HEAT OF FORMATION AND STABILIZATION ENERG Y OF THE PENTAMETHYLCYCLOPENTADIENYL RADICAL

From the thermolysis of 1 in the presence of 1-60 bar oxygen between 7 0 and 100 degrees C the activation energy for its dissociation is foun d to be 27.9 kcal . mol(-1). Combining this result with the enthalpy o f formation of 4 and a forcefield value for 1 leads to DHo (Me(5)C(5)- H)=74.1 kcal . mol(-1) and Delta H-f(0) (2)=16.1 kcal . mol(-1), which differs by 5.4 kcal . mol(-1) from a value published recently by Walt on([1]). The low dissociation enthalpy of 1 is caused only by the reli eve of 28 kcal . mol(-1) of strain. With an intrinsic radical stabiliz ation energy (IRSE) of 17.3 kcal . mol(-1) for 2 there is no justifica tion for an extra stabilization due to cyclic conjugation.