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  • kinetics  (17)
  • mass spectrometry  (13)
  • clusters  (12)
  • 1
    ISSN: 0947-6539
    Keywords: chelate ligands ; hydrogen bonds ; ligand design ; mass spectrometry ; stability constants ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A series of compounds containing the mononuclear complexes [M(tdci)2]3+ (tdci = 1,3,5-trideoxy-1,3,5-tris(dimethylamino)-cis-inositol, M = Al, Fe, Ga, In) and [M(tdci)2]4+ (M = Ti, Ge, Sn) was prepared and characterized by elemental analysis, NMR spectroscopy, and FAB mass spectrometry. Characteristic fragmentation reactions in the mass spectra were elucidated. X-ray analysis of the AlIII, FeIII, GaIII, and InIII complexes revealed that two neutral, zwitterionic tdci ligands coordinate to the metal cation exclusively through deprotonated alkoxo groups. The six coordinated oxygen donors and the six N—H protons form a hydrophilic pocket, whereas the two cyclohexane rings and the twelve methyl groups form two hydrophobic shells. The hydrophilic pocket is filled with twelve water molecules, which are arranged as a second and a third coordination sphere around the metal cation. The reactivity in aqueous solution was investigated by potentiometric measurements. The bis complexes proved to be stable at pH 7. The evaluated formation constants show an increase of stability in the order AlIII〈InIII〈GaIII〈FeIII. The measurements established that tdci is one of the most effective tridentate ligands for small (r ≤ 0.8Å) and highly charged cations. The different chelating properties of tdci and of the unmethylated 1,3,5-triamino- 1,3,5-trideoxy-cis-inositol are discussed in terms of different steric requirements and different types of solvation of the corresponding complexes in aqueous solution.
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  • 2
    ISSN: 0947-6539
    Keywords: clusters ; gas-phase chemistry ; ionmolecule reactions ; iron complexes ; mass spectrometry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We report on the gas phase reactions of small Fe+n clusters (n = 2-10) with O2, N2O and CO2 in an FT-ICR mass spectrometer. Under our experimental conditions, clusters of all sizes reacted readily with O2 and all but the dimer reacted with N2O. Only the smallest Fe+n clusters (n = 2-4) appeared to activate CO2. For all X-O molecules (X = O, N2, CO), reaction pathways were observed that include the transfer of O atoms. In addition, the reactions with O2 and N2O were accompanied by the loss of one or two Fe atoms. Thermochemical considerations based upon the well-known X-O bond energies were used to calculate Fen-O+ bond dissociation energies (BDEs) for sizes n = 2-6; these amount to roughly 550 kJ mol-1 and thus are considerably higher than the BDE of the Fe-O+ ion. All oxidation reactions of the Fe+n clusters (n = 2-6) studied in more detail were terminated by products of FexOx+ stoichiometry (x = 1-4). These “microoxides of iron” are not able to activate any further X-O bonds. Secondary reactions of FexOx+ clusters with C6H6, C2H4 and NH3 were investigated for two selected sizes (x = 2, 3) and compared with reactions of the naked Fe+n clusters.
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  • 3
    ISSN: 0947-6539
    Keywords: clusters ; fullerenes ; nitrenes ; regio-selectivity ; valence isomerization ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis, isolation, and spectroscopic characterization of the bisimino[60]fullerenes C60(NCOOR)2 (1a: R = Et, 1b. R = tBu) with a cis-1 addition pattern as well as of their regioisomeric analogues 2-7 with different addition patterns are described. Whereas compounds 2-7 are typical fulleroaziridines and obey the rule of the minimization of [5,6] double bonds, the cis-1 isomers 1a and 1b represent the first examples of fullerene derivatives with open transannular [6,6] bonds. Characteristic features within the fullerene framework of these valence isomers VI are the presence of a doubly bridged open 14-membered ring with a phenanthrene perimeter as well as of an 8-membered 1,4-diazocine heterocycle. Moreover, it is shown that, by transforming cis-1-C60(NCOOtBu)2 (1b) into cis-1-C60(NH)2 (1c), the fullerene cage can be closed in an intraring 2π→2s̰ isomerization to valence isomer V. These are the first chemical modifications of the fullerene core that allow the synthesis of both open and closed valence isomers with the same addition pattern. Density-functional as well as AM1 calculations corroborate the experimental findings that it depends on the addition pattern as well as on the nature of the addends whether or not the transannular [6,6] bonds are closed. Only in the cis-1 adducts C60(NR)2 that prefer planar imino bridges (e.g., carbamates or amides) are the open forms VI more stable than the closed isomers V.
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  • 4
    ISSN: 0947-6539
    Keywords: iron complexes ; kinetics ; magnetic susceptibility ; Mössbauer spectroscopy ; spin crossover ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The thermal and light-induced spin transitions in [Fe(bpen)(NCS)2] and [Fe(bpen)(NCSe)2] have been investigated by means of magnetic susceptibility and Mössbauer measurements. The HS → LS transition in [Fe(bpen)(NCS)2] is detected at temperatures below 120 K (T1/2 [when the fraction of high-spin species γHS = 0.5] = 68 K) with a residual HS fraction of 8% at 20 K. The transition is fairly abrupt and no hysteresis is observed, in contrast to earlier studies (ref. [8]). The generation of metastable HS states in [Fe(bpen)(NCS)2] is achieved by rapid cooling of the sample and by irradiating the compound at low temperatures with light of appropriate wavelengths (LIESST). The HS → LS relaxation of the metastable spin states after rapid cooling is observed at temperatures between 35 and 42.5 K and shows strong deviations from single exponential behavior. The spin-transition behavior changes drastically on substitution of the thiocyanate ions by NCSe-. In [Fe(bpen)(NCSe)2] a gradual and complete spin transition between 140 and 250 K is observed (T1/2 = 181 K). Irradiation of the compound with green light at low temperatures does not result in formation of long-lived metastable HS states. The spintransition curves derived from magnetic susceptibility and Mössbauer studies show good agreement for each compound, and no hints of significantly different Lamb-Mössbauer factors for the HS and LS states are observed.
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  • 5
    ISSN: 0947-6539
    Keywords: chromium ; clusters ; iron ; molybdenum ; tungsten ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A mixture of Na2MO4, M(CO)6, FeCl3·6H2O, and (EtCO)2O was heated at 120°C to produce the heterooctanuclear clusters Na2Fe2[M3O4-(O2CEt)8]2 (M = Mo, 1; W, 2). The bioxo-capped clusters Na[M3O2(O2CEt)9] (M3 = MoW2, W3) were treated with Fe-(CO)5, Cr(CO)6, and Mo(CO)6 in (EtCO)2O at 120°C to afford Na2M'2-[MoW2O4(O2CEt)8]2 (M' = Fe, 3; Cr, 4; Mo, 5) and Na2Mo2[W3O4(O2CEt)8]2 (6), respectively. The isomorphous clusters 1, 2, 5, and 6 were characterized by X-ray crystallography. The structures, similar to those of the analogues Na2Cr2[M3O4-(O2CEt)8] (M = Mo, 7; W, 8) and Na2V2[M3O4(O2CEt)8]2 (M = Mo, 9; W, 10), consist of two incomplete cubane-type [M3O4(O2Et)8]4- units centrosymmetrically bridged by two M'(III) metal ions through μ-oxo and propionate groups; the resulting cluster dianions are linked by Na+ ions into infinite chains. Clusters 1-10 do not dissolve in water and common organic solvents at room temperature. Unlike the chromium(III) clusters 7 and 8, the iron(III) and vanadium(III) clusters 1 and 9 are soluble in aqueous acid solution, as a result of their decomposition into isolated [Mo3O4]4+ cluster units. Temperature-dependent magnetic susceptibilities of 1, 2, and 7-10 were measured over the range 2-200 K and modeled by means of the spin-Hamiltonian H = - 2JS1S2 to give spin exchange coupling constants J/k of -0.60, -0.72, -1.76, -1.31, -4.80, and -1.46 K, respectively. These figures show that antiferromagnetic spin exchange coupling interactions between the magnetic, bridging M'(III) ions are very weak. These species show characteristic bands in the IR spectra at 740-820 cm-1, which can probably be assigned to ṽ(M-μ-O).
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  • 6
    ISSN: 0947-6539
    Keywords: electron transfer ; indene ; kinetics ; rhodium ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reduction of the indenyl complex [Rh(η5-C9H7)(η4-cod)] has been investigated in the context of structural effects induced by the transfer of one electron. The reduction of this complex occurs in two steps, leading first to the radical anion and then to the highly frangible dianion. Both species eliminate the indenyl anion. In the presence of free cyclooctadiene, the related cleavage leading to the indenyl anion and bis-cyclooctadiene rhodium fragments now follows a Michaelis-Menten-type mechanism involving precoordination of one extra COD ligand to the initial radical anion. These results suggest the modification of the hapticity of the indenyl ligand in connection with 17- and 19-electron metalcentered intermediates.
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  • 7
    ISSN: 0947-6539
    Keywords: aggregates ; clusters ; host-guest chemistry ; iron complexes ; magnetic anisotropy ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of ferric chloride and β-diketones (HL) in alkaline methanol solution represents a good synthetic route to hexairon(III) clusters [MFe6-(OCH3)12(L)6]+ (M = Na, Li), which exhibit an unusual sixfold molecular symmetry. Single-crystal X-ray diffraction showed that the six octahedrally coordinated iron(III) ions define a ring and are linked by twelve bridging methoxide ligands. The resulting [Fe6(OCH3)12] skeleton has the remarkable property of acting as a host for an alkali-metal ion both in the solid state and in organic solution, as demonstrated by 23Na and 7Li NMR experiments. The magnetic behavior of these systems is consistent with the presence of a nonmagnetic S = 0 ground state and of antiferromagnetic exchange interactions between the high-spin ferric ions. The energy of the excited states was studied in detail by high-field DC and pulsed-field differential magnetization experiments at 0.7 and 1.5 K. Single-crystal susceptibility measurements at variable temperature revealed a sizeable magnetic anisotropy, which has been successfully analyzed in terms of single-ion and dipolar contributions. The results are relevant to research into the origin of superparamagnetic-type behavior in transition-metal clusters.
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  • 8
    ISSN: 0947-6539
    Keywords: bipyridine ligands ; helicates ; kinetics ; mass spectrometry ; self-assembly ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The self-assembly of the pentanuclear double helicates Hh, Ha and He from the corresponding oligobipyridine strands Lh, La and Le and Cu1 ions has been investigated by NMR and electrospray mass spectrometry (ESMS). Where as Hh is assembled rapidly (in less than 20 min), He (about 20 h) and especially Ha (about 60 h) form much more slowly. The rate decreases strongly with increasing steric bulk of the substituents in the 4,4′-positions on the bipyridine units; this indicates that the search processes (wrapping, unwrapping) that lead to the final helicate are strongly hindered by the size of the substituents. The ESMS data give information about the species present in solution under different conditions and allow the formulation of possible formation pathways, which may involve, in particular, helicates of hairpin type.
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  • 9
    ISSN: 0947-6539
    Keywords: ab initio calculations ; clusters ; electronic structure ; gallium ; semiempirical calculations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We present idealised geometrical models for the α-Ga and β-Ga structures based on two differently distorted, corrugated 36 nets. We investigated the structural stability of two sets of two-dimensional and three-dimensional model structures consisting of these corrugated nets as a function of the net puckering. We used the simple tight-binding (TB) Hückel model with the structural energy difference theorem and the advanced full-potential linear muffin-tin orbital (FP-LMTO) method in the framework of local-density functional theory. Both methods show the existence of an optimum puckering angle that corresponds well to the situation in the experimental α-Ga structure. The geometrical model of the α-Ga structure follows the building principle of terminally coordinated deltahedral clusters, extended to two-dimensional structures. The chemical bonding in α-Ga is interpreted in terms of multicentre bonding within the corrugated 36 nets and two-electron-two-centre bonds connecting these nets.
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  • 10
    ISSN: 0947-6539
    Keywords: chemoselectivity ; enzyme inhibitors ; glycosidases ; kinetics ; pyridazines ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: (3,4-trans-4,5-trans)-4,5-dihydroxy-3-hydroxymethylhexahydropyrida-zine (16) was synthesized in four steps from 2,4-pentadienol (22) and 4-phenyl-triazolin-3,5-dione (18) in an overall yield of 32%. In the first step a Diels-Alder reaction between 18 and 22 gave (π)-2-hydroxymethyl-8-phenyl-1,6,8-triazabicyclo[4.3.0]non-3-ene-7,9-dione (23c) in 88% yield. Epoxidation of 23c with trifluoromethyl(methyl)dioxirane, generated in situ, gave the trans epoxide 24c in 62% yield. Hydrolysis of the epoxide with perchloric acid gave stereoselectively (2,3-trans-3,4-trans)-3,4-dihydroxy-2-hydroxy-methyl-8-phenyl-1,6,8-triazabicyclo[4.3.0]-nonane-7,9-dione (26) in 73% yield. In the fourth and final step, hydrazinolysis of 26 gave 16 in 84% yield. Pyridazine 16 was found to be a potent inhibitor of α-and β-glucosidase, isomaltase and glyco-gen phosphorylase, while galactosidases and α-mannosidase were not inhibited. The inhibition of β-glucosidase is independent of pH, and was found to be due to unprotonated 16.
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