| 1. |
- Acke, Filip, 1968, et al.
(författare)
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Role of adsorbed surface oxygen in the adsorption of NO on alkaline earth oxides and Pt-promoted CaO surfaces
- 1999
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Ingår i: Journal of Physical Chemistry B Materials. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 103:6, s. 972-978
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Tidskriftsartikel (refereegranskat)abstract
- Adsorbed surface oxygens are formed on CaO, SrO, and BaO during exposure to N2O, and their presence is shown to affect the room-temperature NO adsorption. Information about the adsorbed intermediates is contained in the desorption products and in the desorption temperatures during the subsequent heating ramp in Ar. The presence of adsorbed oxygen species increases the total amount of adsorbed NO for CaO and BaO substrates, whereas for SrO the adsorbed intermediate is stabilized. Two NO desorption peaks are found for CaO and SrO, one at low and one at high temperature. The former is assigned to adsorbed NO, whereas the latter is assigned to adsorbed -NO2 and/or -NO3 species. NO adsorption as -NO2 and/or -NO3 species finds evidence in the corresponding O2 desorption. Only one NO desorption peak is found for BaO. This NO desorption peak disappears in the absence of preadsorbed surface oxygens. O2 desorption is observed, even in the absence of any preadsorbed surface oxygens, for CaO and SrO substrates. This suggests NO bond dissociation upon NO adsorption. The effect of the promotion of CaO by Pt has also been investigated. The respective desorption profiles are similar to those for the unpromoted CaO with preadsorbed surface oxygens, although the amounts are significantly increased.
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| 2. |
- Acke, Filip, et al.
(författare)
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Study of the reduction and reoxidation of a CaO surface
- 1997
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Ingår i: J Phys Chem B. - 1520-5207 .- 1520-6106. ; 101:33, s. 6484-
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Tidskriftsartikel (refereegranskat)abstract
- The reduction of NO with CO and H2 is shown to comprise two basic reactions: a surface oxygen abstraction by the reducing agent and a reoxidation of the surface by NO. The former reaction step has been demonstrated by transient CO2 formation during CO exposure of oxidized CaO surfaces, while the latter was demonstrated by N2 and/or N2O transient formation during NO exposure of a prereduced CaO surface. It was shown that at low temperatures (between room temperature and 500 °C) both N2 and N2O were formed, but at temperatures above 500 °C only N2 was observed. The activation energies of the respective steps have been determined using temperature-programmed reaction experiments. The activation energy of the surface oxygen abstraction was determined to be 25 kcal/mol and is similar to the apparent activation energy of the overall reaction. The activation energy of the NO bond breakage was determined to be maximum 10 kcal/mol as measured by N2O formation. The importance of an N2O2- or N2O22- intermediate in the formation of N2O will be discussed, and the importance of N2O decomposition in forming N2 at temperatures above 500 °C will be compared with a N surface diffusion mechanism.
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| 3. |
- Bertilsson, L, et al.
(författare)
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Adsorption of dimethyl methylphosphonate on self-assembled alkanethiolate monolayers
- 1998
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Ingår i: JOURNAL OF PHYSICAL CHEMISTRY B. - : AMER CHEMICAL SOC. - 1089-5647 .- 1520-6106 .- 1520-5207. ; 102:7, s. 1260-1269
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Tidskriftsartikel (refereegranskat)abstract
- The adsorption of dimethyl methylphosphonate (DMMP), a model molecule for sarin, on three different organic interfaces, prepared by solution self-assembly of alkanethiols on gold, was followed by a surface acoustic wave mass sensor and infrared reflection-absorption spectroscopy at room temperature. The surfaces, characterized by the following tail groups (-OH, -CH3, -COOH), show both quantitative and qualitative differences concerning the interaction with DMMP, the acid surface giving rise to the strongest adsorption. Results obtained in UHV, at low temperatures using infrared spectroscopy and temperature-programmed desorption, support this observation and give complementary information about the nature of the interaction. The hydrogen-bond-accepting properties of the P=O part of DMMP and its impact on the design of sensing interfaces based on hydrogen bonding, as well as the use of self-assembled monolayers to study molecular interactions, are discussed.
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| 4. |
- Bertilsson, Lars, et al.
(författare)
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Interaction of dimethyl methylphosphonate with alkanethiolate monolayers studied by temperature-programmed desorption and infrared spectroscopy
- 1997
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Ingår i: JOURNAL OF PHYSICAL CHEMISTRY B. - : AMER CHEMICAL SOC. - 1089-5647 .- 1520-6106 .- 1520-5207. ; 101:31, s. 6021-6027
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Tidskriftsartikel (refereegranskat)abstract
- The adsorption of dimethyl methylphosphonate (DMMP) on well-defined organic surfaces consisting of self-assembled monolayers (SAMs) of omega-substituted alkanethiolates on gold has been studied. Three different surfaces were examined: one terminated with -OH groups (Au/S-(CH2)(16)-OH), one with -CH3 (Au/S-(CH2)(15)-CH3), and one mixed surface with approximately equal amounts of -OH and -CH3 terminated thiols. Detailed information about the nature and strength of the interaction was gathered by infrared reflection-absorption spectroscopy and temperature-programmed desorption under ultrahigh-vacuum conditions. It is found that the outermost functional groups of the thiol monolayer have a pronounced impact on the interaction with DMMP at low coverage. The -OH surface, allowing for hydrogen bonds with the P=O part of the DMMP molecule, increases the strength of interaction by approximately 3.8 kJ/mol as compared to the -CH3 surface. A preadsorbed layer of D2O leads to stronger interaction on all surfaces. This is explained by additional hydrogen bond formation between free O-D at the ice-vacuum interface and DMMP.
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| 5. |
- Bjorklund, Robert B., et al.
(författare)
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Vapor adsorption in thin silicalite-1 films studied by spectroscopic ellipsometry
- 1998
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 102:12, s. 2245-2250
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Tidskriftsartikel (refereegranskat)abstract
- Thin films of silicalite-1 grown on silicon substrates were studied by spectroscopic ellipsometry. Analysis of spectra using an optical model consisting of a single porous layer on silicon yielded average film thicknesses of 84 and 223 nm for films synthesized for 10 and 30 h. Void fraction for the films was 0.32-0.33. Vapor adsorption from a nitrogen carrier gas at room temperature was monitored by ellipsometry. Isotherms for different adsorbates were obtained by analysis of spectra taken at different vapor concentrations using an optical model where the void volume was filled with both nitrogen and condensed vapors. Quantification of the condensed vapor amount was based on the changes in refractive index when adsorbates replaced nitrogen in the pores. Adsorbate volumes for water, toluene, 1-propanol, and hexane were 0.12, 0.12, 0.15, and 0.17 cm3 liquid g-1 film, respectively.
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| 6. |
- Claesson, PM, et al.
(författare)
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Interactions between a 30% charged polyelectrolyte and an anionic surfactant in bulk and at a solid-liquid interface
- 1998
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Ingår i: Journal of Physical Chemistry B. - 1520-6106 .- 1520-5207. ; 102, s. 1270-1278
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Tidskriftsartikel (refereegranskat)abstract
- The association between a 30% charged cationic polyelectrolyte and an anionic surfactant, sodium dodecyl sulfate (SDS), in 10 mM 1:1 electrolyte was investigated using surface force measurements and dynamic light scattering. The polyelectrolyte employed was a random copolymer of the neutral acrylamide and cationic [3-(2-methylpropionamide)propyl]trimethylammonium chloride (AM-MAPTAC-31). Light scattering measurements show that upon progressive addition of SDS to an AM-MAPTAC-31 solution the single coil size decreases until precipitation occurs at an SDS/MAPTAC ratio of just above 0.4. At SDS/MAPTAC ratios at or above 2, redispersion of the aggregates takes place. The interfacial behavior of AM-MAPTAC-31/SDS complexes was investigated in two ways. In one set of experiments a droplet containing a mixture of SDS and AM-MAPTAC-31 was placed between the surfaces and adsorption was allowed to occur from the aqueous mixture. It was found that the range of the steric force decreased when the SDS/MAPTAC ratio was increased from 0 to 0.4, indicating adsorption in a less extended conformation due to a decreased repulsion between the polyelectrolyte segments. At a ratio of 0.6 a compact interfacial complex was formed and the measured force was attractive over a small distance regime. A further increase in SDS/MAPTAC ratio resulted in precipitation of large aggregates at the surface, and reproducible force data could not be obtained. At an even higher SDS/AM-MAPTAC ratio of 4, individual aggregates were once again adsorbed at the surface. Hence, we find a good correspondence between association in bulk and at the solid surface. In another set of experiments the polyelectrolyte was first preadsorbed to mica surfaces and then SDS was added to the polyelectrolyte-free solution surrounding the surfaces. In this way precipitation of large SDS-polyelectrolyte aggregates onto the surfaces was avoided. Addition of SDS up to a concentration of 0.1 mM hardly affected the long-range interaction but gave an increased compressed layer thickness. A further increase in SDS concentrations to 1 mM results in a dramatic increase in the range of the force, suggesting formation of strongly negatively charged polyelectrolyte-surfactant complexes.
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| 7. |
- Colmenarejo, G., et al.
(författare)
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Electronic Spectra and Transition Moments of 6-(2’-Pyridiniumyl)phenanthridinium Photoactive DNA Intercalators
- 1997
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Ingår i: Journal of Physical Chemistry B Materials. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 101:26, s. 5196-5204
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Tidskriftsartikel (refereegranskat)abstract
- The electronic transitions giving rise to the UV-visible absorption spectra of two pyridinium-phenanthridinium viologens, 6,7-dihydropyridol[2',1':3,4]pyrazinol[1,2-f]phenanthridinediium dication (1) and 7,8-dihydro-6H-pyrido[2',1':3,4]diazepino[1,2-f]phenanthridinediium dication (2), have been investigated with respect to energies, intensities, and transition moment directions. A combination of methods has been applied: UV-visible absorption, circular dichroism, magnetic circular dichroism, fluorescence anisotropy, Linear dichroism In stretched poly(vinyl alcohol) films, and semiempirical molecular orbital calculations. For both drugs, the lowest energy absorption band, occurring around 400 nm, results from two separate transitions. The corresponding electric transition dipole moments lie in the phenanthridine plane and are polarized, respectively, in the direction of the pyridine moiety (the lower energy transition) and parallel to the phenanthridine long axis (the higher energy transition). Up to four additional different pi --> pi* transitions account for a second band that peaks at 250 nm; they show different polarizations within the phenanthridine plane. The lowest energy transition of the whole spectrum of both drugs corresponds to the promotion of an electron from the HOMO to the LUMO, which are molecular orbitals mainly localized in the phenanthridine and pyridine rings, respectively, thereby implying a charge transfer, upon excitation, from the phenanthridine toward the pyridine ring. The experimental and theoretical results are discussed in relation to the spectroscopic, redox, and photochemical properties of these drugs.
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| 8. |
- de Carolis, Stefano, et al.
(författare)
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Structure and electronic properties of Ca-doped CeO2 and implications on catalytic activity : An experimental and theoretical study
- 1999
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Ingår i: J PHYS CHEM B. - : American Chemical Society (ACS). - 1089-5647. ; 103:36, s. 7627-7636
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Tidskriftsartikel (refereegranskat)abstract
- Doping CeO2 with for example, Ca gives an enhanced reactivity toward reduction of SO2 by CO, and total combustion of methane. Theoretical modeling using static minimizations and molecular dynamics (MD) simulations of the doped (110) face in combination with ab initio quantum chemical cluster models shows large effects on the Ce(IV)/Ce(III) balance due to the doping. Computed oxygen-to-cerium charge-transfer energies are strongly reduced as a result of the introduction of defects and oxygen vacancies, but not sufficiently to explain the observed reactivities. The structures resulting from the MD simulations for both the doped and undoped material are in good agreement with recent experimental pulsed neutron scattering results.
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| 9. |
- De Kerpel, Jan O A, et al.
(författare)
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Geometric and Electronic Structure of Co(II)-Substituted Azurin
- 1999
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Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 103:39, s. 8375-8382
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Tidskriftsartikel (refereegranskat)abstract
- The molecular and electronic structures of Co(II)-substituted azurin have been investigated using several realistic models of the metal coordination sphere. The geometry of the models was optimized using the hybrid density functional B3LYP method and compared to the structures obtained for similar Cu(II) models. It is found that Co(II) prefers a distorted tetrahedral structure with four strong bonds to two histidine nitrogens, the cysteine sulphur, and the backbone carbonyl group. This is in contrast to Cu(II), where two weak axial bonds to methionine and the backbone oxygen are found, combined with three strong bonds to the histidines and cysteine in the equatorial plane of a trigonal bipyramidal structure. The optimal structure of the models conforms with experimental crystal data, indicating that the active-site structure in these proteins is determined by the preferences of the metal ion and its ligand and not by protein strain. The electronic structure and spectrum of the Co(imidazole) 2(SH)(SH) 2(HCONH 2) + model have been investigated in detail using multiconfigurational second-order perturbation theory based on a complete active-space wavefunction (CASPT2). Nine ligand-field transitions and six S cys → Co charge-transfer transitions have been calculated, and all experimentally observed absorption bands in the absorption spectrum of Co(II) azurin have been assigned. It is shown that the Co-S cys bond is more ionic than the Cu-S cys bond and that this causes the blue shift and weakening of the charge-transfer states in the spectrum of Co(II)-substituted azurin compared to native copper protein.
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| 10. |
- De Kerpel, Jan O A, et al.
(författare)
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Theoretical study of the structural and spectroscopic properties of stellacyanin
- 1998
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Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 102:23, s. 4638-4647
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Tidskriftsartikel (refereegranskat)abstract
- The electronic spectrum of the azurin Met121Gln mutant, a model of the blue copper protein stellacyanin, has been studied by ab initio multiconfigurational second-order perturbation theory (the CASPT2 method), including the effect of the protein and solvent by point charges. The six lowest electronic transitions have been calculated and assigned with an error of less than 2400 cm-1. The ground-state singly occupied orbital is found to be a predominantly π antibonding orbital involving Cu3d and Scys3pπ. However, it also contains a significant amount (18%) of Cu-Scys σ antibonding character. This σ interaction is responsible for the appearance in the absorption spectrum of a band at 460 nm, with a significantly higher intensity than observed for other, axial, type 1 copper proteins (i.e., plastocyanin or azurin). The π-σ mixing is caused by the axial glutamine ligand binding at a much shorter distance to copper than the corresponding methionine ligand in the normal blue copper proteins. This explains why, based on its spectral properties, stellacyanin is classified among the rhombic type 1 copper proteins, although its structure is clearly trigonal, as it is for the axial proteins. Calculations have also been performed on structures where the glutamine model coordinates to the copper ion via the deprotonated N∈ atom instead of the O∈ atom. However, the resulting transition energies do not resemble the experimental spectrum obtained at normal or elevated pH. Thus, the results do not confirm the suggestion that the coordinating atom of glutamine changes at high pH.
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