环烷烃开环反应动力学的CBS-QB3计算
Detailed kinetic study of the ring opening of cycloalkanes by CBS-QB3 calculations
摘要：
用量子化学方法研究了环丁烷、环戊烷和环己烷的气相单分子分解反应。一个双自由基的机制已设想为每个环烷烃，和形成的双自由基的分解的主要途径进行了研究，在CBS-QB 3理论水平。通过等键反应获得了所有相关物种的热化学数据（δH^0_f，S^0，C^0_p）。还包括了受阻转子的贡献。每个反应的活化势垒进行了分析，以评估1积极最有利的途径分解的biradegates。用过渡态理论导出了在600 ~ 2000 K温度范围内、1个大气压下所有基元反应的速率常数。计算了分子产物中环烷烃的整体分解速率常数。计算和实验结果之间的比较允许验证理论方法。一个重要的结果是，通常被忽略的构象之间的旋转障碍，是最大的双自由基的分解速率的重要性。估算了开环反应过渡态的环应变能，结果表明，在活化过程中，环应变能的主要部分被除去。
英文摘要：
This work reports a theoretical study of the gas phase unimolecular decomposition of cyclobutane, cyclopentane and cyclohexane by means of quantum chemical calculations. A biradical mechanism has been envisaged for each cycloalkane, and the main routes for the decomposition of the biradicals formed have been investigated at the CBS-QB3 level of theory. Thermochemical data (δH^0_f, S^0, C^0_p) for all the involved species have been obtained by means of isodesmic reactions. The contribution of hindered rotors has also been included. Activation barriers of each reaction have been analyzed to assess the 1 energetically most favorable pathways for the decomposition of biradicals. Rate constants have been derived for all elementary reactions using transition state theory at 1 atm and temperatures ranging from 600 to 2000 K. Global rate constant for the decomposition of the cyclic alkanes in molecular products have been calculated. Comparison between calculated and experimental results allowed to validate the theoretical approach. An important result is that the rotational barriers between the conformers, which are usually neglected, are of importance in decomposition rate of the largest biradicals. Ring strain energies (RSE) in transition states for ring opening have been estimated and show that the main part of RSE contained in the cyclic reactants is removed upon the activation process.
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