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| 2. |
- Paul, Jan, et al.
(författare)
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Ab initio calculations of electron distributions in heme-CO models
- 1983
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Ingår i: Biochimica et Biophysica Acta - Bioenergetics. - : Elsevier BV. - 0005-2728 .- 1879-2650. ; 722:1, s. 209-213
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Tidskriftsartikel (refereegranskat)abstract
- We have, by the use of ab initio calculations, found a back-bonding state of π symmetry close to the Fermi level for CO bound to FeN5C14. We thus find it likely that small shifts of the redox potential ¦EF - Ev¦ will cause relatively large changes of the CO vibrational frequency. The separation of Fe 3d orbitals in our heme model is found to agree with what is predicted by ligand field theory for Oh symmetry. This paper presents nonrelativistic Hartree-Fock-Slater calculations of the 5σ bonding and 2π back-bonding between CO and Fe. The effects of up to 19 additional atoms are discussed for models of heme (COFe to COFeN5C14). The filled back-bonding state is found to be strongly influenced by second nearest neighbor atoms. By use of symmetry orbitals we have resolved the Fe 3d orbitals into the T2g and Eg representations of the Oh point group and find the former states to be occupied whereas the latter are unoccupied. The difference in occupancy is reduced when the CO ligand is removed which also causes an increased density of states at the Fermi level, i.e., the highest occupied and lowest unoccupied orbitals. Possible correlations between our data and experimental results are discussed for heme proteins as well as for metal surfaces.
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| 3. |
- Paul, Jan, et al.
(författare)
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Catalytic active sites on sputtered metal surfaces
- 1983
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 127:1, s. L93-L97
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Tidskriftsartikel (refereegranskat)abstract
- By the use of Hartree-Fock-Slater (HFS) cluster calculations, the symmetry of adatom induced electronic states and their possible role in catalytic processes have been investigated. For iron adsorbed on a close packed iron single crystal surface we find an increased density of occupied "π" as well as "σ" states at the Fermi level. Simple group theoretical arguments give that the "π" states, which represent the highest occupied molecular orbitais (HOMO) will mix with the lowest unoccupied molecular orbital (LUMO) of CO, 2π*, assuming CO bound normal to the surface in a terminal position with respect to the adatom. Such a system with an adsorbed atom on a flat surface is a model for a sputtered surface and these "π" states may thus explain the experimentally observed high rate of dissociation for CO on sputtered iron surfaces. A comparative study shows that no such increased density of states (DOS) is found when the Fe adatom is replaced by potassium. Finally for Cu adsorbed on Cu(111) only 4s derived "σ" states are introduced close to the Fermi level.
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| 4. |
- Paul, Jan, et al.
(författare)
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Cluster calculations of intermediate states in the catalyzed conversion of CH3OH to CH2O on a Cu(100) surface
- 1983
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Ingår i: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 84:2, s. 288-296
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Tidskriftsartikel (refereegranskat)abstract
- The Hartree-Fock-Slater (HFS) calculations of a CH3OCu5 cluster symbolizing methoxide bound to a Cu(100) surface are presented. Adsorption geometry, molecular orbitals, and symmetry properties of the CH3O group in terms of one-electron energies, charge distributions, and partial densities of states (PDOS) are discussed. A comparison of calculated ionization energies and measured binding energies supports the occupation by methoxide of the hollow adsorption site. The electron distribution of the hydrogen atoms has been analyzed for different geometries and is shown to be a sensitive probe of the oxidation from methoxide (CH3O)/methyl (CH3) to formaldehyde (CH2O)/methylene (CH2).
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| 5. |
- Paul, Jan, et al.
(författare)
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Cluster calculations of the H2O/Pt(111) system
- 1983
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Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 23:4, s. 1231-1238
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Tidskriftsartikel (refereegranskat)abstract
- The electronic interaction between water and a Pt(111) surface as evaluated for different Ptx(H2O)y clusters is discussed. Hartree-Fock-Slater (HFS) one-electron ground state energies, ionization potentials, partial densities of states, and Mulliken occupation numbers are related to bonding shifts, as well as initial and final state screening for different orientations of the molecule. The formation of Pt--H2O bonds are sensitive to the orientation since surface oriented H atoms bridge the spatial separation between O 2p and Pt 5d orbitals and thus increase the intermixing of metal and adsorbate orbitals. The dipole moment and the net charge of the H2O molecule is also discussed. Finally, approximations of the metal-H2O potential for use in statistical models of the liquid-metal interface are suggested.
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| 6. |
- Paul, Jan, et al.
(författare)
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Electronic structure of CO adsorbed on a Cu(111) surface analyzed with molecular cluster models
- 1982
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 26:8, s. 4073-4077
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Tidskriftsartikel (refereegranskat)abstract
- Using the self-consistent Hartree-Fock-Slater model we have calculated the electronic structure for various Cux(CO)y clusters symbolizing not only CO bound to "on-top" and "bridge" sites but also some lateral CO-CO interaction on a Cu(111) surface. By comparison with experimental photoemission data we are able to reproduce the observed energies of the occupied CO 4σ, 1π, and 5σ orbitals as well as the partly occupied 2π orbital. In our model we assume CO to be adsorbed on "top" sites for coverages less than ⊖=0.33 [(sqrt[3]×sqrt[3])R30°] and on both top and bridge sites for ⊖>0.33. The experimentally observed peak of intensity at the Fermi edge which increases with coverage above 0.33 is in our model explained by the occupation of CO orbitals of the b1 and b2 symmetry types, i.e., "π" orbitals, degenerate for top positions (C3v), split by the change to bridge positions (C2v). Our results further indicate that the broadened 5σ-1π intensity peak at high coverages is a result of CO bound to top and bridge sites.
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| 7. |
- Paul, Jan, et al.
(författare)
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Hydrogen bonding between liganded CO and heme-proteins
- 1984
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 105:2, s. 197-200
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Tidskriftsartikel (refereegranskat)abstract
- Perturbations of the CO bond in carbon monoxide bound to iron in different heme-proteins are discussed. Properties evaluated from Hartree-Fock-Slater electronic structure calculations are correlated with experimental observations. The net charge on oxygen (HFS) is related to the Fe(III)/Fe(II) reduction potential (EMF). Consequently, proteins with low EMF, e.g. peroxidases, have more ability to form FeCOH...H. protein hydrogen bonds than, for example, oxidases with higher EMF. This can explain anomalous vibrational frequencies, νCO, observed for acidified peroxidases. Calculations reveal that direct interaction between CO and polar groups on the protein can cause peturbations of the CO π-bond order similar to those from EMF shifts. Dioxygen and carbon monoxide are compared in regard to hydrogen bonding to heme-proteins.
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| 8. |
- Paul, Jan, et al.
(författare)
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The reactivity of adsorbed Na atoms probed by coadsorbed CH3OH
- 1984
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 146:1, s. 43-60
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Tidskriftsartikel (refereegranskat)abstract
- The chemical properties of sodium atoms adsorbed on a Cu(111) surface have been probed by coadsorbed methanol. Methoxide, stoichiometrically close to CH3ONa2, is identified as a product by photoemission spectroscopy. Sodium atoms are found to react in the same manner at all submonolayer coverages, i.e. independent of atomic charge. Films of closely packed Na atoms - the first sp-metal substrate probed by methanol - are slightly less reactive. Hartree-Fock-Slater cluster calculations as well as observed energy shifts of a Cu electronic surface state indicate a transfer of charge from the copper substrate to the methoxide group. The desorption temperature for a monolayer of methanol molecules from a Cu(111) surface is enhanced by the presence of low sodium concentrations (1/20 of a monolayer). CH3OH is found to adsorb associatively on the uncovered Cu(111) surface.
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