Sauerstoff‐Abfang von Radikalen in überkritischem Kohlendioxid; Bildungsenthalpie und Rekombinationsbarriere des Cycloheptatrienyl‐Radikals

Oxygen Trapping of Radicals in Supercritical Carbon Dioxide; Heat of Formation and Recombination Barrier of the Cycloheptatrienyl Radical The homolytic thermal cleavage of bitropenyl (1) in supercritical carbon dioxide has been measured in the presence of 1 to 180 bar oxygen between 120 and 170°C. From these data the activation parameters of the dissociation and recombination reaction are derived, which lead to a heat of formation for the tropenyl radical (2) of ΔHfo = 64.7 kcal · mol−1. By using this value a radical stabilization energy (RSE) or 31.5 kcal · mol−1 is obtained, which differs by 1.7 kcal · mol−1 from the planar, acyclic isomer (trans,trans‐1,3,6‐trien‐5‐yl: RSE = 29.8 kcal · mol−1), which is practically equivalent to the relief of ring strain due to the formation of the radical. In agreement with the observed stability of 2 the equilibrium constant for its reaction with oxygen at 143°C is Keq = 727.7 mol−1, which corresponds to a bond strengths of 31.5 kcal ˙ mol−1 for R–O2·. The equipment and the method used for these experiments are described in detail.