全文获取类型
收费全文 | 15745篇 |
免费 | 4350篇 |
国内免费 | 9161篇 |
专业分类
化学 | 16420篇 |
晶体学 | 1212篇 |
力学 | 740篇 |
综合类 | 490篇 |
数学 | 1656篇 |
物理学 | 8738篇 |
出版年
2024年 | 162篇 |
2023年 | 567篇 |
2022年 | 629篇 |
2021年 | 725篇 |
2020年 | 548篇 |
2019年 | 736篇 |
2018年 | 509篇 |
2017年 | 703篇 |
2016年 | 757篇 |
2015年 | 902篇 |
2014年 | 1153篇 |
2013年 | 1354篇 |
2012年 | 1245篇 |
2011年 | 1259篇 |
2010年 | 1172篇 |
2009年 | 1208篇 |
2008年 | 1483篇 |
2007年 | 1372篇 |
2006年 | 1535篇 |
2005年 | 1400篇 |
2004年 | 1508篇 |
2003年 | 1230篇 |
2002年 | 1089篇 |
2001年 | 973篇 |
2000年 | 646篇 |
1999年 | 595篇 |
1998年 | 566篇 |
1997年 | 435篇 |
1996年 | 447篇 |
1995年 | 378篇 |
1994年 | 347篇 |
1993年 | 255篇 |
1992年 | 313篇 |
1991年 | 322篇 |
1990年 | 276篇 |
1989年 | 260篇 |
1988年 | 95篇 |
1987年 | 37篇 |
1986年 | 18篇 |
1985年 | 22篇 |
1984年 | 5篇 |
1983年 | 11篇 |
1982年 | 5篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1959年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 421 毫秒
991.
992.
A series of 2-aminochromene derivatives including 2-amino-ben-zo[ h ] chromene, 2-amino-benzo [ f ] chromene and 2-amino-pyrano [3,2-h ]quinolin derivatives was synthesized from aryl-aldehyde, malononitrile or ethyl cyanoacetate and 1-naphthol or 2-naphthoi or 8-hydroxyquinoline in refluxing ethyl alcohol cat-alyzed by KF-Al2O3. The structures were confirmed by X-ray analysis. 相似文献
993.
1-(偶氮苯基)-3-(3,5-二溴-2-吡啶)-三氮烯的合成及其与镉的显色反应 总被引:10,自引:0,他引:10
报道了 1 (偶氮苯基 ) 3 ( 3,5 二溴 2 吡啶 ) 三氮烯的合成及其与镉的显色反应。在TritonX 1 0 0存在下 ,于pH 1 0 .9的Na2 B4O7 NaOH缓冲介质中 ,镉与ABDBPDT形成摩尔比 1∶3型红色配合物。试剂及配合物的最大吸收波长分别为 430nm和 5 30nm。表观摩尔吸光系数为 1 .75× 1 0 5L·mol- 1 ·cm- 1 。镉含量在 0 μg/2 5mL~ 1 2 μg/2 5mL范围内符合比耳定律 相似文献
994.
995.
996.
ZHANG Xiao-Feng HUAND De-Guang CHEN Feng CHEN Chang-Neng LIU Qiu-Tian② 《结构化学》2003,22(5):525-528
1 INTRODUCTION Water oxidation to oxygen gas by photo- synthetic apparatus of green plants and cyano- bacteria is the origin of this gas in the atmosphere. The water oxidation center is a tetranuclear, oxide- bridged manganese cluster with O,N-based peri- pheral ligation by amino acid side-chain group[1, 2]. The binding of aqua to the Mn site may be impor- tant to the oxidation of aqua for producing dioxygen. 1,10-Phenanthroline has been adopted to simulate coordination sphere of manga… 相似文献
997.
Two novel 25,27-dihydroxy-26,28-bis(3-phenylthiapropxy)-calix[4]arene(3) and 25,27-dihydroxy-26,28-bis(3-phenylthiapropoxy)-5,11,17,23-tetra-tert-butylcalix[4] arene (4) were synthesized for the evaluation of their ion-selectivity in ion-selective electrodes(ISEs).ISEs based on 3 and 4 as neutral ionophores were prepared,and their selectivity coefficients for Ag^ (lg KAg,M^pot)were investigated against other alkali metal,alkaline-earth metal,aluminum,thallium(Ⅰ),Lead and some transition metal ions using the separate solution method (SSM).These ISEs showed excellent Ag^ seletivity over most of the interfering cations examined,except for Hg^2 and Fe^2 having relative smaller interference(lg KAg,M^pot≤-2.1). 相似文献
998.
氯代甲苯双电荷离子的单分子解离反应研究 总被引:1,自引:0,他引:1
研究了在70 eV电子轰击电离条件下,氯代甲苯及氯化苄产生的双电荷离子[C_7H_7Cl]~(2+)、[C_7H_6Cl]~(2+·)和[C_7H_5Cl]~(2+)为母体的两种类型单分子解离反应.主要讨论了亚稳双电荷离子的异构化反应、失H解高的“偶电子规则”以及单分子电荷分离过渡态的结构. 相似文献
999.
The relationship among intrinsic surface reaction constant (K) in 1-pK model, point of zero net charge (PZNC) and structural charge density (σst) for amphoteric solid with structural charges was established in order to investigate the effect of σst on pK. The theoretical analysis based on 1-pK model indicates that the independent PZNC of electrolyte concentration (c) exists for amphoteric solid with structural charges. A common intersection point (CIP) should appear on the acid-base titration curves at different c, and the pH at the CIP is pHPZNC. The pK can be expressed as pK=-pHPZNC log[(1 2αPZNC)/(1-2αPZNC)], where αPZNC≡σst/eNANs, in which e is the elementary charge, NA the Avogadro‘s constant and Ns the total density of surface sites. For solids without structural charges, pK=-pHPZNC. The pK values of hydrotalcite-like compounds (HTlc) with general formula of [Mg1-xFex(OH)2](Cl,OH)x were evaluated. With increasing x, the pK increases, which can be explained based on the affinity of metal cations for H^- or OH^- and the electrostatic interaction between charging surface and H^- or OH^-. 相似文献
1000.
(Z)-1-[2-(Tri-o-tolylstannyl)vinyl]-1-indanol (1) and (Z)-1-[2-(tri-p-tolylstannyl)vinyl]-l-indanol (2) were synthesized by the addition reaction of 1-ethynylindanol with tri-o-tolyltin and tri-p-tolyltin hydride. The aryl groups in compound 1 and 2 were substituted by Br2 or I2 to yield monohalide derivatives (3-6). The compounds 1-6 were characterized by elemental analysis, ^1H NMR and FT-IR spectroscopy. The crystal structures of 1, 2 and 4 have been determined by single crystal X-ray diffraction analysis. The Sn atom in 1 and 2 exhibits a tetrahedral geometry distorted towards trigonal bipyramid due to a weak intramolecular interaction between Sn and the hydroxyl O atoms [0.2839(4) nm and 0.2744(5) nm], while the Sn atom in 4 adopts a trigonal bipyramidal geometry with a significant O→Sn(1) interaction [0.2552(5) nm]. 相似文献