| 1. |
- Aziz, Emad F., et al.
(författare)
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Cation-specific interactions with carboxylate in amino acid and acetate aqueous solutions : X-ray absorption and ab initio calculations
- 2008
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 112:40, s. 12567-12570
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Tidskriftsartikel (refereegranskat)abstract
- Relative interaction strengths between cations (X = Li+, Na+, K+, NH4+) and anionic carboxylate groups of acetate and glycine in aqueous solution are determined. These model systems mimic ion pairing of biologically relevant cations with negatively charged groups at protein surfaces. With oxygen ls X-ray absorption spectroscopy, we can distinguish between spectral contributions from H2O and carboxylate, which allows us to probe the electronic structure changes of the atomic site of the carboxylate group being closest to the countercation. From the intensity variations of the COOaq- ls X-ray absorption peak, which quantitatively correlate with the change in the local partial density of states from the carboxylic site, interactions are found to decrease in the sequence Na+ > Li+ > K+ > NH4+. This ordering, as well as the observed bidental nature of the -COOaq- and X-aq(+) interaction, is supported by combined ab initio and molecular dynamics calculations.
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| 2. |
- Aziz, Emad F., et al.
(författare)
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Interaction between liquid water and hydroxide revealed by core-hole de-excitation
- 2008
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 455:7209, s. 89-91
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Tidskriftsartikel (refereegranskat)abstract
- The hydroxide ion plays an important role in many chemical and biochemical processes in aqueous solution(1). But ourmolecular- level understanding of its unusual and fast transport in water, and of the solvation patterns that allow fast transport, is far from complete. One proposal seeks to explain the properties and behaviour of the hydroxide ion by essentially regarding it as a water molecule that is missing a proton(2), and by inferring transport mechanisms and hydration structures from those of the excess proton. A competing proposal invokes instead unique and interchanging hydroxide hydration complexes, particularly the hypercoordinated OH-(H2O)(4) species and tri- coordinated OH-(H2O)(3) that can form a transient hydrogen bond between the H atom of the OH- and a neighbouring water molecule(3-5). Here we report measurements of core- level photoelectron emission and intermolecular Coulombic decay(6-8) for an aqueous hydroxide solution, which show that the hydrated hydroxide ion is capable of transiently donating a hydrogen bond to surrounding watermolecules. In agreement with recent experimental studies of hydroxide solutions(9-12), our finding thus supports the notion that the hydration structure of the hydroxide ion cannot be inferred from that of the hydrated excess proton.
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| 3. |
- Aziz, Emad F., et al.
(författare)
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Photoinduced formation of N-2 molecules in ammonium compounds
- 2007
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 111:39, s. 9662-9669
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Tidskriftsartikel (refereegranskat)abstract
- Via fluorescence yield (FY) and resonant inelastic scattering spectroscopy in the soft X-ray range we find that soft X-rays induce formation of N-2 molecules in solid NH4Cl and in related compounds. The nitrogen molecules form weak bonds in NH4Cl, so that a substantial fraction of the molecules remains in the sample. From measurements of the FY as a function of exposure and temperature, the rates for the photochemical processes are estimated. At elevated temperatures (363 K), several nitrogen atoms are removed from the sample per incoming photon. At lower temperatures (233 K), the rate is reduced to around 0.02 nitrogen atoms for each incoming photon. Virtually all these atoms form N-2 molecules which are bound in the sample. The generality and implications of these results are briefly discussed.
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| 4. |
- Aziz, Emad F, et al.
(författare)
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Probing the electronic structure of the hemoglobin active center in physiological solutions.
- 2009
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 102:6, s. 068103-
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Tidskriftsartikel (refereegranskat)abstract
- Soft-x-ray absorption spectroscopy at the L_{2,3} edge of the iron center in bovine hemoglobin and hemin under physiological conditions is reported for the first time. Spectra of the same compounds in solid form are presented for comparison. Striking differences in the electronic structure of the metalloporphyrin are observed between the liquid and solid compounds. We unambiguously show that hemoglobin and hemin are in a high-spin ferric state in solution, and that the 2p spin-orbit coupling decreases for hemin compared to the hemoglobin, while this is not the case in solids. The spectra were simulated using ligand field multiplet theory, in good agreement with the experiment, allowing quantification of the amount of charge transfer between the porphyrin and Fe3+ ion in hemoglobin and in hemin.
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| 5. |
- Borgström, Magnus, et al.
(författare)
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Sensitized hole injection of phosphorus porphyrin into NiO : Toward new photovoltaic devices
- 2005
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 109:48, s. 22928-22934
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Tidskriftsartikel (refereegranskat)abstract
- This paper describes the preparation and the characterization of a photovoltaic cell based on the sensitization of a wide band gap p-type semiconductor (NiO) with a phosphorus porphyrin. A photophysical study with femtosecond transient absorption spectroscopy showed that light excitation of the phosphorus porphyrin chemisorbed on NiO particles induces a very rapid interfacial hole injection into the valence band of NiO, occurring mainly on the 2-20 ps time scale. This is followed by a recombination in which ca. 80% of the ground-state reactants are regenerated within 1 ns. A photoelectrochernical device, prepared with a nanocrystalline NiO electrode coated with the phosphorus porphyrin, yields a cathodic photocurrent indicating that electrons indeed flow from the NiO electrode toward the solution. The low incident-to-photocurrent efficiency (IPCE) can be rationalized by the rapid back recombination reaction between the reduced sensitizer and the injected hole which prevents an efficient regeneration of the sensitizer ground state from the iodide/triiodide redox mediator. To the best of our knowledge, this work represents the first example of a photovoltaic cell in which a mechanism of hole photoinjection has been characterized.
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| 6. |
- Gråsjö, Johan, et al.
(författare)
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Electronic structure of water molecules confined in a micelle lattice
- 2009
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 113:24, s. 8201-8205
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Tidskriftsartikel (refereegranskat)abstract
- Oxygen K absorption and emission spectra of water molecules confined in dodecyltrimethyl ammonium chloride micelle structures are presented. The local electronic structure of the water molecules is found to be dramatically different from the electronic structure of water molecules in the gas-phase as well as in liquid water. Hybridization with states of the ions in the surrounding ions is directly observed, and evidence for stabilization of the water molecules relative to molecules in bulk water is found.
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| 7. |
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| 8. |
- Nolting, Dirk, et al.
(författare)
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pH-induced protonation of lysine in aqueous solution causes chemical shifts in X-ray photoelectron spectroscopy.
- 2007
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 129:45, s. 14068-73
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate the applicability of X-ray photoelectron spectroscopy to obtain charge- and site-specific electronic structural information of biomolecules in aqueous solution. Changing the pH of an aqueous solution of lysine from basic to acidic results in nitrogen 1s and carbon 1s chemical shifts to higher binding energies. These shifts are associated with the sequential protonation of the two amino groups, which affects both charge state and hydrogen bonding to the surrounding water molecules. The N1s chemical shift is 2.2 eV, and for carbon atoms directly neighboring a nitrogen the shift for C1s is approximately 0.4 eV. The experimental binding energies agree reasonably with our calculated energies of lysine(aq) for different pH values.
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| 9. |
- Ottosson, Niklas, et al.
(författare)
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Large variations in the propensity of aqueous oxychlorine anions for the solution/vapor interface
- 2009
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 131:12, s. 124706-
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Tidskriftsartikel (refereegranskat)abstract
- Core-level photoelectron spectroscopy measurements have been performed of aqueous solutions of NaCl codissolved with NaClOn (n=1-4). Each species has a distinct Cl 2p electron binding energy, which can be exploited for depth-profiling experiments to study the competition between Cl-and ClOn- anions for residing in the outermost layers of the solution/vapor interface. Strongest propensity for the surface is observed for n=4 (perchlorate), followed by n=3 (chlorate), n=2 (chlorite), n=0 (chloride), and n=1 (hypochlorite). Molecular dynamics simulations rationalize the greatest surface propensity of the most oxidized anions in terms of their larger size and polarizability. The anomalous behavior of hypochlorite, being less surface-active than chloride, although it is both larger and more polarizable, is suggested to arise from the charge asymmetry over the anion, increasing its efficiency for bulk solvation. (C) 2009 American Institute of Physics. [doi:10.1063/1.3236805]
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| 10. |
- Winter, Bernd, et al.
(författare)
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Electron dynamics in charge-transfer-to-solvent states of aqueous chloride revealed by Cl- 2p resonant Auger-electron spectroscopy
- 2008
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 130:22, s. 7130-7138
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Tidskriftsartikel (refereegranskat)abstract
- Charge-transfer-to-solvent (CTTS) excited states of aqueous chloride are studied by a novel experimental approach based on resonant inner-shell photoexcitation, Cl-aq(-) 2p -> e(i), i = 1-4, which denotes a series of excitations to lowest and higher CTTS states. These states are clearly identified through the occurrence of characteristic spectator Auger decays to double Cl 3p valence-hole states, where the CTTS states can be more stabilized as compared to single Cl-aq(-) 2p core excitations and optical valence excitations. Furthermore, we have found for the first time that the CTTS electron e(i) bound by a single Cl 2p hole not only behaves as a spectator e(i) -> e'(i), bound by a double valence-hole state before relaxation of the excited electron (i) itself, but also shows electron dynamics to the relaxed lowest state, e(i) -> e'(1*). This interpretation is supported by ab initio calculations. The key to performing photoelectron and Augerelectron spectroscopy studies from aqueous solutions is the use of a liquid microjet in ultrahigh vacuum in conjunction with synchrotron radiation.
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