RESONANCE RAMAN-SPECTRUM AND STRUCTURE OF P-BENZODITHIYL RADICAL-ANION

When excited in resonance with its strong 390-nm transition, p-benzodi thiyl radical anion exhibits Raman bands at 305, 754, 1081, 1194, and 1571 cm(-1) that are assigned, respectively, to the totally symmetric nu(6a), nu(1), nu(7a), nu(9a), and nu(8a) Wilson modes expected for a radical of D-2h symmetry. These assignments are confirmed by ab initio theoretical studies. A modestly intense band is also observed at 1410 cm(-1) which is likely attributable to the nontotally symmetric nu(8b ) mode that acquires intensity by vibronic coupling between the (2)A(u ) <-- B-2(2g) and B-2(3u) <-- B-2(2g) electronic transitions. It is cl ear from both the spectroscopic and theoretical results that the geome tric structure of this radical is closely related to that of p-benzose miquinone radical anion. Of particular note is the frequency of 1081 c m(-1) observed for the nu(7a) symmetrical CS stretch, which indicates that the CS bond is intermediate in character between typical single a nd double CS bonds and slightly closer to the former, in contrast to p -benzosemiquinone radical anion where the CO bond is slightly closer t o a double bond. This structural difference indicates that the unpaire d spin is less delocalized in the dithiyl radical than in the semiquin one radical, explaining the substantial decrease in resonance stabiliz ation energy and increased reactivity of the dithiyl radical as compar ed to its oxygen analogue.