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

Reliable information about isobaric heat capacities C 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 by the homodesmotic method is analyzed. Three cases of C calculation applying the methodology of the complete set of homodesmotic reactions (CS HDRs) are considered: the gas- and liquid-phase C of organic compounds of various classes at 298 K (the mean absolute value of reaction heat capacity, MA ΔC = 1.44 and 2.83 J/mol·K for the gas and liquid phase, correspondingly); and the gas-phase C of n-alkanes C₂-C₁₀ 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 of organic compounds is characterized by high accuracy, which is not inferior to that of the best C-additive schemes and allows us to analyze the reproducibility of the calculation results and eliminate unreliable reference data.