全文获取类型
收费全文 | 7769篇 |
免费 | 1324篇 |
国内免费 | 1233篇 |
专业分类
化学 | 6291篇 |
晶体学 | 69篇 |
力学 | 408篇 |
综合类 | 108篇 |
数学 | 943篇 |
物理学 | 2507篇 |
出版年
2024年 | 29篇 |
2023年 | 145篇 |
2022年 | 203篇 |
2021年 | 207篇 |
2020年 | 301篇 |
2019年 | 331篇 |
2018年 | 269篇 |
2017年 | 214篇 |
2016年 | 358篇 |
2015年 | 399篇 |
2014年 | 441篇 |
2013年 | 584篇 |
2012年 | 662篇 |
2011年 | 622篇 |
2010年 | 513篇 |
2009年 | 505篇 |
2008年 | 528篇 |
2007年 | 473篇 |
2006年 | 463篇 |
2005年 | 381篇 |
2004年 | 373篇 |
2003年 | 339篇 |
2002年 | 386篇 |
2001年 | 247篇 |
2000年 | 217篇 |
1999年 | 206篇 |
1998年 | 149篇 |
1997年 | 122篇 |
1996年 | 96篇 |
1995年 | 79篇 |
1994年 | 69篇 |
1993年 | 52篇 |
1992年 | 60篇 |
1991年 | 53篇 |
1990年 | 42篇 |
1989年 | 35篇 |
1988年 | 21篇 |
1987年 | 17篇 |
1986年 | 29篇 |
1985年 | 32篇 |
1984年 | 18篇 |
1983年 | 16篇 |
1982年 | 9篇 |
1981年 | 7篇 |
1979年 | 6篇 |
1977年 | 2篇 |
1975年 | 2篇 |
1974年 | 2篇 |
1968年 | 3篇 |
1936年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
The reactions of a new cationic complex, [Cp(CO)2Mo(η4-2-methyl-3-SPh-C4H4)]+ PF?6 (3), with carbon, hydride, and nitrogen nucleophiles were found to give only the C-1 addition products in good yield. The X-ray crystal structures of two of the addition products 4a and 4e confirm the regio- and stereochemistry of the nucleophilic additions. 相似文献
82.
The covalent binding of pyrazine on Si(100) have been investigated using high-resolution electron energy loss spectroscopy (HREELS) and x-ray photoelectron spectroscopy. Experimental results clearly suggest that the attachment occurs exclusively through the bonding of the two para-nitrogen atoms with the surface without the involvement of the carbon atoms, as evidenced from the retention of the (sp2) C-H stretching mode in HREELS and a significant down shift of 1.6 eV in the binding energy of N 1s. The binding mechanism for pyrazine on Si(100) demonstrates that reaction channels for heteroatomic aromatic molecules are strongly dependent on the electronic properties of the constituent atoms. 相似文献
83.
Xiao Ling WANG Wei JIAO Xun LIAO Shu Ling PENG Li Sheng DING 《中国化学快报》2006,17(7):916-918
The roots of Paeonia lactiflora Pall. is one of the most important traditional Chinese medicines, which has been used for the treatment of dizziness, spasm, menstrual disorder, spontaneous and night sweating, and as a general analgesic1. Chemical constitu… 相似文献
84.
In continuation of our studies on Aconitum hemsleyanum Pritz.var. pengzhouense, two new norditerpenoid alkaloids, pengshenines A(1) and B(2),have isolated from the roots of the plants and their strucures were elucidated by 1D-and 2D-NMR. 相似文献
85.
Cabeza A Ouyang X Sharma CV Aranda MA Bruque S Clearfield A 《Inorganic chemistry》2002,41(9):2325-2333
Nitrilotris(methylenephosphonic acid) (NTP, [N(CH(2)PO(3)H(2))(3)]) recently has been found to form three-dimensional porous structures with encapsulation of templates as well as layered and linear structures with template intercalation. It was, therefore, of interest to examine the type of organic-inorganic hybrids that would form with metal cations. Mn(II) was found to replace two of the six acid protons, while a third proton bonds to the nitrilo nitrogen, forming a zwitter ion. Two types of compounds were obtained. When the ratio of acid to Mn(II) was less than 10, a trihydrate, Mn[HN(CH(2)PO(3)H)(3)(H(2)O)(3)] (2) formed. Compound 2 is monoclinic P2(1)/c, with a = 9.283(2) A, b = 16.027(3) A, c = 9.7742(2) A, beta = 115.209(3) degrees, V = 1315.0(5) A(3), and Z = 4. The Mn atoms form zigzag chains bridged by two of the three phosphonate groups. The third phosphonate group is only involved in hydrogen bonding. The metal atoms are octahedrally coordinated with three of the sites occupied by water molecules. Adjacent chains are hydrogen-bonded to each other through POH and HN donors, and the additional participation of all the water hydrogens in H-bonding results in a corrugated sheet-like structure. Use of excess NTP at a ratio to metal of 10 to 1 yields an anhydrous compound Mn[HN(CH(2)PO(3)H)(3)] (1), P2(1)/n, a = 9.129(1) A, b = 8.408(1) A, c = 13.453(1) A, beta = 97.830(2) degrees, V = 1023.0(2) A(3), and Z = 4. Manganese is five coordinate forming a distorted square pyramid with oxygens from five different phosphonate groups. The sixth oxygen is 2.85 A from an adjacent Mn, preventing octahedral coordination. All the protonated atoms, three phosphonate oxygens and N, form moderately strong hydrogen bonds in a compact three-dimensional structure. The open-structured trihydrate forms a series of isostructural compounds with other divalent transition metal ions as well as with mixed-metal compositions. This is indicative that the hydrogen bonding controls the type of structure formed irrespective of the cation. 相似文献
86.
87.
[reaction: see text] A microwave-assisted reaction was developed to facilitate the construction of 4,5-disubstituted pyrazolopyrimidines. This one-pot two-step process involves a sequential S(N)Ar displacement of the C4 chloro substituent with various anilines and amines, followed by a Suzuki coupling reaction with different boronic acids. Using microwave irradiation leads to high product conversion, low side product formation, and shorter reactions. 相似文献
88.
Ya‐Ching Shen Yun‐Sheng Lin Shaw‐Man Hsu Ashraf Taha Khalil Shih‐Sheng Wang Ching‐Te Chien Yao‐Haur Kuo Chang‐Hung Chou 《Helvetica chimica acta》2007,90(7):1319-1329
The phytochemical investigation of the more polar fractions from the leaves and twigs of Taxus sumatrana (Taxaceae) afforded five new taxane diterpene esters, tasumatrols P–T ( 1 – 5 ) possessing an 11(15→1),11(10→9)‐diabeotaxane skeleton. Compounds 1, 4 , and 5 contain an α‐hydroxy group at C(14), while 3 has no OH group at either C(13) or C(14). Compound 2 is a natural 4,5‐acetonide derivative, while 4 has an unusual spiro‐connected 2‐hydroxy‐2‐phenyl‐1,3‐dioxolane ring. Ten known taxoids, were also isolated in the course of the chromatographic fractionation. Five additional new O‐acetyl derivatives 3a, 4a, 4b, 5a , and 5b were prepared from the taxanes 3 – 5 . The structures of all new compounds were established on the basis of their spectroscopic analyses. Compound 1 showed mild cytotoxic activity against human Hela and Daoy tumor cells. 相似文献
89.
A numerical investigation for an axisymmetric hypersonic turbulent inlet flow field of a perfect gas is presented for a three-shock configuration consisting of a biconic and a cowl. An upwind parabolized Navier-Stokes solver based on Roe's scheme is used to compute an oncoming flow Mach numberM
=8, temperatureT
=216 K, and pressureP
=5.5293×103 N/m2. In order to assess the flow quantities, the interaction between shock and turbulence, and the inlet efficiency, three different flow calculations — laminar, turbulent with incompressible and compressible two-equationk- turbulence models — have been performed in this work.Computational results show that turbulence is markedly enhanced across an oblique shock with step-like increases in turbulence kinetic energy and dissipation rate. This enhancement is at the expense of the mean kinetic energy of the flow. Therefore, the velocity behind the shock is smaller in turbulent flow and hence the shock becomes stronger. The entropy increase through a shock is caused not only by the amplification of random molecular motion, but also by the enhancement of the chaotic turbulent flow motion. However, only the compressiblek- turbulence model can properly predict a decrease in turbulence length scale across a shock. Our numerical simulation reveals that the incompressiblek- turbulence model exaggerates the interaction between shock and turbulence with turbulence kinetic energy and dissipation rate remaining high and almost undissipated far beyond the shock region. It is shown that proper modeling of turbulence is essential for a realistic prediction of hypersonic inlet flowfield. The performed study shows that the viscous effect is not restricted in the boundary layer but extends into the main flow behind a shock wave. The loss of the available energy in the inlet performance therefore needs to be determined from the shock-turbulence interaction. The present study predicts that the inlet efficiency becomes relatively lower when turbulence is taken into account. 相似文献
90.