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Structures and Infrared Spectra of NH3（H2O）4 and （NH3）2（H2O）2 Complex

Author: YangHua
Tutor: HuWeiJun
School: Shandong Normal University
Course: Theoretical Physics
Keywords: ammonia water clusters structure H-bond density functional theory frequency
CLC: O641
Type: Master's thesis
Year: 2009
Downloads: 25
Quote: 0
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Abstract


Clusters are composed of some atoms or moleculars, and the number of the atoms or moleculars in a cluster has a range from several to thousands. The study of the structure and properties of clusters can not only insight into the nature of the clusters, but also contribute to a better understanding of the material from the micro to the macro transition. Clusters have special physical and chemical nature and the obvious size-dependent, which led to widespread concern of physical, chemical and materials flieds. Clusters exist many isomer structures, and the number of isomers increases with the rapid growth of the size. It is very difficulty to find the most stable structure in theory,and more difficult to the molecular clusters. However, the physical and chemical properties of clusters mainly depend on the ground-state structure, therefore it is a basic problem to find the stable structures of clusters and infer the ground state structure for the cluster problem.In recent years, the hydrogen bond clusters become important research topics. As interaction force between the moleculars, hydrogen bond plays an important role in scientific issues. Dissolution, energy transfer and chemical reactions all can not be separated from hydrogen bonds. Ammonia and water clusters exist O ? ? ? H and N ? ? ? H which are special and important hydrogen bonds, and with the in-depth research, it can better explain the atmosphere, the formation of biological macromolecules. At the same time, as a typical soluble in polar solvents, the ammonia and water have a wide range application in the industrial and agricultural, refrigeration and energy and other fields. Recently, the study of ammonia and water clusters is gradually attracting more and more attention. Paul A. Stockman and others measured microwave and tunable infrared laser spectroscopy of ammonia and water dimer linear at certain conditions. Daniel E. Bacelo explore structure and the spectrum of the neutral ammonia and water cluster with AB initio methods, and found that the possible stable configurations for（NH3） （H2O） 3 and （NH3） （H2O）4 clusters both are parallelogram ring. Subha Pratihar and others research the structure and spectrum of more complex clusters of neutral ammonia （NH3） m （H2O） n （p = m + n = 3-7）,and found the most stable configuration is the planar ring structure for p=3-5 and the cage-like structure for p = 6-7.The structures and Infrared Spectra of NH3（H2O）4 and （NH3）2（H2O）2 clusters were investigated using density functional theory methods （DFT/B3LYP） and M?ller-Plesset second-order perturbation method （MP2） with the 6-31G（d） and 6-311++G（d, p） basis sets. As the first step, 39 among 127 stable geometries of NH3（H2O）4 clusters and 48 among 133 stable geometries of （NH3）2（H2O）2 clusters resulted from an empirical potential model, were selected as the initial input configurations for the ab initio calculations. Secondly, a DFT/B3LYP calculation of structural optimization and infrared spectra was performed with 6-31G（d） basis set for the selected geometries, and 30 stable structures of NH3（H2O）4 clusters and 29 stable structures of （NH3）2（H2O）2 clusters have been obtained. Finally, the B3LYP/6-31G（d） stable structures were further optimized using the same theory level and a bigger 6-311++G（d, p） basis set, and, as a result, 15 stable structures of NH3（H2O）4 clusters and 9 stable structures of （NH3）2（H2O）2 clusters have been obtained. It is found that, to our knowledge, 11 among the 15 structures of NH3（H2O）4 clusters have not been reported in the previous literatures. The result indicates that the pentagon structure is the most stable structure of the NH3（H2O）4,and the square structure exist as the most stable structure of the （NH3）2（H2O）2 clusters. in consistent with the previous reported calculation; the shifts and intensity of the frequencies have close relation to the H-bond types. Also, it is found that the shift is strongly dependent on the number of donors of H-bond, and that there is not certain association between the dipole moment and the stability of the clusters, which of the conclusion is different from previous literatures.

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CLC: > Mathematical sciences and chemical > Chemistry > Physical Chemistry ( theoretical chemistry ),chemical physics > Structural Chemistry
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