Proton Transfer Dynamics in Water and Effect of Water Fluctuation on Chemical Reactions

分析了水动力学的各个方面；(1)液态水的涨落和弛豫及其观测；(2)液态水和冰中的质子转移；(3)超临界水的溶剂化动力学；(4)水的冻结机理。(1)水具有凹凸不平的势能面，包含各种深层能量极小值，具有不同的氢键网络结构。水在这个势能表面上经历缓慢的动力学。因此，在短时间尺度上，液态水是无定形的凝胶状，而在很长的时间尺度上，它作为有序液体表现出扩散运动。在这两个时间尺度之间，氢键网络重排在空间上间歇性地局部发生，涉及数十个水分子的局部集体运动，伴随着较大的能量波动。讨论了观察水中间歇集体运动影响的各种实验技术。特别强调的是高阶非线性光谱，因为这些方法处理系统的相空间动力学。有大量的理论研究提出应用回声型实验，即非共振五阶（二维（2D）拉曼光谱）来区分液体动力学中的均匀和非均匀元素。(2)水中的质子转移受到氢键网络重排（HBNR）的辅助，使得一些水分子三配位。质子转移发生在这三个配位的水分子上。另一方面，质子化的水分子在冰中是四配位的，但它与第四个配位的水分子的相互作用是排斥的（而与前三个配位的水分子是吸引的）；几何上四协调，能量上三协调。由于与第四个水分子的排斥性相互作用，与其他三个水分子的0—(H)—O距离变得很短，因此即使在冰中也可以进行容易的质子转移，而不会引起明显的HBNR。我们还分析了水分子解离的机理。水分子在液态水中解离成H^+和OH（离子通道）。然而，在气相中，已知水会分离成两个自由基，H和OH。这个自由基通道比离子通道稳定约290千卡/摩尔。在水中，离子通道通过水合作用得到广泛的稳定，因此水的产pH = 7。在超临界水中，尽管水分子密度较小，但水合作用比水更强，因此离子通道变得更加稳定。(4)对液态水的冻结机理进行了分子动力学模拟研究。在模拟盒的中心放置一个小的冰结构单元，在其周围形成一个有序的氢键网络的过程被监测。发现在一定的诱导时间后结晶会突然发生。我们进行了各种分析，以找出在这种冻结机制中氢键网络是如何生长的。少

Various aspects of Water Dynamics were analyzed ; (1) Fluctuation and Relaxation in Liquid Water and their Observation, (2) Proton Transfer in Liquid Water and Ice, (3) Solvation Dynamics in Supercritical Water, and (4) Mechanism of Water Freezing.(1) Water has the rugged potential energy surface involving various deep energy minima with different hydrogen bond network structures. Water undergoes the sluggish dynamics on this potential energy surface. In a short time scale liquid water is thus amorphous gel-like, while in a very longer time scale it exhibits diffusional motion as an ordinal liquid. In between these time scales, the hydrogen bond network rearrangement occurs intermittently and locally in space, involving the local collective motions of tens of water molecules accompanied with the large energy fluctuation. The various experimental techniques to observe the effects of intermittent collective motions in water are discussed. Particular emphasis is on the higher-order nonlin … More ear specutroscopies since these methods deal with the phase-space dynamics of a system. There have been intensive theoretical investigations proposing to apply the echo-type experiment, i.e. the off-resonant fifth order (two-dimensional (2D) Raman) spectroscopy to distinguish the homogeneous and the inhomogeneous elements in liquid dynamics.(2) Proton transfer in water is assisted by Hydrogen Bond Network Rearrangement (HBNR), making some water molecules three-coordinated. Proton transfer takes place on these three-coordinated water molecules. On the other hand, the protonated water molecule is four-coordinated in ice, but its interaction with the fourth coordinated water molecule is repulsive (while those with first three are attractive) ; four-coordinated geometrically and three-coordinated energetically. Due to this repulsive interaction with the fourth water molecule, 0-- (H) --O distances with the other three water molecules become short and thus facile proton transfer can take place even in ice without causing a significant HBNR.^2(3) We also analyze the mechanism of the water molecule dissociation. A water molecule dissociates to H^+ and OH (ionic channel) in liquid water. In the gas phase, however, water is known to separate into two radicals, H and OH.This radical channel is about 290 kcal/mol more stable than the ionic channel. In water the ion channel is extensively stabilized by hydration and thus water yields pH = 7. It is also found that in super-critical water the hydration is stronger than in water in spite of the fact that water molecular density is smaller, and hence the ionic channel becomes even more stable.(4) We have investigated the freezing mechanism of liquid water was investigated by using molecular dynamics simulation. Placing a small ice-structured unit in a center of a simulation box, the processes of forming an well-ordered hydrogen bond network around it were monitored. It was found that crystallization suddenly takes place after certain induction-time. Various analyses were made to find how the hydrogen bond network grows in this freezing mechanism. Less