High-level electronic structure calculations combined with empirical adjustments predict the standard enthalpy of the C-H bond dissociation in HC drop C-CH=CH2 to be equal to 115.1 +/-1.4 kcal/mol, i.e., ca, 4.0 kcal/mol higher than that of the analogous bond cleavage in ethene, This difference in bond strengths stems from resonance stabilization of the parent molecule. The standard enthalpy of formation of the 1-buten-3-yn-1-yl radical is estimated at 133.8 +/- 1.5 kcal/mol, which is significantly higher than all of the previously published values. As in the case of polychlorinated alkanes, the BLYP approximation is found to seriously underestimate the strengths of the C-H and C-Cl bonds in chloro derivatives of HC drop C-CH=CH2. On the other hand, the BLYP/6-311G**, MP2/6-311G**, QCISD/6-311G**, and CCSD(T)/6-311G** predictions for the standard enthalpy of ethyne dimerization all closely match their experimental counterpart.