OXYGEN TRAPPING OF RADICALS IN SUPERCRITI CAL CARBON-DIOXIDE - HEAT OF FORMATION AND RECOMBINATION BARRIER OF THE CYCLOHEPTATRIENYL RADICAL

The homolytic thermal cleavage of bitropenyl (1) in supercritical carb on dioxide has been measured in the presence of 1 to 180 bar oxygen be tween 120 and 170 degrees C. From these data the activation parameters of the dissociation and recombination reaction are derived, which lea d to a heat of formation for the tropenyl radical (2) of Delta H-f deg rees = 64.7 kcal . mol(-1). By using this value a radical stabilizatio n 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-t rien-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([5]) In agreement with the observed stability of 2 the equilibrium constan t for its reaction with oxygen at 143 degrees C is K-eq = 727.7 mol(-1 ), which corresponds to a bond strengths of 31.5 kcal . mol(-1) for R- O-2(.). The equipment and the method used for these experiments are de scribed in detail.