全文获取类型
收费全文 | 4504篇 |
免费 | 626篇 |
国内免费 | 172篇 |
专业分类
化学 | 4403篇 |
晶体学 | 3篇 |
力学 | 24篇 |
综合类 | 23篇 |
数学 | 609篇 |
物理学 | 240篇 |
出版年
2024年 | 2篇 |
2023年 | 52篇 |
2022年 | 136篇 |
2021年 | 202篇 |
2020年 | 214篇 |
2019年 | 155篇 |
2018年 | 170篇 |
2017年 | 134篇 |
2016年 | 220篇 |
2015年 | 241篇 |
2014年 | 281篇 |
2013年 | 400篇 |
2012年 | 245篇 |
2011年 | 313篇 |
2010年 | 237篇 |
2009年 | 282篇 |
2008年 | 257篇 |
2007年 | 269篇 |
2006年 | 230篇 |
2005年 | 208篇 |
2004年 | 164篇 |
2003年 | 154篇 |
2002年 | 201篇 |
2001年 | 59篇 |
2000年 | 69篇 |
1999年 | 62篇 |
1998年 | 55篇 |
1997年 | 43篇 |
1996年 | 38篇 |
1995年 | 29篇 |
1994年 | 31篇 |
1993年 | 23篇 |
1992年 | 10篇 |
1991年 | 12篇 |
1990年 | 8篇 |
1989年 | 7篇 |
1988年 | 9篇 |
1987年 | 12篇 |
1986年 | 5篇 |
1985年 | 8篇 |
1984年 | 5篇 |
1983年 | 7篇 |
1982年 | 8篇 |
1981年 | 5篇 |
1980年 | 8篇 |
1979年 | 6篇 |
1976年 | 2篇 |
1973年 | 2篇 |
1969年 | 2篇 |
1968年 | 2篇 |
排序方式: 共有5302条查询结果,搜索用时 31 毫秒
991.
992.
Dr. Amit B. Pawar Prof. Dr. Kavirayani R. Prasad 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(47):15202-15206
A formal total synthesis of the 20‐membered marine macrolide, palmerolide A from chiral pool tartaric acid is described. Elaboration of a γ‐hydroxy amide, which is derived from the desymmetrization of tartaric acid amide, and Boord olefination are the pivotal reactions employed for the synthesis of the chiral building blocks, and Stille coupling and ring‐closing metathesis (RCM) are used to assemble the macrolactone. 相似文献
993.
建立了UHPLC-MS/MS法测定水产品中10种磺胺类(SAs)和7种喹诺酮类(QNs)药物残留的分析方法。样品用200 g/L盐酸羟胺-乙腈溶液提取,以乙酸铵溶液和乙腈为流动相进行梯度洗脱,电喷雾正离子(ESI+)模式电离,多反应监测模式检测,同时对水产品中10种SAs和7种QNs进行定量和定性。在0.25~4.0μg/kg和0.10~2.0μg/kg范围内两类药物的线性良好(r2>0.99);平均回收率为均为80%~120%,RSD为7.4%~14%;10种磺胺药物的检测限(LOD)均为5.0μg/kg,7种喹诺酮药物检测限(LOD)均为2.0μg/kg。该方法适合水产品中这两类药物残留的确证和定量测定。 相似文献
994.
柱后加碱脉冲积分安培检测-离子色谱法测定水产品中十种生物胺 总被引:1,自引:0,他引:1
建立了利用淋洗液自动发生梯度淋洗的离子交换色谱法同时测定水产品中酪胺、5-羟色胺、腐胺、尸胺、组胺、庚二胺、苯乙胺、亚精胺、精胺、色胺等10种生物胺的方法。样品经处理后用Ionpac CS17分离柱和Ionpac CG17型保护柱分离,以EG40自动淋洗液发生器生成的5.0~55 mmol/L的MSA为淋洗液梯度洗脱,脉冲积分安培检测器检测。对梯度进行优化,10种生物胺都能基线分离,并且浓度和峰面积在一定范围内呈良好的线性关系。检出限在0.04 mg/kg以下,回收率在91.2%~102.5%之间,样品的RSD(n=6)小于5%。方法可用于水产品的检测。 相似文献
995.
液相色谱-串联质谱法测定水产品中麻醉剂MS-222残留 总被引:3,自引:0,他引:3
建立了液相色谱-串联质谱法测定水产品中麻醉剂3-氨基苯甲酸乙酯甲基磺酸盐(MS-222)残留量的方法。提取液为50%的甲醇及乙酸-乙酸钠缓冲溶液,提取液经C18固相萃取柱净化处理后用液相色谱-串联质谱仪进行测定,外标法定量。流动相为0.5%的甲酸溶液和乙腈(V:V=60:40),流速为0.2 mL/min。该方法的线性范围为0.001~1.0 mg/L,相关系数大于0.999,检出限为1μg/kg,定量限为2μg/kg。加标回收率可以达到80%~110%。 相似文献
996.
997.
Dr. Yvonne Schmidt Dipl.‐Chem. Konrad Lehr Lucie Colas Prof. Dr. Bernhard Breit 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(23):7071-7081
The determination of the relative configuration of 1,3‐dimethyl‐substituted alkyl chains is possible by interpretation of 1H NMR shift differences. Additionally, assignments are feasible in a variety of deuterated solvents, because the corresponding shift differences are not significantly influenced by the solvent. The trends for Δδ values depending on functional groups adjacent to the stereogenic centers are shown. Based on a thorough comparison with literature data, the relative configuration of natural products can be predicted. For this purpose, we derived an empirical rule for the ranges in which Δδ values usually occur. Furthermore, we were able to proof the validity of our method by the successful prediction of the relative configuration for the polyketide natural product xylarinic acid A, which was confirmed by the asymmetric total synthesis of its enantiomer. Based on the proposed simple analysis of published 1H NMR data and the determination of the relevant chemical‐shift differences, we predicted the relative configurations of several previously unassigned natural products. 相似文献
998.
João M. Batista Jr. Andrea N.L. BatistaMassuo J. Kato Vanderlan S. BolzaniSilvia N. López Laurence A. NafieMaysa Furlan 《Tetrahedron letters》2012,53(45):6051-6054
A reinvestigation of the monoterpene chromane ester enriched fraction from Peperomia obtusifolia using chiral chromatography led to the identification of a minor peak, which was elucidated by NMR and HRMS as fenchyl-3,4-dihydro-5-hydroxy-2,7-dimethyl-8-(3″-methyl-2″-butenyl)-2-(4′-methyl-1′,3′-pentadienyl)-2H-1-benzopyran-6-carboxylate, the same structure assigned to two other fenchyl esters described previously, pointing out a stereoisomeric relationship among them. Further NMR analysis revealed that it was actually a mixture of two compounds, whose absolute configurations were determined by VCD measurements. Although, almost no vibrational transitions could be assigned to the chiral chromane, the experimental VCD spectrum was largely opposite to that obtained for the average experimental VCD [(2S,1?R,2?R,4?S + 2R,1?R,2?R,4?S)/2] for fenchol derivatives. These results allowed us to assign the putative compounds as a racemic mixture of the chiral chromane esterified with the monoterpene (1S,2S,4R)-fenchol, which had not been identified in our early work. 相似文献
999.
A concise total synthesis of (+)-deoxocassine (1) and (−)-deoxoprosophylline (2) has been achieved for the first time from d-xylose. The key steps involved in these synthesis are alkyl Grignard reaction, SN2 substitution of mesylate by azide, Wittig reaction, and reductive amination. 相似文献
1000.
《Analytical letters》2012,45(13):2447-2464
Abstract In this mini‐review we highlight two aspects of ionic liquids that to date have either no or limited studies. They are (1) exploitation of unique features of ionic liquids to develop novel spectroscopic methods and (2) development of novel spectroscopic methods, for the sensitive and accurate determination of thermal physical properties of ionic liquids. In the first category, we will describe several novel methods which were developed utilizing unique properties of ionic liquids for measurements which are not possible otherwise. They include the sensitive and accurate method to determine enantiomeric compositions of a variety of pharmaceutical products with different size, shape, and functional groups. This method is based on the use of a chiral IL which serves both as a solvent and also as a chiral selector. Ionic liquids have also been successfully used to substantially enhance the sensitivity of thermal lens measurements. In the second category, we will describe recent development in which transient grating technique and thermal lens technique have been successfully used for the sensitive, accurate, nondestructive determination of thermal physical properties of ILs. 相似文献