叠氮二氢硼多聚体结构和性质的理论研究(英文)

本文采用DFT-B3LYP方法,以不同基组对叠氮二氢硼多聚体(H2BN3)n (n=1-4)进行计算研究.二聚体(H2BN3)2(C2h对称性)中含B2N2平面四元环结构.船式(Cs对称性)和椅式(C3v对称性)三聚体(H2BN3)3的结合能相近(-122 和 -126 kJ·mol-1),其中均含B3N3六元环结构.拥有B4N4八元环结构的四个四聚体的结合能只有稍微差别.与单体相比,簇合物的结构参数变化较大.由ΔG0T可知,298.2 K下单体形成二聚体在热力学上是不利的,而形成三聚体和四聚体是有利的.     

BN纳米管的结构与性质

采用CASTEP程序包 ,在密度泛函理论 (DFT)框架内 ,在较高的理论水平对BN(n ,0 ) (n =3～ 17)纳米管的几何结构进行了优化 ,优化在a×a×c的正交超原胞中进行 ,并对其结合能和电学性质进行了计算 .结果发现 ,BN(n ,0 )纳米管的结合能随着n的增大而增大 ,并趋于收敛 .BN(n ,0 )纳米管的禁带宽度随着n的增大而增大 ,并收敛于 5 .3 9eV .     

(BN)n富勒烯和单层BN纳米管的图形理论方法研究

应用图形理论方法对(BN)12等富勒烯和单层BN纳米管的能级分布及其稳定性进行研究,发现(BN)n比同构型的(C2)n稳定,且与用DFT方法计算的结果一致.计算结果表明,采用图形理论方法是一种很有意义的解释和预测BN纳米材料的结构和性质的定性研究方法.     

BH+2与C2H2反应势能面的量子化学研究

用B3LYP/6-311G(d,p) 密度泛函方法和高级电子相关的CCSD(T)/6-311G(2df,p)偶合簇法研究BH+2与C2H2反应势能面. 结果表明 该势能面上存在(a) H2B+*C2H2, (b) HBCHCH2+, (c) H2BCCH+2和(d) H2*BHCCH+四种异构体, 其中(b)能量最低且在动力学上最稳定, (a), (c)和(d)在动力学上均不稳定; BH+2通过对C2H2的分步亲电加成以及随后的氢迁移和H2消除等反应形成离解产物HBCCH++H2, 该反应不需要活化能且大量放热. 计算结果有助于深入了解BH+2等缺电子硼氢正离子的反应行为.     

BN纳米管内含C纳米管——结构与电学性质

运用密度泛函理论的PW91/DNP方法对C(6,0)@BN(n,0)体系的结构与稳定性进行了研究, 发现最适合与C(6,0)纳米管形成的嵌套体系的锯齿型BN纳米管是BN(15,0)和BN(16,0), 在形成的C(6,0)@BN(15,0) 和 C(6,0)@BN(16,0)中, 碳壁与氮化硼壁之间的距离分别为0.36和0.40 nm. 在最稳定的C(6,0)@BN(16,0)体系中, 发现内层碳纳米管的电子结构并未受到外层氮化硼纳米管的影响, 然而氮化硼纳米管的能隙缩小了0.5 eV. 对C(6,0)@BN(16,0)的轨道分析表明, 碳纳米管与氮化硼纳米管之间的作用力为范德华力.     

杂原子掺杂的含单空位缺陷BN纳米管的非线性光学性质

采用密度泛函理论(DFT)研究了杂原子M(M=Li, Na, K, Be, Mg, Ca, C和Si)在B/N单空位缺陷处的掺杂对(6,0)BN纳米管体系非线性光学性质的影响. 采用B3LYP方法共得到了14种几何构型, 并采用BHandHLYP方法计算了这些结构的第一超极化率β₀值. 研究结果表明, 单纯的B或N缺陷几乎不影响BN纳米管体系的非线性光学性质; 与B缺陷处掺杂的体系相比, 杂原子在N缺陷处的掺杂更有利于提高BN纳米管体系的第一超极化率β₀值; 对于同周期掺杂原子, 还原性越强的原子掺杂对BN纳米管体系的第一超极化率β₀值的改善越明显, 表现为β₀(Ⅰ族)>β₀(Ⅱ族)>β₀(Ⅳ族); 对比同主族掺杂原子, 第三周期元素Na和Mg的掺杂能更有效地提高体系的第一超极化率β₀值, 原因主要在于原子半径和还原性等因素共同决定其对BN纳米管体系第一超极化率β₀值的改善程度. 本文研究结果为有效提高BN纳米管体系的非线性光学性质提供了一种新思路, 为基于BN纳米管的非线性光学材料设计提供了有价值的理论信息.     

二氧化钛纳米管储氢能力的分子动力学研究

二氧化钛纳米管是一种有前景的储氢材料,因此,在本文中通过卷曲锐钛矿单分子层,获得锯齿型(Zig-zag)和手性型(Chiral)二氧化钛纳米管结构。并采用分子动力学方法(Molecular dynamics)研究了氢分子分别在锯齿型和手性型二氧化钛纳米管和碳纳米管中的分布情况,并计算其储氢能力。结果表明,与碳纳米管一样,锯齿型和手性型二氧化钛纳米管存在管间储氢和管内储氢情况,并且氢分子在管间和管内的分布与二氧化钛纳米管内、外两侧的氧原子相关。Lennard-Jones势能模型表明:氢分子向纳米管内部和管间隙处的低能处聚集,形成氢分子环结构。储氢量计算结果表明,虽然锯齿型和手性型二氧化钛纳米管储存的氢分子数目较多,但由于系统重量较大,储氢量较低,低于美国能源部6%的商业标准,不能满足实际需要,而碳纳米管储氢量接近这一标准。     

螺桨烷型分子BX[(CH_2)_n]_3和BX(CH_2)[CH(CH_2)_nCH](X=N,P)的结构和性质

采用密度泛函理论(DFT)研究了螺桨烷型分子BX[(CH2)n]3和BX(CH2)[CH(CH2)n CH](X=N,P;n=1-6)的结构、稳定性、化学键和电子光谱性质.计算结果表明这些分子都是稳定的.BX[(CH2)n]3(X=N,P;n=1-6)的最高占据分子轨道(HOMO)和最低空分子轨道(LUMO)之间的能隙均大于5.20 eV,其中BN[CH2]3和BP[CH2]3的能隙超过7.0 eV,与C5H6的能隙(7.27 eV)很接近,BX(CH2)[CH(CH2)n CH](X=N,P;n=1-6)的能隙在6.80 eV左右.所研究分子能量的二阶差分表明BN[(CH2)3]3、BP[(CH2)4]3及BX(CH2)[CH(CH2)2CH](X=N,P)是最稳定的.BX[(CH2)n]3的Wiberg键级表明除了BN[(CH2)n]3(n=2和6)中不存在B―N键,其它化合物中B和N均形成了化学键,BP[(CH2)n]3中除了BP[(CH2)2]3不存在B―P键,其它的均存在.电子密度的拓扑分析表明N―B键属于离子键,而P―B键具有共价键特征.BX[(CH2)n]3(X=N,P)的第一垂直激发能分别在191.1-284.8 nm和191.8-270.1 nm之间,BX(CH2)[CH(CH2)n CH](X=N,P)的第一垂直激发能分别在190.5-199.7 nm和209.0-221.3 nm之间.     

金属碳、卡宾阳离子[M—X]~+(M=Au,Ag,Cu;X=C,CH2)与甲烷反应机制研究

在CCSD(T)-REL//B2GP-PLYP水平下构建[Au(CH_2)]~+与甲烷反应的可靠反应势能面,分析了C—H键活化过程中的几何结构变化情况;对反应IRC路径上关键点进行自然键轨道(NBO)电荷和分子轨道分析,从理论上推定该氢转移过程属于氢负离子(H~-)转移.对[M—X]+(M=Au,Ag,Cu;X=C,CH_2)与甲烷反应进行对比,分析了甲烷作为氢供体反应过程的内在影响因素.M—X键能和反应活性中心C上直接参与反应的低能轨道对反应活性均起重要作用,两者协同调控微观反应机制.     

两种烷基碘化物分子理论研究及其发射谱测量

利用密度泛函理论(DFT)的B3LYP方法, 对烷基碘化物分子C2H2F3I和n-C3H4F3I的C—I解离势能曲线进行了理论计算, 并采用B3LYP方法和MPn(n=2, 3, 4)方法精确计算了C—I键解离能. 解离能计算进行了零点振动能(ZPVE)校正, 并运用完全均衡校正法对基函数重叠误差(BSSE)进行校正. 利用微波放电激励方法, 对C2H2F3I和n-C3H4F3I的发射谱进行观测. 实验结果表明, 通过微波放电激励这两种分子, 均可产生1315 nm发射谱, 说明利用微波放电可使C2H2F3I和n-C3H4F3I分子的C—I键解离, 从而产生碘原子.     

Encapsulation of carbon chain molecules in single-walled carbon nanotubes

The vacuum space inside carbon nanotubes offers interesting possibilities for the inclusion, transportation, and functionalization of foreign molecules. Using first-principles density functional calculations, we show that linear carbon-based chain molecules, namely, polyynes (C(m)H(2), m = 4, 6, 10) and the dehydrogenated forms C(10)H and C(10), as well as hexane (C(6)H(14)), can be spontaneously encapsulated in open-ended single-walled carbon nanotubes (SWNTs) with edges that have dangling bonds or that are terminated with hydrogen atoms, as if they were drawn into a vacuum cleaner. The energy gains when C(10)H(2), C(10)H, C(10), C(6)H(2), C(4)H(2), and C(6)H(14) are encapsulated inside a (10,0) zigzag-shaped SWNT are 1.48, 2.04, 2.18, 1.05, 0.55, and 1.48 eV, respectively. When these molecules come inside a much wider (10,10) armchair SWNT along the tube axis, they experience neither an energy gain nor an energy barrier. They experience an energy gain when they approach the tube walls inside. Three hexane molecules can be encapsulated parallel to each other (i.e., nested) inside a (10,10) SWNT, and their energy gain is 1.98 eV. Three hexane molecules can exhibit a rotary motion. One reason for the stability of carbon chain molecules inside SWNTs is the large area of weak wave function overlap. Another reason concerns molecular dependence, that is, the quadrupole-quadrupole interaction in the case of the polyynes and electron charge transfer from the SWNT in the case of the dehydrogenated forms. The very flat potential surface inside an SWNT suggests that friction is quite low, and the space inside SWNTs serves as an ideal environment for the molecular transport of carbon chain molecules. The present theoretical results are certainly consistent with recent experimental results. Moreover, the encapsulation of C(10) makes an SWNT a (purely carbon-made) p-type acceptor. Another interesting possibility associated with the present system is the direction-controlled transport of C(10)H inside an SWNT under an external field. Because C(10)H has an electric dipole moment, it is expected to move under a gradient electric field. Finally, we derive the entropies of linear chain molecules inside and outside an open-ended SWNT to discuss the stability of including linear chain molecules inside an SWNT at finite temperatures.     

First-principles study of interaction between H2 molecules and BN nanotubes with BN divacancies

The interaction between H(2) molecules and boron nitride (BN) single-walled nanotubes with BN divacancies is investigated with density-functional theory. Our calculations reveal that H(2) molecules adsorb physically outside defective BN nanotubes, and cannot enter into BN nanotubes through bare BN divacancies because the energy barrier is as high as 4.62 eV. After the defects are saturated by hydrogen atoms, the physisorption behavior of H(2) molecules is not changed, but the energy barrier of H(2) molecules entering into BN nanotubes through the defects is reduced to 0.58 eV. This phenomenon is ascribed to hydrogen saturation induced reduction of electrostatic potential around the defects.     

A theoretical study of the dissociative chemisorption of hydrogen on carbon nanotubes

Potential profiles were obtained for the chemisorption of hydrogen on (n, n) and (n, 0) carbon nanotubes. The energy barriers and rate constants for hydrogen molecule sorption on and desorption from various nanotubes were determined. The constants for sorption and desorption were used to calculate sorption-desorption equilibrium constants. Sorption on outside nanotube surfaces was found to be more favorable energetically than sorption on inside surfaces.     

Reversible storage of molecular hydrogen by sorption into multilayered TiO2 nanotubes

The sorption of hydrogen between the layers of the multilayered wall of nanotubular TiO2 was studied in the temperature range of -195 to 200 degrees C and at pressures of 0 to 6 bar. Hydrogen can intercalate between layers in the walls of TiO2 nanotubes forming host-guest compounds TiO2 x xH2, where x < or = 1.5 and decreases at higher temperatures. The rate of hydrogen incorporation increases with temperature and the characteristic time for hydrogen sorption in TiO2 nanotubes is several hours at 100 degrees C. The rate of intercalate formation is limited by the diffusion of molecular hydrogen inside the multilayered walls of the TiO2 nanotube. 1H NMR-MAS and XRD data confirm the incorporation of hydrogen between the layers in the walls of TiO2 nanotubes. The nature and possible applications of the observed intercalates are considered.     

Theoretical study of the H(2) reaction with a Pt(4) (111) cluster

The C(s) symmetry reaction of the H(2) molecule on a Pt(4) (111) clusters, has been studied using ab initio multiconfiguration self-consistent field plus extensive multireference configuration interaction variational and perturbative calculations. The H(2) interaction by the vertex and by the base of a tetrahedral Pt(4) cluster were studied in ground and excited triplet and singlet states (closed and open shells), where the reaction curves are obtained through many avoided crossings. The Pt(4) cluster captures and activates the hydrogen molecule; it shows a similar behavior compared with other Pt(n) (n=1,2,3) systems. The Pt(4) cluster in their lowest five open and closed shell electronic states: (3)B(2), (1)B(2), (1)A(1) (3)A(1), (1)A(1), respectively, may capture and dissociate the H(2) molecule without activation barriers for the hydrogen molecule vertex approach. For the threefolded site reaction, i.e., by the base, the situation is different, the hydrogen adsorption presents some barriers. The potential energy minima occur outside and inside the cluster, with strong activation of the H-H bond. In all cases studied, the Pt(4) cluster does not absorb the hydrogen molecule.     

Investigating detailed mechanism of hydrogen molecules adsorbing on single-wall carbon nanotubes using fitted force field parameters containing carbon–carbon interactions

The nonbonded and bonded force field parameters for carbon atoms in single-wall carbon nanotubes (SWNT) are fitted by means of quantum chemistry calculations with considering the periodic boundary conditions. The nonbonded parameters between carbon atoms and hydrogen atoms are fitted as well. All the fitted parameters are verified by comparing to quantum chemistry results and by calculating Young's modulus. Adsorption of Hydrogen molecules are then carried out on a bundle of self-assembled SWNTs. The adsorption isotherms are consistent to the Freundlich equation. Both hydrogen molecules adsorbed outside and inside the SWNTs are counted. According to our result, hydrogen molecules adsorbed inside the SWNTs are more stable at a relatively high temperature and are playing an important part in total amount of the adsorbed molecules. While C(10,10) have the highest adsorption capacities in most of the temperatures, hydrogen molecules inside C(5,5) are the most stable of all the four kinds of SWNTs. Thus, balancing adsorption capacities and strength of interaction can be important in choosing SWNT for gas adsorption. Besides, we deduct an equation that can describe the relation between hydrogen pressure and amount of SWNTs based on our simulation results. The hydrogen pressure may decrease by adding SWNTs in the system. The fitting method in our system is valid to SWNTs and can be tested in further studies of similar systems. © 2018 Wiley Periodicals, Inc.     

Hydrogen adsorption on carbon-doped boron nitride nanotube

The adsorption of atomic and molecular hydrogen on carbon-doped boron nitride nanotubes is investigated within the ab initio density functional theory. The binding energy of adsorbed hydrogen on carbon-doped boron nitride nanotube is substantially increased when compared with hydrogen on nondoped nanotube. These results are in agreement with experimental results for boron nitride nanotubes (BNNT) where dangling bonds are present. The atomic hydrogen makes a chemical covalent bond with carbon substitution, while a physisorption occurs for the molecular hydrogen. For the H(2) molecule adsorbed on the top of a carbon atom in a boron site (BNNT + C(B)-H(2)), a donor defect level is present, while for the H(2) molecule adsorbed on the top of a carbon atom in a nitrogen site (BNNT + C(N)-H(2)), an acceptor defect level is present. The binding energies of H(2) molecules absorbed on carbon-doped boron nitride nanotubes are in the optimal range to work as a hydrogen storage medium.     

(n,m) Selectivity of single-walled carbon nanotubes by different carbon precursors on Co-Mo catalysts

Single walled carbon nanotubes (SWCNTs) were synthesized using four different carbon precursors including CO, C2H5OH, CH3OH, and C2H2 on Co-Mo catalysts. Semiconducting (n,m) abundance was evaluated by a method based on a single-particle tight-binding theoretical model taking into consideration the relative photoluminescence and absorption quantum efficiency for specific (n,m) tubes. (n,m) abundance determined in photoluminescence analysis was used to reconstruct the near-infrared Es11 absorption spectra. Carbon precursor pressure was found to be the key factor to the chirality control in this study. Narrowly (n,m) distributed SWCNTs can only be obtained under high-pressure CO or vacuumed C2H5OH and CH3OH. The majority of these nanotubes are predominately in the same higher chiral-angle region. The carbon precursor chemistry may also play an important role to obtain narrowly (n,m) distributed SWCNTs. (n,m) selectivity on Co-Mo catalysts shifts under different carbon precursors providing the route for (n,m) specific SWCNTs production.