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At least 667 records · Page 37Linked to original sources
PubMed1996

Heteroatomic Centering of Icosahedral Clusters. Crystal and Electronic Structure of the K(6)(NaCd)(2)Tl(12)Cd Compound Containing the Not-So-Naked Tl(12)Cd(12-) Polyanion.

The title compound was synthesized in a niobium container by fusion of the elements followed by slow cooling. In the first stage, the stoichiometric proportion KNaCd(3)Tl(7) yielded a heterogeneous product containing a few single crystals of the compound K(6)(Na(2.36(9))Cd(1.64(9)))Tl(12)Cd, the structure of which was established by a single crystal X-ray diffraction technique (cubic, Im&thremacr;, a = 11.352(2) Å, Z = 2, R(F) = 3.24%, Rw(F) = 4.60%). Occurrence of a stoichiometry range for the compound was indicated after a new reaction starting from the composition K(6)Na(2)Cd(3)Tl(12) gave a quite homogeneous and well-crystallized product (refined composition K(6)(Na(1.93(7))Cd(2.07(7)))Tl(12)Cd, Im&thremacr;, a = 11.321(2) Å, Z = 2, R(F) = 3.98%, Rw(F) = 4.99%). The structure of K(6)(NaCd)(2)Tl(12)Cd is distinguishable from that reported for Na(4)K(6)Tl(13) by replacement of the icosahedron centering thallium and of half the sodium cations by cadmium. Statistical occupation disorder occurs on the 8(c) position of the outer Cd/Na atom. The structure contains the 50-electron closed shell centered Tl(12)Cd(12-) icosahedral cluster with &thremacr;m symmetry (T(h)). Extended Hückel molecular orbital and band calculations were carried out to analyze the centering effect on the anion stability and look at the electron transfer, especially from cadmium lying within the first coordination shell of the icosahedral cluster. Electron localization within the Cd-centered icosahedron is not as evident as in the Tl-centered thallium icosahedral clusters described elsewhere; actually, cadmium is found to bridge icosahedra within a more three-dimensional network than sodium by forming bonds that are mainly covalent. The compound is a semiconducting Zintl phase with closed shell bonding.

Journal Article↗
PubMed1989

Kinetochore structure: electron spectroscopic imaging of the kinetochore.

The structure of the kinetochore in thin section has been studied in the Indian muntjac by an electron spectroscopic imaging technique. This procedures allows the analysis of the distribution of phosphorus within the layers of the kinetochore. The results indicate that this element is a major component of both the inner and outer plates whereas it is largely absent in the middle plate and fibrous corona. The majority of the phosphorus is localized to a 30-nm fiber(s) that is woven through the layers of the kinetochore. The presence of phosphorus within this fiber, along with its morphological and biochemical features, indicates that it contains DNA. The fiber(s) occupies a major portion of the inner and outer plate where it forms a series of rows. It is rarely observed in the middle layer except where it passes between the inner and outer layers. The absence of structure in the middle plate suggests that it may represent a space rather than a plate that in turn may be related to the function of this region. The distribution of phosphorus within the kinetochore is neither altered by treatment with colcemid nor by the presence of microtubules at the kinetochore. Analysis of conventional micrographs of the kinetochore together with structural information obtained by electron spectroscopic imaging suggests that most microtubules insert and terminate between the rows of kinetochore fibers in the outer plate. However, some microtubules continue through the middle layer and terminate at the lower plate. The insertion of microtubules at different levels of the kinetochore may reflect the existence of functionally distinct microtubule classes. Electron spectroscopic imaging indicates that the microtubules associated with the kinetochore are phosphorylated.

Animals↗
PubMed2000

Structural chemistry of polycyclic heteroaromatic compounds. Part XI. Photoelectron spectra and electronic structures of tetracyclic hetarenes of the triphenylene type.

The UV photoelectron spectra of several tetracyclic heteroaromatic compounds (2-9) which are pi-isoelectronic with triphenylene (1) have been recorded and analysed making use of semiempirical AM1 and PM3 as well as ab initio/DFT B3LYP calculations. In one series of compounds (2-7), the peripheral benzene rings of 1 are successively substituted by thiophene rings that are either [b]- or [c]-annellated with the central benzene unit. In 2-7 only marginal shifts are found for most of the IPs of electrons. In the benzotrithiophenes 5-7, a systematic variation is displayed by IP(pi7). Compared to 1, the pi electron system of benzo[c]trithiophene (7) is approximately two times as much destabilized as in the isomers 5 and 6 with [b]annellated thiophene rings. The IP[n(S)] values of the thiophene derivatives 2-7 indicate that these orbitals are clearly destabilized relative to thiophene. The same holds for the n(O) orbital of the furane derivative 9 in comparison with that of furane. In 9, only the higher pi MOs (pi7-pi9) are destabilized whereas the lower levels (pi1-pi4) are stabilized, and those in between (pi5-pi6) remain essentially unshifted. In the pyrrole derivative 8, all pi MOs are substantially destabilized by about 0.5-1.6 eV relative to 1.

Chrysenes↗
PubMed2001

Structural chemistry of polycyclic heteroaromatic compounds. Part 13. Photoelectron spectra and electronic structures of tricyclic hetarenes of the anthracene type.

The ultraviolet photoelectron spectra of two tricyclic heteroaromatic compounds (2,3) that are pi-isoelectronic with anthracene (1) have been recorded and analysed making use of semi-empirical AM1 and PM3, as well as density functional theory (DFT) B3LYP calculations. In compounds 2 and 3, one peripheral benzene ring of compound 1 is substituted by a thiophene ring that is either [b]- or [c]-annellated. In compounds 2 and 3, only small shifts are found for most of the ionization potentials of pi electrons. Since the ionization energies of all occupied pi molecular orbitals of compounds 1-3 could be assigned, a direct comparison of their pi electron energy is possible. Compared with compound 1, the pi-electron system of naphtho[2,3-b]thiophene (2) is stabilized by 0.6 eV, while that of naphtho[2,3-c]thiophene (3) is destabilized by 0.2 eV. [b]-Annellation of the thiophene ring is thus favourable while [c]-annellation is unfavourable.

Electrons↗
PubMed2000

Electronic structure of solids with competing periodic potentials.

When electrons are subject to a potential with two incommensurate periods, translational invariance is lost, and no periodic band structure is expected. However, model calculations based on nearly free one-dimensional electrons and experimental results from high-resolution photoemission spectroscopy on a quasi-one-dimensional material do show dispersing band states with signatures of both periodicities. Apparent band structures are generated by the nonuniform distribution of electronic spectral weight over the complex eigenvalue spectrum.

Journal Article↗
PubMed2002

Electronic structure of some adenosine receptor antagonists. III. Quantitative investigation of the electronic absorption spectra of alkyl xanthines.

Quantitative and comparative investigation of the electronic absorption spectra of theophylline, caffeine and their derivatives is reported. The spectra of theophylline, caffeine and theobromine were compared to establish the predominant tautomeric species in solution. This comparison, analysis of solvent effects and assignments of the observed transitions via MO computations indicate the exits of only one tautomeric species in solution that is the N7 form. A low-lying triplet state was identified which corresponds to a HOMO-LUMO transition. This relatively long-lived T1 state is always less polar than the ground state and may very well underlie the photochemical reactivity of alkyl xanthines. Substituents of different electron donating or withdrawing strengths and solvent effects are investigated and analyzed. The present analysis is facilitated via computer deconvolution of the observed spectra and MO computation.

Caffeine↗
PubMed1995

Electronic structure of Q-A in reaction centers from Rhodobacter sphaeroides. I. Electron paramagnetic resonance in single crystals.

The magnitude and orientation of the electronic g-tensor of the primary electron acceptor quinone radical anion, Q-A, has been determined in single crystals of zinc-substituted reaction centers of Rhodobacter sphaeroides R-26 at 275 K and at 80 K. To obtain high spectral resolution, EPR experiments were performed at 35 GHz and the native ubiquinone-10 (UQ10) in the reaction center was replaced by fully deuterated UQ10. The principal values and the direction cosines of the g-tensor axes with respect to the crystal axes a, b, c were determined. Freezing of the single crystals resulted in only minor changes in magnitude and orientation of the g-tensor. The orientation of Q-A as determined by the g-tensor axes deviates only by a few degrees (< or = 8 degrees) from the orientation of the neutral QA obtained from an average of four different x-ray structures of Rb. sphaeroides reaction centers. This deviation lies within the accuracy of the x-ray structure determinations. The g-tensor values measured in single crystals agree well with those in frozen solutions. Variations in g-values between Q-A, Q-B, and UQ10 radical ion in frozen solutions were observed and attributed to different environments.

Anions↗
PubMed2004

First principles calculations of electronic structures and metal mobility of Na(x)Si(46) and Na(x)Si(34) clathrates.

Energetics, geometry, electronic band structures, and charge transfer for Na(x)Si(46) and Na(x)Si(34) clathrates with different degrees of cavity filling by sodium, and the mobility of the Na atom inside the different cavities are studied using first principles density functional calculations within the generalized gradient approximation. The stabilization of the clathrate lattice and the cell volume variation upon the inclusion of Na (which appears to move easily in the larger cavities of Na(x)Si(34), thus justifying the experimental observations) are discussed in connection with the onset of the repulsion between Na and Si for distances shorter than approximately 3.4 A. For all degrees of filling of the different cavities examined we find that the electron population of the s orbitals in the partially ionized Na atoms increases with a decrease in the size of the cavity, and that the Na states contribute significantly to the density of states at the Fermi level and thus influence the properties of these compounds.

Journal Article↗
PubMed2004

Scaled opposite-spin second order Møller-Plesset correlation energy: an economical electronic structure method.

A simplified approach to treating the electron correlation energy is suggested in which only the alpha-beta component of the second order Møller-Plesset energy is evaluated, and then scaled by an empirical factor which is suggested to be 1.3. This scaled opposite-spin second order energy (SOS-MP2), where MP2 is Møller-Plesset theory, yields results for relative energies and derivative properties that are statistically improved over the conventional MP2 method. Furthermore, the SOS-MP2 energy can be evaluated without the fifth order computational steps associated with MP2 theory, even without exploiting any spatial locality. A fourth order algorithm is given for evaluating the opposite spin MP2 energy using auxiliary basis expansions, and a Laplace approach, and timing comparisons are given.

Algorithms↗
PubMed2005

pH dependence of the electronic structure of glycine.

The carbon, nitrogen, and oxygen K-edge spectra were measured for aqueous solutions of glycine by total electron yield near-edge X-ray absorption fine structure (TEY NEXAFS) spectroscopy. The bulk solution pH was systematically varied while maintaining a constant amino acid concentration. Spectra were assigned through comparisons with both previous studies and ab initio computed spectra of isolated glycine molecules and hydrated glycine clusters. Nitrogen K-edge solution spectra recorded at low and moderate pH are nearly identical to those of solid glycine, whereas basic solution spectra strongly resemble those of the gas phase. The carbon 1s --> pi*(C=O) transition exhibits a 0.2 eV red shift at high pH due to the deprotonation of the amine terminus. This deprotonation also effects a 1.4 eV red shift in the nitrogen K-edge at high pH. Two sharp preedge features at 401.3 and 402.5 eV are also observed at high pH. These resonances, previously observed in the vapor-phase ISEELS spectrum of glycine, have been reassigned as transitions to sigma* bound states. The observation of these peaks indicates that the amine moiety is in an acceptor-only hydrogen bond configuration at high pH. At low pH, the oxygen 1s --> pi*(C=O) transition exhibits a 0.25-eV red shift due to the protonation of the carboxylic acid terminus. These spectral differences indicate that the variations in electronic structure observed in the NEXAFS spectra are determined by the internal charge state and hydration environment of the molecule in solution.

Glycine↗
PubMed2006

Structures, electron affinities, and harmonic vibrational frequencies of C6H5X/C6H5X- (X = N, S, NH, PH, CH2, and SiH2).

The molecular structures and electron affinities of the C6H5X/C6H5X- (X = N, S, NH, PH, CH2, and SiH2) species have been determined using seven different density functional or hybrid Hartree-Fock density functional methods. The basis set used in this work is of double-zeta plus polarization quality with additional diffuse s- and p-type functions, denoted DZP++. These methods have been carefully calibrated (Chem. Rev. 2002, 102, 231). The geometries are fully optimized with each density functional theory (DFT) method, and discussed. Harmonic vibrational frequencies were found to be within 3.2% of available experimental values for most functionals. Three different types of the neutral-anion energy separations reported in this work are the adiabatic electron affinity (EA(ad)), the vertical electron affinity (EA(vert)), and the vertical detachment energy (VDE). The most reliable adiabatic electron affinities, obtained at the DZP++ BPW91 level of theory, are 1.45 (C6H5N), 2.29 (C6H5S), 1.57 (C6H5NH), 1.51 (C6H5PH), 0.91 (C6H5CH2), and 1.48 eV (C6H5SiH2), respectively. Compared with the experimental values, the average absolute error of the BPW91 method is 0.04 eV. The B3LYP and B3PW91 functionals also gave excellent predictions, with average absolute errors of 0.06 and 0.07 eV, respectively.

Journal Article↗
PubMed2005

Manipulating the electronic structures of silicon carbide nanotubes by selected hydrogenation.

We show that the electronic and atomic structures of silicon carbide nanotubes (SiCNTs) undergo dramatic changes with hydrogenation from first-principles calculations based on density-functional theory. The exo-hydrogenation of a single C atom results in acceptor states close to the highest occupied valence band of pristine SiCNT, whereas donor states close to the lowest unoccupied conduction band appear as a Si atom being hydrogenated. Upon fully hydrogenating Si atoms, (8,0) and (6,6) SiCNTs become metallic with very high density of states at the Fermi level. The full hydrogenation of C atoms, on the other hand, increases the band gap to 2.6 eV for (8,0) SiCNT and decreases the band gap to 1.47 eV for (6,6) SiCNT, respectively. The band gap of SiCNTs can also be greatly increased through the hydrogenation of all the atoms.

Journal Article↗
PubMed2006

Controlling the electronic structure of bilayer graphene.

We describe the synthesis of bilayer graphene thin films deposited on insulating silicon carbide and report the characterization of their electronic band structure using angle-resolved photoemission. By selectively adjusting the carrier concentration in each layer, changes in the Coulomb potential led to control of the gap between valence and conduction bands. This control over the band structure suggests the potential application of bilayer graphene to switching functions in atomic-scale electronic devices.

Journal Article↗