Constructing Homodesmic Reactions for Calculating the Enthalpies of Formation of Organic Compounds

The mathematics and software of the homodesmic approach to calculating enthalpies of formation Delta(f) HA degrees of organic compounds are described. The program generates a set of independent homodesmic reactions for a studied compound, increasing the reliability of theoretical determinations of the standard enthalpy of formation. The basic steps of program construction, using the algorithm to determine the basis of homodesmic reactions for organic compounds, and calculations for the enthalpies of formation for a random test set comprising 53 CHNO-containing organic compounds of different classes are described. It is shown that the homodesmic approach provides highly reliable theoretical values of Delta(f) HA degrees virtually regardless of the quantumchemical approach that is used. The average absolute error of MAD calculations for Delta(f) HA degrees of a test set is 2.2 kJ/mol for the simple B3LYP/6-31G(d) approach and 2.1 kJ/mol for the moderately complicated M06-2X/cc-pVTZ approach, while using the complicated method of G3 calculations reduces the MAD value to 1.7 kJ/mol.