Fakultät für Physik and Center for Computational Materials Science, Universität Wien, Sensengasse, Wien 1090, Austria.
J Phys Condens Matter. 2010 Sep 29;22(38):384201. doi: 10.1088/0953-8984/22/38/384201. Epub 2010 Sep 7.
The standard approach to ab initio simulations of activated chemical processes is based on the harmonic-oscillator/rigid-rotor approximation to transition state theory. However, there is increasing evidence that these approximations fail for reactions involving loosely bound reactant and/or transitions states where entropy makes a significant contribution to the free-energy reaction barrier. Examples are provided by the conversion (proton exchange, dehydrogenation, monomolecular cracking) of short alkanes over acidic zeolites. For proton exchange and monomolecular cracking the reaction path may be described reasonably well by simple vectorial reaction coordinates and the free energy of activation may be derived by free-energy integration schemes such as the Blue-Moon ensemble technique in combination with constrained ab initio molecular dynamics simulations. For alkane dehydrogenation, however, several reaction scenarios are in competition and techniques such as transition-path sampling must be used to determine the dominant reaction mechanism. In our paper we describe the fundamental aspects of these techniques and discuss their application to compute free-energy barriers for proton exchange between isobutane and acidic chabazite and for monomolecular cracking of propane. Dehydrogenation of propane has been studied using transition-path sampling. In this case the static approach based on harmonic transition state theory not only fails in producing accurate reaction barriers but even leads to incorrect predictions of reaction intermediates and products.
从头算模拟的标准方法是基于过渡态理论的简谐振子/刚性转子近似。然而，越来越多的证据表明，这些近似在涉及松散结合的反应物和/或熵对自由能反应势垒有重大贡献的过渡态的反应中失效。酸性沸石上短链烷烃的转化（质子交换、脱氢、单分子裂化）就是很好的例子。对于质子交换和单分子裂化，反应途径可以用简单的矢量反应坐标来很好地描述，并且可以通过自由能积分方案（如 Blue-Moon 集合技术与约束从头算分子动力学模拟相结合）来推导激活自由能。然而，对于烷烃脱氢，有几个反应场景是竞争的，必须使用过渡态抽样技术等方法来确定主导反应机制。在我们的论文中，我们描述了这些技术的基本方面，并讨论了它们在计算异丁烷和酸性 chabazite 之间质子交换以及丙烷单分子裂化的自由能势垒方面的应用。丙烷脱氢使用过渡态抽样进行了研究。在这种情况下，基于简谐过渡态理论的静态方法不仅不能产生准确的反应势垒，甚至会导致对反应中间体和产物的错误预测。
