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451.
Structural aspects of the binding of halides in the octaaza cryptand (1,4,11,14,17,24,29,36-octa-azapentacyclo[12.12.12.2(6,9).2(19,22).2(31,34)]-tetratetraconta-6(43),7,9(44),19(41),20,22(42),31(39),32,34(40)-nonaene, N(CH2CH2NHCH2-p-xylyl-CH2NHCH2CH2)3N), L1, were examined for fluoride, chloride, and bromide. Crystallographic results for two different fluoride complexes indicated cascade-like coordination, with two fluoride ions inside the tren-based cavity, bridged by a water molecule. In the two different chloride structures, a single chloride and a water molecule occupied the cavity. The bromide structure contained two crystallographically independent cationic cryptands. Unit A consisted of a bromide on one side of the cavity and three disordered water molecules situated between the cryptand arms on the other side. Unit B also had a bromide inside the cavity at one side, but a single molecule of water was centered at the other side of the cavity. Association constants for the three ions, determined by NMR titrations in aqueous solution at pH 5, revealed log Ka=3.15(5), 3.37(3), and 3.34(4) for fluoride, chloride, and bromide, respectively. 相似文献
452.
The total and semi syntheses of (2R,3R)-epigallocatechin-3-O-(4-hydroxybenzoate), a novel catechin from Cistus salvifolius, were accomplished. The proteasome inhibition and cytotoxic activities of the synthetic compound and its acetyl derivative were studied and compared with (2R,3R)-epigallocatechin-3-gallate (EGCG), the active component from green tea. 相似文献
453.
Christopher J. Hanlon Kristin A. Arnold Isaura Delgado George W. Gokel 《Journal of inclusion phenomena and macrocyclic chemistry》1989,7(4):437-442
The first example of a lariat ether containing a sulfur sidearm donor is reported here along with a comparison of cation binding strengths to Na+ and K+. The binding strength differs remarkably little from analogs completely lacking a sidearm donor group and similar results are observed for the novel sulfoxide analog. 相似文献
454.
Kristin Schröder Bianca Bitterlich Feyissa Gadissa Gelalcha Matthias Beller 《Tetrahedron letters》2007,48(36):6339-6342
A new selective and easily manageable epoxidation method is presented using an inexpensive and efficient FeCl3·6H2O and imidazole derivatives as catalysts. Aqueous hydrogen peroxide as an environmentally benign oxidant is utilized. This novel Fe/imidazole system gives moderate to excellent yields toward aromatic mono-, di-, and tri-substituted olefins. 相似文献
455.
Shultz DA Fico RM Lee H Kampf JW Kirschbaum K Pinkerton AA Boyle PD 《Journal of the American Chemical Society》2003,125(50):15426-15432
The molecular structures and magnetic properties of six dinitroxide biradicals are described. Five of the dinitroxides are trimethylenemethane-type (TMM-type) biradicals; that is, the intramolecular exchange parameter, J, is modulated by a carbon-carbon double bond. However, the efficacy of the carbon-carbon double bond as an exchange coupler is determined by the molecular conformation. Our results show that the exchange parameters correlate with phenyl-ring torsion angles (phi) via a simple Karplus-Conroy-type relation: J = 44 cos(2) phi - 17. Comparison of these results to those obtained for our isostructural series of bis(semiquinone) biradicals shows that both the magnitude of J and the resistance of ferromagnetic J to bond torsions is proportional to the spin density adjacent to the exchange coupler. 相似文献
456.
Siedle AR Webb RJ Behr FE Newmark RA Weil DA Erickson K Naujok R Brostrom M Mueller M Chou SH Young VG 《Inorganic chemistry》2003,42(4):932-934
Triazapentadienides, C(3)F(7)-C(=NR)-N=C(NHR)-C(3)F(7), result from the reaction of primary amines RNH(2) with the fluorinated imine C(3)F(7)-CF=N-C(4)F(9). The aniline derivative (R = Ph) is a weak monoprotic acid in dmso. Its conjugate base exhibits an extensive coordination chemistry. It acts as a bidentate ligand toward the molecular fragments Pd(C(3)H(5)), Rh(c-C(8)H(12)), Ir(c-C(8)H(12)), and Rh(CO)(2). The chelates [C(3)F(7)-C(NPh)-N-C(NPh)-C(3)F(7)](2)M, M = Mg, Mn, Fe, Co, Ni, Cu, Zn, and Pd, were prepared. In the crystallographically characterized Co complex, the metal is 3d(7), S = (3)/(2) and tetrahedrally coordinated. Spin densities at carbon in the C(6)H(5) and C(3)F(7) groups were estimated from the (1)H and (19)F contact shifts. Spin delocalization onto phenyl sp(2) carbons is approximately 10 times greater than onto the fluorinated sp(3) carbons. 相似文献
457.
Mixtures of carbonyl compounds were chromatographed on Dowex-I columns in the bisulfite form. Sodium bisulfite solutions of increasing concentrations were used as eluants. The studied compounds, including acetaldehyde, formaldehyde, 5-hydroxymethylfurfural, furfural and vanillin, were separated completely from each other. The eluate was also analysed quantitatively. 相似文献
458.
Herrmann KA Kuppannan K Wysocki VH 《International journal of mass spectrometry》2006,(249-250):93-105
Doubly-protonated bradykinin (RPPGFSPFR) and an angiotensin III analogue (RVYIFPF) were subjected to hydrogen/deuterium (H/D) exchange with CD(3)OD in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. A bimodal distribution of deuterium incorporation was present for bradykinin after H/D exchange for 90 s at a CD(3)OD pressure of 4 x 10(-7) Torr, indicating the existence of at least two distinct populations. Bradykinin ion populations corresponding to 0-2 and 5-11 deuteriums (i.e., D(0), D(1), D(2), D(5), D(6), D(7), D(8), D(9), D(10), and D(11)) were each monoisotopically selected and fragmented via sustained off-resonance irradiation (SORI) collision-induced dissociation (CID). The D(0)-D(2) ion populations, which correspond to the slower exchanging population, consistently require lower SORI amplitude to achieve a similar precursor ion survival yield as the faster-reacting (D(5)-D(11)) populations. These results demonstrate that conformation/protonation motif has an effect on fragmentation efficiency for bradykinin. Also, the partitioning of the deuterium atoms into fragment ions suggests that the C-terminal arginine residue exchanges more rapidly than the N-terminal arginine. Total deuterium incorporation in the b(1)/y(8) and b(2)/y(7) ion pairs matches very closely the theoretical values for all ion populations studied, indicating that the ions of a complementary pair are likely formed during the same fragmentation event, or that no scrambling occurs upon SORI. Deuterium incorporation into the y(1)/a(8) pseudo-ion pair does not closely match the expected theoretical values. The other peptide, doubly-protonated RVYIFPF, has a trimodal distribution of deuterium incorporation upon H/D exchange with CD(3)OD at a pressure of 1 x 10(-7) Torr for 600 s, indicating at least three distinct ion populations. After 90 s of H/D exchange where at least two distinct populations are detected, the D(0)-D(7) ion populations were monoisotopically selected and fragmented via SORI-CID over a range of SORI amplitudes. The precursor ion survival yield as a function of SORI amplitude falls into two distinct behaviors corresponding to slower- and faster-reacting ion populations. The slower-reacting population requires larger SORI amplitudes to achieve the same precursor ion survival yield as the faster exchanging population. Total deuterium incorporation into the y(2)/b(5) ion pairs matches closely the theoretical values over all ion populations and SORI amplitudes studied. This result indicates the y(2) and b(5) ions are likely formed by the same mechanism over the SORI amplitudes studied. 相似文献
459.
460.
Many solid state systems can be modelled by means of packings of hard spheres of variable size. Therefore the spatial-statistical analysis of the geometrical structure of such packings is of great scientific interest. The present paper starts with the application of classical characteristics such as packing fraction, coordination number and pair correlation function for the characterization of sphere packings. Then, the application of tessellation-based methods follows, which includes the analysis of correlations between cell face and coordination numbers. Finally, a search method for crystalline sub-structures is presented. 相似文献