Molecular Simulation Studies On The Correlations In The Dynamics Of Water Molecules In Liquid State

Water is essential for life in the sense that it not only serves as the necessary en-vironment for life,but also actively participates in lots of biological processes.Water is abundant and seems common;however,its properties are quite unusual,making its role in life irreplaceable.Many of these unusual properties of water originate from its ability to form extensive,dynamic hydrogen-bond networks which undergoes rear-rangement and reconstruction constantly.A major fashion of this rearrangement is via reorientation of individual water molecules,which is shown to occur mainly through large angular jumps.Jump reorientations are an important feature of water molecules;they also play a remarkable role in,e.g.,ion transportation,protein folding and chem-ical reactions in solution.These sorts of dynamic properties of water molecules have been drawing much attention of researchers,with a number of questions to resolve.In this dissertation,by using molecular dynamics simulations,we study some correla-tion properties in this field,including the correlation of jump reorientations of water molecules,and the correlation between jump reorientations and the translational dif-fusions of water molecules.These detailed investigations of the correlation properties in water dynamics at a molecular level not only contribute to our knowledge of water itself,but also serve as a foundation for further understanding of some biological pro-cesses concerned,especially some concerted phenomena therein.The main points of this dissertation are summarized as follows:(1)Correlation of jump reorientations of water molecules in bulk water.We Char-acterize the temporal and spatial correlation of jump reorientations of water molecules in detail,and find that large-angular jump motions of a water molecule can differently enhance the successive jump motions of the same water molecule and the surrounding water molecules.Such a correlation can extend up to a distance of two water layers,with a correlation length of about 5-7 A,and is propagated through the perturbations to the local hydrogen-bond networks.A detailed molecular picture of the correlation propagation is shown as well,in which the over-coordinated defects in water play an important role.(2)Correlated motion in water:large-amplitude angular jumps contribute to the translational motions of water molecules.We present an exploration of the molecu-lar mechanism of translational diffusion in water,from a viewpoint of reorientation-translation correlation.We observe that jump reorientations enhance the translational motions of the water molecules involved,and such fast motions make a major contri-bution to the translational displacements of water molecules with a minor time occupa-tion.Besides,successive jump reorientations promote water translation more dramat-ically,showing that they are correlated jumps accompanied with large perturbations to the local hydrogen-bond networks,not a simple addition of individual jumps.In-vestigation of the dynamics of the jump and diffusive properties of water molecules with adjusted charges further show that the diffusive rates largely depend on the jump frequencies.These results suggest that water molecules diffuse with the accompany of jump reorientations,in a pattern that they conduct fast and slow motions in turn,showing dynamic heterogeneity.(3)The findings together reveal a molecular picture of water motions as below:restricted by four hydrogen-bonds as in normal case,a water molecule only slowly ro-tates and translates near its stable positions,mainly due to thermal fluctuations,which could be accelerated by defects.As the accumulation of local motions achieves an cer-tain level,the water molecule could conduct a fast large-amplitude jump reorientation within a short time of about 250 fs,with the exchange of its hydrogen-bond partner.Besides,within a duration of about 400 fs near the time point of the jump,this water molecule and its neighbors also undergo fast translational motions,which,along with the fast rotations,bring the water molecule to a new stable state with slow rotations and translations again.The water molecule propagates the defects via such a process,resulting in perturbations to local hydrogen-bond networks,and thus lead to correlation effects.Repeating such processes,the water molecules undergo fast and slow motions in turn,in which rotation and translation correlate to each other and promote mutually.The dissertation is organized as follows:Chapter ? serves as a general introduc-tion to the background and significance of our study,and the methods adopted as well.In Chapter ? our study on the correlation of jump reorientations of water molecules in bulk water is presented.In Chapter ? our investigation on the correlation between jump reorientations and translational diffusions of water molecules is shown.In Chap-ter IV we summarize.