Electronic Structures, (d-p)π Conjugation Effects, and Spectroscopic Properties of Polyoxometalates: M6O19 2− (M=Cr, Mo, W)

The electronic structures and bonding of isopoly oxometalates M₆O₁₉ ^2– (M=Cr, Mo, W) have been investigated by using ab initio and relativistic density functional methods. We have discussed the role of the central oxygen atom and the (d-p) conjugation interactions between the metal and bridging oxygen atoms. It is found that there exist 12 three-centered two-electron (d-p-d) bonds for the three M₄(-O)₄ planar rings in M₆O₁₉ ^2– ions and these hexametalates are considered to have quasi-aromaticity. The (d-p) conjugation effects play essential role in stabilizing these cluster compounds, and the reduced (d-p) conjugation effects account for the instability of the isopoly oxochromate ion, Cr₆O₁₉ ^2–. The vibrational spectra and electronic spectra of M₆O₁₉ ^2– ions are evaluated and assigned theoretically and the calculated spectra are in fairly good agreement with the measured experimental results.     

Lindqvist型[M6-nMonO19]p-阴离子（M=W,Nb,Ta）电子性质的密度泛函理论研究

采用密度泛函理论方法探讨了取代Mo原子对[W6-nMonO19]2-,[Nb6-nMonO19]p-和[Ta6-nMonO19]p-体系的M—Ot(M=W,Nb,Ta)键的活化作用.计算结果表明,随着取代Mo原子数的增多,[M6-nMonO19]2-(M=W,Nb,Ta)中M—Ot键的键能逐渐减小,因此Mo原子的引入使M—Ot键活化.在[W6-nMonO19]2-中,Mo—Ot键的键能小于W—Ot键的键能,因此,Mo—Ot键比W—Ot键易断裂,与实验结果一致.而在[Nb6-nMonO19]p-和[Ta6-nMonO19]p-体系中,Mo—Ot键的键能大于M—Ot(M=Nb,Ta)键的键能.Nb和Ta原子的端氧Ot的电荷大于Mo原子的端氧Ot的电荷,初步预测,当[Nb6-nMonO19]p-和[Ta6-nMonO19]p-与有机胺反应时,Nb—Ot和Ta—Ot键优先断裂,易与有机胺的氮原子成键.     

吡啶取代的Lindqvist型多酸的线性和非线性光学性质及阳离子检测功能的理论研究

采用密度泛函理论B3LYP方法计算了吡啶取代的Lindqvist型多酸(POMs)的线性(最大吸收波长, λ_max)和非线性光学(NLO)[超瑞利散射(HRS)的第一超极化率, β_HRS]性质, 探讨了其作为潜在阳离子检测剂的可能性. 金属离子吸附能计算结果表明, 吡啶取代的Lindqvist型多酸配体与金属离子之间均有较强的相互作用, 相互作用强度大小顺序为Ni²⁺>Cu²⁺>Co²⁺>Fe²⁺>Zn²⁺>Mg²⁺>Ca²⁺. 电子光谱和β_HRS计算结果表明, 引入适当的供、 受电子基团对该多酸配体进行修饰可有效调节线性和二阶NLO性质; 同时, 吡啶取代的Lindqvist型多酸对7种金属离子(Cu²⁺, Zn²⁺, Ca²⁺, Mg²⁺, Ni²⁺, Co²⁺, Fe²⁺)表现出了不同的检测行为.     

Delta function model for the helium trimer: origin of three-body forces

The Dirac bubble potential, previously used to model the helium dimer, is applied to the trimer. It is shown that the quantum mechanical kinetic energy operator for a three-body system contains terms over and above the analogues of classical pairwise contributions. The additional terms, the ‘Borromean couplings’, are responsible for a dramatic increase in the binding energy of the trimer. For example, ⁴He₃ is bounded by two orders of magnitude more strongly than ⁴He₂, 279 mK versus 1.31 mK, respectively, according to our calculations. Moreover, the trimer is considerably more compact than the dimer, with (r₁₂) decreasing to 9.01 Å from 51.9 Å.     

A new organic–inorganic hybrid polyoxoniobate based on Lindqvist-type anion and nickel complex

A new organic–inorganic hybrid polyoxoniobate (H₂en)₂[Ni(en)₃][H₂Nb₆O₁₉] · 5.5H₂O (1) (en = ethylenediamine) has been synthesized by the diffusion method and structurally characterized by elemental analyses, infrared spectrum, ultraviolet (UV) spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction, thermogravimetric (TG) analysis, and single-crystal X-ray diffraction. Crystal structure analysis reveals that 1 exhibits a 3-D supramolecular architecture constructed from Lindqvist-type [H₂Nb₆O₁₉]^6? polyoxoanions and [Ni(en)₃]²⁺ via hydrogen-bonding interactions. The XPS measurement indicates that the oxidation state of Ni is +2. TG curve of 1 exhibits two steps of weight loss. In situ UV spectra display that 1 can exist in large pH range in aqueous solution.     

A polyoxoniobate constructed from Lindqvist-type hexaniobates and copper coordinated cations

A new organic–inorganic hybrid polyoxoniobate [Cu(1,3-dap)₂(H₂O)][(H₆Nb₆O₁₉)₂Cu(1,3-dap)₂]?·?4(1,3-dap)·20H₂O (1) (1,3-dap?=?1,3-diaminopropane) has been synthesized by the diffusion method and structurally characterized by elemental analyses, infrared spectrum, ultraviolet spectroscopy, and single crystal X-ray diffraction. Crystal structure analysis reveals that 1 consists of a dimeric dumbbell anion [(H₆Nb₆O₁₉)₂Cu(1,3-dap)₂]^2?, a copper coordinated cation, four 1,3-dap ligands and 20 crystal water molecules. Neighboring units are combined via hydrogen bonds forming a 3-D supramolecular framework.     

Three new polyoxoniobates constructed from Lindqvist-type hexaniobate and copper–amine complexes

Three new polyoxoniobates constructed from Lindqvist-type [Nb₆O₁₉]^8? and copper–amine complexes, [Cu(1,2-dap)₂]{[Cu(1,2-dap)₂]₂[Nb₆O₁₉H₂]}?·?10H₂O (1), [Cu(1,3-dap)₂]₂{[Cu(1,3-dap)]₂[Nb₆O₁₉]}?·?10H₂O (2), and [Cu(en)₂]_0.5{[Cu(en)₂]₂[Nb₆O₁₉H₃]}?·?12H₂O (3) (1,2-dap?=?1,2-diaminopropane, 1,3-dap?=?1,3-diaminopropane, and en?=?ethylenediamine), have been synthesized and characterized by elemental analyses, infrared, powder X-ray diffraction, thermogravimetric analysis, X-ray photoelectron spectrum, and single-crystal X-ray diffraction. Compounds 1–3 exhibit bisupporting hexaniobate cluster structure, each with a [Nb₆O₁₉]^8? cluster decorated by two copper–amine complexes. In 1, adjacent bisupporting clusters are connected with one [Cu(1,2-dap)₂]²⁺ fragment via Cu?···?O weak interactions to generate a 1-D supramolecular chain structure. In contrast to 1, each bisupporting cluster in 2 is linked to another four neighboring ones through four [Cu(1,3-dap)₂]²⁺ fragments by Cu?···?O weak interactions to yield a 2-D supramolecular network. Different from 1 and 2, no detected interaction was found between bisupporting cluster and [Cu(en)₂]²⁺ in 3, the [Cu(en)₂]²⁺ fragment merely acts as an isolated countercation.