Theoretical enthalpies of formation of large compounds using integrated methods

Abstract

The free radicals CH₃CH₂, CH₃CH₂CH₂, n-C₄H₉, i-C₄H₉, neo-C₅H₁₁, and CF₃CH₂ were used as a test set to analyze the performance of integrated methods in the description of standard enthalpies of formation. Using hydrogenation and isodesmic reactions as working chemical reactions, some variants of integrated theoretical approaches (levels) combined with different extended basis sets were applied in the calculation, and the results were compared with the economic hybrid DFT. In general, the integrated and hybrid DFT methods showed similar behaviour in the determination of enthalpies of formation of large compounds, their great advantage being the low computational cost, although the hybrid DFT results were more dependent on the working chemical reaction used. The recommended standard enthalpies of formation (298.15 K and 1.0 atm) are the unweighted averages of the results obtained at the integrated levels with the different working chemical reactions: CH₃CH₂, 29.8 ± 0.5; CH₃CH₂CH₂, 25.1 ± 1.3; n-C₄H₉, 19.6 ± 1.2; i-C₄H₉, 17.8 ± 1.3; neo-C₅H₁₁, 10.0 ± 1.3; CF₃CH₂, −125.6 ± 2.9, kcal mol⁻¹, where the error limits designate the estimated maximum uncertainties. These values are in close agreement with experimental or previous theoretical estimations, although slightly overestimated. This small overestimation is due to the combination of the limited correlation energy and basis set used, and is not associated with the integrated method itself.