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

分析了水动力学和化学反应。我们一直在研究以下三个液体水动力学主题；（1）液体水动力学涉及的全球势能表面的性质是什么，液态水动力学的特征是集体运动和长期放松/波动？ 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 Dynamics Various 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 （与极化波动相关）等。可以通过使用中子散射和X射线散射来检测间歇性局部集体运动的空间性质，而当T…更多的嘿可以测量比目前的较小的角度和较小的角度波动矢量值（即较低的能量和较小的空间区域）。可能检测到这些间歇性集体运动的方法之一是较高的非线性闪光照射实验，因为该方法涉及系统的相空间动力学。该技术类似于自旋回波实验，但使用Photosons，并区分了将这些高阶非线性实验应用于水的问题是，水的信号强度必须非常弱，因为其极化性是比CS2小的数量级。由于该领域的发展非常快，因此我们很快就有可能发现这些集体运动及其在水中的放松。冰块中的质子转移中的普罗顿转移被认为是液体水中的一半。但是它的机制必须与液态水案例完全不同。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的运动被限制在冰上，因此没有发生明显的氢键网络重排，但是质子转移在冰中仍然非常快。我们已经通过分析势能表面，nornal模式以及与缺陷的相互作用来研究冰中过量普罗氏菌转移的机制。已经发现，从长距离壳中的水分子中的溶液对于质子的平稳运输至关重要。水冷冻冷却，水冻结到冰中。这种熟悉的相转换在自然界中广泛发生，但与简单液体的冻结^<4-6>不同，它从未在计算机上成功模拟。困难的事实是，单个水分子之间的氢键产生了无序的三维氢键网络，其坚固而复杂的全球势能表面^<1-3>允许大量可能的网络配置。结果，将“真实”水的冻结重现为具有独特晶体结构的固体非常挑战。对于具有数量有限的无序氢键网络结构（例如受限水）的系统，相对容易地找到从液态到晶体结构^<7-9>的途径。然而，对于纯净和空间不合格的水，冻结的分子动力学模拟严重阻碍了退出的大量可能的网络配置。在这里，我们提出了一个分子动力学轨迹，该动力学轨迹捕获了与纯水冷冻有关的分子过程。我们发现，一旦有足够数量的相对长的氢键在同一位置发育以形成相当紧凑的初始核，就会发生冰成核。然后，初始核慢慢改变形状和大小，直到达到阶段，从而可以快速扩展，从而导致整个系统结晶。较少的