全文获取类型
收费全文 | 3186篇 |
免费 | 77篇 |
国内免费 | 530篇 |
专业分类
化学 | 3278篇 |
晶体学 | 72篇 |
力学 | 18篇 |
综合类 | 16篇 |
数学 | 3篇 |
物理学 | 406篇 |
出版年
2023年 | 48篇 |
2022年 | 48篇 |
2021年 | 63篇 |
2020年 | 74篇 |
2019年 | 72篇 |
2018年 | 65篇 |
2017年 | 72篇 |
2016年 | 86篇 |
2015年 | 97篇 |
2014年 | 148篇 |
2013年 | 229篇 |
2012年 | 181篇 |
2011年 | 191篇 |
2010年 | 181篇 |
2009年 | 202篇 |
2008年 | 209篇 |
2007年 | 217篇 |
2006年 | 199篇 |
2005年 | 193篇 |
2004年 | 183篇 |
2003年 | 143篇 |
2002年 | 109篇 |
2001年 | 88篇 |
2000年 | 78篇 |
1999年 | 81篇 |
1998年 | 65篇 |
1997年 | 59篇 |
1996年 | 69篇 |
1995年 | 62篇 |
1994年 | 54篇 |
1993年 | 49篇 |
1992年 | 34篇 |
1991年 | 22篇 |
1990年 | 21篇 |
1989年 | 22篇 |
1988年 | 22篇 |
1987年 | 10篇 |
1986年 | 7篇 |
1985年 | 9篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1981年 | 4篇 |
1980年 | 2篇 |
1977年 | 2篇 |
1976年 | 4篇 |
1974年 | 3篇 |
1973年 | 1篇 |
1972年 | 2篇 |
1971年 | 1篇 |
排序方式: 共有3793条查询结果,搜索用时 203 毫秒
101.
102.
A simple, efficient, and ecofriendly procedure has been developed using propane-l,2,3-triyl tris(hydrogen sulfate) as a catalyst for the synthesis of biscoumarin derivatives in water and solvent-free conditions. The significant features of the present protocol are simplicity, environmentally benign, high yields, no chromatographic separation, and recyclability of the catalyst. 相似文献
103.
In this study, 1,3-disulfonic acid imidazolium hydrogen sulfate (DSIMHS) is used as an efficient and reusable ionic liquid for the green, mild, and efficient synthesis of xanthenes under solvent-free conditions. Simple and easy work-up, low cost, green process, short reaction times and excellent yields of the products are the advantages of this procedure. Further, the catalyst can be recycled and reused at least for four times without a noticeably decrease in its catalytic activity. 相似文献
104.
105.
The enantioselective reduction of prochiral ketones with NaBH4/Me2SO4/(S)-Me-CBS is described. Borane is generated in situ via the reaction of NaBH4 with Me2SO4 in tetrahydrofuran, which is as efficient as the commercial one. Such in situ–generated borane reagent was applied to reduce prochiral ketones in the presence of chiral oxazaborolidine catalyst directly. The corresponding chiral secondary alcohols were obtained with excellent enantiomeric excesses (93–99% ee) and good to excellent yield (80–99%). 相似文献
106.
Nikhil Reddy Madadi Narsimha Reddy PenthalaLin Song Howard P. HendricksonPeter A. Crooks 《Tetrahedron letters》2014
2H-1,2,3-Triazoles (2) were synthesized by [3+2] cycloaddition of (Z)-2,3-diaryl substituted acrylonitriles (1) with sodium azide and ammonium chloride in DMF/water. This method represents a facile and efficient reaction procedure for the synthesis of 4,5-diaryl-2H-1,2,3-triazoles in modest to good yields. 相似文献
107.
Narsimha Reddy Penthala Nikhil Reddy MadadiVenumadhav Janganati Peter A. Crooks 《Tetrahedron letters》2014
Use of a novel reagent has been established for the synthesis of a series of 4,5-diaryl-2H-1,2,3-triazoles (6a–i and 9a–e) from cyanostilbene analogs of benzo[b]thiophene, benzo[b]furan, and indole, catalyzed by l-proline via Lewis base-catalyzed one-step [3+2]cycloaddition of azide. This method provides an efficient, simple, and environmentally benign procedure that affords good yields and relatively short reaction times. 相似文献
108.
Samuel Piña Jr. Diana M. Cedillo Carlos Tamez Nezhueyotl Izquierdo Jason G. Parsons Jose J. Gutierrez 《Tetrahedron letters》2014
Reported here is the reduction of aromatic nitro compounds using sodium borohydride and transition metal sulfides as catalysts. The reaction conditions were optimized using the reduction of nitrobenzene as a model reaction. The catalysts studied were iron sulfide (Fe3S4), copper sulfide (CuS), zinc sulfide (ZnS), cobalt sulfide (Co3S4), and nickel sulfide (NiS). The reduction was monitored using gas chromatography. Quantitative conversions were achieved using Co3S4 and NiS, representing a ten-fold increase in reactivity compared to the non-catalyzed reaction. Fe3S4 and ZnS had no apparent effect on the reduction of nitrobenzene while the reduction using CuS showed a marginal increase. The reduction method was applied to several aryl-nitro derivatives containing either electron-withdrawing or electron-donating groups. Halogen containing aryl-nitro compounds were reduced without dehalogenation. The reduction had no effect on other functional groups such as carboxylic acids, esters, amides, or alkenes, indicating that the reduction is highly chemoselective. 相似文献
109.
This study investigated the cooling crystallization of aluminum sulfate to explore the basic data for the recovery of aluminum resources from coal spoil. Cooling crystallization process of aluminum sulfate with sodium dodecylbenzenesulfonate (SDBS) was investigated experimentally. The effects of operating conditions, namely rotate speed and cooling rate on the crystal size (Median diameter, D0.5) were studied. Based on single factor experimental results, the response surface method (RSM) with a Box–Behnken design (BBD) was used to determine the key operating conditions, from which a predictive equation was established to quantitatively describe the relationships of D0.5 and there relative parameters. The optimum operating conditions for cooling crystallization of aluminum sulfate were as follows: rotate speed of 200–300 rpm, cooling rate of 4–5 °C /min and n (SDBS)/n (Al2(SO4)3) of around 5E‐4. Molecular dynamics (MD) results reveal that SDBS decreases the diffusion coefficient (D) of Al3+ molecules, which inhibits nucleation and promotes crystal growth. 相似文献
110.
A novel Brönsted acidic room temperature ionic liquid (1‐H‐3‐methyl‐imidazolium bisulfate) is found to catalyze efficiently the acetylation of a wide rang of alcohols as well as phenols with acetic anhydride in good to excellent yields at 50°C under solvent‐free conditions. Products are easily isolated by extraction with ether and the protocol is mild and green, compared to the existing method based on toxic solvents. 相似文献