Structures And Stabilities Of Hydrogen-Bonding Ammonia-Water Clusters

Cluster, an aggregate of atoms or molecules of limited number, is a new form of material between the gaseous and condensed states. The study of the structures and properties of clusters can not only give insight into the nature of the clusters, but also contribute to a better understanding of the material transition from the micro to the macro. In order to understand the natures of clusters adequately in theory, one must first investigate its ground state geometrical structures. Clusters have many isomer structures, and the number of isomers increases rapidly with the size growth. So it is very difficult to find out the most optimized cluster structures, and this problem is called NP-hard problem. So far, a lot of methods have been proposed for cluster structures, such as basin hopping algorithm, genetic algorithm, empirical potential method, etc.Molecular clusters combined with weak hydrogen bonding interaction have attracted attentions of more and more researchers. Hydrogen-bonding clusters widely exist in water,hydrate and lots of organic compound, meanwhile, hydrogen bond plays an important role in many chemical and biological processes, such as dissolution, energy transfer and chemical reactions. In the past few years, preliminary studies have been conducted experimentally and theoretically on water clusters, ammonia clusters and water-ammonia clusters. Experimentally, Mikhail and others obtained (H₂O)_n=2-4 and (NH₃)_n=2,3 clusters via He droplet beam passing through the airtight chamber filled with ammonia and water gas, Qu Hongbo and others acquired acetonitrile and ammonia-water mixed clusters which were mainly binary by Ar-carrier supersonic expansion, C.Desfrancois et. al produced [(NH₃)m(H₂O)n]—through collision between small cold water-ammonia neutral clusters and Rydberg atoms, Paul conducted microwave and far infrared spectra measurement of (NH₃)(H₂O). In theoretical calculation, ab initio method is mostly adopted to confirm ground state structures of water-ammonia clusters. Now there are lots of researches on neutral and electronic clusters of pure water, pure ammonia and water-ammonia dimmer, but reports about largish ammonia-water mixed electronic clusters are very few. According to the research of Piotr Skurski and Maciej Gutowski, the structure of (NH₃H₂O)—just changes a little versus NH₃H₂O by an enhancement of the dipole moment to bound electron. However geometric configurations of [NH₃(H₂O)_n]—(n>1) are distinct from the corresponding neutral clusters.Density functional theory method(DFT) is employed to conduct the structural optimization and frequency calculation for NH₃(H₂O)₅ and [NH₃(H₂O)_n=2,3]—, then the stable structures of clusters are confirmed. Furthermore, further research is taken on the excess electron binding states and vibration spectra of electronic clusters. It is found that all excess electron binding states are surface states and configurations of high stability are prone to have biggish dipole moments. Each peak of infrared spectrum is basically superposition of vibration of molecules, but sometimes the vibration of certain molecule play obviously a dominant role.The main contents of this paper are shown as follows:In chapter 1, firstly we simply introduce the basic concept, classification, main properties, main research directions, present situation, significance and prospect of cluster.In chapter 2, we introduce some theoretic and computing methods of clusters studies, including variation principle, one electron approximation, density functional theory, the basis functions of self-consistent calculation and vibrational frequencies. Then we make a brief analysis of basis set this paper used.In chapter 3, density functional theory method(DFT/B3LYP) is employed to conduct the structural optimization and frequency calculation for initial configurations of the NH₃(H₂O)5 clusters. Finally 20 kinds of stable structures are acquired under the 6-311++G(d,p) basis sets. It is found that the stablest structure is no longer ringlike like NH₃(H₂O) n(n<5).In chapter 4, The geometrical structures and infrared spectra of [NH₃(H₂O)_n]—(n=2,3) clusters are investigated using density functional theory methods(DFT/BLYP) with the 6-31(1+2+)G(d,p) basis sets.Then the stable ground state and the excess electron binding states are confirmed.