Knowledge Bank

The vibrational analysis of the spectrum of the cyclopentadienyl radical (Cp) has not been accomplished, although it was first observed some 40 years ago. The reason lies in the lack of extensive vibrational progressions and the complications due to the Jahn-Teller effect. We have rotationally resolved some of the vibrational bands of the $A¯-X¯$ transition. From its rotational structure, the symmetry of the vibrational mode could be determined. We have thus observed transitions that are conventionally allowed, as well as those induced by the Jahn. Teller effect. One vibrational band that has been assigned to a totally symmetric mode was actually found to be a Jahn-Teller induced transition. These new results and their implications for the Jahn-Teller interaction will be discussed