Theoretical Investigation On The Structures And Properties Of The（MO₃）_n（M=Mo,W;n=7-12）Isomers

As important catalysts in chemical production, Transition metaloxides （TMOs） have gained wide concern of the chemists andphysicists. In this thesis, the structures and properties of transitionmetal oxide （TMO） clusters of the group VIB metals,（MO₃）_n（M=Mo, W;n=7-12）, have been studied with density functional theory （DFT）methods.In the first part, BS1method and BS2method of higher accuracyare respectively adopted to optimize the geometric structures andcalculate the frequency of isomers of （MO₃）_non the basis of densityfunctional theory, so that their geometric structures, energies, NBOcharge and vibrational frequency can be ananlyzed. When the valuesof n are equal to7,9, and11, the stable structures of （MO₃）_ntake theform of the ring and chain; when the values of n are equal to8,10and12,（MO₃）_nthe stable structures of （MO₃）_nare ring, chain and lanterns.And for M10O30, namely another cage structure of isomer should bepaid attention to. The calculation results show that when the valuesof n are equal to7,8,9,10and11, MonO3nring A is the most stablestructure; when the value of n is equal to12, Mo₁₂O₃₆lantern is themost stable structure; when the values of n are equal to9,11, W_nO_3nring B is lowest in energy and the most stable structure; while the values of n are equal to8and12, W_nO_3nlantern is the most stablestructure; when the value of n is equal to10, W10O30cage is the moststable configurations.（MO₃）_nchain has the highest energy and is theleast stable formation. The average binding energy of the stableconfigurations of the （MoO3）nand （WO₃）_nis relatively high, which isrespectively located about67eV and84eV.In the second part, density functional theory at the B3LYP levelwith aug-cc-pVDZ basis sets for H、O and F, and LANL2DZ basis setsfor Mo and W for the geometry optimizations and frequencycalculations of （MO₃）_nH⁺and （MO₃）_nF-. It was found that the addition ofthe proton of the cluster structure does not lead to great changes.Bro¨nsted basicities and Lewis acidities based on a fluoride affinityscale were also calculated. The Bronsted basicities as well as theLewis acidities depend on the size of the cluster and the site to whichthe proton or the fluoride anion binds. These clusters are fairly weakBro¨nsted bases with gas phase basicities comparable to those of H₂Oand NH₃. The clusters are, however, very strong Lewis acids and manyof them are stronger than strong Lewis acids such as SbF₅. Theacid/base properties of these TMO clusters are expected to playimportant roles in their catalytic activities.