L. Triguero

Separate state vs. transition state Kohn-Sham calculations of X-ray photoelectron binding energies and chemical shifts

Abstract The possibilities of using Kohn-Sham density functional theory to make accurate predictions of core photoelectron binding energies and chemical shifts are explored through a series of calculations on compounds of different sizes and types. The recent proposal of an ‘unrestricted generalized transition state’ (UGTS) method, for this purpose [D.P. Chong, Chem. Phys. Lett. 232 (1995) 486; D.P. Chong, J. Chem. Phys. 103 (1995) 1842], is put to test vs. full separate state optimizations of the ground and core hole states, that is a ‘ Δ Kohn-Sham’ ( Δ KS) method. It is found that while internal parametrizations in terms of grid and basis set expansions can be well controlled there is still a notable dependency of the absolute binding energies on the choice of functional for both the UGTS and Δ KS methods. As for pure ab initio Δ SCF the former method must still be viewed as an approximation of the latter. Keeping these dependencies in mind, Kohn-Sham calculations seem to provide a promising tool for predicting binding energies and chemical shifts of an accuracy that approaches that of experiments.

The bonding of CO to metal surfaces

The atom and symmetry specific properties of x-ray emission spectroscopy have been applied to the investigation of CO adsorbed on Ni(100) and Cu(100) surfaces. In comparison to ab initio electronic structure calculations, obtained in density functional theory, we develop a consistent electronic structure model of CO adsorption on transition and noble metals and extend to a conceptual model of the surface chemical bond. A strong CO–substrate interaction is found, characterized by significant hybridization of the initial CO orbitals and the metal bands. In the π system an allylic configuration is found as the result of orbital mixing between the CO 1π, 2π* and the metal dπ-band which is manifested experimentally in the observation of an oxygen lone-pair state. In the σ system experimental evidence of equally strong orbital mixing has been found. Energetically, the adsorbate–substrate complex is stabilized by the π-interaction but is destabilized by the σ-interaction. Furthermore, the internal C–O bond carri...

Spin uncoupling in surface chemisorption of unsaturated hydrocarbons

Unsaturated hydrocarbons, such as acetylene and ethylene, show strong geometrical distortions when coordinated to transition metals or to surfaces; the bonding is normally analysed in terms of a π-donation—π*-backdonation process. In the present work we use chemisorption of the unsaturated hydrocarbons (ethylene, acetylene, and benzene) on cluster models of the copper (100), (110), and (111) surfaces to demonstrate the importance of considering the available excited states of the free molecule in analyzing the bonding scheme of the adsorbate at the surface. By comparison to the structures of the triplet excited states in the gas phase we demonstrate that these must be considered as the states actually involved in the bonding. This implies a spin-uncoupling in both adsorbate and substrate as part of the chemisorption process or bond formation. In particular, for benzene we identify the quinoid gas phase triplet state as the specific state that is most strongly bound to the Cu(110) substrate; the structure ...

Probing Chemical Bonding in Adsorbates using X-ray Emission Spectroscopy

Abstract When a molecule is adsorbed on a metal surface by chemical bonding new electronic states are formed. The direct observation and identification of these states has been an experimental challenge. Their signature is often obscured by bulk substrate states. In the following contribution we will show how X-ray emission spectroscopy (XES), in spite of its inherent bulk sensitivity, can be used to investigate adsorbed molecules. Due to the localization of the core-excited intermediate state, XE spectroscopy allows an atom specific separation of the valence electronic states. Thus the molecular contributions to the surface chemical bond can be separated from those of the substrate. Furthermore, angle dependent measurements make it possible to determine the symmetry of the molecular states, i.e. the separation of π and α type states. Density functional theory calculations in the frozen orbital approximation can describe the XE spectra with a good agreement with experiments. In all we can obtain an atomic view of the electronic states involved in the formation of the chemical bond to the surface. We will show how the electronic structure in simple atomic adsorbates on Cu and Ni surfaces can be related to the concept of less or more noble metals. We also show how new molecular states are formed in adsorbed N 2 and CO on Ni(100). The resulting strength of the adsorbate bond comes from a delicate balance between π bonding and σ repulsion. We can use an additional symmetry selection rule for adsorbed molecules with equivalent atoms where π and π * states can be selectively enhanced depending on the nature of the primary excited state. This will be demonstrated for ethylene and benzene adsorbed on Cu(110). The future prospect is illustrated by the adsorption of formate and ammonia, on Cu(110). These adsorbates represent the interaction of functional groups in amino acids, which are an important class of biological molecules involved in building proteins.

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

We have investigated the local electronic structure of n-octane adsorbed on the Cu(110) surface using symmetry-resolved x-ray absorption spectroscopy (XAS) and x-ray emission spectroscopy (XES) in combination with density functional theory (DFT) spectrum calculations. We found new adsorption-induced states in the XE spectra, which we assign to interaction between the bonding CH orbitals and the metal surface. By performing a systematic investigation of the influence of different structural parameters on the XA and XE spectra, we conclude that the molecular geometry is significantly distorted relative to the gas-phase structure. The bonding to the surface leads to a strengthening of the carbon–carbon bonds and a weakening of the carbon–hydrogen bonds, consistent with a rehybridization of the carbons from sp3 to sp2.8.

CALCULATIONS OF HYDROGEN CHEMISORPTION ENERGIES ON OPTIMIZED COPPER CLUSTERS

Abstract Hydrogen chemisorption on geometry optimized copper clusters with up to nine copper atoms has been studied using both all-electron and one-electron effective core potential methods. The chemisorption energy on the odd clusters are of the order of 20 kcal/mol higher than on the even clusters, in marked contrast to previous results on the corresponding lithium clusters where the odd-even oscillations are much smaller. The difference between lithium and copper clusters is explained in terms of both a large effect of geometry relaxation and a larger tendency to form s-p hybrids in the lithium case.

XPS and XAS investigation of condensed and adsorbed n-octane on a Cu(110) surface

Using x-ray absorption spectroscopy (XAS), x-ray emission spectroscopy (XES) and x-ray photoelectron spectroscopy (XPS) in combination with density functional theory (DFT) the changes in electronic and geometric structure of hydrocarbons upon adsorption are determined. The chemical bonding is analyzed and the results provide new insights in the mechanisms responsible for dehydrogenation in heterogeneous catalysis.In the case of alkanes, n-octane and methane are studied. XAS and XES show significant changes in the electronic structure upon adsorption. XES shows new adsorption induced occupied states and XAS shows quenching of CH*/Rydberg states in n-octane. In methane the symmetry forbidden gas phase lowest unoccupied molecular orbital becomes allowed due to broken symmetry. New adsorption induced unoccupied features with mainly metal character appear just above the Fermi level in XA spectra of both adsorbed methane and n-octane. These changes are not observed in DFT total energy geometry optimizations. Comparison between experimental and computed spectra for different adsorbate geometries reveals that the molecular structures are significantly changed in both molecules. The C-C bonds in n-octane are shortened upon adsorption and the C-H bonds are elongated in both n-octane and methane.In addition ethylene and acetylene are studied as model systems for unsaturated hydrocarbons. The validity of both the Dewar-Chatt-Duncanson chemisorption model and the alternative spin-uncoupling picture is confirmed, as well as C-C bond elongation and upward bending of the C-H bonds.The bonding of ethylene to Cu(110) and Ni(110) are compared and the results show that the main difference is the amount of back-donation into the molecular π* orbital, which allows the molecule to desorb molecularly from the Cu(110) surface, whereas it is dehydrogenated upon heating on the Ni(110) surface. Acetylene is found to adsorb in two different adsorption sites on the Cu(110) surface at liquid nitrogen temperature. Upon heating the molecules move into one of these sites due to attractive adsorbate-adsorbate interaction and only one adsorbed species is present at room temperature, at which point the molecules start reacting to form benzene. The bonding of the two species is very similar in both sites and the carbon atoms are rehybridized essentially to sp2.

MO and DFT approaches to the calculation of X-ray absorption/emission spectra of nitrogen atom adsorbed on Cu(100)

X-ray emission/absorption spectra are calculated for nitrogen chemisorbed on cluster models (up to Cu61) of the four-fold hollow site in the Cu(100) surface. We employ both Hartree-Fock and gradient corrected density functional (DFT) techniques to compute transition energies and intensities involving the fully relaxed core-hole state. Large covalent contributions between the N(2p) and Cu(3d) orbitals are found only at the DFT level, giving rise to bonding and antibonding states that are the main characteristics of the experimental spectra.

DFT and MO calculations of atomic and molecular chemisorption energies on surface cluster models

SummaryDensity functional theory (DFT) (including gradient corrections) and MCPF calculations have been performed for atomic (H, C, N, O) and molecular CHx (x = 1−3) chemisorption on cluster models of different sites of the Cu(100) surface. The DFT and MCPF results are in good agreement once the important effects of core-valence correlation have been accounted for in the MCPF calculations by including contributions from a core polarization potential (CPP); in the DFT approach the core-valence correlation is obtained directly from the total density using the functional. Very large effects on the four-fold hollow site binding energy from core-valence correlation are found for C, N and CH. Several different DFT functionals were employed and compared in the calculations.