G3(MP2) enthalpies of hydrogenation, isomerization, and formation of extended linear polyacetylenes

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 our G3(MP2) calculations of standard enthalpies of hydrogenation, delta(hyd), formation, delta(f), and isomerization, delta(isom), as far as isomeric dodecadiynes. We show that thermochemical stabilization of conjugated polyalkynes is about 1 kcal mol(-1) over most of this range, and that the progression from one polyalkyne to the next is regular and additive. The longest chain polyalkynes, however, begin to revert to classical conjugation stabilization energies. For example, 5,7-dodecadiyne has a thermochemical stabilization enthalpy of 3.1 kcal mol(-1), approaching that of 1,3-butadiene. We also point out some of the difficulties encountered when one departs from Kistiakowsky's operational definition of conjugation stabilization. A cautionary example is drawn from the recent literature in which arguments of hyperconjugation and "virtual states" are used to arrive at, among other things, a value of 8.5 kcal mol(-1) of conjugative stabilization in 1,3-butadiene.