Density functional theory, DFT, and high-level conventional ab initio calculations, together with RRKM calculations, have been employed to study the nature of the transition state geometry for 1,2 elimination of HF from 1,1,1-trifluoroethane-d 0 ,-d 3 ; these serve as test cases for 1,2-HF elimination from fluorocarbons. Quantities calculated include structural parameters, bond indices, energies, atomic charges, vibrational frequencies, and moments of inertia for the reactant and the transition state geometry. The threshold energies for HF and DF elimination were computed and the vibrational frequencies and moments of inertia data were used with the RRKM theory to calculate the entropies of activation, preexponential factors for thermal activation, and also rate constants and the kinetic isotope effect for both thermally and chemically activated 1,1,1-trifluoroethane-d 0 ,-d 3 . Of all the methods employed, the hybrid DFT methods incorporating either the threeparameter exchange functional of Becke with the correlation functional of Perdew and Wang or the correlation functional of Lee, Yang, and Parr were found to give results more consistent with the experimental studies. Both the 6-31G(d′,p′) and cc-PVDZ basis sets gave comparable agreement with experiment and suggest that basis sets of double-quality in the valence region, which include polarization functions, appear to be adequately flexible to describe the systems studied here. There appears to be little to be gained in going from the computationally efficient DFT calculations to the computationally very expensive methods such as G2 or MP2.

Geometries for transition states in the reactions of OH radicals with molecules in the series C2HnF6-n, n = 1-5, have been optimized at the HF/6-31G(d) and MP2/6-31G(d,p) levels of theory. Vibrational frequency analyses were also performed at both levels of theory. Total energies have been calculated for the reactants, transition states, and products of the reactions at the MP2/6-3 1 1G(d,p)//HF/6-3 lG(d,p) level for all species, at the MP2/6-3 1 lG(d,p)/iMP2/6-3 lG(d,p) level for all reactants and products, and transition states containing up to three fluorines, and at Gaussian-2 theory level for reactants and products with up to two fluorines and transition states with one fluorine. These calculations were performed for the abstraction of each inequivalent hydrogen from the most stable conformer of each molecule in the series. Activation entropies, classical barrier heights, with and without zero-point energy corrections at the MP2/6-3 lG(d,p) level, and enthalpies of reaction with zero-point and thermal energy corrections to 25 "C have been calculated. Energy values are affected by the degree and position of fluorine substitution. Trends throughout the series, and differences in the levels of theory, are discussed. In all cases an earlier transition state occurs when electron correlation is included in the geometry optimization. Evidence is seen for an intramolecular hydrogen bond between the hydroxyl hydrogen and a ,&fluorine positioned gauche to the hydrogen being abstracted.

Chemically activated CF 2 ClCF 2 CH 3 and CF 2 ClCF 2 CD 3 , containing 98.5 and 100 kcal/mol of internal energy, respectively, were formed in the gas phase from the combination of CF 2 ClCF 2 and CH 3 or CD 3 radicals, respectively. These radicals were generated from the UV photolysis of CF 2 ClCF 2 I and CH 3 I or CD 3 I. The decomposition products were CF 2 ClCFdCH 2 (CF 2 ClCFdCD 2 ) from a 2,3-HF (DF) elimination and CF 3 -CFdCH 2 (CF 3 CFdCD 2 ) suggesting a 1,3-HCl (DCl) elimination reaction. The 1,3-HCl elimination mechanism appears to be a two-step process; a 1,2-FCl rearrangement, producing CF 3 ClCFClCH 3 (CF 3 CFClCD 3 ), followed by a 2,3-HCl (DCl) elimination. Unimolecular rate constants for CF 2 ClCF 2 CH 3 (CF 2 ClCF 2 CD 3 ) were 5.3 ( 2.1 × 10 5 s -1 (1.8 ( 0.7 × 10 5 s -1 ) for 2,3-HF (DF) loss and 3.6 ( 1.4 × 10 4 s -1 (2.3 ( 0.9 × 10 4 s -1 ) for the 1,2-FCl rearrangement. The branching ratio was 13.5 ( 3 (7.8 ( 1.6) favoring the HF (DF) process. The isotope effect for 2,3-HF/DF was 2.9 ( 0.6, while for the FCl rearrangement, it was considerably smaller at 1.5 ( 0.3. The CF 3 CFClCH 3 and the CF 3 CFClCD 3 , formed by the 1,2-FCl migration, react by loss of HCl (DCl) with rate constants of 2.1 ( 1.3 × 10 7 s -1 (7.9 ( 4.8 × 10 6 s -1 ) and an isotope effect of 2.7 ( 0.8. Theoretical rate constants, branching ratio and isotope effects were calculated using RRKM theory and density functional theory to compute all of the data necessary for the RRKM calculations. The agreement between the experimental and computed kinetic data suggests that the 1,3-HCl elimination is a two-step mechanism consisting of a 1,2-FCl rearrangement followed by a 2,3-HCl elimination.

In search of the optimal combination of basis set and exchange-correlation potential, we have investigated the dependence of the atomization energies (D 0 ) and reaction enthalpies (∆H) for a set of 44 molecules using gradient-corrected density functional theory. Of the six functionals tested, those that include a portion of the exact (Hartree-Fock) exchange perform best and yield D 0 values that generally lie within 3-5 kcal/mol of the experimental value. For the functionals in which pure DFT exchange is employed, the errors in D 0 are instead on the order of 8-10 kcal/mol. Conversely, reaction enthalpies show slightly better agreement with experimental results when pure DFT exchange is employed. For both D 0 and ∆H the four Gaussian basis sets 6-31G(d,p), 6-311G(d,p), cc-pVDZ, and cc-pVTZ show similar behavior. The most accurate predictions were obtained using the largest cc-pVTZ basis. There is a significant variation in these energies obtained with the various functionals depending on the basis set employed.

Geometries for the molecules in the series CzH,F&, (n = 0-6) and for the radicals produced by homolytic cleavage of the C-C and C-H bonds have been optimized at the MP2/6-31G(d,p) level. Total energies have been calculated at the MP2/6-31 lG(d,p)//MP2/6-3lG(d,p) level. A few calculations at the MP4/6-311G-(d,p)//MP2/6-31G(d,p) level have been included to show that higher order correlation effects do not change the observed trends. The preferred conformations of the substituted ethyl radicals indicate that the gauche effect operates in the radicals as well as in the parent molecules. The C -C and C-H bond dissociation energies are reported as Do(298 K). The C-H bond dissociation energies indicate that C-H bonds can be stabilized by an inductive effect from the 8-group and that the length of the bond does not correlate with the dissociation energy. Comparisons of results from HF and MP2 theory are made. Inclusion of electron correlation improves the geometrical parameters, in particular lengthening the C-F bonds, and the total energies. However, the use of correlated geometries as compared to the HartreeFock geometries has little effect on the bond dissociation energies.