全文获取类型
收费全文 | 29353篇 |
免费 | 1627篇 |
国内免费 | 3865篇 |
专业分类
化学 | 19310篇 |
晶体学 | 1057篇 |
力学 | 218篇 |
综合类 | 192篇 |
数学 | 3686篇 |
物理学 | 4079篇 |
综合类 | 6303篇 |
出版年
2024年 | 20篇 |
2023年 | 196篇 |
2022年 | 644篇 |
2021年 | 812篇 |
2020年 | 608篇 |
2019年 | 598篇 |
2018年 | 488篇 |
2017年 | 636篇 |
2016年 | 687篇 |
2015年 | 677篇 |
2014年 | 1049篇 |
2013年 | 1960篇 |
2012年 | 1571篇 |
2011年 | 1843篇 |
2010年 | 1777篇 |
2009年 | 2023篇 |
2008年 | 1945篇 |
2007年 | 2038篇 |
2006年 | 1949篇 |
2005年 | 1661篇 |
2004年 | 1607篇 |
2003年 | 1399篇 |
2002年 | 1215篇 |
2001年 | 907篇 |
2000年 | 881篇 |
1999年 | 742篇 |
1998年 | 643篇 |
1997年 | 562篇 |
1996年 | 469篇 |
1995年 | 470篇 |
1994年 | 433篇 |
1993年 | 398篇 |
1992年 | 376篇 |
1991年 | 309篇 |
1990年 | 184篇 |
1989年 | 224篇 |
1988年 | 172篇 |
1987年 | 111篇 |
1986年 | 72篇 |
1985年 | 77篇 |
1984年 | 91篇 |
1983年 | 23篇 |
1982年 | 44篇 |
1981年 | 60篇 |
1980年 | 51篇 |
1979年 | 38篇 |
1978年 | 27篇 |
1977年 | 22篇 |
1976年 | 11篇 |
1975年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
11.
12.
Valentin A. Semenov Dmitry O. Samultsev Leonid B. Krivdin 《Magnetic resonance in chemistry : MRC》2020,58(6):532-539
The density functional theory calculation of 1H and 13C NMR chemical shifts in a series of ten 10 classically known Strychnos alkaloids with a strychnine skeleton was performed at the PBE0/pcSseg-2//pcseg-2 level. It was found that calculated 1H and 13C NMR chemical shifts provided a markedly good correlation with experiment characterized by a mean absolute error of 0.08 ppm in the range of 7 ppm for protons and 1.67 ppm in the range of 150 ppm for carbons, so that a mean absolute percentage error was as small as ~1% in both cases. 相似文献
13.
Renny Mathew Karolina A. Uchman Lydia Gkoura Chris J. Pickard Maria Baias 《Magnetic resonance in chemistry : MRC》2020,58(11):1018-1025
A combined experimental and computational approach was used to distinguish between different polymorphs of the pharmaceutical drug aspirin. This method involves the use of ab initio random structure searching (AIRSS), a density functional theory (DFT)-based crystal structure prediction method for the high-accuracy prediction of polymorphic structures, with DFT calculations of nuclear magnetic resonance (NMR) parameters and solid-state NMR experiments at natural abundance. AIRSS was used to predict the crystal structures of form-I and form-II of aspirin. The root-mean-square deviation between experimental and calculated 1H chemical shifts was used to identify form-I as the polymorph present in the experimental sample, the selection being successful despite the large similarities between the molecular environments in the crystals of the two polymorphs. 相似文献
14.
Two new divalent copper (C1) and zinc (C2) chelates having the formulae [M(PIMC)2] (where M = Cu(II), Zn(II) and PIMC = Ligand [(E)-3-(((3-hydroxypyridin-2-yl)imino)methyl)-4H-chromen-4-one] were obtained and characterized by several techniques. Structures and geometries of the synthesized complexes were judged based on the results of alternative analytical and spectral tools supporting the proposed formulae. IR spectral data confirmed the coordination of the ligands to the copper and zinc centers as monobasic tridentate in the enol form. Thermal analysis, UV-Vis spectra and magnetic moment confirmed the geometry around the copper center to be tetrahedral, square pyramidal and octahedral. Study of the binding ability of the synthesized compounds with Circulating tumor DNA (CT-DNA) bas been evaluated applying UV-Vis spectral titration and viscosity measurements. The copper and zinc oxides were achieved from the copper and zinc nano-particles structures Schiff base complexes as the raw material after calcination for 5 hr at 600°C. On the other hand, synthesized of C1 and C2 NPs were used as suitable precursors to the preparation of CuO and ZnO NPs. Finally, the synthesized of the two complexes exhibited enhanced activity against the tested bacterial (Staphylococcus aureus and Escherichia Coli) and fungal strains (Candida albicans and Aspergillus fumigatus) as compared to HPIMC. Among all these synthesized compounds, C1 exhibits good cleaving ability compared to other newly synthesized C2. 相似文献
15.
Development and validation of a reversed‐phase HPLC method for CYP1A2 phenotyping by use of a caffeine metabolite ratio in saliva 下载免费PDF全文
Elias Begas Evangelos Kouvaras Andreas K. Tsakalof Maria Bounitsi Eftihia Konstadinos Asprodini 《Biomedical chromatography : BMC》2015,29(11):1657-1663
CYP1A2 is important for metabolizing various clinically used drugs. Phenotyping of CYP1A2 may prove helpful for drug individualization therapy. Several HPLC methods have been developed for quantification of caffeine metabolites in plasma and urine. Aim of the present study was to develop a valid and simple HPLC method for evaluating CYP1A2 activity during exposure in xenobiotics by the use of human saliva. Caffeine and paraxanthine were isolated from saliva by liquid‐liquid extraction (chlorophorm/isopropanol 85/15v/v). Extracts were analyzed by reversed‐phase HPLC on a C18 column with mobile phase 0.1% acetic acid/methanol/acetonitrile (80/20/2 v/v) and detected at 273nm. Caffeine and paraxanthine elution times were <13min with no interferences from impurities or caffeine metabolites. Detector response was linear (0.10–8.00µg/ml, R2>0.99), recovery was >93% and bias <4.47%. Intra‐ and inter‐day precision was <5.14% (n=6). The limit of quantitation was 0.10µg/ml and the limit of detection was 0.018±0.002µg/mL for paraxanthine and 0.032±0.002µg/ml for caffeine. Paraxanthine/caffeine ratio of 34 healthy volunteers was significantly higher in smokers (p<0.001). Saliva paraxanthine/caffeine ratios and urine metabolite ratios were highly correlated (r=0.85, p<0.001). The method can be used for the monitoring of CYP1A2 activity in clinical practice and in studies relevant to exposure to environmental and pharmacological xenobiotics. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
16.
Gas-phase 21Ne nuclear magnetic resonance spectra were measured at the natural abundance of 21Ne isotope for samples consisting of pressurized neon up to 60 bar at room temperature and applying the magnetic field of the strength B0 = 11.7574 T. It showed that the nuclear magnetic resonance frequency is linearly dependent on the density of gaseous neon. The resonance frequency was extrapolated to the zero-density point, and it permitted the determination of the 21Ne nuclear magnetic moment, μ(21Ne) = 0.6617774(10) μN. The present value of μ(21Ne) is not influenced by the bulk magnetic susceptibility of neon and interactions between neon atoms; therefore, it is more precise and reliable than the previous result obtained for μ(21Ne). 相似文献
17.
Pan Huang Dawen Xu Robert M. Reich Felix Kaiser Boping Liu Fritz E. Kühn 《Tetrahedron letters》2019,60(24):1574-1577
A first example of an Et2Zn mediated silylation of 1-aklynes is reported. A series of functional groups are tolerated in this reaction. Mechanistic studies support Zn alkynilides as intermediates in the reaction. This reaction protocol provides a practical method for the preparation of alkynylsilanes and expands the application of organometallic zinc in organic synthesis. 相似文献
18.
雷电波分析与比较研究 总被引:1,自引:0,他引:1
本文对双指数雷电流模型和 IEC1 3 1 2—— 1推荐的雷电流模型进行了分析和研究 ,计算了这两种雷电流模型的雷电流参数。在此基础上 ,用 FFT方法分析了雷电流的频谱分布 相似文献
19.
文章提出一种对MPEG-1静态文件进行索引的方案。该方案把MPEG-1静态文件分解成独立的系统信息、音频流和视频流,并建立索引树。根据索引可以快速地对音频流和视频流进行搜索和定位,还可以按照流媒体传输规则打成RTP包,并介绍了一种简便易行的视频容错策略。 相似文献
20.
在探索综合性大学中教师教育的培养模式研究过程中,根据教师教育双专业的特性,实施了综合性大学师范本科教育3+1课程方案和培养模式的研究与实践.并以化学教师教育培养为例对3+1模式的作了具体说明. 相似文献