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

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^-, BQ^2- and their various isotopomers. Comparing with the reported experimental frequencies and their isotope shifts of BQ, the vibrational frequencies calculated by using Becke's 1988 exchange functional with Perdew-Wang's 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^-, BQ^2- 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 BQ^2-. 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 b_2u, the b_1g and the fourth b_3g fundamental modes, respectively. It seems also reasonable to assign the weaker band at 864 cm^-1 appeared in the spectra of BQ^2- in the aqueous solution to the first b_2g fundamental mode, though we cannot completely exclude the possibility of the first a_u fundamental mode before the RR spectra of the ¹⁶O₂/¹⁸O₂-substituted BQ^2- isotopomer in the region below 800 cm^-1 are obtained.