Thermolabile Hydrocarbons, 33. Thermochemistry and Thermal Decomposition of 9,9′‐Bifluorenyl and 9,9′‐Dimethyl‐9,9′‐bifluorenyl – The Stabilization Energy of 9‐Fluorenyl Radicals

From thermochemical measurements the heats of formation Δ H 0 f (g) forfluorene ( 1 ), 9‐methylfluorene ( 2 ), 9,9′‐bifluorenyl ( 5 ), and 9,9′‐dimethyl‐9,9′‐bifluorenyl ( 6 ) were determined. The homolytic cleavage of the dimers 5 and 6 to 9‐fluorenyl ( 3 ) and 9‐methyl‐9‐fluorenyl radicals ( 4 ), respectively, was studied in mesitylene with thiophenol as trapping agent and found to follow first‐order kinetics. The activation parameters Δ H ≠ ( 5 ) = 212.6 (±2.1) kJ mol −1 , Δ S ≠ ( 5 ) = 70.3 (±4.2) J mol −1 K −1 ΔH ≠ ( 6 ) = 164.9 (±0.7) kJ mol −1 , and Δ S ≠ ( 6 ) = 88.2 (±1.9) J mol −1 K −1 were obtained. For 3 and 4 the radical stabilization energies RSE ( 3 ) = 67 (±7) kJ mol −1 (relative to isopropyl) and RSE ( 4 ) = 64 (±8) kJ mol −1 (relative to tert‐butyl ) were determined by a comparison with the activation parameters of the thermolysis of alkanes possessing the same strain enthalpy. The heats of formation for 3 and 4 and the C–H bond dissociation energies for the C–H bond in the 9–position of 1 and 2 were determined to be Δ H 0 f (g, 3 ) = 300 kJ mol −1 , Δ 0 f (g, 4 ) = 268 kJ mol −1 , BDE CH ( 1 ) = 343 and BDE CH ( 2 ) = 338 kJ mol −1 (estimated errors correspond to those of the RSE values). A good correlation between the heats of vaporization of 37 aromatic hydrocarbons and their calculated solvent‐accessible surfaces was demonstrated. Hence, a new method to estimate heats of vaporization was established.