Unimolecular Reactions in the CF3CH2Cl ↔ CF2ClCH2F System: Isomerization by Interchange of Cl and F Atoms

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 115; no. 6; pp. 1054 - 1062
• Main Authors: Enstice, Erin C, Duncan, Juliana R, Setser, D. W, Holmes, Bert E
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 17-02-2011
• Subjects: A: Kinetics, Spectroscopy
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp108955m

The recombination of CF2Cl and CH2F radicals was used to prepare CF2ClCH2F* molecules with 93 ± 2 kcal mol−1 of vibrational energy in a room temperature bath gas. The observed unimolecular reactions in order of relative importance were: (1) 1,2-ClH elimination to give CF2CHF, (2) isomerization to CF3CH2Cl by the interchange of F and Cl atoms and (3) 1,2-FH elimination to give E- and Z-CFClCHF. Since the isomerization reaction is 12 kcal mol−1 exothermic, the CF3CH2Cl* molecules have 105 kcal mol−1 of internal energy and they can eliminate HF to give CF2CHCl, decompose by rupture of the C−Cl bond, or isomerize back to CF2ClCH2F. These data, which provide experimental rate constants, are combined with previously published results for chemically activated CF3CH2Cl* formed by the recombination of CF3 and CH2Cl radicals to provide a comprehensive view of the CF3CH2Cl* ↔ CF2ClCH2F* unimolecular reaction system. The experimental rate constants are matched to calculated statistical rate constants to assign threshold energies for the observed reactions. The models for the molecules and transition states needed for the rate constant calculations were obtained from electronic structures calculated from density functional theory. The previously proposed explanation for the formation of CF2CHF in thermal and infrared multiphoton excitation studies of CF3CH2Cl, which was 2,2-HCl elimination from CF3CH2Cl followed by migration of the F atom in CF3CH, should be replaced by the Cl/F interchange reaction followed by a conventional 1,2-ClH elimination from CF2ClCH2F. The unimolecular reactions are augmented by free-radical chemistry initiated by reactions of Cl and F atoms in the thermal decomposition of CF3CH2Cl and CF2ClCH2F.