Theoretical Study On The Redox Properties Of Lindqvist And Keggin-type Polyoxometalates

Polyoxometalates (POMs or metal-oxogen clusters) constitute an immense class ofcompounds which are formed by early transition metals. POMs contain a variety of molecularstructure, special physical and chemical property. POMs have the potential application inmaterials science, catalytic and analytical chemistry. The futher research has been forbiddenbecause of the present deficient characterization appoarch, so the quantumn chemicalcomputations as a theoretical method have been introduced into the field of POMs toinvestigate the structure-property relationship at the micro-level. However, the large sizedPOMs system with high negative charge and multiple metal atoms are very computationalcost. Thanks to the development of the density functional theory (DFT) and computertechnology, there is a large improvement on the computational precision and speed for thetransition metals recently.In this thesis, DFT calculations have been performed to investigate the redox properties fortransition metal monosubstituted Lindqvist-and Keggin-type polyoxometalates. The presentwork has been focused on the following two aspects:The redox properties of the Ⅳ-Ⅷ transition metal monosubstituted Lindqvist-typepolyoxometalates have been investigated using a density functional theory method. Theresults show that, when the Mo atom was substituted by VⅤ, CrⅥ, MoⅥ, FeⅤ, FeⅥ, RuⅤ, RuⅥ,OsⅥ, TcⅤ, TcⅥand TcⅦ, the LUMO are mainly concentrated on the substituted metal. Bycomparing the H-L energy gap of these anions, we obtained four anionic has a moderate H-Lenergy gap, the order of the gap between them (from small to large) is [Mo5FeⅤO19]3-<[Mo5CrO19]2-<[MoⅦ5TcO19]-<[Mo5VO19]3-<[Mo6O19]2-. In order to illustrate the electronicscapacity of these anions, we calculated four of the anions with corresponding one-electronreduction species under solvation conditions. The calculation results reveal that [Mo5CrO19]2-is prone to obtain an additional electron than the others, i.e. the order of the oxidation abilityis [Mo5CrO19]2->[Mo5FeO19]3->[Mo6O19]2->[Mo5VO19]3-.The cyclic voltammogram simulation for Keggin-type polyoxometalates has beeninvestigated using a density functional theory method. When the solvent is water or organicsolvents, the calculated reduction potentials are in good agreement with the cathodic peakpotential in the experiment. This is mainly because of the low concentration of protons inthese solvents. Thus, the polyoxometalate anion is difficult to be the protonated. However,when the solvent was changed to acid solution, the protonated of the POMs in these solventsis very easy. And the calculated reduction potentials are deviated from the experimental results. Combined with the peak potential formula, we can get two important points,(Ep, i’p)and (Ep/2, i’p/2), which are similar with the cyclic voltammogram. By these calculations, we areinspired that it is possible to simulate the cyclic voltammogram by theory calculation.