STRUCTURES, BARRIERS FOR INTERNAL-ROTATION, VIBRATIONAL FREQUENCIES, AND THERMODYNAMIC FUNCTIONS OF CH2FCH2, CHF2CH2, AND CF3CH2 RADICALS - AN ABINITIO STUDY

被引:32
作者
CHEN, YH [1 ]
RAUK, A [1 ]
TSCHUIKOWROUX, E [1 ]
机构
[1] UNIV CALGARY, DEPT CHEM, CALGARY T2N 1N4, ALBERTA, CANADA
关键词
D O I
10.1063/1.458929
中图分类号
O64 [物理化学(理论化学)、化学物理学];
学科分类号
070304 ; 081704 ;
摘要
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 CHF2CH 2, respectively. Together with the measured ΔHf 0(CF3CH2), these data are used to evaluate ΔHf,T0, ΔGf,T0, and log Kf for all three radicals as a function of temperature. © 1990 American Institute of Physics.
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页码:6620 / 6629
页数:10
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