s. The oxidized species decays spontaneously to ferric enzyme in a double-exponential manner. By adding excess H2O2 to the system we obtained an inactive derivative identical to horseradish peroxidase P-670. In the presence of one equivalent of reducing substrate and excess H2O2 compound III was formed. The results so indicate that the species obtained in the reaction of WGP C2 with equimolecular amounts of H2O2 is compound I. The resulting compound I spectrum was identical to that of cytochrome c peroxidase, suggesting the formation of a protein radical rather than the typical pi cation radical, a feature which had not been described before for a plant peroxidase.

Enzyme Activation↗
PubMed2006

Molecular and electronic structure of square planar complexes [PdII(tbpy)(L N,O IP)]0, [PdII(tbpy)(LN,O ISQ)](PF6), and [PdII(tbpy)(L N,O IBQ)](PF6)(BF4).2CH2Cl2: an o-iminophenolato based ligand centered, three-membered redox series.

Three Pd(II) complexes which are members of the same electron-transfer series have been synthesized. Refluxing of the reaction mixture containing equimolar amounts of PdCl(2), 2-(2-trifluoromethyl)anilino-4,6-di-tert-butylphenol (H(2)L(N,O)), 4,4'-di-tert-butyl-2,2'-dipyridyl ((t)bpy), and 3 equiv of triethylamine in MeOH under an argon atmosphere followed by exposure to air and addition of KPF(6) after cooling to room temperature yields reddish brown crystals of paramagnetic (S = 1/2) [Pd(L(N,O)(ISQ))((t)bpy)](PF6) (2). Reaction of 2 with one equiv of [CoCp2] in dry and degassed CH(2)Cl(2) using anaerobic conditions gives diamagnetic [Pd(L(N,O)(IP))((t)bpy)] (1), which is the one-electron reduced form of 2. One-electron oxidation of 2 in CH(2)Cl(2) under argon with one equiv of NOBF4 affords diamagnetic [Pd(L(N,O)(IBQ))((t)bpy)](PF6)(BF4).2CH(2)Cl(2) (3). Complexes 1, 2, and 3 constitute three members of the same electron-transfer series. They are ideally suited to distinctly distinguish the geometrical and spectroscopic features of the N,O-coordinated, closed-shell, diamagnetic o-iminophenolate (L(N,O)(IP))2-, the corresponding open-shell pi-radical o-iminobenzosemiquinonate (L(N,O)(ISQ))1-.(S(rad) = 1/2), and the closed-shell o-iminobenzoquinone (L(N,O)(IBQ))0 forms. All complexes were characterized by X-ray crystallography (100 K), cyclic voltammetry, EPR, and UV-vis spectroscopy. Complex 2 exhibits three reversible electron transfer waves in the cyclic voltammogram. Structural characterization of complex 3 reveals an interesting strong ion pairing between the BF4 anion and the complex dication with a short C-F distance of 2.7 A.

Electron Spin Resonance Spectroscopy↗