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.
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