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1.
The activity concentrations of β-emitters including 35S (0.15 MeV) and presumably also 14C (0.14 MeV) may be determined with satisfactory accuracy using an end-window G.M. counter. For single preparations (if weighed), the described procedure warrants that the standard error will not exceed 0.8%, if the amount of substrate is kept below 0.2 mg/cm2, and if carrier-free solutions are avoided. With a minimum of 4 preparations per sample solution, the error of the mean should not exceed 0.4%, 相似文献
2.
《Tetrahedron》1986,42(4):1063-1070
The observable cyclodecyl cation is established as having a 1,5-μ-hydrido-bridged structure and most probably exists as a mixture of conformational isomers. This solution carbocation is unique in organic chemistry in that it evolves molecular H2 at temperatures below -100°C to give the 9-decalyl cation. The mechanism of this reaction is postulated to involve a pentacoordinated carbocation intermediate or transition state with a structure identical to what one would get by protonating the bridgehead C-H bond of decalin. Since conversion of decalin to the decalyl cation in superacids is not accompanied by significant H2 production, the mechanism of these two reactions must be different, and in the latter case could not involve a protonated pentacoordinate carbocation intermediate, as has been postulated. 相似文献
3.
《Journal of Molecular Catalysis #》1992,71(2):261-278
The substrate specificity of some modified α-chymotrypsin derivatives in the synthesis of peptides, using natural and synthetic acyl donors and several nucleophiles, is studied. A systematic study of the alteration of active sites by the modification method is carried out. The method used in the present paper consists of covalent bonding of external lysines of the enzyme with a hydrophobic polymer. Enzymes with several degrees of modification have been obtained. These derivatives present different hydrophobic characteristics, depending on the degree of modification. All the modified enzymes are active in hydrophobic solvents such as benzene, toluene or 1,1,1-trichloroethane but need a small amount of water to be activated (< 1.0%). High yields are obtained in the synthesis of peptidic bonds (100% > × > 60%). The modified enzyme has the same stereospecificity as the native enzyme, because only esters with L-configuration in the α-carbon are accepted as acyl donors. Nevertheless a greater number of acyl donors are accepted by the modified enzyme than by the native α-chymotrypsin, in the synthesis of peptides. On the other hand, the nucleophile subsite is more lipophilic in the modified enzyme than in the native α-chymotrypsin. 相似文献
4.
《Tetrahedron letters》1986,27(41):5007-5010
aldehydes and ketones will add to acrylonitrile, acrylate esters, acrolein and α, β, -enones with the formation of a range of 2(X)-propen-1-ols (X= CN, COOR, COR). The reactions are catalysed by tertiary amines and are very sensitive to pressure. A wide variety of products may be obtained at pressures of 5 kbar or less whereas most of the reactions do not occur at atmospheric pressure. 相似文献
5.
《Thermochimica Acta》1987,122(1):123-133
Firstly, the behavior of droplets (Φ ≈ 1μm) of aqueous saline solutions dispersed within an emulsifying medium and subjected to steady cooling and heating is described. Droplets undergo freezing around a temperature T1(x) and partial ice melting and total salt melting at the eutectic temperature TE. This melting is followed by progressive melting of the remaining ice which ceases when the equilibrium temperature (Te(x)) ice ⇆ solution is reached. Between Te and T1 the droplets are undercooled. Secondly, the results obtained when water crystallization occurs versus time at a fixed temperature C, such as T1(x) < > C < Te(x) are reported. During heating following crystallization at ΘC, an unusual ice melting at 0° and/or ice melting ending at T & >; Te(x) is noticed on the thermogram obtained by differential scanning calorimetry of the emulsion. This shows that pure ice or at all events less concentrated solutions must be present within the emulsion. A possible mechanism of crystallization at ΘC is proposed. 相似文献
6.
Aspects of the radiation-induced polymerization of methacrylic acid in the crystalline state have been investigated and utilized to evaluate the mechanism of polymerization. In particular, results for post-irradiation polymerization at 0°C after radiation doses of 0.1 to 2.0 Mrad support the concepts of Lando and Semen that chain initiation essentially all commences at the same time, that chain propagation continues without termination, and that termination of nonpropagating radicals proceeds simultaneously. 相似文献
7.
Franco Pavese 《The Journal of chemical thermodynamics》2012,44(1):179-180
These are comments to a note from J. Fischer in publication. 相似文献
8.
Sorption of heavy metal (copper and strontium) ions by the meadow chernozem and grey forest soils differing in the content of organic substance was described using the Langmuir equation. The analysis of characteristic sorption coefficients demonstrated that copper possesses the enhanced affinity for the studied soil samples compared to strontium. Maximal values of the sorption of copper (0.085 mmol/g) and strontium (0.045 mmol/g) obtained for the colloidal fraction of meadow chernozem soil (particle diameter d < 1 m) are approximately 1.5 times larger than for the same fraction of grey forest soil; this ratio remains almost the same even when using the coarser fraction (d < 50 m). It was established that up to 90% of metal ions could be present in the soil in an immobile form. An increase in the concentration of heavy metal ions in the soil causes their redistribution between the components of absorbing soil complex and an increase in the fraction of metal in mobile forms (water-soluble, exchange, soluble in weakly acidic medium). Upon the microbiological treatment (bioleaching in the suspension variant) of the soils containing copper or strontium ions, the total content of metal, including its mobile forms, decreases by an order of magnitude. 相似文献
9.
《Tetrahedron: Asymmetry》1998,9(16):2939-2945
Resolution of the enantiomers of the sympathomimetic drug tetrahydrozoline was obtained by chiral HPLC. The isolated enantiomers racemize easily and chiral HPLC experiments allowed the determination of the racemization rate constant. This process occurs via an imine–enamine tautomerism which was studied by UV and 1H NMR spectroscopies. 相似文献
10.
《中国化学》1993,(5)
Arteannuic acid and(-)11R,13-d ydroarteannuic acid are the key intermediates in thebiosynthesis of arteannuin by the leaf homogenate of Artemisia annua L.,and arteannuic acid andepoxyarteannuic acid are the intermediates of arteannuin B.and epoxyarteannuic acid can not betransformed into arteannuin by the homogenate. 相似文献
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12.
Eucommia Ulmoides (EU) gum will crystallize easily at room temperature due to the regular trans-1,4-polyisoprene (TPI) molecular structure. Around the skeleton stretching vibration band of TPI, its FTIR spectrum showed 5 weak crystalline peaks decrease as temperature increase, and there are also two amorphous peaks (845 cm−1 and 668 cm−1) increase as temperature increase. According to this phenomenon, Variable Temperature Fourier Transform Infrared Spectrum (VTFTIR) and Variable Temperature Wide Angle X-ray Diffraction (VTWAXD) have been taken to analyze the degree of crystallinity of EU gum. In addition to the ratio of crystalline zone, the result determined by VTFTIR also involve the mesomorphic zone, which make it higher than the result determined by VTWAXD. Through the comparative analysis of VTFTIR and VTWAXD, it has been proved that the ratio of crystalline zone, mesomorphic zone, amorphous zone in EU gum were respectively 39.8%, 27.6%, 32.6% at room temperature. Based on the Density Measurement and the three-phase model theory, the density gradient function of the mesomorphic zone of EU gum was firstly proposed. Below 60 °C, the density of the mesomorphic does not change with the temperature, approximately 0.931g/cm−3. Above 60 °C, the density of the mesomorphic decreases with the temperature increasing, and finally drops to 0.905g/cm−3 at 80 °C, which is equal to the density of the complete amorphous zone. 相似文献
13.
Jan Dillen 《International journal of quantum chemistry》2013,113(18):2143-2153
The computed electron density of several congested saturated hydrocarbons and halogenated derivatives has been analyzed by the method of interacting quantum atoms (IQA). For all the molecules studied, the calculations show the existence of a bond path between the congested atoms and which, according to the Quantum Theory of Atoms in Molecules, indicates that there is a stabilizing interaction between these atoms. The bond path is found to exist up to interatomic distances well‐beyond the sum of the van der Waals radii. The IQA results indicate that steric hindrance is not a repulsive force between the congested atoms but that is the result of an increase in the intra‐atomic or self‐energy of the congested atoms. This increase in self‐energy is caused by the deformation of the atomic basin of the congested atoms. © 2013 Wiley Periodicals, Inc. 相似文献
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15.
Daniela Chromcová 《European Polymer Journal》2008,44(6):1733-1742
Polyesteramides derived from the cyclic monomers ε-caprolactam and ε-caprolactone have been synthesized by the anionic polymerization and characterized. ε-Caprolactam magnesium bromide was employed as unique initiator giving high yield of copolymer across the whole range of monomer concentrations at 150 °C. Owing to a variation of the content of ester and amide component in polyesteramide backbone a large variety of materials having diverse physico-chemical and mechanical properties have been prepared. Materials prepared were examined for mechanical characteristic. Thermal properties were characterized using DSC, temperature modulated DSC, Hi-ResTM TGA and DMA. Viscosity measurements were employed for molecular characterization. 相似文献
16.
《International Journal for Radiation Physics and Chemistry》1976,8(4):497-501
The broad line N.M.R. spectra of irradiated and unirradiated high density bulk (HD) and single crystals (SC) of polyethylene were found to give rise to two components, e.g. broad and narrow lines, attributed to crystalline and amorphous regions, respectively. An increase in the line width of the narrow component with radiation dose is more marked in bulk HD specimens, and is ascribed to crosslinking in the radiation-sensitive amorphous region; whereas no such increase is apparent for the SC specimen. Though there seems to be a slight increase in the second moment of the total absorption curve with radiation up to about 200 Mrad, both the second moment and the line width of the broad component was found to decrease with radiation dose for both the samples. This indicates an increased chain mobility in the crystalline regions and is considered to be due to the destruction of regularity in the crystal lattices by the combined processes of crosslinking (or endlinking) in the amorphous regions and the formation of lattice defects and imperfections by the ionizing radiation. A relatively faster decrease of line width with radiation dose above about 1000 Mrad may be attributed to the predominance of the latter process at high doses. 相似文献
17.
《Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy》1994,50(10):1789-1790
The Scott equation has been employed to determine ε for p-dinitrobenzene charge transfer complexes with anilines. It is seen that the Benesi—Hildebrand plots show a random scatter, whereas the Scott plots are satisfactory straight lines. 相似文献
18.
Ahmed M. Zaher Ahmad M. Moharram Richard Davis Peter Panizzi Makboul A. Makboul 《Natural product research》2015,29(24):2275-2281
Botryodiplodia theobromae Pat. belongs to the endophytic fungi that live within the tissues of medicinal plants and produce bioactive natural products. The endophyte was isolated from the leaves of Dracaena draco L. The LC–MS-based metabolite fingerprinting of the ethyl acetate extract of B. theobromae with antibacterial activity led to the identification of 13 metabolites pertaining to various classes: dipeptides (maculosin and L,L-cyclo(leucylprolyl), alkaloid (norharman), coumarin and isocoumarins (bergapten, meranzin and monocerin), sesquiterpene (dihydrocumambrin A), aldehyde (formyl indanone), fatty alcohol (halaminol A) and fatty acid amide (palmitoleamide, palmitamide, capsi-amide and oleamide). This study reports for the first time, the LC–MS and LC–MS/MS identification of 13 known bioactive metabolites from the antibacterial ethyl acetate extract of B.theobromae isolated from the leaves of D. draco L. 相似文献
19.
JingCHEN ChenXinCAI 《中国化学快报》2004,15(7):813-816
It is reported that the direct electron transfer of hemoglobin (Hb) can be effectively promoted by carbon nanotubes when Hb was immobilized on the surface of the carbon nanotubes modified electrode. The results indicated that the conversion of Hb-Fe(Ⅲ)/Hb-Fe(Ⅱ) is a one-electron coupled one-proton reaction process. The method presented can be easily extended to study the direct electrochemistry of other proteins or enzymes, 相似文献