Single‐Molecule Study of a Plasmon‐Induced Reaction for a Strongly Chemisorbed Molecule

Chemical reactions induced by plasmons achieve effective solar‐to‐chemical energy conversion. However, the mechanism of these reactions, which generate a strong electric field, hot carriers, and heat through the excitation and decay processes, is still controversial. In addition, it is not fully understood which factor governs the mechanism. To obtain mechanistic knowledge, we investigated the plasmon‐induced dissociation of a single‐molecule strongly chemisorbed on a metal surface, two O₂ species chemisorbed on Ag(110) with different orientations and electronic structures, using a scanning tunneling microscope (STM) combined with light irradiation at 5 K. A combination of quantitative analysis by the STM and density functional theory calculations revealed that the hot carriers are transferred to the antibonding (π*) orbitals of O₂ strongly hybridized with the metal states and that the dominant pathway and reaction yield are determined by the electronic structures formed by the molecule–metal chemical interaction.     

Single-Molecule Study of a Plasmon-Induced Reaction for a Strongly Chemisorbed Molecule

Chemical reactions induced by plasmons achieve effective solar-to-chemical energy conversion. However, the mechanism of these reactions, which generate a strong electric field, hot carriers, and heat through the excitation and decay processes, is still controversial. In addition, it is not fully understood which factor governs the mechanism. To obtain mechanistic knowledge, we investigated the plasmon-induced dissociation of a single-molecule strongly chemisorbed on a metal surface, two O₂ species chemisorbed on Ag(110) with different orientations and electronic structures, using a scanning tunneling microscope (STM) combined with light irradiation at 5 K. A combination of quantitative analysis by the STM and density functional theory calculations revealed that the hot carriers are transferred to the antibonding (π*) orbitals of O₂ strongly hybridized with the metal states and that the dominant pathway and reaction yield are determined by the electronic structures formed by the molecule–metal chemical interaction.     

Role of Reduced Defects for Coupling Reactions of Acetaldehyde on Anatase TiO2(001)-(1×4) Surface

The chemistry of acetaldehyde (CH₃CHO) adsorbed on the anatase TiO₂(001)-(1×4) surface has been investigated by temperature-programmed desorption (TPD) method. Our experimental results provide the direct evidence that the perfect lattice sites on the anatase TiO₂(001)-(1×4) surface are quite inert for the reaction of CH₃CHO, but the reduced defect sites on the surface are active for the thermally driven reductive carbon-carbon coupling reactions of CH₃CHO to produce 2-butanone and butene. We propose that the coupling reactions of CH₃CHO on the anatase TiO₂(001)-(1×4) surface should undergo through the adsorption of paired CH3CHO molecules at the reduced defect sites, since the existing reduced Ti pairs provide the suitable adsorption sites.     

硅苯与HX (X=F,OH, NH2)的1,2-及1,4-加成反应

采用密度泛函理论方法在B3LYP/6-311++G(d,p)水平上, 研究了硅苯与HX (X=F, OH, NH₂)的1,2-及1,4-加成反应的微观机理和势能剖面, 考察了Si 原子上的取代基及四氢呋喃溶剂对反应势能剖面的影响. 研究结果表明, 标题反应有两种可能的机理: (1) 硅苯与一个HX (X=F, OH, NH₂)分子先形成中间复合物, 然后经过四元环过渡态(机理1)生成最终产物; (2) 硅苯与两个HX分子先形成中间复合物, 然后经过六元环过渡态(机理2)生成另一中间复合物, 该中间复合物脱去一个HX分子形成最终产物. 机理2 在动力学上远较机理1 有利. 1,2-及1,4-加成产物哪种优先形成由动力学控制且与X基团的种类有关. HX在气相中参与加成反应从易到难的次序为: HF>H₂O>NH₃. Si 原子上具有较强供电子和吸电子性质的取代基, 在热力学和动力学上均有利于反应的进行, 但具有较大体积的2,4,6-三甲基苯基取代基对反应反而不利. 四氢呋喃溶剂在热力学上不利于硅苯与HX的1,2-及1,4-加成反应, 在动力学上对HF或H₂O作为加成试剂的反应也不利, 但对NH3作为加成试剂的反应反而有利.     

Structures and chemical reactions of SO2 adsorbates studied by surface XAFS

Adsorption and surface chemical reactions of SO₂ on Ni, Cu and Pd metal surfaces are studied by using surface XAFS, as well as XPS and STM. It has turned out that SO₂ lies flat on the Ni(100), Ni(110) and Ni(111) surfaces, while it stands on Pd(100) and Pd(111). By raising the temperature, surface reactions occur on these metal surfaces. Typical reactions on Ni and Cu are 3SO₂ → S+2SO₃, while those on Pd are 2SO₂ → S+SO₄. The structures of the adsorbate species are elucidated.     

Activation of Oxygen on Gold and Silver Nanoparticles Assisted by Surface Plasmon Resonances

Surface plasmon resonances (SPRs) have been found to promote chemical reactions. In most oxidative chemical reactions oxygen molecules participate and understanding of the activation mechanism of oxygen molecules is highly important. For this purpose, we applied surface‐enhanced Raman spectroscopy (SERS) to find out the mechanism of SPR‐assisted activation of oxygen, by using p‐aminothiophenol (PATP), which undergoes a SPR‐assisted selective oxidation, as a probe molecule. In this way, SPR has the dual function of activating the chemical reaction and enhancing the Raman signal of surface species. Both experiments and DFT calculations reveal that oxygen molecules were activated by accepting an electron from a metal nanoparticle under the excitation of SPR to form a strongly adsorbed oxygen molecule anion. The anion was then transformed to Au or Ag oxides or hydroxides on the surface to oxidize the surface species, which was also supported by the heating effect of the SPR. This work points to a promising new era of SPR‐assisted catalytic reactions.     

Ab initio calculations of activation energy of the reaction of hydrogen exchange on strongly acidic centers

Ab initio calculations of fragments of the potential energy surfaces of hydrogen exchange reactions between H₂, CH₄, and alanine molecules and the H₃O⁺ ion were performed by the restricted Hartree-Fock method, at the second-order Møller-Plesset level of perturbation theory, and by the method of coupled clusters using the 6–31G^* and aug-cc-pVDZ basis sets. The one-center synchronous mechanism of hydrogen exchange reaction was studied and the activation energies and structures of transition states were determined. It was found that the geometric parameters of the H₂ and CH₄ molecules in the transition states are close to those of the H₃ ⁺ and CH₅ ⁺ ions. The higher the proton affinity of the reacting molecule in the reaction studied the lower the activiation energy of hydrogen exchange. The one-center mechanism studied can be used to describe the high-temperature solid-state catalytic isotope exchange (HSCIE) reaction. The results ofab initio calculations of synchronous hydrogen exchange between the H₃O⁺ ion and hydrogen atoms in different positions of the alanine molecule are in good agreement with experimental data on the regioselectivity and stereoselectivity of the HSCIE reaction with spillover-tritium.     

甲硫醇在Cu(111)表面的解离吸附: 密度泛函理论研究

采用基于密度泛函理论的第一性原理方法和平板模型研究了CH₃SH分子在Cu(111)表面的吸附反应.系统地计算了S原子在不同位置以不同方式吸附的一系列构型, 第一次得到未解离的CH₃SH分子在Cu(111)表面顶位上的稳定吸附构型,该构型吸附属于弱的化学吸附, 吸附能为0.39 eV. 计算同时发现在热力学上解离结构比未解离结构更加稳定. 解离的CH₃S吸附在桥位和中空位之间, 吸附能为0.75-0.77 eV. 计算分析了未解离吸附到解离吸附的两条反应路径, 最小能量路径的能垒为0.57 eV. 计算结果还表明S―H键断裂后的H原子并不是以H₂分子的形式从表面解吸附而是以与表面成键的形式存在. 通过比较S原子在独立的CH₃SH分子和吸附状态下的局域态密度, 发现S―H键断裂后S原子和表面的键合强于未断裂时S原子和表面的键合.     

Action spectroscopy of single molecules reactions with STM – My personal view back from 2001-

Having obtained an invitation to submit this personal view back to 2001 when I started to work with Prof. Maki Kawai for developing a theory of lateral hopping of a single CO molecule on Pd (1 1 0) with Bo Persson, I briefly describe how I got an idea for elementary processes of vibrationally mediated reactions of single molecules on metal surfaces. During the work with Prof. S.G. Thihodeev on a theory of inelastic electron tunneling spectroscopy (IETS) with scanning tunneling spectrum (STM-IETS), I found that IET current is expressed in terms of a vibrational density of states of a single molecule. This enabled me to propose a formula for a reaction rate $R(V)$ or yield per electron $Y(V)=R(V)/I$, here I is a tunneling current, i.e., action spectrum (STM-AS) of a single molecule reaction. I applied this formula to reproduce the experimental result of a CO molecule hopping on Pd (1 1 0) surface and more insights into the elementary process were revealed. Thomas Frederiksen and Magus Paulsson jointed me to develop a general formula of $Y(V)$ and successfully applied it to analyse the experimental results of H-atom relay reaction of a linear chain, H(D)₂O-OH(D)-O(D) H?→?H(D)-H(D)₂-OH(D) ?→?H(D)-H(D)-OH(D)₂ that was observed by Takashi Kumagai and Hiroshi Okuyama. Actually a hydrogen atom excited at one end of a linear chain composed of H₂O and several OH generates another one at the other end. We employed our formula of to reproduce the experimental result of $Y(V)$. It was found that excitation of the three characteristic vibrational modes (free OH/OD stretch, OH¹?=?OD¹ stretch, and H₂O scissors, where H¹?=?D¹ denotes the shared H/D¹ atom in the H bond) were involved in the relay reaction. It was remarked that the OH(D¹)?=?OD(D¹ stretch modes are significantly redshifted from free OH/OD stretch and also characterized by very large broadening. The significant mode softening with respect to the free stretch modes and spectacular enhancement of the width are known to originate in the strong anharmonic character of a single H bond. Thomas investigated the reaction pathway from total energy calculations for the H-atom transfer reaction by the nudged elastic band method. The initial step is translation of the shared H-atom to the center hydroxyl, which is almost barrierless. The subsequent H-bond cleavage between OH and the center water molecule constitutes the highest barrier in which the displacement of the center water molecule along the [0 0 1] direction is mainly involved. The OH, OH¹ stretch and H₂O scissors modes are therefore postulated to couple to the reaction coordinate for the H-bond cleavage. We have demonstrated a vibrationally induced H-atom-bond relay reaction within H-bonded chains assembled on Cu(1 1 0). In this reaction H-atom transfer results in the ‘structural’ transfer of a water molecule from one end of the chain to the other end without changing the platform of the chain, or actually transferring the molecule.I have been thinking the unresolved issue of C₂H(D)₂ rotation on Cu (1 0 0) since it was published in 1998 by the W. Ho group. This experimental methods and the results obtained as the first demonstration of a single molecule switch are widely recognized as a milestone report of a single molecule manipulation by tunneling current and applied bias voltage which excites the vibrational modes of a molecule. They observed the STM images rotated at 90 degrees before and after applying appropriate bias voltage. They further compared the IETS spectrum and the $Y(V)$ for the rotation. The observed peak beautifully agreed with the threshold bias voltage, which clearly evidenced that a rotation is induced by excitation of a particular vibrational mode of C₂H(D)₂. In particular a crossover from a single electron process to a two electron process with increase in a tunneling current are of great interest. Sergei and his PhD student Yulia E. Shchadilova at that time and Magnus helped me much to reproduce all the experimental results by employing the Keldysh Green’s function theory combined with ab initio density functional theory (DFT) calculation of the optimized adsorption geometry and sophisticated vibrational analysis done by Magus. The experimental result of $Y(V)$ was reproduced by assuming a single electron process to excite the C-H stretch mode, and two electron process (ladder climbing of the C-H vibrational levels) and a excitation of the combination band. I also describe a brief theory of STM-AS I developed with Bo, Sergei, Magnus and Thomas.     

Probing the imine silylenoid HN=SiNaF and its insertions reaction with R–H (R=F,OH, NH<Subscript>2</Subscript>, CH<Subscript>3</Subscript>) using DFT

The geometries and isomerization of the imine silylenoid HN=SiNaF as well as its insertion reactions with some R–H molecules have been systematically investigated theoretically, where R=F, OH, NH₂, and CH₃, respectively. The barrier heights for the four insertion reactions are 67.7, 115.6, 153.5, and 271.5 kJ/mol at the B3LYP/6-311+G* level of theory, respectively. Here, all the mechanisms of the four reactions are identical to each other, i.e., a stable intermediate has been formed during the insertion reaction. Then, the intermediate could dissociate into the substituted silylene (HN=SiHR) and NaF with a barrier corresponding to their respective dissociation energies. Correspondingly, the reaction energies for the four reactions are 71.8, 95.5, 123.3, and 207.6 kJ/mol, respectively, which are linearly correlated with the calculated barrier heights. Furthermore, the effects of halogen substitutions (F, Cl, and Br) on the reaction activity have also been discussed. As a result, the relative reactivity among the four insertion reactions should be as follows: H–F > H–OH > H–NH₂ > H–CH₃.     

Mechanistic Studies of Plasmon Chemistry on Metal Catalysts

Chemical reactions induced by the localized surface plasmon (LSP) of metal nanostructures could be important for a sustainable society to achieve highly efficient conversion from solar energy to chemical energy. However, the reaction mechanism of plasmon chemistry in metal catalysis is still controversial. Mechanistic studies of plasmon chemistry involving direct interactions between the LSP and molecules are reviewed and discussed in terms of the excitation mechanisms of the molecules. We focus on the studies performed using plasmonic metal nanoparticles and highlight the recent progress in plasmon chemistry investigated using scanning probe microscopy with high spatial resolution to obtain mechanistic insights that cannot be obtained by macroscopic analytical methods. This Minireview delivers an overview of the mechanistic understanding of plasmon chemistry in metal catalysis at the current stage, and provides guidance for future studies with respect to clarifying reaction mechanisms.     

HF在α-AlF3(0001)表面吸附的密度泛函理论研究

利用密度泛函理论系统研究了不同覆盖度下HF在3F、2F、1F与Al 终端的α-AlF₃(0001)表面的吸附行为, 分析了HF与不同终端表面相互作用的电子机制. 计算结果表明: HF在3F终端的α-AlF₃(0001)表面物理吸附; 在2F及1F终端表面化学吸附, 形成Al-F键和FHF结构, 使HF分子活化, 可以参加下一步的氟化反应; 在Al 终端表面解离吸附形成Al-F与Al-H键. 3F、2F、1F及Al 终端表面配位不饱和数目分别为0、1、2与3配位.不同覆盖度研究表明, 在2F终端表面上, 吸附一个HF分子使表面Al 配位达到饱和, 后续吸附的HF为物理吸附; 而在1F与Al 终端表面仍可化学吸附. 因此, 推测α-AlF₃暴露不同终端表面中Al 原子配位不饱和数越高, 其对HF吸附与活化能力越强, 可能的氟化催化反应活性越高. 差分电荷密度与电子态密度分析表明, HF与3F终端α-AlF₃(0001)表面发生弱相互作用, 而与2F、1F与Al 终端表面形成较强的电子相互作用.     

Quantum Dynamics of Oxyhydrogen Complex-Forming Reactions for the HO2 and HO3 Systems

Complex-forming reactions widely exist in gas-phase chemical reactions.Various complexforming bimolecular reactions have been investigated and interesting phenomena have been discovered.The complex-forming reactions usually have small or no barrier in the entrance channel, which leads to obvious differences in kinetic and dynamic characteristics compared with direct reactions.Theoretically, quantum state-resolved reaction dynamics can provide the most detailed microscopic dynamic mechanisms and is now feasible for a direct reaction with only one potential barrier.However, it is of great challenge to construct accurate potential energy surfaces and perform accurate quantum dynamics calculations for a complex polyatomic reaction involving deep potential wells and multi-channels.This paper reviews the most recent progress in two prototypical oxyhydrogen complex-forming reaction systems, HO₂ and HO₃, which are significant in combustion, atmospheric, and interstellar chemistry.We will present a brief survey of both computational and experimental work and emphasize on some unsolved problems existing in these systems.     

Quantum chemical study of nickel(II) complexes with cyclic diimine ligands on the base of bis[3,3′-iminopropyl]methylamine

A quantum chemical study of spatial and electronic structures of molecules in the frame complexes, bis[3,3′(RR′)-ketiminepropyl]methylamine nickel dichlorides, where R = H, CH₃, and R′ = H, CH₃, has been performed by DFT(B3LYP/LANL2DZ) method. The molecules of these complexes were found to be in a triplet state. Energy stability of the endo form of the complex molecules was shown. In the molecule of bis[3,3′-aldiminopropyl]methylamine nickel dichloride (R = R′ = H), a considerable strengthening of the bond Ni-N(amine) takes place when passing from the diamagnetic into paramagnetic state, and all bonds Ni-N become equivalent with respect to interatomic distance values. The topology analysis of the electron density for the complexes with R = R′ =H and R = R′ = CH₃ was carried out. It is stated that all Ni-N bonds are covalent in the molecules of paramagnetic complexes.     

磁性分子自旋态检测与调控的STM研究

单分子自旋态检测与可逆调控是目前物理、化学及信息技术等领域的研究热点．本文综述了扫描隧道显微镜在该领域的研究进展,着重论述了酞菁类磁性分子在金属单晶表面或绝缘层薄膜上磁性的检测;自旋交叉配合物分子自旋双稳态的检测与可逆调控;单分子磁体的表面制备及输运性质的检测．     

Prebiotic Synthesis and Isomerization in Interstellar Analog Ice: Glycinal,Acetamide, and Their Enol Tautomers

Glycinal (HCOCH₂NH₂) and acetamide (CH₃CONH₂) are simple molecular building blocks of biomolecules in prebiotic chemistry, though their origin on early Earth and formation in interstellar media remain a mystery. These molecules are formed with their tautomers in low temperature interstellar model ices upon interaction with simulated galactic cosmic rays. Glycinal and acetamide are accessed via barrierless radical-radical reactions of vinoxy (⋅CH₂CHO) and acetyl (⋅C(O)CH₃), and then undergo keto-enol tautomerization. Exploiting tunable photoionization reflectron time-of-flight mass spectroscopy and photoionization efficiency (PIE) curves, these results demonstrate fundamental reaction pathways for the formation of complex organics through non-equilibrium ice reactions in cold molecular cloud environments. These molecules demonstrate an unconventional starting point for abiotic synthesis of organics relevant to contemporary biomolecules like polypeptides and cell membranes in deep space.     

DFT studies of the phosphinidene derivative HPNaF and its insertion reaction with R–H(R=F,OH, NH<Subscript>2</Subscript>, CH<Subscript>3</Subscript>)

The formations of the phosphinidene derivative HPNaF and its insertion reactions with R–H (R=F, OH, NH₂, CH₃) have been systematically investigated employing the density functional theory (DFT), such as the B3LYP and MPW1PW91 methods. A comparison with the results of MP2 calculations shows that MPW1PW91 underestimates the barrier heights for the four reactions considered. Similarly, the same is also true for the B3LYP method depending on the selected reactions, but by much less than MPW1PW91, where the barrier heights of the four reactions are 25.2, 85.7, 119.0, and 142.4 kJ/mol at the B3LYP/6-311+G* level of theory, respectively. All the mechanisms of the four reactions are identical to each other, i.e., an intermediate has been located during the insertion reaction. Then, the intermediate could dissociate to substituted phosphinidane(H₂RP) and NaF with a barrier corresponding to their respective dissociation energies. Correspondingly, the reaction energies for the four reactions are −92.2, −68.1, −57.2, and −44.3 kJ/mol at the B3LYP/6-311+G* level of theory, respectively, where both the B3LYP and MPW1PW91 methods underestimate the reaction energies compared with the MP2 results. The linear correlations between the calculated barrier heights and the reaction energies have also been observed. As a result, the relative reactivity among the four insertion reactions should be as follows: H–F > H–OH > H–NH₂ > H–CH₃.     

CH2XH→CH3X (X＝O, S, Se)异构化的量子拓扑研究

利用从头算和量子拓扑方法讨论了CH₂XH→CH₃X (X＝O, S, Se)异构化过程的反应机理. 着重从电子密度拓扑分析计算了反应进程中的各点, 讨论了反应进程中键的断裂和生成, 上述反应都经历了三元环过渡结构, 找到了这类反应的"能量过渡态"和"结构过渡态", 且结构过渡态均在能量过渡态之后出现. 三元结构过渡态结构出现的范围与反应热成正比.     

Au(111)表面Verdazyl自由基的构象转换

Pure organic radical molecules on metal surfaces are of great significance in exploration of the electron spin behavior. However, only a few of them are investigated in surface studies due to their poor thermal stability. The adsorption and conformational switching of two verdazyl radical molecules, namely, 1, 5-biisopropyl-3-(benzo[b]benzo[4,5]thieno[2, 3-d]thiophen-2-yl)-6-oxoverdazyl (B2P) and 1, 5-biisopropyl-3-(benzo[b]benzo[4,5]thieno[2, 3-d]thiophen-4-yl)-6-oxoverdazyl (B4P), are studied by scanning tunneling microscopy (STM) and density functional theory (DFT). The adsorbed B2P molecules on Au(111) form dimers, trimers and tetramers without any ordered assembly structure in which two distinct appearances of B2P in STM images are observed and assigned to be its "P" and "T" conformations. The "P" conformation molecules appear in the STM image with a large elliptical protrusion and two small ones of equal size, while the "T" ones appear with a large protrusion and two small ones of different size. Likewise, the B4P molecules on Au(111) form dimers at low coverage, strip structure at medium coverage and assembled structure at high coverage which also consists of above-mentioned two conformations. Both B2P molecules and B4P molecules are held together by weak intermolecular interaction rather than chemical bond. STM tip induced conformational switching of both verdayzl radicals is observed at the bias voltage of +2.0 V. The "T" conformation of B2P can be switched to the "P" while the "P" conformation of B4P can be switched to the "T" one. For both molecules, such a conformational switching is irreversible. The DFT calculations with Perdew-Burke-Ernzerhof version exchange-correlation functional are used to optimize the model structure and simulate the STM images. STM images of several possible molecular conformations with different isopropyl orientation and different tilt angle between verdazyl radical and Au(111) surface are simulated. For conformations with different isopropyl orientation, the STM simulated images are similar, while different tilt angles of verdazyl radical lead to significantly different STM simulated images. Combined STM experiments and DFT simulations reveal that the conformational switching originates from the change of tilting angle between the verdazyl radical and Au(111) surface. The tilt angles in "P" and "T" conformations are 0° and 50°, respectively. In this study, two different adsorption conformations of verdazyl radicals on the Au(111) surface are presented and their exact adsorption structures are identified. This study provides a possible way to study the relationship between the electron spin and configuration conversion of pure organic radical molecules and a reference for designing more conformational switchable radical molecules that can be employed as interesting molecular switches.     

Exploring the Dual Characteristics of CH3OH Adsorption to Metal Atomic Structures on Si (111)-7 × 7 Surface

Metal atoms were deposited on an Si (111)-7 × 7 surface, and they were adsorbed with alcohol gases (CH₃OH/C₂H₅OH/C₃H₇OH). Initially, C_nH_2n+1OH adsorption was simply used as an intermediate layer to prevent the chemical reaction between metal and Si atoms. Through scanning tunneling microscopy (STM) and a mass spectrometer, the C_nH_2n+1OH dissociation process is further derived as the construction of a surface quasi-potential with horizontal and vertical directions. With the help of three typical metal depositions, the surface characteristics of CH₃OH adsorption are more clearly presented in this paper. Adjusting the preheating temperature, the difference of thermal stability between CH₃O^– and H⁺ could be obviously derived in Au deposition. After a large amount of H⁺ was separated, the isolation characteristic of CH₃O^– was discussed in the case of Fe deposition. In the process of building a new metal-CH₃O^–-H⁺ model, the dual characteristics of CH₃OH were synthetically verified in Sn deposition. CH₃O^– adsorption is prone to influencing the interaction between the metal deposition and substrate surface in the vertical direction, while H⁺ adsorption determines the horizontal behavior of metal atoms. These investigations lead one to believe that, to a certain extent, the formation of regular metal atomic structures on the Si (111)-7 × 7-CH₃OH surface is promoted, especially according to the dual characteristics and adsorption models we explored.