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

Abstract

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 CH 3CN to the fourth b 2 u , the b 1 g and the fourth b 3 g 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 2 g fundamental mode, though we cannot completely exclude the possibility of the first a u fundamental mode before the RR spectra of the 16O 2/ 18O 2-substituted BQ 2− isotopomer in the region below 800 cm −1 are obtained.