Computational elucidation of the transition state shape selectivity phenomenon
Computational elucidation of the transition state shape selectivity phenomenon
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DOI：
10.1021/ja0381712
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发表时间：
2004-01-28
影响因子：
15
通讯作者：
Sauer, J
中科院分区：
文献类型：
--
作者：
Clark, LA;Sierka, M;Sauer, J
最常被引用的过渡态形状选择性反应的例子，沸石中的间二甲苯歧化，被检查以确定反应的局部空间环境是否可以显著改变选择性。在研究的反应中，经zpe校正的甲氧基途径的限速能垒为136 kJ/mol，二苯基甲烷途径的限速能垒为109 ~ 145 kJ/mol。这两种途径都可能导致选择性，不利于一种产物异构体（1,3,5-trim乙苯），但对其他两种异构体的相对选择性随孔隙几何形状、机理途径和熵效应的包含而变化。最重要的是，在三种不同的常见沸石框架类型（FAU、MFI和MOR）中研究一条途径，可以明确地、面向实际地考虑孔隙形状。因此，在临界过渡态环境形状的变化会影响反应的过程。势垒高度可以达到10-20 kJ/mol的量级。观察到的选择性与这里计算的过渡态特征不一致，因此，很可能是由于产物形状的选择性。对这些途径的进一步研究强调了不产生同分异构体定义键的机制步骤的重要性，并导致了对过渡态形状选择性的更有力的定义。
The most commonly cited example of a transition state shape selective reaction, m-xylene disproportionation in zeolites, is examined to determine if the local spatial environment of a reaction can significantly alter selectivity. In the studied reaction, ZPE-corrected rate limiting energy barriers are 136 kJ/mol for the methoxide-mediated pathway and 109 to 145 kJ/mol for the diphenylmethane-mediated pathway. Both pathways, are likely to contribute to selectivity and disfavor one product isomer (1,3,5-trim ethyl benzene), but relative selectivity to the other two isomers varies with pore geometry, mechanistic pathway, and inclusion of entropic effects. Most importantly, study of one pathway in three different common zeolite framework types (FAU, MFI, and MOR) allows explicit and practically oriented consideration of pore shape. Variation of the environment shape at the critical transition states is thus shown to affect the course of reaction. Barrier height shifts on the order of 10-20 kJ/mol are achievable. Observed selectivities do not agree with the transition state characteristics calculated here and, hence, are most likely due to product shape selectivity. Further examination of the pathways highlights the importance of mechanistic steps that do not result in isomer-defining bonds and leads to a more robust definition of transition state shape selectivity.