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| 2. |
- Myneni, S., et al.
(författare)
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Spectroscopic probing of local hydrogen-bonding structures in liquid water
- 2002
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 14:8, s. L213-L219
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Tidskriftsartikel (refereegranskat)abstract
- We have studied the electronic structure of liquid water using x-ray absorption spectroscopy at the oxygen K edge. Since the x-ray absorption process takes less than a femtosecond, it allows probing of the molecular orbital structure of frozen, local geometries of water molecules at a timescale that has not previously been accessible. Our results indicate that the electronic structure of liquid water is significantly different from that of the solid and gaseous forms, resulting in a pronounced pre-edge feature below the main absorption edge in the spectrum. Theoretical calculations of these spectra suggest that this feature originates from specific configurations of water, for which the H-bond is broken on the H-donating site of the water molecule. This study provides a fingerprint for identifying broken donating H-bonds in the liquid and shows that an unsaturated H-bonding environment exists for a dominating fraction of the water molecules.
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| 3. |
- Goldoni, A., et al.
(författare)
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C-70 adsorbed on Cu(111) : Metallic character and molecular orientation
- 2002
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 116:17, s. 7685-7690
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Tidskriftsartikel (refereegranskat)abstract
- We investigated the interaction of C-70 with the Cu(111) surface using x-ray photoelectron spectroscopy and x-ray absorption spectroscopy. The data point to a net charge transfer from the Cu substrate to the C-70 molecules directly bonded to the Cu atoms, providing a metallic character for a single layer of C-70 (monolayer) adsorbed on this surface. A strong dependence of the x-ray absorption spectra from the light polarization is also observed. By comparing these experimental data with density-functional calculations of the contribution to the x-ray absorption spectral line shape of the five inequivalent carbon atoms in the C-70 cage, we observed that the molecules are mainly oriented with the C5V axis perpendicular to the Cu surface.
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| 4. |
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| 5. |
- Oxelman, Bengt, et al.
(författare)
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RPB2 gene phylogeny in flowering plants, with particular emphasis on asterids.
- 2004
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Ingår i: Molecular Phylogenetics and Evolution. ; 32:2, s. 462-479
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Tidskriftsartikel (refereegranskat)abstract
- Two, apparently functional, paralogues of the RPB2 gene, which encodes the second largest subunit of RNA polymerase II, are shown to be present in two major groups of asterid plants. Although all other land plants surveyed so far have been found to have only one of these two copies, the RPB2 gene phylogeny inferred from the 3' half of the gene for 35 angiosperm taxa and six other land plants indicates that the duplication of the RPB2 gene occurred earlier than the time for origin of the asterid group, probably near the origin of "core eudicots." The d copy is present in all plants which are unambiguously assigned to the core eudicots, whereas the I copy is retained only in the lamiid clade, Ericales, and Escallonia, all belonging to the asterid group of plants. Both parsimony and likelihood analyses of sequences from the 3' half of the gene give strong bootstrap support for these conclusions. There is no support for monophyly of the taxa having both copies. Thus, numerous losses of one of the copies must be inferred. Structurally, both paralogues appear functional, and transcription is demonstrated for both copies. In the lamiid group, the d copy has lost introns 18-23. The well supported phylogenetic relationships implied by the RPB2 gene phylogeny are largely congruent with well supported phylogenetic hypotheses based on other sequence data. However, Ilex, usually assigned to the campanuliid clade, is instead supported as being a member of the lamiid clade, both from sequence data and the presence of an I copy as well as the loss of introns 18-23 in the d copy. Escallonia, supported as a member of the campanuliid clade both by RPB2-d-sequences and previously published DNA sequence data, has all the introns 18-23 in its d copy, as do all other members studied from the campanuliid group. We used the Markov Chain Monte Carlo (MCMC) approach of the MrBayes program to implement Maximum Likelihood bootstrapping. Under the same model of evolution, bootstrapping frequencies are significantly lower than the Bayesian posterior probabilities inferred from the MCMC chain.PMID: 15223030 [PubMed - indexed for MEDLINE]
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| 6. |
- Pettersson, L. G. M., et al.
(författare)
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Electronic structure effects from hydrogen bonding in the liquid phase and in chemisorption : an integrated theory and experimental effort
- 2001
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Ingår i: Journal of Synchrotron Radiation. - 0909-0495 .- 1600-5775. ; 8, s. 136-140
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Tidskriftsartikel (refereegranskat)abstract
- A closely integrated theoretical and experimental effort to understand chemical bonding using X-ray spectroscopic probes is presented. Theoretical techniques to simulate XAS (X-ray absorption spectroscopy), XES (X-ray emission spectroscopy), RIXS (resonant inelastic X-ray scattering) and XPS (X-ray photoelectron spectroscopy) spectra have been developed and implemented within a density functional theory (DFT) framework. In combination with new experimental techniques, such as high-resolution XAS on liquid water under ambient conditions and XES on complicated surface adsorbates, new insight into e.g. hydrogen-bonded systems is obtained. For the (3 x 2) overlayer structure of glycine/Cu(110), earlier work has been extended to include adsorbate-adsorbate interactions. Structures are optimized for large cluster models and for periodic boundary conditions. It is found that specific features in the spectra arise from hydrogen-bonding interactions, which thus have important effects at the molecular-orbital level. XAS on liquid water shows a pronounced pre-edge feature with significant intensity, while the spectrum of ice shows only little intensity in this region. Theoretical spectrum calculations, based on instantaneous structures obtained from molecular-dynamics (MD) simulations, show that the pre-edge feature in the liquid is caused by water molecules with unsaturated hydrogen bonding. Some aspects of the theoretical simulations will be briefly discussed.
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| 7. |
- Triguero, L., et al.
(författare)
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Direct experimental measurement of donation/back-donation in unsaturated hydrocarbon bonding to metals
- 2000
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 122:49, s. 12310-12316
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Tidskriftsartikel (refereegranskat)abstract
- Using resonant and off-resonant X-ray emission spectroscopy, in combination with ground-state density functional calculations, we examine the electronic structure of chemisarbed ethylene and benzene on thr Cu(110) surface to investigate the suitability of the donation/back-donation bonding model given by Dewar(1) and Chart and Duncanson(2) (DCD) for the interaction of unsaturated hydrocarbons with metal surfaces. We give an experimental verification of the DCD model and find donation/back-donation to be twice as large for ethylene as for benzene. In particular, the degree of sigma-pi mixing (rehybridization) is found to correspond to the amount of donation/back-donation, which is put in relation to the aromatic and nonaromatic pi characters of benzene and ethylene, respectively.
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| 8. |
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| 9. |
- Huang, Z. L., et al.
(författare)
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Novel heterocycle-based organic molecules with two-photon induced blue fluorescent emission
- 2003
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Ingår i: Journal of Materials Chemistry. - : Royal Society of Chemistry (RSC). - 0959-9428 .- 1364-5501. ; 13:4, s. 708-711
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Tidskriftsartikel (refereegranskat)abstract
- Two-photon absorption and two-photon induced blue emission characteristics of a series of heterocycle-based organic molecules are investigated experimentally and by quantum-chemical computations. The molecules consist of a typical A-pi-A' structure, where heterocycle, styryl and formyl groups are employed as A, pi-conjugated and A' moieties, respectively. Experimental results indicate that significant enhancements in the blue emission efficiency and two-photon absorption cross-sections can be achieved by replacing S and O atoms with an N atom in the heterocycle acceptor moiety, which is also supported by the quantum-chemical computations. Additionally, larger two-photon absorption cross-sections can be obtained by choosing appropriate solvents, as indicated by the computations.
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| 10. |
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