Structures, barriers for internal rotation and inversion, vibrational frequencies, and thermodynamic functions of CH2FCHF, CHF2CHF, and CF3CHF radicals: An a b i n i t i o study

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Abstract
The equilibrium geometries, rotational and inversion barriers, and harmonic vibrational frequencies for α, β difluoroethyl (CH2FCHF), α, β, β trifluorethyl (CHF2CHF), and α, β, β, β tetrafluoroethyl (CF3CHF) radicals have been determined by ab initio molecular orbital techniques using the gaussian 86 system of programs at the unrestricted Hartree–Fock (UHF)/6–31G* level of theory. Three conformational minima in the potential energy surface were found for the CH2FCHF, and CHF2CHF radicals, while only one was found for the CF3CHF radical. The radical centers for all three species are nonplanar. The barriers of rotation about the C–C bond and the inversion barriers of the radical center were located for each compound by analytical methods. Vibrational frequencies, as well as moments of inertia for overall and internal rotation are reported for each species. Calculated heat capacities, entropies, and enthalpy and free energy functions are tabulated as a function of temperature. Several isodesmic/homodesmic reactions have been studied for the purpose of obtaining theoretical heats of formation of the three radicals for which experimental values are not available. The theoretical heats of formation (ΔH0f,298 ) thus evaluated are −56.3 kcal/mol, −108.0 kcal/mol, and −164.5 kcal/mol for CH2FCHF, CHF2CHF, and CF3CHF, respectively. These data are used to evaluate ΔH0f,T, ΔG0f,T, and Kf,T for all three radicals as a function of temperature.

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