Carbon-hydrogen bond dissociation energies: The curious case of cyclopropene

The ionization energy (IE) of the 3-cyclopropenyl radical (6.00 ± 0.17 eV) was measured in the gas phase by reacting 3-cyclopropenium cation (c-C₃H₃⁺) with a series of reference reagents of known IEs. This result was combined in a thermodynamic cycle to obtain the heat of formation of c-C₃H₃^• (118.9 ± 4.0 kcal mol^-1) and the allylic C-H bond dissociation energy (BDE) of cyclopropene (104.4 ± 4.0 kcal mol^-1). These experimental values are well reproduced by high level G3 and W1 computations and reveal that the BDE is similar to that for cyclopropane and the vinyl position of cyclopropene. This is unprecedented and is a reflection of the unusual nature of cyclopropene.