Structures, barriers for internal rotation and inversion, vibrational frequencies, and thermodynamic functions of CH2FCF2, CHF2CF2, and CF3CF2 radicals: An a b i n i t i o study

Abstract

The equilibrium geometries, rotational and inversion barriers, and harmonic vibrational frequencies for α,α,β-trifluoroethyl (CH2FCF2), α,α,β,β-tetrafluoroethyl (CHF2CF2), and pentafluoroethyl (CF3CF2) radicals have been determined by ab initio molecular orbital techniques using the GAUSSIAN 86 system of programs at the unrestricted Hartree–Fock level of theory with the 6-31G* basis set (UHF/6-31G*). Three conformational minima in the potential energy surface were found for the CH2FCF2 and the CHF2CF2 radicals, while only one was found for the CF3CF2 radical. The radical centers for all three species are nonplanar. The barriers hindering rotation about the C–C bond and inversion 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 free energy functions are tabulated as a function of temperature. Several homodesmic reactions have been studied for the purpose of obtaining theoretical heats of formation of CH2FCF2 and CHF2CF2 radicals for which experimental values are not available. The theoretical heats of formation thus evaluated are −106.6 kcal/mol and −157.8 kcal/mol for CH2CF2 and CHF2CF2, respectively. These data and the experimental heat of formation of CF3CF2 (ΔH0f,298 =−213.0±1.3 kcal/mol) are used to evaluate ΔH0f ,T, ΔG0f ,T, and Kf ,T for all three radicals as a function of temperature.