Determination of reaction volumes and reaction enthalpies by photoacoustic calorimetry

The photoinduced reaction of diphenylcyclopropenone (DPCP) has been investigated by photoacoustic calorimetry (PAC) in a homologous series of alkanes at constant temperature and in micellar systems at different temperatures. The results remove severe contradictions in the literature and demonstrate that both PAC methods can actually be used for the determination of reliable data for reaction volumes and reaction enthalpies. The following reaction parameters have been determined for the dissociation of DPCP: Gamma V = 22.3 +/- 2.5 ml mol(-1) and Delta H= -18 +/- 5 kJ mol(-1) in alkanes and Delta V = 22.1 +/- 0.8 ml mol(-1) and Delta H = -25 +/- 36 kJ mol(-1) in micelles, respectively. It is demonstrated that the usual separation of Delta V into contributions caused by bond breaking or bond making processes and by electrostriction of the solvent is insufficient. Additionally, a third contribution to Delta V has to be considered, which results from the perturbation of the solvent structure by the reactant and product molecules. This latter part can be important and has mostly been disregarded up to now.