Theoretical Study On The Redox Properties Of Double Replaced And Single-absence Keggin-type Polyxometalate Derivatives

Polyacids（polyoxometalates,POMs）are a class of oxygen-containing polyacids formed by the attachment of oxygen to transition metal atoms.Keggin-type polyacids are one of the most typical and reported polyacid structures.Due to its excellent properties and abundant structure,Keggin POMs have attracted the theoretical research of polyacids to make a great breakthroughs.Since Keggin-type POMs have excellent properties and abundant structures,which are highly concerned in materials science,photocatalysis,medicine and other fields.With the development of quantum chemistry,people recently begin to pay much attention in the theoretical research of polyacids.In this thesis,the application of density functional theory（DFT）to calculate and analyze geometric structure,electronic structure and redox reaction of polyacids in different configurations are explored.The third chapter uses DFT method to investigate geometries and energies of double replacedα-[SiW₁₀M₂O₄₀]^n-（M=Ti^Ⅳ,V^Ⅴ,Cr^Ⅵ,Mn^Ⅴ,Fe^Ⅲ）type silicotungstic acid in the same three metal cluster.When metal Ti,V,Cr,Mn,Fe are used to instead of W,the frontier molecular orbital structure do not change by Ti double replacement,but the highest occupied orbital（HOMO）are changed when double replaced Mn and Fe takes place,electrons are mainly distributed in the replaced metal atoms.For LUMO,main delocalization are on metal atoms except for Ti.By calculating the energy gap（GHL）of different systems,as[SiW₁₂O₄₀]^4->[SiW₁₀V₂O₄₀]^6->[SiW₁₀Cr₂O₄₀]^4->[SiW₁₀Mn₂O₄₀]^6->[SiW₁₀Fe₂O₄₀]^10-,and the decrease of LUMO after Cr substitution,it can be suggested that[SiW₁₀Cr₂O₄₀]^4-has been verified to be the best oxidative reducibility of them.In chapter 4,DFT and TDDFT methods are used to analyze the geometrical structures,electronic properties,oxidative reducibility and absorption spectra ofα-[XMo₁₁O₃₉]^n-（X=S,P,Si）single-absence systems.With the change of central heteroatoms S,P and Si,the central tetrahedral cavity XO₄ does not change much,and GHL remains basically unchanged,indicating that the heteroatoms do not affect the stability and energy gap of the system,but the oxidation ability of different series are[SMo₁₁O₃₉]^6->[PMo₁₁O₃₉]^7->[SiMo₁₁O₃₉]^8-.Through the analysis of the excited states,the absorption spectra of the three systems are mainly distributed in the ultraviolet region,corresponding to charge transfer from oxygen to transition metal transition（OMCT）.