An electron spin resonance study of cyclopentadiene radical cations

Abstract

If solutions of hexa-,penta-, tetra-, or tri-alkylcyclopentadienes in trifluoroacetic acid are irradiated with u.v. light, the e.s.r. spectra of the corresponding alkylcyclopentadiene radical cations can be observed. The radical cation Me₅C₅H⁺˙ can also be obtained from decamethylbi (cyclopentadienyl), Me₅C₅–C₅Me₅ under similar conditions. The spectra of Me₆C₅⁺˙, Me₅C₅H⁺˙, and Me₄C₅H₂⁺˙ can be analysed in terms of single isomers. The sp³ carbon atom of the ring lies in the nodal plane of the SOMO, and substituents at this position show only small hyperfine coupling constants. The radical cation Me₅C₅H⁺˙ shows no evidence of fluxionality on the e.s.r. timescale up to 328 K. The spectra of Me₄EtC₅H⁺˙, Me₃Et₂C₅H⁺˙, and Me₃C₅H₃^+˙ apparently relate to mixtures of prototropic isomers. Experiments with optical cut-off filters show that the primary photochemical process involves excitation of the protonated diene.