Unimolecular Reactions of 1,1,1-Trichloroethane, 1,1,1-Trichloropropane, and 3,3,3-Trifluoro-1,1,1-trichloropropane: Determination of Threshold Energies by Chemical Activation

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• 作者：Martha A. Turpin ; Kylie C. Smith ; George L. Heard ; D. W. Setser ; Bert E. Holmes
• 刊名：Journal of Physical Chemistry A
• 出版年：2014
• 出版时间：October 9, 2014
• 年：2014
• 卷：118
• 期：40
• 页码：9347-9356
• 全文大小：393K
• ISSN：1520-5215

文摘

The recombination of CCl₃ radicals with CH₃, CH₃CH₂, and CF₃CH₂ radicals was used to generate CH₃CCl₃, CH₃CH₂CCl₃, and CF₃CH₂CCl₃ molecules with approximately 87 kcal mol^鈥? of vibrational energy in a bath gas at room temperature. The competition between collisional deactivation and unimolecular reaction by HCl elimination was used to obtain the experimental rate constants for each molecule. These experimental rate constants were matched to calculated statistical unimolecular rate constants to assign threshold energies to the three HCl elimination reactions. The models needed for the calculations of the rate constants were obtained from molecular structure calculations using density functional theory (DFT) with the hybrid density-functional MO6-2X recommended by Truhlar for transition states. The assigned threshold energies are 52 卤 2, 50 卤 2, and 52 卤 2 kcal mol^鈥? for CH₃CCl₃, CH₃CH₂CCl₃, and CF₃CH₂CCl₃, respectively, and the CH₃ and CF₃ groups have only a minor effect on the threshold energies for HCl elimination. The DFT calculated threshold energies are in agreement with the experimentally assigned values. The addition of Cl atoms to the same carbon atom lowers the threshold energy for HCl elimination in the CH₃CH₂Cl, CH₃CHCl₂, and CH₃CCl₃ series. This trend, which is the opposite of that for CH₃CH₂F, CH₃CHF₂, and CH₃CF₃, is discussed in terms of transition-state structure and correlated with the relative stabilities of CH₃CH₂⁺, CH₃CHCl⁺, and CH₃CCl₂⁺ ions; the relative stabilities are based on the hydride affinities obtained from calculations. Comparison of the reactions of CH₃CCl₃ and CH₂ClCHCl₂ shows that the threshold energy is much higher for the isomer with chlorine atoms on both carbon atoms.