全文获取类型
收费全文 | 29200篇 |
免费 | 1106篇 |
国内免费 | 150篇 |
专业分类
化学 | 21355篇 |
晶体学 | 239篇 |
力学 | 530篇 |
数学 | 3447篇 |
物理学 | 4885篇 |
出版年
2023年 | 195篇 |
2022年 | 246篇 |
2021年 | 392篇 |
2020年 | 526篇 |
2019年 | 521篇 |
2018年 | 348篇 |
2017年 | 292篇 |
2016年 | 782篇 |
2015年 | 667篇 |
2014年 | 816篇 |
2013年 | 1380篇 |
2012年 | 1746篇 |
2011年 | 1986篇 |
2010年 | 1103篇 |
2009年 | 958篇 |
2008年 | 1657篇 |
2007年 | 1584篇 |
2006年 | 1562篇 |
2005年 | 1504篇 |
2004年 | 1292篇 |
2003年 | 979篇 |
2002年 | 1030篇 |
2001年 | 545篇 |
2000年 | 505篇 |
1999年 | 416篇 |
1998年 | 399篇 |
1997年 | 373篇 |
1996年 | 404篇 |
1995年 | 320篇 |
1994年 | 367篇 |
1993年 | 335篇 |
1992年 | 313篇 |
1991年 | 234篇 |
1990年 | 257篇 |
1989年 | 224篇 |
1988年 | 247篇 |
1987年 | 213篇 |
1986年 | 221篇 |
1985年 | 327篇 |
1984年 | 313篇 |
1983年 | 213篇 |
1982年 | 233篇 |
1981年 | 219篇 |
1980年 | 211篇 |
1979年 | 195篇 |
1978年 | 224篇 |
1977年 | 206篇 |
1976年 | 158篇 |
1975年 | 164篇 |
1974年 | 184篇 |
排序方式: 共有10000条查询结果,搜索用时 750 毫秒
241.
M Sugiyama T Sakamoto K Tabata K Endo K Ito M Kobayashi H Fukumi 《Chemical & pharmaceutical bulletin》1989,37(8):2122-2131
Thiazolo[3,2-a]thieno[3,2-d]-, [3,4-d]- and [2,3-d]pyrimidin-5-one derivatives (6, 11 and 16), and polymethylene condensed thieno[3,2-d]-, [3,4-d]- and [2,3-d]pyrimidin-5-one derivatives (19-21), in which the oxygen atom of the oxazolidine moiety in 3 was replaced by a sulfur atom or methylene groups, were synthesized and evaluated for gastric antisecretory activity in pylorus-ligated rats. The structure-activity relationships of these compounds are discussed. 相似文献
242.
Thomas Bally Peter Diehl Edwin Haselbach Alan S. Tracey 《Helvetica chimica acta》1975,58(8):2398-2402
The NMR. spectrum of guanidinium ion 1 is studied in anisotropic liquid crystalline nematic solution. Assuming an HNH-angle of 120°, the distance ratio NH /NC = 0.784 is obtained, from which using NC = 1.330 Å (from X-ray data) NH = 1.043 Å results. An upper bound for the free energy of activation for bond rotation of ΔG+ ≤ 13 kcal/mol is deduced. The bondrotational mobility of 1 is also investigated using the MINDO/3-SCF-procedure. The results obtained for the three conceivable consecutive activation energies for bond-rotation indicate that the observed bond-rotational mobility of 1 does not involve cooperative two- or three-bond rotations. The ‘conjugative stabilization’ of 1 has been estimated to be of the order of 24–26 kcal/mol. 相似文献
243.
Charles L. Watkins Larry K. Krannich Colacot J. Thomas Dileep Srivastava 《Polyhedron》1994,13(24):3299-3307
The reactions of (Me2AlH)3 with Me2AsNMe2, MeAs(NMe2)2, and As(NMe2)3 were investigated as a function of time at room temperature and over the temperature range −90 to 24°C by use of 1H and 13C NMR spectroscopy. (Me2AlH)3 was found to be very reactive toward the aminoarsines, even at −90°C, and no stable Me2AlH-aminoarsine adducts were observed. Instead, the initial stages of the reactions involved AS---N bond cleavage with the generation of highly reactive AlN- and AsH-bonded species. The subsequent course of each reaction and the final arsenic-containing product distribution depended upon the original AL:N stoichiometric ratio and the respective aminoarsine. When the Al:N ratio was 1:1, the reactions were straightforward for each aminoarsine. However, in every case, [Me2AlNMe2]2 was the final AlN-containing product. Independent reactions were carried out to verify many of the proposed decomposition pathways that lead to thermodynamically stable products. The results of this study are compared with those of the analogous, previously reported (Me3Al)2-aminoarsine systems. Additionally, a new synthetic route to [Me2AlAsMe2]3 has been established from the reaction of (Me2AlH)3 with Me2AsH. 相似文献
244.
I. Kunugiyama N. Ito T. Ishida S. Suzuki Y. Katoh Y. Furukawa 《Journal of Radioanalytical and Nuclear Chemistry》1992,165(5):309-317
Bromine and chlorine concentrations in some tissues (liver, spleen, thymus) of healthy mice (normal group) and lymphomatous mice (lymphoma group) were determined by neutron activation analysis. There were significant differences in the Br concentrations of the tissues between the normal and lymphomatous group, while the Cl concentrations were found relatively constant in all the tissues. As a result, the Br/Cl ratios in the tissues of the lymphomatous group were significantly higher than those of the normal group. It was difficult to conclude that Br was absorbed into the bodies of lymphomatous mice more easily than was Cl. It is suggested that Br excretion from the body is suppressed. 相似文献
245.
246.
247.
248.
249.
Liu B. Thomas P. S. Ray A. S. Williams R. P. 《Journal of Thermal Analysis and Calorimetry》2004,76(1):115-122
The effect of sampling conditions on the decomposition of electrolytic manganese dioxide using thermal methods is reported.
Significant differences were observed in the mechanism of the decomposition by simply changing the reaction environment from
a closed pan to an open pan configuration. The purge gas atmosphere was also observed to influence the decomposition mechanism.
As a product of the decomposition is oxygen, the change in the mechanism observed between the experimental conditions may
be explained in terms of the ease of removal of oxygen from the reaction site.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
250.
Fumio Hamada Kyoko Ishikawa Riko Ito Hisanori Shibuya Sanyo Hamai Iwao Suzuki Tetsuo Osa Akihiko Ueno 《Journal of inclusion phenomena and macrocyclic chemistry》1994,20(1):43-51
-Cyclodextrin with two sodium anthranilate moieties (1) has been prepared as a sensor for detecting organic compounds including terpenoids and steroids. Compound1 shows a pure monomer fluorescence whose intensity is increased or decreased upon addition of the guest species examined. In this system, the sodium anthranilate moieties act either as a spacer, which enables the cyclodextrin to form a 1:1 host-guest complex by narrowing the -cyclodextrin cavity, or as a hydrophobic cap.1 recognizes steroids with much higher sensitivity than terpenoids, in which the appended moieties act as a hydrophobic cap for terpenoids and a spacer for steroids, respectively. 相似文献