全文获取类型
收费全文 | 4890篇 |
免费 | 313篇 |
国内免费 | 32篇 |
专业分类
化学 | 3365篇 |
晶体学 | 60篇 |
力学 | 174篇 |
数学 | 779篇 |
物理学 | 857篇 |
出版年
2023年 | 32篇 |
2022年 | 46篇 |
2021年 | 147篇 |
2020年 | 101篇 |
2019年 | 201篇 |
2018年 | 161篇 |
2017年 | 109篇 |
2016年 | 244篇 |
2015年 | 157篇 |
2014年 | 254篇 |
2013年 | 493篇 |
2012年 | 294篇 |
2011年 | 306篇 |
2010年 | 217篇 |
2009年 | 204篇 |
2008年 | 229篇 |
2007年 | 214篇 |
2006年 | 208篇 |
2005年 | 178篇 |
2004年 | 163篇 |
2003年 | 132篇 |
2002年 | 182篇 |
2001年 | 73篇 |
2000年 | 110篇 |
1999年 | 67篇 |
1998年 | 41篇 |
1997年 | 47篇 |
1996年 | 38篇 |
1995年 | 36篇 |
1994年 | 37篇 |
1993年 | 39篇 |
1992年 | 32篇 |
1991年 | 24篇 |
1990年 | 15篇 |
1989年 | 28篇 |
1988年 | 24篇 |
1987年 | 20篇 |
1986年 | 17篇 |
1985年 | 32篇 |
1984年 | 24篇 |
1983年 | 23篇 |
1982年 | 21篇 |
1981年 | 27篇 |
1980年 | 17篇 |
1979年 | 18篇 |
1978年 | 16篇 |
1976年 | 14篇 |
1975年 | 18篇 |
1974年 | 9篇 |
1973年 | 10篇 |
排序方式: 共有5235条查询结果,搜索用时 390 毫秒
41.
Elías Molins Amílcar Labarta Javier Tejada Amparo Caubet Santiago Alvarez 《Transition Metal Chemistry》1983,8(6):377-380
Summary Magnetic moments and57Fe Mössbauer spectra at room temperature and at liquid nitrogen temperature of polynuclear ligand-bridged iron phthalocyanines, FePcL (L = pyrazine and 4,4-bipyridyl), are reported. The experimental data are in agreement with a pseudo-octahedral environment around the iron(II) cation. Partially oxidized derivatives, FePc(pz)I0.26 and FePc(bipy)I1.06 have been synthesized and their57Fe Mössbauer spectra and magnetic moments within the same temperature range obtained. Oxidation is shown to be metal centred and the iodine is found to be in I– form for FePc(pz)0.26 and as both I– and I
3
–
species for FePc(bipy)I1.06. While the magnetic moment of FePc(pz) increases upon oxidation, the opposite occurs for FePc(bipy). 相似文献
42.
2‐Amino‐4‐ethoxycarbonylpyridine 1 was used as a starting material in the synthesis of some 4‐substituted‐N1‐2‐pyridylsulfanilamide derivatives to evaluate their antimicrobial activity. The obtained compounds were of no particular effect against the tested organisms except for a noticeable inhibition of B. subtilis, which was of varying extents but remained clearly significant. 相似文献
43.
The one pot, three‐components condensation of aromatic aldehydes, hydrazine and sulfur in ethanol under microwave irradiation provided symmetrically 3,5‐disubstituted 1,3,4‐thiadiazoles in high yields and good purity. This reaction must be conducted under pressure of hydrogen sulfide produced in‐situ. The structure of the compounds was confirmed by 1H, 13C NMR, MS and elemental analysis. 相似文献
44.
Novel synthesis of (1H)‐pyridin‐2‐one, pyrazolo[1,5‐a]pyrimidine and isoxazole derivatives incorporating N‐methylphthalimide moiety are reported. Reaction of enaminone 2 with malononitrile affords 4. Condensation of 2 with cyanothioacetamide or benzoylacetonitrile affords compounds 6 and 7 respectively. Reaction of 2 with hydrazine hydrate afford 2,3‐dihydrophthalazine‐1,4‐dione ( 10 ). Condensation of 2 with hydroxylamine and 3‐aminopyrazole derivatives affords compounds 12 and 15a,b respectively. Antimicrobial and antifungal activity were determined for representative compounds and most of them showed moderate activity as antimicrobial agents, while compounds 2 and 7 show strong activity against Aspergillus niger. The structure of the newly synthesized compounds was elucidated by elemental analyses and 1H nmr spectra and some cases by 13C nmr investigation. 相似文献
45.
Wagner-Czauderna Elżbieta Orlik Marek Zajączkowska Agnieszka Kalinowski Marek K. 《Transition Metal Chemistry》2004,29(3):276-279
Microsolvation of the [Ni(acac)(tmen)]+ complex by a series of aliphatic n-alcohols (Solv) has been studied in ClCH2CH2Cl solutions by spectrophotometry. Based on the changes in the electronic spectrum of the afore-mentioned complex, observed under the influence of any alcohol, the equilibrium constants for the formation of the [Ni(acac)(tmen)Solv]+ and [Ni(acac)(tmen)Solv2]+ species have been computed according to the algorithm presented in this work. It was found that, in all the systems studied, the stability of five-coordinated [Ni(acac)(tmen)Solv]+ is higher than that of octahedral [Ni(acac)(tmen)Solv2]+. The resulting values are discussed in terms of the Lewis basicity of alcohols. 相似文献
46.
J. Yves Laronze Rachida El Boukili Dominique Patigny Seloua Dridi Dominique Cartier Lean L vy 《Tetrahedron》1991,47(48):10003-10014
Upon acid catalyzed rearrangement after Sheradsky, the aryloximes A gave the tricyclic aminals C, which suffered hydrolysis to lactols E. The unique alcohol 29 was then prepared through a highly stereoselective equilibration-reductive alkylation of the epimeric mixture of lactols 22a,b. Two routes, one of which was stereospecific, allowed cyclization of 29 to (±)-aplysin 34. The yield was 2.5 % from oximes 2a,b. The isomeric epi-aplysin 35 and filiformin 36 were also obtained from 29. The debromo analogues 37,38 and 39 and their trideutero derivatives 41,42 and 43 were synthesized along similar line and allowed unequivocal structure elucidation by NMR spectroscopy. 相似文献
47.
ApparentpK a values of thymol blue solubilized in Aerosol-OT reversed micelles in carbon tetrachloride at 25 °C were determined spectrophotometrically. The effects of the Aerosol-OT concentration and the (water)/(surfactant)R ratio were investigated. The apparentpK a values increase as a function of increasing (R). All the measuredpK a values are less than that in water. The decrease ranges from 1.23 units (detergent=0.4 M,R=1.39) to 0.42 units (Aerosol-OT=0.6 M,R=9,25). These results are rationalized in terms of decreased hydronium ion activity in the micellar core due to its binding to the detergent SO3 ? groups. 相似文献
48.
The asymmetric total synthesis of (-)-reveromycin A is described. The key steps involved a Lewis acid catalyzed inverse electron demand hetero-Diels-Alder reaction followed by hydroboration/oxidation to afford the spiroketal core 4 in a highly stereoselective manner and introduction of the C18 hemisuccinate by high-pressure acylation. 相似文献
49.
Treatment of derivatives of 5-phenylthioureido-3H-imidazole-4-carboxylic acid with bromine afforded 2-(imidazolylamino)benzothiazoles. 相似文献
50.
Fathy Abdel‐Kader Amer Metwally Hammouda Abdel‐Aziz Sayed El‐Ahl Bakr Fathy Abdel‐Wahab 《中国化学会会志》2007,54(6):1543-1552
Treatment of heterocyclic β‐ketonitriles 1a,b with hydrazine hydrate and phenylhydrazine afforded the hydrazine derivatives 2a‐d which cyclized in PPA into pyrrolo[3,4‐c]pyrazoles 3a‐d. Reaction of 1a,b with cyanoacetohydrazide furnished the cyanoacetyl pyrrolo[3,4‐c]pyrazoles 4a,b. The hydrazine 2c reacted with β‐diketone and β‐ketoesters to afford pyrazolyl‐pyrrolines 5‐7. Also the later hydrazine reacted with some D‐aldoses and aceteophenone to give the corresponding hydrazones 10‐12 and hydrazine carboxamide derivatives 15a,b respectively. 相似文献