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681.
Alexander Engström 《Discrete Mathematics》2009,309(10):3299-2211
First we prove that certain complexes on directed acyclic graphs are shellable. Then we study independence complexes. Two theorems used for breaking and gluing such complexes are proved and applied to generalize the results by Kozlov.An interesting special case is anti-Rips complexes: a subset P of a metric space is the vertex set of the complex, and we include as a simplex each subset of P with no pair of points within distance r. For any finite subset P of R the homotopy type of the anti-Rips complex is determined. 相似文献
682.
Gina C. Vlahopoulou Dimitris I. Alexandropoulos Catherine P. Raptopoulou Spyros P. Perlepes Albert Escuer Theocharis C. Stamatatos 《Polyhedron》2009
The use of pyridine-2,6-dimethanol (pdmH2) in copper(II) nitrate chemistry is reported. The reaction of Cu(NO3)2·3H2O with one equivalent of pdmH2 in MeCN affords the known mononuclear complex [Cu(pdmH2)2](NO3)2 (1) in high-yield. The reaction of 1 and NaOMe in an 1:1 ratio, as well as the reaction between Cu(NO3)2·3H2O, pdmH2 and NaOMe in an 1:1:1 ratio, in MeOH gives the tetranuclear complex [Cu4(NO3)2(pdmH)4(H2O)(MeOH)](NO3)2 (2) in moderate yields. The cation of 2 possesses a slightly distorted tetrahedral Cu4 topology with a [Cu4(μ2-OR)4]4+ core. The pdmH− ions behave as η1:η1:η2:μ2 ligands. Strong intramolecular hydrogen bonds and π–π stacking interactions provide thermodynamic stability on compound 2. Variable-temperature, solid-state dc magnetic studies were carried out on complex 2 in the 2.0–300 K range. The data indicate predominant antiferromagnetic exchange interactions and a resulting S = 0 ground state, which is expected for a solely, μ2-alkoxide-bridged system with obtuse Cu–O–Cu bond angles that magnetically behaves as a Cu4 ring. A simplified 1 − J model was found to be adequate to describe the variable-temperature dc susceptibility data. The data were fitted to the appropriate equation derived from the Hamiltonian H = −J1(S1 · S2 + S2 · S4 + S3 · S4 + S1 · S3), giving the parameters J1 = −99.5 cm−1 and g = 2.11(4). The combined work demonstrates the ligating flexibility of the pdmH2 chelate and its usefulness in the synthesis of oligo- and polynuclear CuIIx clusters with interesting structural and magnetic properties, without requiring the co-presence of carboxylate ligands. 相似文献
683.
Xylenol orange (XO) is a suitable reagent for the spectrophotometric determination of niobium in a weakly acidic medium. The
present study shows that the addition of 3-hydroxy-2-methyl-1-phenyl-4-pyridone (HX) influences the complex formation as well
as the spectroscopic properties of this colored system. To prevent formation of niobium(V) hydrolyzed species in water, tartaric
acid was used when preparing the niobium stock solution. The red-violet colored complex formed by heating niobium(V) with
xylenol orange (XO) in the presence of HX at pH=3 has a maximum absorption wavelength at 565 nm. The complex can be extracted
by a chloroform solution of tetraphenylphosphonium (TPP) chloride. The optimum reaction conditions and other parameters for
complex formation have been evaluated. The mechanism of extraction is probably based on the formation of the associated ion
pair between the tetraphenylphosphonium cation and the mixed Nb(V)-XO-HX anion. The extracted complex in chloroform showed
a maximum absorbance at 585 nm with the corresponding molar absorption coefficient being 3.72×104 L⋅mol−1⋅cm−1, and obeys Beer’s law in the range 3×10−6 to 3×10−5 mol⋅L−1. 相似文献
684.
Unsymmetrically substituted metal-free and metallated benzoporphyrazines including pyridino[3,4]tribenzoporphyrazinic macrocycles have been synthesized in good yields by ring expansion reactions of boron(III) subphthalocyanines with phthalonitriles in functional ammonium ionic liquids in the presence of 1,8-diazabicyclo-[5,4,0]-undec-7-ene (DBU) under different reaction conditions. The concentration dependent UV/vis, fluorescence, NMR and ESI-MS spectroscopic data indicate that zinc(II) pyridino[3,4]tribenzoporphyrazines exist as head-to-tail self-aggregates in the solution phase. 相似文献
685.
686.
687.
《光谱学快报》2013,46(4):327-340
Electrospray/mass spectrometry ESI/MS analyses were performed to study the stability of calix[4]‐arene‐crown‐6/alkali cation complexes in the gas phase, and in acetonitrile/water mixtures. This approach allowed a comparison with previous investigations by molecular‐dynamic simulations,which demonstrated a complementarity between calculation and experiment. Experimental results obtained from ESI/MS confirm that the stability of calixarene/cation complexes depends upon the medium used. Indeed, the calixarene in solution presents a strong affinity for cesium, whereas in the gas phase, it has a stronger affinity for sodium. Similarly, the stability of [calixarene + Na]+‐type complexes in the solvent phase is increased by the presence of water in the dilution system (up to 40% in acetonitrile), whereas other alkaline complexes are destabilized by water in any proportion. Finally, calixarenes that bear benzo groups on their crowns have an affinity for sodium, which is weak in solution, but considerably stronger in the gas phase. 相似文献
688.
《Analytical letters》2012,45(15):2329-2342
The synthesis, characterization, and mass spectra of oxorhenium(V) complexes with 1,2-dihydroxybenzene, 1,2,3-trihydroxybenzene, and 2,3-dihydroxynaphtalene are reported. Electrospray ionization, atmospheric pressure photoionization, and laser desorption/ionization mass spectra of the complexes showed abundant negatively charged molecular anions and low fragmentation. Calculated similarity indexes showed significant conformity between the computed and experimental isotopic patterns of selected ions and confirmed correct assignment of elemental composition to m/z values. Electrospray tandem mass spectrometry provided essential information about fragments from molecular ions of studied complexes, making it possible to distinguish among fragment ions and the ions arising from compounds present in the reaction mixture. Based on the results, mass spectrometry utilizing soft common ionization techniques is useful for monitoring complex formation reaction kinetics and the stabilities of the complexes. Representative spectra were recorded for micromolar concentrations of the analytes. 相似文献
689.
《Journal of Coordination Chemistry》2012,65(6):467-480
Mononuclear and binuclear Mn(II), Co(II), Ni(II) and Cu(II) complexes of new semicarbazone ligands derived from sulfonamide were synthesized and characterized by elemental analysis and IR spectra. In mononuclear complexes, the semicarbazone behaves as a monoanionic terdentate or neutral terdentate ligand towards the metal ion. However, in binuclear complexes, it behaves as a monoanionic terdentate towards one of the bivalent metal ions and monoanionic bidentate ligand towards the other metal ion in the same complex. Electronic spectra and magnetic susceptibility measurements of the solid complexes indicated octahedral geometry around Mn(II), Co(II) and Ni(II) and square planar around the Cu(II) ion. These geometries were confirmed by the results obtained from thermal analyses. The antifungus properties of the ligands and their complexes were investigated. 相似文献
690.
《Journal of Coordination Chemistry》2012,65(8):691-700
Equilibrium studies in aqueous solution are reported for dimethyltin(IV) complexes of zwitterionic buffers, such as bicine and tricine (L). Stoichiometry and stability constants for the complexes formed were determined at 25°C and ionic strength 0.1M NaNO3. The results showed the best fit of the titration curves were obtained for complexes MLH, ML, ML2, MLH-1, and MLH-2 with the hydrolysis products of the dimethyltin(IV) cation. The bonding sites of the dimethyltin(IV) complexes with bicine and tricine at different pH were characterized in the solid state by elemental analyses, FTIR, and TG analysis. The molecular formula of the complexes synthesized at pH=3.0 is [(CH3)2Sn(L)(H2O)]Cl while in neutral and alkaline media the hydrolytic species are formed. 相似文献