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

When excited in resonance with its strong 390-nm transition, p-benzodithiyl 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 clear from both the spectroscopic and theoretical results that the geometric structure of this radical is closely related to that of p-benzosemiquinone radical anion. Of particular note is the frequency of 1081 cm(-1) observed for the nu(7a) symmetrical CS stretch, which indicates that the CS bond is intermediate in character between typical single and double CS bonds and slightly closer to the former, in contrast to p-benzosemiquinone radical anion where the CO bond is slightly closer to a double bond. This structural difference indicates that the unpaired spin is less delocalized in the dithiyl radical than in the semiquinone radical, explaining the substantial decrease in resonance stabilization energy and increased reactivity of the dithiyl radical as compared to its oxygen analogue.