[推荐]分子聚集体的计算机模拟
h,d,F m%?【作者】郭森立 D0M Ws(@7I T1i
【学号】19910041 !R$f2pG Ra
【毕业学校】北京大学
【答辩时间】2002.05 8W t{*p Wz?
【培养单位】化学与分子工程学院
【学位名称】理学硕士
【中文题名】分子聚集体的计算机模拟 B:^ NX$r+~O
【外文题名】Computational simulation of molecular aggregation
【专业】物理化学
【导师】徐筱杰
【导师单位】化学与分子工程学院
【关键词/主题词】介观模型方法;介观动力学;电荷转移盐;量子力学;嵌段共聚物 ;_x2Ti0WZ-pe*u
【中文摘要】用计算机模拟方法来处理化学问题已经发展了好几十年，随着计算机计算能力的提高计算机模拟在化学研究中的地位显得越来越重要，已经形成一个独立的化学分支—计算化学（Computational Chemistry）。经典的计算机模拟方法主要包括基于电子轨道或电子分布的量子力学方法和基于分子模型的分子力学方法。随着研究体系变得越来越复杂，近年来也出现了一些粗粒度的（Coarse Grained）模拟方法，如介观动力学(Mesodyn)和耗散颗粒动力学(Dissipative Particle Dynamics)。这类方法对解决介观层次（体系的特征尺度为10-1000nm）上的化学问题有较好的效果。本论文的主要内容是用不同的计算机模拟方法对聚集体体系进行了研究，具体包括以下两个方面：1．用介观动力学模拟Pluronic L64/水/p-Xylene体系我的工作是在原有李有勇同学的工作基础上，用介观动力学在介观领域里研究了(EO)_(13)(PO)_(30)(EO)_(13) (Pluronic L64)/水/p-xylene 体系的相行为。在模拟中用“等效链”方法参数化高分子链。通过对比不同溶剂体系的介观形貌讨论了水在体系胶团形成中的作用。结算显示Pluronic L64 在纯p-Xylene或是存在少量水的溶剂中不生成聚合物分子胶团，但是随着体系中水浓度的增加，生成了不同形状的聚合物分子胶团。通过分析1000模拟步骤后体系中水的分布证实了在胶团核中确实存在自由水。我们还研究了温度对体系相行为的影响和PEO，PPO嵌段长度对体系相行为的影响。模拟结果表明，形成反胶团的过程是一个放热过程，随着温度的升高，反胶团形成变得越来越困难，形成的速度也变得越来越慢。PEO嵌段长度比PPO嵌段长度对反胶团的形成有更大的影响。2. 一个新BEDT-TTF电荷转移盐的晶体结构预测用分子力学方法预测了一个新的电荷转移盐(ET)_2FeCl_4的晶体结构。 用量子力学方法中的密度泛函理论(DFT)首次进行(ET)_2FeCl_4系列电荷转移盐的单晶电子能量计算，通过对比相近晶体的晶体结构和电子能量解释了计算所得晶体结构的合理性。
【外文摘要】Using computational simulation methods to solve the chemical problems have been developed for several decades, and the function of computational simulation in Chemistry is becoming more and more significant along with increasing computational power of Computers，then now it is developed as an independent branch of chemistry, named Computational Chemistry. Traditional methods of computational simulation mainly include the quantum mechanics, which is based on the electron orbital or the electron distribution, and the molecular mechanics, which is based on the model of the molecular. With increasing complexity of system studied, recently, several coarse grained simulation methods were developed, such as Mesodyn, Dissipative Particle Dynamics. These kinds of methods got good results in solving the chemical problems in mesoscale, whose characteristic length is 10-1000nm.
The main work of the thesis is to study complex system using several computational simulation methods and the details of the thesis mainly be divided into two parts: Z r }1\ Kk3|#|Jk;q
1. Simulation of the Phase Separation of the Pluronic L64/Water/p-Xylene System Using MesoDyn. MH!oi1IT Z8o3^"T
The basic idea in the Mesodyn method is the density functional theory. It is based on the idea that the free energy F of an inhomogeneous liquid is a functional of the local density function ρ. From the free energy, all thermodynamic functions can be derived, so that for instance phase transitions can be investigated as a functional of the density distribution in the system. It is largely used to simulate the process of micelle formation and the manufacturing procedure of polymer materials.
Pluronic are the trade names of block copolymers of (PEO)x-(PPO)y-(PEO)x family, where PEO is poly(ethylene oxide) and PPO is poly(propylene oxide). As one type of nonionic surfactant, they were widely used in detergency, foam formation, dispersion stabilization, lubrication and drug delivery, and has received wide attention in the literature. 7W(o6v J5g.Sh z
Following the previous work of Li Youyong, the phase behavior of the (EO)13(PO)30(EO)13 (Pluronic L64)/water/p-xylene system in mesoscopic region was investigated using MesoDyn. The ‘equivalent chain’ method was used to perform the parameterization of the Gaussian chain. This paper compared the mesoscopic morphology formations of systems at different concentrations of solvents and discussed the effect of water on the formation of micelles. The calculated results show that Pluronic L64 does not form polymolecular micelles in pure p-xylene or in the presence of a small amount of water, but with the increase of the concentration of water, polymolecular micelles in different shapes were formed. By analyzing the water distributions of the systems after 1000 simulation steps, we proved that free water did exist in micellar core.
The effects of temperature and PEO and PPO block sizes are also investigated. The results indicate that the formation of reverse micelles is exothermic, and so the formation of reverse micelles becomes more difficult and the rate becomes slower with increasing temperature. Moreover, PEO block size has a stronger effect on the formation of reverse micelles compared with PPO block size. *n {2K"C9h z4| D1N
2. Crystal Structure Prediction of A New BEDT-TTF Charge Transfer Salt A U;zM]V7i
Since Parkin and his co-worker first discovered superconductivity of BEDT-TTF[bis(et-hylenedithio)tetrathiafulvalene] charge transfer salts (also called ET salt), plenty of ET charge transfer salts have been synthesized in an attempt to discover new superconducting materials and to understand the mechanisms which lead to superconductivity in these salts. 4@D7q A ~Q_6r,J1x2s
Quantum mechanics and molecular mechanics are two basic simulation methods of computational chemistry. Theories of these two methods are introduced and their advantages and disadvantages are compared each other.
The crystal structure of a new charge transfer salt， (ET)2FeCl4 has been predicted by molecular mechanics. Unit crystal electron energies of charge transfer salts of (ET)2FeCl4 family are first calculated using density functional Theory (DFT). Rationality of the crystal structure is explained by comparing it with other two similar charge transfer salts in crystal structure and electron energy.
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