全文获取类型
收费全文 | 4002篇 |
免费 | 127篇 |
国内免费 | 34篇 |
专业分类
化学 | 2984篇 |
晶体学 | 34篇 |
力学 | 86篇 |
综合类 | 1篇 |
数学 | 538篇 |
物理学 | 520篇 |
出版年
2023年 | 16篇 |
2022年 | 24篇 |
2021年 | 31篇 |
2020年 | 56篇 |
2019年 | 55篇 |
2018年 | 33篇 |
2017年 | 39篇 |
2016年 | 89篇 |
2015年 | 90篇 |
2014年 | 100篇 |
2013年 | 213篇 |
2012年 | 207篇 |
2011年 | 321篇 |
2010年 | 122篇 |
2009年 | 129篇 |
2008年 | 258篇 |
2007年 | 262篇 |
2006年 | 258篇 |
2005年 | 253篇 |
2004年 | 224篇 |
2003年 | 164篇 |
2002年 | 160篇 |
2001年 | 44篇 |
2000年 | 51篇 |
1999年 | 42篇 |
1998年 | 38篇 |
1997年 | 44篇 |
1996年 | 62篇 |
1995年 | 36篇 |
1994年 | 43篇 |
1993年 | 46篇 |
1992年 | 35篇 |
1991年 | 36篇 |
1990年 | 24篇 |
1989年 | 22篇 |
1988年 | 24篇 |
1987年 | 16篇 |
1986年 | 18篇 |
1985年 | 46篇 |
1984年 | 39篇 |
1983年 | 30篇 |
1982年 | 55篇 |
1981年 | 43篇 |
1980年 | 43篇 |
1979年 | 32篇 |
1978年 | 40篇 |
1977年 | 33篇 |
1976年 | 24篇 |
1975年 | 31篇 |
1974年 | 19篇 |
排序方式: 共有4163条查询结果,搜索用时 15 毫秒
141.
At least one Holy Grail for many academic researchers and pharmaceutical research divisions alike has been to identify therapeutically useful selective PI3K inhibitors. There are several different but closely related PI3Ks which are thought to have distinct biological roles. Until now, however, researchers have been frustrated by poor selectivity of the available pharmacological inhibitors, which are unable to distinguish the different isoforms of PI3K adequately. Fortunately, recently published work gives cause for optimism; there are now several patent specifications published that describe new PI3K inhibitors, including some that are more selective for the delta isoform of PI3K. Given the involvement of PI3Ks in a plethora of biological settings, such isoform-selective inhibitors may have immense potential use for the treatment of patients with inflammatory and autoimmune disorders as well as cancer and cardiovascular diseases. 相似文献
142.
Aloui M Chambers DJ Cumpstey I Fairbanks AJ Redgrave AJ Seward CM 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(11):2608-2621
The technique of intramolecular aglycon delivery (IAD), whereby a glycosyl acceptor is temporarily appended to a hydroxyl group of a glycosyl donor is an attractive method that can allow the synthesis of 1,2-cis glycosides in an entirely stereoselective fashion. 2-O-Allyl protected thioglycoside donors are excellent substrates for IAD, and may be glycosylated stereoselectively through a three-step reaction sequence. This sequence consists of quantitative yielding allyl bond isomerisation, to produce vinyl ethers that can then undergo N-iodosuccinimide mediated tethering of the desired glycosyl acceptor, and subsequent intramolecular glycosylation, to yield either alpha-glucosides or beta-mannosides accordingly. Although attempted one-pot tethering and glycosylation is hampered by competitive intermolecular reaction with excess glycosyl acceptor, this problem can be simply overcome by the use of excess glycosyl donor. Allyl mediated IAD is a widely applicable practical alternative to other IAD approaches for the synthesis of beta-mannosides, that is equally applicable for alpha-gluco linkages. It is advantageous in terms of both simplicity of application and yield, and in addition has no requirement for cyclic 4,6-protection of the glycosyl donor. 相似文献
143.
Jaska CA Dorn H Lough AJ Manners I 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(1):271-281
Reaction of [Pt(PEt(3))(3)] with the primary and secondary phosphine-borane adducts PhRPH x BH(3) (R=H, Ph) resulted in oxidative addition of a P-H bond at the Pt(0) center to afford the complexes trans-[PtH(PPhR x BH(3))(PEt(3))(2)] (1: R=H; 2: R=Ph). The products 1 and 2 were characterized by (1)H, (11)B, (13)C, (31)P, and (195)Pt NMR spectroscopy, and the molecular structures were verified by X-ray crystallography. In both cases, a trans arrangement of the hydride ligand with respect to the phosphidoborane ligand was observed. When 2 was treated with PhPH(2) x BH(3), a novel phosphidoborane ligand-exchange reaction occurred which yielded 1 and Ph(2)PH x BH(3). Treatment of 2 with one equivalent of depe (depe=1,2-bis(diethylphosphino)ethane) resulted in the formation of the complex cis-[PtH(PPh(2) x BH(3))(depe)] (3), in which the hydride ligand and the phosphidoborane ligand are in a cis arrangement. Treatment of 3 with PhPH(2) x BH(3) was found to result in an exchange of the phosphidoborane ligands to give the complex cis-[PtH(PPhH x BH(3))(depe)] (4) and Ph(2)PH x BH(3). Complex 4 was found to undergo further reaction in the presence of PhPH(2) x BH(3) to give meso-cis-[Pt(PPhH x BH(3))(2)(depe)] (5) and rac-cis-[Pt(PPhH x BH(3))(2)(depe)] (6). 相似文献
144.
145.
Ian Hamerton John R. Jones David C. Povey Gallienus W. Smith John M. Barton 《Journal of chemical crystallography》1992,22(1):101-108
The structure of the title compound (I) was determined by direct methods using MoK diffractometer data, and refined by full-matrix least squares toR=0.066 for 1536 reflections (I3 (I)). The structure shows a central tetrahedral carbon atom surrounded by two methyl and two 3-allyl-4-cyanatophenyl groups. The geometry of the cyanato group in this molecule compares well with those in 2,2-bis(4-cyanatophenyl)isopropylideneII) and 4-chloro-3,5-dimethyl-phenylcyanate (III), the only other examples of organic compounds bearing the cyanato moiety in the Cambridge Crystallographic Database (V.3). 相似文献
146.
The synthesis of the 1-phenylimidazobenzodiazepine 5 from 1 and the anion of the nitrone 2 is described. The 3-phenyl-derivative 14 was prepared via the ammo alcohol 11 which was obtained by condensation of the nitrosamine 9 with benzaldehyde followed by catalytic hydro-genolysis of the nitroso group. 相似文献
147.
A series of aryl iodides, with a range of substituents, has been successfully coupled using 10 mol % Pd catalyst with samarium thiolates, derived from the corresponding aryl thiocyanates upon reductive cleavage with SmI(2). Reactions proceed in THF at 65 degrees C in yields ranging from good to excellent and are compatible with both electron-donating and electron-withdrawing substituents, except NO(2). The reactions may also be conducted with aryl bromides although with somewhat lower yields. 相似文献
148.
Denning MS Watts ID Moussa SM Durand P Rosseinsky MJ Tanigaki K 《Journal of the American Chemical Society》2002,124(19):5570-5580
The high symmetry and resulting electronic degeneracy of the C(60)(3)(-) anion is viewed as the key molecular feature in the high superconducting transition temperatures of fulleride and oxidized fullerene systems. The experimental evaluation of this hypothesis requires the synthesis of face-centered cubic (fcc) trivalent fulleride anion salts derived from higher fullerenes such as C(70), which have thus far proved elusive with only stable A(1)C(70), A(4)C(70), and A(6)C(70) phases known. In this paper, we report the synthesis of fcc A(3)C(70) phases stabilized by size-matching the tetrahedral site with the sodium cation. The structures are strongly dependent on the cooling protocol due to the existence of metastable partially or completely orientationally disordered phases. EPR data indicate that the phases are metallic but not superconducting. The densities of states at the Fermi level appear too low to give superconductivity at above 5 K, consistent with recent observations that four electrons per C(70) anion are required for superconductivity. Size-matching on both the octahedral and tetrahedral sites is required for A(3)C(70) stability - K(2)CsC(70) is only stable at elevated temperature and Na(2)C(70) is unstable, the composition corresponding to C(70) and a sodium-rich trigonal phase. 相似文献
149.
The enantioselective reduction of 2-pentanone to (R)- and (S)-2-pentanol by Thermoanaerobacter (formerly Thermoanaerobium) brockii alcohol dehydrogenase (TBADH) in mixtures of water and water-miscible organic solvents was investigated. Significant enzymatic activity was retained in up to 87% methanol, ethanol and acetonitrile. The changes in enzyme activity as a function of organic solvent were correlated to structural alterations of TBADH with a series of spectroscopic studies (fluorescence, fluorescence quenching and circular dichroism (CD)). Interestingly, this study shows that the tetrameric form of TBADH is not critical for catalytic performance. 相似文献
150.
Jae C. Schwartz John E. P. Syka Ian Jardine 《Journal of the American Society for Mass Spectrometry》1991,2(3):198-204
By using a modified ion trap mass spectrometer, resolution in excess of 30,000 (FWHM) at m I z 502 is demonstrated. The method of increasing resolution in the ion trap mass spectrometer operated in the mass-selective instability mode depends on decreasing the rate of scanning the primary radio frequency amplitude as well as using resonance ejection at the appropriate frequency and amplitude. A theoretical basis for the method is introduced. 相似文献