全文获取类型
收费全文 | 1138篇 |
免费 | 32篇 |
国内免费 | 8篇 |
专业分类
化学 | 861篇 |
晶体学 | 15篇 |
力学 | 17篇 |
数学 | 75篇 |
物理学 | 210篇 |
出版年
2024年 | 6篇 |
2023年 | 13篇 |
2022年 | 96篇 |
2021年 | 58篇 |
2020年 | 48篇 |
2019年 | 39篇 |
2018年 | 52篇 |
2017年 | 34篇 |
2016年 | 58篇 |
2015年 | 34篇 |
2014年 | 49篇 |
2013年 | 82篇 |
2012年 | 76篇 |
2011年 | 73篇 |
2010年 | 43篇 |
2009年 | 42篇 |
2008年 | 29篇 |
2007年 | 37篇 |
2006年 | 38篇 |
2005年 | 38篇 |
2004年 | 33篇 |
2003年 | 28篇 |
2002年 | 31篇 |
2001年 | 7篇 |
2000年 | 15篇 |
1997年 | 2篇 |
1996年 | 6篇 |
1995年 | 5篇 |
1993年 | 6篇 |
1992年 | 12篇 |
1991年 | 5篇 |
1990年 | 7篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1986年 | 3篇 |
1984年 | 3篇 |
1983年 | 6篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1978年 | 11篇 |
1977年 | 3篇 |
1976年 | 4篇 |
1975年 | 7篇 |
1974年 | 4篇 |
1972年 | 7篇 |
1971年 | 5篇 |
1967年 | 2篇 |
1958年 | 1篇 |
排序方式: 共有1178条查询结果,搜索用时 15 毫秒
71.
Ahmad VU Khan A Farooq U Kousar F Khan SS Nawaz SA Abbasi MA Choudhary MI 《Chemical & pharmaceutical bulletin》2005,53(4):378-381
Three new tricyclic cis-clerodane type diterpenoids trivially named as limbatolide A (1), limbatolide B (2) and limbatolide C (3) have been isolated from the roots of Otostegia limbata along with two known compounds; oleanic acid and beta-sitosterol. The structure elucidation of the new compounds was based primarily on two-dimensional (2D) NMR techniques. Compounds 1-3 displayed inhibitory potential in a concentration-dependent manner against acetylcholinesterase (AChE; EC 3.1.1.7) and butyrylcholinesterase (BChE; EC 3.1.1.8) enzymes, respectively. 相似文献
72.
Elin S. Karlsson Caroline H. Johnson Sunil Sarda Lisa Iddon Mazhar Iqbal Xiaoli Meng John R. Harding Andrew V. Stachulski Jeremy K. Nicholson Ian D. Wilson John C. Lindon 《Rapid communications in mass spectrometry : RCM》2010,24(20):3043-3051
The use of high‐performance liquid chromatography/mass spectrometry (HPLC/MS) and proton nuclear magnetic resonance (1H NMR) spectroscopy for the kinetic analysis of acyl glucuronide (AG) isomerisation and hydrolysis of the 1‐β‐O‐acyl glucuronides (1‐β‐O‐AG) of phenylacetic acid, (R)‐ and (S)‐α‐methylphenylacetic acid and α,α‐dimethylphenylacetic acid is described and compared. Each AG was incubated in both aqueous buffer, at pH 7.4, and control human plasma at 37°C. Aliquots of these incubations, taken throughout the reaction time‐course, were analysed by HPLC/MS and 1H NMR spectroscopy. In buffer, transacylation reactions predominated, with relatively little hydrolysis to the free aglycone observed. In human plasma incubations the calculated rates of reaction were much faster than for buffer and, in contrast to the observations in buffer, hydrolysis to the free aglycone was a significant contributor to the overall reaction. A diagnostic analytical methodology based on differential mass spectrometric fragmentation of 1‐β‐O‐AGs compared to the 2‐, 3‐ and 4‐positional isomers, which enables selective determination of the former, was confirmed and applied. These findings show that HPLC/MS offers a viable alternative to the more commonly used NMR spectroscopic approach for the determination of the transacylation and hydrolysis reactions of these AGs, with the major advantage of having the capability to do so in a complex biological matrix such as plasma. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
73.
Chengzhi Chuai Mahmood Iqbal Shixiong Tian 《Journal of Polymer Science.Polymer Physics》2010,48(3):267-275
Graft copolymerization of low‐density polyethylene (LDPE) with a maleic anhydride (MAH) was performed using intermeshing corotating twin‐screw extruder in the presence of benzoyl peroxide (BPO). The LDPE/polyamide 6 (PA6) and LDPE‐g‐MAH/PA6 blends were prepared in a corotating twin‐screw extruder. The melt viscosity of the grafted LDPE was measured by a capillary rheometer. The grafted copolymer was characterized by Fourier transform infrared spectroscopy (FTIR) and scanning electron microcopy (SEM). The influence of the variation in temperature, BPO and MAH concentration, and temperature on the grafting degree and on the melt viscosity was studied. The grafting degree increased appreciably up to about 0.45 phr and then decreased continuously with an increasing BPO concentration. According to the FTIR analysis, it was found that the amount of grafted MAH on the LDPE chains was ~5.1%. Thermal analysis showed that melting temperature of the graft copolymers decreases with increasing grafting degree. In addition to this, loss modulus (E″) of the copolymers first increased little with increasing grafting and then obviously decreased with increasing grafting degree. Furthermore, the results revealed that the tensile strength of the blends increased linearly with increasing PA6 content. The results of SEM and mechanical test showed that the blends have good interfacial adhesion and good stability of the phase structure, which is reflected in the mechanical properties. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 267–275, 2010 相似文献
74.
Farhat A Fabiano-Tixier AS Visinoni F Romdhane M Chemat F 《Journal of chromatography. A》2010,1217(47):7345-7350
Without adding any solvent or water, we proposed a novel and green approach for the extraction of secondary metabolites from dried plant materials. This "solvent, water and vapor free" approach based on a simple principle involves the application of microwave irradiation and earth gravity to extract the essential oil from dried caraway seeds. Microwave dry-diffusion and gravity (MDG) has been compared with a conventional technique, hydrodistillation (HD), for the extraction of essential oil from dried caraway seeds. Essential oils isolated by MDG were quantitatively (yield) and qualitatively (aromatic profile) similar to those obtained by HD, but MDG was better than HD in terms of rapidity (45min versus 300min), energy saving, and cleanliness. The present apparatus permits fast and efficient extraction, reduces waste, avoids water and solvent consumption, and allows substantial energy savings. 相似文献
75.
Mazhar Iqbal 《European Polymer Journal》2010,46(11):2174-2180
In this paper the synthesis and characterization of a new family reactive nematic oligomers based on 4-hydroxybenzoic acid (4-HBA) will be presented. We modified the backbone using para- and meta-substituted aromatic monomers such as terephthalic acid (TA), isophthalic acid (IA), hydroquinone (HQ), resorcinol (RS), 4,4′-bisphenol (BP) and 3-hydroxybenzoic acid (3-HBA). All oligomers, with a target Mn of 5000 and 9000 g mol−1, were end-capped with reactive phenylethynyl functionalities and synthesized using standard melt condensation techniques. Curing of the phenylethynyl reactive functionalities proceeds through chain extension and crosslinking, depending upon the temperature and time and can be carried out between 310 and 400 °C. Fully cured nematic thermosets could be obtained with glass-transition temperatures previously not accessible (Tg > 400 °C). The cured polymers exhibit excellent tensile properties, i.e. tensile strength (83 MPa) and elongation at break (9%). This approach allows us to prepare all-aromatic polymers with a combination of useful properties such as ease of processing, high Tg’s, and excellent mechanical properties. 相似文献
76.
Synthesis and characterization of molybdenum complex with tolbutamide (an oral antidiabetic drug) have been studied. The conductometric titration using monovariation method indicates that complex is non-ionic and of L2M type. Analytical data agree with the molecular formula (C12H17N2O3S)2Mo·2H2O. Structure of the complex was assigned as octahedral in which ligand molecules lie horizontally joining the central molybdenum atom and one water molecule, each attached vertically with the metal. Infra-red spectral studies confirm the coordination of sulphonyl oxygen on one side and enolic oxygen attached from other side with the metal ion. IR and 1H NMR studies support structure II proposed on the basis of analytical data. Magnetic susceptibility data suggested that the complex is diamagnetic. Thermal studies support the presence of ligand moieties and coordinated water. X-ray diffraction data also support the complex formation. 相似文献
77.
78.
79.
Shahan Zeb Khan Muhammad Kashif Amir Imdad Ullah Asma Aamir John M. Pezzuto Tamara Kondratyuk Francine Bélanger‐Gariepy Akbar Ali Sajid Khan Zia‐ur‐Rehman 《应用有机金属化学》2016,30(6):392-398
Two new heteroleptic palladium(II) complexes have been synthesized by reacting equimolar quantities of palladium(II) chloride, sodium 4‐(2‐methoxyphenyl)piperazine‐1‐carbodithioate and diphenyl‐p‐tolylphosphine ( 1 ) or tri‐p‐tolylphosphine ( 2 ). Complexes 1 and 2 have been characterized using elemental analysis, Fourier transform infrared spectroscopy, multinuclear NMR (1H, 13C and 31P) spectroscopy and single‐crystal X‐ray analysis. The latter technique confirms a pseudo square‐planar geometry in which two adjacent positions are occupied by bidentate dithiocarbamate while chloro and substituted triphenylphosphine are present at the remaining two positions. The anticancer activity of both complexes against five different cancer cell lines (LU – human lung carcinoma, established at UIC, Department of Surgical Oncology; MCF7 – human breast adenocarcinoma, ATCC number HTB‐22?; MDA‐MB‐231 – human breast adenocarcinoma, ATCC number HTB‐26?; Hepa‐1c1c7 – mouse liver hepatoma, ATCC number CRL‐2026?; PC‐3 – human prostate adenocarcinoma, ATCC number CRL‐1435?) was determined by MTT assay, revealing 2 has higher activity than 1 . A drug–calf thymus DNA binding study with UV–visible spectroscopy reveals a higher DNA binding affinity of 2 (3.511 × 104 M?1) than 1 (4.213 × 103 M?1). Density functional theory studies confirm the relatively more stable nature of 2 than 1 . Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
80.
Muhammad Umer Faroque Sammer Yousuf Salman Zafar M. Iqbal Choudhary Maqsood Ahmed 《Acta Crystallographica. Section C, Structural Chemistry》2016,72(5):398-404
Biotransformation is the structural modification of compounds using enzymes as the catalysts and it plays a key role in the synthesis of pharmaceutically important compounds. 10β,17β‐Dihydroxy‐17α‐methylestr‐4‐en‐3‐one dihydrate, C19H28O3·2H2O, was obtained from the fungal biotransformation of methyloestrenolone. The structure was refined using the classical independent atom model (IAM) and a transferred multipolar atom model using the ELMAM2 database. The results from the two refinements have been compared. The ELMAM2 refinement has been found to be superior in terms of the refinement statistics. It has been shown that certain electron‐density‐derived properties can be calculated on the basis of the transferred parameters for crystals which diffract to ordinary resolution. 相似文献