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

Abstract

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.

Rate Constants and Kinetic Isotope Effects for Unimolecular 1,2-HX or DX (X = F or Cl) Elimination from Chemically Activated CF3CFClCH3-d0, -d1, -d2, and -d3

“…On the basis of our investigation of several HX elimination reactions, this similarity in calculated frequencies can be taken as a general conclusion. 6,15 On the other hand, these calculations frequently do not provide reliable threshold energies, which must be derived experimentally from Arrhenius constants or from fitting chemical activation rate constants to calculated RRKM rate constants. The calculated frequencies for the transition states for reactions 1, 2, and 4 are listed in Table 5, and the geometry of the molecule and the transition states are illustrated in Figure 1.…”

Mixed Quantum-Classical Reaction Path Dynamics of C2H5F → C2H4 + HF

“…The dynamics and kinetics of three-and four-centered elimination of HF and HCl from halogenated alkanes have been the subject of a considerable number of experimental 1-25 and computational [26][27][28][29][30][31][32][33][34][35] studies. Initially, chemically activated decomposition by radical recombination was performed to obtain dynamical data, [1][2][3][4][5][6][7][8][9][10][11] and shock tube pyrolysis was used to obtain kinetic data.…”

Ab initio, DFT and transition state theory calculations on 1,2-HF, HCI and CIF elimination reactions from CH2F–CH2Cl

“…The HX elimination reactions from haloethanes have continued to attract both experimental [1][2][3][4][5][6] and theoretical efforts. [6][7][8][9][10] One of the main reasons for this interest is the fact that these reactions offer an excellent example for a truly unimolecular reaction involving complicated motions along at least 4 bonds. It is ideal for discussing the intricacies of ' the reaction coordinate ' in a classroom.…”