Unimolecular rate constants, kinetic isotope effects and threshold energies for FH and FD elimination from CF3CHFCH3 and CF3CHFCD3

The combination of CF(3)CHF and CH(3) or CD(3) radicals was used to prepare vibrationally excited CF(3)CHFCH(3) or CF(3)CHFCD(3) molecules with 97 kcal mol(-1) of internal energy. The experimental unimolecular rate constants were 3.7 x 10(6) s(-1) for 2,3-FH elimination from CF(3)CHFCH(3) and 1.3 x 10(6) s(-1) for 2,3-DF elimination from CF(3)CHFCD(3). Unimolecular rate constants for 1,2-FH elimination reaction were approximately 230 and 98 times smaller for CF(3)CHFCH(3) and CF(3)CHFCD(3), respectively, than the corresponding rate constants for 2,3-FH elimination. Density functional theory (DFT) was used to calculate the structures and vibrational frequencies of the molecules and transition states; this information was subsequently employed for calculations of RRKM rate constants. Comparison of the experimental and calculated rate constants gave a threshold energy of 73 +/- 2 kcal mol(-1) for the 1,2-FH elimination process and 60.5 +/- 1.5 kcal mol(-1) for the 2,3-FH elimination reaction from CF(3)CHFCH(3). The calculated kinetic-isotope effects agree with the experimental results. The experimentally derived threshold energies for 1,2-FH and 2,3-FH elimination reactions from several fluoropropanes and fluorochloropropanes are summarized and compared to those from DFT calculations.