Thermochemical Properties Enthalpy, Entropy, and Heat Capacity of C1–C4 Fluorinated Hydrocarbons: Fluorocarbon Group Additivity

Enthalpies of formation for 14 C2–C4 fluorinated hydrocarbons were calculated with nine popular ab initio and density functional theory methods: B3LYP, CBS-QB3, CBS-APNO, M06, M06-2X, ωB97X, G4, G4­(MP2)-6X, and W1U via several series of isodesmic reactions. The recommended ideal gas phase ΔH_f298^° (kcal mol^–1) values calculated in this study are the following: −65.4 for CH₃CH₂F; −70.2 for CH₃CH₂CH₂F; −75.3 for CH₃CHFCH₃; −75.2 for CH₃CH₂CH₂CH₂F; −80.3 for CH₃CHFCH₂CH₃; −108.1 for CH₂F₂; −120.9 for CH₃CHF₂; −125.8 for CH₃CH₂CHF₂; −133.3 for CH₃CF₂CH₃; −166.7 for CHF₃; −180.5 for CH₃CF₃; −185.5 for CH₃CH₂CF₃; −223.2 for CF₄; and −85.8 for (CH₃)₃CF. Entropies (S₂₉₈^° in cal mol^–1 K^–1) were estimated using B3LYP/6-31+G­(d,p) computed frequencies and geometries. Rotational barriers were determined and hindered internal rotational contributions for S₂₉₈^°, and C_p(T) were calculated using the rigid rotor harmonic oscillator approximation, with direct integration over energy levels of the intramolecular rotation potential energy curve. Thermochemical properties for the fluorinated carbon groups C/C/F/H₂, C/C₂/F/H, C/C/F₂/H, C/C₂/F₂, and C/C/F₃ were derived from the above target fluorocarbons. Previously published enthalpies and groups for 1,2-difluoroethane, 1,1,2-trifluoroethane, 1,1,2,2-tetrafluoroethane, 1,1,1,2-tetrafluoroethane, 1,1,1,2,2-pentafluoroethane, 2-fluoro-2-methylpropane that were previously determined via work reaction schemes are revised using updated reference species values. Standard deviations are compared for the calculation methods.