| Title
| - Simultaneous Adjustment of Experimentally Based Enthalpies of Formation of CF3X, X =nil, H, Cl, Br, I, CF3, CN, and a Probe of G3 Theory
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| Abstract
| - The enthalpies of formation of the title compounds have been determined by simultaneous least-squares solutionof a thermochemical network containing 28 experimental measurements extracted from the literature. Thenew enthalpies differ considerably from the older tabulated values and now show a high degree of internalconsistency achieved through excellent agreement with newer experiments, as well as older calorimetricdeterminations. The fitted values are: ΔHf298°(CF3) = −111.3 ± 0.5 kcal/mol (−110.6 ± 0.5 kcal/mol at 0K), ΔHf298°(CF3H) = −165.6 ± 0.5 kcal/mol (−163.9 ± 0.5 kcal/mol at 0 K), ΔHf298°(CF3Cl) = −169.5 ±0.7 kcal/mol (−168.3 ± 0.7 kcal/mol at 0 K), ΔHf298°(CF3Br) = −155.3 ± 0.5 kcal/mol (−152.4 ± 0.5kcal/mol at 0 K), ΔHf298°(CF3I) = −140.1 ± 0.5 kcal/mol (−138.7 ± 0.5 kcal/mol at 0 K), ΔHf298°(C2F6) =−321.3 ± 0.8 kcal/mol (−319.3 ± 0.8 kcal/mol at 0 K), and ΔHf298°(CF3CN) = −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 beenperformed 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 withexperiment within <2 kcal/mol. An isodesmic bond-separation scheme brings the calculated values evencloser to the experiment. In addition, the G3 value for the ionization energy of CF3, 9.08 eV, was found tobe in good agreement with the recent experimental value of 9.055 ± 0.011 eV. Further indications that theexperimental value for ΔHf°(C2F4) is probably too high are also presented.
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