Theoretical Study Of Ammonia Borane Compound Structure And Hydrogen Storage Thermodynamic Properties

Ammonia borane (NH3BH3, AB) has attracted much interest because of its extremely high gravimetric hydrogen storage and relatively favorable thermal stability. The structures and thermodynamic properties of ammonia borane and its derivatives have been studied both experimentally and theoretically. In this thesis, the structures and properties of a kind of lantern-like compounds derived from ammonia borane have been investigated by the density functional theory (DFT). In addition, thermochemistry of the dehydrogenation of the cycloalkanes CnH2n(n=3-6) and boron-nitrogen-containing heterocycles Cx(BN)yH2x+4y with LiH/NaH, as well as the chemical bond of Cx(BN)yH2x+4y were carried out using DFT The main contents of this thesis contain three parts as follows:1. On the basis of ammonia borane, we designed nineteen lantern-like compounds derived from ammonia borane, three isomers of three compounds B2N2H6(XH2)3 (X=C, Si and Ge) were calculated by B3LYP/6-311++G** and MP2/6-311++G**, and the most stable isomers are H3NB(CH2)3BNH3, H3BN(SiH2)3BNH3 and H3NB(GeH2)3BNH3, indicating that B3LYP method is realiable to study this system. Additionally, other sixteen kinds of lantern-like compounds derived from ammonia borane, B2N2H6(C2H2)3, B2N2H6(C2H4)3, B2N2H6(C6H6), B2N2H6(NH)3, B2N2H6(B3N3H6), B2N2H6(BH)n (n=3-5,8 and 10), B2N2H6(B)n (n=4,6), B2N2H6(B)5- and B2N2H6(X)3 (X=O, S and Se) were designed. Moreover, the most stable structures, energy gaps, aromaticities, electronic spectra and enthalpies of formation of nineteen lantern-like compounds have been obtained at B3LYP/6-311++G** level. The energy gap of H3NB(B)5BNH3- is 1.53eV, which is the smallest, and the energy gaps of other compounds are between 2.81 and 5.88eV. The centers of the cages of these compounds have strong aromaticity except for H3NB(C2H2)3BNH3, H3NB(C2H4)3BNH3, H3NB(C6H6)BNH3 and H3NB(B3N3H6)NBH3. Specially, the NICS(0) value of H3NB(B)4BNH3 is-78.62, showing the center of the cage has very strong aromaticity. The wavelengths of the origin band of the electronic transition of these complexes H3NB(B)4BNH3 and H3NB(B)5BNH3- are greater than 700.00nm, while the wavelengths of the other complexes are between 220.00 and 422.00nm. The enthalpies of formation of the nineteen compounds at 0 and 298K are given.2. The reaction enthalpies at OK and 298K of the dehydrogenation reactions of CnH2n with LiH (and NaH) calculated at both B3LYP/6-311++G** and B3LYP/aug-cc-pVTZ levels are greater than 10.0kcal·mol-1, thus CnH2n are unfavorable to storage hydrogen. However, calculated reaction enthalpies at OK and 298K of the dehydrogenation reactions of Cx(BN)yH2X+4y with one, two and three LiH (and NaH) at the same level of theory and the values are negative, which reaction enthalpies are lower than -16.0kcal·mol-1, then, LiH or NaH together with boron-nitrogen-containing heterocycles Cx(BN)yH2x+4y are a kind of good storage medium for hydrogen.3. The natural bond orbital and AIM analysis of Cx(BN)yH2x+4y (x=1-4, y=1; x=1-2, y=2; x=0; y=2-3) show that the B and N atoms are close to sp3 hybrid, the WBIs of B-N are closes to ammonia borane. Furthermore, the C-N and C-B bonds of (BN)2H8, C3(BN)H10, (BN)3H12 and C4(BN)H,2 are covalent bond, which the B-N bond is a characteristics of coordinate bond.