Resonance Raman and FTIR Spectra of Isotope-Labeled Reduced 1,4-Benzoquinone and Its Protonated Forms in Solutions

Abstract

Resonance Raman (RR) and FTIR spectra were observed for 1,4-benzoquinone (BQ) and its 18O2 and d4 isotopomers in their reduced states and their protonated forms, including the anion radical (BQ•-), semiquinone radical (BQH•), dianion (BQ2-), and protonated anion (BQH-) in water and acetonitrile. The reduced species were generated electrochemically, and the RR spectra were excited in situ at 441.6, 363.8, 351.1, 325, and 235 nm in resonance with the three lowest electronic transitions. The observed bands were assigned empirically on the basis of the observed 18O2- and d4-isotopic frequency shifts and depolarization ratios and further with ab initio MO calculations at the MP2 level. In contrast with neutral BQ, the 18O atoms of BQ•- and BQ2- were rapidly exchanged with bulk water during the RR measurements for aqueous solutions. Upon reduction from BQ to BQ•- and BQ2-, the CO stretching mode (ν7a in Wilson's notation) shifts from 1639 to 1452 and 1275 cm-1, respectively, while the ν8a mode exhibits smaller shifts from 1666 to 1609 and 1596 cm-1, respectively. The ν8a and ν8b separation was 293, 149, and 81 cm-1 for BQ, BQ•-, and BQ2-, respectively, indicating that the six-membered ring increasingly approaches a benzenoid ring as reduction proceeds. These frequency changes are in qualitative agreement with those expected from the optimized bond lengths of the MO calculations. The RR spectra of BQH- were close to that of BQ2-, suggesting that the extra charges of BQ2- are mainly localized on the two oxygen atoms. Three b3g vibrations were resonance enhanced for BQ•- via vibronic coupling, and this determines the symmetry of the first allowed electronic excited state to be 2B3u in the D2h group. Assuming D2h symmetry for BQ•- predicts the presence of five polarized RR bands in the region below 1700 cm-1, but nine fundamentals were observed in addition to several overtones and combinations. In addition, one band appears both in the IR and the RR spectra. Therefore, practical molecular symmetry is reduced to Cs owing to solvation.