Simultaneous Adjustment of Experimentally Based Enthalpies of Formation of CF₃X, X = nil, H, Cl, Br, I, CF₃, CN, and a Probe of G3 Theory

The enthalpies of formation of the title compounds have been determined by simultaneous least-squares solution of a thermochemical network containing 28 experimental measurements extracted from the literature. The new enthalpies differ considerably from the older tabulated values and now show a high degree of internal consistency achieved through excellent agreement with newer experiments, as well as older calorimetric determinations. The fitted values are: Δ H f298 °(CF 3 ) = −111.3 ± 0.5 kcal/mol (−110.6 ± 0.5 kcal/mol at 0 K), Δ H f298 °(CF 3 H) = −165.6 ± 0.5 kcal/mol (−163.9 ± 0.5 kcal/mol at 0 K), Δ H f298 °(CF 3 Cl) = −169.5 ± 0.7 kcal/mol (−168.3 ± 0.7 kcal/mol at 0 K), Δ H f298 °(CF 3 Br) = −155.3 ± 0.5 kcal/mol (−152.4 ± 0.5 kcal/mol at 0 K), Δ H f298 °(CF 3 I) = −140.1 ± 0.5 kcal/mol (−138.7 ± 0.5 kcal/mol at 0 K), Δ H f298 °(C 2 F 6 ) = −321.3 ± 0.8 kcal/mol (−319.3 ± 0.8 kcal/mol at 0 K), and Δ H f298 °(CF 3 CN) = −118.4 ± 0.4 kcal/mol (−117.5 ± 0.4 kcal/mol at 0 K). Theoretical calculations at the Gaussian-3 (G3) level of theory have been performed for a subgroup of these molecules, which were problematic at the Gaussian-2 (G2) level of theory. In comparison to G2 theory, the G3 results show a remarkable improvement in accuracy and agree with experiment within <2 kcal/mol. An isodesmic bond-separation scheme brings the calculated values even closer to the experiment. In addition, the G3 value for the ionization energy of CF 3, 9.08 eV, was found to be in good agreement with the recent experimental value of 9.05 5 ± 0.01 1 eV. Further indications that the experimental value for Δ H f °(C 2 F 4 ) is probably too high are also presented.