全文获取类型
收费全文 | 1712篇 |
免费 | 52篇 |
国内免费 | 7篇 |
专业分类
化学 | 1197篇 |
晶体学 | 25篇 |
力学 | 28篇 |
数学 | 179篇 |
物理学 | 342篇 |
出版年
2023年 | 13篇 |
2022年 | 33篇 |
2021年 | 35篇 |
2020年 | 34篇 |
2019年 | 37篇 |
2018年 | 28篇 |
2017年 | 23篇 |
2016年 | 56篇 |
2015年 | 42篇 |
2014年 | 75篇 |
2013年 | 124篇 |
2012年 | 94篇 |
2011年 | 121篇 |
2010年 | 64篇 |
2009年 | 50篇 |
2008年 | 91篇 |
2007年 | 65篇 |
2006年 | 78篇 |
2005年 | 78篇 |
2004年 | 51篇 |
2003年 | 40篇 |
2002年 | 56篇 |
2001年 | 25篇 |
2000年 | 20篇 |
1999年 | 13篇 |
1998年 | 16篇 |
1997年 | 24篇 |
1996年 | 19篇 |
1995年 | 15篇 |
1994年 | 18篇 |
1993年 | 21篇 |
1992年 | 15篇 |
1991年 | 16篇 |
1990年 | 10篇 |
1989年 | 13篇 |
1988年 | 16篇 |
1987年 | 15篇 |
1986年 | 15篇 |
1985年 | 11篇 |
1984年 | 17篇 |
1983年 | 10篇 |
1982年 | 21篇 |
1981年 | 12篇 |
1980年 | 15篇 |
1979年 | 15篇 |
1978年 | 17篇 |
1977年 | 10篇 |
1975年 | 13篇 |
1973年 | 14篇 |
1972年 | 9篇 |
排序方式: 共有1771条查询结果,搜索用时 15 毫秒
991.
Goverdhan Mehta Saikat Sen 《Acta Crystallographica. Section C, Structural Chemistry》2010,66(2):o59-o63
The crystal structures of three conformationally locked esters, namely the centrosymmetric tetrabenzoate of all‐axial perhydronaphthalene‐2,3,4a,6,7,8a‐hexaol, viz.trans‐4a,8a‐dihydroxyperhydronaphthalene‐2,3,6,7‐tetrayl tetrabenzoate, C38H34O10, and the diacetate and dibenzoate of all‐axial perhydronaphthalene‐2,3,4a,8a‐tetraol, viz. (2R*,3R*,4aS*,8aS*)‐4a,8a‐dihydroxyperhydronaphthalene‐2,3‐diyl diacetate, C14H22O6, and (2R*,3R*,4aS*,8aS*)‐4a,8a‐dihydroxyperhydronaphthalene‐2,3‐diyl dibenzoate, C24H26O6, have been analyzed in order to examine the preference of their supramolecular assemblies towards competing inter‐ and intramolecular O—H...O hydrogen bonds. It was anticipated that the supramolecular assembly of the esters under study would adopt two principal hydrogen‐bonding modes, namely one that employs intermolecular O—H...O hydrogen bonds (mode 1) and another that sacrifices those for intramolecular O—H...O hydrogen bonds and settles for a crystal packing dictated by weak intermolecular interactions alone (mode 2). Thus, while the molecular assembly of the two crystalline diacyl derivatives conformed to a combination of hydrogen‐bonding modes 1 and 2, the crystal packing in the tetrabenzoate preferred to follow mode 2 exclusively. 相似文献
992.
Ogunwande IA Walker TM Bansal A Setzer WN Essien EE 《Natural product communications》2010,5(11):1815-1818
Peristrophe bicalyculata (Retz) Nees (Acanthaceae) or 'The Goddess of Mercy' and Borreria verticillata (L.) G.F.W. Mey., (Rubiaceae), or 'Irawo-Ile' (Yoruba, South-west, Nigeria), are annual herbs, which are poorly exploited. The volatile oils obtained by hydrodistillation in an all glass Clevenger-type apparatus from the plant samples have been investigated by gas chromatography-mass spectrometry (GC-MS). With respect to the oil of P. bicalyculata, beta-caryophyllene (33.9%), alpha-zingiberene (10.4%), germacrene D and globulol (5.0%) were the compounds occurring in abundance. The oil of B. verticillata had an abundance of phytol (56.3%) and 1, 8-cineole (20.4%), with sizeable proportions of alpha-pinene (7.1%) and p-cymene (4.0%). In addition, the volatile oils displayed promising in-vitro antimicrobial activity against the tested micro-organisms, (MIC 12.5-22.3 microg/mL), while only the oil of P. bicalyculata displayed in-vitro cytotoxicity to MCF-7 (human breast tumor) and MDA-MB-468 (human breast tumor) cells. The present investigation may be the first of its kind for the evaluation of the volatile oil constituents of the studied plants. 相似文献
993.
Seung Kyu Min Dr. Mina Park Prof. N. Jiten Singh Prof. Han Myoung Lee Dr. Eun Cheol Lee Prof. Dr. Kwang S. Kim Dr. Anita Lagutschenkov Prof. Dr. Gereon Niedner‐Schatteburg 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(34):10373-10379
Protonated and deprotonated adipic acids (PAA: HOOC? (CH2)4? COOH2+ and DAA: HOOC? (CH2)4? COO?) have a charged hydrogen bond under the influence of steric constraint due to the molecular skeleton of a circular ring. Despite the similarity between PAA and DAA, it is surprising that the lowest energy structure of PAA is predicted to have (H2O???H???OH2)+ Zundel‐like symmetric hydrogen bonding, whereas that of DAA has H3O+ Eigen‐like asymmetric hydrogen bonding. The energy profiles show that direct proton transfer between mirror image structures is unfavorable. Instead, the chiral transformation is possible by subsequent backbone twistings through stepwise proton transfer along multistep intermediate structures, which are Zundel‐like ions for PAA and Eigen‐like ions for DAA. This type of chiral transformation by multistep intramolecular proton transfers is unprecedented. Several prominent OH???O short hydrogen‐bond stretching peaks are predicted in the range of 1000–1700 cm?1 in the Car–Parrinello molecular dynamics (CPMD) simulations, which show distinctive signatures different from ordinary hydrogen‐bond peaks. The O? H? O stretching peaks in the range of 1800–2700 cm?1 become insignificant above around 150 K and are almost washed out at about 300 K. 相似文献
994.
Coales SJ E SY Lee JE Ma A Morrow JA Hamuro Y 《Rapid communications in mass spectrometry : RCM》2010,24(24):3585-3592
Backbone amide hydrogen exchange rates can be used to describe the dynamic properties of a protein. Amide hydrogen exchange rates in a native protein may vary from milliseconds (ms) to several years. Ideally, the rates of all amide hydrogens of the analyte protein can be determined individually. To achieve this goal, monitoring of a wider time window is critical, in addition to high sequence coverage and high sequence resolution. Significant improvements have been made to hydrogen/deuterium exchange mass spectrometry methods in the past decade for better sequence coverage and higher sequence resolution. On the other hand, little effort has been made to expand the experimental time window to accurately determine exchange rates of amide hydrogens. Many fast exchanging amide hydrogens are completely exchanged before completion of a typical short exchange time point (10–30 s) and many slow exchanging amide hydrogens do not start exchanging before a typical long exchanging time point (1–3 h). Here various experimental conditions, as well as a quenched‐flow apparatus, are utilized to monitor cytochrome c amide hydrogen exchange behaviors over more than eight orders of magnitude (0.0044–1 000 000 s), when converted into the standard exchange condition (pH 7 and 23°C). Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
995.
Rajeshwari V. Hosahalli Anita P. Savanur Sharanappa T. Nandibewoor Shivamurti A. Chimatadar 《Transition Metal Chemistry》2010,35(2):237-246
The oxidation of d-panthenol by MnO4
− was studied in the absence and in the presence of ruthenium(III) catalyst in alkaline medium at 298 K and at constant ionic
strength of 0.50 mol dm−3 by spectrophotometry. The stoichiometry in both the cases was [panthenol]: [MnO4
−] = 1:4. The oxidation products were identified by IR and GC–MS. The reaction was first-order with respect to both MnO4
− and ruthenium(III), while the orders with respect to both panthenol and alkali varied from first order to zero order as the
concentrations increased. The effects of added products, ionic strength and dielectric constant were studied. The reaction
constants, activation parameters and thermodynamic quantities were calculated for both the uncatalysed and catalysed reactions. 相似文献
996.
Orendt AM 《Magnetic resonance in chemistry : MRC》2006,44(3):385-389
The chemical shift tensors of the acetate anions in cadmium acetate dihydrate are calculated using a cluster approach, the embedded ion method (EIM), and a combination of the two in the EIM/cluster method. The results of these calculations are compared with those completed on the isolated acetate anion and show the need for the inclusion of intermolecular interactions. The RMS difference between experiment and theory improves from over 60 ppm when the calculation is completed on an isolated anion, to below 10 ppm when interactions to nearby atoms are included. The best cluster model includes three cadmium acetate dihydrate and gives an RMS result of 4.4 ppm. The EIM method, which uses point charges to account for the intermolecular effects, achieves an RMS of 7.7 ppm on individual anions alone. A combination of the two, the EIM/cluster method, shows that the only necessary atom to explicitly add is the nearest cadmium; this addition results in an RMS of 4.1 ppm. These results are also discussed in terms of the computational cost of the different calculations. 相似文献
997.
998.
A strong indication about the existence of a (7p, 4, 1) difference family with p ≡ 7 (mod 12) a prime has been given in [11]. Here, developing some ideas of that paper, we give, much more generally, a strong
indication about the existence of a cyclic (pq, 4, 1) difference family whenever p and q are primes congruent to 7 (mod 12) and of a cyclic (pq, 5, 1) difference family whenever p and q are primes congruent to 11 (mod 20). Indeed we give an algorithm for their construction that seems to be always successful
and we have checked it works whenever both primes p and q do not exceed 1,000. All our (pq, 4, 1) and (pq, 5, 1) difference families have the nice property of admitting a multiplier of order 3 or 5, respectively, that fixes almost
all base blocks. As an intermediate result we also find an optimal (p, 5, 1) optical orthogonal code for every prime p ≡ 11 (mod 20) not exceeding 10,000. 相似文献
999.
Anita Pasotti 《Discrete Mathematics》2010,310(22):3080-3087
J.A. Gallian has proved [J.A. Gallian, Labeling prisms and prism related graphs, Congr. Numer. 59 (1987) 89-100] that every cubic graph M2k obtainable from a 2k-cycle by adding its k diameters (the so-called Moebius Ladder of order 2k) is graceful. Here, in the case of k even, we propose a new graceful labeling that besides being simpler than Gallian’s one is able to give, at the same time, a graceful labeling of the prism of order 2k. Most importantly in the case of k odd, namely in the bipartite case, we prove that M2k also admits an α-labeling. This implies that there exists a cyclic decomposition of the complete graph K6kt+1 into copies of M2k for every pair of positive integers k and t with k odd.In some cases we are able to give such decompositions also when k is even. Apart from the case of t=1 that is an obvious consequence of the gracefulness of M2k, this happens, for instance, when k≡2 (mod 4) and 6kt+1 is a prime. 相似文献
1000.
The emergence of solar cells on flexible and bendable substrates has made the printing process a ubiquitous tool for the fabrication of these devices. The various printing techniques available now such as inkjet, screen and flexography offer cost- effectiveness, user-friendliness and suitability for mass production. While downscaling the fill factor and efficiency of organic solar cells. A multilayered structure, the combination of different printing techniques avails the variety of thickness and resolution required for each layer in the production of an organic solar cell. In this review article, we discuss the suitability of the inkjet and screen printing processes to produce organic solar cells. We also discuss various challenges involved in the fabrication of organic solar cells using these two techniques and the possible solutions for the same. We also provide an analogy that both processes share. Further, we consider future possibilities of combining these printing technologies to produce organic solar cells to improve device performance. 相似文献