15 research outputs found

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

Methane strongly
adsorbs on the (110) surface of IrO<sub>2</sub>, a rutile-type metal
dioxide. Its Câ€“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 Câ€“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 methaneâ€™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 Câ€“H bond scission of
methane is correlated well with the activation barrier. A substantial
bonding interaction between CH<sub>3</sub>â€™s nonbonding orbital
and the d<sub><i>z</i><sup>2</sup></sub> orbital hints at
the strong Irâ€“CH<sub>3</sub> bond and hence high catalytic
activity ensues. Last but not least, Î²-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 Câ€“H bond activation of methane

### Frontier Orbital Perspective for Quantum Interference in Alternant and Nonalternant Hydrocarbons

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 Ï€â€“Ï€ Stacking in Multilayered Donorâ€“Acceptor Cyclophanes

Extended Ï€-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 Ï€â€“Ï€ stacked structures
in the multilayered cyclophanes are performed by using nonequilibrium
Greenâ€™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 Ã…<sup>â€“1</sup>, which lies for the values between
Ï€-conjugated molecules and Ïƒ-bonded molecules. Applied
bias calculations provide currentâ€“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 Aviramâ€“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 donorâ€“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 Ï€â€‘Conjugated System

Spintronic properties of cyclobutadiene (CBD) systems
are investigated
based on a qualitative frontier orbital analysis. CBD undergoes a
Jahnâ€“Teller distortion from the square triplet state to the
rectangular singlet state. According to the qualitative HuÌˆckel
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-Î± and spin-Î² polarized conductance,
respectively. The qualitative orbital analysis is useful as a guiding
principle for designing molecular spintronics

### Molecular Rectifier Based on Ï€â€“Ï€ Stacked Charge Transfer Complex

Electron transport through Ï€â€“Ï€ stacked materials has been studied theoretically and experimentally so far with versatile applications in mind. In this paper a novel Ï€â€“Ï€ stacked molecular rectifier is proposed. Electron transport properties through cyclophane-type quinhydrone are investigated by using nonequilibrium Greenâ€™s function method combined with density functional theory. The investigated molecule has a quinhydrone structure comprised of Ï€â€“Ï€ 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 currentâ€“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 Ï€â€“Ï€ stacked system will provide motivation for design of future molecular devices

### Orbital Control of Single-Molecule Conductance Perturbed by Ï€â€‘Accepting Anchor Groups: Cyanide and Isocyanide

Electron transport properties through benzene molecules
disubstituted
with Ï€-accepting cyanide and isocyanide anchor groups at their
para and meta positions are investigated on the basis of a qualitative
orbital analysis at the HuÌˆckel 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 Ï€-accepting
anchor groups, where the HOMOâ€“LUMO symmetry in the molecular
orbital energies relative to the Fermi level is removed. The conservation
of the HOMOâ€“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 Greenâ€™s
function (NEGF) method. The orbital analysis is a powerful tool for
the understanding and rational design of molecular devices composed
of Ï€-conjugated hydrocarbons and those perturbed by the Ï€-accepting
anchor groups

### Quantum Interference, Graphs, Walks, and Polynomials

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 Cayleyâ€“Hamilton theorem for characteristic
polynomials and the Coulsonâ€“Rushbrooke pairing theorem for
alternant hydrocarbons, it is possible to derive a finite series expansion
of the Greenâ€™s function for electron transmission in terms
of the odd powers of the vertex adjacency matrix or HuÌˆckel
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 Greenâ€™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 Greenâ€™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 Greenâ€™s function

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

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, Hâ€™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. Câ€“H bond activation, i.e.,
oxidative addition, and Câ€“C and Hâ€“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 HuÌˆckel 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

How
heteroatomic substitutions affect electron transport through
Ï€-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 Ï€-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 HuÌˆckel 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

Electron transport properties of boron- and nitrogen-doped
polycyclic
aromatic hydrocarbons and cyclophanes are investigated with the nonequilibrium
Greenâ€™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-Ï€
bridge-acceptor and donor-Ïƒ 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 orbitalâ€“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 Greenâ€™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