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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Threshold Energies and Unimolecular Rate Constants for Elimination of HF from Chemically Activated CF_3CH_2CH_3 and CF_3CH_2CF_3: Effect of CH_3 and CF_3 Substituents at the β-Carbon and Implications about the Transition State
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Threshold Energies and Unimolecular Rate Constants for Elimination of HF from Chemically Activated CF_3CH_2CH_3 and CF_3CH_2CF_3: Effect of CH_3 and CF_3 Substituents at the β-Carbon and Implications about the Transition State

机译：消除化学活化的CF_3CH_2CH_3和CF_3CH_2CF_3中的HF的阈值能量和单分子速率常数：CH_3和CF_3取代基在β-碳上的作用及其对过渡态的影响

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Chemically activated CF_3CH_2CF_3 was prepared with 104 kcal/mol of internal energy by the combination of CF_3CH_2 and CF_3 radicals, and chemically activated CF_3CH_2CH_3 was prepared with 101 and 95 kcal/mol by combination of CF_3 and CH_2CH_3 radicals and by combination of CF_3CH_2 and CH_3 radicals, respectively. The experimental rate constants for unimolecular 1, 2-dehydrofluorination were 1.2 * 10~5 s~(-1) for CF-3CH_2CF_3 and 3.2 * 10~6 s~(-1) for CF_3CH_2CH_3 with 95 kcal/mol and 2.0 * 10~7 s~(-1) with 101 kcal/mol of energy. Fitting the calculated rate constants for HF elimination from RRKM theory to the experimental values provided threshold energies, E_0, of 73 kcal/mol for CF_3CH_2CF-3 and 62 kcal/mol for CF_3CH_2CH_3. Comparing these threshold energies to those for CF-3CH_3 and CF_3CH_2Cl illustrates that replacing the hydrogen of CF_3CH_3 with CH)3 lowers the E_0 by 6 kcal/mol and replacing with CF_3 or Cl raises the E_0 by 5 and 8 kcal/mol, respectively. The CF-3 substituent, an electron acceptor, increases the E_0 an amount similar to Cl, suggesting that chlorine substituents also prefer to withdraw electron density from the β-carbon. As the HF transition state forms, it appears that electron density flows from the departing hydrogen to the β-carbon and from the β to the α-carbon, to the α-carbon from its substituents, but the α-carbon releases most of the incoming electron density to the departing fluorine. The present work supports this scenario because electron-donating substituents, such as CH_3, on either carbon would reduced the E_0 as they aid the flow of negative charge, while electron-withdrawing substituents such as Cl, F, and CF_3 would raise the E_0 for HF elimination because they hinder the flow of electron density.
机译：通过CF_3CH_2和CF_3自由基的组合，以104 kcal / mol的内部能量制备化学活化的CF_3CH_2CF_3，并通过CF_3和CH_2CH_3自由基的组合以及CF_3CH_2和CH_3自由基的组合，以101和95 kcal / mol的化学活化CF_3CH_2CH_3制备， 分别。 CF-3CH_2CF_3的单分子1，2-脱氟化氢的实验速率常数为1.2 * 10〜5 s〜（-1），95 kcal / mol和2.0 * 10的CF_3CH_2CH_3的实验速率常数为3.2 * 10〜6 s〜（-1）。能量为101 kcal / mol时〜7 s〜（-1）。将根据RRKM理论计算出的用于消除HF的速率常数与实验值进行拟合，可以为CF_3CH_2CF-3提供73 kcal / mol的阈值能量E_0，为CF_3CH_2CH_3提供62 kcal / mol的阈值能量。将这些阈值能量与CF-3CH_3和CF_3CH_2Cl的阈值能量进行比较表明，用CH）3替换CF_3CH_3的氢会使E_0降低6 kcal / mol，而用CF_3或Cl替代则使E_0分别提高5 kcal / mol和8 kcal / mol。 CF-3取代基（一种电子受体）使E_0的含量类似于Cl，表明氯取代基也更喜欢从β-碳中吸取电子密度。当形成HF过渡态时，似乎电子密度从离开的氢流向β-碳，从β-流向α-碳，从其取代基流向α-碳，但α-碳释放出大部分进入的氟的电子密度。当前的工作支持这种情况，因为任一碳上的给电子取代基（例如CH_3）将减少E_0，因为它们有助于负电荷的流动，而吸电子取代基（例如Cl，F和CF_3）会提高E_0对消除HF是因为它们阻碍了电子密度的流动。

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