Comparisons between Density Functional Theory and Conventional ab Initio Methods for 1,2-Elimination of HF from 1,1,1-Trifluoroethane: Test Case Study for HF Elimination from Fluoroalkanes

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-d0,-d3; 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-d0,-d3. Of all the methods employed, the hybrid DFT methods incorporating either the three-parameter exchange functional of Becke with the corre...