Radiative lifetime and hyperfine-structure studies on laser-evaporated boron

The neutral boron atom has been investigated using laser spectroscopic techniques for the first time. Radiative lifetimes have been measured for the 3p ² P and 4p ² P states and the hyperfine-structure splitting of the 4p ² P _3/2 level has been determined. Laser evaporation was used for production of free atoms. The experimental results are compared with theoretical ab initio calculations.     

水在HfO2(111)和(110)表面的吸附与解离

运用广义梯度近似密度泛函理论方法(GGA-PW91)结合周期平板模型, 研究水分子在二氧化铪(111)和(110)表面不同吸附位置在不同覆盖度下的吸附行为. 通过比较不同吸附位的吸附能和几何构型参数发现:(111)和(110)表面铪原子(top 位)是活性吸附位. 水分子与表面的吸附能值随覆盖度的变化影响较小. 在(111)和(110)表面, 水分子都倾向以氧端与表面铪原子相互作用. 同时也计算了羟基、氧和氢在表面的吸附, Mulliken 电荷布居, 态密度及部分频率. 结果表明, 在两种表面羟基以氧端与表面铪相互作用, 氧原子与表面铪和氧原子同时成键, 而氢原子直接与表面氧原子相互作用形成羟基. 通过过渡态搜索, 水分子在(111)和(110)表面发生解离, 反应能垒分别为9.7和17.3 kJ·mol^-1, 且放热为59.9和47.6 kJ·mol^-1.     

Ab initio study of the potential energy surface and product branching ratios for the reaction of O(D) with CH3CH2F

The potential energy surface of O(¹D) + CH₃CH₂F reaction has been studied using QCISD(T)/6-311++G(d,p)//MP2/6-311G(d,p) method. The calculations reveal an insertion–elimination reaction mechanism of the title reaction. The insertion process has two possibilities: one is the O(¹D) atom inserting into C–F bond of CH₃CH₂F produces one energy-rich intermediate CH₃CH₂OF and another is the O(¹D) atom inserting into one of the C–H bonds of CH₃CH₂F produces two energy-rich intermediates, IM1 and IM2. The three intermediates subsequently decompose to various products. The calculations of the branching ratios of various products formed though the three intermediates have been carried out using RRKM theory at the collision energies of 0, 5, 10, 15, 20, 25 and 30 kcal/mol. CH₃CH₂O is the main decomposition product of CH₃CH₂OF. HF and CH₃ are the main decomposition products for IM1; CH₂OH is the main decomposition product for IM2. Since IM1 is more stable and more likely to form than CH₃CH₂OF and IM2, HF and CH₃ are probably the main products of the O(¹D) + CH₃CH₂F reaction. Our computational results can give insight to reaction mechanism and provide probable explanations for future experiments.     

A Synergy of ZnO and ZnWO4 in Composite Nanostructures Deduced from Optical Properties and Photocatalysis

ZnO/ZnWO₄ composite rod-like nanoparticles were synthesized by low-temperature soft solution method at 95 °C with different reaction times (1–120 h), in the presence of non-ionic copolymer surfactant Pluronic F68. Obtained nanoparticles had diameters in the range around 10 nm and length of 30 nm. Optical properties such as reflection and room temperature photoluminescence of obtained samples showed strong dependence on their crystallinity and composition. Photocatalytic activity of ZnO/ZnWO₄ nanopowders was checked using photodegradation of selected dyes as model system. Obtained results were correlated with specific surface area, particle sizes, crystallinity and ZnO/ZnWO₄ ratio of the samples. As crystallinity of ZnWO₄ component in the ZnO/ZnWO₄ increase, photocatalytic activity also increases. The main findings can be explained by charge transfer reactions that follow light absorption by ZnO and ZnWO₄ in nanocomposite.     

Quantum chemical ab initio calculations for excited states of F IV

Quantum chemical ab initio calculations have been performed for the ground state and for several excited states of the F³⁺ ion (F IV). Three levels of accuracy have been used: Frozen-core SCF calculations (FRC-SCF) to determine orbital energies ε _nl and quantum defects δ _l for excited Rydberg orbitalsnl; frozen-core SCF followed by CI calculations (FRC-CI) which account for multiplet splittings and configuration mixings, and multi-configuration coupled-electron-pair approximation (MC-CEPA) calculations which include dynamic correlation effects. The accuracy of the calculated excitation energies is in the order of 5000 cm^?1 at the FRC-CI level and in the order of 500 cm^?1 at the MC-CEPA level. This latter error amounts to about 0.1% for excitation energies in the range of 400000 to 600000 cm^?1. The MC-CEPA calculations have been performed for 17 experimentally known states and for 14 experimentally unknown states, in particular for the configurations 2s2p ² (² D)3s, 2s 2p ²(² S)3s, 2s ² 2p 4p, and 2s ² 2p 5p.     

Quantal close-coupling calculation of the cross sections for the j 1 m 1 → j 2 m 2 transitions in the 4p 2P and 3d 2D excited states of Ca+ induced by collisions with helium

Cross sections for the j ₁ m ₁ → j ₂ m ₂ transitions in the resonance 4p ²P and metastable 3d ²D states of the singly charged calcium ion induced by collisions with the ground-state He atom have been calculated using the quantal close-coupling method. The calculations are based on the earlier obtained Ca⁺-He pseudopotential SCF potential energy curves. The calculated cross sections are discussed in the energy range from threshold to 1.5 eV. Satisfactory agreement with other theoretical results has been found for the 4p ²P state. However, relatively large discrepancy between theory and available experimental data still exists for both the Ca⁺ states.     

Ab initio calculations of group IVA tetrachloride complexes: VII. Structure and dynamics of the formation of GeCl<Subscript>4</Subscript> complexes with hexamethylphosphoric triamide

Quantum-chemical calculations by the RHF/6-31G(d) method were performed for the systems GeCl₄←[N(CH₃)₂]₃ and GeCl₄←₂OP[N(CH₃)₂]₃ with full geometry optimization and varied Ge←O distances. The calculations with full geometry optimization gave trigonal-pyramidal and trans-octahedral structures, respectively, which agrees with experimental NQR data. As the components of a system get closer together, mutual polarization followed by electron density transfer from H atoms of the electron donor onto Cl atoms of the electron acceptor take place. The O and Ge atoms act as conductors in this electron density transfer. Nonempirical quantum-chemical calculations do not reproduce adequately the p _σ density of the axial Cl atom in the trigonal-bipyramidal complex.     

Comparative computational investigation of N and F substituted polyoxoanionic compounds: The case of Li2FeSiO4 electrode material

First principles calculations are used to anticipate the electrochemistry of polyoxoanionic materials consisting of XO_{4 − y}A_y (A = F, N) groups. As an illustrative case, this work focuses on the effect of either N or F for O substitution upon the electrochemical properties of Li₂FeSiO₄. Within the Pmn2₁–Li₂FeSiO₄ structure, virtual models of Li₂Fe₂^2.5+SiO_3.5N_0.5 and Li_1.5Fe²⁺SiO_3.5F_0.5 have been analyzed. We predict that the lithium deinsertion voltage associated to the Fe^3+/Fe⁴⁺ redox couple is decreased by both substituents. The high theoretical specific capacity of Li₂FeSiO₄ (330 mAh/g) could be retained in N-substituted silicates thanks to the oxidation of N³⁻ anions, whilst Li_1.5Fe²⁺SiO_3.5F_0.5 has a lower specific capacity inherent to the F substitution. Substitution of N/F for O will respectively improve/worsen the electrode characteristics of Li₂FeSiO₄.     

The electronic structures of vanadate salts: Cation substitution as a tool for band gap manipulation

The electronic structures of six ternary metal oxides containing isolated vanadate ions, Ba₃(VO₄)₂, Pb₃(VO₄)₂, YVO₄, BiVO₄, CeVO₄ and Ag₃VO₄ were studied using diffuse reflectance spectroscopy and electronic structure calculations. While the electronic structure near the Fermi level originates largely from the molecular orbitals of the vanadate ion, both experiment and theory show that the cation can strongly influence these electronic states. The observation that Ba₃(VO₄)₂ and YVO₄ have similar band gaps, both 3.8 eV, shows that cations with a noble gas configuration have little impact on the electronic structure. Band structure calculations support this hypothesis. In Pb₃(VO₄)₂ and BiVO₄ the band gap is reduced by 0.9-1.0 eV through interactions of (a) the filled cation 6s orbitals with nonbonding O 2p states at the top of the valence band, and (b) overlap of empty 6p orbitals with antibonding V 3d-O 2p states at the bottom of the conduction band. In Ag₃VO₄ mixing between filled Ag 4d and O 2p states destabilizes states at the top of the valence band leading to a large decrease in the band gap (E_g=2.2 eV). In CeVO₄ excitations from partially filled 4f orbitals into the conduction band lower the effective band gap to 1.8 eV. In the Ce_1−xBi_xVO₄ (0≤x≤0.5) and Ce_1−xY_xVO₄ (x=0.1, 0.2) solid solutions the band gap narrows slightly when Bi³⁺ or Y³⁺ are introduced. The nonlinear response of the band gap to changes in composition is a result of the localized nature of the Ce 4f orbitals.     

13C NMR and SCCCMO studies on substituted benzophenonetricarbonylchromium complexes

¹³C NMR studies on benzophenonetricarbonylchromium and its p-F, p-Cl and p-OCH₃ derivatives, with the substitutents on the uncomplexed rings, show a small substituent effect on the complexed ring and on the carbons of the Cr(CO)₃ group. The SCCCMO calculations show π-electron donation from the ring to the metal but greater σ-electron back-donation which leaves the ring more negatively charged than before complexation. The chromium atom is more positively charged than it is in benzenetricarbonylchromium. The trends is calculated CO bond orders are in agreement with the trends in CO infrared stretching frequencies.     

CdWO4 polymorphs: Selective preparation, electronic structures, and photocatalytic activities

This work explored the selective synthesis of polymorphs of CdWO₄ in either tetragonal or monoclinic phase by optimizing the experimental parameters. Systematic characterization indicated that both polymorphs possessed similar spherical morphologies but different structural building blocks. Electronic structures calculations for both polymorphs demonstrated the same constructions of conduction band or valence band, while the conduction band widths of both polymorphs were quite different. Both CdWO₄ polymorphs exhibited good photocatalytic activity for degradation of methyl orange under UV light irradiation. When comparing to some other well-known tungstate oxide materials, the photocatalytic activity was found to follow such a consequence, monoclinic CdWO₄≈monoclinic ZnWO₄>tetragonal CdWO₄>tetragonal CaWO₄. The specific photocatalytic activity of monoclinic CdWO₄ was even higher than that of commercial TiO₂ photocatalyst (Degussa P25). The increased activity from the tetragonal CdWO₄ to the monoclinic was consistent with the trend of the decreased symmetry, and this could be explained in terms of the geometric structures and electronic structures for both polymorphs.     

Syntheses, crystal structures and Raman spectra of Ba(BF4)(PF6), Ba(BF4)(AsF6) and Ba2(BF4)2(AsF6)(H3F4); the first examples of metal salts containing simultaneously tetrahedral BF4 and octahedral AF6 anions

In the system BaF₂/BF₃/PF₅/anhydrous hydrogen fluoride (aHF) a compound Ba(BF₄)(PF₆) was isolated and characterized by Raman spectroscopy and X-ray diffraction on the single crystal. Ba(BF₄)(PF₆) crystallizes in a hexagonal space group with a=10.2251(4) Å, c=6.1535(4) Å, V=557.17(5) Å³ at 200 K, and Z=3. Both crystallographically independent Ba atoms possess coordination polyhedra in the shape of tri-capped trigonal prisms, which include F atoms from BF₄⁻ and PF₆⁻ anions. In the analogous system with AsF₅ instead of PF₅ the compound Ba(BF₄)(AsF₆) was isolated and characterized. It crystallizes in an orthorhombic Pnma space group with a=10.415(2) Å, b=6.325(3) Å, c=11.8297(17) Å, V=779.3(4) Å³ at 200 K, and Z=4. The coordination around Ba atom is in the shape of slightly distorted tri-capped trigonal prism which includes five F atoms from AsF₆⁻ and four F atoms from BF₄⁻ anions. When the system BaF₂/BF₃/AsF₅/aHF is made basic with an extra addition of BaF₂, the compound Ba₂(BF₄)₂(AsF₆)(H₃F₄) was obtained. It crystallizes in a hexagonal P6₃/mmc space group with a=6.8709(9) Å, c=17.327(8) Å, V=708.4(4) Å³ at 200 K, and Z=2. The barium environment in the shape of tetra-capped distorted trigonal prism involves 10 F atoms from four BF₄⁻, three AsF₆⁻ and three H₃F₄⁻ anions. All F atoms, except the central atom in H₃F₄ moiety, act as μ₂-bridges yielding a complex 3-D structural network.     

Synergistic Effect of SnO2/ZnWO4 Core–Shell Nanorods with High Reversible Lithium Storage Capacity

High reversible lithium storage capacity is obtained from novel SnO₂/ZnWO₄ core–shell nanorods. At C/20 (20 h per half cycle) rate, the reversible capacity of SnO₂/ZnWO₄ core–shell nanorods is as high as 1000 mAh g^?1, much higher than that of pure ZnWO₄, SnO₂, or the traditional theoretical result of the simple mixture. Such performance can be attributed to the synergistic effect between the nanostructured SnO₂ and ZnWO₄. The distinct electrochemical activity of ZnWO₄ nanorods probably activates the irreversible capacity of the SnO₂ nanoparticles. These results indicate that high‐performance lithium ion batteries can be realized by introducing the synergistic effect of one‐dimensional core–shell nanocomposites.     

Electronic structure and photoluminescence properties of Eu-activated Ca2BN2F

The electronic structure of undoped and luminescence properties of Eu²⁺-doped Ca₂BN₂F have been investigated. First-principles calculations for Ca₂BN₂F show that the valence band is mainly composed of F and N 2p, B 2s and 2p orbitals, while the Ca 4s and 3d are almost empty, indicating that Ca₂BN₂F is a very ionic compound. The valence band close to the Fermi level is dominated by the N 2p states, and the bottom of the conduction band is determined by the Ca 3d and N/B 3s orbitals. The direct energy gap is calculated to be about 3.1 eV, in fair agreement with the experimental data of ∼3.6 eV derived from the diffuse reflection spectrum. Due to the high degree of ionic bonding of the coordinations of Eu with (N, F) on the Ca sites, Ca₂BN₂F:Eu²⁺ shows strong blue emission with a maximum at about 420 nm upon UV excitation in the absorption range of 330-400 nm.     

一步醇-水热法合成高效的F掺杂BiVO4光催化剂

以NH₄F为掺杂前体,采用简单的一步醇-水热法制备了F掺杂BiVO₄光催化剂。利用X射线衍射（XRD）、扫描电子显微镜（SEM）、X射线光电子能谱（XPS）、紫外-可见漫反射光谱（UV-Vis）和光致发光光谱（PL）表征了这些光催化剂的物理化学性质。在少量H₂O₂存在条件下,以可见光照射下光催化降解苯酚的反应测定了这些光催化剂的催化活性。研究表明,相较于未掺杂的BiVO₄样品而言,F掺杂BiVO₄样品不仅仍保留了单斜结构,而且有更高的结晶度、表面氧空位密度和光生电荷载流子分离效率,更强的光吸收和更低的带隙能。在这些F掺杂BiVO₄样品中,以n_F/n_Bi的理论值为1.0且带隙能为2.43 eV的F掺杂BiVO₄样品的光催化活性最好（90 min内苯酚的降解率可达95%）。这一优良的光催化性能与其具有最高的结晶度、表面氧空位密度和光生电荷载流子分离效率,最强的光吸收和最低的带隙能有关。     

Low-dimensional compounds containing cyano groups. XV. Preparation,crystal structure and spectral properties of three copper(II) nitrosodicyanomethanide complexes

We describe the preparation and crystal structures of the ionic complexes [Cu(bipy)₂{ONC(CN)₂}]CF₃SO₃ (1b), [Cu(phen)₂{ONC(CN)₂}]PF₆ (2p) and [Cu(bipy)₂{ONC(CN)₂}]PF₆ (2b). In the complex cations [Cu(L)₂{ONC(CN)₂}]⁺ (L is 2,2′-bipyridine (bipy) or 1,10-phenanthroline (phen)) the two molecules of bipy or phen coordinate to the copper atom through two nitrogen atoms along with the oxygen atom of the nitrosodicyanomethanide anion, ONC(CN) ₂ ^? , to form a {CuN₄O} chromophore with a distorted square pyramidal coordination sphere in (1b) and (2b) and a distorted trigonal bipyramidal geometry in (2p). The basal plane in (1b) and (2b) is formed by an oxygen atom coordinated at the Cu1–O1 distance of 1.990(2) and 2.002(2) Å, respectively, and three nitrogen atoms coordinated to the copper atom at similar distances with the average of 2.01(2) and 2.00(3) Å, respectively. The axial position is occupied by the fourth N atom at the longer distance of 2.222(2) and 2.185(2) Å, respectively. The trifluoromethanesulfonate anion (triflate), CF₃SO ₃ ^? , in (1b) might be considered as very weakly coordinated in the opposite axial position (Cu1–O2 = 2.719(2) Å). The equatorial plane in (2p) is formed by an oxygen atom coordinated at the Cu1–O1 distance of 1.975(3) Å, and two nitrogen atoms from different phen molecules coordinated to the copper atom at the same distance within 2 σ with the average distance of 2.124(2) Å. The axial positions are occupied by remaining two nitrogen atoms coordinated at shorter distance (average Cu–N = 1.99(3) Å). The hexafluorophosphate anions, PF ₆ ^? , in (2p) and (2b) remain uncoordinated. Besides the ionic forces, the structures of (2p) and (2b) may be stabilized by very weak C–H···F whereas the structure of (1b) by very weak C–H···F, C–H···O and C–H···N hydrogen bonds. The structural–spectral correlations are also discussed.     

Valence bond studies of AH2 molecules

Minimal basis set (STO) molecular orbital and valence-bond calculations are reported for the³ B ₁ and¹ A ₁ states of CH₂. The open-shell molecular orbital calculations used the Roothaan formulation. The valence-bond calculations used the Prosser-Hagstrom biorthogonalisation technique to evaluate the cofactors required in using Löwdin's formulae. Optimisation of geometry and orbital exponents in the molecular orbital calculation on the³ B ₁ state gave a geometry of R_C-H=2.11 a.u. and H-C-H=123.2 °. The energy obtained was ?38.8355 a.u. The molecular orbital and valencebond calculations are compared. In the valence-bond calculations the variation with bond-length and bond-angle of the configuration energies was studied. Valence bond “build-up” studies are also reported. Valence-bond calculations using hybrid orbitals instead of natural atomic orbitals showed that the perfect-pairing approximation is not as good for CH₂ as BeH₂. The nature of the lone-pair and bonding orbitals is found to be significantly different between the³ B ₁ and¹ A ₁ states. In the³ B ₁ state the 2s and 2p orbitals are fairly equally mixed between both types of orbital. However in the¹ A ₁ state the bonding orbitals have mainly 2p character and the lone pair orbitals have mainly 2s character. As was found for H₂O, the bonding hybrid orbitals do not follow the hydrogen nuclei as the bond angle varies but continue to point approximately in their equilibrium directions.     

Electronic structure and photocatalytic properties of ABi2Ta2O9 (A=Ca, Sr, Ba)

A new series of layered perovskite photocatalysts, ABi₂Ta₂O₉ (A=Ca, Sr, Ba), were synthesized by the conventional solid-state reaction method and the crystal structures were characterized by powder X-ray diffraction. The results showed that the structure of ABi₂Ta₂O₉ (A=Ca, Sr) is orthorhombic, while that of BaBi₂Ta₂O₉ is tetragonal. First-principles calculations of the electronic band structures and density of states (DOS) revealed that the conduction bands of these photocatalysts are mainly attributable to the Ta 5d+Bi 6p+O 2p orbitals, while their valence bands are composed of hybridization with O 2p+Ta 5d+Bi 6s orbitals. Photocatalytic activities for water splitting were investigated under UV light irradiation and indicated that these photocatalysts are highly active even without co-catalysts. The formation rate of H₂ evolution from an aqueous methanol solution is about 2.26 mmol h^-1 for the photocatalyst SrBi₂Ta₂O₉, which is much higher than that of CaBi₂Ta₂O₉ and BaBi₂Ta₂O₉. The photocatalytic properties are discussed in close connection with the crystal structure and the electronic structure in details.     

The nickel-group IV molecules NiC,NiSi, and NiGe

All electron ab initio calculations have been applied to elucidate the electronic states and the nature of the chemical bonds in the molecules NiC, NiSi, and NiGe. The calculations have revealed that the ground states of all three molecules are¹Σ⁺, but due to the open 3d shell of the Ni atom the molecules have many low-lying electronic states. The NiC molecule is strongly polar, and the low-lying electronic states have been identified as those arising when the angular momenta of the³F_g Ni⁺ ion are coupled to the angular momenta of the⁴S_uC^? anion. The chemical bond in the NiC molecule has triple bond character due to the valence bond couplings between the Ni 4s and 3dπ electrons and theC 2p electrons. The chemical bonds in the molecules NiSi and NiGe are very much alike; they are double bonds composed of oneσ and oneπ bond. Theσ bond is due to the doubly occupied delocalized molecular orbital composed of the Ni 4s orbital and the Si 3pσ or the Ge 4pσ orbital. Theπ bond originates from the valence bond coupling between the localized hole in the Ni 3dπ orbital and the valencepπ electron of Si or Ge.