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排序方式: 共有750条查询结果,搜索用时 46 毫秒
101.
Haisong Qi Tim Liebert Frank Meister Lina Zhang Thomas Heinze 《Macromolecular Symposia》2010,294(2):125-132
In this work, the carboxymethylation of cellulose in a new alkaline cellulose solvent, LiOH/urea aqueous solution, was investigated. Carboxymethyl cellulose (CMC) samples were characterized with FT-IR, NMR, HPLC, and viscosity measurements. Water-soluble CMC with DS = 0.36∼0.65 was prepared, from both Avicel cellulose and cotton linters in the LiOH/urea system. The total DS of CMC could be controlled by varying the molar ratio of reagents and the reaction temperature. The results from structure analysis by HPLC after complete depolymerization showed that the mole fractions of the different carboxymethylated repeating units as well as those of unmodified glucose follow a simple statistic pattern. A distribution of the carboxymethyl groups of the AGU was determined to be in the order O-6 > O-2 > O-3 position at the level. 相似文献
102.
G. Schmidt 《Isotopes in environmental and health studies》2013,49(3):185-188
Abstract The increasing application of 13C-labelled urea in medicine requires simple and reasonable methods for measuring highly enriched 13C in urea. The combination: ultimate organic analysis—mass spectrometry so far prescribed is complicated and expensive. For medical diagnosis, however, isotope selective nondispersive infrared spectrometers (NDIRS) have been available for many years. One of these tools is FANci2 which is very reasonable and easily to be operated. By means of such devices also urea highly enriched in 13C can be analysed, provided that the samples are first diluted with a defined amount of urea of natural isotopic composition and then transformed into carbon dioxide by means of urease. The relative abundance of 13C in this carbon dioxide, measured by nondispersive infrared spectrometry, is then a measure of the 13C abundance in the initial urea sample. Comparison of results of such measurements with those attained by mass spectrometry proves that this procedure is feasible and yields precise results. 相似文献
103.
In der Arbeit wird untersucht, unter welchen analytischen Bedingungen eine quantitative Sticksoffbestimmung mit dem 15 N-Analysator NA-5A mittels Isotopenverdünnung durchzuführen ist. Es wird auf die Berechnung der N-Menge, die Abschätzung des maximalen Fehlers unter den speziellen Verhāltnissen der 15N-Analysator-Anwendung und die Auswertungsverfahren näher eingegangen. Testanalysen an unterschiedlich markierten Modell-Lösungen, bestehendaus Ammonium-, Harnstoff- und Aminosäurestickstoff, ergaben gute Übereinstimmung mit den vorgegebenen Werten. Das Verfahren wurde zur N-Bestimmung von biologish-medizinischem Material eingesetzt. Ein Arbeitsschema zur quantitativan Bestimmung von Ammonium-N, Harnstoff-N und Gesamt-N direkt in Harn wird mitgeteilt. 相似文献
104.
János Mink László Hajba Imre Pápai Judith Mihály Csaba Neméth Mikhail Yu. Skripkin 《应用光谱学评论》2013,48(4):274-326
Abstract Mid-infrared, far-infrared, and Raman vibrational spectroscopic studies were combined with density functional theory (DFT) calculations and normal coordinate force field analyses for N,N′-dimethylurea (DMU), N,N,N′,N′-tetramethylurea (TMU), and N,N′-dimethylpropyleneurea (DMPU: IUPAC name 1,3-dimethyltetrahydropyrimidin-2(1H)-one). The equilibrium molecular geometry of DMU (all three conformers), TMU, and DMPU and the frequencies, intensities, and depolarization ratios of their fundamental infrared (IR) and Raman vibrational transitions were obtained by DFT calculations. The vibrational spectra were fully analyzed by normal coordinate methods as well. A starting force field for DMPU was obtained by adapting corresponding force constants for DMU and TMU, resulting after refinements in the stretching force constants C=O (7.69, 7.30, 7.68 N·cm?1), C–N (5.16, 5.55, 5.05 N·cm?1), and C-Me (5.93, 4.00, 4.22 N·cm?1) for DMU, TMU, and DMPU, respectively. The dominating conformer of liquid DMU was identified as trans-trans, strong intermolecular hydrogen bonding was verified in solid DMU, and weak dipole–dipole association was found in liquid TMU and in DMPU. Special attention was paid to analyzing the methyl group frequencies, which revealed deviations from local C3v symmetry. A linear correlation was found between the CH stretching force constants and the inverse of the CH bond lengths (1/r 2). The averaged NH stretching frequencies of gaseous, dissolved, and solid urea and of DMU, with variations for hydrogen bonding of different strength, are linearly correlated to the NH stretching force constants. Characteristic skeletal vibrations were assigned for a broad variety of urea derivatives and also for pyrimidine derivatives, which all contain the N2C=O entity. The very strong IR bands of C=O stretching (1,676 ± 40 cm?1) and asymmetric CN2 stretching (1,478 ± 60 cm?1), and the very intense Raman feature of symmetric CN2 stretching or ring breathing (757 ± 80 cm?1), can be recognized as fingerprint bands also for the pyrimidine derivatives cytosine, thymine, and uracil, which all are nucleobases in DNA and RNA nucleotides. 相似文献
105.
Robert Szczsny Tuan K. A. Hoang Liliana Dobrzaska Duncan H. Gregory 《Molecules (Basel, Switzerland)》2021,26(16)
Herein we describe an alternative strategy to achieve the preparation of nanoscale Cu3N. Copper(II) oxide/hydroxide nanopowder precursors were successfully fabricated by solution methods. Ammonolysis of the oxidic precursors can be achieved essentially pseudomorphically to produce either unsupported or supported nanoparticles of the nitride. Hence, Cu3N particles with diverse morphologies were synthesized from oxygen-containing precursors in two-step processes combining solvothermal and solid−gas ammonolysis stages. The single-phase hydroxochloride precursor, Cu2(OH)3Cl was prepared by solution-state synthesis from CuCl2·2H2O and urea, crystallising with the atacamite structure. Alternative precursors, CuO and Cu(OH)2, were obtained after subsequent treatment of Cu2(OH)3Cl with NaOH solution. Cu3N, in the form of micro- and nanorods, was the sole product formed from ammonolysis using either CuO or Cu(OH)2. Conversely, the ammonolysis of dicopper trihydroxide chloride resulted in two-phase mixtures of Cu3N and the monoamine, Cu(NH3)Cl under similar experimental conditions. Importantly, this pathway is applicable to afford composite materials by incorporating substrates or matrices that are resistant to ammoniation at relatively low temperatures (ca. 300 °C). We present preliminary evidence that Cu3N/SiO2 nanocomposites (up to ca. 5 wt.% Cu3N supported on SiO2) could be prepared from CuCl2·2H2O and urea starting materials following similar reaction steps. Evidence suggests that in this case Cu3N nanoparticles are confined within the porous SiO2 matrix. 相似文献
106.
Abstract The crystal structure of N′-(6-chloropyridin-3-formyl)-N-t-butyl urea, C11H14ClN3O2, has been established, and which belongs to monoclinic crystal system, space group Pn with unit cell dimensions a = 9.335(3) ?, b = 12.715(3) ?, c = 21.813(6) ?, β = 95.417(6)°, V = 2577.6(12) ?3 and Z = 8. An intramolecular N–H···O hydrogen bond forms a six-membered ring in the central part of the molecule. Both two N–H
atoms of single unit participate in intermolecular hydrogen bonds and an intramolecular hydrogen bond, respectively.
Index Abstract The crystal structure ofN′-(6-chloropyridin-3-formyl)-N-t-butyl urea, C11H14ClN3O2, has been established, and which belongs to monoclinic crystal system withZ = 8, space group Pn.
相似文献
107.
An unprecedented base promoted domino approach has been developed for the synthesis of pyridin-2-yl urea derivatives via the reaction of 2-aminopyridinium salts and arylamines. The developed strategy tolerated a wide range of functional groups and afforded pyridin-2-yl ureas in moderate to good yields. The reaction was postulated to involve tandem cyclization, intermolecular nucleophilic addition, ring opening, and demethylation. 相似文献
108.
Silver coated ZnO nanorods and nanoflakes with different crystallographic orientations were synthesized by a combination of sputter deposition and solution growth process. Catalytic properties of morphology‐dependent Ag/ZnO nanostructures were then investigated for urea sensors without enzyme. Ag/ZnO nanorods on carbon electrodes exhibit a higher catalytic activity and an improved efficiency than Ag/ZnO nanoflakes on carbon electrodes. Ag/ZnO nanorod catalysts with more electrochemically surface area (169 cm2 mg?1) on carbon electrode facilitate urea electrooxidation due to easier electron transfer, which further promotes the urea electrolysis. The Ag/ZnO nanorod catalysts also show a significant reduction in the onset voltage (0.410 V vs. Ag/AgCl) and an increase in the current density (12.0 mA cm?2 mg?1) at 0.55 V vs Ag/AgCl. The results on urea electrooxidation show that Ag/ZnO nanostructures can be a potential catalyst for non‐enzymatic biosensors and fuel cells. 相似文献
109.
在B3LYP/6.31 G(2DF,2PD)水平优化气相中的尿素和硫脲分子构型,得到C2点群的非平面分子.对此构型的尿素和硫脲分子用CPHF和FF方法进行了非线性光学性质的计算,比较了尿素和硫脲非线性光学性质,讨论了极化函数、弥散函数和相关效应对(超)极化率的影响,还与固定C2v对称相同水平优化得到的平面分子计算结果进行了比较. 相似文献
110.
The diagnosis of Helicobacter pylori (H. pylori) infection by GC-MS detection of the (13)CO(2) enrichment in (13)C-urea breath test ((13)C-UBT) samples is reported. This study aimed to optimize the (13)C-UBT with regards to the diagnostic cut-off value, sampling time, and frequency. The H. pylori status of 103 dyspeptic patients was obtained by histological examination, the rapid urease test as well as with the GC-MS (13)C-UBT. Analytical and diagnostic accuracies were determined by comparison of the GC-MS (13)C-UBT results with that of the analytical and diagnostic gold standards, namely GC-isotope ratio MS (IRMS) and histology. The (13)CO(2) enrichment values obtained with GC-MS analysis, correlated favorably (r(2) = 0.993) with those obtained by GC-IRMS analysis. When compared to histology, the GC-MS (13)C-UBT had a diagnostic sensitivity of 92% and a specificity of 93%. The positive predictive value (PPV), negative predictive value (NPV), and accuracy were 95, 89, and 92%, respectively. It was concluded that SIM GC-MS is capable of analyzing nonradioactive (13)C-UBT samples, with a precision and accuracy sufficient to distinguish between H. pylori positive and negative patients. 相似文献