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

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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