首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   210篇
  免费   3篇
化学   151篇
力学   14篇
数学   10篇
物理学   38篇
  2022年   7篇
  2021年   14篇
  2020年   9篇
  2019年   7篇
  2018年   10篇
  2017年   6篇
  2016年   7篇
  2015年   7篇
  2014年   3篇
  2013年   27篇
  2012年   8篇
  2011年   11篇
  2010年   4篇
  2009年   8篇
  2008年   5篇
  2007年   6篇
  2005年   4篇
  2003年   2篇
  2002年   4篇
  2001年   1篇
  2000年   2篇
  1999年   1篇
  1998年   3篇
  1997年   1篇
  1996年   1篇
  1994年   1篇
  1993年   1篇
  1992年   4篇
  1991年   1篇
  1989年   2篇
  1987年   1篇
  1986年   4篇
  1985年   2篇
  1984年   2篇
  1983年   1篇
  1982年   1篇
  1981年   3篇
  1980年   3篇
  1979年   5篇
  1978年   4篇
  1977年   2篇
  1976年   4篇
  1974年   1篇
  1973年   1篇
  1972年   1篇
  1968年   2篇
  1967年   1篇
  1960年   2篇
  1959年   3篇
  1958年   1篇
排序方式: 共有213条查询结果,搜索用时 218 毫秒
61.
Polyethylene acrylic acid copolymer (PEAA) was modified with different types of alkanols and hydroxybenzoic acids to produce three types of esterified grafts. The molecular structures of the synthesized dispersants were performed using 1H NMR analysis. The molecular weights of the synthesized dispersants were determined by GPC technique. The activity of PEAA derivatives as asphaltene stabilizer in the crude oil was evaluated by means of flocculation measurements. The effect of asphaltenes on the rheological behavior of highly asphaltenic crude oils was studied to evaluate the transportation parameters of crude oils. The rheological properties were measured at different temperature ranges of 5°C to 25°C in presence and absence of PEAA additives. It was observed that the PEAA having side chain lengths below C16 show non‐Newtonian pseudoplastic relationships at temperatures from 25°C to 5°C. While, PEAA additives having side chain lengths equal or above C16 show Newtonian relationship even at low temperature (5°C) for all tested crude oils.  相似文献   
62.
Ahmad Farouk Eweas 《合成通讯》2013,43(10):1541-1552
The ring‐closure metathesis of the diene (2S,3R,4S)‐1‐(tert‐butyldiphenylsilyloxy)‐2,4‐dimethylhex‐5‐en‐3‐yl acrylate produced the dihydropyrone with the correct stereochemistry for Soraphen A synthesis. The C2,C3 stereocenters were introduced by the addition of the (Z)‐crotyl‐n‐butylstannane to the β‐alkoxyaldehyde(S)‐3‐(benzyloxy)‐2‐methylpropanal in presence of TiCl4 as a chelating catalyst to give the desired anti,syn homoallyic alcohol (2S,3R,4S)‐1‐(tert‐butyldiphenylsilyloxy)‐2,4‐dimethylhex‐5‐en‐3‐ol.  相似文献   
63.
Palladium(II) and platinum(II) complexes of N-ethyl-N′-pyrimidin-2-ylthiourea(HL1) and N-phenyl-N′-pyrimidin-2-ylthiourea (HL2) have been prepared, and the complexes [M(HL)Cl2], [Pt(L)2], [Pd(HL1)2]Cl2, and [Pd(L2)2] (where M = PdII or PtII) were characterized. The spectroscopic data are consistent with coordination of thioureas as neutral or monoanionic ligands to PdII and PtII through S and a pyrimidine-N. The IR spectra show shifts of CS and pyrimidine ring stretch bands to lower and higher frequencies, respectively. The 1H NMR spectra differentiate between H(4′) and H(6′) resonances and indicate downfield shifts for all protons of pyrimidine [H(4′), H(5′), and H(6′)], two resonances for two N?H protons for complexes containing the neutral ligand (HL), and only one N?H proton chemical shift for complexes containing the monoanion (L). 13C NMR chemical shifts of pyrimidine carbons are correlated with the type of bonding between PdII or PtII and pyrimidine-N. The magnetic susceptibilities suggest a diamagnetic planar structure for all complexes.

Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file.  相似文献   
64.
Abstract

N-Phenylmorpholine (1) reacted with chlorosulfonic acid to give the p-sulfonyl chloride (2), which was characterized as the sulfonamides (35). Benzothiazole (6) was converted into the sulfonyl chloride (7) by sequential treatment with hot chlorosulfonic acid and thionyl chloride. Reaction of (7) with amines afforded the derivatives (810); NMR spectral analysis of the dimethylamide (8) indicated that it was a mixture of the 4- and 7-isomers. Chlorosulfonation of 2-methylbenzothiazole (11) was achieved by heating with chlorosulfonic acid with or without thionyl chloride. The chloride (12) was converted into amides (1319). Study of the NMR spectra indicated that mixtures of the 5- and 6-isomers were formed. 2,4,5-Triphenyloxazole (20) reacted with chlorosulfonic acid to give either the mono-(21), bis (23) or bis-tris sulfonylchlorides (23, 34); these were converted into 14 sulfonamides. 2-(p-Nitrophenyl)-4,5-diphenyloxazole (41) reacted with hot chlorosulfonic acid to give the bis-sulfonyl chloride (42), characterized as the dimethylsulfonamide (43). Attempts to form the pure monosulfonyl chloride and to mono nitrate 2,4,5-triphenyloxazole (20) were unsuccessful.  相似文献   
65.
Novel β‐enaminonitrile of 1‐(6‐phenyl‐pyridazin‐3‐yl)‐pyrazole derivative 2 was formed using (6‐phenyl‐pyridazin‐3‐yl)‐hydrazine ( 1 ) and 2‐ethoxymethylene‐malononitrile. The β‐enaminonitrile derivative 2 was in turn used as precursors for the preparation of 1‐(6‐phenyl‐pyridazin‐3‐yl)‐pyrazoles ( 3 , 9 , 11 ), 1‐(6‐phenyl ‐pyridazin‐3‐yl)‐pyrazolo[3,4‐d]pyrimidines ( 4 , 5 , 6 , 7 , 8 , 13 , 14 , 15 , 16 ) and some of their corresponding N‐acyclic nucleosides ( 17 , 18 ). All synthesized compounds were tested for their antimicrobial evaluation, and compounds 3 , 9 , 17 , and 18 showed more significant activity than the other tested compounds and some known drugs (standers).  相似文献   
66.
Reactions of 3-aryl-1-phenyl-2-propen-1-ones IIa-d with ethyl phenylacetate (I) in the presence of sodium ethoxide under reflux for one hour gave only the corresponding 3,5-diaryl-2,4-dibenzoyl-6-phenyl-cyclo-hexanones IIIa-d. The reaction of these compounds with hydrazine hydrate afforded the corresponding 5-benzoyl-4,6-diaryl-3,7-diphenyl-1,4,5,6,7-pentahydro-1H-indazole IVa-d. The structures of the products were established by spectroscopic (ir, uv, nmr and mass spectra), single-crystal X-ray analysis and chemical evidence.  相似文献   
67.
This article discusses a chemical route to prepare new ethylene/propylene copolymers (EP) containing a terminal reactive group, such as ?‐CH3 and OH. The chemistry involves metallocene‐mediated ethylene/propylene copolymerization in the presence of a consecutive chain transfer agent—a mixture of hydrogen and styrene derivatives carrying a CH3 (p‐MS) or a silane‐protected OH (St‐OSi). The major challenge is to find suitable reaction conditions that can simultaneously carry out effective ethylene/propylene copolymerization and incorporation of the styrenic molecule (St‐f) at the polymer chain end, in other words, altering the St‐f incorporation mode from copolymerization to chain transfer. A systematic study was conducted to examine several metallocene catalyst systems and reaction conditions. Both [(C5Me4)SiMe2N(t‐Bu)]TiCl2 and rac‐Et(Ind)2ZrCl2, under certain H2 pressures, were found to be suitable catalyst systems to perform the combined task. A broad range of St‐f terminated EP copolymers (EP‐t‐p‐MS and EP‐t‐St‐OH), with various compositions and molecular weights, have been prepared with polymer molecular weight inversely proportional to the molar ratio of [St‐f]/[monomer]. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1858–1872, 2005  相似文献   
68.
The stability constants of UO 2 2+ -squarate complexes are measured at ionic strengths of 0.05M, 0.06M, 0.075M, 0.09M, 0.1M (squaric acid-perchloric acid) using a solvent extraction method at a pH of 1.1 and a temperature of 25 °C. The extractant used is dinonylnaphthalenesulfonic acid in n-heptane. The aqueous phase was made of a mixture of squaric and perchloric acid and233U radio tracer. The stability constants of squarate complexes of UO 2 2+ is seen to decrease linearly with the square root of the ionic strength.  相似文献   
69.
A highly selective and sensitive method was developed for simultaneous determination of the antihistaminic drug hydroxyzine (HZ) and its pharmacologically active metabolite cetirizine (CZ) in human serum using haloperidol as internal standard. The method was based on fluorescence labeling of both drugs with a fluorescent arylboronic acid 4-(4,5-diphenyl-1H-imidazol-2-yl)phenyl boronic acid followed by separation on silica column using a mobile phase consisting of acetonitrile and water (90:10, v/v%) containing triethylamine and acetic acid. The labeling reaction conditions were optimized and the liquid-liquid extraction method was successfully applied to extract the both drugs from serum. The linearity range was 0.025-2.00 microg/mL for HZ and CZ. The limit of detection (S/N = 3) was 10 and 5 ng/mL for HZ and CZ, respectively.  相似文献   
70.
Salsola cyclophylla, an edible halophyte, is traditionally used for inflammation and pain. To confirm the claimed anti-inflammatory and analgesic properties, a detailed study on respective pharmacological actions was undertaken. The activities are contemplated to arise from its phytoconstituents. The LC-MS analysis of S. cyclophylla 95% aqueous-ethanolic extract revealed the presence of 52 compounds belonging to phenols, flavonoids, coumarins, and aliphatics class. A high concentration of Mn, Fe, and Zn was detected by atomic absorption spectroscopic analysis. The ethyl acetate extract showed the highest flavonoid contents (5.94 ± 0.04 mg/g, Quercetin Equivalents) and Fe2+-chelation (52%) potential with DPPH radicals-quenching IC50 at 1.35 ± 0.16 mg/mL, while the aqueous ethanolic extract exhibited maximum phenolics contents (136.08 ± 0.12 mg/g, gallic acid equivalents) with DPPH scavenging potential at IC50 0.615 ± 0.06 mg/mL. Aqueous ethanolic extract and standard quercetin DPPH radicals scavenging’s were equal potent at 10 mg/mL concentrations. The aqueous ethanolic extract showed highest analgesic effect with pain reduction rates 89.86% (p = 0.03), 87.50% (p < 0.01), and 99.66% (p = 0.0004) after 60, 90, and 120 min, respectively. Additionally, aqueous ethanolic extract exhibited the highest anti-inflammation capacity at 41.07% (p < 0.0001), 34.51% (p < 0.0001), and 24.82% (p < 0.0001) after 2, 3, and 6 h of extract’s administration, respectively. The phytochemical constituents, significant anti-oxidant potential, remarkable analgesic, and anti-inflammatory bioactivities of extracts supported the traditionally claimed anti-inflammatory and analgesic plant activities.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号