A photoacoustic calorimetric characterization of the reaction enthalpy and volume for the preparation of a reactive intermediate from CpMn(CO)3

Photoacoustic calorimetry (PAC) allows measurement of the energetics of reactive intermediates. Here, we report the examination of the metal carbonyl η⁵-CpMn(CO)₃ (Cp, cyclopentadiene) via time-independent PAC, in a homologous series of solvents. The measured heat releases allow one to determine simultaneously the enthalpy and volume change resulting from the photodissociation of CpMn(CO)₃. While the photoacoustic signal results from both of these processes, it has often been assumed that the volume change contribution to the observed photoacoustic signal is negligible for small molecules undergoing photodissociation. The current study tests the assumption of a negligible reaction volume by using a more complete treatment. The reaction of an equimolar number of photons and CpMn(CO)₃ molecules, the subsequent photodissociation of the Mn–CO bond, and the ligation of a solvent molecule in an alkane solvent yields ΔH_obs = 32.7 ± 0.7 kcal/mol and ΔV_chem = 11.0 ± 1.3 mL/mol, both of which are independent of the quantum yield of photodissociation. A detailed analysis of the quantum yield is included (using both previously reported measurements, and new data from this work), from which we determine Φ_diss = 0.635. This quantum yield allows us to determine ΔH_rxn = 51.6 kcal/mol and ΔV_rxn = 17.3 mL/mol. These results demonstrate that if the contribution of the reaction volume change to the photoacoustic signal is ignored, the reaction enthalpy derived would underestimate the true value by 7%. We also estimate the BDE{Cp(CO)₂Mn–CO} to be 59.4 kcal/mol.