DEPENDENCE OF STABILIZATION ENERGY OF AN (E,Z)-PENTADIENYL RADICAL ON SUBSTITUTION - ROLE OF SOLVENT FRICTION - NEGLIGIBLE AT HIGHER TEMPERATURES

Pentaene 6 has the same configuration and degree of substitution as the previously reported pentaene, 2,2'-bicholestadienylidene 2, and is thus a more suitable model of low extensivity for comparison with 2 of much higher extensivity. The rates of their thermal anti-syn rearrangements and derived Arrhenius parameters are essentially identical. The previous, tentatively proffered suggestion that solvent friction might have been responsible for the otherwise inexplicably slower rate of thermal cis-trans isomerization of 2 relative to compound 4 is no longer warranted. Instead it now appears that the enthalpy of stabilization in the pentadienyl radical is sensitive to substitution, at least at its central carbon atom, contrary to the earlier assumption based on the apparent insensitivity of the allyl radical to substitution.