全文获取类型
收费全文 | 59455篇 |
免费 | 4679篇 |
国内免费 | 4058篇 |
专业分类
化学 | 28560篇 |
晶体学 | 353篇 |
力学 | 6221篇 |
综合类 | 875篇 |
数学 | 19583篇 |
物理学 | 12600篇 |
出版年
2024年 | 108篇 |
2023年 | 532篇 |
2022年 | 1219篇 |
2021年 | 1519篇 |
2020年 | 1549篇 |
2019年 | 1495篇 |
2018年 | 1395篇 |
2017年 | 1764篇 |
2016年 | 2267篇 |
2015年 | 1624篇 |
2014年 | 2488篇 |
2013年 | 4475篇 |
2012年 | 3363篇 |
2011年 | 3081篇 |
2010年 | 2692篇 |
2009年 | 3437篇 |
2008年 | 3483篇 |
2007年 | 3838篇 |
2006年 | 3228篇 |
2005年 | 2915篇 |
2004年 | 2746篇 |
2003年 | 2327篇 |
2002年 | 1995篇 |
2001年 | 1681篇 |
2000年 | 1593篇 |
1999年 | 1514篇 |
1998年 | 1309篇 |
1997年 | 1163篇 |
1996年 | 1026篇 |
1995年 | 983篇 |
1994年 | 836篇 |
1993年 | 694篇 |
1992年 | 619篇 |
1991年 | 553篇 |
1990年 | 459篇 |
1989年 | 325篇 |
1988年 | 300篇 |
1987年 | 224篇 |
1986年 | 188篇 |
1985年 | 228篇 |
1984年 | 199篇 |
1983年 | 95篇 |
1982年 | 150篇 |
1981年 | 92篇 |
1980年 | 86篇 |
1979年 | 114篇 |
1978年 | 52篇 |
1977年 | 42篇 |
1976年 | 38篇 |
1974年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 109 毫秒
991.
环氧合酶-2抑制剂的三维定量构效关系研究 总被引:2,自引:0,他引:2
建立三环系COX-1和COX-2抑制剂结构与活性的三维定量构效关系模型,为设 计新型的具有选择性的COX-2抑制剂提供线索。通过与酶的对接并优化,确定化合 物在受体结合腔中的构象,利用比较分子力场分析方法建立三维定量构效关系模型 。模型1R_(cv)~2=0.685,最佳主成分数为6,传统相关系数为R~2=0.988, F-726. 2,标准偏差S = 0.080;模型2 R_(cv)~2 = 0.573,最佳主成分数为6,传统相关 系数为R~2=0.996, F = 1147.6,标准偏差S = 0.034。所得的模型不仅解释了化合 物的构效关系,而且对预测集中的化合物有很好的预测能力;比较不同模型的系数 相关图,分析了结构与活性、结构与选择性的关系,得到的结果可以指导新化合物 的设计与合成。 相似文献
992.
Direct CI mass spectrometry profiling of fatty acid methyl esters (FAMEs) from in situ thermal hydrolysis/methylation (THM) of whole bacterial cells with tetramethylammonium hydroxide (TMAH) has been demonstrated as a potential method for real time and fieldable detection/identification of microorganisms. Bacillus anthracis (Ames), Yersinia pestis (Nair. Kenya), Vibrio cholerae (E1 Tor), Brucella melitensis (Abortus wild) and Francisella tularensis (LVS vaccine) were profiled by this method during a 10-month period. Repeatability of the in situ FAME data was calculated using one-way analysis of variance (ANOVA) and a t-test. Artificial neural network (ANN) and multivariate statistics of the FAME profiles were also compared for bacterial identification/classification. Equivalent results were obtained with a multivariate rule building expert system (MuRES) and the ANN. However, the ANN analysis required much less computer time and was deemed the best choice for this application. In situ THM FAME profiles of the bacterial samples provided comparable results with those obtained from the Microbial Identification System (MIDI) (Newark, DE) wet chemistry-gas chromatographic based system. 相似文献
993.
评述了近几年来电化学检测在流动注射分析中的应用,展望了流动注射电化学分析法的发展动向。 相似文献
994.
A direct quantitative method is presented that is based upon the use of multiple headspace solid phase microextraction (HS-SPME) to monitor biogenic volatile organic compounds (BVOCs) released from a living leaf of Pelargonium hortorum in situ. Seventeen BVOCs were detected by GC-MS after a single SPME extraction using a CAR/DVB/PDMS fibre. An internal standard was employed to determine the absolute amounts of seven terpenoid compounds released from a P. hortorum leaf. The quantitative analysis was performed over two days, with extraction preformed for 20 min every 3 h. The amount of volatiles extracted varied with the time of day, with two maxima recorded at 14:00 (day 1) and 17:00 (day 2), corresponding to 236 and 277 ng of the seven terpenoids recorded, respectively. These results indicate that multiple HS-SPME in combination with an internal standard is a simple, quick, and quantitative technique for analysising BVOC emissions from a live plant sample. 相似文献
995.
996.
H. Saglam T. Gozler B. Kivcak B. Demirci K. H. C. Baser 《Chemistry of Natural Compounds》2001,37(5):439-441
The chemical composition of the essential oil ofHaplophyllum myrtifoliumBoiss., endemic to Turkey, was examined by GC/MS. Ninety-seven compounds were characterized with linalool (12.8%), -caryophyllene (10.3 %), and methyleugenol (5.9 %) as the main constituents. 相似文献
997.
The semiempirical AM I method was used for the calculation of conformational potential energy surfaces of hydrogen trioxide and tetraoxide. The most stable conformation of trioxide is itsanti-form, which is characterized by the torsion angle = 82.3°, and that of tetraoxide is theanti, anti-form with = 80.7° (H-O-O-O) and 74.1° (O-O-O-O). The structures of the stable conformers are determined by the interaction between lone clectron pairs of oxygen atoms. The dipole-dipole interaction of OH groups is weaker than that in hydrogen peroxide. The results were used for the estimation of the polyoxide increment of the Benson's additivity scheme, f
H°[O-(O)2] = 48±6 k1 mol–1.Translated from lzveshyn Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1356-1361, June, 1996. 相似文献
998.
Peri F Jiménez-Barbero J García-Aparicio V Tvaroska I Nicotra F 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(6):1433-1444
N(OMe)-linked disaccharide analogues, isosteric to the corresponding natural disaccharides, have been synthesized by chemoselective assembly of unprotected natural monosaccharides with methyl 6-deoxy-6-methoxyamino-alpha-D-glucopyranoside in an aqueous environment. The coupling reactions were found to be chemo- and stereoselective affording beta-(1-->6) disaccharide mimics when using Glc and GlcNAc; in the case of Gal, the beta-anomer was prevalent (beta:alpha=7:1). An iterative method for the synthesis of linear N(OMe) oligosaccharide analogues was demonstrated, based on the use of an unprotected monosaccharide building block in which an oxime functionality at C-6 is converted during the synthesis into the corresponding methoxyamino group. The conformational analysis of these compounds was carried out by using NMR spectroscopy, ab initio, molecular mechanics, and molecular dynamics methods. Optimized geometries and energies of fourteen conformers for each compound have been calculated at the B3LYP/6-31G* level. Predicted conformational equilibria were compared with the results based on NMR experiments and good agreement was found. It appears that N(OMe)-linked disaccharide analogues exhibit a slightly different conformational behavior to their parent natural disaccharides. 相似文献
999.
We report on capillary isoelectric focusing-mass spectrometry (CIEF-MS) of complex peptide mixtures in the absence of carrier ampholytes. Furthermore, the use of low concentrations of carrier ampholytes as mere spacers is investigated. Carrier ampholytes are complex mixtures of amphoteric compounds with high buffering capacity. Since all peptides are amphoteric compounds by themselves, the use of carrier ampholytes may be superfluous to establish a stable pH gradient in CIEF analysis of protein digests. Our research showed that when carrier ampholytes are omitted, the analyte ions are not focused at their isoelectric point. The analytes are charged, leading to electrophoretic mobility uncharacteristic for CIEF. The method was tested for a five-protein-mixture at 0.02 mg/mL per protein and 0.05 mg/mL per protein. At the lower concentration, the analytes were stacked during the focusing process in only a limited length of the capillary. Therefore, the higher concentration led to better separation efficiency. It was found that at low concentration (0.20%) the carrier ampholytes could work as spacers. Though it led to sensitivity losses of 15-45%, this was compensated by the higher separation efficiencies seen. The method was evaluated with an eight-protein-mixture, of which all could be identified after performing MS/MS. 相似文献
1000.
Polymer melting and polymer powder sintering by thermal analysis 总被引:10,自引:0,他引:10
Sintering of polymeric powders is a peculiar characteristic of many processing technologies, including rotational moulding
and selective laser sintering (SLS). During polymer sintering, viscosity reduction in the melt state promotes densification
of polymer powders, through a double stage mechanism, involving powder coalescence and bubble removal. In particular, sintering
of semi-crystalline polymers is strongly influenced by the melting behaviour. Nevertheless, melting itself in absence of pressure
is not necessarily accompanied by powder sintering, unless low viscosities are achieved. In this work, the melting and sintering
behaviour of recycled high density polyethylene (rHDPE) have been analysed through differential scanning calorimetry (DSC)
and Thermomechanical Analysis (TMA). Efficient models capable of describing the melting temperature distribution and rate
of sintering of rHDPE powders have been developed, highlighting the inherent differences between the two distinct processes.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献