全文获取类型
收费全文 | 6203篇 |
免费 | 505篇 |
国内免费 | 241篇 |
专业分类
化学 | 4237篇 |
晶体学 | 54篇 |
力学 | 249篇 |
综合类 | 6篇 |
数学 | 691篇 |
物理学 | 1712篇 |
出版年
2024年 | 24篇 |
2023年 | 75篇 |
2022年 | 203篇 |
2021年 | 251篇 |
2020年 | 224篇 |
2019年 | 207篇 |
2018年 | 240篇 |
2017年 | 176篇 |
2016年 | 262篇 |
2015年 | 229篇 |
2014年 | 249篇 |
2013年 | 446篇 |
2012年 | 418篇 |
2011年 | 456篇 |
2010年 | 260篇 |
2009年 | 273篇 |
2008年 | 271篇 |
2007年 | 289篇 |
2006年 | 251篇 |
2005年 | 241篇 |
2004年 | 161篇 |
2003年 | 139篇 |
2002年 | 129篇 |
2001年 | 133篇 |
2000年 | 108篇 |
1999年 | 122篇 |
1998年 | 64篇 |
1997年 | 74篇 |
1996年 | 89篇 |
1995年 | 74篇 |
1994年 | 46篇 |
1993年 | 57篇 |
1992年 | 63篇 |
1991年 | 68篇 |
1990年 | 41篇 |
1989年 | 30篇 |
1988年 | 37篇 |
1987年 | 37篇 |
1986年 | 35篇 |
1985年 | 46篇 |
1984年 | 32篇 |
1983年 | 30篇 |
1982年 | 33篇 |
1981年 | 17篇 |
1980年 | 32篇 |
1979年 | 26篇 |
1978年 | 30篇 |
1977年 | 29篇 |
1976年 | 23篇 |
1973年 | 15篇 |
排序方式: 共有6949条查询结果,搜索用时 15 毫秒
91.
Yiming Mo Yanfei Guan Pritha Verma Jiang Guo Mike E. Fortunato Zhaohong Lu Connor W. Coley Klavs F. Jensen 《Chemical science》2021,12(4):1469
With recent advances in the computer-aided synthesis planning (CASP) powered by data science and machine learning, modern CASP programs can rapidly identify thousands of potential pathways for a given target molecule. However, the lack of a holistic pathway evaluation mechanism makes it challenging to systematically prioritize strategic pathways except for using some simple heuristics. Herein, we introduce a data-driven approach to evaluate the relative strategic levels of retrosynthesis pathways using a dynamic tree-structured long short-term memory (tree-LSTM) model. We first curated a retrosynthesis pathway database, containing 238k patent-extracted pathways along with ∼55 M artificial pathways generated from an open-source CASP program, ASKCOS. The tree-LSTM model was trained to differentiate patent-extracted and artificial pathways with the same target molecule in order to learn the strategic relationship among single-step reactions within the patent-extracted pathways. The model achieved a top-1 ranking accuracy of 79.1% to recognize patent-extracted pathways. In addition, the trained tree-LSTM model learned to encode pathway-level information into a representative latent vector, which can facilitate clustering similar pathways to help illustrate strategically diverse pathways generated from CASP programs.Tree-structured long short-term memory neural model learns to understand the retrosynthesis design strategies from patent-extracted retrosynthetic pathway data. 相似文献
92.
Kinetic study on the cleavage of N‐phenylphthalimide (NPhPT) in the presence of 0.05 M NH2NH2 and mixed H2O‐CH3CN solvents reveals the occurrence of reaction scheme where A, B, C, C1, An, E, and F represent NPhPT, o‐CO?2C6H4CONHC6H5, o‐CONHNH2C6H4‐ CONHC6H5, N‐aminophthalimide, aniline, o‐CO?2C6H4CONHNH2, and o‐CONHNH2C6H4‐CONHNH2, respectively. But, in the presence of either nonbuffered ?0.20 M NH2NH2 hydrazine buffer of pH ~7.30–8.26 with total buffer concentration ([Buf]T) of >0.02 M, further conversion of F to 2,3‐dihydrophthalazine‐1,4‐dione (DHPD) has been detected depending upon the length of the reaction time (t), the values of [Buf]T, and pH. It has been shown that the rate of conversion of C1 to F is much faster than that of C to C1 which is much faster than that of F to DHPD. The reaction step A → C involves general base (GB) catalysis, while step C → C1 seems to involve specific base–general acid (GA) and GB‐GB catalysis. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 147–161, 2005 相似文献
93.
Protonation of acylferrocenes (FeCOR) in FSO3H-SO2CIF(SO2) solution was studied by PMR spectroscopy. The site of protonation is found to be at the carbonyl oxygen atom. Temperature dependent PMR spectra of protonated acylferrocenes FeCROH+ (R = CH3, C2H5, C6H5, OCH3) were observed indicating intermolecular hydrogen exchange with the acid solvent system. In addition the PMR spectra of acylferrocenes in FSO3 H-SO2 CIF(SO2) were found to be dependent upon the acid concentration. 相似文献
94.
Muhammad Adil Khan Shahid Khan Samet Erden Muhammad Samraiz 《Mathematical Methods in the Applied Sciences》2022,45(1):36-48
There are many useful applications of Jensen's inequality in several fields of science, and due to this reason, a lot of results are devoted to this inequality in the literature. The main theme of this article is to present a new method of finding estimates of the Jensen difference for differentiable functions. By applying definition of convex function, and integral Jensen's inequality for concave function in the identity pertaining the Jensen difference, we derive bounds for the Jensen difference. We present integral version of the bounds in Riemann sense as well. The sharpness of the proposed bounds through examples are discussed, and we conclude that the proposed bounds are better than some existing bounds even with weaker conditions. Also, we present some new variants of the Hermite–Hadamard and Hölder inequalities and some new inequalities for geometric, quasi-arithmetic, and power means. Finally, we give some applications in information theory. 相似文献
95.
96.
Summary
Bis(cyclopentadienyl)titanium(IV) diisothiocyanate [(Cp)2-Ti(NCS)2] reacts with MCl2 (M = Cu, Pd or Pt), [CuCl(PPh3)3], [RuCl2(PPh3)3] and [RuCl2(DMSO)4] (DMSO = dimethylsulphoxide) giving solid compounds of stochiometry [(Cp)2Ti(-NCS)2MCl2] (M = Cu, Pd or Pt), [(Cp)2Ti(-NCS)2CuCl(PPh3)], [(Cp)2Ti(-NCS)2-RuCl(PPh3)2]Cl and [(Cp)2Ti(-NCS)2RuCl2(DMSO)2]. These products have been characterized by physicochemical and spectroscopic methods. 相似文献
97.
98.
We report the first experimental observation of the excitation of the 4p
2
P
3/2, 1/2 resonance states of Ca II (located at 74 720.4 and 74 497.5 cm–1 above the ground state of Ca I) following pulsed-laser pumping of the 4s
2
1
S
O–4s4p
3P1 intercombination transition of Ca I (E
ex = 15 210 cm–1). Large scale collisional transfer of energy between the laser-excited atoms is believed to be responsible for this. This is possibly because sufficient time is available (rad of the 4s4p
3
P
1 state is approximately 350 µs) for collisions to build such a high level of excitation. Some interesting additional features of the fluorescence spectra of the laser-pumped Ca vapor, such as temperature dependence of the fluorescence intensities and evolution in time of some selected states, are also presented. 相似文献
99.
100.