全文获取类型
收费全文 | 15820篇 |
免费 | 2804篇 |
国内免费 | 1859篇 |
专业分类
化学 | 11325篇 |
晶体学 | 167篇 |
力学 | 1149篇 |
综合类 | 67篇 |
数学 | 1707篇 |
物理学 | 6068篇 |
出版年
2024年 | 36篇 |
2023年 | 342篇 |
2022年 | 448篇 |
2021年 | 544篇 |
2020年 | 640篇 |
2019年 | 597篇 |
2018年 | 538篇 |
2017年 | 493篇 |
2016年 | 843篇 |
2015年 | 746篇 |
2014年 | 933篇 |
2013年 | 1148篇 |
2012年 | 1481篇 |
2011年 | 1461篇 |
2010年 | 991篇 |
2009年 | 940篇 |
2008年 | 1087篇 |
2007年 | 948篇 |
2006年 | 859篇 |
2005年 | 766篇 |
2004年 | 532篇 |
2003年 | 439篇 |
2002年 | 424篇 |
2001年 | 355篇 |
2000年 | 312篇 |
1999年 | 326篇 |
1998年 | 271篇 |
1997年 | 240篇 |
1996年 | 272篇 |
1995年 | 266篇 |
1994年 | 188篇 |
1993年 | 147篇 |
1992年 | 152篇 |
1991年 | 145篇 |
1990年 | 125篇 |
1989年 | 101篇 |
1988年 | 85篇 |
1987年 | 67篇 |
1986年 | 50篇 |
1985年 | 41篇 |
1984年 | 30篇 |
1983年 | 25篇 |
1982年 | 14篇 |
1981年 | 16篇 |
1980年 | 9篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1972年 | 1篇 |
1957年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
T.-T. Xu X.-Y. Xu J. Gao Sh.-Sh. Ni D.-Q. Wang L.-D. Lu X.-J. Yang X. Wang 《Journal of Structural Chemistry》2007,48(1):193-197
Two new Schiff bases were prepared by the condensation reaction of paeonol with ethylenediamine (for compound 1) and 1,3-propylenediamine (for compound 2). The compounds were studied with single crystal X-ray diffraction method. Compound 1 is monoclinic, space group P21/c, a = 10.956(15) Å, b = 6.767(10) Å, c = 12.494(18) Å; β = 104.66(2)°, V = 896(2) Å3, Z = 2. Compound 2 is triclinic, space group P-1, a = 6.878(2) Å, b = 12.141(4) Å, c = 12.414(4) Å; α = 101.023(6)°, β = 103.415(6)°, γ = 102.337(6)°, V = 952.8(6) Å3, Z = 2. The molecular structures of 1 and 2 are similar; the molecule being centrosymmetric in 1 but asymmetric in 2. 相似文献
992.
共轭亚麻酸的制备、表征和生物活性研究进展 总被引:2,自引:0,他引:2
近年来的研究发现共轭亚麻酸(CLN)具有多种非常有益的生理功能,已成为化学家、药理学家、营养学家争相研究的热点。本文就CLN的制备、表征、分离分析以及生物活性等方面,对国内外的研究进行了综述。CLN的制备包括化学法、生物法和天然产物提取法三大类。CLN的表征方法主要有紫外光谱、红外光谱、核磁共振等方法,以及采用气相色谱法(GC)、高效液相色谱法(HPLC)和毛细管电泳法(CE)等方法对合成的CLN混合物进行分离分析。CLN的生物活性主要有抗癌、降血脂、减肥、抗致癌等。 相似文献
993.
Zhihui Zhao Hui Dai Xiangmei Wu Haitao Chang Xiaoming Gao Mengjun Liu Pengfei Tu 《Chemistry of Natural Compounds》2007,43(4):374-376
A pectic polysaccharide was isolated from the fruits of Ziziphus jujuba Mill. cv. jinsixiaozao Hort. and its structure was characterized by acid hydrolysis, methylation, and NMR spectroscopies.
The purified fraction Ju-B-7 had a molecular mass of over 2000 kDa. Chemical and spectroscopic analyses indicated that Ju-B-7
is composed mainly of α-1,4-linked D-galactopyranosyluronic acid and 1,2-linked L-rhamnose with a molar ratio of 8.1:1. It can significantly stimulate
spleen cell proliferation in vitro (P<0.01, 10 μmg/mL).
Published in Khimiya Prirodnykh Soedinenii, No. 4, pp. 311–312, July–August, 2007. 相似文献
994.
995.
Development of a Chromatographic Fingerprint of Tobacco Flavor by Use of GC and GC-MS 总被引:7,自引:0,他引:7
A new chromatographic fingerprinting method has been established for quality control of tobacco flavor. Three different extraction techniques, simultaneous distillation extraction (SDE), liquid–liquid extraction, and solid-phase microextraction were evaluated for isolation of the components of interest. After comparison of their performance, a combination of SDE and GC-MS was used for simple, reliable, and reproducible development of a chromatographic fingerprint of tobacco flavor. Twelve samples of the flavor from different batches were used to establish the fingerprint. Thirty-nine volatile components of the tobacco flavor samples, accounting for 86.54% of the total content, were identified and quantified. The 12 samples had 28 peaks in common. The method of fingerprint analysis was then validated on the basis of the relative retention times and relative peak areas of the common peaks, sample stability, and similarity analysis. The similarities of the 12 samples of tobacco flavor were >0.80, showing that samples from different batches were, to some extent, consistent. The chromatographic fingerprint developed was successfully used to differentiate tobacco flavor samples from tobacco extract prepared from tobacco leaf, both by similarity comparison and by principal-components projection analysis. The method can be used for quality control of tobacco flavor. 相似文献
996.
荧光法对光诱导野生型肌红蛋白和突变体(D44K)去氧的对照研究 总被引:1,自引:0,他引:1
用荧光法对光诱导野生型肌红蛋白(Mb)和突变体(D44K)去氧的过程进行对照研究。发现430nm是研究Mb(WT)和Mb(D44K)光照去氧的最佳激发波长。430nm激发时,Mb(D44K)在597.9nm和628.8nm处出现两个荧光发射峰,不同于Mb(WT)仅在597nm处出现一个荧光发射峰。经研究证明,628.8nm处荧光峰是Mb3 -H2O型中的H2O峰。光照也使此峰的荧光强度下降,但比去氧的速率慢。研究发现,597nm处Mb(D44K)的荧光效率比Mb(WT)的荧光效率低。传能实验表明Mb表面44位氨基酸由天冬氨酸突变为赖氨酸后,不影响Mb(D44K)中色氨酸和酪氨酸残基传递给铁卟啉的荧光效率,但使Mb(D44K)中色氨酸和酪氨酸残基的荧光效率变高。 相似文献
997.
998.
A rapid, selective and convenient liquid chromatography–mass spectrometric method for the simultaneous determination of paracetamol and caffeine in human plasma was developed and validated. Analytes and theophylline [internal standard (I.S.)] were extracted from plasma samples with diethyl ether-dichloromethane (3:2, v/v) and separated on a C18 column (150 × 4.6 mm ID, 5 μm particle size, 100 Å pore size). The mobile phase consisted of 0.2% formic acid–methanol (60:40, v/v). The assay was linear in the concentration range between 0.05 and 25 μg mL?1 for paracetamol and 10–5,000 ng mL?1 for caffeine, with the lower limit of quantification of 0.05 μg mL?1 and 10 ng mL?1, respectively. The intra- and inter-day precision for both drugs was less than 8.1%, and the accuracy was within ±6.5%. The single chromatographic analysis of plasma samples was achieved within 4.5 min. This validated method was successfully applied to study the pharmacokinetics of paracetamol and caffeine in human plasma. 相似文献
999.
1000.
Feng H Liu X Gao W Chen X Wang J Chen L Lüdemann HD 《Physical chemistry chemical physics : PCCP》2010,12(45):15007-15017
Self-diffusion and structural properties of ammonia, methylamine and trimethylamine have been studied by molecular dynamics simulation in the temperature range between the melting pressure curve and 700 K at pressures up to 400 MPa. The calculation results agree well with the experiment, which suggests that one can use the simulation method as a powerful tool to obtain self-diffusion coefficients over wide range of temperatures and pressures, under which it is rather difficult for experiments. The local structures of such fluids are investigated by calculating radial distribution functions (RDFs), the numbers of hydrogen bonds and coordination numbers. The correlation between self-diffusion and structural properties, and the influence of temperature and pressure on them are discussed. The simulation results demonstrate that the temperature effects are more pronounced than the pressure effects on self-diffusion and structural properties, and the effect of hydrogen bonding on the translational dynamics in any of these systems is a minor factor, while it is mainly affected by the close packing of amine molecules. 相似文献