Theoretical studies of the bonding of oxygen to models of the (100) surface of nickel

Electronic wavefunctions have been obtained as a function of geometry for an O atom bonded to Ni clusters (consisting of one to five atoms) designed to model bonding to the (100) surface of Ni. Electron correlation effects were included using the generalized valence bond and configuration interaction methods. For the (100) surface, we find that the charge distribution for the full O overlay er is consistent with taking a positively charged cluster. The four surface atoms in the surface unit cell and the atom beneath the surface are important in determining the geometry, leading to a Ni⁺₅O cluster as the model for the (100) surface. The optimum oxygen position with this model is 0.96 Å above the surface (four-fold coordinate site) in good agreement with the value (0.90 ± 0.10 Å) from dynamic LEED intensity analysis. The atom beneath the surface allows important polarization effects for the positively charged cluster. The bonding to the surface involves bridging two diagonal surface Ni atoms. There is an O(2pπ) pair which overlaps the other diagonal pair of Ni atoms leading to nonbonded repulsions which increase the distance above the surface. There are two equivalent such structures, the resonance leading to a c(2 × 2) structure for the O overlayer.     

Molecular cluster calculations for the interpretation of CO chemisorption on a Ni(100) surface

Self-consistent Hartree-Fock-Slater molecular cluster calculations for the chemisorption of carbon monoxide on a Ni(100) surface are presented. In earlier calculations of this type the CO molecule has been assumed to be chemisorbed in a hollow position of C_4v symmetry. A recent EELS experiment shows however that in the most stable configuration CO is linearly bonded to the Ni atoms, i.e. a top position of the CO-molecule. This experiment indicates also that there exists an additional bridge bonding of the CO molecule to the two nearest neighbour Ni atoms. The variation of the energy levels, binding energies and charge distribution with the height of the CO molecule above the nickel surface is calculated for the top position using the NiCO and Ni₅CO clusters and for the bridge bonding configuration using the Ni₂CO cluster. The CO 1π level is found to be split by about 0.8 eV in bridge bonding geometry. For both hollow and top positions the 1π and 5σ levels are separated by about 0.5 eV. The energy separation to the 4σ level is about 3 eV, which is in good agreement with experimental data. Theoretical ionization energies relative to the Fermi energy for top position geometry at a bond distance of 3.5 au between the carbon atom and nickel surface were found to be 25.7, 11.7, 8.7 and 8.2 eV for the 3σ, 4σ, 5σ and 1π levels which should be compared with the experimental data of 29.0, 10.8, 8.4 and 7.8 eV, respectively. The corresponding ionization energies for a bond angle of 99° in bridge bonding were 23.7, 12.1, 7.3, 7.0 and 7.9 eV. The two last values represent the 1π level which is split into two levels in this geometry. The variation of the C-O stretch vibrational frequencies with the height of the CO molecule above the surface for the top-position geometry is estimated from the 5σ and 2π gross orbital populations and from the CO σ and π overlap populations.     

A theoretical deduction of the shape and size of nanocarbons suitable for hydrogen storage

We evaluated the adsorption energy of a hydrogen molecule in nanocarbons consisting of graphene sheets. The nanocarbon shapes were a pair of disks with separation 2d, a cylinder with radius d, and a truncated sphere with radius d. We obtained the adsorption energy in the form of a 10–4 Lennard–Jones function with respect to 1/d. The values of the potential depth (D) and equilibrium distance (d _e), respectively, were 94 meV and 2.89 Å for the disk pair, 158 meV and 3.14 Å for the cylinder, and 203 meV and 3.37 Å for the sphere. When d=d _e, the adsorption energy of the disk pair (cylinder) became deeper than ?0.9D, and it approached ?D when the radius (length) increased to more than twice its separation (radius). The adsorption energy of the sphere was increased from ?D to ?0.5D when the radius of the opening increased from 0 to d _e. These results suggest that porous carbon materials can increase the adsorption energy by up to ～200 meV if the carbon atoms are arranged on a spherical-like surface with ～7 Å separation. This may lead to practical hydrogen storage for fuel cells.     

Surface diffusion of lead on tungsten

Field electron microscopy is used to study the surface diffusion of lead on tungsten. A simple method to measure rough values of the diffusion coefficient and its dependence on sub-monolayer coverage is described and tested. In the region around (001) the displacement energy found is about 1.30 eV/atom up to 10¹⁵ atoms/cm² where it decreases to 0.78 eV/atom. In the residual region except (110) this energy at 1.5×10¹⁴ atoms/cm² is 1.22 eV/atom, it decreases at 4 × 10¹⁴ atoms/cm² to 0.61 eV/atom and increases at 10¹⁵ atoms/cm² to 0.78 eV/atom. Corresponding values of the diffusion coefficient D and of the preexponential D₀ are given. The dependence of D on submonolayer coverage is discussed.     

Ab initio all-electron fully relativistic Dirac—Fock self-consistent field calculations for UCI6

Ab initio all-electron fully relativistic Dirac-Fock and non-relativistic Hartree-Fock self-consistent field (SCF) calculations are reported at four UCl bond distances, assuming octahedral UCl₆. The results are fitted to a polynomial, obtaining thereby the optimized values of the bond distance and the corresponding total electronic energy for the UCl₆. The nonrelativistic Hartree-Fock (HF) and Dirac-Fock (DF) SCF calculations predict UCl₆ to be bound, with a predicted dissociation (atomization) energy D _e of 11.88 eV and 17.89 eV, respectively. Relativistic effects lead to ~51% increment in the predicted atomization energy of UCl₆. The UCl bond lengths predicted for UCl₆ with the relativistic DF and non-relativistic HF wave-functions are 2.46 Å and 2.58 Å, respectively. Complete neglect in the SCF step of the two-electron [SS|SS] integrals involving the small components of the spinors (NOSS) in the DF SCF calculation for UCl₆ predicts a D _e of 18.25 eV and essentially the same bond length (2.48 Å) as that predicted with the full SCF procedure. Thus the small components contribute an antibinding relativistic effect of ~0.4 eV to the D _e of UCl₆ and have a negligible effect on the bond length. The calculations show that relativistic effects are significant for the bonding and the dissociation (atomization) energy of UCl₆, and that these may be treated accurately using Dirac's fully relativistic equation for an electron.     

Theoretical studies of chemisorbed oxygen on Ag(110)

The interaction of an oxygen atom with a 26-atom cluster model of the (110) face of Ag has been investigated with ab initio self-consistent-field and configuration-interaction theory. The SCF results for the bridge site (C_2v) predict $\text{r = 0.3}\text{A}\text{?}$ and ω_e = 327 cm^?1, in good agreement with the available experimental evidence. The calculated binding energy (D_e = 9 kcal/mole) is roughly an order of magnitude too small. The inclusion of electron correlation increases r_e and ω_e only slightly, but should have a dramatic effect on D_e. The ground state corresponds to a “surface oxide” state. The theoretical projected density-of-states curves exhibit “bonding” and “anti-bonding” O(2p) peaks, separated by ~ 6 eV, in good agreement with recent angle-resolved photoemission data.     

Adsorption and diffusion of a single Pt atom on γ-Al2O3 surfaces

Motivated to better understand the interactions between Pt and γ-Al₂O₃ support, the adsorption and diffusion of a single Pt atom on γ-Al₂O₃ was studied using density functional theory. Two different surface models with atoms of various coordination (3–5) were used, one derived from a defected spinel structure, and another derived from the dehydration of boehmite (AlOOH). Adsorption energies are similar for the two surfaces, about −2 eV for the most stable sites, and involve Pt binding to surface O atoms. An unusually strong trapping geometry whereby Pt moves into the surface was identified over the boehmite-derived surface. In all cases the surface transfers 0.2–0.3 e⁻ to the Pt atom. The bonding is explained as being a combination of charge transfer between the surface and Pt atom, polarization of the metal atom, and some weak covalent bonding. The similarity of the two surfaces is attributed to the similar local environments of the surface atoms, as corroborated by geometry analysis, density of states, and Bader charge analysis. Calculated activation barriers (0.3–0.5 eV) for the defected spinel surface indicate fast diffusion and a kinetic Monte Carlo model incorporated these barriers to determine exact diffusion rates and behavior. The kinetic Monte Carlo results indicate that at low temperatures (<500 K) the Pt atom can become trapped at certain surface regions, which could explain why the sintering process is hindered at low temperature. Finally we modeled the adsorption of Pt on hydrated surfaces and found adsorption to be weaker due to steric repulsion and/or decreased electron-donating ability of the surface.     

Bond length inequalities and relaxations at the Rh(110)-c(2 × 2)-S surface structure studied by LEED crystallography

A tensor LEED analysis has been made for the Rh(110)-c(2 × 2)-S surface structure using intensity-versus-energy curves measured for twelve independent beams at normal incidence. Each S atom chemisorbs on a centre site of the Rh(110) surface. It bonds to the second layer Rh atom directly below, with a bond distance equal to about 2.27 Å, and to four neighbouring first layer Rh atoms at close to 2.47 Å. A significant feature of this structure is that the second metal layer is buckled; those Rh atoms directly below the S atoms relax down by about 0.11 Å compared with the other second layer Rh atoms. This buckling is apparently driven by the need to reduce the difference that would otherwise occur between these two types of S-Rh bond lengths. A component in the observed difference between the S-Rh distances appears to be dependent on the metallic coordination number for the Rh atoms; in this regard, a comparison is made with the structural details for O chemisorbed on reconstructed Ni(110).     

Size dependent 2p3/2 binding-energy shift of Ni nanoclusters on SiO2 support: Skin-depth local strain and quantum trapping

An in situ X-ray photoelectron emission investigation revealed that the size trend of the 2p_3/2 binding-energy shift (BES) of Ni nanoclusters grown on SiO₂ substrate follows the prediction of the bond order-length-strength (BOLS) correlation theory [30]. Theoretical reproduction of the measurements turns out that the 2p_3/2 binding energy of an isolated Ni atom is 850.51 eV and its intrinsic bulk shift is 2.70 eV. Findings confirmed that the skin-depth local strain and potential well quantum trapping induced by the shorter and stronger bonds between under-coordinated surface atoms provide perturbation to the Hamiltonian and hence dominate the size dependent BES.     

c-C5H5 on a Ni(1 1 1) surface: Theoretical study of the adsorption, electronic structure and bonding

In the present work the ASED-MO method is applied to study the adsorption of cyclopentadienyl anion on a Ni(1 1 1) surface. The adsorption with the centre of the aromatic ring placed above the hollow position has been identified to be energetically the most favourable. The aromatic ring remains almost flat, the H atoms are tilted 17° away from the metal surface. We modelled the metal surface by a two-dimensional slab of finite thickness, with an overlayer of c-C₅H₅⁻, one c-C₅H₅⁻ per nine surface Ni atoms. The c-C₅H₅⁻ molecule is attached to the surface with its five C atoms bonding mainly with three Ni atoms. The NiNi bond in the underlying surface and the CC bonds of c-C₅H₅⁻ are weakened upon adsorption. We found that the band of Ni 5dz² orbitals plays an important role in the bonding between c-C₅H₅⁻ and the surface, as do the Ni 6s and 6p_z bands.     

The structure and bonding of furan on Pd(111)

The structure and bonding of molecular furan, C₄H₄O, on Pd(111) has been investigated using density functional theory (DFT) calculations and the results compared with those of a recent experimental investigation using scanned-energy mode photoelectron diffraction (PhD). The DFT results confirm the orientation of the molecular plane to be essentially parallel to the surface and show a clear energetic preference for one of the two possible structures identified in the PhD study, namely that with the molecule centred over the hollow sites of the surface. Two slightly different geometries at the hollow sites are found to be essentially energetically equivalent; in both cases, one Pd surface atom bonds to two C atoms, while two other Pd atoms each bond to one C atom. These structures differ in that in one case the pair of C atoms bonding to a single Pd atom are both β-C (C atoms not bonded to O in the furan molecule), whereas in the second case this pair of C atoms comprises one β-C and one α-C (adjacent to the O atom in furan). In both structures the C–Pd bonding is accompanied by displacements of the H and O atoms away from the surface and out of the molecular plane and local C–Pd coordination consistent with a rehybridisation of the C bonding to sp³ character.     

Extended X‐ray absorption fine structure and multiple‐scattering simulation of nickel dithiolene complexes Ni[S2C2(CF3)2]2n (n = −2, −1, 0) and an olefin adduct Ni[S2C2(CF3)2]2(1‐hexene)

A series of Ni dithiolene complexes Ni[S₂C₂(CF₃)]₂ⁿ (n = ?2, ?1, 0) ( 1 , 2 , 3 ) and a 1‐hexene adduct Ni[S₂C₂(CF₃)₂]₂(C₆H₁₂) ( 4 ) have been examined by Ni K‐edge X‐ray absorption near‐edge structure (XANES) and extended X‐ray absorption fine‐structure (EXAFS) spectroscopies. Ni XANES for 1 – 3 reveals clear pre‐edge features and approximately +0.7 eV shift in the Ni K‐edge position for `one‐electron' oxidation. EXAFS simulation shows that the Ni—S bond distances for 1 , 2 and 3 (2.11–2.16 Å) are within the typical values for square planar complexes and decrease by ～0.022 Å for each `one‐electron' oxidation. The changes in Ni K‐edge energy positions and Ni—S distances are consistent with the `non‐innocent' character of the dithiolene ligand. The Ni—C interactions at ～3.0 Å are analyzed and the multiple‐scattering parameters are also determined, leading to a better simulation for the overall EXAFS spectra. The 1‐hexene adduct 4 presents no pre‐edge feature, and its Ni K‐edge position shifts by ?0.8 eV in comparison with its starting dithiolene complex 3 . Consistently, EXAFS also showed that the Ni—S distances in 4 elongate by ～0.046 Å in comparison with 3 . The evidence confirms that the neutral complex is `reduced' upon addition of olefin, presumably by olefin donating the π‐electron density to the LUMO of 3 as suggested by UV/visible spectroscopy in the literature.     

The adsorption of sulfur on the platinum (100) surface

Clean platinum (100) surfaces of 1× 1 and 5 × 20 structure were exposed to H₂S. Surface coverage with Sulfur followed Langmuir kinetics, which, together with LEED data, points to a repulsive interaction between sulfur atoms. Sulfur adsorption causes a decrease in the work function of platinum by 0.7 eV at saturation coverage. This is attributed to polarization, rather than ionization, of the adsorbed sulfur. Photoemission measurements are difficult to interpret because of two-dimensional periodicity and the overlap of electronic structure of the adsorbate with the platinum d band. We observe peaks due to sulfur at 6.3, 4.5, and 2.5 eV below the Fermi level for the c(2 × 2) overlayer and at 6.8, 4.5, and 2.0 eV below E_F for the p(2 × 2) surface. A tentative interpretation in terms of sulfur orbitals is given. The decrease in work function and analogy with the properties of PtS₂ lead us to propose covalent bonding of sulfur to platinum, in which every sulfur atom is bonded to four Pt neighbors in both structures. The repulsive interaction between sulfur atoms is indirect through the platinum substrate.     

Theoretical studies of the geometries of O and S overlayers on the (100) surface of nickel

Geometries for O and S overlayers on the (100) surface of Ni have been calculated using $\text{a b initio}$ wavefunctions for O and S bonded to small clusters of Ni atoms (1 to 5 Ni atoms). The calculated distance of the adatom from the surface is 0.96 Å and 1.33 Å for O and S, respectively, in excellent agreement with the results of dynamic LEED intensity calculations, 0.9 ± 0.1 Å and 1.3 ± 0.1 Å, respectively. This indicates that accurate geometries of chemisorbed atoms may be obtained from calculations using clusters.     

Hartree-Fock-Slater-LCAO studies of the acetylene-transition metal interaction: I. Chemisorption on Ni surfaces; cluster models

The interaction of C₂H₂ with Ni surfaces has been studied by the Hartree-Fock-Slater-LCAO method (with core pseudopotentials). Different adsorption sites (π, di-σ, μ₂, μ₃) at the Ni(111) surface have been modelled by clusters of 1 to 4 Ni atoms; the structure of C₂H₂ and the Ni-C distance have been varied (3 structures, 2 distances). The acetylene-metal bonding can be interpreted in terms of π to metal donation and, especially, metal to π¹ back donation effects which considerably weaken the C-C bond. These effects become increasingly important when more metal atoms are directly involved in the adsorption bonding: π < di-σ < μ₂ < μ₃. The calculated shifts in the ionization energies are in fair agreement with the experimentally observed shifts (by UPS) for C₂H₂ adsorbed on Ni(111) (and other Ni surfaces); these shifts do not depend very sensitively on the bonding situation, however, so that we could not assign the structure of adsorbed C₂H₂ solely on this basis. From the comparison between the measured C-C stretch frequency (by ELS) and the calculated C-C overlap populations, using a relation calibrated on Ni-acetylene complexes, we find that μ₃ bonding of C₂H₂ with a Ni-C distance of about 1.9 Å is most probable on the Ni(111) surface; the CCH angle is estimated to be somewhat smaller than 150°. We have suggested an explanation for the surface specific dissociation of C₂H₂: C₂ fragments (C-H bond breaking) have been observed on stepped Ni surfaces (at low temperature), CH fragments (C-C bond breaking) have been found on ideal surfaces (at higher temperatures).     

The H atom in CaTiO 3 : Structure and electronic properties

In the present work we explore the effects that an H impurity produces upon the geometry and electronic structure of the CaTiO 3 crystal considering the cubic and orthorhombic crystallographic lattices of the material. A quantum-chemical method based on the Hartree-Fock formalism and the periodic large-unit-cell model is used throughout the work. The analysis of the results shows that the interstitial H impurity binds to one of the O atoms forming the so-called O-H group. At equilibrium, the O-H distances are found to be equal to 0.89 and 1.04 Å for cubic and orthorhombic lattices respectively. Atomic displacements and relaxation energies are analysed comparing the obtained results in the cubic lattice with those in the orthorhombic lattice. In the cubic phase the computed relaxation energy of vicinity of the O-H group is found to be equal to 1.1 eV and the atomic displacements generally obey the Coulomb law. So, the negatively charged O atoms move outwards from the defective region by about 0.09 Å while the positively charged Ti and Ca atoms move towards the defective region by about 0.05 and 0.01 Å respectively. A similar effect is observed in the orthorhombic lattice of CaTiO 3 doped with an H atom. It is necessary to mention that different O positions in the orthorhombic structure are considered for the O-H bond creation. The computed relaxation energies of the atomic displacements in this structure are found to be equal to 2.3 and 2.1 eV depending on the crystallographic type of the bonding O atom.     

Noble gas halides: The B → X and D → X systems of 136Xe35Cl

The B → X (2870–3100 Å) and D → X (2250–2370 Å) band systems of ¹³⁶Xe³⁵Cl are photographed and vibrationally analyzed. A simultaneous least-squares fit of 41 band-heads in the B-X system and 35 in D-X yields, in part, the following constants (in cm^?1): T_eB = 32 405.8, T_eD = 42 347.9, ω_eB = 194.75, ω_eD = 204.34, ω_eX = 26.22. The ground state dissociation energy ($\text{D}$″_e) is estimated to be 281 ± 10 cm^?1. Potential curves are derived for all three states through Franck-Condon calculations. From these curves the D-state internuclear distance is 0.09 ± .02 Å smaller than the B-state distance.     

Point-defect properties of,and sputtering events in,the {001} surfaces of Ni3Al. II. Sputtering events at and near surfaces

Atomic recoil events at and near {001} surfaces of Ni₃Al due to elastic collisions between electrons and atoms have been simulated by molecular dynamics to obtain the sputtering threshold energy as a function of atomic species, recoil direction and atomic layer of the primary recoil atom. The minimum sputtering energy occurs for adatoms and is 3.5 and 4.5?eV for Al and Ni adatoms on the Ni–Al surface (denoted ‘M’), respectively, and 4.5?eV for both species on the pure Ni surface (denoted ‘N’). For atoms within the surface plane, the minimum sputtering energy is 6.0?eV for Al and Ni atoms in the M plane and for Ni atoms in the N surface. The sputtering threshold energy increases with increasing angle, θ, between the recoil direction and surface normal, and is almost independent of azimuthal angle, ?, if θ<60°; it varies strongly with ? when θ>60°, with a maximum at ??=?45° due to ?{110}? close-packed atomic chains in the surface. The sputtering threshold energy increases significantly for subsurface recoils, except for those that generate efficient energy transfer to a surface atom by a replacement collision sequence. The implications of the results for the prediction of the mass loss due to sputtering during microanalysis in a FEG STEM are discussed.     

Photoelectron spectroscopic and thermodynamic studies of the free surface of benzyl alcohol

He(I) photoelectron spectra of the liquid surface and the vapour of benzyl alcohol (C₆H₅CH₂OH) are reported. The gas-phase spectrum resembles that of benzene except that between the 1e_1g(π) and the 3e_2g benzenoid bands there is an additional peak in benzyl alcohol at 10.69 eV. In the spectrum of the liquid the additional peak is either absent or of much reduced intensity and this, together with consideration of the spectra of related molecules, leads to its assignment as the ionization of a “lone pair” orbital. In the condensed phase the lone pair electrons become involved in hydrogen bonding. Considerations of the excess energy and entropy of the surface of liquid benzyl alcohol point to a surface in which the hydrogen bonds are not broken, in agreement with the spectroscopic result.     

Halogen bond between hypervalent halogens YF3/YF5 (Y=Cl,Br, I) and H2X (X= O,S, Se)

ABSTRACT

The complexes of H₂X (X?=?O, S, Se) with hypervalent halogens YF₃ and YF₅ (Y?=?Cl, Br, I) have been studied. The σ-hole on the Y atom participates in a halogen bond with the lone pair on the chalcogen atom. In addition, some secondary interactions coexist with the halogen bond in most complexes. The interaction energy correlates with the nature of both X and Y atoms. In most cases, the complex is more stable for the heavier Y atom and the lighter X atom. Of course, there are some exceptions in H₂X···YF₃. YF₃ forms a more stable complex with H₂X than does YF₅. These complexes are dominated by electrostatic interaction and the halogen bond involving H₂S and H₂Se exhibits some covalent character.

Halogen bond plays an important role in chemical reactions and multivalent halogens can regulate chemical reactions by participating in a halogen bond. Thus we compare the effect of the chalcogen electron donor on the strength and nature of halogen bonding involving multivalent halogens.