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Theoretical Studies of Fluctuations and Reaction Dynamics in Many-Body Chemical Systems

多体化学系统中波动和反应动力学的理论研究

基本信息

批准号：
10206201
负责人：
OHMINE Iwao
金额：
$ 33.6万
依托单位：
Nagoya University
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research on Priority Areas (B)
财政年份：
1998
资助国家：
日本
起止时间：
1998 至 2000
项目状态：
已结题

项目摘要

Water dynamics and chemical reactions are analyzed.We have been studying the following three subjects of liquid water dynamics ;(1) What is the nature of the global potential energy surface involved in the liquid water dynamics, which characterized by collective motions and long-time relaxation/fluctuations? How can we detect directly these collective motions and relaxations experimentally?(2) How does an excess proton move in water (liquid water and ice)?(3) How does water freeze into a crystalline ice structure?We will present the results for (1)--(3), especially (3).Liquid Water DynamicsVarious relaxations associated with these collective motions in liquid water yield so-called 1/f spectra, which appears in potential energy fluctuation the low frequency profile of Raman signal (associated with the polarization fluctuation), and others. The spatial-temporal nature of the intermittent local collective motions can be detected by using the neutron scattering and X-ray scattering, when t … More hey can measure for the smaller angle and lager wave vector values (i. e. the lower energy and smaller spatial region) than the present ones. One of the methods, which may detect these intermittent collective motions, is a higher nonlinear flash photolysis experiment, since this method deals with the phasspace dynamics of a system. This technique is analogous to the spin-echo experiment but uses photons, and distinguishes the homogeneous and the inhomogeneous elements in liquid dynamics. The problem of applying these higher order nonlinear experiments to water at present is that the signal intensity from water must be very weak, as its polarizability is one order of magnitude smaller than CS2. As the development of this field is very fast, it may become soon possible that we detect these collective motions and their relaxation in water directly.Proton Transfer in IceThe proton transfer in ice is known to be very fast ; its rate is considered to be about half of that in liquid water. But its mechanism must be quite different from the liquid water case. The geometry and the motions of water molecules in ice are confined due to the strong structural constraint from the surrounding water molecules and thus no significant hydrogen bond network rearrangement takes place, but the proton transfer is still very fast in ice. We have investigated the mechanism of the excess-proton transfer in ice by analyzing the potential energy surface, the norrnal modes and the interaction with a defect. It is found that the solvation from water molecules in long-distance shells is essential for the smooth transport of the proton.Water FreezingUpon cooling, water freezes to ice. This familiar phase transition occurs widely in nature, yet unlike the freezing of simple liquids^<4-6>, it has never been successfully simulated on a computer. The difficulty lies with the fact that hydrogen bonding between individual water molecules yields a disordered three-dimensional hydrogen-bond network whose rugged and complex global potential energy surface^<1-3> permits a large number of possible network configurations. As a result, it is very challenging to reproduce the freezing of 'real' water into a solid with a unique crystalline structure. For systems with a limited number of possible disordered hydrogen-bond network structures, such as confined water, it is relatively easy to locate a pathway from a liquid state to a crystalline structure^<7-9>. For pure and spatially unconfined water, however, molecular dynamics simulations of freezing of are severely hampered by the large number of possible network configurations that exit. Here we present a molecular dynamics trajectory that captures the molecular processes involved in the freezing of pure water. We find that ice nucleation occurs once a sufficient number of relatively long-lived hydrogen bonds develop spontaneously at the same location to form a fairly compact initial nucleus. The initial nucleus then slowly changes shape and size until it reaches a stage that allows rapid expansion, resulting in crystallization of the entire system. Less

本文分析了水动力学和化学反应，研究了以下三个问题：（1）以集体运动和长时间弛豫/涨落为特征的液态水动力学所涉及的全球势能面的性质是什么？我们怎样才能在实验上直接探测到这些集体运动和弛豫呢？(2)多余的质子如何在水中移动（液态水和冰）？(3)水是如何冻结成结晶冰结构的？我们将介绍（1）-（3）的结果，特别是（3）.液态水动力学液态水中与这些集体运动有关的各种弛豫产生所谓的1/f谱，它出现在势能涨落、拉曼信号的低频轮廓（与偏振涨落有关）和其他谱中。利用中子散射和X射线散射可以探测到间歇性局部集体运动的时空性质，当 ...更多信息它们可以测量较小的角度和较大的波矢量值（即，e.较低的能量和较小的空间区域）。可以检测这些间歇性集体运动的方法之一是更高的非线性闪光光解实验，因为这种方法涉及系统的相空间动力学。这种技术类似于自旋回波实验，但使用光子，并区分液体动力学中的均匀和非均匀元素。目前将这些高阶非线性实验应用于水的问题是，来自水的信号强度必须非常弱，因为其极化率比CS2小一个数量级。由于这一领域的发展非常迅速，我们可能很快就能直接探测到这些集体运动及其在水中的弛豫。冰中的质子转移众所周知，冰中的质子转移非常快，其速率被认为是液态水中的一半左右。但它的机制肯定与液态水的情况大不相同。由于周围水分子的强结构约束，冰中水分子的几何形状和运动受到限制，因此没有发生显著的氢键网络重排，但质子在冰中的转移仍然非常快。通过对势能面、简正模以及与缺陷相互作用的分析，研究了冰中过剩质子的转移机理。研究发现，长距离壳层中的水分子的溶剂化作用对质子的顺利传输是必不可少的。水的冻结水冷却后会冻结成冰。这种熟悉的相变在自然界中广泛存在，但与简单液体的冻结不同，它从未在计算机上成功模拟过。<4-6>困难在于单个水分子之间的氢键产生无序的三维氢键网络，其崎岖而复杂的全局势能表面允许大量可能的网络构型。<1-3>因此，将"真实的"水冷冻成具有独特晶体结构的固体是非常具有挑战性的。对于具有有限数量的可能的无序氢键网络结构的系统，例如承压水，定位从液态到结晶结构的路径相对容易。<7-9>然而，对于纯净水和空间无压水，冻结的分子动力学模拟严重阻碍了大量可能的网络配置退出。在这里，我们提出了一个分子动力学轨迹，捕捉分子过程中涉及的纯水冻结。我们发现，一旦足够数量的相对长寿命的氢键自发地在同一位置形成一个相当紧凑的初始核，冰成核发生。最初的核然后慢慢地改变形状和大小，直到它达到允许快速膨胀的阶段，导致整个系统的结晶。少