Characterization of the 1,1-HF Elimination Reaction from the Competition between the 1,1-HF and 1,2-DF Unimolecular Elimination Reactions of CD₃CD₂CHF₂

Abstract

The recombination of CHF2 and C2D5 radicals was used to produce CD3CD2CHF2* molecules with 96 kcal mol(-1) of vibrational energy in a room temperature bath gas. The formation of CD3CD═CHF and CD3CD═CDF was used to identify the 1,2-DF and 1,1-HF unimolecular elimination channels; CD3CD═CDF is formed by isomerization of the singlet-state CD3CD2CF carbene. The total unimolecular rate constant is 1.6 × 10(6) s(-1), and the branching ratio for 1,1-HF elimination is 0.25. Threshold energies of 64 ± 2 and 73 ± 2 kcal mol(-1) were assigned to the 1,2-DF and 1,1-HF reaction channels. The E and Z isomers of 1-fluoropropene were observed for each reaction; approximately 30% of the CD3CD═CDF molecules derived from 1,1-HF elimination retained enough energy to undergo cis-trans isomerization. Electronic structure calculations with density-functional theory were used to characterize the transition-state structures and the H atom migration barrier for CD3CD2CF. Adjustment of the rate constants to account for kinetic-isotope effects suggest that the branching ratio would be 0.20 for 1,1-HF elimination from C2H5CHF2. The results from an earlier study of CD3CHF2 and CH3CHF2 are also reinterpreted to assign a threshold energy of 74 kcal mol(-1) for the 1,1-HF elimination reaction. Because CHF2CHF2* is generated in the photolysis system, the 1,1-and 1,2-HF-elimination reactions of CHF2CHF2* are discussed. The 1,1-HF channel was identified by trapping the CF2HCF carbene with cis-butene-2.

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 rate constants from C 2 D 5 CHFCl are 1.4-times larger for DCl + HCl relative to C 2 D 5 CHCl 2 10 and 2.1-times smaller for DF + HF reactions relative to C 2 D 5 CHF 2. 4 The difference is mainly a consequence of the difference in <E>, 94 versus 88 kcal mol −1 and 94 versus 96 kcal mol −1 for the three molecules, and the two-fold larger reaction path degeneracy for dichloro-and difluoropropanes. All energies in the table are in kcal mol −1 and all include zero-point energy.…”

“…The details needed to calculate k M are given in footnote b . The rate constants ,, for CD 3 CHFCl*, C 2 D 5 CHF 2 , and C 2 D 5 CHCl 2 * are included for comparison to C 2 D 5 CHFCl*. The uncertainty in the rate constants for C 2 D 5 CHFCl* was increased at least to 10% to account for the uncertainty in k M . b Collision rate constants of the bath gases with C 2 D 5 CHFCl* were calculated from the following collision diameters, Å, and ε/k,°K, values: CF 3 CH=CH 2 (4.90 and 250), C 2 D 5 l (5.0 and 394), CHFCl 2 (4.6 and 405), C 2 D 5 CHFCl (5.3 and 370).…”

“…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

“…As previously noted, the E (cis) and Z (trans) isomers are combined so there are only four rate constants. In fact, the cis isomers are always preferred for 1-halopropene products 8,10,12 respectively. It should be noted that 1,3-fluorobromopropane was used instead of 1,1-fluorobromopropane in the calibration of the GCMS; therefore, some systematic error could exist for these rate constants.…”