| Polyoxometalates (POMs) constitute an immense class of compounds that are formed by early transition metals. POMs have potential applications in many fields including medicine, catalysis, multifunctional materials, and chemical analysis as they have determined structures, multiple components and remarkable chemical and physical properties. Following the development of the computer technology, quantum chemistry calculation as a theoretical study method has been introduced to the field of POMs. In early time, the ab initio Hartree-Fock (HF) approximation provided a reasonable starting point for understanding structures and electronic properties of POMs. However, relative to a mass of experimentation reports,the development of theoretical study of POMs is slow. In practice, important computation is limited and the ab initio and density-functional theory (DFT) modeling of polyoxoanions is still incipient.In this thesis, quantum chemistry calculations have been carried out to investigate electronic properties, stability, protonation, and redox properties of a series of Lindqvist- and Keggin-type POM derivatives. The present work has been focus on the following five aspects:1. Calculations based on density functional theory (DFT) have been carried out to investigate the geometry structures, electronic properties, redox potential, and electronic spectrum of the peroxohexaniobate species, [H3Nb6O13(O2)6]5-. Theη2-(O(I))22- segments in the terminal positions effectively modify the electronic properties. The HOMO in [H3Nb6O13(O2)6]5- mainly concentrates on theη2 oxygen atoms in the terminal positions. The LUMO delocalizes over the niobiums and bridge oxygens, butη2 oxygen atoms in the terminal positions have contribution. The LUMO energy and the HOMO-LUMO energy gap (Eg) of [H3Nb6O13(O2)6]5- are lower than that of [H3Nb6O19]5-, so [H3Nb6O13(O2)6]5- is easily reduced and the electron transition between HOMO and LUMO is much easier. Furthermore, the theoretical prediction of the redox potential of [H3Nb6O13(O2)6]5- confirms that the first reduction step of [H3Nb6O13(O2)6]5- becomes more easier as all six terminal oxygen atoms are replaced byη2-(O(I))22-. In addition, the calculation of electronic spectrum shows that the transition nature is also different between [H3Nb6O13(O2)6]5- and [H3Nb6O19]5- and the photochemistry activity of terminal oxygen (Ot) in [H3Nb6O13(O2)6]5- is strong.2. The electronic structure of a new type of polyoxometalate [Ti12Nb6O44]10- has been investigated using density functional theory (DFT). The calculations represent that the LUMO in fully oxidized [Ti12Nb6O44]10- delocalizes among the titanium (Nb) and niobium (Ti). So, both Ti and Nb have the probability to accept extra electron when [Ti12Nb6O44]10- as catalyst is reduced, which has been reinforced by the spin density for the monoreduced specie [Ti12Nb6O44]11-. Three kinds of possible protonated isomers [HTi12Nb6O44]9- are discussed. The results reveal that the preferred protonation sites correspond to bridging oxygens Nb-O-Ti. In addition, the calculation of electronic spectrum shows that there is obvious intramolecular charge transfer from oxygen to metal.3. Calculations based on density functional theory (DFT) have been carried out to investigate the electronic and redox properties of a series of Lindqvist-type polyanions [W5O18Zr(H2O)3-n(DMSO)n]2- (n = 0, 1, and 2); [W5O18M(H2O)3]2- (M=Ti and Hf); [Mo5O18M(H2O)3]2- (M=Ti, Zr, and Hf). The results show that the fragments of {Zr(H2O)3-n(DMSO)n}4+ (n = 0, 1, and 2) modify the HOMO-LUMO energy gaps of [W5O18Zr(H2O)3-n(DMSO)n]2- (n = 0, 1, and 2), which are lower than that of related parent [W6O19]2-. So, they are more easily reduced than their parent species. The LUMO in [W5O18Zr(H2O)3]2- mainly localizes on the d-orbitals of W (61.71%), and the LUMO+1 mainly concentrates on the W atoms and the contribution of Zr is only 2.38%. Thus, W atoms will prefer to accept electron when [W5O18Zr(H2O)3]2- as catalysts is reduced. However, the electronic property of [W5O18Ti(H2O)3]2- is different from that of the [W5O18Zr(H2O)3]2- and [W5O18Hf(H2O)3]2-. The substituting metal Ti effectively modifies the electronic property of [W5O18Ti(H2O)3]2-. The LUMO in [W5O18Ti(H2O)3]2- mainly concentrates on Ti atom. Thus, Ti atom in [W5O18Ti(H2O)3]2- becomes the reduced center, which means that different metal substitution can alter the reduced center. In addition, the replacements of five W atoms with five Mo atoms in [Mo5O18M(H2O)3]2- (M=Zr, Ti, and Hf) significantly reduce the HOMO-LUMO energy gaps, which indicates that Ti/Zr/Hf-Mo polynuclear Lindqvist complexes may be the promising catalysts.4. The electronic structures of [CpTi·SiW9V3O40]4– constructed from Keggin-type polyoxometalates functionalized by CpTi3+ group have been investigated by Density Functional Theory (DFT) methods. We discuss the relative stability affected by incorporating the CpTi3+ group into the different sites of the [SiW9V3O40]7- framework on the basis of geometrical parameters, total bonding energies and fragment analysis, and frontier molecular orbitals analysis. The calculated results indicate that the structure of the CpTi3+ group coordinating to one terminal oxygen and two bridging oxygen atoms of the Keggin-type polyoxoanion (system a) is more stable than that to three bridging oxygen atoms (system b).
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