ROTATIONALLY RESOLVED LASER SPECTROSCOPY OF THE JET-COOLED METHYLCYCLOPENTADIENYL RADICAL (CH3-C5H4 AND CD3-C5H4)

The rotationally resolved, electronic, excitation spectra of supersonically cooled methylcyclopentadienyl radicals, CH3-C5H4 and CD3-C5H4, have been obtained. Analysis of these spectra characterizes both the ground and electronically excited states involved in this transition. The ground state barrier to internal rotation of the methyl group is very high compared to similar closed-shell species. Additionally it shows a strong isotopic dependence and increases in magnitude further in the excited state. The doubly degenerate 2E"1 ground state of C5H5 is split into two components, X approximately 2B2 and A approximately 2A2. The rotational analysis shows that the cyclopentadienyl ring undergoes a significant, static distortion from a regular pentagon. The measured distortion is consistent with and related to the dynamic Jahn-Teller distortion in C5H5 itself.