HIGH-RESOLUTION SPECTROSCOPY OF JET-COOLED SUBSTITUTED CYCLOPENTADIENYL RADICALS

Rotationally resolved electronic spectra have been observed for a number of substituted cyclopentadienyl radicals, C5H4X, with X = F, Cl, CN, and CH3. This paper reports the spectral analysis for the radicals with X = F and Cl; results for X = CN and CH3 have been reported previously. The geometric structural parameters for all the radicals are consistent with a distortion of the five-membered ring to a diene-like structure in the ground state, with C-C bond length alternations in excess of 0.1 angstrom in some cases. The analysis also quite clearly shows a strengthening and shortening of the C-X bond in the radical compared to that of the similarly substituted benzene derivatives. The observed results are rationalized in terms of a simple molecular orbital picture involving conjugation and hyperconjugation.