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1.
  • Acke, Filip, 1968, et al. (författare)
  • Role of adsorbed surface oxygen in the adsorption of NO on alkaline earth oxides and Pt-promoted CaO surfaces
  • 1999
  • Ingår i: Journal of Physical Chemistry B Materials. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 103:6, s. 972-978
  • 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.
  • Adebahr, J, et al. (författare)
  • Ion and solvent dynamics in gel electrolytes based on ethylene oxide grafted acrylate polymers
  • 2002
  • Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106 .- 1089-5647. ; 106:47, s. 12119-12123
  • Tidskriftsartikel (refereegranskat)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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3.
  • Bauer, C., et al. (författare)
  • Electron injection and recombination in Ru(dcbpy)(2)(NCS)(2) sensitized nanostructured ZnO
  • 2001
  • Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 105:24, s. 5585-5588
  • Tidskriftsartikel (refereegranskat)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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4.
  • Bergström, Lars Magnus, et al. (författare)
  • A small-angle neutron scattering study of surfactant aggregates formed in aqueous mixtures of sodium dodecyl sulfate and didodecyldimethylammonium bromide
  • 2000
  • Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 104:17, s. 4155-4163
  • Tidskriftsartikel (refereegranskat)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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5.
  • Bertilsson, L, et al. (författare)
  • Adsorption of dimethyl methylphosphonate on self-assembled alkanethiolate monolayers
  • 1998
  • Ingår i: JOURNAL OF PHYSICAL CHEMISTRY B. - : AMER CHEMICAL SOC. - 1089-5647 .- 1520-6106 .- 1520-5207. ; 102:7, s. 1260-1269
  • 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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6.
  • Bertilsson, Lars, et al. (författare)
  • Interaction of dimethyl methylphosphonate with alkanethiolate monolayers studied by temperature-programmed desorption and infrared spectroscopy
  • 1997
  • Ingår i: JOURNAL OF PHYSICAL CHEMISTRY B. - : AMER CHEMICAL SOC. - 1089-5647 .- 1520-6106 .- 1520-5207. ; 101:31, s. 6021-6027
  • 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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7.
  • Biesuz, R., et al. (författare)
  • Estimation of deprotonation coefficients for chelating ion exchange resins. Comparison of different thermodynamic model
  • 2001
  • Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 105:20, s. 4721-4726
  • Tidskriftsartikel (refereegranskat)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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8.
  • Boschloo, Gerrit, et al. (författare)
  • Spectroelectrochemistry of nanostructured NiO
  • 2001
  • Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 105:15, s. 3039-3044
  • Tidskriftsartikel (refereegranskat)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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9.
  • Brinck, Tore, et al. (författare)
  • Solvation of carbanions in organic solvents : A test of the polarizable continuum model
  • 2000
  • Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 104:42, s. 9887-9893
  • Tidskriftsartikel (refereegranskat)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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10.
  • Cho, K. B., et al. (författare)
  • The substrate reaction mechanism of class III anaerobic ribonucleotide reductase
  • 2001
  • Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 105:27, s. 6445-6452
  • Tidskriftsartikel (refereegranskat)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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11.
  • Claesson, Per M., et al. (författare)
  • Mixtures of cationic polyelectrolyte and anionic surfactant studied with small-angle neutron scattering
  • 2000
  • Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 104:49, s. 11689-11694
  • Tidskriftsartikel (refereegranskat)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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12.
  • Colmenarejo, G., et al. (författare)
  • Electronic Spectra and Transition Moments of 6-(2’-Pyridiniumyl)phenanthridinium Photoactive DNA Intercalators
  • 1997
  • Ingår i: Journal of Physical Chemistry B Materials. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 101:26, s. 5196-5204
  • 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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13.
  • Cummins, D., et al. (författare)
  • Ultrafast electrochromic windows based on redox-chromophore modified nanostructured semiconducting and conducting films
  • 2000
  • Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 104:48, s. 11449-11459
  • Tidskriftsartikel (refereegranskat)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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14.
  • de Carolis, Stefano, et al. (författare)
  • Structure and electronic properties of Ca-doped CeO2 and implications on catalytic activity : An experimental and theoretical study
  • 1999
  • Ingår i: J PHYS CHEM B. - : American Chemical Society (ACS). - 1089-5647. ; 103:36, s. 7627-7636
  • 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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15.
  • D'Ercole, A, et al. (författare)
  • Embedded-cluster study of hydrogen interaction with an oxygen vacancy at the magnesium oxide surface
  • 1999
  • Ingår i: JOURNAL OF PHYSICAL CHEMISTRY B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 103:19, s. 3872-3876
  • Tidskriftsartikel (refereegranskat)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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16.
  • Engstrom, Maria, et al. (författare)
  • Density functional theory calculations of electron paramagnetic resonance parameters of a nitroxide spin label in tissue factor and factor VIIa protein complex
  • 2002
  • Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207 .- 1089-5647. ; 106:47, s. 12354-12360
  • Tidskriftsartikel (refereegranskat)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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17.
  • Furo, Istvan, et al. (författare)
  • Structure and dynamics of associative water-soluble polymer aggregates as seen by F-19 NMR spectroscopy
  • 2000
  • Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 104:3, s. 485-494
  • Tidskriftsartikel (refereegranskat)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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18.
  • Gisselfält, Katrin, 1969, et al. (författare)
  • 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
  • Ingår i: Journal of Physical Chemistry B Materials. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 104:15, s. 3651-3659
  • Tidskriftsartikel (refereegranskat)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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19.
  • Hedin, N., et al. (författare)
  • Fast diffusion of the Cl- ion in the headgroup region of an oppositely charged micelle. A Cl-35 NMR spin relaxation study
  • 2000
  • Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 104:35, s. 8544-8547
  • Tidskriftsartikel (refereegranskat)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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20.
  • Himo, Fahmi, et al. (författare)
  • Very stable ribonucleotide substrate radical relevant for class I ribonucleotide reductase
  • 2000
  • Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 104:31, s. 7502-7509
  • Tidskriftsartikel (refereegranskat)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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21.
  • Keil, M, et al. (författare)
  • Influence of the morphology on the electronic structure of hexa-peri-hexabenzocoronene thin films
  • 2000
  • Ingår i: JOURNAL OF PHYSICAL CHEMISTRY B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 104:16, s. 3967-3975
  • Tidskriftsartikel (refereegranskat)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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22.
  • Koehorst, R. B. M., et al. (författare)
  • Spectral sensitization of TiO2 substrates by monolayers of porphyrin heterodimers
  • 2000
  • Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 104:10, s. 2371-2377
  • Tidskriftsartikel (refereegranskat)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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23.
  • Kyrychenko, Alexander, et al. (författare)
  • Sterically induced conformational relaxation and structure of meso-diaryloctaalkyl porphyrins in the excited triplet state: Experimental and DFT studies
  • 2002
  • Ingår i: Journal of Physical Chemistry B Materials. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 106:48, s. 12613-12622
  • Tidskriftsartikel (refereegranskat)abstract
    • The excited triplet state conformations of zinc and free base 5,15-diaryloctaalkylporhyrins are studied by experimental and computational (density functional theory, DFT) methods. From the observations of an unusual triplet state dynamics, i.e., fast nonradiative and biexponential decay, it has been suggested that these porphyrins exist in two distinguishable conformers in the lowest excited triplet state. X-ray crystallography and,DFT (B3LYP/6-31G(d)) optimization of the ground state show that the porphyrins are planar prior to excitation. However, in the excited triplet state, the planar structure relaxes to an out-of-plane distorted saddle-shaped conformer. This distorted conformer and the lowest triplet potential energy surface are characterized by DFT calculations. It is suggested that the conformational relaxation explains the unusual triplet dynamics of this class of porphyrins.
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24.
  • Li, Xueqiang, et al. (författare)
  • Noncovalent assembly of a metalloporphyrin and an iron hydrogenase active-site model : Photo-induced electron transfer and hydrogen generation
  • 2008
  • Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207 .- 1089-5647. ; 112:27, s. 8198-8202
  • Tidskriftsartikel (refereegranskat)abstract
    • A noncovalent assembly of a pyridyl-functionalized hydrogenase active-site model complex and zinc tetraphenylporphyrin has been obtained and characterized. Upon light irradiation, fluorescence quenching by electron transfer was observed from the singlet excited state of the porphyrin to the diiron center, and the mechanism was verified by fluorescence lifetime and transient absorption spectroscopic measurements. In contrast to molecular dyads linked by covalent bonds, the assembled system was designed to avoid charge recombination via complex dissociation after photo-induced electron transfer. Visible light-driven hydrogen generation was observed from this self-assembled system. The assembling strategy employed in this study has the potential to be used for any other hydrogenase models in the future.
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25.
  • Liedberg, Bo, et al. (författare)
  • Self-assembly of alpha-functionalized terthiophenes on gold
  • 1997
  • Ingår i: JOURNAL OF PHYSICAL CHEMISTRY B. - : AMER CHEMICAL SOC. - 1089-5647 .- 1520-6106 .- 1520-5207. ; 101:31, s. 5951-5962
  • Tidskriftsartikel (refereegranskat)abstract
    • alpha-Functionalized terthiophenes containing disulfide (-S-T-3-H)(2) and alkanethiol (HS-(CH2)(11)-T-3-H) anchoring groups have been synthesized for direct immobilization onto gold. Monolayer structures of these compounds are prepared by spontaneous assembly from ethanol solutions on evaporated gold substrates and thoroughly characterized by ellipsometry, contact angle goniometry, infrared and X-ray photoelectron spectroscopy, and cyclic voltammetry. The two molecules coordinate to the gold substrate exclusively via the anchoring groups upon formation of gold-thiolate bonds. The kinetics of monolayer formation vary dramatically for the two compounds. The alkanethiol analogue assembles rapidly, within a few minutes, and forms a densely packed and highly organized monolayer, with the alkyl chains in an almost perfect all-trans conformation and the C-alpha-C-alpha axis of the alpha-T-3 units tilted about 14 degrees away from the surface normal. The assembly process is much slower for the disulfide, but an organized monolayer with an average alpha-T-3 chain tilt of about 33 degrees will eventually form when the assembly is allowed to equilibrate with a solution containing the disulfide for at least 1 day. Moreover, the two monolayer assemblies also display a remarkably different electrochemical, behavior. The heterogeneous electron-transfer rate at the disulfide-covered gold substrate is almost indistinguishable from that at bare gold, suggesting that the assembly contains a large number of easily accessible defects. An alternative mechanism for explaining the large electron-transfer rate involving electronic coupling via the conjugated pi-system of the alpha-T-3 units is also proposed. The electrochemical response is significantly reduced for the HS-(CH2)(11)-T-3-H assembly, but another type of defects, the so-called shallow defects originating from sparsely populated areas on the electrode surface, can be identified.
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26.
  • Lincoln, Per, 1958, et al. (författare)
  • DNA Binding Geometries of Ruthenium(II) Complexes with 1,10-Phenanthroline and 2,2'-Bipyridine Ligands Studied with Linear Dichroism Spectroscopy. Borderline Cases of Intercalation
  • 1998
  • Ingår i: Journal of Physical Chemistry B Materials. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 102:47, s. 9583-9594
  • Tidskriftsartikel (refereegranskat)abstract
    • The DNA binding geometries for a series of enantiomerically purr substitution-inert ruthenium(II) complexes, containing 1,10-phenanthroline (phen) and 2,2'-bipyridine (bpy) as ligands, have been studied with linear dichroism (LD) spectroscopy, Based on flow LD and emission anisotropy (EA) spectra of the complexes in the presence of calf thymus DNA, their absorption spectra in the visible region have been resolved into three component spectra that are polarized nearly perpendicular to each other. Spectral similarities between the D-3 symmetric [Ru(phen)(3)](2+) and C-2 symmetric [Ru(phen)(2)dppz](2+) (dppz dipyrido[3,2-a :2',3'-c]phenazine), in which dppz is known to be intercalated between the DNA basepairs, indicates a symmetry breaking in the former chromophore, bringing up the long debated question whether one of the three phen ligands of [Ru-(phen)(3)](2+) may be intercalated. Indeed, the complete analysis of the flow LD spectra for [Ru(L)(2)Y](2+) complexes (L bpy or phen) reveals similar angular binding geometries whether Y = dppz or phen. When Y = phen, for both the Delta- and Lambda-enantiomers this ligand is thus found oriented nearly perpendicular to the DNA helix axis as if intercalated and, similar to the orientation of dppz in dppz complexes, with a small but distinct clockwise rotation around the pseudo-dyad axis when viewed toward the minor groove ("positive roll"). A markedly less ordered binding observed when passing from [Ru(bpy)(2)phen](2+) to [Ru(bpy)(3)](2+) supports a stacking interaction of the phen ligand with the nucleobases. However, in contrast to the lengthening of DNA observed with [Ru(phen)(2)dppz](2+) and classical intercalators, the absence of lengthening with [Ru-(phen)(3)](2+) indicates that although a phen may have stacking interaction with the nucleobases, it does not enter a fully opened intercalation pocket. Two alternative binding modes are discussed: semi-intercalation (only edge of the intercalation pocket opened) and quasi-intercalation (indenture of a basepair allowing stacking of adjacent bases with the intruding phen ligand). Fair agreement is found between the proposed binding geometry of [Ru(phen)(3)](2+) and previously reported 2D-NMR NOE data.
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27.
  • Naidoo, Kevin J., et al. (författare)
  • Glucose orientation and dynamics in α-, β-, and γ-cyclodextrins
  • 2008
  • Ingår i: The Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 112:47, s. 15151–15157-
  • Tidskriftsartikel (refereegranskat)abstract
    • We investigate, using molecular dynamics (MD) computer simulations, the conformational behavior of α-, β-, and γ-cyclodextrins (CDs). Our analysis of a 30 ns trajectory of CD solution dynamics reveals the underlying conformational behaviours of the CDs that explain their relative flexibility. The distributions of the torsion angles related to the glycosidic linkages, P(ϕ,ψ) were calculated for the three CDs. Most noticeable is the limited range in ϕ torsion rotations compared with ψ rotations for all the CDs. This difference between the three CDs is amplified in the motion and dynamics of their glucose monomers when we monitor their orientational and librational positions relative to the macrocyclic mean plane. The relaxation times of the monomers to their equilibrium orientations follow the pattern γ-CD > α-CD > β-CD. The root-mean-square deviations of the motion of the monomer centers of mass from the mean macrocyclic planes exhibit the same trend.
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28.
  • Olsson, Louise, 1974, et al. (författare)
  • A Kinetic Study of NO Oxidation and NOx Storage on Pt/Al2O3 and Pt/BaO/Al2O3
  • 2001
  • Ingår i: Journal of Physical Chemistry B Materials. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 105:29, s. 6895-6906
  • Tidskriftsartikel (refereegranskat)abstract
    • Modeling and flow reactor experiments were used to study the kinetics of NO, storage/release on a Pt/BaO/Al2O3 model catalyst. The mechanism for this concept can be divided into four steps: (i) NO to NO2 oxidation on Pt, (ii) NO2 storage on BaO, (iii) NO, release, and (iv) NO, reduction to N2. In this paper, we have focused on the first three steps. From the NO oxidation study on Pt/Al2O3 compared to Pt/BaO/Al2O3, we observed that the presence of BaO decreases the formation of NO2. To test the importance of this step for effective storage, experiments were performed with a Pt/Al2O3 catalyst placed before the Pt/BaO/Al2O3 catalyst. This resulted in increased NO, storage for the combined system compared to the Pt/BaO/Al2O3 case. To resolve the second and third steps, an experimental investigation of NOx storage/release on BaO/Al2O3 was performed using only NO2 and N2 in the gas feed. We propose a kinetic model, which first includes adsorption of NO2, which oxidizes the surface, followed by nitrate formation. Finally, NO3-BaO-NO2, i.e., Ba(NO3)2, is formed. By using the kinetic parameters from the NO oxidation on Pt/BaO/Al2O3 and the NO, storage on BaO/Al2O3, a kinetic model was constructed to describe NO, storage/release experiments on Pt/BaO/Al2O3. However, the rate for NO, release was increased when Pt was present, and the kinetic model could not accurately describe this phenomenon. Therefore, the mechanism was modified by including a reversible surface spillover step of NO2 between Pt sites and BaO sites. Further, experiments with NO2 exposure followed by a temperature ramp with NO/N2 showed that the desorption behaviors from the BaO/Al2O3 and Pt/BaO/Al2O3 were significantly different, which further supports the spillover mechanism. Finally, the models describing NO, storage on BaO/Al2O3 and on Pt/BaO/Al2O3 were successfully validated with independent experiments.
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29.
  • Olsson, Louise, 1974, et al. (författare)
  • A kinetic study of oxygen adsorption/desorption and NO oxidation over Pt/Al2O3 catalysts
  • 1999
  • Ingår i: Journal of Physical Chemistry B Materials. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 103:47, s. 10433-10439
  • Tidskriftsartikel (refereegranskat)abstract
    • Laboratory tests and kinetic modeling were carried out in order to provide kinetic input data to a systematic investigation of the mechanism of nitrogen oxides (NO,) storage in catalysts used for lean-burn engines. In particular, we present a kinetic model of the NO oxidation to NO2 over a Pt/Al2O3 model catalyst for the temperature range 250-450 degrees C. Since the oxygen behavior at atmospheric pressure is critical for such a model, we have also studied the adsorption/desorption of oxygen by temperature-programmed desorption (TPD) experiments. The experiments show that oxygen starts to desorb at about 300 degrees C. Furthermore, the NO oxidation was studied in a temperature ramp with NO and oxygen in the gas feed. The data from this experiment and the above-determined values for the oxygen adsorption/desorption were used to construct a kinetic model for the NO oxidation. Finally, the model was validated with some transient experiments with either NO or NO2 and different oxygen concentrations in the gas feed. We found a good agreement between these experiments and the model.
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30.
  • Ortegren, J., et al. (författare)
  • Improved thermal stability of pyroelectric polymers by crosslinking of ferroelectric liquid crystals
  • 2001
  • Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 105:42, s. 10223-10227
  • Tidskriftsartikel (refereegranskat)abstract
    • Monofunctional ferroelectric liquid crystalline monomers and a blend of monofunctional/lbifunctional ferroelectric liquid crystalline monomers were photopolymerized, yielding a side-chain liquid crystalline polymer and a cross-linked polymer, respectively. The cross-linked polymer exhibited higher thermal stability than the side-chain liquid crystalline polymer and was pyroelectric up to 170 degreesC, whereas the side-chain liquid crystalline polymer lost most of its pyroelectricity at 38 degreesC. It is shown by electrooptic and birefringence measurements that cross-linking in the unwound SmC* phase prevented the reoccurrence of the helical superstructure.
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31.
  • O'Shea, J. N., et al. (författare)
  • Hydrogen-bond induced surface core-level shift in isonicotinic acid
  • 2001
  • Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 105:10, s. 1917-1920
  • Tidskriftsartikel (refereegranskat)abstract
    • Intermolecular hydrogen-bonding in thick films of isonicotinic acid evaporated onto rutile TiO2(110) has been investigated with X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The rate of deposition is found to be a key factor in overcoming the kinetic barriers to extensive hydrogen-bond formation, which, when present, gives rise to large energy shifts between bulk and surface in both the N1s XPS and XAS. The origin of the surface core-level shift is attributed to the presence of non-hydrogen-bonded nitrogen atoms in the surface layer.
  •  
32.
  • Petersson, A, et al. (författare)
  • Injection time in the metaloxide-molecule interface calculated within the tight-binding model
  • 2000
  • Ingår i: JOURNAL OF PHYSICAL CHEMISTRY B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 104:35, s. 8498-8502
  • Tidskriftsartikel (refereegranskat)abstract
    • The injection time tau of the electron transfer from an excited electronic state of a dye molecule to a semiconducting surface is studied, An analytical expression for the injection time as a function of excitation energy is derived. The semiconductor is described as a one-dimensional chain in the tight-binding approximation, and the dye molecule is included via the coupling to the semiconductor. For a TiO2 semiconductor the injection time is estimated to be approximately 5 fs. The results support and characterize recent experimental studies.
  •  
33.
  • Pithawalla, Y. B., et al. (författare)
  • Synthesis of magnetic intermetallic FeAl nanoparticles from a non-magnetic bulk alloy
  • 2001
  • Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 105:11, s. 2085-2090
  • Tidskriftsartikel (refereegranskat)abstract
    • We report the synthesis of intermetallic FeAl nanoparticles using the laser vaporization controlled condensation technique. The nanoparticles have, an average particle diameter between 6 and 9 nm. Measurements of the d spacing from X-ray and electron diffraction studies confirm that the FeAl nanoparticles have the same crystal structure (B2) as the bulk FeAl. High-resolution TEM images reveal that the nanoparticles consist of a crystalline core encased within a thin amorphous layer formed upon the exposure of the particles to air. We also report the synthesis of FeAl oxide and carbide nanoparticles. The oxide nanoparticles have the hercynite FeAl2O4 composition and the carbide nanoparticles have the AlFe3C0.5 composition. While the crystalline bulk FeAl (20-30% weight Al) materials are nonmagnetic, the analogues nanoparticles are found to be ferromagnetic even well above room temperature. Coercivities of the order of 50 Oe almost independent of temperature are observed from room temperature down to 40 K, below which the coercivities values increase up to 700 Oe at 5 K. The nanoparticles show a definitive exchange anisotropy effect probably arising from the intersurface anisotropy at the adsorbed oxygen layers on these particles. The observed displacement of the hysteresis loops along the field and the magnetization axes are reversible with the direction of the applied field. The FeAl nanoparticles may lead to a novel form of advanced materials that combine high electrical resistivity, oxidation resistance, ductility and unique magnetic properties.
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34.
  • Rojas, O. J., et al. (författare)
  • X-ray photoelectron spectroscopy in the study of polyelectrolyte adsorption on mica and cellulose
  • 2000
  • Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 104:43, s. 10032-10042
  • Tidskriftsartikel (refereegranskat)abstract
    • X-ray photoelectron spectroscopy was used to estimate the absolute amount of cationic polyelectrolytes that adsorbs on mica and cellulose surfaces in aqueous media. The calculation takes advantage of the knowledge of the mica crystal composition at the basal plane and its ion-exchange properties in aqueous solution. The XPS was operated under monochromatic and unmonochromatic mode and good agreement was observed in the resulting adsorbed amount. The evaluation of the amount of cationic polyelectrolyte adsorbed on cellulose was achieved using calibration curves obtained from adsorption data for the same polyelectrolytes on bare mica surfaces. The adsorption isotherm for polyelectrolytes of low charge density adsorbed on cellulose reveals that their affinity toward cellulose is weaker compared to that observed for highly charged surfaces such as mica. The effect of the polyelectrolyte charge density on the adsorbed amount and the number density of charged segments adsorbed on cellulose were also investigated. From these results it can be concluded that nonelectrostatic interactions are the main contributors to the adsorption of polyelectrolytes on cellulose, but it cannot be ruled out that electrostatic effects also take part in the adsorption mechanism. Finally, it is demonstrated that it is not correct to use the adsorbed amount of polyelectrolytes to determine the surface charge on cellulose surfaces.
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35.
  • Ruseckas, A, et al. (författare)
  • Intra- and interchain luminescence in amorphous and semicrystalline films of phenyl-substituted polythiophene
  • 2001
  • Ingår i: JOURNAL OF PHYSICAL CHEMISTRY B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 105:32, s. 7624-7631
  • Tidskriftsartikel (refereegranskat)abstract
    • We report time-resolved photoluminescence (PL) measurements of spin-cast amorphous films of a regioregular polythiophene derivative poly[3-(4-octylphenyl)thiophene] and of annealed semicrystalline films of the same polymer with an interchain stacking distance of 5 Angstrom. Red-shifted PL appears at long delay times in both pristine and annealed films, which we assign to interchain aggregates populated by excitation energy migration. Aggregate luminescence in annealed films exhibits a pronounced vibronic structure indicating the coupling to a C=C bond stretch with a Huang-Rhys factor S = 2. Two types of aggregates are distinguished in annealed films: in a few picoseconds most excitations are trapped by aggregates with a large energy gap (about 1.83 eV) between the lowest excited singlet state and the ground state. Excitation energy is transferred or the aggregates relax structurally with a time constant of about 200 ps to aggregates with a smaller energy gap (about 1.75 eV). The radiative lifetime of aggregates with smaller energy gap is estimated to be about 35 ns, two times longer than that of aggregates with the larger energy gap. In spin-cast amorphous films, emission from intrachain singlet excitons has a longer lifetime than in annealed films and only high-energy-gap aggregates are populated.
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36.
  • Samori, P., et al. (författare)
  • Growth of ordered hexakis-dodecyl-hexabenzocoronene layers from solution : A SFM and ARUPS study
  • 2001
  • Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 105:45, s. 11114-11119
  • Tidskriftsartikel (refereegranskat)abstract
    • The layer growth of a polycyclic aromatic hydrocarbon moiety, hexakis-dodecyl-hexabenzocoronene (HBC-C12), from solution onto a conductive flat solid substrate has been studied. Scanning Force Microscopy (SFM), together with the analysis of the intensities of p-structures in spectra of Angle-Resolved Ultraviolet Photoelectron Spectroscopy (ARUPS) measurements, revealed that the HBC-C12 molecules can self-assemble as dry layers with the conjugated disklike molecules lying flat on the (0001) plane of highly oriented pyrolitic graphite (HOPG). By varying the rate of the molecular physisorption it was possible to orient these molecular architectures along preferential directions according to the symmetry of the substrate. Additionally, the film morphology is affected by the concentration of the solution. This indicates that the growth of these organic layers on HOPG is a kinetically governed process which, if carried out sufficiently slowly, leads to the growth of hetero-epitaxial crystallites.
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37.
  • Sandberg, L., et al. (författare)
  • Calculated solvation free energies of amino acids in a dipolar approximation
  • 2001
  • Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 105:1, s. 273-281
  • Tidskriftsartikel (refereegranskat)abstract
    • The solvation foe energies of amino acids (hydrophobicities) are rationalized here by using a simple electrostatic ab initio model. The parameters of the model are the surface energy density (gamma = 9.55 kJ/mol/nm(2), the same for all atoms), the fractional charges, and the radii of the atoms of the amino acids. From the fractional charges, dipoles are constructed, and the polarization energies (self-energies) of the dipoles are calculated. The saturation effect of the solvent is included in the model. The dipole electrostatic contribution is in general not very sensitive to the choice of parameters. In contrast, the self-energies of single net charges are extremely sensitive to the choice of the Born radius. However, for amino acids that may carry a net charge, this energy will be prohibitively large in a low-dielectric medium. Therefore, they will in general change their protonation state to become neutral. The energetic cost of this is calculated from the difference between the amino acid side chain pK(a)'s and the pH of the solvent. This results in a hydrophobicity scale for amino acid side chains based on fundamental physics that agrees well with experimental hydrophobicity scales. The solvation energy for the amino acid backbone (the peptide bond) can also be calculated in this way. This gives good agreement with available experimental data.
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38.
  • Spangberg, D., et al. (författare)
  • Model extended X-ray absorption fine structure (EXAFS) spectra from molecular dynamics data for Ca2+ and Al3+ aqueous solutions
  • 2000
  • Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 104:45, s. 10467-10472
  • Tidskriftsartikel (refereegranskat)abstract
    • Theoretical extended X-ray absorption fine structure (EXAFS) spectra have been computed from molecular dynamics (MD) simulation data. Based on MD-generated molecular geometries, EXAFS spectra were generated with the FEFF6 program, commonly used in the analysis of experimental EXAFS spectra. The effects of multiple scattering and second-shell neighbors on the theoretical spectra were evaluated for CaCl2(aq) and AlCl3(aq), i.e., for two solutions with very different cation hydration structures. The effects are significantly larger for the more structured Al3+ solution, The theoretical Ca2+(aq) spectra are compared with new experimental EXAFS data.
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39.
  • Tornblom, M., et al. (författare)
  • Field-dependent NMR relaxation study of aggregation and dynamics in dilute to concentrated micellar decylammonium chloride solutions
  • 2000
  • Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 104:7, s. 1529-1538
  • Tidskriftsartikel (refereegranskat)abstract
    • The surfactant aggregation behavior in aqueous decylammonium chloride solutions for concentrations between 5 and 29 wt % was studied by H-2 nuclear magnetic relaxation dispersion in the 0.5-77-MHz frequency range. Below the second cmc the micelles can be described as polydisperse fairly small aggregates undergoing unhindered Brownian rotation. Above the second cme the micelles are rod shaped and grow with increasing concentration, and as a consequence the tumbling of the micelles eventually becomes restricted by neighboring aggregates, which is demonstrated by the low-frequency relaxation data. The restricted reorientation of the micelles in the concentrated solution has been modeled as rotational diffusion in a hard-walled double cone potential of mean torque.
  •  
40.
  • Valiokas, Ramunas, et al. (författare)
  • Influence of specific intermolecular interactions on the self-assembly and phase behavior of oligo(ethylene glycol)-terminated alkanethiolates on gold
  • 2001
  • Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 105:23, s. 5459-5469
  • Tidskriftsartikel (refereegranskat)abstract
    • A comparative study of the self-assembly and phase behavior of seven different oligo(ethylene glycol) (OEG)-terminated alkanethiols on polycrystalline gold surfaces is presented. The general structure of the compounds is HS(CH2)m-X-EGn, where m = 11, 15, n = 2, 4, 6, and the linkages X are amide (-CONH-), ester (-COO-), or ether (-O-) groups. The amide and ester groups give rise to the intermolecular hydrogen bonding and dipole-dipole interactions, respectively, whereas the ether lacks specific interactions. The results from contact angle goniometry, null ellipsometry, and infrared reflection-absorption spectroscopy (IRAS) indicate that the intermolecular interactions can be partly used to control the conformation and order of the OEG portion of the self-assembled monolayers (SAMs). It is shown that the lateral hydrogen bonding stabilizes the all-trans conformation of the EG4 tails in the SAMs. Further on, the mechanism behind the thermal phase behavior of the OEG SAMs is investigated using temperature-programmed IRAS in ultrahigh vacuum. In the present study we show that the earlier reported helix-to-all-trans conformational transition at 60°C in the SAM of HS(CH2)15CONH-EG6 (Valiokas, R., Östblom, M., Svedhem, S., Svensson, S. C. T., Liedberg, B. J. Phys. Chem. 2000, 104, 7565-7569.) is a result of the particular molecular design of the SAMs through the specifically built-in lateral hydrogen bonds. A shortening of the alkyl chain to 11 methylenes has no effect on the amide-EG6 phase behavior. Contrary, the ester- and ether- containing SAMs undergo a melting type of transitions at 52 and 68°C, respectively, similar to that observed for poly(ethylene glycol).
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41.
  • Valiokas, Ramunas, et al. (författare)
  • Thermal stability of self-assembled monolayers : Influence of lateral hydrogen bonding
  • 2002
  • Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 106:40, s. 10401-10409
  • Tidskriftsartikel (refereegranskat)abstract
    • Temperature-programmed desorption (TPD) of self-assembled monolayers (SAMs) on gold is investigated by using in parallel mass spectrometry (MS) and infrared reflection-absorption spectroscopy (IRAS). Monolayers formed by HS(CH2)n-OH (n = 18, 22) and HS(CH2)15-CONH-(CH2CH2O)-H (EG1) are compared to reveal the influence of specifically introduced hydrogen-bonding groups on their thermal stability. The overall desorption process of the above molecules is found to occur in two main steps, a disordering of the alkyl chains followed by a complex series of decomposition/desorption reactions. The final step of the process involves desorption of sulfur from different chemisorption states. The amide-group-containing SAM, which is stabilized by lateral hydrogen bonds, displays a substantial delay of the alkyl chain disordering by about 50 K, as compared to the linear chain alcohols HS(CH2)n-OH. Moreover, the decomposition of the alkyls and the onset of sulfur desorption occur at a temperature that is higher by approximately 25 K as compared to the HS(CH2)18-OH SAM. The desorption process is also studied for two oligo(ethylene glycol)-terminated SAMs, HS(CH2)15-X-(CH2CH2O)4-H (EG4-SAMs), where X is -CONH- and -COO- linking groups. In addition to the molecular chain disordering, the decomposition/desorption process of the EG4-SAMs occurs in two steps. The first is associated with the loss of the oligomer portion and the second with the desorption of the alkylthiolate part of the molecule. Our study points out that lateral hydrogen bonding, introduced via amide groups, is a convenient way to improve the thermal stability of alkanthiolate SAMs.
  •  
42.
  • Wang, H. L., et al. (författare)
  • Electrochemical investigation of traps in a nanostructured TiO2 film
  • 2001
  • Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 105:13, s. 2529-2533
  • Tidskriftsartikel (refereegranskat)abstract
    • Electron traps at the nanostructured TiO2 (anatase)/aqueous electrolyte interface have been studied by means of electrochemical methods. The transient current decays at different potentials positive of the flat band potential clearly illustrate the trap-filling process. The more negative the potential, the shorter the trap-filling period. It is found that most traps locate positive of -0.9 V vs Ag/AgCl at pH 13, or positive of -0.3 V vs Ag/AgCl at pH 4.7. It is proposed that the trap distribution as a function of potential is directly proportional to dQ/dU, i.e., to the current density in a linear sweep voltammetry experiment. The trap densities in aqueous electrolytes are estimated to be 4 x 10(11) cm(-2) (microscopic area) at pH 4.7 and 5 x 10(13) cm(-2) (microscopic area) at pH 13. The pH dependency of the trap density indicates that traps investigated are surface-related.
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43.
  •  
44.
  • Baev, Alexander, et al. (författare)
  • Theoretical Simulations of clamping levels in optical power limiting
  • 2006
  • Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207 .- 1520-5215. ; 110:42, s. 20912-20916
  • Tidskriftsartikel (refereegranskat)abstract
    • Multiphysics modeling, combining quantum mechanical and classical wave mechanical theories, of clamping levels has been performed for a platinum(II) organic compound in a sol-gel glass matrix. A clamping level of 2.5 mu J is found for a pulse duration of 10 ns. The excited-state absorption in the triplet manifold is shown to be crucial for clamping to occur.
  •  
45.
  • Malysheva, Lyuba, et al. (författare)
  • Molecular Orientation in Helical and All-Trans Oligo(ethylene glycol)-Terminated Assemblies on Gold:  Results of ab Initio Modeling
  • 2005
  • Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215 .- 1520-6106 .- 1520-5207. ; 109:34, s. 7788-7796
  • Tidskriftsartikel (refereegranskat)abstract
    • The structural properties of self-assembled monolayers (SAMs) of oligo(ethylene glycol) (OEG)-terminated and amide-containing alkanethiols (HS(CH2)15CONH(CH2CH2O)6H and related molecules with shorter alkyl or OEG portions) on gold are addressed. Optimized geometry of the molecular constituents, characteristic vibration frequencies, and transition dipole moments are obtained using density-functional theory methods with gradient corrections. These data are used to simulate IR reflection-absorption (RA) spectra associated with different OEG conformations. It is shown that the positions and relative intensities of all characteristic peaks in the fingerprint region are accurately reproduced by the model spectra within a narrow range of the tilt and rotation angles of the alkyl plane, which turns out to be nearly the same for the helical and all-trans OEG conformations. In contrast, the tilt of the OEG axis changes considerably under conformational transition from helical to all-trans OEG. By means of ab initio modeling, we also clarify other details of the molecular structure and orientation, including lateral hydrogen bonding, the latter of which is readily possessed by the SAMs in focus. These results are crucial for understanding phase and folding characteristics of OEG SAMs and other complex molecular assemblies. They are also expected to contribute to an improved understanding of the interaction with water, ions, and ultimately biological macromolecules.
  •  
46.
  • Abbas, Zareen, 1962, et al. (författare)
  • Monte Carlo Simulations of Salt Solutions: Exploring the Validity of Primitive Models
  • 2009
  • Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 113:17, s. 5905-5916
  • Tidskriftsartikel (refereegranskat)abstract
    • An extensive series of Monte Carlo (MC) simulations were performed in order to explore the validity of simple primitive models of electrolyte solutions and in particular the effect of ion size asymmetry on the bulk thermodynamic properties of real salt solutions. Ionic activity and osmotic coefficients were calculated for 1:1, 2:1, and 3:1 electrolytes by using the unrestricted primitive model (UPM); i.e., ions are considered as charged hard spheres of different sizes dissolved in a dielectric continuum. Mean ionic activity and osmotic coefficients calculated by the MC simulations were fitted simultaneously to the experimental data by adjusting only the cation radius while keeping the anion radius fixed at its crystallographic value. Ionic radii were further optimized by systematically varying the cation and anion radii at a fixed sum of ionic radii. The success of this approach is found to be highly salt specific. For example, experimental data (mean ionic activity and osmotic coefficients) of salts which are usually considered as dissociated such as HCl, HBr, LiCl, LiBr, LiClO4, and KOH were successfully fitted up to 1.9, 2.5, 1.9, 3, 2.5, and 4.5 M concentrations, respectively. In the case of partially dissociated salts such as NaCl, the successful fits were only obtained in a more restricted concentration range. Consistent sets of the best fitted cation radii were obtained for acids, alkali, and alkaline earth halides. A list of recommended ionic radii is also provided. The reliability of the optimized ionic radii was further tested in simulations of the osmotic coefficients of LiCl−NaCl−KCl salt mixtures. A very good agreement between the simulated and experimental data was obtained up to ionic strength of 4.5 M.
  •  
47.
  • Abdurrokhman, Iqbaal, 1991, et al. (författare)
  • Protic Ionic Liquids Based on the Alkyl-Imidazolium Cation: Effect of the Alkyl Chain Length on Structure and Dynamics
  • 2019
  • Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 123:18, s. 4044-4054
  • Tidskriftsartikel (refereegranskat)abstract
    • Protic ionic liquids are known to form extended hydrogen-bonded networks that can lead to properties different from those encountered in the aprotic analogous liquids, in particular with respect to the structure and transport behavior. In this context, the present paper focuses on a wide series of 1-alkyl-imidazolium bis(trifluoromethylsulfonyl)imide ionic liquids, [HC n Im][TFSI], with the alkyl chain length (n) on the imidazolium cation varying from ethyl (n = 2) to dodecyl (n = 12). A combination of several methods, such as vibrational spectroscopy, wide-angle X-ray scattering (WAXS), broadband dielectric spectroscopy, and 1 H NMR spectroscopy, is used to understand the correlation between local cation-anion coordination, nature of nanosegregation, and transport properties. The results indicate the propensity of the -NH site on the cation to form stronger H-bonds with the anion as the alkyl chain length increases. In addition, the position and width of the scattering peak q 1 (or the pre-peak), resolved by WAXS and due to the nanosegregation of the polar from the nonpolar domains, are clearly dependent on the alkyl chain length. However, we find no evidence from pulsed-field gradient NMR of a proton motion decoupled from molecular diffusion, hypothesized to be facilitated by the longer N-H bonds localized in the segregated ionic domains. Finally, for all protic ionic liquids investigated, the ionic conductivity displays a Vogel-Fulcher-Tammann dependence on inverse temperature, with an activation energy E a that also depends on the alkyl chain length, although not strictly linearly.
  •  
48.
  • Acebes, S., et al. (författare)
  • Mapping the Long-Range Electron Transfer Route in Ligninolytic Peroxidases
  • 2017
  • Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 121:16, s. 3946-3954
  • Tidskriftsartikel (refereegranskat)abstract
    • Combining a computational analysis with site-directed mutagenesis, we have studied the long-range electron transfer pathway in versatile and lignin peroxidases, two enzymes of biotechnological interest that play a key role for fungal degradation of the bulky lignin molecule in plant biomass. The in silico study established two possible electron transfer routes starting at the surface tryptophan residue previously identified as responsible for oxidation of the bulky lignin polymer. Moreover, in both enzymes, a second buried tryptophan residue appears as a top electron transfer carrier, indicating the prevalence of one pathway. Site-directed mutagenesis of versatile peroxidase (from Pleurotus eryngii) allowed us to corroborate the computational analysis and the role played by the buried tryptophan (Trp244) and a neighbor phenylalanine residue (Phe198), together with the surface tryptophan, in the electron transfer. These three aromatic residues are highly conserved in all the sequences analyzed (up to a total of 169). The importance of the surface (Trp171) and buried (Trp251) tryptophan residues in lignin peroxidase has been also confirmed by directed mutagenesis of the Phanerochaete chrysosporium enzyme. Overall, the combined procedure identifies analogous electron transfer pathways in the long-range oxidation mechanism for both ligninolytic peroxidases, constituting a good example of how computational analysis avoids making extensive trial-error mutagenic experiments.
  •  
49.
  • Acke, Filip, et al. (författare)
  • Study of the reduction and reoxidation of a CaO surface
  • 1997
  • Ingår i: J Phys Chem B. - 1520-5207 .- 1520-6106. ; 101:33, s. 6484-
  • 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.
  •  
50.
  • Agrell, H. G., et al. (författare)
  • Conductivity studies of nanostructured TiO2 films permeated with electrolyte
  • 2004
  • Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 108:33, s. 12388-12396
  • Tidskriftsartikel (refereegranskat)abstract
    • Charge transport in nanostructured TiO2 films permeated with an electrolyte was studied, using temperature-dependent conductivity and electron accumulation measurements. Two regions for charge transport were distinguished from the relationship between conductivity and electron concentration. In the first region (similar to1-20 electrons per TiO2 particle), the effective electron mobility is dependent on the electron concentration and values between 7 x 10(-4) and 78 x 10(-4) cm(2) V-1 s(-1) were determined. The activation energy of the mobility was similar to0.3 eV. The charge transport can be described with a trapping/detrapping model that involves localized band-gap states. In the second region (> 20 electrons per TiO2 particle), the effective electron mobility is independent of electron concentration and values of similar to150 x 10(-4) cm(2) V-1 s(-1) are calculated. The activation energy of mobility is in the range of 0-0.15 eV, depending on the electrolyte. Transport of electrons in the conduction band seems to be the most applicable model.
  •  
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