TDDFT studies on the determination of the absolute configurations and chiroptical properties of strandberg-type polyoxometalates

摘要

The electronic circular dichroism (ECD) and UV-visible absorption (UV-vis) spectra of Strandberg-type polyoxometalates (POMs) (R, R)-[(RPO _3)_2M_5O_(15)]~(2-) (R* = CH_3CH(NH_3), (M = Mo, W)) have been explored using the time-dependent density functional theory (TDDFT) method. It demonstrates that the absolute configurations of chiral systems can be determined by chiroptical spectroscopic methods combined with DFT calculations. The calculated ECD spectra of the Strandberg-type molybdate were produced over the range of 3.3-6.5 eV, which are generally in agreement with the experimental spectra. In addition, the ECD spectra of (R, R)-[(RPO_3)_2W_5O _(15)]~(2-) (R* = CH_3CH(NH_3)) were produced over the range of 4.5-8.5 eV. The Becke's half-and-half hybrid exchange-correlation functional (BHandHLYP) with the HF exchange fraction to 55% hybrid functional was found to well predict the excitation energies of studied systems. The origins of the ECD bands of two systems are mainly ascribed to charge-transfer (CT) transitions from oxygen atoms to metal atoms in polyanion. The results suggest that the polyanion are chiroptical chromophores. The polyanion plays a role as an optically active chromophore and contribute to the absorptions of ECD spectra. The difference of the UV-vis/ECD spectra between two systems shows that the transition metal atom significantly influences on the chiroptical properties of the studied Strandberg-type POMs.