Energetics of alkylbenzyl radicals: A time-resolved photoacoustic calorimetry study

The PhCH2-H bond dissociation enthalpy in solution was determined for the first time as 380.5 +/- 5.1 kJ mol(-1), from a time-resolved photoacoustic calorimetry study (TR-PAC) in toluene. The derived gas-phase result, 375.5 +/- 5.0 kJ mol(-1), allows the calculation of the enthalpy of formation of the benzyl radical as Delta(f)Hdegrees (PhCH2., g) = 208.0 +/- 5.0 kJ mol(-1), which is in excellent agreement with a value recently published in the literature, obtained from gas-phase experiments, and demonstrates the accuracy of TR-PAC as a tool for the study of organic radical energetics. A detailed discussion of the methods and assumptions used to derive those values from the TR-PAC experiments was made. The study was then extended to ethylbenzene and cumene. The results support the assumption that solvation enthalpies of the alkylbenzenes and their corresponding radicals are similar. The trend of C-H bond dissociation enthalpies in those alkylbenzenes is in keeping with early gas-phase literature results, but it is in clear contrast with the values obtained from a proton affinity cycle using data from the NIST Chemistry WebBook.