| Résumé | The thermal dissociation of bis(pentamethylcyclopentadienyl) to pentamethylcyclopentadienyl radicals has been studied by EPR spectroscopy in the temperature range 365-393 K. The enthalpy of dissociation of the dimer is found to be 18.8 kcal mol-1 and combination of this result with empirical and semi-empirical estimates of the enthalpies of formation of the dimer and of pentamethylcyclopentadiene, leads to ΔH°f (Me 5C5.) = 10.7 kcal mol-1, DH°(Me5C5-H) = 70.6 kcal mol-1 and E sMeK(Me5C5.) = 34.2 kcal mol-1 (where EsMeH is the methane-based stabilisation energy). Steric strain is shown to be a major factor leading to weakening of the inter-ring bond of the dimer. The pentaethyl and pentamethoxycarbonyl analogues have been investigated, but neither is suitable for quantitative measurements. Thermal decomposition of trans,trans-deca-1,3,7, 9-tetraene and trans,trans-3,8-dimethyldeca-1,3,7,9-tetraene gives rise to pentadienyl and 3-methylpentadienyl radicals respectively, each of which isomerises to form a mixture of trans- and cis-radicals which then couple in both the 'end to end' and 'end to centre' modes to give mixtures of tetraenes. Hydrogen abstraction from 1,2-diphenylhydrazine is not competitive with pentadienyl radical combination in the temperature range 450-490 K. The EPR spectra of pentadienyl and 3-methylpentadienyl radicals have been observed in this temperature range. |
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