全文获取类型
收费全文 | 1017篇 |
免费 | 43篇 |
国内免费 | 2篇 |
专业分类
化学 | 728篇 |
晶体学 | 17篇 |
力学 | 32篇 |
数学 | 67篇 |
物理学 | 218篇 |
出版年
2023年 | 9篇 |
2022年 | 9篇 |
2021年 | 13篇 |
2020年 | 17篇 |
2019年 | 26篇 |
2018年 | 15篇 |
2017年 | 14篇 |
2016年 | 43篇 |
2015年 | 18篇 |
2014年 | 36篇 |
2013年 | 65篇 |
2012年 | 60篇 |
2011年 | 86篇 |
2010年 | 34篇 |
2009年 | 34篇 |
2008年 | 61篇 |
2007年 | 56篇 |
2006年 | 57篇 |
2005年 | 30篇 |
2004年 | 37篇 |
2003年 | 35篇 |
2002年 | 25篇 |
2001年 | 26篇 |
2000年 | 16篇 |
1999年 | 6篇 |
1998年 | 4篇 |
1997年 | 8篇 |
1996年 | 8篇 |
1995年 | 10篇 |
1994年 | 13篇 |
1993年 | 6篇 |
1992年 | 14篇 |
1991年 | 8篇 |
1990年 | 9篇 |
1989年 | 10篇 |
1988年 | 7篇 |
1987年 | 16篇 |
1986年 | 8篇 |
1985年 | 8篇 |
1984年 | 16篇 |
1983年 | 10篇 |
1982年 | 12篇 |
1981年 | 15篇 |
1980年 | 12篇 |
1978年 | 5篇 |
1977年 | 4篇 |
1976年 | 4篇 |
1975年 | 4篇 |
1973年 | 4篇 |
1972年 | 3篇 |
排序方式: 共有1062条查询结果,搜索用时 359 毫秒
121.
Sankar C Bhunia Sutanuka Pal Gopal C Patra Sudhir C Pal 《Journal of heterocyclic chemistry》2014,51(6):1679-1688
Some 6‐disubstituted, 8‐disubstituted, and/6,8‐disubstituted compounds have been prepared from coumarin, 7‐methylcoumarin, and 3,4‐benzocoumarin. The Reimer–Tiemann reaction, Lederer–Manasse reaction, bromination using molecular bromine as well as 2,4,4,6‐tetrabromocyclohex‐2,5‐dien‐1‐one, Elbs reaction, and diazocoupling have been carried under controlled conditions to obtain various derivatives. Further, several reactions of aldehyde derivatives of these coumarins have been carried on to prepare important functional compounds including some heterocycles. It is noteworthy that these aldehydes behave as phenolic aldehydes under alkaline conditions to undergo the Dakin reaction. The reactions are mostly carried in aqueous media involving a dianionic intermediate and hence fulfill one important criterion of green chemistry. 相似文献
122.
A multicomponent method for scaffold-modification of nucleobases (adenine, guanine, and cytosine) was developed. This modification approach, as an alternative to usual synthetic routes involving protection-deprotection or SNAr of halo (or leaving group-equivalent)-purines, affords in one step therapeutically-relevant substituted aminoimidazole-[i]-condensed adenine, [b]-condensed guanine, [c]-condensed cytosine. These derived nucleobases possess enhanced lipophilicity and solubility and contain the functionalities useful for further chemical manipulations. 相似文献
123.
Nanoporous layered silicate materials contain 2D-planar sheets of nanoscopic thickness and ordered porous structure. In comparison to porous 3D-framework materials such as zeolites, they have advantages such as significantly increased surface area and decreased diffusion limitations because the layers can potentially be exfoliated or intercalated into polymers to form nanocomposite materials. These properties are particularly interesting for applications as materials for enhancing molecular selectivity and throughput in composite membranes. In this report, the swelling and surface modification chemistry of two attractive nanoporous layered silicate materials, AMH-3 and MCM-22, were studied. We first describe a method, using long-chain diamines instead of monoamines, for swelling of AMH-3 while preserving its pore structure to a greater extent during the swelling process. Then, we describe a stepwise functionalization method for functionalizing the layer surfaces of AMH-3 and MCM-22 via silane condensation reactions. The covalently attached hydrocarbon chain molecules increased the hydrophobicity of AMH-3 and MCM-22 layer surfaces and therefore allow the possibility of effectively dispersing these materials in polymer matrices for thin film/membrane applications. 相似文献
124.
Sankar M Nowicka E Tiruvalam R He Q Taylor SH Kiely CJ Bethell D Knight DW Hutchings GJ 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(23):6524-6532
In the solvent-free oxidation of benzyl alcohol to benzaldehyde using supported gold-palladium nanoparticles as catalysts, two pathways have been identified as the sources of the principal product, benzaldehyde. One is the direct catalytic oxidation of benzyl alcohol to benzaldehyde by O(2), whereas the second is the disproportionation of two molecules of benzyl alcohol to give equal amounts of benzaldehyde and toluene. Herein we report that by changing the metal oxide used to support the metal-nanoparticles catalyst from titania or niobium oxide to magnesium oxide or zinc oxide, it is possible to switch off the disproportionation reaction and thereby completely stop the toluene formation. It has been observed that the presence of O(2) increases the turnover number of this disproportionation reaction as compared to reactions in a helium atmosphere, implying that there are two catalytic pathways leading to toluene. 相似文献
125.
A diethylpyrrole‐bridged dizinc(II) bisporphyrin (Zn2DEP) is reported that encapsulates fluorescent probe pyrene molecules through strong π–π interactions, which can relay information about the chemical environment in the interior of the host–guest supramolecular assembly. X‐ray structures of both Zn2DEP and the encapsulated pyrene complex are reported, which provides a rare opportunity to investigate the structural changes upon guest binding. A comparative structural analysis demonstrated the exceptional ability of this bisporphyrin platform to open its binding pocket for pyrene encapsulation by a vertical displacement of more than 2.45 Å, although both Zn2DEP and the pyrene complex have nearly parallel porphyrin ring orientations. The 1H NMR spectrum of the encapsulated pyrene complex in solution shows the upfield shifts of the pyrene protons due to a strong ring current effect, which demonstrates the retention of the solid‐state structure in solution. To further assess the extent to which pyrene guests remain encapsulated in solution, a known fluorescence quencher, dimethylaniline, was added to the host–guest assembly, which shows no exciplex formation for days in nonpolar solvents. Thus, the assembly also retained the structural integrity in solution for a long time. The association constant (Kasso) for such a complexation process in solution was observed to be 1.78×105 M ?2 for 1:2 binding. Steady‐state fluorescence and lifetime studies indicate significant photoinduced singlet–singlet energy transformation from the excited state of pyrene to zinc bisporphyrin. 相似文献
126.
Price-sensitive demand for perishable items - an EOQ model 总被引:1,自引:0,他引:1
Shib Sankar Sana 《Applied mathematics and computation》2011,217(13):6248-6259
This paper develops a finite time-horizon deterministic EOQ (Economic Order Quantity) model where the rate of demand decreases quadratically with selling price. Prices at different periods are considered as decision variables. The objective is to find the optimal ordering quantity and optimal sales prices that maximizes the vendor’s total profit. The results are discussed with numerical examples. Sensitivity analysis of the optimal solution with respect to the key parameters of the system is carried out. 相似文献
127.
Shib Sankar Sana 《Applied mathematics and computation》2011,218(7):3277-3288
The paper deals with an inventory model to determine the retailer’s optimal order quantity for similar products. It is assumed that the amount of display space is limited and the demand of the products depends on the display stock level where more stock of one product makes a negative impression of the another product. Besides it, the demand rate is also dependent on selling price and salesmen’s initiatives. Also, the replenishment rate depends on the level of stock of the items. The objective of the model is to maximize the profit function, including the effect of inflation and time value of money by Pontryagin’s Maximal Principles. The stability analysis of the concerned dynamical system has been done analytically. 相似文献
128.
129.
G. Chandra Sekhar Reddy A. Uma Ravi Sankar M. Veera Narayana Reddy C. Radha Rani C. Suresh Reddy 《Journal of heterocyclic chemistry》2011,48(6):1229-1233
A series of a new class of phosphorus analogue macrocycles was accomplished by condensation of N‐substituted‐[bis(3,5‐dimethyl‐2‐hydroxybenzyl)]‐amines with various phosphorus dichlorides in dry toluene in the presence of triethylamine at 0–50°C. All the title compounds were evaluated for antimicrobial activity to determine their efficacy and were effective in suppressing the growth of bacteria and fungi. The chemical structures of the title products were characterized by IR, 1H, 13C, 31P‐NMR, mass spectral studies, and elemental analysis. J. Heterocyclic Chem., (2011). 相似文献
130.