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31.
Bert Lutz Jan A. Kanters John van der Maas Jan Kroon Thomas Steiner 《Journal of Molecular Structure》1998,440(1-3)
Structural and IR-spectroscopic evidence is given that directed contacts from terminal alkynes to C=C double bonds possess the essential characteristics of weak hydrogen bonds. The contacts are directed at the center of the π-bond rather than at one of the individual C-atoms. The contact distances from H to the center of the C=C bond are typically 2.8 Å, with the shortest distances being 2.5 Å. The interaction is of pronounced long-range nature and can be detected in the infrared spectrum even with a long H…π distance of almost 3.0 Å. The sample used is mainly composed of ethynyl steroids belonging to the progestine family. 相似文献
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For Wiener-Hopf integral equations with an operator or matrix valued kernel and with an invertible symbol which is analytic on the real line and at infinity an indicator is introduced. In general this indicator is a bounded linear operator, but when the kernel is matrix valued and the symbol is rational it is a (possibly non-square) matrix. From the indicator the invertibility properties and Fredholm characteristics of the integral equation can be read off. The class of Wiener-Hopf equations studied here is also described in terms of growth conditions on the derivatives of the kernel. 相似文献
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Influence of a TiCl4 post-treatment on nanocrystalline TiO2 films in dye-sensitized solar cells 总被引:2,自引:0,他引:2
Sommeling PM O'Regan BC Haswell RR Smit HJ Bakker NJ Smits JJ Kroon JM van Roosmalen JA 《The journal of physical chemistry. B》2006,110(39):19191-19197
In this study, the influence of the TiCl(4) post-treatment on nanocrystalline TiO(2) films as electrodes in dye-sensitized solar cells is investigated and compared to nontreated films. As a result of this post-treatment cell efficiencies are improved, due to higher photocurrents. On a microscopic scale TiO(2) particle growth on the order of 1 nm is observed. Despite a corresponding decrease of BET surface area, more dye is adsorbed onto the oxide surface. Although it seems trivial to match this finding with the improved photocurrent, this performance improvement cannot be attributed to higher dye adsorption only. This follows from comparison between incident photon to current conversion efficiency (IPCE) and light absorption characteristics. Since the charge transport properties of the TiO(2) films are already more than sufficient without treatment, the increase in short circuit current density J(SC) cannot be related to improvements in charge transport either. Transient photocurrent measurements indicate a shift in the conduction band edge of the TiO(2) upon TiCl(4) treatment. It is concluded that the main contribution to enhanced current originates from this shift in conduction band edge, resulting in improved charge injection into the TiO(2). 相似文献
38.
LaF3 pure host, LaF3:Ce, LaF3:Tb as well as LaF3:Ce,Tb phosphors were synthesized by the hydrothermal method. X-ray diffraction measurements were in good agreement with the standard data of LaF3 from JCPDS card No. 32-0483 and indicated that the material was nanocrystalline with an average particle size of about 36 nm. Photoluminescence spectra of co-doped samples revealed that the Ce3+ emission was quenched while Tb3+ emission was enhanced, implying that energy was transferred from Ce3+ (the donor) to Tb3+ (the acceptor) in this system. The luminescence intensities and lifetimes of the donor for different concentrations of the acceptor were fitted to theoretical models in order to investigate the energy transfer mechanism. The quadrupole–quadrupole and exchange interaction mechanisms gave the best fit between the experimental data and the theoretical curves. The effective average Bohr radius from the fit to the exchange model is 0.095 nm. Since this is close to the ionic radii of the Ce3+ and Tb3+ ions, it suggests that the exchange interaction mechanism contributes to the energy transfer. 相似文献
39.
Mia C. Brown Andrew C. Mutter Ronald L. Koder Renee D. JiJi Jason W. Cooley 《Journal of Raman spectroscopy : JRS》2013,44(7):957-962
The molten globule (MG) state can aid in the folding of a protein to a functional structure and is loosely defined as an increase in structural disorder with conservation of the ensemble secondary structure content. Simultaneous observation of persistent secondary structure content with increased disorder has remained experimentally problematic. As a consequence, modeling how the MG state remains stable and how it facilitates proper folding remains difficult due to a lack of amenable spectroscopic techniques to characterize this class of partially unfolded proteins. Previously, deep‐UV resonance Raman (dUVRR) spectroscopy has proven useful in the resolution of global and local structural fluctuations in the secondary structure of proteins. In this work, dUVRR was employed to study the MG to ordered transition of a model four‐helix bundle protein, HP7. Both the average ensemble secondary structure and types of local disorder were monitored, without perturbation of the solvent, pH, or temperature. The MG to ordered transition is induced by stepwise coordination of two heme molecules. Persistent dUVRR spectral features in the amide III region at 1295–1301 and 1335–1338 cm−1 confirm previous observations that HP7 remains predominantly helical in the MG versus the fully ordered state. Additionally, these spectra represent the first demonstration of conserved helical content in a MG protein. With successive heme binding, significant losses are observed in the spectral intensity of the amide III3 and S regions (1230–1260 and 1390 cm−1, respectively), which are known to be sensitive to local disorder. These observations indicate that there is a decrease in the structural populations able to explore various extended conformations with successive heme binding events. DUVRR spectra indicate that the first heme coordination between two helical segments diminishes exploration of more elongated backbone structural conformations in the inter‐helical regions. A second heme coordination by the remaining two helices further restricts protein motion. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
40.
Vanessa Renee Little Hilary Jenkins Keith Vaughan 《Journal of chemical crystallography》2008,38(6):447-452
Abstract The crystal structures of methyl 4-{(E)-2-(4-methylpiperazino)-1-diazenyl}benzoate (2a) and 1,4-di[(E)-2-(2-nitrophenyl)-1-diazenyl]piperazine (3a) have been determined by single crystal X-ray diffraction analysis. The bis-triazene (3a) adopts an unusual pseudo-boat conformation in the piperazine ring, with a dihedral angle of 52.20(0.06)° between the two
planes defined within the piperazine ring. The crystal structures of 2a and 3a are compared with the structure of the triazene (4) and the closely related bis-triazene (5). The piperazine ring of 2a adopts a typical chair conformation, whereas the piperazine ring of 3a adopts an unusual boat conformation. Crystal data: 2a C13H18N4O2, monoclinic, space group P21
/n, a = 13.849(3) ?, b = 6.577(1) ?, c = 14.904(3) ?, α = 90°, β = 96.098(3)°, γ = 90° and V = 1,349.8(4) ?3, for Z = 4. 3a C16H16N8O4, triclinic, space group P-1, a = 7.6066(6) ?, b = 8.3741(7) ?, c = 14.507(1) ?, α = 78.673(1)°, β = 81.877(1)°, γ = 73.445(1)° and V = 865.0(1) ?3, for Z = 2.
Index abstract
The crystal structures of methyl 4-{(E)-2-(4-methylpiperazino)-1-diazenyl}benzoate (2a) and 1,4-di[(E)-2-(2-nitrophenyl)-1-diazenyl]piperazine (3a) have been determined by single crystal X-ray diffraction analysis. 相似文献