Vibrational and electronic excitations of H(2)O on thymine films induced by low-energy electrons

We investigated vibrational and electronic excitations of 0.1-layer up to 2.4-layer film of H(2)O deposited on a 1.4-layer film of thymine condensed on Ar at a temperature of 18 K using high-resolution electron-energy loss (EEL) spectroscopy at the incident energy of 12 eV. The spectral contribution originating essentially from the H(2)O overlayer is obtained by separating the measured contribution from the underlying film of thymine, considering the electron beam attenuation in the H(2)O overlayer. The vibrational EEL spectrum of submonolayer amount of H(2)O on thymine, which excepts for small energy shift of the vibrational bands, is found to compare in intensity to that of the same amount of H(2)O deposited directly on the argon. The electronic energy-loss intensity near 8.6 eV, which is attributed to the excitation of (3,1)B(1) states of H(2)O in condensed phase, is observed to decrease by a factor of about 3 by the presence of the underlying film of thymine. This indicates that the corresponding cross section for excitation the (3,1)B(1) states of H(2)O by the electron impact is reduced significantly by the close proximity of the thymine molecules.     

Practical aziridinations II: electronic modifications to poly(pyrazolyl)borate-copper catalysts

A significant influence of the electronic features of poly(pyrazolyl)borate ligands on the efficiency of the copper-catalyzed aziridination reaction has been noted. Electron-deficient, bidentate di(pyrazolyl)borates in conjunction with copper(II) chloride generated the most effective catalyst system for the aziridination of a variety of olefins.     

Spectroscopic and microscopic investigations of organic ultrathin films: Correlation between geometrical structures and unoccupied electronic states

In this review, we summarize recent progress in experimental approaches to the investigation of the unoccupied electronic structures of organic ultrathin films, based on a combination of spectroscopic and microscopic techniques. At the molecule/substrate interface, electronic structures are greatly affected by the geometrical structures of adsorbed molecules. In addition, a delicate balance between substrate-molecule and intermolecular interactions plays an important role in the formation of complex polymorphism. In this context, we have clarified the correlation between geometric and electronic structures using a combination of two-photon photoemission (2PPE) spectroscopy, low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Organic ultrathin films of metal phthalocyanines and polycyclic aromatic hydrocarbons (naphthalene, rubrene and perylene) on graphite substrates were examined as model systems. Depending on the substrate temperature and coverage, unique morphologies, including well-ordered films, a metastable phase and a two-dimensional gas-like phase, were determined at the molecular level. The data show that variations in molecular orientation have a significant impact on the occupied/unoccupied electronic structures. In addition to static information regarding electronic states, ultrafast electron excitation and relaxation dynamics can be tracked in real time on the femtosecond scale by time-resolved 2PPE spectroscopy. The excited electron dynamics of rubrene films are discussed herein, taking into account structural information, in the presence and absence of an overlap of the wave function with the substrate. Spatial resolution at the molecular level is also obtainable via STM-based local spectroscopy and mapping, which have been utilized to elucidate the spatial extent of unoccupied orbitals in real space. Visible photon emissions from the unoccupied states of perylene monolayer films were observed using 2PPE, representing a characteristic deexcitation process from electronically excited states, depending on the surface structure. These spectroscopic and molecular level microscopic investigations provide fundamental insights into the electronic properties of organic/substrate interfaces.     

Electrochemical catalysis of styrene epoxidation with films of MnO(2) nanoparticles and H(2)O(2)

Films of polyions and octahedral layered manganese oxide (OL-1) nanoparticles on carbon electrodes made by layer-by-layer alternate electrostatic adsorption were active for electrochemical catalysis of styrene epoxidation in solution in the presence of hydrogen peroxide and oxygen. The highest catalytic turnover was obtained by using applied voltage -0.6 V vs SCE, O(2), and 100 mM H(2)O(2). (18)O isotope labeling experiments suggested oxygen incorporation from three different sources: molecular oxygen, hydrogen peroxide, and/or lattice oxygen from OL-1 depending on the potential applied and the oxygen and hydrogen peroxide concentrations. Oxygen and hydrogen peroxide activate the OL-1 catalyst for the epoxidation. The pathway for styrene epoxidation in the highest yields required oxygen, hydrogen peroxide, and a reducing voltage and may involve an activated oxygen species in the OL-1 matrix.     

Density functional study on geometrical features and electronic structures of di-mu-oxo-bridged [Mn2O2(H2O)8]q+ with Mn(II), Mn(III), and Mn(IV)

We report the geometrical features and electronic structures of di-mu-oxo-bridged Mn-Mn binuclear complexes with H2O ligands [Mn2O2(H2O)8]q+ in the iso- and mixed-valence oxidation states. All of the combinations among Mn(II), Mn(III), and Mn(IV) ions are considered the oxidation states of the Mn-Mn center, and the changes in molecular structure induced by the different electron configurations of Mn-based orbitals are investigated in relation to the oxygen-evolving complex (OEC) of photosystem II. The stable geometries of complexes are determined by using the hybrid-type density functional theory for both of the highest- and lowest-spin couplings between Mn sites, and the lowest-spin-coupled states are energetically more favorable than the highest-spin-coupled states except in the case of the complexes with the Mn(II) ion. The coordination positions of H2O ligands at the Mn(II) site tend to shift from the octahedral positions in contrast to those at the Mn(III) and Mn(IV) sites. The shape of the Mn2O2 core and the distances between the Mn ions and the H2O ligands vary depending on the electron occupations of the octahedral eg orbitals on the Mn site with an antibonding nature for the Mn-ligand interactions, indicating the trend as Mn(II)-O > Mn(III)-O and Mn(IV)-O, O-Mn(II)-O > O-Mn(III)-O > O-Mn(IV)-O among the iso-valence Mn2O2 cores, and O-Mn(lower)-O < O-Mn(higher)-O within the mixed-valence Mn2O2 core, and as Mn(II)-OH2 and Mn(III)-OH2 > Mn(IV)-OH2 for the axial H2O ligand. The optimized geometries of model complexes are compared with the X-ray structure of the OEC, and it is suggested that the cubane-like Mn cluster of the active site may not contain a Mn(II) ion. The effective exchange integrals are estimated by applying the approximate spin projection to clarify the magnetic coupling between Mn sites, and the superexchange pathways through the di-mu-oxo bridge are examined on the basis of the singly occupied magnetic orbitals derived from the singlet-coupled natural orbitals in the broken-symmetry state. The comparisons of the calculated results between [Mn2O2(H2O)8]q+ in this study and [Mn2O2(NH3)8]q+ reported by McGrady et al. suggest that the symmetric pathways are dominant to the exchange coupling constant, and the crossed pathway would be less important for the former than it would for the latter in the Mn(III)-Mn(III), Mn(IV)-Mn(IV), and Mn(III)-Mn(IV) oxidation states.     

2-甲基-6-二乙胺基乙氧基-3(2H)-哒嗪酮的合成及其电子性质

分别利用两种方法合成2－甲基－6－二乙胺基乙氧基－3（2H）－哒嗪酮，用^1H NMR,IR,UV和MS进行结构表征，利用ab initio HF和密度泛函理论（DFT）B3LYP方法，对反应物、中间物、产物及其同分异构体进行几何结构的优化和总能量计算，系统分析前线分子轨道特征和能级分布规律，结果表明，2－甲基－6－二乙胺基乙氧基－3（2H）－哒嗪酮比其同分异构体哒嗪氮烷基化合物的能量低，前比后稳定，与实验事实相符合。     

Evolution of the geometrical and electronic structures of Gan(n=2-26) clusters: a density-functional theory study

Density-functional theory with generalized gradient approximation for the exchange-correlation potential has been used to calculate the lowest-energy geometries and electronic structure of neutral gallium clusters containing up to 26 atoms. Harmonic vibrational frequency analysis is undertaken to assure that the lowest-energy geometries are real local minima. With increasing cluster size, we find that the gallium clusters tend to adopt compact structures. The structures comprise triangular units that connect each other with different dihedral angles. The lowest-energy structure can be obtained by capping an atom on the structure of smaller one. The capping site occurs at a site where interactions with more atoms are available. The binding energy evolves monotonically with size, but Ga(8), Ga(14), and Ga(20) exhibit particularly higher stability. Except Ga(2) and Ga(4), all even-numbered gallium clusters we studied are closed-shell singlet states with a substantial highest occupied and lowest unoccupied molecular orbitals gap. The odd-numbered clusters are open shell with a small gap. The size dependence of cluster's ionization potentials and electron affinities is discussed and compared with available experiment.     

A computational investigation of the geometrical structure and protodeboronation of boroglycine, H2N-CH2-B(OH)2

In this article the geometrical structure of the simple, achiral, alpha-amino boronic acid boroglycine, H2N-CH2-B(OH)2, was investigated using density functional theory (DFT), second-order M?ller-Plesset (MP2) perturbation theory, and coupled cluster methodology with single- and double-excitations (CCSD); the effects of an aqueous environment were incorporated into the results by using a few explicit water molecules and/or self-consistent reaction field (SCRF) calculations with the IEF polarizable continuum model (PCM). Neutral reaction mechanisms were investigated for the direct protodeboronation (hydrolysis) of boroglycine (H2O+H2N-CH2-B(OH)2-->B(OH)3+H2N-CH3), for which DeltaH degrees 298 was -21.9 kcal/mol at the MP2(FC)/aug-cc-pVDZ level, and for the 1,2-carbon-to-nitrogen shift of the -B(OH)2 moiety (H2N-CH2-B(OH)2-->H3C-NH-B(OH)2), for which the corresponding value of DeltaH degrees 298 was -18.2 kcal/mol. A boron-oxygen double-bonded intermediate was found to play an important role in the 1,2-rearrangement mechanism.     

Enhanced selectivity towards O(2) and H(2) dissociation on ultrathin Cu films on Ru(0001)

The reactivity of Cu monolayer (ML) and bilayer films grown on Ru(0001) towards O(2) and H(2) has been investigated. O(2) initial sticking coefficients were determined using the King and Wells method in the incident energy range 40-450 meV, and compared to the corresponding values measured on clean Ru(0001) and Cu(111) surfaces. A relative large O(2) sticking coefficient (～0.5-0.8) was measured for 1 ML Cu and even 2 ML Cu/Ru(0001). At low incident energies, this is one order of magnitude larger than the value observed on Cu(111). In contrast, the corresponding reactivity to H(2) was near zero on both Cu monolayer and bilayer films, for incident energies up to 175 meV. Water adsorption on 2 ML Cu/Ru(0001) was found to behave quite differently than on the Ru(0001) and Cu(111) surfaces. Our study shows that Cu/Ru(0001) is a highly selective system, which presents a quite different chemical reactivity towards different species in the same range of collision energies.     

Tubing modifications for countercurrent chromatography: Investigation of geometrical parameters

Countercurrent chromatography (CCC) is a separation technique which may be described as a combination of a great number of liquid–liquid distributions of analytes in a two-phase solvent system with liquid chromatography (LC) features. Even optimized CCC separations currently provide a lower number of theoretical plates when compared to LC. For this reason, instrumental advancements are indispensable to, at least partly, overcome this drawback. Recently, we found that improvement of the classic CCC coil, that is using a long hollow tubing, may be achieved by the introduction of tubing crimpings which increase the stationary phase retention. In this study, we systematically investigated the effects of three geometrical parameters (crimping depth, distance between two crimpings as well as partial or complete crimping of the tubing) on the stationary phase retention by a factorial design of experiments approach. Separation efficiency tests were performed with two groups of analytes: fatty acid methyl esters (FAME) in the n-hexane/acetonitrile (HAcn) and alkyl p-hydroxybenzoates in the n-hexane/tert-butylmethylether/methanol/water solvent system. The most narrow crimping distance and the deepest crimping of the tubing were the best configurations in the examined flow rate range.     

Crystal structures of two polymorphic modifications of trans-diisothiocyanato-bis-(ethylenediamine)nickel(II) [Ni(C2H8N2)2(NCS)2]

Institute of Inorganic Chemistry and Institute of Solid-State Chemistry and Mineral Ore Processing, Siberian Branch, Academy of Sciences of the USSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 32, No. 5, pp. 96–102, September–October, 1991.     

"Stapled" bis(phthalocyaninato)niobium(IV), Pc2Nb: X-ray crystal structure,chemical and electrochemical behavior,and theoretical studies. Perspectives for the use of Pc2Nb (thin films) as an "optically passive electrode" in electrochromic devices

Recrystallization of the previously reported monosolvated bis(phthalocyaninato)niobium(IV), [Pc2Nb].CINP (CINP = 1-chloronaphthalene), has allowed isolation of a single crystal of a new solvated form, i.e. [Pc2Nb]. 3.5CINP, whose structure has been elucidated by X-ray work: space group P2(1)/n (No. 14); a = 16.765(3), b = 23.800(4), c = 19.421(4) A; alpha = gamma = 90 degrees, beta = 92.51(2) degrees; Z = 4. The sandwiched material is a "stapled" molecule, characterized by the presence of two intramolecular interligand C-C sigma bonds and highly strained phthalocyanine units, as formerly observed by crystallographic work for its Ti(IV) analogue, [Pc2Ti], and the +1 corresponding fragment, [Pc2Nb]+, present in the species [Pc2Nb](l3)(l2)0.5.3.5CINP. [Pc2Nb] appears to be reluctant to undergo further oxidation above the +1 oxidation state. Detailed theoretical studies by DFT and TDDFT methods have been developed on [Pc2Nb] and [Pc2Nb]+, also extended for comparison to the Ti(IV) complex [Pc2Ti], and an adequate picture of the ground-state electronic structure of these species has been achieved. Moreover, the excitation energies and oscillator strengths calculated for the closed-shell systems, [Pc2Ti] and [Pc2Nb]+, provide a satisfactory interpretation of their characteristic visible optical spectra and help to rationalize the similar features observed in the visible spectrum of the open-shell "stapled" complex, [Pc2Nb]. Thin solid films (100-250 nm) of [Pc2Nb] deposited on ITO (indium-doped tin oxide) show a reversible redox process in neutral or acidic aqueous electrolytes. The electrochemical and electrochromic properties of the sandwiched complex, combined with impedance and UV/visible spectral measurements, are presented and discussed. The achieved electrochemical information, while substantially in keeping with the observed chemical redox behavior and theoretical predictions, qualifies [Pc2Nb] as an "optically passive" electrode and a promising material for its use in electrochromic devices.     

Ultrathin metals films on W(221): Structure, electronic properties and reactivity

Nanoscale pyramidal facets with (211) faces are formed when W(111) surface is covered by monolayer film of certain metals (including Pt, Pd and Au) and annealed to T ≥ 750 K. In the present work, we focus on the structure, electronic properties and reactivity of planar W(211) covered by ultrathin films of platinum and palladium. The measurements include soft X-ray photoelectron spectroscopy using synchrotron radiation, Auger electron spectroscopy, low energy electron diffraction (LEED) and thermal desorption spectroscopy. The metal film growth and evolution during annealing has been investigated for coverages ranging from 0 to 8 monolayers. The films grow initially in a layer-by-layer mode at 300 K. LEED, Auger, and Surface Core Level Shift (SCLS) measurements reveal that for coverages of one monolayer, the films are stable up to temperatures at which desorption occurs. In contrast, at higher coverages, SCLS data indicate that surface alloys are formed upon annealing films of Pt and Pd; surface alloy formation is not seen for Au overlayers. These findings are discussed in terms of structural and electronic properties of these bimetallic systems. Relevance to catalytic properties for acetylene cyclization over Pd/W(211) is also discussed.     

Initiated chemical vapor deposition of poly(1H,1H,2H,2H-perfluorodecyl acrylate) thin films

A solvent-free initiated chemical vapor deposition (iCVD) process was used to create low surface energy poly(1H,1H,2H,2H-perfluorodecyl acrylate) (PPFDA) thin films at deposition rates as high as 375 nm/min. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy showed full retention of the fluorine moieties, and no measurable cross-linking was detected. Additionally, the FTIR studies support the hypothesis that film deposition results from vinyl polymerization. For all iCVD PPFDA films, the static contact angle was found to be 120.8 +/- 1.2 degrees. The roughness of the films was found to be between 14.9 and 19.8 nm RMS, and the refractive index of the films was found to be between 1.36 and 1.37. The deposition rate was studied as a function of the substrate temperature and the partial pressure of the monomer. It was found that the deposition rate increases with decreasing substrate temperature and increasing monomer partial pressure. It was also found that the molecular weight increases with decreasing substrate temperature and increases with increasing monomer partial pressure. The highest molecular weight measured was 177 300 with a polydispersity of 2.27. Quartz crystal microbalance (QCM) measurements showed that these effects correlated with an increased monomer concentration at the surface. The deposition rate data and the QCM data were quantitatively analyzed to find the rate constants of the process using a previously published model for the iCVD process involving nonfluorinated monomers. The determined values of the rate constants of the surface polymerization were found to be similar to the rate constants measured in liquid-phase free radical polymerization. The kinetic data found in this paper can now be used to study iCVD deposition onto substrates with more complex geometries.     

Alkali-induced Transformation of Ni-MOF into Ni(OH)2 Nanostructured Flowers for Efficient Electrocatalytic Methanol Oxidation Reaction

Post treatment of metal-organic frameworks (MOFs) is widely employed to develop efficient electrocatalysts with better catalytic properties. But the complex processes of post treatment generally led to the collapse of the original structures of MOFs, making the preservation of their pristine hierarchical porous structure a great challenge. Herein, we propose the strategy of alkali treatment of Ni-MOF to transform it into Ni(OH)₂ with similar morphology and enhanced electrocatalytic properties for methanol oxidation reaction (MOR). The structure and electrocatalytic properties of as-obtained Ni(OH)₂ nanostructured flowers were seriously depended on the alkali concentrations. As the result, Ni(OH)₂ obtained from Ni-MOF treated by 0.25 M NaOH (noted as Ni(OH)₂-0.25) performs 1.5 and 2.5 times larger current density than those of Ni(OH)₂-0.025 and Ni(OH)₂-0.5 for MOR. Moreover, the electrocatalytic process and mechanism of MOR on the catalyst of Ni(OH)₂-0.25 are also revealed. Hence, this ex situ conversion strategy of alkali treatment for Ni-MOF uncovered the transformation of MOFs in alkaline solution and develops robust electrocatalyst for practical application of methanol fuel cells.     

Photoabsorption cross sections of sodium clusters: electronic and geometrical effects

We study the origin of the discrepancy between the photoabsorption cross sections of small jellium spheres calculated by the time dependent local density approximation (TDLDA) and experiments for small metallic clusters. We have specifically studied Na ₂₁ ⁺ . We conclude that both non-local exchange-correlation effects beyond the LDA and geometrical effects beyond the jellium approximation should be taken in the same calculation. We also present local and non-local calculations for Na _n ^? (n=19, 91 and 197) within the framework of the jellium model. The large anions show a fragmentation of the plasmon due to its interference with the ionization threshold. This feature is absent in the TDLDA results.     

The bivalent metal hypophosphites Sr(H2PO2)2, Pb(H2PO2)2 and Ba(H2PO2)2

The structures of isomorphous monoclinic strontium and lead bis­(di­hydrogenphosphate), Sr(H₂PO₂)₂ and Pb(H₂PO₂)₂, and orthorhombic barium bis­(di­hydrogen­phos­phate), Ba(H₂PO₂)₂, consist of layers of hypophosphite anions and metal cations exhibiting square antiprismatic coordination by O atoms. The Sr and Pb atoms are located on sites with point symmetry 2, and the Ba atoms are on sites with point symmetry 222. Within the layers, each anion bridges four metal cations.