On the lack of conjugation stabilization in polyynes (Polyacetylenes)

By analogy to conjugated polyenes, conjugative stabilization of polyynes with the -Cequivalent toC-Cequivalent toC-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 groundstate 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.