Unimolecular reactions of mutual isomerization of cyclopentyl and 1‐penten‐5‐yl radicals have been investigated by chemical activation. The radicals were generated by adding energized hydrogen atoms (EH about 23 kcal mol−1) to the double bond of either cyclopentane or 1,4‐pentadiene. Based on the extensive steady‐state RRKM calculations employing the experimental data from this work as well as from the literature, the threshold energies for the cyclopentyl ring opening and closure are 32 ± 0.3 and 16.2 ± 0.3 kcal mol−1, respectively. The entropy of activation for the ring opening is close to zero.
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