THE HEAT OF FORMATION OF THE BENZOPHENONE KETYL RADICAL BY TIME-RESOLVED PHOTOACOUSTIC CALORIMETRY

Photoacoustic calorimetry is used to determine the heat of hydrogen abstraction from benzhydrol by the triplet state of benzophenone, which yields a pair of ketyl radicals. A thermodynamic cycle using this value, experimentally determined solution heats of benzophenone and benzhydrol crystals in benzene, and tabulated formation heats of these crystals, gives the formation heat of the ketyl radical, the dissociation energy of the C(Ph)2(OH)-H bond, and other thermochemical parameters. While the thermochemistry of producing radicals R1 and R2 from even-electron precursors cannot yield information about R1 or R2 separately, in the present case R1=R2=Ph2COH and consequently no (potentially erroneous) assumption about another radical need be made.