Theoretical thermochemistry of the 1-buten-3-yn-1-yl radical and its chloro derivatives

High-level electronic structure calculations combined with empirical adjust ments 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 hig her than that of the analogous bond cleavage in ethene, This difference in bond strengths stems from resonance stabilization of the parent molecule. T he standard enthalpy of formation of the 1-buten-3-yn-1-yl radical is estim ated at 133.8 +/- 1.5 kcal/mol, which is significantly higher than all of t he 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 o ther hand, the BLYP/6-311G**, MP2/6-311G**, QCISD/6-311G**, and CCSD(T)/6-3 11G** predictions for the standard enthalpy of ethyne dimerization all clos ely match their experimental counterpart.