[reaction: see text] In contrast to 1,3-butadiene, the textbook example of "conjugation stabilization", G3(MP2) calculations yielding the enthalpy of hydrogenation Delta(hyd)H(298) of 1,3-butadiyne indicate that it is not stabilized by the conjugated configuration of its triple bonds. Differences between ethylenic and acetylenic pi bonds are examined in the light of CAS-MCSCF calculations on 1,3-butadiene and 1,3-butadiyne.

By analogy to conjugated polyenes, conjugative stabilization of polyynes with the -CC-CC- group might be expected to be substantial. On the contrary, consistent with our recent report of a surprising lack of conjugative stabilization in butadiyne, we find by G3(MP2) calculations and by comparisons with available experimental data from these and other laboratories that the ground-state stabilization of conjugated polyynes is in fact quite small, amounting to <1 kcal mol(-)(1). By similar calculations, the 2,4-pentadiyn-1-yl radical shows no enhanced stabilization relative to 2-propyn-1-yl radical, despite the potential stabilization of the odd electron by two conjugated triple bonds and unlike the behavior of 2,4-pentadien-1-yl radical. The thermochemistry of straight-chain alkynes and polyynes is very self-consistent. Enthalpies of hydrogenation, leading to enthalpies of formation, are predictable with a high degree of accuracy (absolute mean deviation = +/-0.39 kcal mol(-)(1) vs theoretical values and +/-0.52 vs experimental) from three molecular structure enthalpies and one conjugation stabilization parameter.

We have calculated the standard enthalpies of isomerization, hydrogenation, and formation (Δhyd H 298, Δisom H 298, and Δf H 298) of norbornadiene, quadricyclane, norbornene, nortricyclane, and norbornane by the G2, G2(MP2), G2(MP2,SVP), G3(MP2), and B3LYP/6-311+(3df,2p) procedures. Comparison with combustion and hydrogenation thermochemical results shows that agreement between G-n theory and experiment remains within the target interval of 1 to 2 kcal mol-1 for these C7 hydrocarbons, but that B3LYP/6-311+(3df,2p) results do not fall within this range.