全文获取类型
收费全文 | 19215篇 |
免费 | 645篇 |
国内免费 | 108篇 |
专业分类
化学 | 12428篇 |
晶体学 | 116篇 |
力学 | 397篇 |
综合类 | 2篇 |
数学 | 3833篇 |
物理学 | 3192篇 |
出版年
2023年 | 117篇 |
2022年 | 221篇 |
2021年 | 405篇 |
2020年 | 372篇 |
2019年 | 345篇 |
2018年 | 734篇 |
2017年 | 533篇 |
2016年 | 637篇 |
2015年 | 519篇 |
2014年 | 573篇 |
2013年 | 1133篇 |
2012年 | 1165篇 |
2011年 | 1239篇 |
2010年 | 799篇 |
2009年 | 756篇 |
2008年 | 1100篇 |
2007年 | 1053篇 |
2006年 | 984篇 |
2005年 | 909篇 |
2004年 | 725篇 |
2003年 | 647篇 |
2002年 | 566篇 |
2001年 | 333篇 |
2000年 | 380篇 |
1999年 | 240篇 |
1998年 | 186篇 |
1997年 | 191篇 |
1996年 | 226篇 |
1995年 | 158篇 |
1994年 | 170篇 |
1993年 | 151篇 |
1992年 | 136篇 |
1991年 | 122篇 |
1990年 | 127篇 |
1989年 | 115篇 |
1988年 | 92篇 |
1987年 | 94篇 |
1986年 | 96篇 |
1985年 | 116篇 |
1984年 | 135篇 |
1983年 | 97篇 |
1982年 | 104篇 |
1981年 | 106篇 |
1980年 | 79篇 |
1979年 | 64篇 |
1978年 | 83篇 |
1977年 | 68篇 |
1976年 | 68篇 |
1975年 | 61篇 |
1973年 | 44篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
The reactions of the 3-methoxy-isoxazole quinones 1 and 9 in a highly polar solution of dimethyl sulfoxide have been studied. Quinones of type 2, 4/11, 5/12, 6 and 7/13 have been isolated; nmr measurements give a detailed insight into the reaction mechanism. 相似文献
142.
Riedl Z Kövér P Soós T Hajós G Egyed O Fábián L Messmer A 《The Journal of organic chemistry》2003,68(14):5652-5659
2-Arylsulfanyl and 2-benzylsulfanylpyridinium N-arylimides (2) easily prepared from 3-aryltetrazolopyridinium salts (1) with aryl and benzylthiolates, respectively, reacted with various dipolarophiles yielding cycloadducts that underwent transformation to give tetrahydropyrrolo[3,2-b]pyridines (5, 6, and 8) in good yields. A similar rearrangement (formation of 15) was also observed in the case of parent derivatives being unsubstituted in position 2 (12). The abscence of any significant solvent effect, comparison of the sulfur and non-sulfur analogues, as well as the stereoselective nature of the observed ring transformation seem to support a sigmatropic mechanism. Structure elucidation of the products has been carried out by single-crystal X-ray diffraction and (1)H NMR experiments. 相似文献
143.
The aim of this research work was to clarify the role of the mixed emulsifier in the structure formation and water binding
mode in the case of o/w creams prepared with different surfactants. The swelling behavior of mixed emulsifiers was examined
by means of direct investigation methods such as transmission electron microscopy (TEM) and X-ray diffraction. The detailed
structure image of the creams was created with the help of the latter. The influence of the structure of the hydrophilic gel
phase, and the structural changes during storage were studied with rheological methods. On the basis of the results, it can
be stated that the investigated creams had different structures from those mentioned in the literature: surfactant did not
create a mixed bilayer with the structure to furnish fatty amphiphile; instead, micelles were formed. These results correlated
well with the results of the rheological tests. 相似文献
144.
Asymmetry and magnetism in bis(oximato)-bridged heterobimetallic compounds: a computational approach
Cano J Rodríguez-Fortea A Alemany P Alvarez S Ruiz E 《Chemistry (Weinheim an der Bergstrasse, Germany)》2000,6(2):327-333
A density functional study of exchange coupling was carried out for a series of heterobinuclear oximato-bridged transition metal complexes. Model calculations were used to examine the influence of the electronic configuration of the metal atoms on the coupling constants. This analysis was complemented by a study of the variation of the coupling constant with the most usual structural distortions within this family of compounds. The influence of the nature of the terminal ligands as well as that of the symmetry on the bridge were also investigated. 相似文献
145.
Microchimica Acta - Mit der turbidimetrischen Strychninnitrat-Methode für Phosphorspurenbestimmung nach der Version von Hegedüs und Dvorszky kann man nur dann ausgezeichnete... 相似文献
146.
Alberto Avenoza Jesús H Busto Carlos Cativiela Francisco Corzana Jesús M Peregrina María M Zurbano 《The Journal of organic chemistry》2002,67(2):598-601
A short and efficient asymmetric synthesis of the (6R,7S)-7-tert-butoxycarbonylamino-2-ketocarbacepham is described. The key step involves the hetero Diels-Alder reaction of the benzylimine derived from the enantiomer of Garner's aldehyde with Danishefsky's diene. 相似文献
147.
A detailed analysis of one- and two-dimensional 1H and 13C NMR data for the endo and the exo adducts, obtained by Diels-Alder reaction of thuriferic and epithuriferic acids with cyclopentadiene is described. The unequivocal spectral data assignment of the endo and exo structures was complemented with molecular modelling studies and confirmed through X-ray diffraction studies. 相似文献
148.
149.
Pau Bernadó Renato Barbieri Esteve Padrós Claudio Luchinat Miquel Pons 《Journal of the American Chemical Society》2002,124(3):374-375
Addition of Tb3+ to purple membrane (PM) suspensions changes the orientation of the menbrane normal from parallel to perpendicular with respect to the magnetic field. Residual dipolar couplings measured in protein L in the presence of PM are scaled by a factor of -1/2. NMR line broadening and cross-correlation effects induced by the addition of PM are partially reversed by Tb3+ but not by Tm3+ that has no effect on the orientation of PM. This is interpreted as the result of anisotropically restricted motion of protein L in the transiently PM-bound form. 相似文献
150.
Thermal behaviour of a polyhedral oligomeric silsesquioxane with epoxy resin cured by diamines 总被引:1,自引:0,他引:1
Ramírez C. Abad M. J. Barral L. Cano J. Díez F. J. López J. Montes R. Polo J. 《Journal of Thermal Analysis and Calorimetry》2003,72(2):421-429
A new material belongs to the family of polyhedral oligomeric silsesquioxanes, the 1-(3-glycidyl) propoxy-3,5,7,9,11,13,15-isobutylpentacyclo-[9.5.1.1(3,9).1(5,15).1(7,13)]octasiloxane
(glycidylisobutyl-POSS) is characterized by differential scanning calorimetry, thermogravimetric analysis and atomic force
microscopy. Epoxy systems based on diglycidyl ether of bisphenol A (DGEBA) cured with the diamines, 4,4'-diamine-diphenylmethane
(DDM) and 1,4-phenylenediamine (pPDA), were kinetically studied by differential scanning calorimetry in isothermal and dynamic
modes. The thermal behaviour of these systems as the glycidylisobutyl-POSS was added, is discussed later.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献