Carbon–Hydrogen Bond
Dissociation Energies:
The Curious Case of Cyclopropene
- Publication date
- Publisher
Abstract
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 (<i>c</i>-C<sub>3</sub>H<sub>3</sub><sup>+</sup>)
with a series of reference reagents of known IEs. This result was
combined in a thermodynamic cycle to obtain the heat of formation
of <i>c</i>–C<sub>3</sub>H<sub>3</sub><sup>•</sup> (118.9 ± 4.0 kcal mol<sup>–1</sup>) and the allylic
C–H bond dissociation energy (BDE) of cyclopropene (104.4 ±
4.0 kcal mol<sup>–1</sup>). 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