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121.
Schoonbeek FS van Esch JH Hulst R Kellogg RM Feringa BL 《Chemistry (Weinheim an der Bergstrasse, Germany)》2000,6(14):2633-2643
Several geminal bis-urea compounds were synthesised by means of an acid-catalysed condensation of various benzaldehydes with different monoalkylureas. Many of these compounds form thermoreversible gels with a number of organic solvents at very low concentrations (<3mM) and which are stable to temperatures higher than 100 degrees C. Electron microscopy revealed a three-dimensional (3D) network of intertwined fibres, which are several tens of micrometers long and have a width ranging from approximately 30 to 300 nm. The possible aggregate forms and aggregate symmetries were evaluated by means of molecular mechanics calculations. 1H NMR, 2D NMR, 13C NMR and 13C-CP/MAS NMR techniques were used to obtain information about the aggregation and possible aggregate symmetry of geminal bis-ureas in solution, in the gel state, and in the solid state. 相似文献
122.
O-Protected 3-hydroxyoxazolidin-2,4-diones have been prepared in a novel one-pot reaction by subsequent treatment of cyanohydrins with 1,1'-carbonyldiimidazole and O-protected hydroxylamines followed by acidic hydrolysis of the intermediate 4-imino-oxazolidin-2-ones. Decarbonylation of O-protected 3-hydroxyoxazolidin-2,4-diones by catalytic amounts of sodium methoxide, lithium hydroxide, sodium carbonate and caesium carbonate in methanol afforded O-protected alpha-hydroxyhydroxamic acids in excellent yields. Their deprotection provided a series of novel alpha-hydroxyhydroxamic acids. 相似文献
123.
Marsman AW Havenith RW Bethke S Jenneskens LW Gleiter R van Lenthe JH Lutz M Spek AL 《The Journal of organic chemistry》2000,65(15):4584-4592
Long-range sigma-pi interactions in tetrahydro4H-thiopyran end-capped oligo(cyclohexylidenes) were identified by He(I) photoelectron spectroscopy (PES) and ab initio RHF/6-31G* calculations. The vertical ionization energies Ivj of the highest occupied molecular orbitals (MO's) were assigned using Koopmans' theorem (Iv,j = -epsilonj) and by correlation with the ionizations of related reference compounds. The experimental (PES) and theoretical (RHF/6-31G*) results are in good agreement. For tercyclohexylidene derivatives which contain two nonconjugated pi-bonds splittings deltaIv,j of the pi-bands in the range from approximately 0.5 to 0.7 eV (delta-epsilonj approximately 0.6 to 0.9 eV). For the bi- and tercyclohexylidene compounds containing two sulfur atoms at their alpha- and omega-end positions the pi-type sulfur lone pair bands [Lppi(S)] split significantly by deltaIvj approximately 0.3 to 0.4 eV (delta-epsilonj approximately 0.3 to 0.4 eV), i.e. sigma-pi interactions over distances of ca. 8 and 12 A, respectively, occur. The magnitude of the interactions and the observed splittings are independent of the anti and syn conformations of the oligo(cyclohexylidene) hydrocarbon skeletons. RHF/6-31G* Natural Bond Orbital analyses reveal that the Hax-C-C-Hax precanonical MO's (PCMO's) centered on the cyclohexyl-type rings are paramount for the relay of the through-bond sigma-pi interactions; no through-space sigma-pi interactions were identified. 相似文献
124.
Comparison of methods for the gas-chromatographic determination of PCB congeners and chlorinated pesticides in marine reference materials 总被引:1,自引:0,他引:1
Michele M. Schantz Reenie M. Parris Joachim Kurz Karlheinz Ballschmiter Stephen A. Wise 《Fresenius' Journal of Analytical Chemistry》1993,346(6-9):766-778
Summary Three gas-chromatographic (GC) columns with different selectivity (DB-5, DB-1701, and C-18) and two different GC detectors (electron-capture and mass-spectrometric) were used to analyze three Standard Reference Materials (SRMs), which are available from the National Institute of Standards and Technology (NIST), and two Certified Reference Materials (CRMs), which are available from the Community Bureau of Reference (BCR), for polychlorinated biphenyl (PCB) congeners and chlorinated pesticides. The materials analyzed were: SRM 1588, Organics in Cod Liver Oil; SRM 1941, Organics in Marine Sediment; SRM 1974, Organics in Mussel Tissue (Mytilus edulis); CRM 349, Chlorobiphenyls in Cod Liver Oil; and CRM 350, Chlorobiphenyls in Mackerel Oil. Results from these different methods are compared, and concentrations for additional PCB congeners and chlorinated pesticides in these reference materials, which have not been measured previously, are reported.Dedicated to Professor Dr. Wilhelm Fresenius on the occasion of his 80th birthday 相似文献
125.
126.
The phase diagram Cu2SeAs2Se3 was investigated by thermal and X-ray methods. Cu2Se has a limited solubility for As2Se3 (5 mole% at 769 K). The stoichiometric compound Cu3AsSe3 exists between 696 and 769 K. Cu4As2Se5, a phase at 66.6 mole% Cu2Se, decomposes peritectically at 746 K. The narrow homogeneity range (4 mole% at 683 K) extends far into the ternary space. CuAsSe2 also decomposes peritectically at 683 K. A degenerated eutectic between CuAsSe2 and As2Se3 was found at 641 K. Single crystals of Cu4As2Se5 were grown in a salt melt. A metastable modification of the high-temperature phase Cu3AsSe3 can be obtained by quenching. Cu4As2Se5 (space group R3, lattice constants a = 1404.0(1) pm, c = 960.2(1) pm), Cu6As4Se9, obtained by Cambi and Elli, and Cu7As6Se13 of Takeuchi and Horiuchi are different versions of a sphalerite-type compound with a broad homogeneity range in the system CuAsSe. CuAsSe2 is possibly monoclinic with lattice parameters of a = 946.5(1) pm, b = 1229.3(1) pm, c = 511.7(1) pm, and β = 98.546(4)°. The enthalpy of mixing of Cu2Se and As2Se3 in the liquid state is endothermic. 相似文献
127.
Karl Schneeberger Karl F. Doerner Andrea Kurz Michael Schilde 《Central European Journal of Operations Research》2016,24(1):1-27
This article deals with the relocation of ambulance vehicles from their origin location (i.e., position before a crisis occurs) to a crisis area and to undercovered areas (i.e., no ambulance vehicle is available for a potential emergency in a given region). Support for a crisis area can lead to insufficient coverage of other emergency patients in other regions, so decision makers need assistance with useful relocation information, including the relocation of vehicles to the crisis area and to undercovered areas. As optimization criteria, this study considers two objectives: undercoverage (i.e., unsupported demand) and the total time needed to get to the crisis area with all vehicles. A proposed model aims to minimize the time required for the vehicle relocation process and avoid relocation mistakes (e.g., sending a vehicle to support the crisis area when it would be better to leave it at its current location). Devising the relocation plan consists of three phases. First, a location model allocates all available vehicles to potential vehicle locations (positioning vehicles before the crisis occurs). Second, the same location model allocates the remaining vehicles in the case of a crisis (all vehicles not needed to manage the crisis). Third, the relocation model (phase three) moves vehicles from their position before the crisis occurs (phase one) to their position during the crisis (phase two). 相似文献
128.
J. D'ans R. Walcher C. Zerbe D. Witt F. Schuster U. Stolzenburg A. Orlicek W. Liesegang A. Splittgerber R. Strohecker P. Sander F. Scheffer W. Wcihlbier Fr. Specht B. Wandrowsky L. Schmitt A. Dietzel G. Wittmann W. Heimsoeth R. Grün Fr. Heinrich Fr. Petzold Hellmut Fischer F. Kurz G. Darius K. Wagenmann G. Siebel H. Toussaint G. W. Scott Blair H. Kauffmann O. E. Radczewski J. Winckelmann Verein Deutscher Ingenieure 《Analytical and bioanalytical chemistry》1940,119(3-4):125-130
129.
130.