Revision and Extension of a Generally Applicable Group-Additivity Method for the Calculation of the Standard Heat of Combustion and Formation of Organic Molecules

The calculation of the heats of combustion ΔH°c and formation ΔH°f of organic molecules at standard conditions is presented using a commonly applicable computer algorithm based on the group-additivity method. This work is a continuation and extension of an earlier publication. The method rests on the complete breakdown of the molecules into their constituting atoms, these being further characterized by their immediate neighbor atoms. The group contributions are calculated by means of a fast Gauss-Seidel fitting calculus using the experimental data of 5030 molecules from literature. The applicability of this method has been tested by a subsequent ten-fold cross-validation procedure, which confirmed the extraordinary accuracy of the prediction of ΔH°c with a correlation coefficient R2 and a cross-validated correlation coefficient Q2 of 1, a standard deviation σ of 18.12 kJ/mol, a cross-validated standard deviation S of 19.16 kJ/mol, and a mean absolute deviation of 0.4%. The heat of formation ΔH°f has been calculated from ΔH°c using the standard enthalpies of combustion for the elements, yielding a correlation coefficient R2 for ΔH°f of 0.9979 and a corresponding standard deviation σ of 18.14 kJ/mol.

On the Prediction of Standard Enthalpy of Formation of C2-C4 Oxygenated Species

In this study, to determine an efficient and accurate method for predicting standard enthalpy of formation (Δ_fH^o) of oxygenated species, we calculated Δ_fH^o for several typical C2-C4 oxygenated species using atomization and isodesmic reactions in combination with various quantum chemical methods, including six density functional theory methods, three compound methods, and CCSD(T)/CBS. Compared with experimental values, at the same quantum chemical level, Δ_fH^o values predicted by using isodesmic reactions are more accurate than those using atomization reactions. Comparing various quantum chemical methods when isodesmic reactions are used, the performance of G4 is the best with a mean unsigned deviation (MUE) of 0.3 kcal/mol and a standard deviation (SD) of 0.3 kcal/mol, while M06-2X can predict Δ_fH^o efficiently and accurately with an MUE of 0.6 kcal/mol and SD of 0.5 kcal/mol. Using the best methods we have found, we calculated the enthalpies of formation and other thermodynamic properties for dimethyl carbonate (DMC) and its associated species and then applied them in a DMC combustion model for predicting ignition delay times. Better agreement with the experiments is achieved when the newly computed thermodynamic properties are adopted.