全文获取类型
收费全文 | 101821篇 |
免费 | 6216篇 |
国内免费 | 15250篇 |
专业分类
化学 | 81956篇 |
晶体学 | 1402篇 |
力学 | 2888篇 |
综合类 | 965篇 |
数学 | 14463篇 |
物理学 | 21613篇 |
出版年
2024年 | 136篇 |
2023年 | 803篇 |
2022年 | 2169篇 |
2021年 | 2121篇 |
2020年 | 2573篇 |
2019年 | 2446篇 |
2018年 | 2152篇 |
2017年 | 3022篇 |
2016年 | 3213篇 |
2015年 | 2691篇 |
2014年 | 3847篇 |
2013年 | 7541篇 |
2012年 | 7096篇 |
2011年 | 5589篇 |
2010年 | 4753篇 |
2009年 | 6452篇 |
2008年 | 6715篇 |
2007年 | 7013篇 |
2006年 | 6417篇 |
2005年 | 5484篇 |
2004年 | 5085篇 |
2003年 | 4273篇 |
2002年 | 5459篇 |
2001年 | 3245篇 |
2000年 | 3039篇 |
1999年 | 2846篇 |
1998年 | 2481篇 |
1997年 | 1990篇 |
1996年 | 1707篇 |
1995年 | 1655篇 |
1994年 | 1464篇 |
1993年 | 1199篇 |
1992年 | 1198篇 |
1991年 | 809篇 |
1990年 | 670篇 |
1989年 | 616篇 |
1988年 | 453篇 |
1987年 | 358篇 |
1986年 | 333篇 |
1985年 | 273篇 |
1984年 | 270篇 |
1983年 | 151篇 |
1982年 | 229篇 |
1981年 | 191篇 |
1980年 | 203篇 |
1979年 | 188篇 |
1978年 | 172篇 |
1977年 | 128篇 |
1976年 | 111篇 |
1973年 | 70篇 |
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
991.
薄层扫描法测定冬眠过程中刺猬活性组织酶的周期变化冯金城(天津师范大学天津300074)孙金生(天津水产研究所天津300221)1前言为了跟踪观察刺猬在冬眠过程中体内乳酸脱氢酶的周期性变化,利用薄层扫描仪测定了其相关的组织同工酶的差异。实践证明,此法简... 相似文献
992.
The Gas Complex MnAlF5 and its Influence on the Purification of AlF3 by Sublimation The gas complex MnAlF5 has been determined mass spectroscopically by the ions MnAlF5+ and MnAlF4+. The gas complex MnAlF5 is formed above 973 K by heating up mixtures of AlF3/MnF2 or AlF3 · 3 H2O endowed with Mn2+ or by heating up solid MnAlF5 too. At 1 008 K the enthalpie of dissociation is 197 kJ/mole. The equilibrium structures of the high spin molecule MnAlF5 (S = 5/2) were examined with ab initio calculations at the HF-level by comlete gradient optimizing. Two minimum structures were found on the potential energy surface. A bidentate fluorine bridged structure was found to be the most stable at the HF-level. Vibrational frequencies and thermodynamic functions of complex formation were estimated for both minimum structures. The importance of the formation of the gas complex for the separation of MnF2 and AlF3 by sublimation is discussed. 相似文献
993.
Hidetake Sakuraba Yoshio Tananaka Fujio Toda 《Journal of inclusion phenomena and macrocyclic chemistry》1991,11(2):195-204
The asymmetric Michael addition of aromatic thiols to 2-cyclohexenone and maleic acid esters has been carried out by utilizing their crystalline cyclodextrin complexes suspended in water. The best chiral induction, 30% enantiomeric excess (ee), was achieved in combinations of 2-cyclohexenone and octyl maleate with the crystalline -cyclodextrin complex of benzenethiol (method A) to afford (S)-3-phenylthiocyclohexanone and (S)-octyl-2-phenylthiosuccinate, respectively, whereas the reaction of benzenethiol with 2-cyclohexenone included in -cyclodextrin (method B) inversely induced the chiral recognition to give the (R)-adduct with 4–9% ee. 相似文献
994.
995.
The properties of thermoresponsive poly(N-isopropylacrylamide) (PNIPAAm) hydrogel in tetrahydrofuran/H2O mixtures were studied. Scanning electron microscopic (SEM) images demonstrate that the hydrogel changes from homogeneous to heterogeneous microstructure upon the addition of tetrahydrofuran to water. This heterogeneous PNIPAAm hydrogel in the mixture solvent exhibits a very slow response rate at temperatures above its lower critical solution temperature. The decreased response rate is attributed to the formation of special ternary complexes including the polymer and the two solvents in the tetrahydrofuran/H2O mixture. Factors controlling the thermoresponse rate are discussed further and several suggestions are provided for designing and developing fast-response PNIPAAm hydrogels in the future. 相似文献
996.
997.
Marit Rolandsgard 《Tetrahedron》2005,61(16):4129-4140
Preparation of α-oxo derivatives of spiro[4.4]nonane, spiro[4.5]decane and spiro[5.5]undecane derivatives is described. An efficient method for spiroannulation by Rh(I)-catalysed intramolecular hydroacylation provides α,α′-difunctionalised spiro[4.5]decanes. The α,α′-dioxo groups have been converted into vinyl triflates for arylation by Pd-catalysed cross-coupling reactions under Stille, Negishi or Suzuki conditions depending on relative reactivities. Stereoselective saturation of the conjugated aryl olefinic bonds by catalytic hydrogenation over Pd-carbon provides methodology for stereoselective preparation of α-aryl- and α,α′-cis,cis-diaryl spiranes, the latter with a sandwich like structure. Single crystal X-ray analyses have been used in the structural assignments. 相似文献
998.
999.
利用单核配合物Na2 CuTS·H2 O在溶剂中的不稳定性质 ,自分解并重新组装得到了双核配合物 [Cu2 (TS)(H2 O) ]·H2 O .配合物为单斜晶系 ,空间群P2 1/C ,晶胞参数为a =0 .80 830 (16 )nm ,b =1.70 79(3)nm ,c =1.4 0 73(3)nm ,β =10 5 .0 8(3)° ,V =1.86 5 4 (6 )nm3 ,Z =4 ,最终一致性因子R =0 .0 6 4 4 .该分子结构为双核单元 ,金属原子通过两个酚氧原子桥联在一起 .结合晶体结构对配合物作了电化学研究 相似文献
1000.
A facile and short synthesis of 2-oxo-4-sec-amino-2,3,5,6-tetrahydrobenzo[f]isoquinoline-1-carbonitriles has been delineated through base catalyzed ring transformation of 5,6-dihydro-2-oxo-4-sec-amino-2H-benzo[h]chromene-3-carbonitrile by cyanoacetamide in excellent yields. 相似文献