Heat Capacity Estimation Using a Complete Set of Homodesmotic Reactions for Organic Compounds

Reliable information about isobaric heat capacities C(P) is necessary to determine the energies of organic compounds and chemical processes at an arbitrary temperature. In this work, the possibility of theoretical estimation of C(P) by the homodesmotic method is analyzed. Three cases of C(P) calculation applying the methodology of the complete set of homodesmotic reactions (CS HDRs) are considered: the gas- and liquid-phase C(P) of organic compounds of various classes at 298 K (the mean absolute value of reaction heat capacity, MA ΔC(P) = 1.44 and 2.83 J/mol·K for the gas and liquid phase, correspondingly); and the gas-phase C(P) of n-alkanes C(2)–C(10) in the temperature range of 200–1500 K with an average error in calculating the heat capacity of 0.93 J/mol·K. In the latter case, the coefficients of the Shomate equation are determined for all n-alkanes that satisfy the homodesmoticity condition. New values of gas- and liquid-phase heat capacities are obtained for 41 compounds. The CS HDRs-based approach for estimating the C(P) of organic compounds is characterized by high accuracy, which is not inferior to that of the best C(P)-additive schemes and allows us to analyze the reproducibility of the calculation results and eliminate unreliable reference data.