Theoretical Studies On Electronic Structures And ECD Spectra Of Chiral Dawson-and Strandberg-type Polyoxometalate Derivatives

Polyoxometalates （POMs） are a rich and diverse family of metal–oxygen clusters madeup of early transitional metals with unique photonic, electronic, and magnetic properties andchemical reactivity that have promised dramatic applications in quite diverse disciplines,including catalysis, medicine, and materials sciences. In addition to the importantachievement in the applications, the breakthrough has been made in the theoretical aspects.Due to the large sizes of POMs, including many transition metal atoms, high negative charge,the theoretical studies of POMs are relatively difficult.In this thesis, quantum chemical calculations have been used to investigate the electronicproperties and chiroptical properties of a series of organic groups substituted POM derivatives.The research work mainly includes the following four aspects:1. The UV/CD spectra of tin-bearing acetonyl-substituted Dawson polyoxotungstates1-and2-[P2W17O61{SnCH₂CH₂C（=O）}]6were systematically investigated using thetime-dependent density functional theory （TDDFT） method. The electronic circular dichroism（ECD） spectra were produced over the range of3.35.8eV. The calculated ECD spectra of theα1-R isomer were generally in agreement with the experimental spectra. The fact that theUV/ECD spectra of1-isomers are different from those of2-isomers demonstrates the effectof the tin substitution site on the chiroptical properties of the studied isomers. The origins ofthe ECD bands are mainly ascribed to charge-transfer （CT） transitions from oxygen atoms toW atoms, from organic fragments to W atoms, or from the combination of two CT transitions.The results suggest that the organic fragment and polyoxometalate （POM） cluster arechiroptical chromophores.2. The ultraviolet-visible and electronic circular dichroism （UV–vis/ECD） spectra ofdiphosphonate-functionalized asymmetric cantilever-type chiral polyoxomolybdate （POM）enantiomer R-{Mo2O5[（Mo2O6）NH₃CH₂CH₂CH₂C（O）（PO₃）₂]₂}^6-（R） were systematicallyinvestigated using the time-dependent density functional theory （TDDFT） method. From theview of molecular structure and relative energy, we inferred that there is likely a structuralconversion from enantiomers R to S-{Mo2O5[（Mo2O6）NH₃CH₂CH₂CH₂C（O）（PO₃）₂]₂}^6-（S）via the intermediate configuration （IN）. The ECD spectra of the enantiomer R were produced over the range of3.0⁶.3eV. The UV-vis and ECD spectra of enantiomer R in the gas phaseand different solvents were calculated. The results reveal that the UV-vis and ECD spectra ofthe chiral POM in gas phase, polar solvent, or non-polar solvent are different. The calculatedelectron density difference maps （EDDMs） display that the POM cluster is a chiropticalchromophore in studied compound.3. The electronic circular dichroism （ECD） and UV visible absorption （UV vis） spectraof Strandberg-type polyoxometalates （POMs）（R, R）-[（R*PO₃）₂M₅O₁₅]^2-（R*=CH₃CH（NH₃）,（M=Mo, W）） have been explored using the time-dependent density functional theory（TDDFT） method. It demonstrates that the absolute configurations of chiral system can bedetermined by chiroptical spectroscopic methods combined with DFT calculations. Thecalculated ECD spectra of the Strandberg-type molybdate were produced over the range of3.36.5eV, which are generally in agreement with the experimental spectra. In addition, theECD spectra of （R, R）-[（R*PO₃）₂W₅O₁₅]^2-（R*=CH₃CH（NH₃）） were produced over the rangeof4.58.5eV. The Becke’s half-and-half hybrid exchange-correlation functional（BHandHLYP） with the HF exchange fraction to55%hybrid functional was found to wellpredict the excitation energies of studied systems. The origins of the ECD bands of twosystems are mainly ascribed to charge-transfer （CT） transitions from oxygen atoms to metalatoms in polyanion. The results suggest that the polyanion are chiroptical chromophores. Thepolyanion plays a role as an optically active chromophore and contribute to the absorptions ofECD spectra. The difference of the UV-vis/ECD spectra between two systems shows that thetransition metal atom significantly influences on the chiroptical properties of the studiedStrandberg-type POMs.4. The activated effect of Mo atom on the M―O_t（M=W, Nb, Ta） of substituted systems[W_6-nMo_nO₁₉]^2-,[Nb_6-nMo_nO₁₉]^p-and [Ta_6-nMo_nO₁₉]^p-has been investigated using thetime-dependent density functional theory （TDDFT） method. The results show that thebonding energy of M―O_tin [M_6-nMo_nO₁₉]^2-（M=W, Nb, Ta） decrease with the increasingnumber of Mo atom. Therefore, the M―O_tbond was activated when Mo atom was introduced.While the Mo―O_tbonding energy is less than that of W―O_t, so the bond Mo―O_tis easilybroken than the bond W―O_t, which was in well agreement with experimental results. Forsystems [Nb_6-nMo_nO₁₉]^p-and [Ta_6-nMo_nO₁₉]^p-, the bonding energy of Mo―O_tis larger thanthat of M―O_t（M=Nb, Ta）. The natural charge on terminal oxygen of Nb―O_tand Ta―O_tare larger than that of terminal oxygen of Mo―O_t. It predicts that Nb―O_tand Ta―O_tarepreferred to break, and easily form Nb≡N bond and Ta≡N bond when [Nb_6-nMo_nO₁₉]^p-and[Ta_6-nMo_nO₁₉]^p-react with organic amines.