Orbital rehybridization in n-octane adsorbed on Cu(110)

Simulations of x-ray absorption spectra for CO desorbing from Ru(0001) with transition-potential and time-dependent density functional theory approaches

The desorption of a carbon monoxide molecule from a Ru(0001) surface was studied by means of X-ray Absorption Spectra (XAS) computed with Transition Potential (TP-DFT) and Time Dependent (TD-DFT) DFT methods. By unraveling the evolution of the CO electronic structure upon desorption, we observed that at 2.3 Å from the surface, the CO molecule has already predominantly gas-phase character. While C 1s XAS is quite insensitive to changes in the C-O bond length, the O 1s excitation is very sensitive with the π* coming down in energy upon CO bond stretching, which competes with the increase in orbital energy due to the repulsive interaction with the metallic surface. We show in a systematic way that the TP-DFT method can describe the XAS rather well at the endpoints (chemisorbed and gas phase) but is affected by artificial charge transfer and/or incorrect spin treatment in the transition region in cases like CO, where there are low-lying π* orbitals and large exchange interactions between the core 1s and valence-acceptor π* orbitals. As an alternative, we demonstrate by comparing with experimental data that a linear response approach using TD-DFT employing common exchange-correlation functionals and finite-size clusters can yield a good description of the spectral evolution of the 1s → π* transition with correct spin and gas-to-chemisorbed chemical shifts in good agreement with experiment.

Direct transformation of n-alkane into all-trans conjugated polyene via cascade dehydrogenation

Selective C(sp³) −H activation is of fundamental importance in processing alkane feedstocks to produce high-value-added chemical products. By virtue of an on-surface synthesis strategy, we report selective cascade dehydrogenation of n-alkane molecules under surface constraints, which yields monodispersed all-trans conjugated polyenes with unprecedented length controllability. We are also able to demonstrate the generality of this concept for alkyl-substituted molecules with programmable lengths and diverse functionalities, and more importantly its promising potential in molecular wiring.

Role of Metal Lattice Expansion and Molecular π-Conjugation for the Magnetic Hardening at Cu-Organics Interfaces

Magnetic hardening and generation of room-temperature ferromagnetism at the interface between originally nonmagnetic transition metals and π-conjugated organics is understood to be promoted by interplay between interfacial charge transfer and relaxation-induced distortion of the metal lattice. The relative importance of the two contributions for magnetic hardening of the metal remains unquantified. Here, we disentangle their role via density functional theory simulation of several models of interfaces between Cu and polymers of different steric hindrance, π-conjugation, and electron-accepting properties: polyethylene, polyacetylene, polyethylene terephthalate, and polyurethane. In the absence of charge transfer, expansion and compression of the Cu face-centered cubic lattice is computed to lead to magnetic hardening and softening, respectively. Contrary to expectations based on the extent of π-conjugation on the organic and resulting charge transfer, the computed magnetic hardening is largest for Cu interfaced with polyethylene and smallest for the Cu-polyacetylene systems as a result of a differently favorable rehybridization leading to different enhancement of exchange interactions and density of states at the Fermi level. It thus transpires that neither the presence of molecular π-conjugation nor substantial charge transfer may be strictly needed for magnetic hardening of Cu-substrates, widening the range of organics of potential interest for enhancement of emergent magnetism at metal-organic interfaces.

Compression-Induced Conformation and Orientation Changes in an n-Alkane Monolayer on a Au(111) Surface

The influence of the preparation method and adsorbed amount of n-tetratetracontane (n-C₄₄H₉₀) on its orientation in a monolayer on the Au(111) surface is studied by near carbon K-edge X-ray absorption fine structure spectroscopy (C K-NEXAFS), scanning tunneling microscopy (STM) under ultrahigh vacuum, and infrared reflection-absorption spectroscopy (IRAS) at the electrochemical interface in sulfuric acid solution. The n-C₄₄H₉₀ molecules form self-assembled lamellar structures with the chain axis parallel to the surface, as observed by STM. For small amounts adsorbed, the carbon plane is parallel to the surface (flat-on orientation). An increase in the adsorbed amount by ∼10-20% induces compression of the lamellar structure either along the lamellar axis or alkyl chain axis. The compressed molecular arrangement is observed by STM, and induced conformation and orientation changes are confirmed by in situ IRAS and C K-NEXAFS.

High Spin Polarization at Ferromagnetic Metal-Organic Interfaces: A Generic Property

A high spin polarization of states around the Fermi level, EF, at room temperature has been measured in the past at the interface between a few molecular candidates and the ferromagnetic metal Co. Is this promising property for spintronics limited to these candidates? Previous reports suggested that certain conditions, such as strong ferromagnetism, i.e., a fully occupied spin-up d band of the ferromagnet, or the presence of π bonds on the molecule, i.e., molecular conjugation, needed to be met. What rules govern the presence of this property? We have performed spin-resolved photoemission spectroscopy measurements on a variety of such interfaces. We find that this property is robust against changes to the molecule and ferromagnetic metal's electronic properties, including the aforementioned conditions. This affirms the generality of highly spin-polarized states at the interface between a ferromagnetic metal and a molecule and augurs bright prospects toward integrating these interfaces within organic spintronic devices.

Hybridization trends for main group elements and expanding the Bent's rule beyond carbon: more than electronegativity

Trends in hybridization were systematically analyzed through the combination of DFT calculations with NBO analysis for the five elements X (X = B, C, N, O, and F) in 75 HnX-YHm compounds, where Y spans the groups 13-17 of the periods 2-4. This set of substrates probes the flexibility of the hybridization at five atoms X through variations in electronegativity, polarizability, and orbital size of Y. The results illustrate the scope and limitations of the Bent's rule, the classic correlation between electronegativity and hybridization, commonly used in analyzing structural effects in carbon compounds. The rehybridization effects are larger for fluorine- and oxygen-bonds than they are in the similar bonds to carbon. For bonds with the larger elements Y of the lower periods, trends in orbital hybridization depend strongly on both electronegativity and orbital size. For charged species, the effects of substituent orbital size in the more polarizable bonds to heavier elements show a particularly strong response to the charge introduction at the central atom. In the final section, we provide an example of the interplay between hybridization effects with molecular structure and reactivity. In particular, the ability to change hybridization without changes in polarization provides an alternative way to control structure and reactivity, as illustrated by the strong correlation of strain in monosubstituted cyclopropanes with hybridization in the bond to the substituent.

Two-dimensional superstructures and softened C-H stretching vibrations of cyclohexane on Rh(111): effects of preadsorbed hydrogen

Adsorption structures and interaction of cyclohexane molecules on the clean and hydrogen-preadsorbed Rh(111) surfaces were investigated using scanning tunneling microscopy, spot-profile-analysis low-energy electron diffraction, temperature-programmed desorption, and infrared reflection absorption spectroscopy (IRAS). Various ordered structures of adsorbed cyclohexane were observed as a function of hydrogen and cyclohexane coverages. When the fractional coverage (θ(H)) of preadsorbed hydrogen was below 0.8, four different commensurate or higher-order commensurate superstructures were found as a function of θ(H); whereas more densely packed incommensurate overlayers became dominant at higher θ(H). IRAS measurements showed sharp softened C-H vibrational peaks at 20 K, which originate from the electronic interaction between adsorbed cyclohexane and the Rh surface. The multiple softened C-H stretching peaks in each phase are due to the variation in the adsorption distance from the substrate. At high hydrogen coverages they became attenuated in intensity and eventually diminished at θ(H) = 1. The gradual disappearance of the soft mode correlates well with the structural phase transition from commensurate structures to incommensurate structures with increasing hydrogen coverage. The superstructure of adsorbed cyclohexane is controlled by the delicate balance between adsorbate-adsorbate and adsorbate-substrate interactions which are affected by preadsorbed hydrogen.

Adsorption of linear alkanes on Cu(111): Temperature and chain-length dependence of the softened vibrational mode

The vibrational spectra of linear alkanes, with lengths ranging from n-propane to n-octane, were examined on a copper surface by reflection-absorption infrared spectroscopy. The appearance and frequency of the "soft mode," a feature routinely seen in studies of saturated hydrocarbons adsorbed on metals, were examined and compared between the different adsorbates. The frequency of the mode was found to be dependent on both the number of methylene units of each alkane as well as specific aspects of the order of the monolayer phase. Studies of monolayer coverages at different temperatures provide insights into the nature of the two-dimensional (2D) melting transitions of these adlayer structures, ones that can be inferred from observed shifts in the soft vibrational modes appearing in the C-H stretching region of the infrared spectrum. These studies support recently reported hypotheses as to the origins of such soft modes: the metal-hydrogen interactions that mediate them and the dynamics that underlay their pronounced temperature dependencies. The present data strongly support a model for the 2D to one-dimensional order-order phase transition arising via a continuous rather than discrete first-order process.

Are recent water models obtained by fitting diffraction data consistent with infrared/Raman and x-ray absorption spectra?

X-ray absorption (XA) spectra have been computed based on water structures obtained from a recent fit to x-ray and neutron diffraction data using models ranging from symmetrical to asymmetrical local coordination of the water molecules [A. K. Soper, J. Phys.: Condens. Matter 17, S3273 (2005)]. It is found that both the obtained symmetric and asymmetric structural models of water give similar looking XA spectra, which do not match the experiment. The fitted models both contain unphysical structures that are allowed by the diffraction data, where, e.g., hydrogen-hydrogen interactions may occur. A modification to the asymmetric model, in which the non-hydrogen-bonded OH intramolecular distance is allowed to become shorter while the bonded OH distance becomes longer, improves the situation somewhat, but the overall agreement is still unsatisfactory. The electric field (E-field) distributions and infrared (IR) spectra are also calculated using two established theoretical approaches, which, however, show significant discrepancies in their predictions for the asymmetric structural models. Both approaches predict the Raman spectrum of the symmetric model fitted to the diffraction data to be significantly blueshifted compared to experiment. At the moment no water model exists that can equally well describe IR/Raman, x-ray absorption spectroscopy, and diffraction data.

Physisorption-induced C-H bond elongation in methane

Physisorption of methane to a Pt surface was studied by x-ray absorption spectroscopy in combination with density functional theory spectrum calculations. The experiment shows new electronic states appearing upon physisorption. We find that these states are due to orbital mixing causing charge polarization as a means to minimize Pauli repulsion. The results can be explained by elongation of 1 C-H bond by 0.09 A in the physisorbed state even though no covalent chemical bond is formed.

Photochemistry of cyclohexane on Cu(111)

The photochemistry of cyclohexane on Cu(111) and its excitation mechanism have been studied by temperature-programmed desorption, ultraviolet and X-ray photoelectron spectroscopy. Cyclohexane weakly adsorbed on Cu(111) has been known to show a broadened and redshifted CH stretching band, i.e., CH vibrational mode softening. Although no dehydrogenation takes place thermally on this surface and by the irradiation of photons at 5.0 eV, adsorbed cyclohexane is dissociated to cyclohexyl and hydrogen by the irradiation of photons at 6.4 eV. This is a marked contrast to cyclohexane in the gas phase where the onset of absorption is located at 7 eV. When the surface irradiated by 6.4-eV photons is further annealed, cyclohexyl is dehydrogenated to form cylcohexene that desorbs at 230 K. The systematic measurements of photochemical cross sections at 6.4 eV with linearly polarized light as a function of incident angle indicate that the electronic transition from the highest occupied band of cyclohexane to a partially occupied hybridized band near the Fermi level is responsible for the photochemistry. The hybridized band is formed by the interactions between the electronic states of cyclohexane and the metal substrate. The role of the hybridized band in the photochemistry and the CH vibrational mode softening is discussed.

Bonding of saturated hydrocarbons to metal surfaces

The adsorption of octane on Cu(110) was studied by x-ray absorption and x-ray emission spectroscopy, in combination with spectrum calculations in the framework of density functional theory, as a model system for alkane adsorption on transition metals. Significant electron sharing between the adsorbate and metal surface and involvement of both bonding and antibonding C-H molecular orbitals in the molecule-metal bond was found. The calculations were extended to the case of octane adsorbed on Ni(110), and the position of the metal d band was found to be important for the bonding. The results were generalized to show that this is important for the efficiency as an alkane dehydrogenation catalyst.