Unimolecular Rate Constant and Threshold Energy for the HF Elimination from Chemically Activated CF₃CHFCF₃

Abstract

Combination of CF(3)CHF and CF(3) radicals at room temperature generated chemically activated CF(3)CHFCF(3) molecules with 95 +/- 3 kcal/mol of internal energy that decompose by loss of HF, initially attached to adjacent carbons, with an experimental unimolecular rate constant of (4.5 +/- 1.1) x 10(2) s(-1). Density functional theory was used to model the unimolecular rate constant for HF elimination, k(HF), to determine a threshold energy of 75 +/- 2 kcal/mol.

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

“…Higher-level treatments seem to be required for satisfactory analysis of the whole fluoroethane series by a single computational method. The threshold energy for 1,2-H atom transfer is so high that the reaction is only competitive for C 2 F 5 H. It is not even important for CF 3 CHFCF 3 . It is of interest to note that replacing an F atom by a CF 3 -group lowers E 0 (2,1-HF) of CF 3 CHFCF 3 to 75 kcal mol –1 .…”

“…The threshold energy for 1,2-H atom transfer is so high that the reaction is only competitive for C 2 F 5 H. It is not even important for CF 3 CHFCF 3 . 48 It is of interest to note 48 that replacing an F atom by a CF 3 -group lowers E 0 (2,1-HF) of CF 3 CHFCF 3 to 75 kcal mol −1 . There is a strong correlation of increased threshold energies with the reduced dissociation energies of the product fluoroethenes.…”

Reinvestigation of the Unimolecular Reactions of CHF2CHF2: Identification of the 1,1-HF Elimination Component from Addition of :CFCHF2 to trans-2-Butene

“…This competition exists because the low vibrational frequencies of the 1,1-HF transition state compensate for the higher threshold energy for the 1,1-HF process. This competition has been experimentally documented from chemical activation studies of CD 3 CHF 2 , C 2 D 5 CHF 2 , CD 3 CHFCl, and CD 2 FCHF 2 . For all of these cases, the 1,1-HF channel was identified from the fluoropropene or fluoroethene corresponding to the D atom migration in the carbene following 1,1-HF elimination.…”

“…This competition has been experimentally documented from chemical activation studies of CD 3 CHF 2 , 5 C 2 D 5 CHF 2 , 5 CD 3 CHFCl, 6 and CD 2 FCHF 2 . 3 For all of these cases, the 1,1-HF channel was identified from the fluoropropene or fluoroethene corresponding to the D atom migration in the carbene following 1,1-HF elimination. The 1,1-HF component from CHF 2 CHF 2 and CF 3 CHF 2 has not been identified, because the deuterium labeling technique cannot be utilized.…”

“…Numerous thermal and chemical activation − experiments have demonstrated that the threshold energy for 1,2-HF elimination from fluorinated alkanes increases with the number of fluorine atoms attached to the two carbon atoms involved in the four-membered ring of the transition state. If two F atoms are located on the terminal carbon atom of the fluoroalkane, 1,1-HF elimination via a three-membered ring can become competitive with 1,2-HF elimination for high-temperature or high-vibrational energies.…”