Thermodynamic Properties ΔHf°298, S°298, and Cp(T) for 2-Fluoro-2-Methylpropane, ΔHf°298 of Fluorinated Ethanes, and Group Additivity for Fluoroalkanes

Abstract

G2(MP2) composite calculations are performed to obtain thermodynamic properties (ΔHf°298, S°298 and Cp(T)'s of 2-fluoro-2-methylpropane. ΔHf°298 is calculated from the G2(MP2) calculated enthalpy of reaction (ΔHrxn°298) and use of isodesmic reactions. Standard entropy (S°298 in cal/(mol·K)) and heat capacities (Cp(T)'s in cal/(mol·K)) are calculated using the rigid-rotor−harmonic-oscillator approximation with direct integration over energy levels of the intramolecular rotation potential energy curve. These thermodynamic properties are used to estimate data for the C/C3/F group. Enthalpies of formation (ΔHf°298 in kcal/mol) for 1,2-difluoroethane (−102.7), 1,1,2-trifluoroethane (−156.9), 1,1,2,2- and 1,1,1,2-tetrafluoroethane (209.6 and 213.3), and pentafluoroethane (−264.1), are calculated using total energies obtained from G2(MP2) composite ab initio methods. Isodesmic reactions with existing literature values of ΔHf°298 for ethane, 1-fluoroethane, 1,1-difluoroethane and 1,1,1-trifluoroethane are used. Fluorine/fluorine interaction terms, F/F, 2F/F, 3F/F, 2F/2F, and 3F/2F, where “/” indicates interaction for alkane compounds, for ΔHf°298 are reevaluated based on ΔHf°298 of the above five fluoroethanes. Thermodynamic properties (ΔHf°298, S°298 and Cp(T)'s (300 ≤ T/K ≤ 1500)) for fluorinated carbon groups, C/C3/F, C/C2/F/H, C/C2/F2, are calculated using data from ab initio methods and existing literature data. Fluorine−methyl (alkyl) group additivity corrections for gauche interactions are also evaluated.