全文获取类型
收费全文 | 6448篇 |
免费 | 1117篇 |
国内免费 | 680篇 |
专业分类
化学 | 5374篇 |
晶体学 | 99篇 |
力学 | 209篇 |
综合类 | 35篇 |
数学 | 1234篇 |
物理学 | 1294篇 |
出版年
2024年 | 6篇 |
2023年 | 76篇 |
2022年 | 195篇 |
2021年 | 145篇 |
2020年 | 145篇 |
2019年 | 163篇 |
2018年 | 129篇 |
2017年 | 163篇 |
2016年 | 183篇 |
2015年 | 472篇 |
2014年 | 514篇 |
2013年 | 325篇 |
2012年 | 307篇 |
2011年 | 427篇 |
2010年 | 351篇 |
2009年 | 560篇 |
2008年 | 517篇 |
2007年 | 387篇 |
2006年 | 413篇 |
2005年 | 324篇 |
2004年 | 381篇 |
2003年 | 305篇 |
2002年 | 293篇 |
2001年 | 197篇 |
2000年 | 144篇 |
1999年 | 136篇 |
1998年 | 121篇 |
1997年 | 114篇 |
1996年 | 93篇 |
1995年 | 98篇 |
1994年 | 97篇 |
1993年 | 68篇 |
1992年 | 71篇 |
1991年 | 48篇 |
1990年 | 44篇 |
1989年 | 31篇 |
1988年 | 23篇 |
1987年 | 17篇 |
1986年 | 20篇 |
1985年 | 18篇 |
1984年 | 16篇 |
1983年 | 12篇 |
1982年 | 5篇 |
1981年 | 15篇 |
1980年 | 20篇 |
1979年 | 15篇 |
1978年 | 14篇 |
1977年 | 12篇 |
1976年 | 5篇 |
1974年 | 4篇 |
排序方式: 共有8245条查询结果,搜索用时 15 毫秒
91.
Synthesis and photochemistry of acridin-9-ylmethoxycarbonyl(Amoc)as a new photochemically removableprotecting group for alcohols were described.Three carbonates of alcohols 1—3 were synthesized throughcondensation of 9-hydroxymethylacridine and chloroformates of alcohols,including benzyl alcohol,phenethylalcohol and one galactose derivative.The photolysis of protected alcohols can efficiently release the correspondingalcohol in the efficiencies(Q_(u1)ε)of 100—200(quantum yield Q_(u1)=0.011—0.023,and molar absorptivity ε=9.1×10~3—9.8×10~3 mol~(-1)·L·cm~(-1))under 360 nm light. 相似文献
92.
Preparation, Characterization and Catalytic Properties of S2O8^2-/ZrO2 Supported by Tungsten Carbide
XUEHua-xin CHENJian-min 《高等学校化学研究》2004,20(1):68-72
A WC-supported S2O8^2-/ZrO2(PSZ) catalyst was prepared and characterized by means of XRD, BET, FTIR and XPS. The isomerization of n-pentane over the catalyst was investigated as well. The results show that the skeletal isomerization and the crack of n-pentane proceed simultaneously on WC-supported S2O8^2-/ZrO2 catalyst. The addition of tungsten carbide showed a significant enhancement in the activity and stability of the catalyst for n-pentane isomerization. The catalyst showed evidently a better activity than S2O8^2-/ZrO2 supported by Pt and WO3. The results can be interpreted by the existence of the tungsten oxycarbide compound(WCxOy) with carbidic, oxide and acidic sites. 相似文献
93.
G. Yu. Ishmuratov M. P. Yakovleva R. Ya. Kharisov G. I. Kunafina E. V. Gorobets A. V. Kuchin G. A. Tolstikov 《Russian Chemical Bulletin》1997,46(8):1465-1467
A convergent synthesis of octadeca-2E, 13Z-dienyl acetate, a pheromone component ofSynanthedon tipuliformis C., has been developed. The synthesis is based on cross-coupling of 8-iodooct-2E-en-I-ol THP ether with dec-5Z-enyl bromide catalyzed by CuBr.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1529–1531, August, 1997. 相似文献
94.
The reaction of titanocene dichloride,Cp
2TiCl2 (Cp=5-C5H5), with one or two equivalents of sodium cyanodithioformate affords the new mono- or bis(dithiocarboxylato) derivativesCp
2TiCl(S2CCN) (1) andCp
2Ti(S2CCN)2 (2). Elimination of sulfur converts2 into the metallacyclicCp
2TiS2C2(CN)2 (3), which does not react with the diene isoprene, but can be reconverted into the appropriate titanocene dihalides by chlorine or bromine. 相似文献
95.
M. Schubert-Zsilavecz D. Gusterhuber W. Likussar G. Färber 《Monatshefte für Chemie / Chemical Monthly》1994,125(6-7):735-742
Summary Regio-, stereo- and siteselectivity of the cycloadditions of trimethylbenzonitrile oxide with 2H-3,4-Dihydro-1,4-benzoxazin-6(8aH)-ones have been investigated. The structures of the obtained products were elucidated by means of homonuclear NOE difference spectroscopy and HMQC and HMBC spectra. The structure of 6-Benzyl-9a-methoxy-3-mesityl-9b-methyl-7,8,9a,9b-tetrahydro-3aH-1,2-oxazolo-[4,5-h]1,4-benzoxazin-4(6H)-on5 was elucidated from a single crystal X-ray structure analysis at ambient temperature: C27H30N2O4, M=446.55g/mol, monoclinic, P21/c,a=10.372 (34) Å,b=11.908 (21) Å,c=20.040 (34) Å, =97.16 (17)°,V=2455.8 (1.9)Å3,Z=4,d
c=1.208g/cm3, =0.08mm–1,R=0.0546.Dedicated to Prof. Ott with best wishes for his 70th birthday 相似文献
96.
V. F. Traven N. Ya. Podkhalyuzina D. S. Kanevskii 《Chemistry of Heterocyclic Compounds》2003,39(7):866-871
5,7-Dihydroxy-4-methyl- and 5,7-dihydroxy-4-trifluoromethylcoumarins are starting compounds in the synthesis of new 10-methyl-8H-difuro[2,3-f;2,3-h]-8-ones and 10-trifluoromethyl-8H-difuro-[2,3-f;2,3-h]chromen-8-ones with high yields. 相似文献
97.
2-(8-羟基喹啉-5-磺酸-7-偶氮)-1,8-二羟基-3,6-萘二磺酸与牛血清白蛋白的相互作用 总被引:5,自引:0,他引:5
用平衡透析法和分光光度法研究了 2 (8 羟基喹啉 5 磺酸 7 偶氮 ) 1,8 二羟基 3,6 萘二磺酸与牛血清白蛋白 (BSA)在酸性溶液中的结合反应 ,认为 8Q5SAC与BSA之间的结合力是以静电引力为主的非共键作用力 ,并探讨了其结合模型。在 2 98K下 ,测得这一反应的最大结合数为 35~ 40 ,结合常数为 6 .1× 10 5L mol。还研究了溶液基本条件如酸度和离子强度等对 8Q5SAC与牛血清白蛋白分子复合物形成的影响 ,在pH =3.34条件下 ,标准工作曲线的线性范围为 0 .2 0~ 46 .90mg L。 相似文献
98.
Grimme S 《Journal of computational chemistry》2003,24(13):1529-1537
Based on a partitioning of the total correlation energy into contributions from parallel‐ and antiparallel‐spin pairs of electrons, a modified third‐order Møller–Plesset (MP) perturbation theory is developed. The method, termed SCS–MP3 (SCS for spin‐component‐scaled) continues previous work on an improved version of MP2 (S. Grimme, J Chem Phys 2003, 118, 9095). A benchmark set of 32 isogyric reaction energies, 11 atomization energies, and 11 stretched geometries is used to assess to performance of the model in comparison to the standard quantum chemical approaches MP2, MP3, and QCISD(T). It is found, that the new method performs significantly better than usual MP2/MP3 and even outperforms the more costly QCISD method. Opposite to the usual MP series, the SCS third‐order correction uniformly improves the results. Dramatic enhancements are especially observed for the more difficult atomization energies, some of the stretched geometries, and reaction and ionization energies involving transition metal compounds where the method seems to be competitive or even superior to the widely used density functional approaches. Further tests performed for other complex systems (biradicals, C20 isomers, transition states) demonstrate that the SCS–MP3 model yields often results of QCISD(T) accuracy. The uniformity with which the new approach improves for very different correlation problems indicates significant robustness, and suggests it as a valuable quantum chemical method of general use. © 2003 Wiley Periodicals, Inc. J Comput Chem 24: 1529–1537, 2003 相似文献
99.
Summary The acid promoted decomposition of 2-(10-diazo-10H-anthracen-9-ylidene)-malonodinitrile in the presence of water, methanol, ethanol, acetic and propionic acid, ethyl thioglycolate,p-thiocresole, and acetyl acetone yielding 9,10-disubstituted 9,10-dihydroanthracenes was investigated. Treatment of the reaction products with tri-ethyl amine followed by hydrochloric acid caused their tautomerization to the corresponding 9,10-disubstituted anthracenes. A mechanism for this reaction is proposed. The first example of an intermolecular C-C-bond formation during the protic acid promoted decomposition of a diazo compound in the presence of CH-acids was found. An improved procedure for the preparation of the starting diazo compound, which may serve as a convenient precursor of 9,10-disubstituted anthracenes, is described.Cordially dedicated to Prof. Dr.K. Winsauer on the occasion of his 70th birthday 相似文献
100.
The reaction of 2,5-diamino-4-benzylamino-pyrimidin-6(III)-one (6) with benzoyl isothiocyanate furnished 2-amino-4-benzylamino-5-[1-(3-benzoylthioureido)]-pyrimidin-6(1H)-one (7) in good yield. The title compound I was synthesized from compound 7 via a cyclodesulfurative reaction with DCC in DMF at 80°C directly to form 8-benzoylamino-9-benzylguanine (9) which was subsequently treated with 1 N sodium hydroxide. 相似文献