1. |
- Acke, Filip, et al.
(author)
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Effects of O-2 on the reduction of NO over prereduced CaO surfaces: A mechanistic understanding
- 1999
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In: Journal of Physical Chemistry B Materials. - 1089-5647. ; 110:12, s. 2195-2201
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Journal article (peer-reviewed)abstract
- The effect of O-2 on the reduction of NO over prereduced CaO surfaces is investigated. The experimental results suggest the existence of at least three different reaction channels, of which two are related to the high-temperature reduction of the CaO surfaces and involve the use of extra electrons in breaking the NO bond. The third reaction channel does not employ extra electrons for bond breaking, but the activity is affected by the amount of adsorbed surface oxygens. The difference between the former two reaction channels is found in the temperature needed for an observable activity. The reaction channel which is already active at low temperatures is described by a model based on F-centers, whereas the one which needs elevated temperatures involves a hole transport through the bulk. The activation energy for this transport is determined experimentally using a temperature-programmed reaction technique as well as theoretically by means of ab initio quantum chemistry calculations. Room-temperature exposure to O-2 is suggested to result in a poisoning of the F-centers, but has only a minor effect on the reaction channel proposed for high temperatures. Effects on the reduction of NO of time as well as temperature for the O-2 exposure step are also investigated and found to be consistent with an understanding based on the coexistence of different reaction channels.
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2. |
- Acke, Filip, et al.
(author)
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Kinetic study of heterogeneous oxygen-exchange reactions and bulk self-diffusion of oxygen
- 1998
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In: Journal of Physical Chemistry B Materials. - 1089-5647. ; 102:26, s. 5158-5164
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Journal article (peer-reviewed)abstract
- Apparent activation energies are determined for oxygen-isotope exchange reactions between O2, CO, or NO and preoxidized or prereduced CaO surfaces. Oxygen exchange between N16O or C16O and isotope labeled Ca18O surfaces produced an apparent activation energy of 2 kcal/mol. Similar values are obtained for single- isotope-exchange between 18O18O and prereduced Ca16O surfaces. Apparent activation energies of 15−18 kcal/mol are found for single and double exchange between 18O18O and preoxidized Ca16O surfaces, as well as for double exchange between 18O18O and prereduced Ca16O surfaces. The low apparent activation energies are believed to result from adsorbed intermediates, whereas the high values may involve the formation of a singlet O2 transient. It is shown that eventually the self-diffusion of oxygen ions in the bulk becomes the rate-determining step in the isotope-exchange reaction. Apparent activation energies are determined, and the values are found to depend on surface treatment, (i.e., 44 kcal/mol under reducing and 78 kcal/mol under oxidizing conditions). The involvement of oxygen vacancies under reducing conditions is discussed.
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3. |
- Acke, Filip, et al.
(author)
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Promoting effects of Na and Fe impurities on the catalytic activity of CaO in the reduction of NO by CO and H-2
- 1998
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In: Journal of Physical Chemistry B Materials. - 1089-5647. ; 102:26, s. 5127-5134
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Journal article (peer-reviewed)abstract
- The heterogeneous reduction of NO by H-2 and CO over different CaO materials is investigated. The dependence of the specific NO reduction rate on the impurity content is demonstrated for both reducing species. The roles of two specific impurities, i.e., Na and Fe, as well as their combined effect are investigated. The apparent activation energies for the NO + CO and NO + Hz reactions are determined for three different calcium oxides. Values between 26 and 28 kcal/mol are obtained. The influence of impurity content is found in the preexponential factor of the Arrhenius equation. A reaction mechanism based on a rate-determining surface-oxygen-abstraction step is suggested. This mechanistic understanding is explored to compare the activities of other alkaline-earth oxides. Particularly, a linear correlation between the apparent activation energy and the lattice parameter is observed.
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4. |
- Acke, Filip, 1968, et al.
(author)
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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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In: 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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Journal article (peer-reviewed)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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5. |
- Adebahr, J, et al.
(author)
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Ion and solvent dynamics in gel electrolytes based on ethylene oxide grafted acrylate polymers
- 2002
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In: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106 .- 1089-5647. ; 106:47, s. 12119-12123
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Journal article (peer-reviewed)abstract
- Multinuclear pulsed field gradient NMR measurements and theological viscosity measurements were performed on three series of polymer gel electrolytes. The gels were based on a lithium salt electrolyte swollen into a copolymer matrix comprising an acrylate backbone and ethylene oxide side chains. In each series the side chains differed in length and number, but the acrylate-to-ethylene oxide ratio was kept constant. It was found that the self-diffusion coefficient of the cations was much lower than that of the anions, and that it decreased rapidly when the side chains got longer. In contrast, the self-diffusion coefficient of the anions was found to be independent of chain length. In the gel electrolytes, the diffusion coefficients of the solvent molecules are relatively constant despite an increased viscosity with increasing length of the side chains. However, in saltfree gels made for comparison, the diffusion coefficients of the solvent molecules decreased with, increasing length of the side chains, which is consistent with an increased viscosity.
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6. |
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7. |
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8. |
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9. |
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10. |
- Bales, BL, et al.
(author)
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Time-resolved fluorescence quenching and electron paramagnetic resonance studies of the hydration of lithium dodecyl sulfate micelles
- 2000
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In: JOURNAL OF PHYSICAL CHEMISTRY B. - : AMER CHEMICAL SOC. - 1089-5647. ; 104:2, s. 256-263
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Journal article (peer-reviewed)abstract
- A spin-probe method to study the surface hydration of sodium dodecyl sulfate (SDS) micelles (Bales, B. L.; Messina, L.; Vidal, A.; Peric, M.; Nascimento, O. R. J. Phys. Chem. 1998, 102, 10347; referred to as I) is applied to lithium dodecyl sulfate (LiDS)
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11. |
- Bauer, C., et al.
(author)
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Electron injection and recombination in Ru(dcbpy)(2)(NCS)(2) sensitized nanostructured ZnO
- 2001
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In: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 105:24, s. 5585-5588
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Journal article (peer-reviewed)abstract
- The dynamics of electron-transfer processes between bis(tetrabutylammonium) cis-bis(thiocyanato)bis(2,2'-bypiridine-4,4'-dicarboxylato)ruthenium(II) (called N719) and nanostructured ZnO films have been investigated by femtosecond and nanosecond spectroscopy. The incident photon to current conversion efficiency (IPCE) for these dye-sensitized electrodes was 36% in the maximum of 530 nm, corresponding to a quantum efficiency of 80%. The highest: IPCE values were obtained when the electrodes were prepared under conditions where formation of dye aggregates in the pores of the nanostructured films is avoided. For such films, the electron injection time was in the subpicosecond regime (< 300 fs), which is comparable to the N719-TiO2 system. The back electron-transfer kinetics between conduction band electrons and oxidized dye molecules were biexponential with time constants of 300 mus and 2.6 us. Variation of the light intensity did not affect the time constants, but only their relative weights. The kinetics of back electron transfer in the N719-ZnO and N719-TiO2 systems were found to be identical.
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12. |
- Bauer, Christophe, et al.
(author)
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Electron injection and recombination in Ru(dcbpy)(2)(NCS)(2) sensitized nanostructured ZnO
- 2001
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In: JOURNAL OF PHYSICAL CHEMISTRY B. - : AMER CHEMICAL SOC. - 1089-5647. ; 105:24, s. 5585-5588
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Journal article (peer-reviewed)abstract
- The dynamics of electron-transfer processes between bis(tetrabutylammonium) cis-bis(thiocyanato)bis(2,2'-bypiridine-4,4'-dicarboxylato)ruthenium(II) (called N719) and nanostructured ZnO films have been investigated by femtosecond and nanosecond spectrosco
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13. |
- Bergström, Lars Magnus, et al.
(author)
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A small-angle neutron scattering study of surfactant aggregates formed in aqueous mixtures of sodium dodecyl sulfate and didodecyldimethylammonium bromide
- 2000
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In: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 104:17, s. 4155-4163
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Journal article (peer-reviewed)abstract
- The structure of aggregates formed in aqueous mixtures of a single-chain anionic surfactant, sodium dodecyl sulfate (SDS), and a double-chain cationic surfactant, didodecyldimethylammonium bromide (DDAB), has been investigated at 38 degrees C using small-angle neutron scattering (SANS). Several overall surfactant concentrations [SDS] + [DDAB] between 0.1 and 5 wt % were measured at the two SDS-rich compositions [SDS]:[DDAB] = 90:10 and 95:5. Samples with a concentration above about [SDS] + [DDAB] = 1 wt % at [SDS]:[DDAB] = 95:5 contained only somewhat elongated tablet-shaped micelles (triaxial ellipsoids) with typical values of the half-axes a (related to the thickness) = 14 Angstrom, b (related to the width) = 23 Angstrom, and c (related to the length) = 27 Angstrom. When a sample at [SDS]:[DDAB] = 95:5 is diluted below about [SDS] + [DDAB] = 1 wt %, an increasing amount of small unilamellar vesicles forms, and in the samples below about 0.2 wt %, only vesicles are observed. The average radius of the vesicles [R] increases from about 90 Angstrom at 0.3 wt % to 110 Angstrom at 0.1 wt %. The transition from micelles to vesicles with decreasing surfactant concentration was also observed in the samples at [SDS]:[DDAB] = 90:10 in which, however, an additional amount of bilayer sheets was seen to be always present. Compared with the micelles at [SDS]:[DDAB] = 95:5, the micelles formed at [SDS]:[DDAB] = 90:10 were considerably longer (c approximate to 40 Angstrom), but with similar cross section dimensions, and the vesicles formed were seen to be somewhat larger than the corresponding aggregates at 95:5. The relative standard deviation sigma(R)/[R] of the (number-weighted) vesicle size distributions was in the range 0.2 < sigma(R)/[R] < 0.3.
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14. |
- Bertilsson, L, et al.
(author)
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Adsorption of dimethyl methylphosphonate on self-assembled alkanethiolate monolayers
- 1998
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In: JOURNAL OF PHYSICAL CHEMISTRY B. - : AMER CHEMICAL SOC. - 1089-5647 .- 1520-6106 .- 1520-5207. ; 102:7, s. 1260-1269
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Journal article (peer-reviewed)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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15. |
- Bertilsson, Lars, et al.
(author)
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Interaction of dimethyl methylphosphonate with alkanethiolate monolayers studied by temperature-programmed desorption and infrared spectroscopy
- 1997
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In: JOURNAL OF PHYSICAL CHEMISTRY B. - : AMER CHEMICAL SOC. - 1089-5647 .- 1520-6106 .- 1520-5207. ; 101:31, s. 6021-6027
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Journal article (peer-reviewed)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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16. |
- Biesuz, R., et al.
(author)
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Estimation of deprotonation coefficients for chelating ion exchange resins. Comparison of different thermodynamic model
- 2001
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In: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 105:20, s. 4721-4726
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Journal article (peer-reviewed)abstract
- The deprotonation of quinolic resin P-127 and iminodiacetic resin Amberlite IRC-718 has been studied. The process of salt transfer into the resin phase is considered to be an important contributor to the deprotonation process. Estimation of the salt transfer was based on the principle of equal activity of the salt in both phases at equilibrium. Two assumptions were made: sorbed alkali metal ions are not associated with functional groups, while all hydrogen ions are associated with functional groups. The associated hydrogen ions and functional groups do not contribute to the internal ionic strength value. Two thermodynamic models, describing the deprotonation of ion-exchange resin, were used and compared: the Gibbs-Donnan-based model of Bukata and Marinsky and the model proposed by Erik Hogfeldt. Thermodynamic characteristics of the resins' deprotonation are obtained using two different thermodynamic approaches. Hogfeldt's three-parameter model provides a better agreement with experimental data. The fitting of the data to Marinsky's method can be improved by taking into account the influence of the resins' macroporosity; however, this requires an additional empirical parameter to describe the resin.
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17. |
- Boschloo, Gerrit, et al.
(author)
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Spectroelectrochemical investigation of surface states in nanostructured TiO2 electrodes
- 1999
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In: JOURNAL OF PHYSICAL CHEMISTRY B. - : AMER CHEMICAL SOC. - 1089-5647. ; 103:12, s. 2228-2231
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Journal article (other academic/artistic)abstract
- Surface states at the nanostructured TiO2 (anatase)/aqueous electrolyte interface have been investigated using spectroelectrochemical methods. It is found that electrons trapped in these states have an absorption spectrum that differs significantly from t
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18. |
- Boschloo, Gerrit, et al.
(author)
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Spectroelectrochemistry of nanostructured NiO
- 2001
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In: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 105:15, s. 3039-3044
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Journal article (peer-reviewed)abstract
- Transparent nanostructured NiO electrodes have been prepared by heating Ni(OH)(2) sol-gel films at a temperature of 300-320 degreesC. Nanostructured NiO (bunsenite) behaves as a p-type semiconductor and has an indirect band gap of 3.55 eV. It shows a strong anodic electrochromic effect, as it changes color from transparent to brown-black upon application of positive potentials. This effect is caused by oxidation of Ni atoms located at the NiO/electrolyte interface. Electrochemical oxidation reactions are highly reversible in both aqueous and nonaqueous electrolytes. In aqueous electrolyte, the half-potentials show a Nernstian pH dependence, whereas in nonaqueous electrolytes, the type of cation present determines the shape and position of the cyclic voltammogram.
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19. |
- Boschloo, Gerrit, et al.
(author)
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Spectroelectrochemistry of nanostructured NiO
- 2001
-
In: JOURNAL OF PHYSICAL CHEMISTRY B. - : AMER CHEMICAL SOC. - 1089-5647. ; 105:15, s. 3039-3044
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Journal article (peer-reviewed)abstract
- Transparent nanostructured NiO electrodes have been prepared by heating Ni(OH)(2) sol-gel films at a temperature of 300-320 degreesC. Nanostructured NiO (bunsenite) behaves as a p-type semiconductor and has an indirect band gap of 3.55 eV. It shows a stro
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20. |
- Brinck, Tore, et al.
(author)
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Solvation of carbanions in organic solvents : A test of the polarizable continuum model
- 2000
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In: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 104:42, s. 9887-9893
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Journal article (peer-reviewed)abstract
- The solvation of carbanions in the solvents N,N-dimethylformamide (DMF) and tetrahydrofuran (THF) has been analyzed on the basis of experimental and theoretical data. Experimental solvation energies are obtained from present and previously reported electrochemical measurements of reduction potentials of the corresponding radicals. Theoretical solvation energies are obtained from quantum chemical calculations using the polarizable continuum model (PCM). It is found that the solvation energy is relatively independent of molecular size and structure for the saturated carbanions. This indicates that the negative charge is strongly localized to the anionic carbon. The conjugated carbanions have considerably lower absolute solvation energies (\ DeltaG degrees (sol)\) than the saturated carbanions. This is a consequence of the strong delocalization of the negative charge in the former group. The propargyl anion is also found to have a surprisingly low absolute solvation energy. However, high-level quantum chemical calculations show that the electronic structure has large contributions from two different resonance structures, CH=CCH2- and -CH=C=CH2, which results in a significant charge delocalization. There is good agreement between calculated and experimental solvation energies for both the conjugated and nonconjugated primary anions. However, the PCM method consistently underestimates the absolute solvation energies of the secondary and tertiary carbanions. This is attributed to an insufficient treatment of first-layer solvation effects in the method. According to the experimental measurements, the absolute solvation energies are on average 2-3 kcal mol(-1) lower in THF than in DMF. The theoretical data indicate a considerably larger solvent effect, 7-10 kcal mol(-1). The discrepancy between theory and experiment may partly be attributed to the use of a supporting electrolyte in the measurements, but the main cause seems to be that the short-range interaction tendencies of the solvent cannot be Fully characterized by its dielectric constant.
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21. |
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22. |
- Cho, K. B., et al.
(author)
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The substrate reaction mechanism of class III anaerobic ribonucleotide reductase
- 2001
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In: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 105:27, s. 6445-6452
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Journal article (peer-reviewed)abstract
- The substrate mechanism of class III anaerobic ribonucleotide reductase has been studied using quantum chemical methods. The study is based on the previously suggested mechanism for the aerobic class I enzyme, together with the recently determined X-ray structure of the anaerobic enzyme. The initial steps are similar in the mechanisms of these enzymes, but for the suggested rate-limiting steps there are key differences. In the class I enzyme, the 3 ' -keto group of the substrate is protonated in a step involving formation of a sulfur-sulfur bond between two cysteines, One of these cysteines is not present in the anaerobic enzyme. Instead, carbon dioxide is formed in this step from formate, which is present as a cofactor. In line with previous suggestions from experimental observations, the formate first forms a formyl radical. The next step, where the formyl radical protonates the 3 ' -keto group of the substrate, is suggested to be rate limiting with a calculated total barrier of 19.9 kcal/mol, in reasonable agreement with the experimental rate-limiting barrier of 17 kcal/mol. Zero-point and entropy effects are found to be quite significant in lowering the barrier. The mechanism for the entire cycle is discussed in relation to known experimental facts.
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23. |
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24. |
- Claesson, Per M., et al.
(author)
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Mixtures of cationic polyelectrolyte and anionic surfactant studied with small-angle neutron scattering
- 2000
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In: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 104:49, s. 11689-11694
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Journal article (peer-reviewed)abstract
- Small-angle neutron scattering (SANS) data for solutions containing a highly charged cationic polyelectrolyte and an anionic surfactant are presented. The scattering data were obtained in pure D2O, emphasizing the scattering from the polyelectrolyte, and in a H2O/D2O mixture that contrast matches the polyelectrolyte. In the absence of surfactant, a broad scattering peak due to the mesh size of the polyelectrolyte solution is the most characteristic feature. This peak moves to larger q-values (smaller distances) as the polyelectrolyte concentration is increased, as expected for a semidilute polyelectrolyte solution. Addition of a small amount of surfactant reduces and finally removes this peak. Instead a sharp diffraction peak appears at high q-values. This Bragg peak corresponds to a characteristic distance of 37-39 Angstrom, and it is observed when either the polyelectrolyte or the surfactant is contrast matched by the solvent. Once this peak has appeared, its position does not change when the surfactant concentration is increased. The intensity of the peak grows, however, until a stoichiometric polyelectrolyte-surfactant complex has been formed. The Bragg peak remains in excess surfactant solution. These results are discussed in relation to the structure of the polyelectrolyte-surfactant aggregates and in connection with recent results from surface force and turbidity measurements using the same polyelectrolyte-surfactant pair.
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25. |
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26. |
- Colmenarejo, G., et al.
(author)
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Electronic Spectra and Transition Moments of 6-(2’-Pyridiniumyl)phenanthridinium Photoactive DNA Intercalators
- 1997
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In: 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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Journal article (peer-reviewed)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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27. |
- Cummins, D., et al.
(author)
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Ultrafast electrochromic windows based on redox-chromophore modified nanostructured semiconducting and conducting films
- 2000
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In: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 104:48, s. 11449-11459
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Journal article (peer-reviewed)abstract
- Described is the construction of an ultrafast electrochromic window. One electrode of this window is based on a transparent nanostructured TiO2 (anatase) film (4.0 mum. thick) supported on conducting glass (F-doped tin oxide, 10 Ohm cm(-2), 0.5 mum thick) and modified by chemisorption of a monolayer of the redox chromophore bis(2-phosphonoethyl)-4,4'-bipyridinium dichloride. The other electrode is based on a transparent nanostructured SnO2 film (3.0 mum thick) supported on conducting glass (F-doped tin oxide, 10 Ohm cm-2, 0.5 mum thick) and modified by chemisorption of a monolayer of the redox chromophore [beta-(10-phenothiazyl)propoxy]phosphonic acid. The electrolyte used is LiClO4 (0.2 mol dm(-3)) in gamma -butyrolactone. The excellent performance of a 2.5 cm x 2.5 cm window over 10 000 electrochromic test cycles-switching times (coloring and bleaching) of less than 250 ms, coloration efficiency of 270 cm(2) C-1, steady-state currents (colored and bleached) of less than 6 muA cm(-2), and memory of greater than 600 s (time required for low end transmittance to increase by 5%)-suggest a practical technology.
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28. |
- de Carolis, Stefano, et al.
(author)
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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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In: J PHYS CHEM B. - : American Chemical Society (ACS). - 1089-5647. ; 103:36, s. 7627-7636
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Journal article (peer-reviewed)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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29. |
- D'Ercole, A, et al.
(author)
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Embedded-cluster study of hydrogen interaction with an oxygen vacancy at the magnesium oxide surface
- 1999
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In: JOURNAL OF PHYSICAL CHEMISTRY B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 103:19, s. 3872-3876
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Journal article (peer-reviewed)abstract
- An embedded-cluster Hartree−Fock approximation is adopted for simulating the formation of Fs(H) color centers at the (001) surface of magnesium oxide. This process is assumed to take place in two steps at an isolated surface anion vacancy: first, a hydrogen molecule is adsorbed dissociatively at the defect; second, following UV irradiation, a neutral hydrogen atom is removed and an electron remains trapped at the vacancy with a hydroxyl group nearby. According to the present calculations, the activation energy for the dissociation is appreciable (about 25 kcal/mol) and the products (a proton bound to a low-coordinated oxygen and a hydride ion above the vacancy) are considerably less stable than the reactants. The excitation of the adsorbed species owing to the UV irradiation is simulated by considering a singlet−triplet transition of the hydride−vacancy complex, which then dissociates into an H atom and a trapped lone electron. The electronic structure and the EPR parameters of the resulting paramagnetic state are explored. The theoretical results agree in many respects with the experimental data as concerns one of the forms of heterolitically dissociated hydrogen which are found at the defective MgO surface. However, from the viewpoint of the energetics, this model is untenable because that species is known to form irreversibly at room temperature with low activation energy.
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30. |
- Engstrom, Maria, et al.
(author)
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Density functional theory calculations of electron paramagnetic resonance parameters of a nitroxide spin label in tissue factor and factor VIIa protein complex
- 2002
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In: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207 .- 1089-5647. ; 106:47, s. 12354-12360
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Journal article (peer-reviewed)abstract
- The electron paramagnetic resonance (EPR) g and N-14 hyperfine coupling (A) tensors of a nitroxide spin label are calculated with density-functional theory (DFT). The influence on the spin label from nearby amino acids in the extracellular part of tissue factor (sTF) and activated factor VII (FVIIa) protein complex is investigated. For that purpose, the nitroxide unit and six surrounding amino acids within 5 Angstrom are selected on the basis of a molecular mechanics structure of the protein complex. The effects of the surroundings on the EPR parameters of the spin label can be divided into indirect effects caused by the induced structure changes of the spin label and direct effects. The structural changes are larger in the present case. The experimentally measurable hyperfine tensor component perpendicular to the molecular plane of the spin label, A(zz), as well as the g tensor component along the NO direction, g,,, are significant probes of the intramolecular structure of the spin label. This indicates the possibility of relating EPR properties to the geometric structure of radical sites. The direct environmental effects on the g tensor from the surrounding amino acids mainly affect the second-order spin-orbit/orbital Zeeman cross-term contributions from the spin label itself. The direct effects originating elsewhere in the model are small. Neither the g nor A tensors display additivity of the effects of individual amino acids on the final observable. The results underline the feasibility of DFT calculations of the EPR parameters in large molecular systems, such as spin labels and other radicals in proteins.
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31. |
- Furo, Istvan, et al.
(author)
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Structure and dynamics of associative water-soluble polymer aggregates as seen by F-19 NMR spectroscopy
- 2000
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In: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 104:3, s. 485-494
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Journal article (peer-reviewed)abstract
- The microstructure and the molecular dynamics of aqueous solutions of an associative polymer (AP), consisting of a polyelectrolyte backbone and attached perfluorocarbon side chains, have been studied by F-19 NMR. The spectral shape and the distributions of the molecular self-diffusion coefficient and of the transverse and the longitudinal relaxation rates along the F-19 spectrum have been measured, together with the magnetic field dependence of the relaxation rates. A comprehensive analysis yields that the spectral lines are broadened by a chemical shift distribution that reflects the distribution of the average hydrophobicity of the environment within the aggregates in which the perfluorocarbon side chains reside. A fraction of the AP chains remain unaggregated with all their side chains immersed in water. As shown by the NMR data, the exchange time of the APs among different states of aggregation is long, > 1 s, while the exchange time of individual side chains among aqueous and hydrophobic environments within the same aggregate is fast, approximately microseconds. Among the aggregates, there is a considerable heterogeneity in the average hydrophobicity. Some implications of the F-19 NMR findings for H-1 NMR studies of APs with hydrocarbon side chains are discussed.
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32. |
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33. |
- Gisselfält, Katrin, 1969, et al.
(author)
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Interactions of tris(phenanthroline)ruthenium(II) enantiomers with DNA: Effects on helix flexibility studied by the electrophoretic behavior of reptating DNA in agarose gel
- 2000
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In: Journal of Physical Chemistry B Materials. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 104:15, s. 3651-3659
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Journal article (peer-reviewed)abstract
- A combination of measurement of mobility and orientational dynamics of long reptating DNA in agarose gel has been used to reveal how the binding of Delta and Lambda enantiomers of the tris(phenanthroline)ruthenium(II) ion affects the flexibility of the DNA helix. The mobility data, and data over the step length and period time of the reptation cycle, obtained in the presence of the respective enantiomer, are compared with those of free DNA and with data obtained earlier on DNAs with known variations in helix flexibility. The results suggest that the Delta form induces kinks in the DNA helix while the Lambda form gives rise to a local stiffening of the helix.
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34. |
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35. |
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36. |
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37. |
- Hammarstrom, Leif, et al.
(author)
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Two-dimensional emission quenching and charge separation using a Ru(II)-photosensitizer assembled with membrane-bound acceptors
- 1997
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In: JOURNAL OF PHYSICAL CHEMISTRY B. - : AMER CHEMICAL SOC. - 1089-5647. ; 101:38, s. 7494-7504
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Journal article (peer-reviewed)abstract
- Novel syntheses of the bipyridine ligand 1, dcHb (dcHb = 4,4'-dicarboxy-2,2'-bipyridine), by anionic oxidation of 4,4'-dimethyl-2',2-bipyridine (dmb) using molecular oxygen (4 atm), and of the sensitizer precursor 4, tris(4,4'-diethoxycarbonyl-2,2'-bipyri
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38. |
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39. |
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40. |
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41. |
- He, JJ, et al.
(author)
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Dye-sensitized nanostructured p-type nickel oxide film as a photocathode for a solar cell
- 1999
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In: JOURNAL OF PHYSICAL CHEMISTRY B. - : AMER CHEMICAL SOC. - 1089-5647. ; 103:42, s. 8940-8943
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Journal article (peer-reviewed)abstract
- Nanostructured NiO film was prepared by depositing nickel hydroxide slurry on conducting glass and sintering at 500 degrees C to a thickness of about 1 mu m. The photocurrent-voltage (IV) characteristics of the plain nanostructured NiO electrode recorded
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42. |
- Hedin, N., et al.
(author)
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Fast diffusion of the Cl- ion in the headgroup region of an oppositely charged micelle. A Cl-35 NMR spin relaxation study
- 2000
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In: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 104:35, s. 8544-8547
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Journal article (peer-reviewed)abstract
- Field-dependent Cl-35 spin relaxation rates have been measured in the micellar solution of C(16)TACl-D2O. The data are analyzed in terms of a fast hydration dynamics of the ion and a slower motion that consists of ion diffusion and micellar tumbling. This model provides a distinct motional correlation time of the Cl- ion from which it is concluded that, on the nanosecond time scale, the Cl- ion diffuses on the micellar surface close to its bulk diffusion speed. The hydration dynamics on the picosecond time scale differs from that in a dilute solution. These features are similar to that observed for Br- ions in the vicinity of aqueous micelles of C(16)TABr (Hedin, N.; Furo, I. J. Phys. Chem. B 1999, 103, 9640).
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43. |
- Hermansson, K, et al.
(author)
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Water exchange around Li+ and Na+ in LiCl(aq) and NaCl(aq) from MD simulations
- 1998
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In: JOURNAL OF PHYSICAL CHEMISTRY B. - : AMER CHEMICAL SOC. - 1089-5647. ; 102:31, s. 6089-6097
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Journal article (other academic/artistic)abstract
- The solvent exchange mechanisms around alkali cations in dilute aqueous solution have been investigated from cation-oxygen distance vs time curves, molecular animations, activation volumes, and lifetime statistics. Constant pressure (0 atm)-constant tempe
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44. |
- Himo, Fahmi, et al.
(author)
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Very stable ribonucleotide substrate radical relevant for class I ribonucleotide reductase
- 2000
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In: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 104:31, s. 7502-7509
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Journal article (peer-reviewed)abstract
- In recent experimental studies on the E441Q mutant of ribonucleotide reductase, a new substrate radical was discovered on the minute time scale. This radical is kinetically coupled to a cysteine-based radical appearing on the 10 s time scale. In the present study, density functional calculations have been performed to investigate the nature of these radicals. The most interesting result is that a very stable substrate radical was found, which lies outside the normal substrate pathway. This radical is so stable that its creation has to be avoided by the enzyme, or the substrate reactions would be slowed by several orders of magnitude. It is suggested that the enzyme accomplishes this task by considerably straining the mobility of the Cys225 residue. A previously suggested reaction mechanism is modified to take these recent findings into account. The modification does not significantly change the energetics of the model reactions.
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45. |
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46. |
- Kariis, H, et al.
(author)
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Thioethylpyrrole monolayers on gold. A spectroscopic study in ultrahigh vacuum
- 1998
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In: JOURNAL OF PHYSICAL CHEMISTRY B. - : AMER CHEMICAL SOC. - 1089-5647. ; 102:34, s. 6529-6538
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Journal article (other academic/artistic)abstract
- The adsorption of 3-(2-thioethyl)pyrrole (3-TEP) and 1-(2-thioethyl)pyrrole (1-TEP) on gold has been studied in ultrahigh vacuum with infrared reflection absorption spectroscopy, X-ray photoelectron spectroscopy, and temperature-programmed desorption mass
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47. |
- Keil, M, et al.
(author)
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Influence of the morphology on the electronic structure of hexa-peri-hexabenzocoronene thin films
- 2000
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In: JOURNAL OF PHYSICAL CHEMISTRY B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 104:16, s. 3967-3975
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Journal article (peer-reviewed)abstract
- Ultrathin films of hexa-peri-hexabenzocoronene (HBC), prepared in ultrahigh vacuum by sublimation and studied using a combination of atomic force microscopy, X-ray absorption spectroscopy, and photoelectron spectroscopy were interpreted with the help of results of quantum chemical calculations. The important role of the nature of the surface of the substrate as well as the effect of postdeposition annealing on the morphology and the subsequent electronic structure were studied. In particular, on the (0001) surfaces of highly oriented pyrolitic graphite (HOPG) or molybdenum disulfide (MoS2), there is a high degree of molecular order induced by the crystal structure of the substrate surface. When deposited at room temperature on HOPG, the molecules are epitaxially ordered, while crystals from the epitaxially ordered films nucleate and grow during heating. On oxidized silicon(001) or polycrystalline gold surfaces, only totally disordered molecular solid films are formed. The epitaxial films assume a layered structure and exhibit a graphite-like electronic structure. In particular, the relationship between electronic structure and topology is discussed. The results of these studies indicate methods for the preparation of the crystalline seeds to molecular wires.
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48. |
- Kilså-Jensen, Kristine, 1972, et al.
(author)
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Interlayer energy transfer between carbazole and two 9-anthroyloxy derivatives in Langmuir-Blodgett films
- 1999
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In: Journal of Physical Chemistry B Materials. - 1089-5647. ; 103:40, s. 8514-8523
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Journal article (peer-reviewed)abstract
- The interlayer excitation energy transfer between 11-(9-carbazole)undecanoic acid (II-CU) and two 9-anthroyloxy derivatives, 9-(9-anthroyloxy)stearic acid (9-AS) and 2-(9-anthroyloxy)stearic acid (2-AS), in alternating multilayer Langmuir-Blodgett films has been studied. The 11-CU fluorescence is quenched by energy transfer to 9-AS or 2-AS as judged by steady-state and picosecond time-resolved fluorescence measurements. The fluorescence decay curves of 11-CU in the films were analyzed in the framework of several models: (1) a general model for interlayer energy transfer, (2) a two-exponential decay, (3) a Forster model for energy transfer in a two-dimensional system, and (4) a stretched-exponential decay, characteristic of Forster energy transfer in self-similar fractal-like structures. The recovered decay parameters suggest an inhomogeneous mixing of the acceptor molecules in LB films leading to a two-phase system. The phase separation during compression of the acceptor monolayers forms regions of acceptor concentration about 3 times that of the intended and regions with very low acceptor concentration.
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49. |
- Kilså, Kristine, 1972, et al.
(author)
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Mediated electronic coupling: Singlet energy transfer in porphyrin dimers enhanced by the bridging chromophore
- 1999
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In: Journal of Physical Chemistry B Materials. - 1089-5647. ; 103:34, s. 7329-7339
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Journal article (peer-reviewed)abstract
- We have studied singlet electronic energy transfer (EET) in two donor-bridge-acceptor series (D-B -A), in which the donor (zinc porphyrin or its pyridine complex) and the acceptor (free base porphyrin) were covalently connected by a geometrically well-defined bridging chromophore. We have investigated how the medium between a donor and an acceptor influences EET by separating the influence of the electronic structure of the bridging chromophore from other effects known to influence the energy transfer. The electronic structure of the bridging chromophore was varied by changing the central unit (bicyclo[2.2.2]octane, benzene, naphthalene, or anthracene) in the bridging chromophore. In all systems the excited state energy separation donor-bridge and bridge-acceptor is large enough to prevent stepwise singlet energy transfer. In addition, the systems were designed to minimize conjugation to preserve the identity of the separate chromophores (donor, bridge, acceptor). Compared with the rate constant expected from the Forster theory, the bridging chromophore with bicyclo[2.2.2]octane as the central unit did not significantly enhance the energy transfer rate constant. However, the bridging chromophores with benzene and naphthalene as the central unit showed a moderate increase, whereas the bridging chromophore with anthracene as the central unit showed the largest increase in energy transfer rate constant. This increase is ascribed to a mediating effect of the bridging chromophore and it is proposed to be strongly correlated to the energy splitting between the singlet excited states of donor and bridging chromophores.
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50. |
- Koehorst, R. B. M., et al.
(author)
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Spectral sensitization of TiO2 substrates by monolayers of porphyrin heterodimers
- 2000
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In: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 104:10, s. 2371-2377
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Journal article (peer-reviewed)abstract
- Photoelectrochemical cells have been constructed by depositing monolayers of oriented covalently linked zinc/free base porphyrin heterodimers onto similar to 30 nm nonporous layers of TiO2 on ITO, deposited by metalorganic chemical vapor deposition (MO-CVD), and onto similar to 100 nm porous, nanostructured TiO2 layers, spin-coated from a suspension of P25 (Degussa) on ITO. Fluorescence quenching of the dyes on both types of TiO2 substrates is compared with that of dilute solutions of the dyes and with that of dye-coated, porous ZrO2 (Degussa) substrates. By functionalizing one of the porphyrin dimer components with carboxylic substituents, which bind to the TiO2 or ZrO2 substrate surface, either the zinc porphyrin (ZnP) or the free base porphyrin (H2P) component of the dimer can be made to be in diner contact with the substrate. These dimer-substrate arrangements are denoted ZnP-H2P- -TiO2 (dimer 1) and H2P-ZnP- -TiO2 (dimer 2), respectively, where - - denotes binding of the carboxyl-substituted porphyrin in the heterodimer to the substrate surface. In solution as well as deposited on ZrO2, in contact with the solvent without a redox couple, both types of dimers show efficient internal ZnP to H2P energy transfer. Deposited on TiO2, in the presence of the solvent, monolayers of both types of dimers show less efficient energy transfer than the dimers on ZrO2, For a ZnP-H2P- -TiO2 electrochemical cell the photocurrent action spectrum reproduces the absorption spectrum, i.e., contains contributions of both the ZnP and H2P moieties. By contrast, for H2P-ZnP- -TiO2 cells mainly the ZnP dimer component contributes to the photocurrent, demonstrating that in H2P-ZnP- -TiO2 cells electron transfer from the ZnP into the TiO2 substrate is faster than energy transfer to the adjacent free base porphyrin. The photocurrent action spectrum of the ZnP-H2P- -TiO2 cell also demonstrates that energy transfer in monolayers of this dimer results in sensitization of the semiconductor substrate, since the spectral response of a cell is enhanced with respect to that of a cell with a monolayer of a monomeric sensitizer. These results are relevant for the construction of a solar cell containing a supramolecular, light-collecting antenna.
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