RESONANCE RAMAN-SPECTRA AND STRUCTURE OF PHENYLTHIYL RADICAL

Time-resolved resonance Raman spectroscopic and theoretical studies of phenylthiyl radical (PhS.) show that the CS bond in this radical is essentially a single bond with the unpaired electron localized on the sulfur atom. This structure contrasts with that of phenoxy radical (PhO.) where the CO bond is close to a double bond and the unpaired spin is largely delocalized onto the ring. The nu-8a ring stretching and nu-9a CH bending frequencies in the PhS. (1551 and 1180 cm-1) and in PhO. (1552 and 1157 cm-1) radicals are comparable, but the substituent-sensitive modes nu-7a, nu-12, and nu-6a in PhS. (1073, 724, and 436 cm-1) are at much lower frequencies than in PhO. (1505, 840, and 528 cm-1). While a drop of 92 cm-1 in the nu-6a frequency from PhO. to PhS. is almost entirely due to increased mass of sulfur atom, a downward shift of more than 400 cm-1 in thc nu-7a frequency indicates a drastically reduced CS bond strength. The structural differences between the two radicals account for their distinctly different reactivities, e.g., 4 times faster second-order decay of PhS. radicals (2k = 9.6 X 10(9) M-1 s-1) as compared to PhO., and combination at the sulfur site in PhS. in contrast to reaction at the ring carbon sites in PhO.. It has been found that the PhS' absorption in the 400-510-nm region involves contributions from at least two electronic transitions, 2A2 <-- B-2(1) and B-2(1) <-- B-2(1), which are vibronically coupled via a non-totally symmetric ring distortion mode (nu-6b).