全文获取类型
收费全文 | 4862篇 |
免费 | 248篇 |
国内免费 | 16篇 |
专业分类
化学 | 3728篇 |
晶体学 | 19篇 |
力学 | 72篇 |
数学 | 463篇 |
物理学 | 844篇 |
出版年
2023年 | 42篇 |
2022年 | 53篇 |
2021年 | 177篇 |
2020年 | 143篇 |
2019年 | 124篇 |
2018年 | 62篇 |
2017年 | 58篇 |
2016年 | 181篇 |
2015年 | 183篇 |
2014年 | 176篇 |
2013年 | 265篇 |
2012年 | 362篇 |
2011年 | 406篇 |
2010年 | 230篇 |
2009年 | 188篇 |
2008年 | 347篇 |
2007年 | 309篇 |
2006年 | 306篇 |
2005年 | 239篇 |
2004年 | 252篇 |
2003年 | 187篇 |
2002年 | 165篇 |
2001年 | 49篇 |
2000年 | 54篇 |
1999年 | 43篇 |
1998年 | 28篇 |
1997年 | 27篇 |
1996年 | 38篇 |
1995年 | 29篇 |
1994年 | 30篇 |
1992年 | 14篇 |
1991年 | 9篇 |
1990年 | 19篇 |
1989年 | 18篇 |
1988年 | 10篇 |
1987年 | 13篇 |
1986年 | 13篇 |
1985年 | 21篇 |
1984年 | 26篇 |
1983年 | 19篇 |
1982年 | 25篇 |
1981年 | 24篇 |
1980年 | 17篇 |
1979年 | 13篇 |
1978年 | 13篇 |
1977年 | 17篇 |
1976年 | 14篇 |
1975年 | 9篇 |
1974年 | 10篇 |
1973年 | 10篇 |
排序方式: 共有5126条查询结果,搜索用时 15 毫秒
91.
[reaction: see text] Condensation of N-substituted glycines with chiral 3-allyl-2-formyl perhydro-1,3-benzoxazines forms an azomethine ylide that cyclizes to give octahydropyrrolo[3,4-b]pyrrole derivatives. The [3 + 2] dipolar cycloadditions are stereoespecific leading to a single diastereoisomer. The chemical yields are dependent on the reaction temperature and the presence or absence of a base. 相似文献
92.
The origin of enantioselectivity in the Jacobsen-Katsuki reaction has been investigated by applying density functional calculations in combination with molecular mechanics methodologies. The calculations suggest that a high enantiomeric excess is connected to three specific features: 1) a chiral diimine bridge, which induces folding of the salen ligand(H2salen = bis(salicylidene)ethylenediamine), and hence the formation of a chiral pocket; 2) bulky groups at the 3,3'-positions of the salen ligand, which cause a preferential approach from the side of the aromatic rings; and 3) pi conjugation of the olefinic double bond, which confers regioselectivity and, consequently, enantioselectivity. In combination with experimental studies, the model also provides a rationale for the decrease in ee values when one of these components is missing. 相似文献
93.
The redox catalysis of oxygen reduction was performed on a platinum rotating disk electrode. The Fe(III)/Fe(II)/H2SO4 system at different pH's was used as a MEDIATOR. The catalytic effect of mediator was directly related to the solvation sphere of Fe(III) and Fe(II). Only the redox couple FeHSO
4
2+
/FeHSO
4
+
(pH<0) showed a catalytic effect on oxygen reduction. 相似文献
94.
[reactions: see text] Two complementary and efficient strategies have been developed for the regiospecific synthesis of unsymmetrical indolopyrrolocarbazoles (IPCs) mono-N-substituted with a pentacycle. A halogen in position 2 of the intermediate bisindolylmaleimides 3a-e allows a selective Mitsunobu coupling by exploiting the increased acidity of the 2-chloro-substituted indole nitrogen. It also promotes an easier cyclization of bisindolylmaleimides 4a-e and 7b-e to IPCs. Alkylation of the 2-unsubstituted indole-3-carboxamides 2a,b and further processing to the corresponding IPCs gives access to the opposite regioisomers. 相似文献
95.
Jan Becher Klaus Brsndum Poul Hansen Jens Peter Jan Jacobsen 《Journal of heterocyclic chemistry》1988,25(3):795-798
The anions of 1,4-diaryl-3-buten-2-ones 1 reacts with arylisothiocyanates, yielding intermediates 4 which can ring close to 5,6-dihydro-4H-thiopyran-4-ones 5 . Under similar reaction conditions ethyl 3-oxo-4-pentenoates 7 gives the 6-spiropyrans 9 . Methylation of 3 gives the S-methylated open form 6 . 相似文献
96.
The Sm+O chemiionization reaction has been investigated theoretically using a method that allows for correlation and relativistic effects. Potential energy curves have been calculated for several electronic states of SmO and SmO+. Comparison with available spectroscopic and thermodynamic values for these species is reported and a mechanism for the chemiionization reaction Sm+O is proposed. The importance of spin-orbit coupling in the excited states of SmO, in allowing this chemiionization reaction to take place, has been revealed by these calculations. This paper shows the metal-plus-oxidant chemiionization reaction. 相似文献
97.
The tetrahedral zinc complex [(Tp(Ph,Me))ZnOH] (Tp(Ph,Me) = hydrotris(3,5-phenylmethylpyrazolyl)borate) was combined with 2-thenylmercaptan, ethyl 4,4,4-trifluoroacetoacetate, salicylic acid, salicylamide, thiosalicylic acid, thiosalicylamide, methyl salicylate, methyl thiosalicyliate, and 2-hydroxyacetophenone to form the corresponding [(Tp(Ph,Me))Zn(ZBG)] complexes (ZBG = zinc-binding group). X-ray crystal structures of these complexes were obtained to determine the mode of binding for each ZBG, several of which had been previously studied with SAR by NMR (structure-activity relationship by nuclear magnetic resonance) as potential ligands for use in matrix metalloproteinase inhibitors. The [(Tp(Ph,Me))Zn(ZBG)] complexes show that hydrogen bonding and donor atom acidity have a pronounced effect on the mode of binding for this series of ligands. The results of these studies give valuable insight into how ligand protonation state and intramolecular hydrogen bonds can influence the coordination mode of metal-binding proteinase inhibitors. The findings here suggest that model-based approaches can be used to augment drug discovery methods applied to metalloproteins and can aid second-generation drug design. 相似文献
98.
99.
100.
An azaspiro[5.5]undecadiene corresponding to a subunit of the shellfish toxin gymnodimine was synthesized by Diels-Alder cycloaddition. One member of the pair of stereoisomeric adducts was transformed to a spiroimine, which will serve as the core around which the macrocyclic portion of the toxin will be assembled. [structure: see text] 相似文献