PDF(Pubmed)
Abstract：
One of the most investigated properties of porous crystalline metal-organic frameworks (MOFs) is their potential flexibility to undergo large changes in unit cell size upon guest adsorption or other stimuli, referred to as \"breathing\". Computationally, such phase transitions are usually investigated using periodic boundary conditions, where the system\'s volume can be controlled directly. However, we have recently shown that important aspects like the formation of a moving interface between the open and the closed pore form or the free energy barrier of the first-order phase transition and its size effects can best be investigated using non-periodic nanocrystallite (NC) models [Keupp et al. (Adv. Theory Simul., 2019, 2, 1900117)]. In this case, the application of pressure is not straightforward, and a distance constraint was used to mimic a mechanical strain enforcing the reaction coordinate. In contrast to this prior work, a mediating particle bath is used here to exert an isotropic hydrostatic pressure on the MOF nanocrystallites. The approach is inspired by the mercury nanoporosimetry used to compress flexible MOF powders. For such a mediating medium, parameters are presented that require a reasonable additional numerical effort and avoid unwanted diffusion of bath particles into the MOF pores. As a proof-of-concept, NCs of pillared-layer MOFs with different linkers and sizes are studied concerning their response to external pressure exerted by the bath. By this approach, an isotropic pressure on the NC can be applied in analogy to corresponding periodic simulations, without any bias for a specific mechanism. This allows a more realistic investigation of the breathing phase transformation of a MOF NC and further bridges the gap between experiment and simulation.
摘要：
多孔晶体金属有机骨架（MOFs）研究最多的特性之一是其潜在的灵活性，可以在客体吸附或其他刺激下发生晶胞大小的巨大变化，称为“呼吸”。计算上,这种相变通常使用周期性边界条件进行研究，可以直接控制系统的音量。然而,我们最近表明，重要的方面，如开放和封闭的孔隙形式之间的移动界面的形成或一级相变的自由能垒及其尺寸效应可以最好地研究使用非周期性纳米晶体（NC）模型[Keupp等。（Advant.理论模拟。,2019年，2,1900117)]。在这种情况下,施加压力并不简单，并且使用距离约束来模拟执行反应坐标的机械应变。与之前的工作相比，这里使用中介粒子浴对MOF纳米微晶施加各向同性的静水压力。该方法受到用于压缩柔性MOF粉末的汞纳米粒度测定法的启发。对于这样的媒介，提出的参数需要合理的额外数值努力，并避免浴液颗粒不必要的扩散到MOF孔中。作为一个概念证明,研究了具有不同接头和大小的柱状层MOF的NC，涉及它们对浴施加的外部压力的响应。通过这种方法，NC上的各向同性压力可以类似于相应的周期性模拟，对特定机制没有任何偏见。这允许对MOFNC的呼吸相位转换进行更现实的研究，并进一步弥合了实验和仿真之间的差距。
