THERMOLABILE HYDROCARBONS .34. THERMAL-ST ABILITY OF 9,9'-BIXANTHENE AND AHYDRO-10,10,10',10'-TETRAMETHYL-9,9'-BIANTHRACENE - STABILIZATIONENERGIES OF PLANAR BENZHYDRYL RADICALS

The thermal cleavage of the central CC bonds in the title compounds wa s investigated. The activation parameters were obtained from kinetics and the equilibrium enthalpies and entropies from the temperature depe ndence of the equilibrium radical concentrations, determined by quanti tative ESR. DELTAH(not-equal) and DELTAH(Diss) are nearly identical bu t not DELTAS(not-equal) and DELTAS(Diss), as expected. The radical sta bilisation 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 alpha-phenylethyl radicals (8.0 +/- 1.5 kcal/mol) determined previo usly. The smaller RSE of the benzhydryl radical (12.9 +/- 0.9 kcal/mol ), therefore, must be due to its nonplanarity.