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Electronic Structure Analysis (electronic + structure_analysis)
Selected AbstractsPhotocatalytic Activity and Electronic Structure Analysis of N-doped Anatase TiO2: A Combined Experimental and Theoretical StudyCHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 6 2009H. Gao Abstract N-Doped TiO2 photocatalysts were prepared by a hydrothermal method with tetra- n -butyl titanate (TTNB) and triethanolamine as precursors. The obtained samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and UV-visible diffuse reflectance spectra (DRS), respectively. Photocatalytic activities of the anatase products were investigated on the degradation of methyl orange (MO). The incorporation of nitrogen impurity in anatase TiO2 was studied by the first-principles calculations based on the density functional theory (DFT). The calculated electronic band structures for substitutional and interstitial N-doped TiO2 indicated the formation of localized states in the band gap, which lied above the valence band. Excitation from the impurity states of N 2p to the conduction band could account for the optical absorption edge shift toward the lower energies. It was consistent with the experimentally observed absorption of N-doped samples in the visible region. [source] Electronic structure analyses of BN network materials using high energy-resolution spectroscopy methods based on transmission electron microscopyMICROSCOPY RESEARCH AND TECHNIQUE, Issue 7 2006M. Terauchi Abstract Electronic structures of boron-nitride (BN) nanotubes and a BN cone-structure material were studied by using a high energy-resolution electron energy-loss spectroscopy (EELS) microscope. A trial of the whole electronic structure study of hexagonal BN (h-BN), which consists of flat BN honeycomb layers, was conducted by a combination of EELS and X-ray emission spectroscopy (XES) based on transmission electron microscopy (TEM) (TEM-EELS/XES). The , and ,+, plasmon energies of BN nanotubes (BNT) were smaller than those of h-BN. The ,+, energy was explained by the surface plasmon excitation. The spectrum of a two-wall BNT of 2.7 nm in diameter showed a new spectral onset at 4 eV. The valence electron excitation spectra obtained from the tip region of the BN cone with an apex angle of 20° showed similar intensity distribution with those of BNTs. The B K-shell electron excitation spectra obtained from the bottom edge region of the BN cone showed additional peak intensity when compared with those of h-BN and BNT. The B K-shell electron excitation spectra and B K-emission spectra of h-BN were compared with a result of a LDA band calculation. It showed that high symmetry points in the band diagram appear as peak and/or shoulder structures in the EELS and XES spectra. Interband transitions appeared in the imaginary part of the dielectric function of h-BN experimentally obtained were assigned in the band diagram. The analysis also presented that the LDA calculation estimated the bandgap energy smaller than the real material by an amount of 2 eV. Those results of TEM-EELS/XES analysis presented that high energy-resolution spectroscopy methods combined with TEM is a promising method to analyze whole electronic structures of nanometer scale materials. Microsc. Res. Tech., 2006. © 2006 Wiley-Liss, Inc. [source] Electronic structure and reactivity of guanylthiourea: A quantum chemical studyJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 6 2010Ahmed Mehdi Abstract Electronic structure analysis of guanylthiourea (GTU) and its isomers has been carried out using quantum chemical methods. Two major tautomeric classes (thione and thiol) have been identified on the potential energy (PE) surface. In both the cases conjugation of pi-electrons and intramolecular H-bonds have been found to play a stabilizing role. Various isomers of GTU on its PE surface have been analyzed in two different groups (thione and thiol). The interconversion from the most stable thione conformer (GTU-1) to the most stable thiol conformer (GTU-t1) was found to take place via bimolecular process which involves protonation at sulfur atom of GTU-1 followed by subsequent CN bond rotation and deprotonation. The detailed analysis of the protonation has been carried out in gas phase and aqueous phase (using CPMC model). Sulfur atom (S1) was found to be the preferred protonation site (over N4) in GTU-1 in gas phase whereas N4 was found to be the preferred site of protonation in aqueous medium. The mechanism of S-alkylation reaction in GTU has also been studied. The formation of alkylated analogs of thiol isomers (alkylated guanylthiourea) is believed to take place via bimolecular process which involves alkyl cation attack at S atom followed by CN bond rotation and deprotonation. The reactive intermediate RS(NH2)CNC(NH2)2+ belongs to the newly identified ,N(,L)2 class of species and provides the necessary dynamism for easy conversion of thione to thiol. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010 [source] 4d Electronic structure analysis of ruthenium in the perovskite oxides by Ru K - and L -edge XASJOURNAL OF SYNCHROTRON RADIATION, Issue 2 2001Jong-Young Kim 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 ,,4.5 Å are well reproduced by ab-initio calculation based on crystallographic data, which supports the long-range structure presented by Rietveld refinement. [source] [Fe-Fe]-hydrogenase reactivated by residue mutations as bridging carbonyl rearranges: A QM/MM studyINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 14 2010Stefan Motiu Abstract In this work, we found aqueous enzyme phase reaction pathways for the reactivation of the exogenously inhibited [Fe-Fe]-hydrogenases by O2, or OH,, which metabolizes to H2O (Dogaru et al., Int J Quantum Chem 2008, 108; Motiu et al., Int J Quantum Chem 2007, 107, 1248). We used the hybrid quantum mechanics/molecular mechanics (QM/MM) method to study the reactivation pathways of the exogenously inhibited enzyme matrix. The ONIOM calculations performed on the enzyme agree with experimental results (Liu et al., J Am Chem Soc 2002, 124, 5175), that is, wild-type [Fe-Fe]-hydrogenase H-cluster is inhibited by oxygen metabolites. An enzyme spherical region with a radius of 8 Å (from the distal iron, Fed) has been screened for residues that prevent H2O from leaving the catalytic site and reactivate the [Fe-Fe]-hydrogenase H-cluster. In the screening process, polar residues were removed, one at a time, and frequency calculations provided the change in the Gibbs' energy for the dissociation of water (due to their deletion). When residue deletion resulted in significant Gibbs' energy decrease, further residue substitutions have been carried out. Following each substitution, geometry optimization and frequency calculations have been performed to assess the change in the Gibbs' energy for the elimination of H2O. Favorable thermodynamic results have been obtained for both single residue removal (,G,Glu374 = ,1.6 kcal/mol), single substitution (,GGlu374His = ,3.1 kcal/mol), and combined residue substitutions (,GArg111Glu;Thr145Val;Glu374His;Tyr375Phe = ,7.5 kcal/mol). Because the wild-type enzyme has only an endergonic step to overcome, that is, for H2O removal, by eliminating several residues, one at a time, the endergonic step was made to proceed spontaneously. Thus, the most promising residue deletions which enhance H2O elimination are ,Arg111, ,Thr145, ,Ser177, ,Glu240, ,Glu374, and ,Tyr375. The thermodynamics and electronic structure analyses show that the bridging carbonyl (COb) of the H-cluster plays a concomitant role in the enzyme inhibition/reactivation. In gas phase, COb shifts towards Fed to compensate for the electron density donated to oxygen upon the elimination of H2O. However, this is not possible in the wild-type enzyme because the protein matrix hinders the displacement of COb towards Fed, which leads to enzyme inhibition. Nevertheless, enzyme reactivation can be achieved by means of appropriate amino acid substitutions. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [source] | ||||||||||