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The Theoretical Study On The Electronic Structures And Nonlinear Optical Properties Of Lindqvist-and Wells-Dawson-type Polyoxometalates

Posted on:2013-05-09Degree:MasterType:Thesis
Country:ChinaCandidate:S CongFull Text:PDF
GTID:2231330395472664Subject:Materials Physics and Chemistry
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Polyoxometalates (POMs or Metal-Oxogen Clusters) are a rich family of polynuclearcompounds which contain a variety of molecular structure, special physical and chemicalproperty. POMs have the potential application in catalytic and analytical chemistry, andmaterials science. The futher research has been restricted because of the present deficientcharacterization appoarch, so the quantumn chemistry calculation as a theoretical method hasbeen introduced into the field of POMs and investigated the structure-property relationship atthe micro-level. However, the large sized POMs system with high negative charge andmultiple metal atoms are very computational cost. Thanks to the development of the densityfunctional theory (DFT) and computer technology, there is a large improvement on thecomputational precision and speed for the transition metals.In this thesis, DFT calculations have been performed to investigate the redox properties andsecond-order nonlinear optical (NLO) response for a series of organic-inorganic hybridLindqvist-type POM derivatives. These results may provide a theoretical basis for designingnovel POM materials with electronic, redox and optical properties. The present work has beenfocused on the following three aspects:(1) The electronic properties and stabilities of five [Nb2W4O18OCH3]3- isomers have beeninvestigated using a density functional theory method. The results show that the isomer withthe methoxy group occupying a bridging position between two tungsten atoms (two tungstenatoms in the plane that contains two niobium atoms) in the [Nb2W4O18OCH3]3framework isthe most stable isomer in acetonitrile. The stability of the one-electron-reduced isomerschanges little. The most stable one-electron-reduced isomer has the methoxy group occupyinga bridging position between niobium atoms in the [Nb2W4O18OCH3]4- framework. The M–Ob(M=Nb, W; b denotes bridging) bond lengths in anions in which the metal atoms areconnected by a methoxy group are longer than those in [Nb2W4O19]4. The highest occupiedmolecular orbitals (HOMO) in [Nb2W4O19]4- mainly delocalize over the bridging oxygenatoms of two niobium atoms and two tungsten atoms located in the equatorial plane, and thebridging oxygen atoms on the axial surface. The lowest unoccupied molecular orbitals(LUMO) of [Nb2W4O19]4- are mainly concentrated on the tungsten atoms and antibondingoxygen atoms. Methoxy substitution modifies the electronic properties of the[Nb2W4O18OCH3]-3isomers. The HOMOs in the five isomers formally delocalize over thebridging oxygen atoms, which are distant from the surface containing the methoxy group andfour metal atoms. The LUMOs delocalize over the d-shells of the four metal atoms that are close to the methoxy group, and the p-orbitals of oxygen. One-electron reduction occurred atthe tungsten atoms, not the niobium atoms.(2) The electronic and NLO properties of Lindqvist-type tungstate containing late3dtransition metals [(py)MW5O18]4-(M=Fe, Co, Ni) have been systematically investigatedusing DFT method. The bonding character of M-N bond is analyzed using natural bondorbital (NBO) method. The first hyperpolarizabilities of [(py)MW5O18]4-anions have beeninvestigated by coulomb-attenuating method (CAM-B3LYP). The NLO properties of[(py)MW5O18]4-with different spin states are also studied. The results show that the staticsecond-order polarizability (β0) of [(py)5FeW5O18]4-(Fe=quintet state) is525.10×10-30esu,which is larger than those of [(py)4CoW5O18]4-(β0=120.72×10-30esu) and [(py)3NiW5O18]4-(β0=30.45×10-30esu) anions. Time-dependent DFT (TDDFT) results reveal that thesubstituted transition metals-to-pyridine charge transfer may be responsible for the NLOproperties of this kind of polyoxometalates.(3) The geometry structure of Wells-Dawson derivatives [X-C(CH2O)3P2M′3M15O59]6-(X=NO2, NH2, and CH3, M′=V and Nb, M=W and Mo) have been firstly calculated by DFTmethod. The results show that the distances of P···P and P···C are controlled by the {M′3}triads, and also {M′3} triads affect the sizes of W6hexagonal belts. The firsthyperpolarizabilities and origin of NLO properties of those clusters have been investigated byTDDFT method. The NLO analysis reveals that [X-C(CH2O)3P2V3Mo15O59]6-(X=NH2)anions have the largest βvecvalues,75.064×10-30esu. The major electron transitions orbitalsdisplay that the charge transfer of organic group to the metal atom is responsible for the NLOproperties of this kind of POM derivative.
Keywords/Search Tags:Polyoxometalates, Electronic Structure, Redox Property, Nonlinear OpticalProperty, Natural Bond Orbital, Density Functional Theory (DFT)
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