Resonance raman and FTIR spectra of isotope-labeled reduced 1,4-benzoquinone and its protonated forms in solutions

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 (BQ^2-), 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 ¹⁸O_2- 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 BQ^.- were rapidly exchanged with bulk water during the RR measurements for aqueous solutions. Upon reduction from BO to BQ^.- and BQ^2-, the C=O stretching mode (v_7a in Wilson's notation) shifts from 1639 to 1452 and 1275 cm^-1 respectively, while the v_8a mode exhibits smaller shifts from 1666 to 1609 and 1596 cm^-1, respectively. The v_8a and v_8b separation was 293, 149, and 81 cm^-1 for BQ, BQ^.-, and BQ^2-, 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 BQ^2-, suggesting that the extra charges of BO^2- are mainly localized on the two oxygen atoms. Three b_3g vibrations were resonance enhanced tor BQ^.- via vibronic coupling, and this determines the symmetry of the first allowed electronic excited state to be ²B_3u in the D_2h group. Assuming D_2h 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 C_s owing to solvation.