Rate Constants and Kinetic Isotope Effects for Unimolecular 1,2-HX or DX (X = F or Cl) Elimination from Chemically Activated CF3CFClCH3-d 0, -d 1, -d 2, and -d 3

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 110; no. 4; pp. 1506 - 1517
• Main Authors: Zhu, Li, Simmons, J. G, Burgin, M. O, Setser, D. W, Holmes, B. E
• Format: Journal Article
• Language: English
• Published: American Chemical Society 02.02.2006
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp053233r

Chemically activated CF3CFClCH3, CF3CFClCD3, CF3CFClCH2D, and CF3CFClCHD2 molecules with 94 kcal mol-1 of internal energy were formed by the combination of CF3CFCl radicals with CH3, CD3, CH2D, and CHD2 radicals, which were generated from UV photolysis of CF3CFClI and CH3I, CD3I, CH2DI, or CHD2I. The total (HF + HCl) elimination rate constants for CF3CFClCH3 and CF3CFClCD3 were 5.3 × 106 and 1.7 × 106 s-1 with product branching ratios of 8.7 ± 0.6 in favor of HCl (or DCl). The intermolecular kinetic isotope effects were 3.22 and 3.18 for the HCl and HF channels, respectively. The product branching ratios were 10.3 ± 1.9 and 11.8 ± 1.8 (10.8 ± 3.8 and 11.6 ± 1.7) for HCl/HF and DCl/DF, respectively, from CF3CFClCH2D (CF3CFClCHD2). The intramolecular kinetic-isotope effects (without correction for reaction path degeneracy) for HCl/DCl and HF/DF elimination from CF3CFClCH2D (CF3CFClCHD2) were 2.78 ± 0.16 and 2.98 ± 0.12 (0.82 ± 0.04 and 0.91 ± 0.03), respectively. Density function theory at the B3PW91/6-311+G(2d,p) and B3PW91/6-31G(d‘,p‘) levels was investigated, and the latter was chosen to calculate frequencies and moments of inertia for the molecules and transition states. Rate constants, branching ratios and kinetic-isotope effects then were calculated using RRKM theory with torsional motions treated as hindered internal rotations. Threshold energies for HF and HCl elimination from CF3CFClCH3 were assigned as 61.3 ± 1.5 and 58.5 ± 1.5 kcal mol-1, respectively. The threshold energy for Cl−F interchange was estimated as 67 kcal mol-1. The difference between the transition states for HCl and HF elimination is discussed.