| Water is a widely distributed substance in nature, which participates in various physical and chemical processes and provides basic environments for biochemical reactions. Investigating the existent form, properties and behaviors of water systems from atomic level is essential for understanding fundamental problems in related fields. The microstructure of water can generally be taken as the clusters constructed by hydrogen-bonding(H-bonding) water molecules, which is the elementary unit of hydrogen-bond networks. Owing to the structural freedom of water clusters and the complex behaviors of hydrogen bond(H-bond) networks, the properties of microscopic water systems are not only limited by those of individual water molecules, but can also show size-dependent evolution of structures and chirality as well as processes like conformation conversion and proton transfer. It is noted that, the H-bond between water molecules, which is closely related to the electronic structure of a single molecule, determines the structure and properties of the whole network. And recently, the covalent-like electronic structure has been observed in intermolecular hydrogen bonds. This implies, from the angle of electronic structure, a systematic theoretical research on water clusters may provide fundamental understanding for properties and behaviors of above hydrogen-bond system. Meanwhile, water system related problems occupy important positions in various subjects and interdisciplinary fields. In this work, based on first-principles methods and from the perspective of electronic structures, we present a systemic research composed of five works in order: the molecular orbital expression of covalent-like characteristic in H-bond between water molecules, the delocalized electronic structure of small sized cyclic water clusters, the correlation between delocalized molecular orbitals and geometrical planarization in cyclic clusters, the evolution of H-bond electronic structure respect to size effect for four coordinated central water molecule in medium size water clusters and finally, the chiral conversion and recognition in prism water clusters during concerted proton transfer.Specifically, at first, we choose the basic model of water dimer(H2O)2 to study the covalent-like characteristic of H-bond between water molecules, using high-precision first-principles methods and from the angle of molecular orbitals. Molecular orbital analysis shows two orbitals cross the H-bond region, whose components are mainly from the O(2p) orbital at the donor side. Energy decomposition analysis show a non-negligible induction term, which amounts to more than 10% of the total attractive interaction energy. This further confirms the covalent-like characteristic of H-bonds. Our research provides a molecular orbital view for understanding of H-bonds in ice, liquid water, functional materials and biological systems as well as a fundamental guide for further studying water systems in this thesis based on electronic structureSecondly, considering quasiplanar small water rings(H2O)n(n = 3, 4, 5, 6) are elementary structural and functional units of more complex three dimensional water H-bond networks, the understanding of intermolecular H-bonding mechanism in these structures is of fundamental significance for further exploration of water systems. Through density functional theory methods combining bond analysis, we studied such small cyclic water clusters. Results show, both clusters with n = 3 and 4 possess delocalized molecular orbitals which spread over the whole ring frame and cover the ring centers. For n = 5 and 6, the distribution of corresponding delocalized orbitals around the ring center are weakened. The delocalized electronic structure of small cyclic water clusters extends the understanding of the H-bond in water clusters. The indicated interplay between electronic structure of H-bonds and geometrical structure as well as cluster size provides inspirations for following researches.Thirdly, based on the last part, we further explore the correlation between delocalized electronic structure and structural planarization in small cyclic water clusters(H2O)n(n = 3-6). Results show three kinds of delocalized molecular orbitals(MO) closely relate to the stabilization of planar structure. Respectively, they are denoted as MO-I, which mainly originates from the 2p lone pair electrons of oxygen(O), MO-II, which are bonding orbitals along the the H-bond direction and mainly composed of the 2p electrons of O and the 1s electrons of H and the last one, MO-III, the bonding orbitals mainly composed of 2s electrons of O and 1s electrons of H. When geometric planarization is introduced, the maximized overlap of monomeric orbitals of single water molecules respect to these three kinds of MOs is achieved. Further energy decomposition analysis demonstrates that the orbital interaction takes over 30% of the total attraction terms, highlighting the covalent-like characteristic of H-bonds.Fourthly, towards the size effect which should be considered when taken water clusters as models for liquid water and based on the basic understanding of small sized water clusters including their size-dependent H-bond electronic structure variance, we studied the evolution of H-bond electronic structure of the four coordinated central water molecule respect to cluster sizes in medium sized water clusters(H2O)n(n = 17, 19, 20, 21, 23, 25). Results show, with the increase of n, the interaction energy between central water molecule and its nearest neighbor water molecules decreases while that between central water molecule and second nearest neighbor water molecules increases. This indicates a competition between the interaction with two group of neighbors for the central four coordinated water molecule. Spectra and electron density analysis show no obvious size-dependence in the lowest-frequency intermolecular vibrations involving the central molecule and also the charge transfer between the central molecule and the outer cage. Besides, topology analysis further shows the covalent-like characteristic of H-bond electronic structure for the four coordinated central molecule. We expect this theoretical research can contribute to researches on medium-sized water clusters and understanding of H-bonding interaction in liquid water.Finally, as a further step based on previous studies, we explore the dynamic process of H-bond networks. Proton transfer and chiral conversion via H-bonds are fundamental problems in physics and interdisciplinary fields involving subjects like life science and material science. These processes are also critical for chiral recognition and chemical industries like enzyme catalysis and drug preparation. Here we investigated the chiral conversion and recognition during concerted intralayer proton transfer(CIPT) processes in small prismatic water clusters which can be taken as bilayer n-membered water rings(Bn WRs, n = 4, 5, 6). Results show that, in spite of small energy variations between initial and final states which is less than 0.3 kcal/mol, the vibrational circular dichroism(VCD) spectrum provides clear chiral recognition regions ranging from 3000 cm-1 to 3500 cm-1 which can be used to distinguish the chiral initial and final clusters. Vibrational modes in this region correspond to the intralayer stretching of the H-bonds. These modes also exhibit strong signals in infrared and Raman spectra, facilitating possible observations. The electronic circular dichroism spectrum(ECD) also shows obvious recognizable signals. Further molecular orbital analysis indicates the molecular orbitals involved in interlayer interaction are dominated by O(2p) atomic orbitals and the CIPT process will induces energy increase up to 0.1 e V for these orbitals. In addition, the isotopic substitution by deuterium atom could result in shifts for characteristic peaks in the VCD spectra, which provides a principle design for experimental recognition on chiral water clusters. We hope our findings could promote the clarification of chirality concept in water cluster systems, from the atomic level, and even the implement of corresponding chiral recognition in experiments. |
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