Stable Structures And IR Spectra Of Protonated Ammonia Water Clusters

Atomic and molecular clusters (referred as clusters) are relatively stable micro or sub-micro aggregates, composed of thousands of atoms or molecules by some physical or chemical bonding forces^[1-2], and the physical and chemical properties of clusters change with the size. The cluster size ranges from a few to hundreds of angstroms. It is too big to describe with the concept of inorganic molecules, and too small to describe with the concept of solid grains. The cluster has many special properties, which are different from the individual atoms and molecules, and different from solid and liquid, also can not be obtained with simple linear extension or interpolation. Therefore, people view clusters as a new material level between atoms, molecules and macro solid state matter. It is a transition state of substance from atom, molecule to bulk material, or represents initial state of condensed matter.As we all known, the hydrogen bonds widely exist in varies substances, and as the interaction force between molecules, they play an important role in many scientific issues. Many problems can be solved by the research of the hydrogen bonds, such as energy transfer, dissolution and chemical reactions and so on. It can provide useful value for the development of water resources. O…H and N…H are two kinds of special and important hydrogen bonds. They exist in ammonia water clusters. Exploring the stable structures of the ammonia water clusters can provide theory groundwork for researching the componen -ts of atmosphere and the formation of biological macromolecules. Through the research of clusters, people can set a bridge between atoms, molecules and macro solid . Clusters science is an cross subject, which is among atom, molecular and solid state physics. In recent years, researching of clusters has gotten more and more extensive attention^[1-9]. The proton transfer in hydrogen bonding system is one basic processes in nature. To understand basic physical, chemical, and biological processes, we have to research proton transfer reaction mechanism.First this paper a model potential is constructed for the protonated ammonia water clusters, and we obtain many model potential isomers. Then, we selected the stable structures from many model potentia isomers as the initial structure. The stable structure and harmonic vibration spectra of protonated ammonia water cluster NH₄⁺(H₂O)₄ and NH₄⁺(H₂O)₇ are calculated at the density functional theory(B3LYP) level with 6-31G(d) and 6-311++G(d,p) basis sets.The main conclusions of paper are as follows:1. The stable structure of protonated ammonia water clusters is that a proton and an ammonia molecule form a NH 4+ core ion, and the water molecules are bound to hydrogen atoms of this core ion by hydrogen bonds.2. The stable structures of protonated ammonia water were studied by density functional theory(B3LYP),we get lower-energy structure than in the literature.3. Through the analysis of the harmonic vibration spectral of the most stable protonated ammonia water clusters, we found that the strongest peak of the spectrum resul -t from the superposition of various vibration modes. The main contents of this dissertation are shown as follows:In chapter 1, firstly we simply introduce hydrogen bonding influence structure and properties of the material, and hydrogen bonding application prospect, the concept and research background, classification, the main character of cluster. Secondly, we presented the main research directions and difficult problems of cluster. Lastly, we show the research of cluster and our work.In chapter 2, we introduc some theoretical calculation methods, including variation principle and perturbation theory, density functional theory, the basis functions of self-consistent calculation, the calculation method of vibrational frequencies, a brief description of the Gaussian 03 program. In chapter3, The structures and infrared spectra of NH₄⁺(H₂O)₄ clusters were studied by density functional theory methods (DFT/B3LYP) with the 6-31G(d) and 6-311++G(d,p) basis sets. We found 9 stable structure of NH₄⁺(H₂O)₄clusters, and we research infrared spectrum of four stable structure.In chapter 4, the structures and infrared spectra of NH₄⁺(H₂O)₇ cluster were studied by density functional theory methods (DFT/B3LYP) with the 6-31G(d) and 6-311++G(d,p) basis sets. We found 10 stable structure of NH₄⁺(H₂O)₇clusters, and we study infrared spectrum of four stable structure. We found various vibration modes superposition caused the highest peak of the spectrum.In chapter 5,we have a summary and outlook of our work.