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 funct ional calculations have been carried out for evaluating fundamental vi brational frequencies of BQ, BQ(-), BQ(2-) and their various isotopome rs. Comparing with the reported experimental frequencies and their iso tope shifts of BQ, the vibrational frequencies calculated by using Bec ke's 1988 exchange functional with Perdew-Wang's 91 gradient-corrected correlation functional (BPW91) are closest to the corresponding exper imental data. Due to the reliability of the BPW91 results. the vibrati onal frequencies of BQ, BQ(-), BQ(2-) and their various isotopomers ca lculated with this unified method have been employed to further discus s the assignments of the recently reported resonance Raman (RR) bands of the BQ(-) and BQ(2-). The previous assignments of the most vibratio nal bands are well confirmed. However, based on the calculated results , it is more reasonable to reassign the three bands at 1046, 748 and 6 38 cm(-1) in the spectra of the BQ(-) existing in CH3CN to the fourth b(2u), the b(1g) and the fourth b(2g) fundamental modes, respectively. It seems also reasonable to assign the weaker band at 864 cm(-1) appe ared in the spectra of BQ(2-) in the aqueous solution to the first b(2 g) fundamental mode, though we cannot completely exclude the possibili ty of the first a(u) Fundamental mode before the RR spectra of the O-1 6(2)/O-18(2)-substituted BQ(2-) isotopomer in the region below 800 cm( -1) are obtained. (C) 1998 Elsevier Science B.V.