Complete set of isodesmic reactions for the accurate thermochemistry of three- and four-atom CNO-compounds

The methodology of the complete set of isodesmic reactions, CS IDR, was developed and applied to a test set of 154 small organic compounds, including three- and four-atom linear and cyclic CNO-containing structures, to evaluate their standard enthalpies of formation, Δ_fH°. The CS IDR approach was tested using ten quantum-chemical methods: seven DFT approximations, DLPNO-CCSD(T), and two composite methods, G4 and W1BD. It was found that DFT methods do not provide high accuracy in estimating Δ_fH°. In terms of calculation quality, the ab initio methods are ranked as G4 > W1BD > DLPNO-CCSD(T), with the accuracy characterized by mean absolute deviations (MAD) of 1.29, 1.52, and 2.24 kJ mol^-1, respectively. Analysis of the calculation results revealed that composite methods are characterized by a systematic error, the magnitude of which is proportional to the enthalpy of the reference reaction. Eliminating the systematic error of composite methods and improving the agreement between calculation and experiment is possible using averaged estimate according to the equation Δ_fH° = 0.5×(Δ_fH°(G4) + Δ_fH°(W1BD)) - 0.78 kJ mol^-1.