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排序方式: 共有578条查询结果,搜索用时 31 毫秒
91.
Gunther Buehrdel Rainer Beckert Birgit Friedrich Helmar Goerls 《Journal of heterocyclic chemistry》2008,45(3):845-851
92.
Zawisza I Nullmeier M Pust SE Boukherroub R Szunerits S Wittstock G 《Langmuir : the ACS journal of surfaces and colloids》2008,24(14):7378-7387
Ultrathin titanium layers when deposited on the surface of gold can be successfully applied for infrared reflection absorption spectroscopy (IRRAS) investigations. It was shown that the reflectivity, the phase shift, and the mean square electric field of the p- and s-polarized IR radiation in up to 20 nm thick titanium layers covered with a 3-4 nm thick layer of native oxide are comparable to those of the air/gold interface. The surface selection rule is fulfilled. Thus, qualitative and quantitative analysis of 1,2-dimyristoyl- sn-glycero-3-phosphocholine (DMPC) bilayers transferred in liquid expanded (LE) and liquid condensed (LC) states can be performed. Differences are found in the hydration state and molecular arrangement of the two investigated bilayers. In the DMPC bilayer in the LE state, the C-N bond in the positively charged choline moiety is inclined by approximately 70 degrees toward the surface of the negatively charged titanium substrate. In the phosphate moiety, the in-plane vector of the O-P-O group makes a small angle of approximately 15 degrees to the surface normal. This open structure of the lipid molecule corresponds to the B crystal structure of the DMPC molecule and provides space for strong hydration of the polar headgroup. In the DMPC bilayer in the LC state, the intermolecular distances are reduced; the C-N bond of the choline group makes a smaller angle to the surface normal, and the in-plane vector of the O-P-O group in the phosphate moiety displays a larger tilt. The degree of hydration is reduced. The arrangement of the polar headgroup region corresponds to the A crystal structure of the DMPC molecule. 相似文献
93.
Khan M Brunklaus G Enkelmann V Spiess HW 《Journal of the American Chemical Society》2008,130(5):1741-1748
13C-CPMAS and other solid-state NMR methods have been applied to monitor the solid-state reactions of trans-cinnamic acid derivatives, which are the pioneer and model compounds in the field of topochemistry previously studied by X-ray diffraction, AFM, and vibrational spectroscopy. Single-crystal X-ray analyses of photoirradiated alpha-trans-cinnamic acid where the monomers are arranged in a head-to-tail manner have revealed the formation of a centrosymmetric alpha-truxillic acid photodimer. For a centrosymmetric dimer, however, two cyclobutane carbon signals and one carbonyl carbon signal were expected apart from other aromatic carbon signals. Instead, four cyclobutane and two carbonyl carbon signals were observed suggesting the formation of a non-centrosymmetric photodimer. Removing hydrogen bonds from the system by esterfication of alpha-truxillic acid yield a centrosymmetric photodimer. Careful analysis of the obtained products via solid-state NMR clearly showed that the observed peak splittings in the 13C-CPMAS spectra did not originate from packing effects but rather result from asymmetric hydrogen bonds distorting the local symmetry. Further evidence of this rather dynamic hydrogen-bonding stems from high-temperature X-ray data revealing that only the joint approach of both X-ray analysis and solid-state NMR at similar temperatures allows for the successful characterization of dynamic processes occurring in topochemical reactions, thus, providing detailed insight into the reaction mechanism of organic solid-state transformations. 相似文献
94.
In this paper, for a frame L, we characterize modified sobriety in stratified L-topological spaces and strong L-topological spaces internally using certain classes of stratified L-filters. While the first characterization using completely prime L-filters is trivial and applies to all stratified L-topological spaces, the other one using irreducible L-filters generalizes an approach of R.E. Ho?mann to the lattice-valued case but is restricted to either the case that the lattice is a complete Boolean algebra or to the case of completely distributive lattices and strong L-topological spaces. 相似文献
95.
Two‐Dimensional Core‐Shelled Porous Hybrids as Highly Efficient Catalysts for the Oxygen Reduction Reaction
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Kai Yuan Dr. Xiaodong Zhuang Haiyan Fu Dr. Gunther Brunklaus Dr. Michael Forster Prof. Dr. Yiwang Chen Prof. Dr. Xinliang Feng Prof. Dr. Ullrich Scherf 《Angewandte Chemie (International ed. in English)》2016,55(24):6858-6863
Two‐dimensional (2D) transition‐metal dichalcogenides (TMDs) have drawn much attention due to their unique physical and chemical properties. Using TMDs as templates for the generation of 2D sandwich‐like materials with remarkable properties still remains a great challenge due to their poor solvent processability. Herein, MoS2‐coupled sandwich‐like conjugated microporous polymers (M‐CMPs) with high specific surface area were successfully developed by using functionalized MoS2 nanosheets as template. As‐prepared M‐CMPs were further used as precursors for preparation of MoS2‐embedded nitrogen‐doped porous carbon nanosheets, which were revealed as novel electrocatalysts for oxygen reduction reaction with mainly four‐electron transfer mechanism and ultralow half‐wave potential in comparison with commercial Pt/C catalyst. Our strategy to core–shelled sandwich‐like hybrids paves a way for a new class of 2D hybrids for energy conversion and storage. 相似文献
96.
Brunklaus G Koch A Sebastiani D Spiess HW 《Physical chemistry chemical physics : PCCP》2007,9(32):4545-4551
We studied the incorporation of various small guest molecules into calix[4]hydroquinone nanotubes and nanoclusters using solid-state proton NMR spectroscopy in combination with quantum chemical calculations. While the molecules exhibit different types of hydrogen bonding and van der Waals interactions, they show different affinities to the nanotube host structures. As the guest molecules are located inside the complexes, they experience a shift in the NMR resonance line caused by screening effects from the aromatic electrons of the host superstructure. The abilities to fill the otherwise empty space within the tubes can hence be measured indirectly by the displacement of the NMR lines relative to the free molecules. In this way, we can probe which guest molecules are recognized by the calix[4]hydroquinones as suitable for filling their nanoporous superstructures. Selective guest-host interactions have been successfully achieved for the three component mixture of water and acetone with either 2-methyl-2-propanol or 2-propanol. In both cases, the alcohols were superior to acetone in filling the CHQ tubes. 相似文献
97.
Titanocene complexes with chelating N-heterocyclic ligand bridges react with ferrocenium salts as selective oxidants to afford air-stable cationic complexes and allow the preparation of exceptional mixed valence hexaazatrinaphthylene complexes [(Cp2Ti)3(mu3-HATNMe6)]n+ (1n+) (n=1, 2, 3, 4). Cyclic voltammograms (CV) and differential pulse voltammograms (DPV) show that nine oxidation states of 1 are generated without decomposition. Comproportionation constants Kc have been calculated in order to determine the extent of electronic communication between the titanium centers. The Kc values of the mixed valence states are indicative of uncoupled (14+), moderately coupled (15+), and strongly coupled (1-, 1+, and 12+) systems. Small but significant structural changes occurring upon oxidation of neutral 1 are observed by X-ray structural analysis on 1+-14+. Anion-pi interactions between the electron-deficient central ring of the HATNMe6 moiety and PF6- and BF4- counterions, respectively, are found for 12+, 13+, and 14+. The short cation-anion contacts cause interesting molecular allignments in terms of molecular architecture. For 12+ the assembly of an one-dimensional (1D) polymer is observed. Electrochemical investigations on the mononuclear cationic titanocene complexes [(Cp2Ti)(L)]+ (L=2,2'-biquinoline (2+), 4,4'-dimethyl-2,2'-biquinoline (3+), and 5,8'-dimethyl-2,3'-biquinoxaline (4+)) showed similar oxidation and reduction characteristics among each other. Conversion to monoanionic, neutral, and dicationic states is enabled. As found for the trinuclear compounds 1n+, the molecular structures of 2+-4+ reveal significant differences compared to their neutral parents. 相似文献
98.
PM IRRAS investigation of thin silica films deposited on gold. Part 1. Theory and proof of concept 总被引:1,自引:0,他引:1
Zawisza I Wittstock G Boukherroub R Szunerits S 《Langmuir : the ACS journal of surfaces and colloids》2007,23(18):9303-9309
Polarization modulation infrared reflection absorption spectroscopy (PM IRRAS) was successfully used for the first time to characterize an optically transparent thin oxide film. SiO2 layers of 7 nm thickness were synthesized by plasma enhanced chemical vapor deposition (PECVD) on 200 nm thick gold covered glass slides. Despite the fact that silica is transparent and absorptive to IR radiation when deposited in the form of thin films on a gold surface, it preserves the high metallic reflectivity for the IR light. At grazing angles of incidence of the IR beam, the enhancement of the normal component of the electric field at the interface is comparable to that of Au alone. In addition, the analysis of the structure of a 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) lipid monolayer deposited using the Langmuir-Blodgett technique is demonstrated. 相似文献
99.
Randy W. Larsen Michael K. Helms W. Russell Everett David M. Jameson 《Photochemistry and photobiology》1999,69(4):429-434
Abstract— We report the formation of an electrostatic complex between (16-pyrimidinium crown-4)tetranitrate (16PC4) and tetrakis-(4-sulfonatophenyl)porphyrin (4SP) in aqueous solution. Ground-state complex formation results in a red shift of the 4SP visible absorption bands and a decrease in absorbance of the Soret band. The equilibrium constant for complex formation (determined from optical titrations) is found to be (2.0 ± 0.2) × 105 M −1 . In addition, the data fit to an expression describing a 1:1 stoichiometry. Excitation of the complex results in quenching of both the excited singlet and triplet states of the associated porphyrin. The singlet-state lifetime decreases from 10 ns for the free porphyrin to 1.5 ns in the presence of 16PC4 at low solution ionic strengths. In addition, evidence is presented for triplet-state quenching within the complex with k q = (1.1 ± 0.1) × 104 s−1 . The mechanism of quenching is tentatively assigned to electron transfer from either the excited singlet or excited triplet state of the porphyrin to the ground state of the 16PC4. 相似文献
100.
Gunther Mair Hans Georg von Schnering Michael Wrle Reinhard Nesper 《无机化学与普通化学杂志》1999,625(7):1207-1211
Li2B6 is formed from the elements as transparent red microcrystalline compound (Li : B = 1 : 3; Mo crucible in closed Nb ampoule; 1723 K; 4 h). Single crystals are grown from a lithium silicide melt with large Li excess at 1923 K. Li2B6 is a semiconductor with electron as well as Li+ ionic conductivity which dominates above 600 K. Microcrystalline samples react with H2O liberating gases and forming a brownish amorphous product, but larger crystals are not very sensitive. – Li2B6 crystallizes tetragonally in a new tP16 structure type which is a variant of the CaB6 structure (a = 5.975 Å, c = 4.189 Å; Z = 2; space group P4/mbm). The [B62–] net of the polymeric octahedro-anion is slightly distorted to give space for the insertion of a (32434) net of the Li+ cations in the cavities (d(B–B)endo = 1.766 Å; d(B–B)exo = 1.720 Å; d(Li–B) = 2.363 Å; d(Li–Li) = 3.094 Å). The incomplete occupancy of the Li position (80%) and the electron density at a further position (20%) indicate the mobility of the Li+ cations. 相似文献