| Abstract
| - By analogy to conjugated polyenes, conjugative stabilization of polyynes with the −C⋮C−C⋮C−group might be expected to be substantial. On the contrary, consistent with our recent report of asurprising lack of conjugative stabilization in butadiyne, we find by G3(MP2) calculations and bycomparisons 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. Bysimilar calculations, the 2,4-pentadiyn-1-yl radical shows no enhanced stabilization relative to2-propyn-1-yl radical, despite the potential stabilization of the odd electron by two conjugated triplebonds and unlike the behavior of 2,4-pentadien-1-yl radical. The thermochemistry of straight-chainalkynes and polyynes is very self-consistent. Enthalpies of hydrogenation, leading to enthalpies offormation, are predictable with a high degree of accuracy (absolute mean deviation = ±0.39 kcalmol-1 vs theoretical values and ±0.52 vs experimental) from three molecular structure enthalpiesand one conjugation stabilization parameter.
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