The Unimolecular Reactions of CF₃CHF₂ Studied by Chemical Activation: Assignment of Rate Constants and Threshold Energies to the 1,2-H Atom Transfer, 1,1-HF and 1,2-HF Elimination Reactions, and the Dependence of Threshold Energies on the Number of F-Atom Substituents in the Fluoroethane Molecules

Abstract

The recombination of CF and CHF radicals in a room-temperature bath gas was used to prepare vibrationally excited CFCHF* molecules with 101 kcal mol of vibrational energy. The subsequent 1,2-H atom transfer and 1,1-HF and 1,2-HF elimination reactions were observed as a function of bath gas pressure by following the CHF, CF(F)C: and CF product concentrations by gas chromatography using a mass spectrometer as the detector. The singlet CF(F)C: concentration was measured by trapping the carbene with trans-2-butene. The experimental rate constants are 3.6 × 10, 4.7 × 10, and 1.1 × 10 s for the 1,2-H atom transfer and 1,1-HF and 1,2-HF elimination reactions, respectively. These experimental rate constants were matched to statistical RRKM calculated rate constants to assign threshold energies (E) of 88 ± 2, 88 ± 2, and 87 ± 2 kcal mol to the three reactions. Pentafluoroethane is the only fluoroethane that has a competitive H atom transfer decomposition reaction, and it is the only example with 1,1-HF elimination being more important than 1,2-HF elimination. The trend of increasing threshold energies for both 1,1-HF and 1,2-HF processes with the number of F atoms in the fluoroethane molecule is summarized and investigated with electronic-structure calculations. Examination of the intrinsic reaction coordinate associated with the 1,1-HF elimination reaction found an adduct between CF(F)C: and HF in the exit channel with a dissociation energy of ∼5 kcal mol. Hydrogen-bonded complexes between HF and the H atom migration transition state of CH(F)C: and the F atom migration transition state of CF(F)C: also were found by the calculations. The role that these carbene-HF complexes could play in 1,1-HF elimination reactions is discussed.

Experimental and Computational Studies of Unimolecular 1,1-HX (X = F, Cl) Elimination Reactions of C2D5CHFCl: Role of Carbene:HF and HCl Adducts in the Exit Channel of RCHFCl and RCHCl2 Reactions

“…The lower frequencies of 1,1-HX transition states relative to 2,1-DX transition states has been discussed previously. [4][5][6][7][8]10 The statistical rate constant at energy E was obtained in the usual way from eq 6:…”

“…The current study of C 2 D 5 CHFCl provides additional examples of carbene:HX adducts. Low vibrational frequencies of the transition state enable 1,1-HX elimination reactions to compete with traditional 2,1-HX elimination reactions at high temperatures or high vibrational energies. − For most chemical activation systems, the carbene product has sufficient energy to isomerize to an olefin. Several examples of 1,1-HF elimination reactions have been successfully characterized by experimental and computational studies. − According to computational results, , hydrogen-bonded carbene:HF adducts with dissociation energies between 6 and 10 kcal mol –1 exist in the exit channels.…”

“…Low vibrational frequencies of the transition state enable 1,1-HX elimination reactions to compete with traditional 2,1-HX elimination reactions at high temperatures or high vibrational energies. − For most chemical activation systems, the carbene product has sufficient energy to isomerize to an olefin. Several examples of 1,1-HF elimination reactions have been successfully characterized by experimental and computational studies. − According to computational results, , hydrogen-bonded carbene:HF adducts with dissociation energies between 6 and 10 kcal mol –1 exist in the exit channels. Calculated threshold energies for 1,1-HF elimination are a few kcal mol –1 higher than the enthalpy of reaction, and those calculated values are in accord with threshold energies derived from comparing experimental rate constants with statistical rate constants obtained from models of transition states.…”

Unimolecular HBr and HF Elimination Reactions of Vibrationally Excited C2H5CH2Br and C2D5CHFBr: Identification of the 1,1-HBr Elimination Reaction from C2D5CHFBr and Search for the C2D5(F)C:HBr Adduct

“…The results for 2,1-HBr, 2,1-HF, and 1,1-HBr reactions are summarized in Table . Threshold energies for 1,1-HF and 2,1-HF reactions of fluoroethane molecules (see ref ) also increase with additional F atoms, and HF elimination from the bromofluoroethane molecules in Table remains as a minor component. Threshold energies for 1,1-HBr elimination increases by about 5 kcal mol –1 for each F atom, but E 0 (2,1-HBr) increases by about 10 kcal mol –1 with each F atom.…”

“…Passing the transition-state barrier should be rate-limiting, and eq should apply to 1,1-HF elimination, that is, inner and outer transition states need not be considered. The 1,1-HF elimination reaction from several molecules has been discussed extensively, − and it does not merit further attention, except as a contrast to the exit channel description for the 1,1-HBr reaction. The MO6-2X (74 kcal mol –1 )- and MP2 (76 kcal mol –1 )-calculated E 0 (1,1-HF) values are about 5 kcal mol –1 too high.…”