Determination of the enthalpy and reaction volume changes of organic photoreactions using photoacoustic calorimetry

Abstract

Photoacoustic calorimetry (PAC) can be used to measure both the thermal and reaction volume changes for photoinitiated reactions. The photoreactions of 2,3-diazabicyclo(2.2.1)hept-2-ene (DBH), diphenylcyclopropenone (DPC), and trans-stilbene (TS) are investigated by PAC. The resolution of these experimental volume changes is accomplished by either a temperature dependence or a binary solvent mixture method. The thermal volume changes yield the enthalpies of reaction in solution, which can be compared to literature values. In two cases (DPH and DPC), the values are more endothermic than those predicted from gas-phase heats of formation. The differences can possibly be attributed to differential solvation of the reactants and products in the polar solvents employed. Absolute reaction volume changes for the photoreactions are also obtained for the photoreactions. PAC is a useful alternative technique to pressure-dependence studies to obtain this information. These volume changes can further be time-resolved to provide kinetic information about the photoprocesses.