密度泛函理论研究银上吸附对巯基吡啶的SERS化学增强效应

基于密度泛函理论计算和拉曼光谱理论分析,我们研究了对巯基吡啶(4MPY)分子的拉曼光谱和其在银上的表面增强拉曼光谱(SERS),并进一步探讨了SERS与界面吸附结构、异构化、质子化和氢键作用以及低能激发态的关系。首先,我们对两种分子异构体的相对稳定性和拉曼光谱进行了理论分析。在此基础上,进而研究了该分子与不同银簇作用时的拉曼光谱,结果表明,4MPY以巯基硫与银簇作用形成强的Ag―S键,导致拉曼光谱的线型不依赖于所选银簇的大小。接着我们考虑了吡啶氮端作用的两种情况。(1)当4MPY-银簇复合物同时以吡啶氮与水簇或水合质子簇形成氢键时,结果表明吡啶环的部分振动频率随氢键和质子化发生蓝移。(2)当考虑吡啶氮与银簇作用时,吡啶环三角畸变振动发生蓝移。上述情况不仅解释了实验观测的振动频率变化,而且表明了化学环境改变对相对拉曼强度的影响。最后,我们计算了当对巯基吡啶分子以单端或双端与银簇作用,在考虑激发光与低能激发态的能量匹配时,拉曼光谱强度与低能激发态的关系。计算结果表明,在双端吸附构型下,与吡啶氮成键的银簇受激发产生电荷转移态,不仅导致吡啶环v_(12)、v_1和v_(8a)振动的拉曼信号增强,而且选择性地增强吡啶环C―H面内对称弯曲振动v9a的拉曼信号。     

簇合物Mo2X4(X＝S,O)电子结构和光谱性质的Ab Initio研究

在abinitio水平上对含cis/trans-MO₂S₄核和含Mo₂O₄核簇合物的电子结构进行了研究.对钼原子选取和构造了一组适合于含Mo-Mo金属键的双核钼簇合物从头算的(4s4p2d)基组,利用该基组并结合自然键轨道(NBO)方法,对含上述簇胳的3个簇合物的电子结构进行了研究,并与相应的钨簇合物进行了比较.     

Formation and structure of B24N24 clusters

Boron nitride (BN) nanocage clusters of B₂₄N₂₄ were synthesized and detected by laser desorption time-of-flight mass spectrometry. The B₂₄N₂₄ clusters consisted of 4-, 6- and 8-membered BN rings satisfying the isolated tetragonal rule, which was optimized by molecular orbital calculations. The electronic structure showed a bandgap energy of 4.9 eV, which is smaller than that of B₃₆N₃₆ cluster.     

The valence orbitals of NH3 by electron momentum spectroscopy: Quantitative comparisons using Hartree-Fock limit and correlated wavefunctions

The complete valence shell binding energy spectra and valence orbital electron momentum distributions for NH₃ have been measured by high-momentum-resolution electron momentum spectroscopy (EMS). The results are quantitatively compared with theoretical calculations using SCF wavefunctions ranging from DZ quality to a newly developed 126-GTO wavefunction essentially at the Hartree-Fock limit. The 3a₁ and to a lesser extent the 2a₁ valence orbital are not adequately described even at the Hartree-Fock limit with basis set saturation including diffuse functions. The differences between theory and experiment are largely resolved by ion-neutral overlap calculations using CI wavefunctions to incorporate the effects of electron correlation. The 126-G (CI) wavefunctions provide accurate calculation of a wide range of electronic properties of NH₃ and also give good quantitative prediction of the three valence orbital momentum distributions as well as a reasonable prediction of the many-body pole strength distribution observed in the (2a₁)⁻¹ inner valence binding energy spectrum. The present EMS results are compared with recent investigations of wavefunction tails by exterior electron distribution calculations and Penning ionization electron spectroscopy measurements reported by Ohno et al.     

Ab initio calculation of the two lowest excited states of H2S relevant for the photodissociation in the first continuum

We present extensive MRD CI ab initio calculations for the first two excited states of ¹A″ symmetry (¹B₁ and ¹A₂ in the C_2v point group). Full three-dimensional potential energy surfaces are constructed by varying both H---S-bond lengths as well as the HSH bending angle. The lower adiabatic potential is dissociative whereas the upper one is binding. These surfaces, together with the transition dipole moments with the ground electronic state, form the basis for future investigations of the photodissociation of hydrogen sulfide.     

硝基甲烷光重排反应机理的从头算和组态相互作用的研究

硝基甲烷是最简单的硝基化合物,同时也是一种高能物质,在工业上有着重要的作用。对于硝基甲烷形成亚硝酸甲酯的光反应,不同的实验研究得到了不同的结论。     

Ih对称性硼、碳原子簇化合物的从头计算(I)──B32、C2B30及B32H322-的构型及稳定性

本文用从头计算方法研究了含32个顶点的硼烷B₃₂H₃₂^2-及裸原子簇B₃₂与C₂B₃₀的构型、稳定性及反应活性。B₃₂的优化计算结果表明,12个5配位与20个6配位两类硼原子不在同一球面时最稳定,而且所含两种B-B核间距离(R₅₆R₆₆)的优化值验证了Lipscomb的经验数值。B₃₂、B₃₂H₃₂^2-电子结构及原子布居的计算表明骨架成键轨道满足Wade规则。相应的计算还表明,中性BB₃₂H₃₂可能稳定存在,但构型的对称性将低于I_h对称,另外。对C₂B₃₀结合能的计算及前线轨道性质的分析表明,C₂B₃₀热稳定性较高,并且有较强的得电子反应活性。     

正二十面体Au13和Pt13团簇上肉桂醛的吸附

运用广义梯度近似(GGA)密度泛函理论的Perdew-Burke-Ernzerh (PBE)方法, 研究了肉桂醛在正二十面体Au₁₃和Pt₁₃团簇上的吸附行为. 通过分析不同吸附模式的吸附能和几何构型发现: 同一金属团簇, 顺式肉桂醛的吸附能强于反式肉桂醛的吸附能. 对于Au₁₃团簇, 肉桂醛的稳定吸附构型为C＝C和C＝O共吸附模型; 对于Pt₁₃团簇, 肉桂醛的稳定吸附构型为C＝O吸附. 比较二者发现, 肉桂醛在Pt₁₃团簇的吸附能力强于Au₁₃团簇.分析Au₁₃和Pt₁₃团簇上肉桂醛最稳定吸附构型的电子结构表明, 电子由肉桂醛原子的2s、2p轨道向金属表面转移, 同时金属部分电子反馈到肉桂醛的反键轨道, 最终肉桂醛稳定吸附于金属团簇. 此外, 肉桂醛在团簇模型上的吸附能大于其在平板模型上的吸附能.     

纳米团簇Au19Pd和Au19Pt催化解离N2O

采用密度泛函理论研究Au-Pd和Au-Pt 纳米团簇催化解离N₂O. 首先根据计算得到Au₁₉Pd和Au₁₉Pt 团簇的最优构型(杂原子均位于团簇的表面). 以Au₁₉Pd催化解离N₂O为例研究催化解离的反应机理. 对此主要考虑两个反应机理, 分别是Eley-Rideal (ER)和Langmuir-Hinshelwood (LH). 第一个机理中N₂O解离的能垒是1.118 eV, 并且放热0.371 eV. N₂分子脱附后, 表面剩余的氧原子沿着ER路径消除需要克服的能垒是1.920eV, 这比反应沿着LH路径的能垒高0.251 eV. 此外根据LH机理, 氧原子在表面的吸附能是-3.203 eV, 而氧原子在表面转移所需的能垒是0.113 eV, 这表明氧原子十分容易在团簇表面转移, 从而促进氧气分子的生成. 因此, LH为最优反应路径. 为了比较Au₁₉Pd和Au₁₉Pt 对N₂O解离的活性, 根据最优的反应路径来研究Au₁₉Pt 催化解离N₂O, 得到作为铂族元素的铂和钯对N₂O的解离有催化活性, 尤其是钯. 同时, 将团簇与文献中的Au-Pd合金相比较, 得到这两种团簇对N₂O 解离有较高的活性, 尤其是Au₁₉Pd团簇. 再者, O₂的脱附不再是影响反应的主要原因, 这可以进一步提高团簇解离N₂O的活性.     

Photoinduced intramolecular charge transfer (ICT) reaction in trans-methyl p-(dimethylamino) cinnamate: A combined fluorescence measurement and quantum chemical calculations

The photophysical behaviour of trans-methyl p-(dimethylamino) cinnamate (t-MDMAC) donor–acceptor system has been investigated by steady-state absorption and emission spectroscopy and quantum chemical calculations. The molecule t-MDMAC shows an emission from the locally excited state in non-polar solvents. In addition to weak local emission, a strong solvent dependent red shifted fluorescence in polar aprotic solvents is attributed to highly polar intramolecular charge transfer state. However, the formation of hydrogen-bonded clusters with polar protic solvents has been suggested from a linear correlation between the observed red shifted fluorescence band maxima with hydrogen bonding parameters (). Calculations by ab initio and density functional theory show that the lone pair electron at nitrogen center is out of plane of the benzene ring in the global minimum ground state structure. In the gas phase, a potential energy surface along the twist coordinate at the donor (–NMe₂) and acceptor (–CH = CHCOOMe) sites shows stabilization of S₁ state and destabilization S₂ and S₀ states. A similar potential energy calculation along the twist coordinate in acetonitrile solvent using non-equilibrium polarized continuum model also shows more stabilization of S₁ state relative to other states and supports solvent dependent red shifted emission properties. In all types of calculations it is found that the nitrogen lone pair is delocalized over the benzene ring in the global minimum ground state and is localized on the nitrogen centre at the 90° twisted configuration. The S₁ energy state stabilization along the twist coordinate at the donor site and localized nitrogen lone pair at the perpendicular configuration support well the observed dual fluorescence in terms of proposed twisted intramolecular charge transfer (TICT) model.     

Energetics and bonding in beryllium metallized carbon clusters

Small carbon clusters C, C₂ and C₃ metallized with beryllium were studied by first principles within the hybrid density functional approach with generalized gradient correction. Cluster isomer structures for the ground state and several excited states where systematically calculated for C_xBe_y with x = 1–3 and y = 1–4 including the vibrational analysis of all states. Ionization potentials and electron affinities are calculated for the ground state cluster isomers. The thermal stability of the ground state isomers was verified within the harmonic approximation of the Helmholtz free energy up to temperatures of about 1000 K. Within the family of clusters studied, C₂Be₃ undergoes a solid-like structural transition at about 260 K changing from a planar structure at low temperatures to a linear isomer at high temperatures.     

A theoretical study of the ground and first excited singlet state proton transfer reaction in isolated 7-azaindole–water complexes

A systematic study of the proton transfer in the 7-azaindole–water clusters (7-AI(H₂O)_n; n=1–4) in both the ground and first excited singlet electronic states is undertaken. DFT(B3LYP) calculations for the ground electronic state shows that the more stable geometry of the initial normal tautomer presents a cyclic set of hydrogen bonds that links the two nitrogen atoms of the base across the waters. For the n=4 cluster the water molecules adopt a double ring structure so that two cycles of hydrogen bonds are found there. From this structure full tautomerization implies only one transition state so that a concerted but non-synchronous process is predicted by our theoretical calculations. This behavior is found both in the ground and the excited states where CIS geometry optimizations and TD(B3LYP) energy calculations are performed. The difference between both states is the height of the energy barrier that is much lower in the excited state. Another clear difference between both electronic states is that full tautomerization is an endergonic process in the ground state whereas it is clearly exergonic (then favorable) in the excited state. This is so because electronic excitation implies a charge transfer from the five-member cycle to the six-member one of 7-azaindole so that the proton transfer from the pyrrolic side to the pyridinic one is favored. These results clearly indicate that full tautomerization will not likely occur in the ground state but it will be quite easy (and fast) in the excited state. Reaction is already feasible in the S₁ 1:1 complex but it is faster in the 1:2 complex. However the reaction slows again for the 1:3 complex and, finally, reaches a new maximum for the largest cluster studied here, the n=4 case. These results, which are in agreement with experimental data, are explained in terms of the number of hydrogen bonds that are involved in the transfer. The proton transfer through a ring formed by the substrate and two water molecules is found to be the more efficient one, at least in this system.     

单个金或银原子掺杂的氧化钒团簇上的甲烷活化反应

运用密度泛函理论系统研究了甲烷在MV_3O_y~q (M=Au/Ag,y=6–8,q=0或±1)团簇上的吸附和活化。研究得到了吸附体系的微观几何构型、吸附能、电荷分布等性质,找到了5个可以明显活化甲烷分子的含Au团簇。在这些体系中,Au均吸附在基底团簇V_3O_y~q的O位置,而CH_4均在Au原子上被活化。团簇电荷对活化能力有明显影响,阳离子团簇的活化能力最强,中性体系次之,阴离子团簇的活化能力很弱。测试计算表明引入D3色散矫正对于体系结构和能量的计算结果影响不大。本文作为单原子催化剂上甲烷吸附和活化反应的团簇模型研究,为进一步研究单原子催化剂上甲烷的活化机理提供了基础,也为合理设计低温下甲烷转化的单原子催化剂提供了有益的线索。     

Al6ONa2组装Zintl相晶体的理论研究

结合遗传算法和从头算方法确定了Al₆ONa₂的低能异构体. 揭示了以非金属为中心的Al-Na团簇的幻数行为. 金属团簇离域的凝胶轨道和非金属原子的价轨道作用形成定域在O原子上的成键轨道和离域在整个团簇上的反键轨道, 26个价电子形成s ²p ⁶S ²P ⁶D ¹⁰的闭合壳层强幻数电子结构. Al₆O ^2-与Na ⁺通过离子键结合, Al₆O ^2-的电子结构符合Wade-Mingos规则. 基于超原子阴离子Al₆O ^2-具有较强的热稳定性和化学惰性, 组装了具有Pn \bar{3} m和Fd \bar{3} m空间群的AB₂型晶体结构, Al₆ONa₂团簇的结合能为-23.44 eV, 组装结构中团簇单元间的相互作用能分别为-1.19和-1.53 eV. 晶体结构的电子结构分析表明, 2种组装晶体均为宽带隙半导体. 声子色散曲线和弹性常数的计算表明组装晶体具有良好的动力学稳定性.     

内嵌金属碳氮化物团簇富勒烯的稳定性与生成机理

Fullerene molecules have nano-scale cavities in which various metal or metal clusters of different sizes can be embedded to form metallofullerenes with unique core-shell structures. The physical and chemical properties of metallofullerenes can be modified through the interaction between the encapsulated metals and the fullerene cages. As such, the investigation of metallofullerenes with novel structures has been a principal research focus in the field of fullerenes. In this study, we investigated the size matching effect between encapsulated clusters and fullerene cages for the endohedral metal carbonitride clusterfullerenes in order to discover new metallofullerenes. The stability and electronic structure of the metallofullerenes formed by encapsulating M₃NC clusters (M = Y, La, Gd) into D₂(186)-C₉₆ and D₂(35)-C₈₈ fullerenes were studied using quantum chemical calculations. It was found that the fullerene cages formed stable structures by accepting six electrons transferred from the encapsulated clusters. The change in configuration of the encapsulated clusters was clarified by a comparison with the corresponding M₃N@C_2n metal nitride clusterfullerenes; the size matching effect between M₃NC cluster and fullerene cage was elucidated on the basis of the calculated results and previous studies on Sc₃NC@I_h(7)-C₈₀. For the D₂(186)-C₉₆ fullerene, the Gd₃NC cluster was found to have smaller changes in the configuration as compared with the La₃NC cluster, proving that Gd₃NC is more suitable than La₃NC for encapsulation in the D₂(186)-C₉₆ fullerene cage. In addition, it was determined that the La₃NC cluster requires a large structural change to maintain its planar configuration. For the D₂(35)-C₈₈ fullerene cage, the Y₃NC cluster is more suitable than Gd₃NC for encapsulation owing to the smaller size of the Y₃NC cluster. The spatial distribution of the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) of Gd₃NC@D₂(186)-C₉₆ were found to be similar to those of Gd₃N@D₂(186)-C₉₆. However, a unique endohedral cluster-based occupied molecular orbital was found for Gd₃NC@D₂(186)-C₉₆. This orbital is derived from the interaction between the NC unit and the Gd atoms. The spatial distribution of the HOMO of Y₃NC@D₂(35)-C₈₈ is similar to that of Y₃N@D₂(35)-C₈₈, while the LUMO of Y₃NC@D₂(35)-C₈₈ has a much larger contribution from the endohedral cluster as compared to Y₃N@D₂(35)-C₈₈. Thus, the addition of a carbon atom in the cluster has a remarkable impact on the electronic structure of the metallofullerenes. With respect to structural characteristics, we found that the three fullerene cages, D₂(186)-C₉₆, D₂(35)-C₈₈, and I_h(7)-C₈₀, have a uniform distribution of five-membered carbon atom rings; these fullerenes can be greatly stabilized in the form of C_2n^6- anions. However, the formation mechanism of fullerenes and metallofullerenes, at present, is poorly understood. Based on the structural analysis, we propose a direct mechanism for the formation of fullerenes without the Stone-Wales isomerization, i.e., the rearrangement of five-membered rings through the addition of carbon atoms and the transformation into larger carbon cages while maintaining stable structural units.     

5-巯基-1, 3, 4-噻二唑-2-硫酮微溶剂团簇的光谱和理论计算研究

微溶剂作用(即溶剂化过程)广泛存在于所有物理、 化学和生命过程中. 在液相化学反应体系中, 几乎是一切化学反应的基础. 通过傅里叶变换拉曼光谱(FT-Raman)并结合密度泛函理论(DFT), 表征了固态5-巯基- 1, 3, 4-噻二唑-2-硫酮(MTT)的结构, 并进一步确认了MTT在乙腈、 甲醇和水中微溶剂团簇的大小和氢键位点. 通过探究MTT在不同溶剂及pH条件下的紫外-可见吸收光谱(UV-Vis), 结合含时密度泛函理论(TD-DFT)计算, 揭示了溶剂和pH对MTT电子跃迁带的影响, 进一步解释了其光谱位移. 结合能量计算可以得出, MTT分别与1个乙腈、 2个甲醇和2个水分子形成MTT(CH₃CN), MTT(CH₃OH)₂ 和MTT(H₂O)₂团簇.     

单层石墨相氮化碳负载Pt4团簇吸附O2的第一性理论研究

采用第一性原理密度泛函理论结合周期性平板模型模拟研究了Pt₄团簇吸附单层石墨相氮化碳(g-C₃N₄)的几何结构和电子性质,以及氧气在其表面上的吸附行为。同时,对比分析了氧气在纯净的石墨相氮化碳和Pt₄团簇上的吸附行为。计算结果表明, Pt₄团簇吸附在3-s-三嗪环石墨相氮化碳表面,并与四个边缘氮原子成键,形成两个六元环时为最稳定构型。Pt₄团簇倾向于吸附在三嗪环石墨相氮化碳的空位并与邻近三个氮原子成键。由于Pt与N原子较强的杂化作用,以及金属与底物之间较多电子转移增强了Pt₄团簇吸附g-C₃N₄的稳定性。另外,对比分析了氧气在纯净的g-C₃N₄和金属吸附的g-C₃N₄上吸附行为,发现金属原子的加入促进了电子转移,同时拉长了O―O键长。Pt₄吸附3-s-三嗪环g-C₃N₄比Pt₄吸附三嗪环g-C₃N₄表现出微弱的优势,表现出明显的基底扭曲以及较大的吸附能。这些结果表明,化学吸附通过调节电子结构和表面性质增强催化性能的较好方法。     

Scanning tunneling microscopy studies of metal clusters supported on TiO2 (110): Morphology and electronic structure

A brief review of our laboratory's recent scanning tunneling microscopy (STM) studies on nanoclusters supported on TiO₂(110) is presented. Particular emphasis is placed on the system Au/TiO₂(110). The nucleation and growth of the clusters, which were vapor-deposited on TiO₂(110) under ultra high vacuum (UHV) conditions, were investigated using STM. It was found that Au, Pd, and Ag clusters all grow in a three-dimensional (3D) (Volmer-Weber) fashion on TiO₂(110), but that at low coverages, quasi-two dimensional (quasi-2D) Au and Pd clusters were observed. These quasi-2D clusters are characterized by heights of 1–2 atomic layers. Annealing studies show that Au and Pd clusters form large microcrystals with well-defined hexagonal shapes. Al clusters, which have a strong interaction with the substrate, are oxidized upon deposition, “wetting” the surface and forming small clusters. In addition to the topographic studies, the local electronic properties of these clusters have been studied using scanning tunneling spectroscopy (STS) to measure the cluster band gaps. The electronic structure was found to be cluster size-dependent, as seen by the appearance of a band gap as the cluster size decreased. More specifically, the onset of cluster metallic properties correlates with the transition from quasi-2D to 3D cluster growth.