Electron Density Topology Of The Structure And Reaction Mechanism Of Electron-deficient Boron Hydride

In this thesis, BH₂⁺ , BH4 and some simple boranes are studied using quantum chemistry method and the topological analysis of electronic density. Structures, chemical bonds and relevant reaction mechanisms of these species are discussed. All studies consist of three parts, the first part presents the reaction mechanisms of BH₂⁺ with H₂O and H₂S, the second part shows the topological analysis of electronic density on BH4 and its ions, and the last part gives the structures of some simple boranes and the possible isomers.At the beginning of this thesis, the previous studies on borohydrides are surveyed. Since boranes were firstly discovered by Stock and coworkers in 1912, electron deficient borides are particularly appealing for theoretical and experimental investigations because of their unique properties and potential applications.In the second chapter, the developments and applications of modern quantum chemistry theory and quantum topological theory are described briefly. The application areas of quantum chemistry is increasingly growing, bonding characters and electronic structures in molecules are still the main research area of quantum chemistry, and its theoretical predictive ability becomes more and more obviously. In the past several years, the topological analysis of electronic density has been applied to the studies of bonding characters and reaction mechanisms and gained noteworthy achievements.The studies on the reaction mechanism of BH₂⁺ with H₂O and H₂S are presented in the third chapter. The possible reaction channels are found for each reaction. According to Bader's theory of "AIM", the characteristics of the chemical bonds are described quantitatively according to the change of electronic density and the Laplacian. The structure transition state and the energy transition state are found in the reaction of BH₂⁺ with H₂O and H₂S.In the fourth chapter, the characteristics of structures and bonds of BH4 and its ions are studied using B3LYP method at the 6-311+G(d,p) level. The following conclusions are drawn: The symmetries of BH₄⁺, BH₄ and BH₄^- are C2v C₂v and T_d respectively. There are B-H bond, H-H bond and atom-molecular bond in BH₄⁺ and BH4. While there are four equivalent B-H bonds in BH4". In the case of BH4 there are three pairs of electrons with one single electron surrounding the boron.The structures for the tetraborane, pentaborane, pentaborane, hexaborane, octaborane and nanaborane are investigated with a view to finding the existence of possible isomers in the fifth chapter. The symmetries of these simple boranes are C>^ C*v> C/> Co Cs and C, respectively. The highly symmetric structures proposed by Elk are not supported by our theoretical calculations. There is an isomer for tetraborane, tetraborane dication, hexaborane and nanaborane respectively, and all geometric parameters of these boranes and isomers are shown in this chapter.The novel conclusions and ideas of this work are listed as follows:1. The mechanisms for the reactions of BH2+ with H2O and H2S are investigated respectively, and the possible reaction channels are found for each reaction. In these reactions, B in BH2+ attacks O and S forming transition states, then the transfer of H and breakage of B-H bond result in the formation of the products.2. B3LYP/6-311+G(d,p) method is used to investigate the characteristics of structures and bonds of BH4 and its ions. The following conclusions are drawn: There are B-H bond, H-H bond and atom-molecular bond in BH4"1" and BH4, while there are four equivalent B-H bonds in BHLf. In the case of BH4 there are three pairs of electrons with one single electron surrounding the boron.3. The structures for the boranes and the possible isomers are investigated using B3LYP methods. The symmetries of these simple boranes are C^v^ C*v> Cj^Cs^ Cs and Cs respectively, and there is an isomer for tetraborane, tetraborane dication, hexaborane and nanaborane.