Structures, barriers for internal rotation, vibrational frequencies, and thermodynamic functions of CH2FCH2, CHF2CH2, and CF3CH2radicals: Anabinitiostudy

摘要

The equilibrium geometries, rotational barriers, and harmonic vibrational frequencies for bgr;hyphen;fluoroethyl (CH2FCH2), bgr;,bgr;hyphen;difluoroethyl (CHF2CH2), and bgr;,bgr;,bgr;hyphen;trifluoroethyl (CF3CH2) radicals have been determined byabinitiomolecularhyphen;orbital techniques using thegaussian86 system of programs at the unrestricted Hartreendash;Fock, UHF/6hyphen;31Gast; level of theory. Three conformational minima in the potentialhyphen;energy surface were found for the CH2FCH2and CHF2CH2radicals, 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 bgr;hyphen;position hydrogen by fluorine atoms is much weaker than is the case for agr;hyphen;position fluorine substituted ethyl radicals. Transition structures for internal rotation were also located. Estimates for the correction of electron correlation effects were obtained by singlehyphen;point calculations using secondhyphen;order Mollerndash;Plesset perturbation theory (UMP2). The rotation about the Cndash;C bond is almost free for all three radicals, which again is much different from the situation in the agr;hyphen;fluorine substituted ethyl radicals. Based on calculated vibrational frequencies and moments of inertia, thermodynamic properties including heat capacities, entropies, enthalpy, and freehyphen;energy functions are tabulated as a function of temperature. Several isodesmicndash;homodesmic reactions have been studied for the purpose of obtaining theoretical heats of formation of the bgr;hyphen;fluoroethyl and bgr;, bgr;hyphen;difluoroethyl radicals for which experimental values are not available. The theoretical heats of formation thus evaluated are minus;10.65 and minus;66.26 kcal/mol for CH2FCH2and CHF2CH2, respectively. Together with the measured Dgr;H0f(CF3CH2), these data are used to evaluate Dgr;H0f,T,thinsp;Dgr;G0f,T, and logKffor all three radicals as a function of temperature.