| Abstract
| - Motivated by our recent finding that, in contrast to their olefinic counterparts, linear alternant polyacetylenes(polyynes) show no appreciable thermodynamic evidence of conjugation stabilization, we have extended ourG3(MP2) calculations of standard enthalpies of hydrogenation, Δhyd, formation, Δf, and isomerization, Δisom, as far as isomeric dodecadiynes. We show that thermochemical stabilization of conjugatedpolyalkynes is about 1 kcal mol-1 over most of this range, and that the progression from one polyalkyne tothe next is regular and additive. The longest chain polyalkynes, however, begin to revert to classical conjugationstabilization energies. For example, 5,7-dodecadiyne has a thermochemical stabilization enthalpy of 3.1 kcalmol-1, approaching that of 1,3-butadiene. We also point out some of the difficulties encountered when onedeparts from Kistiakowsky's operational definition of conjugation stabilization. A cautionary example isdrawn from the recent literature in which arguments of hyperconjugation and “virtual states” are used toarrive at, among other things, a value of 8.5 kcal mol-1 of conjugative stabilization in 1,3-butadiene.
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