Heats of Formation of Hydrofluorocarbons Obtained by Gaussian-3 and Related Quantum Chemical Computations

The heats of formation of ∼120 C 1 and C 2 hydrofluorocarbon and oxidized hydrofluorocarbon molecules, as well as of hexafluoropropene and the hexafluoropropyl radical, were computed using the Gaussian-3 (G3) method, along with two approximations to G3, denoted G3[MP2(full)] and G3(MP4SDQ), and the G2(MP2) method. The performance of G3 is clearly superior to that of the other methods when the heats of formation are computed via atomization energies, and in general, the G3 results agree with the available good-quality experimental and theoretical data to within 2 kcal mol -1 . The use of isodesmic reaction schemes improves the overall accuracy of the computed heats of formation and results in a consistent set of predictions that are largely method-independent. Although, for the majority of molecules, the G3 data agree well with the earlier theoretical predictions of Zachariah et al. ( J. Phys. Chem ., 1996, 100, 8737), who used the bond-additivity-corrected MP4 (BAC-MP4) method, there are significant discrepancies as well. The heats of formation of a group of small molecules consisting of the fluoroacetylenes (HCCF and C 2 F 2, as well as C 2 H 2 ) and the C 2 H, C 2 F, and formyloxyl (HCOO) radicals were also computed using the coupled-cluster method with basis sets ranging from cc-pVDZ to aug-cc-pVQZ and cc-pCVQZ, followed by extrapolation to the CBS limit and corrections for core-valence correlation and scalar relativistic effects. The predicted CBS heats of formation (in kcal mol -1 ) are Δ f (HCCF) = 24.6 ± 1.0, Δ f (C 2 F 2 ) = 0.5 ± 1.0, Δ f (C 2 H) = 135.9 ± 1.0, Δ f (C 2 F) = 109.1 ± 1.0, and Δ f (HCOO) = −30.1 ± 1.0, in good agreement with the G3 results. The current work on formyloxyl provides strong support for the experimental value of Δ f = 28.6 ± 0.7 kcal mol -1 obtained by Langford et al. ( J. Chem. Soc., Faraday Trans . 1997, 93, 3757).