Simultaneous adjustment of experimentally based enthalpies of formation ofCF3X, 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 enthalp ies differ considerably from the older tabulated values and now show a high degree of internal consistency achieved through excellent agreement with n ewer experiments, as well as older calorimetric determinations. The fitted values are: Delta H(f298)degrees(CF3)= - 111.3 +/- 0.5 kcal/mol (-110.6 +/- 0.5 kcal/mol at 0 K), Delta H(f298)degrees(CF3H) = -165.6 +/- 0.5 kcal/mol (-163.9 +/- 0.5 kcal/mol at 0 K), Delta H(f298)degrees(CF3Cl) = -169.5 +/- 0.7 kcal/mol (-168.3 +/- 0.7 kcal/mol at 0 K), Delta H(f298)degrees(CF3Br) = -155.3 +/- 0.5 kcal/mol (-152.4 +/- 0.5 kcal/mol at 0 K), bH(f298)degree s(CF3I) = -140.1 +/- 0.5 kcal/mol (-138.7 +/- 0.5 kcal/mol at 0 K), Delta H (f298)degrees(C2F6) = -321.3 +/- 0.8 kcal/mol (-319.3 +/- 0.8 kcal/mol at 0 K), and Delta H(f298)degrees(CF3CN) = 1118.4 +/- 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 wer e problematic at the Gaussian-2 (G2) level of theory. In comparison to G2 t heory, the G3 results show a remarkable improvement in accuracy and agree w ith experiment within <2 kcal/mol. An isodesmic bond-separation scheme brin gs the calculated values even closer to the experiment. In addition, the G3 value for the ionization energy of CF3, 9.08 eV, was found to be in good a greement with the recent experimental value of 9.05(5) +/- 0.01(1) eV. Furt her indications that the experimental value for Delta H(f)degrees(C2F4) is probably too high are also presented.