15 research outputs found

    Adsorption and Activation of Methane on the (110) Surface of Rutile-type Metal Dioxides

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    Methane strongly adsorbs on the (110) surface of IrO<sub>2</sub>, a rutile-type metal dioxide. Its C–H bond is facilely dissociated even below room temperature, as predicted in a few theoretical works and actually observed in a recent experimental study. Thence, three questions are posed and answered in this paper: First, why does methane adsorb on the IrO<sub>2</sub> surface so strongly? Second, why is the surface so active for the C–H bond breaking reaction? Third, is there any other rutile-type metal dioxide that is more active than IrO<sub>2</sub>? A second-order perturbation theoretic approach is successfully applied to the analysis of the electronic structure of methane, which is found to be significantly distorted on the surface. Regarding the first point, it is clarified that an attractive orbital interaction between the surface Ir 5d<sub><i>z</i><sup>2</sup></sub> orbital and the distorted methane’s highest occupied molecular orbital leads to the strong adsorption. As for the second point, the bond strength between the surface metal atom and the CH<sub>3</sub> fragment generated after the C–H bond scission of methane is correlated well with the activation barrier. A substantial bonding interaction between CH<sub>3</sub>’s nonbonding orbital and the d<sub><i>z</i><sup>2</sup></sub> orbital hints at the strong Ir–CH<sub>3</sub> bond and hence high catalytic activity ensues. Last but not least, β-PtO<sub>2</sub>, a distorted rutile-type dioxide, is identified as a more active catalyst than IrO<sub>2</sub>. Here again, a perturbation theoretic line of explanation is found to be of tremendous help. This paper is at the intersection of theoretical, catalytic, inorganic, and physical chemistry. Also, it should serve as a model for the design and study of metal-oxide catalysts for the C–H bond activation of methane

    Frontier Orbital Perspective for Quantum Interference in Alternant and Nonalternant Hydrocarbons

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    The wave-particle duality of electrons gives rise to quantum interference (QI) in single molecular devices. A significant challenge to be addressed in molecular electronics is to further develop chemical intuition to understand and predict QI features. In this study, an orbital rule is markedly ameliorated so that it can capture the manifestation of QI not only in alternant hydrocarbons but also in nonalternant ones. The orbital-based prediction about the occurrence of QI in a nonalternant hydrocarbon shows good agreement with experimental results. A simple perturbation theoretic line of reasoning suggests that frontier orbital phase and splitting play a pivotal role in QI phenomena

    Current Rectification through π–π Stacking in Multilayered Donor–Acceptor Cyclophanes

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    Extended π-stacked molecules have attracted much attention since they play an essential role in both electronic devices and biological systems. In this article electron transport properties of a series of multilayered cyclophanes with the hydroquinone donor and quinone acceptor units in the external positions are theoretically studied with applications to molecular rectifiers in mind. Calculations of electron transport through the π–π stacked structures in the multilayered cyclophanes are performed by using nonequilibrium Green’s function method combined with density functional theory. Calculated transmission spectra show that the conductance decreases exponentially with the length of the molecule with a decay factor of 0.75 Å<sup>–1</sup>, which lies for the values between π-conjugated molecules and σ-bonded molecules. Applied bias calculations provide current–voltage curves, which exhibit good rectifying behavior. The rectification mechanism in the coherent transport regime is qualitatively explained by the response of the frontier orbital energy levels, especially LUMO levels, to the applied bias, where the rectifying direction is expected to be opposite to the Aviram–Ratner model. The maximum value of rectification ratio increases with an increase in the number of stacking layers due to the effective separation of the donor and acceptor parts, where effects from the opposite electrodes to the donor and acceptor are negligible. Multilayered donor–acceptor cyclophanes are suitable materials for investigating the relationship among electron transport properties, rectification properties, and molecular length (separation between the donor and acceptor parts)

    Orbital Determining Spintronic Properties of a π‑Conjugated System

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    Spintronic properties of cyclobutadiene (CBD) systems are investigated based on a qualitative frontier orbital analysis. CBD undergoes a Jahn–Teller distortion from the square triplet state to the rectangular singlet state. According to the qualitative Hückel molecular orbital analysis, the electron transport through the square triplet state is symmetry allowed, whereas that through the rectangular singlet state is symmetry forbidden. The magnetic triplet state is a possible coexisting system of conductivity and magnetism. Sophisticated first-principles quantum chemical calculations are performed by using a realistic molecular junction model. Obtained results are in good agreement with the prediction based on the qualitative orbital analysis. Interesting spin filtering properties are found in the square-shaped CBD system. The high- and low-spin states of the square-shaped CBD system produce the spin-α and spin-β polarized conductance, respectively. The qualitative orbital analysis is useful as a guiding principle for designing molecular spintronics

    Molecular Rectifier Based on π–π Stacked Charge Transfer Complex

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    Electron transport through π–π stacked materials has been studied theoretically and experimentally so far with versatile applications in mind. In this paper a novel π–π stacked molecular rectifier is proposed. Electron transport properties through cyclophane-type quinhydrone are investigated by using nonequilibrium Green’s function method combined with density functional theory. The investigated molecule has a quinhydrone structure comprised of π–π stacked donor (hydroquinone) and acceptor (benzoquinone) pair due to the in-phase orbital interaction between the HOMO of hydroquinone and the LUMO of benzoquinone. A computed current–voltage curve shows rectifying behavior in the direction perpendicular to the ring plane. The maximum value of rectification ratio of 2.37 is obtained at 0.8 V. In this system the LUMO level plays a key role, and asymmetrical evolution of the LUMO level for positive and negative biases leads to the rectifying behavior. The present study is a basic step for further functionalization of a molecular rectifier based on transannular electron transport. The understanding of insight into the electron transport through a π–π stacked system will provide motivation for design of future molecular devices

    Orbital Control of Single-Molecule Conductance Perturbed by π‑Accepting Anchor Groups: Cyanide and Isocyanide

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    Electron transport properties through benzene molecules disubstituted with π-accepting cyanide and isocyanide anchor groups at their para and meta positions are investigated on the basis of a qualitative orbital analysis at the Hückel molecular orbital level of theory. The applicability of a previously derived orbital symmetry rule for electron transport is extended to the systems perturbed by the π-accepting anchor groups, where the HOMO–LUMO symmetry in the molecular orbital energies relative to the Fermi level is removed. The conservation of the HOMO–LUMO symmetry in the spatial distribution of the molecular orbitals between the unperturbed benzene molecule and the perturbed molecules with the anchor groups rationalizes symmetry-allowed electron transport through the para isomers. On the other hand, destructive interferences between the nearly 2-fold degenerate frontier orbitals constructed from the 2-fold degenerate orbitals of the unperturbed benzene molecule and the anchor groups lead to symmetry-forbidden electron transport through the meta isomers. The qualitative orbital thinking is supported by more quantitative density functional theory (DFT) calculations combined with the nonequilibrium Green’s function (NEGF) method. The orbital analysis is a powerful tool for the understanding and rational design of molecular devices composed of π-conjugated hydrocarbons and those perturbed by the π-accepting anchor groups

    Quantum Interference, Graphs, Walks, and Polynomials

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    In this paper, we explore quantum interference (QI) in molecular conductance from the point of view of graph theory and walks on lattices. By virtue of the Cayley–Hamilton theorem for characteristic polynomials and the Coulson–Rushbrooke pairing theorem for alternant hydrocarbons, it is possible to derive a finite series expansion of the Green’s function for electron transmission in terms of the odd powers of the vertex adjacency matrix or Hückel matrix. This means that only odd-length walks on a molecular graph contribute to the conductivity through a molecule. Thus, if there are only even-length walks between two atoms, quantum interference is expected to occur in the electron transport between them. However, even if there are only odd-length walks between two atoms, a situation may come about where the contributions to the QI of some odd-length walks are canceled by others, leading to another class of quantum interference. For nonalternant hydrocarbons, the finite Green’s function expansion may include both even and odd powers. Nevertheless, QI can in some circumstances come about for nonalternants from cancellation of odd- and even-length walk terms. We report some progress, but not a complete resolution, of the problem of understanding the coefficients in the expansion of the Green’s function in a power series of the adjacency matrix, these coefficients being behind the cancellations that we have mentioned. Furthermore, we introduce a perturbation theory for transmission as well as some potentially useful infinite power series expansions of the Green’s function

    Oxidative Addition of Methane and Reductive Elimination of Ethane and Hydrogen on Surfaces: From Pure Metals to Single Atom Alloys

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    Oxidative addition of CH4 to the catalyst surface produces CH3 and H. If the CH3 species generated on the surface couple with each other, reductive elimination of C2H6 may be achieved. Similarly, H’s could couple to form H2. This is the outline of nonoxidative coupling of methane (NOCM). It is difficult to achieve this reaction on a typical Pt catalyst surface. This is because methane is overoxidized and coking occurs. In this study, the authors approach this problem from a molecular aspect, relying on organometallic or complex chemistry concepts. Diagrams obtained by extending the concepts of the Walsh diagram to surface reactions are used extensively. C–H bond activation, i.e., oxidative addition, and C–C and H–H bond formation, i.e., reductive elimination, on metal catalyst surfaces are thoroughly discussed from the point of view of orbital theory. The density functional theory method for structural optimization and accurate energy calculations and the extended Hückel method for detailed analysis of crystal orbital changes and interactions play complementary roles. Limitations of monometallic catalysts are noted. Therefore, a rational design of single atom alloy (SAA) catalysts is attempted. As a result, the effectiveness of the Pt1/Au(111) SAA catalyst for NOCM is theoretically proposed. On such an SAA surface, one would expect to find a single Pt monatomic site in a sea of inert Au atoms. This is desirable for both inhibiting overoxidation and promoting reductive elimination

    The Influence of Linkers on Quantum Interference: A Linker Theorem

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    How heteroatomic substitutions affect electron transport through π-conjugated hydrocarbons has been the subject of some debate. In this paper we investigate the effect of heteroatomic linkers in a molecular junction on the electron-transmission spectrum, focusing on the occurrence of quantum interference (QI) close to the Fermi level, where conductivity can be significantly suppressed. We find that the substitution or addition of heteroatoms to a carbon skeleton at the contact positions does not change the main feature of QI due to the underlying carbon skeleton. QI in the overall system thus remains a robust feature. This empirical observation leads us to derive, in two mathematical ways, that these findings can be generalized. We note that addition or substitution of a carbon atom by a heteroatom at the contact positions will increase or decrease the number of electrons in the π-system, which will lead to a change in the alignment of the molecular orbitals of the isolated system relative to the electrode Fermi level. Both Hückel and density functional theory calculations on model systems probe the effect of this Fermi level change and confirm qualitatively the implications of the underlying mathematical proofs

    Current Rectification in Nitrogen- and Boron-Doped Nanographenes and Cyclophanes

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    Electron transport properties of boron- and nitrogen-doped polycyclic aromatic hydrocarbons and cyclophanes are investigated with the nonequilibrium Green’s function method and compared to transport properties of the unsubstituted species. The aim of the study is to derive the effect of the heteroatomic defects on the conductance of nanographenes and to propose new effective ways for current control and design of carbon devices. Of special interest are the electrical current rectifying properties of asymmetrically doped nanographenes and cyclophanes, as well as the rectification mechanism. The mechanisms of donor-π bridge-acceptor and donor-σ bridge-acceptor rectification are used to explain the diode-like properties of asymmetrically doped nanographenes and cyclophanes. The electron-rich nitrogen and electron-poor boron heteroatoms introduce conductance channels within the highest occupied molecular orbital–lowest unoccupied molecular orbital gaps of the hydrocarbons and cyclophanes and significantly enhance the conductance. The combination of nitrogen and boron impurities in one polycyclic aromatic hydrocarbon leads to asymmetrical I/V curves. The rectification is further enhanced in the cyclophanes where the boron impurities are located in one of the layers and the nitrogen impurities in the other. Owing to the efficient separation of the donor and acceptor parts, a higher rectification ratio is estimated. The rectifying properties of the asymmetrically doped carbon materials are derived from the nonequilibrium Green’s function theory. The main reason for the rectification is found to be the interaction of the external electric field induced between the electrodes with the molecular orbitals of the asymmetrically doped hydrocarbons
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