| Polyoxometalates (POMs) have been traditionally the subject of study of molecular inorganic chemistry. Yet, these polynuclear molecules, reminiscent of oxide clusters, present a wide range of structures and with them ideal frameworks for the deployment of a plethora of useful magnetic, electronic, catalytic, bioactive and photochemical properties. With the splendid experimental studies, POMs have been recently paid much attention on their electronic structures, redox properties, bonding features, and molecular stabilities.In this thesis, the density functional theory (DFT) has been employed to study the geometrical and electronic structures, bonding character, and redox properties of polyanions. All finished work has been focused on the following aspects:1. The geometrical structures and electronic properties of [VIV18O42(X)]13- (X=Cl-, Br-, I-, SH-, NO2-) were investigated by DFT method. The results show that the guest anion affects the size of the host cluster. With the increasing size of guest anion, the size of host cluster increases. The frontier molecular orbital and the LUMO energy of[VIV18O42(X)]13- are slightly influenced by the guest anion. It indicates that the interaction between guest anion and the host cluster is weak.2. The investigations of the structural and electronic properties of proposed novel rhenium-nitrido isoelectronic analogues toward several typical polyoxometalates were performed by theoretical approach. The structures of rhenium-nitrido isoelectronic analogues are similar to their parents POMs. The cluster sizes of rhenium-nitrido analogues are slightly smaller than that of related polyoxoanions. The charge populations computed for the [XRe12(N12O28]n- (X= P, Si, Al) clusters support the hypothesis that an oxidized anion is an XO4n- clathrate inside a neutral Re12N12O24 cage. The vibrational frequency analysis show that [Re6N6O13]2- and [PRe12(N12O28]3- are minima. Comparing with Re–Ob bonds vibrations, Re≡N bonds occur at high frequencies. The HOMO-LUMO gaps and LUMO energies of [Re6N6O13]2- and [PRe12(N12O28]3- are lower than related parents [W6O19]2- and [PW12O40]3- as the d-rhenium orbitals are lower in energy than tungsten orbitals. It suggests that [Re6N6O13]2- and [PRe12(N12O28]3- could be more reactive than their parent species. The orbital characters in rhenium-nitrido analogues resemble with their related species. The rhenium-nitrido analogue core is a reservoir of electrons that resemble polyoxoanions and can accept many electrons without significantly deforming the framework. |
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