| Budget Amount *help |
¥52,500,000 (Direct Cost: ¥52,500,000)
Fiscal Year 2000: ¥5,600,000 (Direct Cost: ¥5,600,000)
Fiscal Year 1999: ¥38,900,000 (Direct Cost: ¥38,900,000)
Fiscal Year 1998: ¥8,000,000 (Direct Cost: ¥8,000,000)
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| Research Abstract |
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 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 (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
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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 Ice
The 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 Freezing
Upon 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
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