Ab initio MO and density functional studies on the vibrational spectra of 1,4-benzoquinone, and its anion and dianion Original Research Article

In order to reassign recently reported resonance Raman spectra of the reduced forms of 1,4-benzoquinone (BQ), ab initio MO and density functional calculations have been carried out for evaluating fundamental vibrational frequencies of BQ, BQ−, BQ2− and their various isotopomers. Comparing with the reported experimental frequencies and their isotope shifts of BQ, the vibrational frequencies calculated by using Beckeʹs 1988 exchange functional with Perdew–Wangʹs 91 gradient-corrected correlation functional (BPW91) are closest to the corresponding experimental data. Due to the reliability of the BPW91 results, the vibrational frequencies of BQ, BQ−, BQ2− and their various isotopomers calculated with this unified method have been employed to further discuss the assignments of the recently reported resonance Raman (RR) bands of the BQ− and BQ2−. The previous assignments of the most vibrational bands are well confirmed. However, based on the calculated results, it is more reasonable to reassign the three bands at 1046, 748 and 638 cm−1 in the spectra of the BQ− existing in CH3CN to the fourth b2u, the b1g and the fourth b3g fundamental modes, respectively. It seems also reasonable to assign the weaker band at 864 cm−1 appeared in the spectra of BQ2− in the aqueous solution to the first b2g fundamental mode, though we cannot completely exclude the possibility of the first au fundamental mode before the RR spectra of the 16O2/18O2-substituted BQ2− isotopomer in the region below 800 cm−1 are obtained.