STABILIZATION ENERGY OF POLYENYL RADICALS - ALL-TRANS-NONATETRAENYL RADICAL BY THERMAL REARRANGEMENT OF A SEMIRIGID (4-1-2) HEPTAENE - MODEL FOR THERMAL LABILITY OF BETA-CAROTENE

Evaluation of the stabilization energy of the nonatetraenyl radical directly from the enthalpy of activation of the thermal anti-syn rearrangement of a semirigid nonaene having a symmetrical distribution {4-1-4} of double bonds is thwarted by insolubility. Indirect comparison of the enthalpy of activation of an unsymmetrical {4-1-2} heptaene with that of an already determined symmetrical {2-1-2} pentaene leads to an inferred enthalpy of activation of 24.5 kcal/mol for the {4-1-4} nonaene. Perhaps the point of greatest theoretical interest is the rapidity with which successive increments in stabilization energies (SE(n)) decrease with increasing number of double bonds in the conjugated polyenyl radicals. Values of SE(n) for n = 1 (allyl), 2 (pentadienyl), 3 (heptatrienyl), and 4 (nonatetraenyl) are 13.5, 16.9, 19.2, and 20.7 kcal/mol, respectively.