全文获取类型
收费全文 | 5863篇 |
免费 | 105篇 |
国内免费 | 125篇 |
专业分类
化学 | 3361篇 |
晶体学 | 125篇 |
力学 | 243篇 |
综合类 | 49篇 |
数学 | 1205篇 |
物理学 | 1110篇 |
出版年
2023年 | 23篇 |
2022年 | 118篇 |
2021年 | 131篇 |
2020年 | 106篇 |
2019年 | 118篇 |
2018年 | 129篇 |
2017年 | 128篇 |
2016年 | 134篇 |
2015年 | 96篇 |
2014年 | 104篇 |
2013年 | 199篇 |
2012年 | 95篇 |
2011年 | 92篇 |
2010年 | 93篇 |
2009年 | 141篇 |
2008年 | 144篇 |
2007年 | 150篇 |
2006年 | 127篇 |
2005年 | 266篇 |
2004年 | 484篇 |
2003年 | 382篇 |
2002年 | 330篇 |
2001年 | 271篇 |
2000年 | 220篇 |
1999年 | 158篇 |
1998年 | 127篇 |
1997年 | 123篇 |
1996年 | 183篇 |
1995年 | 147篇 |
1994年 | 192篇 |
1993年 | 136篇 |
1992年 | 133篇 |
1991年 | 69篇 |
1990年 | 60篇 |
1989年 | 57篇 |
1988年 | 51篇 |
1987年 | 69篇 |
1986年 | 49篇 |
1985年 | 39篇 |
1984年 | 42篇 |
1982年 | 39篇 |
1981年 | 37篇 |
1980年 | 44篇 |
1979年 | 58篇 |
1978年 | 48篇 |
1977年 | 50篇 |
1976年 | 37篇 |
1975年 | 11篇 |
1974年 | 13篇 |
1973年 | 17篇 |
排序方式: 共有6093条查询结果,搜索用时 15 毫秒
991.
992.
Raffaele Montagnani Pierluigi Riani Oriano Salvetti 《Theoretical chemistry accounts》1973,32(2):161-170
The energies of some low-lying electronic excited states of methane are calculated by using wave functions built up in terms of plane waves modulated by multicenter Gaussian factors. The wave functions of the various states are evaluated by a two steps iterative process. In the first step, each excited orbital is determined while keeping all other rigid; in the second, rearrangement effects are introduced. Final results are in good agreement with experimental data and allow to enhance an assignement hypothesis for the first electronic transitions. 相似文献
993.
Chemical Composition of Certain Sedum Species of Kazakhstan 总被引:2,自引:0,他引:2
D. Yu. Korul''kin 《Chemistry of Natural Compounds》2001,37(3):219-223
The chemical composition of sevenSedumspecies growing in Kazakhstan was studied by chromatography. The contents of seven groups of natural compounds were determined. Comparison with markers and literature data identified 24 compounds including 9 that have not been described previously inSedumspecies 相似文献
994.
G. A. Tolstikov I. V. Kresteleva A. Yu. Spivak A. A. Fatykhov V. R. Sultanmuratova 《Russian Chemical Bulletin》1993,42(3):557-563
Aluminum and manganese derivatives of lithiated bis-lactim ethers of cyclo-(L-Ala-L-Ala) were used for the enantioselective synthesis of -hydroxy and polyhydroxy -amino acids of the 2-methyIserine series. A new variant of the synthesis of these acids via 3-acyl-2, 5-dialkoxy-3, 6-dimethyl-3, 6-dihydropyrazines is proposed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 590–595, March, 1993. 相似文献
995.
Cathodic reduction of -azidocinnamic ester under aprotic conditions on Hg, Pt, or graphite electrodes can be directed to high yields of N,N-diacylated dehydroaminoacid derivates (f.i. addition of acetic anhydride) or to almost quantitative yields of -aminocinnamic ester in very pure form by careful addition of H+-donors. The dehydroamino compounds in turn can be further reduced to the corresponding saturated compounds by following H+-addition and changed electrolysis potential. Almost no dimerization occurs. 相似文献
996.
A general algorithm of evaluation of the coefficients of molecular integrals (coupling constants) appearing in the direct configuration interaction method is derived. The configurations are assumed to be spin-adapted antisymmetrized products of orthonormal orbitals. No limitation is imposed either upon the reference state (the number of the singly occupied orbitals may be arbitrary) or upon the excitation multiplicity. 相似文献
997.
The adsorption of ovalbumin, -globulin, and lysozyme on uniform spherical hematite and chromium hydroxide particles in aqueous media has been studied as a function of the pH at a constant ionic strength. The uptake of ovalbumin and -globulin was greatest at their isoelectric points and differed little at 10–2 and 10–3 mol dm–3 NaNO3. The adsorption of lysozyme was strongly influenced by the ionic strength.The deposition of ovalbumin on hematite in the presence of Mg (NO3)2 was significantly greater than that with NaNO3 under otherwise comparable conditions. Dialysis experiments with ovalbumin against magnesium nitrate solutions showed Mg2+ to be specifically bound to the protein.The shapes of isotherms indicated monolayer coverage for ovalbumin and multilayer coating for lysozyme for both adsorbents. The shapes of isotherms of -globulin on hematite point to a rearrangement of the protein on the particle surface, while a monolayer was found on chromium hydroxide particles.Supported by the NSF Grant CHE-9108420Part of a Ph.D. thesis 相似文献
998.
江汉平原上空QTEC变化特征的统计分析 总被引:1,自引:0,他引:1
提出了一种观测台网用多接收机性能一致性的标定方法,并利用江汉平原GPS接收台网的观测数据,分析了该地区上空QTEC的日变化、季节变化、年变化和逐年变化的统计特征.标定结果表明,不同接收机之间的仪器偏差是从双频GPS卫星信标推算QTEC的主要误差来源之一,必须校正.统计分析证实,江汉平原上空QTEC的日变化特征同时受电离层和等离子层控制,其年变化以半年周期为主,峰值和谷值分别出现在两分点和两至点附近,QTEC值的逐年变化从太阳活动峰年到低年逐年下降,与以太阳黑子数为代表的太阳活动密切相关. 相似文献
999.
聚异丁烯高活性端基含量及相对分子质量测定方法的研究 总被引:2,自引:0,他引:2
综合使用核磁,VPO和近红外光谱分析技术对聚异丁烯活性端基含量以及相对分子质量的测定进行了详细研究,分别建立核测定聚异丁烯活性端基含量以及其他烯键含量和近红外光谱快速测量聚异聚丁烯活性端基含量。其他烯键含量和相对分子质量的分析方法。成对t检验结果表明,近红外光谱分析方法测定结果与核磁和VPO方法测定结果之间无显著性差异。 相似文献
1000.
Zofia Urbańczyk-Lipkowska Kinga Suwińska Danuta Mostowicz Marek Chmielewski 《Journal of chemical crystallography》1995,25(10):693-699
Geometry and molecular conformation of theN-toluenesulfonyl1 andN-acetyl-2 derivatives of peracetylated 2-C:1-N-carbonyl-2-deoxy--d-glucopyranosylamine were investigated with the use of X-ray diffraction methods. Compound1 (C20H23NSO10) crystallizes in the monoclinic P21 space group, withZ=2 anda=8.238(1),b=7.988(1),c=16.928(2)Å, =99.12(1)°. Compound2 (C15H19NO9) crystallizes in orthorhombic P212121 space group withZ=4 anda=8.385(1),b=8.550(1),c=24.000(2) Å. Analysis of differences in bond lengths and angles between compounds1 and2 and other compounds of this class showed that the electronwithdawing effect by the residue located at the nitrogen atom can be manifested by lengthening of the -lactam C-N bonds, with simultaneous shortening of the distance between two carbon atoms at the ring fusion. Semi-empirical calculations suggested that the title compounds displayed two positively charged centers, susceptible for attack of nucleophiles, one at the carbonyl group of -lactam and the second at the anomeric carbon atom. Atomic charges, however, calculated for compounds1 and2 did not explain their different reaction directions during alcoholysis.Part 3. For part 1 and 2, see Refs. 5 and 6. 相似文献