Electronic structures and chemical bonding in transition metal monosilicides MSi (M=3d, 4d, 5d elements)

Bond distances, vibrational frequencies, electron affinities, ionization potentials, dissociation energies, and dipole moments of the title molecules in neutral, positively, and negatively charged ions were studied using the density functional method. Ground state was assigned for each species. The bonding patterns were analyzed and compared with both the available data and across the series. It was found that besides an ionic component, covalent bonds are formed between the metal s, d orbitals and the silicon 3p orbital. The covalent character increases from ScSi (YSi) to NiSi (PdSi) for 3d (4d) metal monosilicides, then decreases. For 5d metal monosilicides, the covalent character increases from LaSi to OsSi, then decreases. For the dissociation of cations, the dissociation channel depends on the magnitude of the ionization potential between metal and silicon. If the ionization potential of the metal is smaller than that of silicon, channel MSi+-> M++Si is favored. Otherwise, MSi+-> M+Si+ will be favored. A similar behavior was observed for anions, in which the dissociation channel depends on the magnitude of electron affinity.

Chemical bonding and electronic structure of 4d-metal monocarbides

Bond distances, vibrational frequencies, dissociation energies, electron affinities, ionization potentials and dipole moments of the title molecules in neutral and charged ions were studied by use of density functional method. Ground states for each molecule were assigned. For neutral and cationic molecules, the bond distance decreases from YC (YC+) to RhC (RhC+), then increases, while for anionic molecules, the bond distance decreases from YC- to RuC-, then increases. Opposite trend was observed for vibrational frequency. The bond ionic character decreases from ZrC to PdC for neutral molecules. The bonding patterns are discussed and compared with the available studies.

Geometries and electronic properties of Ta-n, TanO and TaOn (n=1-3) clusters

Geometries, vibrational frequencies, electron affinities, ionization potentials and dissociation energies of the title clusters in both neutral and positively and negatively charged states were studied by use of density functional theory. For both neutral and charged species, different initial isomers were studied in order to determine the structure with the lowest energy. Vibrational analysis was also performed in order to characterize these isomers. For Ta-2, Ta-Ta metallic bond is strengthened by adding or removing an electron, i.e. the charged species are much more stable than the neutral counterpart. For Ta-3, equilateral triangle with D-3h symmetry has the lowest energy for both neutral and charged species (near equilateral triangle for cation). TaO and its charged species have much larger dissociation energy compared with other tantalum oxides. For Ta2O and TaO2. structure with C-2v symmetry is much more stable than linear chains. For Ta3O, planar structure with doubly bridging oxygen atoms of C-2v, symmetry is the global minimum for both neutral and charged species. While for TaO3, three-dimensional structures are favored for both neutral (C-1 symmetry) and charged species (C-3v symmetry).

Substitution effect of nitrogen atoms with carbon atoms in porphyrin on the electronic structure and excited states properties

Density functional theory (DFT) electronic structure calculations were carried out to predict the structures and the absorption and emission spectra for porphyrin and a series of carbaporphyrins-carbaporphyrin, adj-dicarbaporphyrin, opp-dicarbaporphyrin, tricarbaporphyrin and tetracarbaporphyrin. The ground- and excited-state geometries were optimized at the B3LYP/6-31g(d) and CIS/6-31g(d) level, respectively. The optimized ground-state geometry and absorption spectra of porphyrin, calculated by DFT and time-dependent DFT (TDDFT), are comparable with the available experimental values. Based on the optimized excited-state geometries obtained by CIS/6-31g(d) method, the emission properties are calculated using TDDFT method at the B3LYP/6-31g(d) level. The effects of the substitution of nitrogen atoms with carbon atoms at the center positions of porphyrin are discussed. The results indicate that the two-pyrrole nitrogens are important to the chemical and physical properties for porphyrin.

Geometries and electronic properties of AunPdm (n=1-4, m =-1, 0, 1) clusters

Possible conformers for AunPdm (n = 1-4, m = -1, 0, 1) clusters have been presented and studied by use of density functional theory. The results indicate that for n = 2, linear conformer with C-infinityv symmetry is the most stable for anion species, while for cation and neutral species, conformer with C-2v symmetry is the most stable. For n = 3, 4, conformers with C-2v symmetry (kite-shape) are energetically favored. The calculated electron affinities (EAs) and vertical detachment energies (VDEs) are in good agreement with experiments for n = 1-4. It is also interesting to note that for even n (n = 2, 4), the most stable conformers do not give the best agreement between calculated and experimental EA and VDE values, while for odd n (n = 3), the lowest energy conformer also gives the best agreement. The ionization potentials (IPs) of AunPd clusters are calculated as well.

Investigation of Pr valence and relationship between bond covalency and T-c in Y1-xPrxBa2Cu3O7 (x = 0-1)

The valence of Pr and relationship between bond covalency and T-c in Y1-xPrxBa2Cu3O7 (x = 0-1) have been studied using complex chemical bond theory. The results indicate that the depression of superconductivity in Y1-xPrxBa2Cu3O7 can be reasonably explained by bond covalency difference for the bonds between CuO2 plane and CuO chain. T-c decreases with the decreasing of bond covalency difference and reaches zero when bond covalency difference is zero (or bond covalency in CuO2 exceeds that in CuO chain) at Pr concentration 0.55 and valence +3.30. These are in good agreement with the experiments and meanwhile suggest that the valence of Pr is + 3.30 in Y1-xPrxBa2Cu3O7. The results also indicate that for Pr valence less than +3.15, superconductivity always exists for whatever Pr concentration, whereas for Pr with a valence of +4.0, superconductivity disappears as soon as Pr concentration exceeds 0.19. This supports with the viewpoint that higher valence Pr will contribute more electrons to CuO2 plane, filling the mobile holes responsible for conduction. For PrBa2Cu3O7 with no Ba-site Pr, our calculation suggests that it will be a superconductor if the average valence of Pr is less than +3.15. (C) 1998 Published by Elsevier Science B.V. All rights reserved.

Crystal Radii and Their Application on the Study of Electronic Structures of LaX(X=N,P,As,Sb)

以Ｐｈｉｌｉｐｓ－ＶａｎＶｅｃｈｔｅｎ理论和晶体光学性质为基础，计算了ＬａＸ（Ｘ＝Ｎ，Ｐ，Ａｓ，Ｓｂ）晶体的离子半径（单位：ｎｍ）：（Ｌａ：Ｎ）为（０．１４１４，０．１２３６），（Ｌａ：Ｐ）为（０．１５１８，０．１４８９），（Ｌａ：Ａｓ）为（０．１５３６，０．１５２６），（Ｌａ：Ｓｂ）为（０．１５８６，０．１６５１）．用ＬＭＴＯ－ＡＳＡ方法对ＬａＸ系列晶体的能带结构进行了计算．所得到的能隙是：ＬａＮ为２．３０ｅＶ，ＬａＰ为２．０５ｅＶ，ＬａＡｓ为１．６６ｅＶ，ＬａＳｂ为１．３４ｅＶ．与实验结果相符．这也证明了作者得到的半径的合理性.

Synthesis, extraction and electronic structure of Ce@C-2n

In view of the growing interest in endohedral lanthanide fullerenes, Ce, as a typical +4 oxidation state lanthanide element, has been systematically studied. The synthesis, extraction and electronic structure of Ce@C-2n are investigated. Soot containing Ce@C-2n was synthesized in high yield by carbonizing CeO2-containing graphite rods and are back-burning the CeC2-enriched cathode deposit in a DC are plasma apparatus. Ce@C-2n, dominated by Ce@C-82, can be efficiently extracted from the insoluble part of the soot after toluene Soxhlet extraction by pyridine at high temperature and high pressure in a closed vessel. About 60% Ce@C-2n (2n = 82,80,78,76) and 35% Ce@C-82 can be enriched in the pyridine extract. This fact is identified by desorption electron impact mass spectrometry (DEI MS). The electronic structure of Ce@C-2n is analyzed by using X-ray photoemission spectroscopy (XPS) of pyridine-free film. It is suggested that the encapsulated Ce atom is in a charge state close to +3 and was effectively protected from reaction with water and oxygen by the enclosing fullerene cage. Unlike theoretical expectation, the electronic state of Ce@C-82 is formally described as Ce+3@C-82(3-). (C) 1997 Elsevier Science Ltd.

Conversion from electronic conducting phase to redox conducting phase of polypyrrole

Square-wave voltommetry is used to study the oxidation of polypyrrole doped with dodecylsulfate. The net current curve in this experiment shows why the oxidation current does not display the capacitive-like shape common in cyclic voltammetry. In cyclic voltammetry, the redox behavior of polypyrrole is attributed to the size of dodecylsulfate, irreversible incorporation and the complete consumption of dodecylsulfate. After the polypyrrole film was scanned in aqueous NaCl solution, square wave voltammetric measurements show different results, indicating the change of the polymer nature with regard to the charge transport. This is explained by anion replacement, exclusion and the change of the charge transport mechanism.

IONIC-CONDUCTION OF A NOVEL COMB POLYMER ELECTROLYTE BASED ON MALEIC-ANHYDRIDE COPOLYMER WITH OLIGO-OXYETHYLENE SIDE-CHAINS

A comb-shaped polymer (BM350) with oligo-oxyethylene side chains of the type -O(CH2CH2O)(7)CH3 was prepared from methyl vinyl ether/maleic anhydride copolymer. Homogeneous amorphous polymer electrolyte complexes were made from the comb polymer and LICF(3)SO(3) by solvent casting from acetone, and their conductivities were measured as a function of temperature and salt concentration. Maximum conductivity close to 5.08 X 10(-5) Scm(-1) was obtained at room temperature and at a [Li]/[EO] ratio of about 0.12. The conductivity which displayed non-Arrhenius behaviour was analyzed using the Vogel-Tammann-Fulcher equation and interpreted on the basis of the configurational entropy model. The results of mid-IR showed that the coordination of Li+ to side chains made the C-O-C band become broader and shift slightly. X-ray photoelectron spectroscopy analysis indicated that the oxygen atoms in the two situations could coordinate to Li+ and this coordination resulted in the reduction of the electron orbit binding energy of F and S.

PROTONIC ACID-DOPING OF POLY-ORTHO-METHYLANILINE

Poly-o-methylaniline (poly-o-toluidine) was doped by some protonic acids. It was found that the acidity, molecular size and oxidizing ability of protonic acids affected the doping level and conductivity of polymer obtained to some extent. The organic acid

THE INFLUENCE OF PROTONIC ACIDS ON THE CHEMICAL POLYMERIZATION OF ORTHO-METHYLANILINE

The chemical polymerization of ortho-methylaniline (MAn) is performed in aqueous solution of six protonic acids. The MAn polymerization conversion, and the electrical conductivity and doping level as well as molecular chain structure of the polymers obtained depend not only on the acid concentration but also on their acidity and molecular size.