Gas-phase energetics of organic free radicals using time-resolved photoacoustic calorimetry

A generalized method for the determination of thermochemical data of transient species, using time-resolved photoacoustic calorimetry (TR-PAC), is described in detail. Taking phenol as an example, the procedure for the determination of the PhO–H bond dissociation enthalpy from photoacoustic experiments, in various solvents, is presented, and its assumptions discussed. To derive gas-phase bond dissociation enthalpies from the solution values, a widely used procedure is compared with a computational chemistry (CC) microsolvation method. Results from the combined TR-PAC/CC approach show that the established “hydrogen bond only” model (to describe the difference between the solvation enthalpies of phenol and phenoxy radical) leads to an underestimation of the derived gas-phase bond dissociation enthalpy. When that differential solvation is properly accounted for, the agreement between our results and a recommended gas-phase value improves, indicating that the combined TR-PAC/CC approach is a valid tool for the study of organic free radical energetics.