Spectrum of the Cyclopentadienyl Radical

Abstract

THE great stability of benzene has aroused much interest in the properties of other planar symmetrical cyclic species of formula (CH)_n. For even values of n, only cyclo-octatetraene is stable, and is of limited interest owing to puckering of the ring¹. The odd series can exist only as radicals or ions and, while several calculations have been made of their resonance energies^2–4, only C₇H₇+ has been observed spectroscopically⁵, although the acidity of cyclopentadiene⁶ provides evidence for the stability of the ion C₅H₅−. Like benzene, both these species have six π-electrons and are expected to have higher resonance energies than the 38 k.cal. predicted for the Cyclopentadienyl radical (C₅H₅)^3,4.