Department of Chemical Engineering, Kyoto University, Katsura, Nishikyo, Kyoto 615-8510, Japan.
J Chem Phys. 2013 Feb 7;138(5):054708. doi: 10.1063/1.4789810.
We conduct grand canonical Monte Carlo simulations and a free-energy analysis for a simplified model of a stacked-layer porous coordination polymer to understand the gate phenomenon, which is a structural transition of a host framework induced by the adsorption of guest particles. Our calculations demonstrate that stabilization of the system due to the guest adsorption causes host deformation under thermodynamic equilibrium. We also investigate spontaneous transition behaviors (gate opening and closing under metastable conditions). The structural transition should occur when the required activation energy, which is determined using the free-energy analysis, becomes equal to the system energy fluctuation. To estimate the system energy fluctuation, we construct a kinetic transition model based on the transition state theory. In this model, the system energy fluctuation can be calculated by setting the adsorption time and transition domain size of the host framework. The model demonstrates that a smaller domain size results in a gate-opening transition at lower pressure. Furthermore, we reveal that the slope of the logarithm of the equilibrium structural transition pressure versus reciprocal temperature shows transition enthalpy, and that slopes of the gate-opening and -closing transition pressures versus reciprocal temperature show activation enthalpies.
我们对堆叠层多孔配位聚合物的简化模型进行了巨正则蒙特卡罗模拟和自由能分析，以了解门控现象，即客体粒子吸附引起的主体框架的结构转变。我们的计算表明，由于客体吸附导致系统稳定化，在热力学平衡下引起主体变形。我们还研究了自发转变行为（亚稳条件下的门控开启和关闭）。当所需的激活能（通过自由能分析确定）等于系统能量涨落时，结构转变应该发生。为了估计系统能量涨落，我们基于过渡态理论构建了一个动力学转变模型。在该模型中，通过设置主体框架的吸附时间和转变域大小，可以计算系统能量涨落。该模型表明，较小的域大小导致在较低压力下发生开门转变。此外，我们揭示了平衡结构转变压力的对数与倒数温度的斜率显示转变焓，而开门和关门转变压力的斜率与倒数温度的斜率显示激活焓。
