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91.
The equilibrium energetics and the kinetics of cis-trans isomerization of some bis(dialkylsulfide)dihaloplatinum(II) complexes have been examined by 1H-NMR. spectroscopy. The isomers are stable in chloroform but each form isomerizes to an equilibrium mixture when free dialkylsulfide is added. The cis to trans process is endothermic and the position of the equilibrium is markedly dependent on the nature of the donor atoms and of the solvent. The rate of isomerization of Pt(Me2S)2Cl2 is first order in complex and in Me2S. The isomerization proceeds by a double displacement mechanism as it is shown that the tris(dimethylsulfide)chloroplatinum(II) cation is an isolable intermediate of the reaction. When free Me2S is added to trans-Pd(Me2S)2Cl2, isomerization does not occur and one observes instead a fast ligand exchange. Its mechanism is the usual associative one for substitutions in square planar d8complexes. 相似文献
92.
Tula Alev-Behmoaras Jean-Jacques Toulme Claude Hélène 《Photochemistry and photobiology》1979,30(5):533-539
Abstract— In order to test the ability of phosphate groups to quench the tyrosine fluorescence in nucleic acid-protein complexes, we have studied the effect of several phosphate ions on the fluorescence of tyrosine derivatives. Mono and bianions (H2 PO4 and HPO4 2– ) which are good proton acceptors quenched the fluorescence of all the phenolic compounds studied except that of O -methyl tyrosine. With the other derivatives (tyrosine, N -acetyl tyrosinamide and lysyl-tyrosyl-α lysine) fluorescence inhibition was accompanied by the appearance of a long wavelength emission (345 nm) attributed to tyrosinate anions. The quenching of tyrosine emission was due to the deprotonation of the phenolic group promoted in the excited state by phosphate ions and leading to the weakly fluorescent tyrosinate ion. Mono and dianions of phosphate mono ester inhibited tyrosine fluorescence as did unesterified phosphates. However, phosphate diester did not have any effect on the fluorescence of tyrosine derivatives. We conclude from this study that in nucleic acid-protein complexes phosphate groups are not able to quench tyrosine fluorescence except at the end of polynucleotide chains. Since monoester and diester monoanions have a different behavior, we propose that quenching of tyrosine fluorescence by monoanions requires the formation of two hydrogen bonds. This complex cannot form with diesters which consequently do not quench tyrosine fluorescence. 相似文献
93.
The oscillator strengths of the symmetry-forbidden transitions of the ion [PdCl4]2– were calculated by a method based on the evaluation of the MO of the distorted ion. It is not very likely that the transition to1
B
1g
(a
1g
(d
z
2)b
1g
) state, weak, mainlyz polarized, explains the band that appears as a shoulder observed towards 30 000 cm–1 in the absorption spectrum. If this band is specific of the ion [PdCl4]2–, the assignment to the forbidden charge-transfer transition,1
A
1g
1
B
2g
, is more plausible. 相似文献
94.
β-ketonitriles R1COCH2CN and R1COCH(R2)CN are respectively prepared from (CH3)3SiOCOCHLiCN or R2CHLiCN by acylation reaction with mixed anhydrides RCOOCO2Et. 相似文献
95.
Gérard Hopfgartner Claude Piguet Jack D. Henion 《Journal of the American Society for Mass Spectrometry》1994,5(8):748-756
Double-helical [M2L2] n+, triple-helical [M2L3] n+, and toroidal [M3L3] n+ (M = Cu, Co, Fe, Ni, La, Eu, Gd, Tb, or Lu) supramolecular complexes have been fully characterized by ion spray mass spectrometry (IS-MS). The IS-MS spectra from pure acetonitrile solutions reflect the nature of the cations present in solution with conservation of the charge state and allow an efficient qualitative speciation of the compounds. The mass spectrometry results can be correlated with other powerful techniques (nuclear magnetic resonance and electronic spectroscopy) for the characterization of supramolecular complexes in solution, Structural information is obtained by collision-induced dissociation, which strongly depends on the metal ions used in the supramolecular complexes and on the various connectivities and topologies of the ligands. When the ligand contains 3,5dimethoxybenzyl groups bound to the benzimidazole rings, the partial fragmentation of the complexes is associated with a decrease of the total charge of the complexes and the appearance of the characteristic fragment at m/z 151 that corresponds to the 3,5-dimethoxybenzyl cation. A detailed analysis of the fragmentation pathways of these supramolecular complexes suggests that the metal-nitrogen coordination bonds are very strong in the gas phase. 相似文献
96.
Micrometric BN powders used as catalyst support: influence of the precursor on the properties of the BN ceramic 总被引:1,自引:0,他引:1
Thin powders and foams of boron nitride have been prepared from molecular precursors for use as noble metal supports in the catalytic conversion of methane. Different precursors originating from borazines have been tested. The best results were obtained using a precursor derived from trichloroborazine (TCB) which, after reacting with ammonia at room temperature and then thermolyzing up to 1800°C, led to BN powders with a specific area of more than 300 m2 g−1 and a micrometric spherical texture. Comparable results were obtained using polyborazylene under similar conditions. Aminoborazine-derived precursors did not yield such high specific area ceramics but the BN microstructure resembled a foam with a crystallized skin and amorphous internal part. These differences were related to the chemical mechanism of the conversion of the precursor into BN. Polyhaloborazines and polyborazines yielded BN through gas-solid reactions whereas aminoborazine polymers could be kept waxy up to high temperatures, which favored the glassy foam. Catalysts composed of BN support and platinum have been prepared using two routes: from a mixture of precursor or by impregnation of a BN powder leading to very different catalysts. 相似文献
97.
Claude Delseth Luhata Tolela Carl Djerassi Paul J. Scheuer Robert J. Wells 《Helvetica chimica acta》1979,62(1):101-109
The steroidal components of 2 marine sponges, Terpios zeteki (from Hawaii) and Dysidea herbacea (from Australia) were fractionated through a combination of chromatographic methods, including reversed phase HPLC., and were analyzed by a combination of physical methods, including high resolution GC.-MS. and 360 MHz 1H-NMR. T. zeteki contains 6 conventional 5α-stanols which comprise 91% of the sterol mixture, and traces (0.5%) of a new C26 sterol, 5α-24-norcholestan-3β-ol. Minor amounts of conventional Δ5-sterols (6.5%) and of a single Δ4-3-ketosteroid (1.5%) were also present. In contrast, the Australian sponge (D. herbacea) contains 3 Δ5,7-sterols which comprise 1.5% of the sterol mixture, and one new C29 sterol, (24 Z)-stigmasta-5,7,24(28)-trien-3β-ol, as the major component (75%). In addition, minor amounts of conventional 5α-stanols (0.5%), Δ5-sterols (5%) and 5α-Δ7-sterols (18%) were present in this complex sterol mixture. The possible dietary or endosymbiotic origins of these sterols are discussed. 相似文献
98.
99.
100.
Christian Bellec Daniel Bertin Roger Colau StPhane Deswarte Pierre Maitte Claude Viel 《Journal of heterocyclic chemistry》1979,16(8):1657-1659
Nitration of α-acylstilbenes with dinitrogen tetroxide leads to corresponding Z-α-acyl-β-nitrostilbenes. By chemical or electochemical reduction of these compounds, trisubstituted isoxazoles were prepared in good yields. 相似文献