STRUCTURES, BARRIERS FOR ROTATION AND INVERSION, VIBRATIONAL FREQUENCIES, AND THERMODYNAMIC FUNCTIONS OF ETHYL, ALPHA-FLUOROETHYL, AND ALPHA,ALPHA-DIFLUOROETHYL RADICALS - AN ABINITIO STUDY

The potential energy surfaces of CH3CHF and CH 3CF2 radicals were studied by ab initio molecular orbital methods using the GAUSSIAN 86 system of programs at the UHF/6-31G* level of theory. CH3CHF has C1 symmetry and exists as a single pair of enantiomeric conformations. The difluoro species has only a single stable structure, of Cs symmetry. The barriers to rotation about the C-C bond, and the barriers hindering pyramidal inversion at the nonplanar radical center were located for each compound by analytical methods. The monofluoro and difluoro species had similar rotation barriers at the UMP2/6-31G*// 6-31G* level, 1.68 and 2.26 kcal/mol, respectively, after inclusion of zero-point vibrational energy differences. The inversion barriers were substantially different, 0.54 and 10.45 kcal/mol, respectively. Vibrational frequencies, as well as moments of inertia for overall and internal rotations, are reported for each species and for the ethyl radical. Calculated heat capacities, entropies, enthalpies, and free energy functions are tabulated as a function of temperature. Use of several isodesmic or homodesmic reactions to obtain a value for the heat of formation of CH3CHF at 298 K is discussed. Good experimental values are available for ethyl and CH 3CF2. Adopting reference values for ΔH f,2980 of 28.36, - 17.3, and - 72.3 kcal/mol for CH 3CH2, CH3CHF, and CH3CF2, respectively, values for ΔHf,T0, ΔG f,T0 and log10Kf for all three radicals are reported as a function of temperature in the range 0-1500 K. Comparison of the theoretical and experimental data for CH3CH 2 suggests that the error introduced by use of harmonic HF/6-31G*//6-31G* frequencies is ± 1 cal mol-1 K-1 for both heat capacity and entropy. © 1990 American Institute of Physics.