Unimolecular Elimination of HF and HCl from Chemically Activated CF ₃CFClCH ₂Cl

Abstract

The unimolecular reactions of CF3CFClCH2Cl molecules formed with 87 kcal mol(-1) of vibrational energy by recombination of CF3CFCl and CH2Cl radicals at room temperature have been characterized by the chemical activation technique. The 2,3-ClH and 2,3-FH elimination reactions, which have rate constants of (2.5 +/- 0.8) x 10(4) and (0.38 +/- 0.11) x 10(4) s(-1), respectively, are the major reactions. The 2,3-FCl interchange reaction was not observed. The trans (or E)-isomers of CF3CFCHCl and CF3CClCHCl are favored over the cis (or Z)-isomers. Density functional theory at the B3PW91/6-31G(d',p') level was used to evaluate thermochemistry and structures of the molecule and transition states. This information was used to calculate statistical rate constants. Matching the calculated to the experimental rate constants for the trans-isomers gave threshold energies of 62 and 63 kcal mol(-1) for HCl and HF elimination, respectively. The threshold energy for FCl interchange must be 3-4 kcal mol(-1) higher than for HF elimination. The results for CF3CFClCH2Cl are compared to those from CF3CFClCH3; the remarkable reduction in rate constants for HCl and HF elimination upon substitution of one Cl atom for one H atom is a consequence of both a lower E and higher threshold energies for CF3CFClCH2Cl.

Isomerisation of CF2ClCH2Cl and CFCl2CH2F by Interchange of Cl and F Atoms with Analysis of the Unimolecular Reactions of Both Molecules

“…Given the uncertainties in k expt , < E >, and the assumptions in calculating k E, the E 0 values have a 8-13 kJ mol À1 absolute uncertainty. However, the relative E 0 values for processes in the same molecule should be [22][23][24][25] The present work shows that this generalization also extends to the CFCl 2 group. The small rate constant for HCl elimination from CFCl 2 CH 2 F requires an E 0 (HCl) that is 17-21 kJ mol À1 higher than for CF 2 ClCH 2 Cl.…”

“…The statistical, RRKM, rate constants were calculated with the same methodology as used in previous papers in this series. [20][21][22][23] Structures of the molecules and transition states from the B3PW91 method were obtained with the 6-31G(d',p') basis set using the Gaussian [41] code. Some calculations also were done with the 6-311 + G(2d,p) basis set for confirmation of results.…”

“…The transition states for Cl/F interchange, HF elimination and HCl elimination were located and tested in the usual ways. [7,[20][21][22][23] The vibrational frequencies and moments of inertia are input parameters for computation of the rate constants as a function of E 0 and total energy, E. Therefore, the frequencies and moments, including the reduced moment for internal rotation, were averaged over the three conformers. The difference in energies of the conformers is small, just 4.2 and 0.7 kJ mol À1 for CF 2 ClCH 2 Cl and CFCl 2 CH 2 F, respectively.…”

“…assignment of threshold energies to the various unimolecular reactions of CF 2 ClCH 2 Cl and CFCl 2 CH 2 F. The methodology is the same as that employed in similar chemical activation studies from this laboratory. [9,[20][21][22][23] Density functional theory was used to develop models for the transition states and molecules. These models are then used to calculate statistical rate constants for a specific vibrational energy.…”

Studies of 1,2-Dihalo Shifts in Carbon–Carbon, Carbon–Silicon, and Silicon–Silicon Systems: A Computational Study

“…4 from F 2 C[F,Cl]CH 2 , undergoes elimination, leading to CF 2  CHCl as well. Similar transition-state structures and energetics have been found for other systems by Holmes and co-workers as well as others; these include not only doubly bridged species with halogen, 4,6,7,[9][10][11][12][13]15 but ones with chlorine and methyl. 5,8 Turning to examples containing silicon, there also are cases of dihalogen exchange across a carbon−silicon single bond.…”