Thermische Stabilität von 9,9′‐Bixanthen und 9,9′,10,10′‐Tetrahydro‐ 10,10,10′,10′‐tetramethyl‐9,9′‐bianthracen Stabilisierungsenergie planarer Benzhydryl‐Radikale[2]

The thermal cleavage of the central CC bonds in the title compounds was investigated. The activation parameters were obtained from kinetics and the equilibrium enthalpies and entropies from the temperature dependence of the equilibrium radical concentrations, determined by quantitative ESR. ΔH≠ and ΔHDiss are nearly identical but not ΔS≠ and ΔSDiss, as expected. The radical stabilisation enthalpies RSE obtained from these data (RSE: 7a = 17.9 ± 1.3; 7b = 15.6 ± 1.4 kcal/mol) correspond closely to twice the RSE of α‐phenylethyl radicals (8.0 ± 1.5 kcal/mol) determined previously. The smaller RSE of the benzhydryl radical (12.9 ± 0.9 kcal/ mol), therefore, must be due to its nonplanarity.