Experimental and density functional study on electronic structure and electronic circular dichroism of the phenylpyrazole insecticides enantiomers and the probable chiral catabolites

The stable conformations of three phenylpyrazole insecticides enantiomers and their probable chiral catabolites, marked A, B, C, a, b and c, are identified via Monte Carlo searching with the MMFF94 molecular mechanics force field. The electronic circular dichroism of flufiprole (A) and ethiprole (B) was recorded. Moreover, the DFT method is utilized to optimize for the searched conformers. Then, the electronically excited states involving the first 30 excited states were computed using the time-dependent density functional theory method. The conformations selected to investigate in this article were the most stable conformers, and the UV and ECD spectra were in accord with the experimental values. The delocalized transition between benzene ring and pyrazole groups causes the strong absorptions of the electronic transitions at the UV-visible range. Three bands appear in the ECD spectra, in which the lower-energy ECD band is a delocalized pi(py) -> pi(ph)* electronic transition. In addition, the higher and highest energy bands originate from the localized pi(py) -> pi(ph)* electronic transition. The calculated UV and ECD spectra were in good accordance with reported experimental values. Since the ECD spectrum at the B3LYP/6-311++G** level is indistinctively different from that at the B3LYP/6-31+G* level, it is necessary for the diffuse functions to be added to the 6-311++G** basis set, especially considering the solvent effect.