氨硼烷类化合物结构和储氢热力学性质的理论研究

【作者】 何李宁；

【导师】 张聪杰；

【作者基本信息】 陕西师范大学 ， 物理化学， 2011， 硕士

【摘要】 氨硼烷(NH3BH3, AB)具有氢含量高(19.6wt.%)和良好的热稳定性的优点而成为新型的储氢材料,在实验和理论上,人们研究了氨硼烷及其衍生物的结构和性质。本论文采用密度泛函理论(DFT),研究了两类氨硼烷类化合物的结构和储氢性质：一是灯笼状氨硼烷类化合物；另外一个是BN单元取代环烷烃的杂环化合物与LiH/NaH的脱氢反应。主要的研究内容包括如下三个方面：1.在氨硼烷的基础上,我们设计了19种灯笼状氨硼烷类衍生物,采用B3LYP/6-311++G**和MP2/6-311++G**方法研究化合物B2N2H6(XH2)3(X=C、Si和Ge)及其异构体的结构和振动频率,发现在这两种方法下三类化合物最稳定的构型分别为H3NB(CH2)3BNH3、H3BN(SiH2)3BNH3、H3NB(GeH2)3BNH3,表明采用B3LYP/6-311++G**方法研究笼状氨硼烷类化合物是可行的。因此,在该方法下研究了其它16种灯笼状化合物(B2N2H6(C2H2)3. B2N2H6(C2H4)3、B2N2H6(C6H6)、B2N2H6(NH)3、B2N2H6(B3N3H6)、B2N2H6(BH)n(n=3-5、8和10)、B2N2H6(B)n (n=4、6)、B2N2H6(B)5及B2N2H6(X)3(X=O、S和Se))的结构和稳定性,进一步我们计算了这19种化合物的芳香性、电子光谱和热力学性质。计算结果表明,除离子化合物H3NB(B)5BNH3的能隙值为1.53eV外,其它18种化合物的能隙值在2.81和5.88eV之间,计算的NICS(0)表明化合物H3NB(C2H2)3BNH3、H3NB(C2H4)3BNH3、H3NB(C6H6)BNH3和H3NB(B3N3H6)NBH3的芳香性较弱,其它化合物的芳香性较强,其中化合物H3NB(B)4BNH3芳香性最强,它的NICS(0)值为-78.62。电子光谱计算表明除化合物H3NB(B)4BNH3和H3NB(B)5BNH3-的第一激发波长值大于700.00nm外,其它17种化合物的波长在220.00和422.00nm之间,同时,计算得到了0K和298K时19种化合物的生成焓。2.在B3LYP/6-311++G**和B3LYP/aug-cc-pVTZ水平下,我们研究了环烷烃CnH2n(n=3-6)的稳定构型与LiH/NaH的脱氢反应焓变,研究发现虽然在这两种水平下计算的反应焓变值非常接近,然而在B3LYP/aug-cc-pVTZ水平下计算的298K时H2、LiH、NaHH及CnH2n(n=3-6)的生成焓值(ΔHft)与实验值较接近,其值分别为-1.2、32.6、33.1、11.9、7.7、-14.4及-23.5kcal·mol-1。因此采用该方法研究了BN取代的杂环化合物Cx(BN)yH2x+4y (x=1-4, y=1; x=1-2, y=2; x=0;y=2-3)与LiH/NaH脱氢反应的热力学性质。计算表明,环烷烃CnH2n (n=3-6)与Li/NaH的脱氢反应焓变皆大于10.0kcal-mol-1,而BN取代的杂环化合物与一个或多个LiH/NaH脱氢反应焓变皆小于16.0kcal·mol-1,说明BN取代的环烷烃降低了脱氢焓变,使脱氢反应更容易进行,因此该类化合物可作为潜在的储氢材料,并且LiH与杂环化合物的脱氢反应比NaH与化合物的反应在热力学上更易发生。3.在B3YP/aug-cc-pVTZ水平下,我们采用自然布居分析和AIM方法研究了杂环化合物Cx(BN)yH2x+4y (x=1-4, y=1; x=1-2, y=2; x=0; y=2-3)的化学成键性质,Wiberg键级数(WBI)表明B、N原子皆采用sps杂化且符合八隅规则,AIM分析得到BN)2H8、C3(BN)H10、椅式(BN)3H12和C4(BN)H12中的C-N和C-B为共价键,而B-N为配位键。更多还原

【Abstract】 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.更多还原