Why does cyclopropene have the acidity of an acetylene but the bond energy of methane?

The gas-phase acidity of 3,3-dimethylcyclopropene (1) has been measured by bracketing and equilibrium techniques. Consistent with simple hybridization arguments, our value (ΔH^o_acid = 382.7 ± 1.3 kcal mol^-1) is indistinguishable from that for methylacetylene (i.e., ΔΔH^o_acid(1 - CH₃C≡CH) = 1.6 ± 2.5 kcal mol^-1). The electron affinity of 3,3- dimethylcyclopropenyl radical (1r) was also determined (EA = 37.6 ± 3.5 kcal mol^-1), and these quantities were combined in a thermodynamic cycle to afford the homolytic C-H bond dissociation energy. To our surprise, the latter quantity (107 ± 4 kcal mol^-1) is the same as that for methane, which cannot be explained in terms of the s-character in the C-H bonds. An orbital explanation (delocalization) is proposed to account for the extra stability of 1r. All of the results are supplemented with G3 and B3LYP computations, and both approaches are in good accord with the experimental values. We also note that for simple hydrocarbons which give localized carbanions upon deprotonation there is an apparent linear correlation between any two of the following three quantities: ΔH^o_acid, BDE, and EA. This observation could be of considerable value in many diverse areas of chemistry.