全文获取类型
收费全文 | 13139篇 |
免费 | 835篇 |
国内免费 | 1582篇 |
专业分类
化学 | 4780篇 |
晶体学 | 101篇 |
力学 | 213篇 |
综合类 | 152篇 |
数学 | 8309篇 |
物理学 | 2001篇 |
出版年
2024年 | 17篇 |
2023年 | 121篇 |
2022年 | 211篇 |
2021年 | 276篇 |
2020年 | 289篇 |
2019年 | 366篇 |
2018年 | 346篇 |
2017年 | 364篇 |
2016年 | 273篇 |
2015年 | 264篇 |
2014年 | 427篇 |
2013年 | 1278篇 |
2012年 | 473篇 |
2011年 | 696篇 |
2010年 | 612篇 |
2009年 | 774篇 |
2008年 | 871篇 |
2007年 | 1069篇 |
2006年 | 826篇 |
2005年 | 695篇 |
2004年 | 613篇 |
2003年 | 638篇 |
2002年 | 530篇 |
2001年 | 458篇 |
2000年 | 430篇 |
1999年 | 397篇 |
1998年 | 375篇 |
1997年 | 335篇 |
1996年 | 248篇 |
1995年 | 191篇 |
1994年 | 194篇 |
1993年 | 130篇 |
1992年 | 119篇 |
1991年 | 87篇 |
1990年 | 72篇 |
1989年 | 68篇 |
1988年 | 56篇 |
1987年 | 55篇 |
1986年 | 38篇 |
1985年 | 57篇 |
1984年 | 34篇 |
1983年 | 24篇 |
1982年 | 32篇 |
1981年 | 25篇 |
1980年 | 23篇 |
1979年 | 15篇 |
1978年 | 18篇 |
1977年 | 15篇 |
1976年 | 12篇 |
1974年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
Bis‐trimethylamine‐ethynyl‐di‐bis(trifluoromethyl)borane [Me3N(CF3)2BCCB(CF3)2NMe3] ( 1 ) has been prepared from trimethylamine‐ethynyl‐bis(trifluoromethyl)borane, [HCCB(CF3)2NMe3], and dimethylamino‐bis(trifluoromethyl)borane, (CF3)2BNMe2. The structure of 1 has been determined by x‐ray crystallography. In the solid state the molecule possesses crystallographic Ci symmetry. The acetylenic attachment to the boron atom is characterized by a short B–C bond length of 1.565(4) Å and an essentially linear B–C–C′ bond angle of 178.1(4)°. 相似文献
962.
Ana Erica Paramo Susana Palmero Aranzazu Heras Alvaro Colina David Ibañez 《Electroanalysis》2016,28(4):890-897
The purpose of this work is to develop simple and reproducible ways to fabricate carbon nanofibers (CNFs) electrodes. Disposable CNFs electrodes were fabricated after filtering CNFs through polytetrafluoroethylene filters (PTFE), using directly as electrodes these filters with the CNFs film. The electrochemical behaviour of CNFs/PTFE electrodes were tested with ferrocenemethanol and tris(2,2′‐bypyridyl)dichlororuthenium(II) (Ru(bipy)32+). As a proof of concept of the analytical usefulness of the CNFs/PFTE electrodes, the study of vitamin B12 was carried out. Multivariate calibrate has been successfully used to determine vitamin B12 in a complex medium with vitamins B1 and B6 as interferents. 相似文献
963.
Screening and isolation of potential lactate dehydrogenase inhibitors from five Chinese medicinal herbs: Soybean,Radix pueraria,Flos pueraria,Rhizoma belamcandae,and Radix astragali 下载免费PDF全文
Ying Tang Senlin Li Sainan Li Xiaojing Yang Yao Qin Yuchi Zhang Chunming Liu 《Journal of separation science》2016,39(11):2043-2049
Stroke is among the leading causes of death and severe disability worldwide. Flavonoids have been extensively used in the treatment of ischemic stroke by reducing lactate dehydrogenase levels and thereby enhancing blood perfusion to the ischemic region. Here, we used ultrafiltration high‐performance liquid chromatography coupled with diode array detection and mass spectrometry for the rapid screening and identification of flavonoids from five Chinese medicinal herbs: soybean, Radix pueraria, Flos pueraria, Rhizoma belamcandae, and Radix astragali. Using PC12 cells as a suitable in vitro model of toxicity, cell viability was quantitated using the 3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide assay. The results showed that the extracts of soybean and the six major components, namely, acetyldaidzin, malonylgenistin, daidiain, glycitin, genistin, and acetylcitin; the extract of R. pueraria and its main component daidzein; the extract of F. pueraria and its three major components, tectorigenin, tectoridin, and tectorigenin‐7‐O‐xylosylglucosid; and the extract of R. belamcandae and its main component, tectoridin, were strong lactate dehydrogenase inhibitors. Also, the components of R. astragali showed no bioactivity. These findings indicate that the ultrafltration high‐performance liquid chromatography coupled with diode array detection and mass spectrometry method could be utilized in rapid screening and separation of bioactive compounds from a complex matrix. 相似文献
964.
The structural and thermodynamic properties of the donor-acceptor (DA) complexes of Group 13 metal halides (MX3) with ethylenediamine and their decomposition products have been studied theoretically at the B3LYP/LANL2DZ(d,p) level of theory. Gas-phase dissociation into various components and HX elimination reactions are considered. Both processes are endothermic but favored by entropy. Complexes of 2:1 composition are predicted to be stable in the gas phase up to 640-1000 K. It is found that complexation with the second acceptor molecule lowers the HX elimination enthalpy; in turn, HX elimination increases DA bonding with a second MX3 molecule. Exceptionally high values of the dissociation enthalpies (310-390 kJ mol(-1)) and HX elimination reactions (360-420 kJ mol(-1)) of the amido compounds MX2NHC2H4NH2 and MX2NHC2H4NHMX2 make them important intermediates in the decomposition processes. Dissociation reactions of the complexes are more favorable than HX elimination reactions; however, the subsequent oligomerization and cyclization processes of coordinationally unsaturated amido and imido compounds may facilitate HX elimination. Since HI elimination reactions are predicted to be the least endothermic, and aluminum-containing compounds have the strongest M-N dissociation enthalpies, it is expected that compounds based on aluminum iodide are promising objects for experimental studies. 相似文献
965.
966.
通过2,5-二溴苯乙烯与对甲酰基苯硼酸的Suzuki偶联反应得到2,5-二(4'-甲酰基苯基)苯乙烯.在催化剂量的冰乙酸存在下,与光学纯的(S)-(-)-α-甲基苄胺或(R)-( )-α-甲基苄胺发生缩和反应,得到了一对手性非外消旋单体,( )-2,5-二{4'-[(N-(S)-α-甲基苄亚胺基)次甲基]苯基}苯乙烯和(-)-2,5-二{4'-[(N-(R)-α-甲基苄亚胺基)次甲基]苯基}苯乙烯.以偶氮二异丁腈(AIBN)或过氧化苯甲酰(BPO)为引发剂,经自由基溶液聚合得到光学活性聚合物.比旋光度、紫外-可见吸收光谱以及圆二色光谱研究表明,聚合物主链可能形成了某一方向占优的稳定螺旋构象,且该螺旋构象的旋光方向与单体的旋光方向相反.聚合条件对聚合物的光学活性有很大影响,在极性较大的芳香族溶剂和较高温度下得到的聚合物具有和单体相差更大的比旋光度.侧基的手性基团脱除后,聚合物仍具有一定的旋光性,说明聚合过程中形成的螺旋手性具有一定的记忆效应. 相似文献
967.
M. A. Bastrakov A. M. Starosotnikov V. V. Kachala E. N. Nesterova S. A. Shevelev 《Russian Chemical Bulletin》2007,56(8):1603-1607
Reaction of different anionic S-nucleophiles with 3-R-2-aryl-4,6-dinitroindoles led to a regiospecific nucleophilic substitution
of the nitro group in position 4 with 6-NO2 group remaining intact. The representatives of some peri-annulated polycyclic systems were synthesized on the basis of the substitution products.
Dedicated to Academician V. A. Tartakovsky in honor of his 75th anniversary.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1543–1547, August, 2007. 相似文献
968.
Thomas Müller 《Silicon Chemistry》2007,3(3-4):123-130
Quantum mechanical calculations at the B3LYP/6-311+G(d,p) and MP2/6-311+G(d,p) level of theory reveal that higher congeners
of the aromatic imidazolium ion, e.g. 2-E-imidazolium ions (E = Si, Ge, Sn), adopt either planar or pyramidal structures,
depending on the substituent R
2 attached to the element and on the group 14 element itsself. In the case of 2-silaimidazolium ions chemically significant
energy differences in favour of non-planar cations are predicted only for strongly σ-electron withdrawing substituents R
2 such as F or CF3. The pyramidalization computed for the germanium and tin analogues are however significant for all investigated substituents
R
2 and are accompanied by a substantial stabilization compared to the corresponding planar structures. A detailed bonding analysis
reveals that the non-planar cations are best described as complexes of monovalent group 14 element cations R
2E+ with the diazabutadiene ligand. 相似文献
969.
Hatsuo Yamamura Kousuke Satake Rieko Hirasawa Shinichi Yamada Masao Kawai 《Journal of inclusion phenomena and macrocyclic chemistry》2007,57(1-4):261-264
A novel cyclodextrin (CD) dimer linked with a bis(picolinyl)cystine (Cys) moiety was prepared by the coupling of Boc-protected
Cys with amino-modified CDs, followed by deprotection of the Boc groups and bispicolinylation. The dimer showed less affinity
to an organic guest molecule compared to that of a native CD monomer. It was attributed to an intramolecular inclusion of
the pyridine moiety into CD cavity. The dimer caused significant increase of its organic guest affinity by an addition of
a copper ion. The included pyridine group may come out of a CD cavity to bind the copper ion and the two CDs included cooperatively
and intermolecularly a guest molecule with high affinity. 相似文献
970.
A new compound of 4,4'-diamino-N,N'-diethyl bisbenzenesulfamide (C18H26N4O4S2,Fw = 426.55) has been synthesized and its structure was determined by X-ray crystallography method. The crystal belongs to the monoclinic system, space group P21/c with a = 10.0623(9), b =13.6759(13), c =15.5309(14) (A),β = 100.482(2)°, V= 2101.6(3)(A)3, Dc = 1.348 g/cm3, F(000) = 904,μ = 0.285 mm-1, Z = 4, the final R = 0.0512 and wR = 0.1363 for 3485 observed reflections with I >2σ(Ⅰ). The structure of the title compound is pseudo secondary axisymmetric, and the two sulfamide-groups show distorted tetrahedral configurations. 相似文献