Structures, barriers for internal rotation, vibrational frequencies, and thermodynamic functions of CH2FCH2, CHF2CH2, and CF3CH2 radicals: An a b i n i t i o study

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Abstract
The equilibrium geometries, rotational barriers, and harmonic vibrational frequencies for β-fluoroethyl (CH2FCH2), β,β-difluoroethyl (CHF2CH2), and β,β,β-trifluoroethyl (CF3CH2) 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 CH2FCH2 and CHF2CH2 radicals, while only one was found for CF3CH2. The radical centers for all three species are nonplanar, but the effect on the geometries of these radicals in replacing β-position hydrogen by fluorine atoms is much weaker than is the case for α-position fluorine substituted ethyl radicals. Transition structures for internal rotation were also located. Estimates for the correction of electron correlation effects were obtained by single-point calculations using second-order Moller–Plesset perturbation theory (UMP2). The rotation about the C–C bond is almost free for all three radicals, which again is much different from the situation in the α-fluorine substituted ethyl radicals. Based on calculated vibrational frequencies and moments of inertia, thermodynamic properties including heat capacities, entropies, 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 β-fluoroethyl and β, β-difluoroethyl radicals for which experimental values are not available. The theoretical heats of formation thus evaluated are −10.65 and −66.26 kcal/mol for CH2FCH2 and CHF2CH2, respectively. Together with the measured ΔH0f(CF3CH2), these data are used to evaluate ΔH0f,T, ΔG0f,T, and log Kf for all three radicals as a function of temperature.