Gaseous Phase Heat Capacity of Benzoic Acid

The gaseous phase heat capacity of benzoic acid (BA) was proven using the experimental technique called the “in vacuum sublimation/vaporization Calvet microcalorimetry drop method”. To overcome known experimental shortfalls, the gaseous phase heat capacity of BA monomer was estimated by ab initio calculations and compared with experimental results. Gaseous phase heat capacities of BA were directly derived via calculated harmonic frequencies obtained by density functional theory (DFT) (B3LYP, BLYP, BP86, with 6-311++G(d,p), TZVP, cc-pVTZ basis sets) and the second-order Møller−Plesset theory, MP2/6-311++G(d,p). To increase the accuracy of estimation of the thermal properties, a procedure based on the calculation of the heat capacity from quantum chemical calculations in combination with a heat capacity balance of isodesmic reactions is described and applied to calculate the gaseous phase heat capacity, C_p,m, of the monomeric species over the temperature range of (298.15 to 600) K. The gaseous phase thermodynamic properties of the monomeric form of the BA were also derived from the assignment of the fundamental vibrational frequencies using experimental IR spectra. An excellent agreement among the experimental gaseous phase heat capacities, the results obtained using the proposed ab initio procedure, and the results derived from the assignment of fundamental vibrational frequencies was found. The results for the monomeric form of the BA, directly or indirectly obtained, and conclusions of this work strongly support the thesis that the gaseous phase heat capacity data as currently found in the literature are underestimated to the order of 20 %.