| This thesis details our efforts to establish photoacoustic calorimetry (PAC) as a reliable probe of reaction energetics on the nano- and microsecond time scales. PAC offers unique insight into the thermochemistry of transient species by detection of the volume expansion initiated by photoexcitation. This work emphasizes two concepts central to PAC, the reaction volume problem and time resolution of the photoacoustic waveform.; Because PAC measures volume changes, thermal expansion contributes only a fraction of the signal. The volume of reaction, often ignored in earlier work, contributes the remainder of the signal. Two methods to dissect the thermal from the non-thermal volume changes in organic and aqueous solutions are presented. In organic solvents, the contributions were dissected using a homologous series of alkanes to examine the photodecarbonylation of diphenylcyclopropenone. In aqueous systems, the contributions were dissected by considering the temperature dependence of the signal in the study of the dissociation energies of the cobalt-carbon bonds of methylcobalamin and methylcobinamide. The remarkable agreement between these photoacoustic bond energies and the values measured by an independent method is a testament to the increasing confidence in PAC. These cases dramatically illustrate the importance of reaction volume in the interpretation of the photoacoustic signal.; Originally, PAC was limited to processes occurring within 10 ns, returning the enthalpy with high precision. More recently, the introduction of time-resolved analysis has expanded the scope of PAC to include processes occurring on a microsecond regime and has allowed the simultaneous recovery of both reaction energetics and dynamics. The study of several pertinent problems demonstrates the efficacy of this analysis. The examination of the capture of methyl radical by oxygen illustrates the versatility of PAC to probe reactivity even in the absence of a convenient chromophore. Secondly, the triplet states of C{dollar}sb{lcub}60{rcub}{dollar} and C{dollar}sb{lcub}70{rcub}{dollar} and their interactions with oxygen were precisely characterized. Our value for the triplet energy of C{dollar}sb{lcub}60{rcub}{dollar} was subsequently confirmed by phosphorescence. Finally, the simultaneous observation of intramolecular charge transfer in both the Marcus inverted and normal regions highlights the unique ability of PAC to quantify partitioning among divergent non-radiative pathways. |
