全文获取类型
收费全文 | 905篇 |
免费 | 17篇 |
国内免费 | 6篇 |
专业分类
化学 | 553篇 |
晶体学 | 8篇 |
力学 | 26篇 |
数学 | 152篇 |
物理学 | 189篇 |
出版年
2022年 | 8篇 |
2021年 | 11篇 |
2020年 | 21篇 |
2019年 | 10篇 |
2018年 | 12篇 |
2017年 | 6篇 |
2016年 | 12篇 |
2015年 | 12篇 |
2014年 | 19篇 |
2013年 | 24篇 |
2012年 | 35篇 |
2011年 | 38篇 |
2010年 | 30篇 |
2009年 | 24篇 |
2008年 | 52篇 |
2007年 | 32篇 |
2006年 | 42篇 |
2005年 | 38篇 |
2004年 | 32篇 |
2003年 | 27篇 |
2002年 | 26篇 |
2001年 | 16篇 |
2000年 | 10篇 |
1999年 | 8篇 |
1997年 | 10篇 |
1996年 | 10篇 |
1994年 | 9篇 |
1993年 | 18篇 |
1992年 | 22篇 |
1991年 | 6篇 |
1990年 | 11篇 |
1989年 | 11篇 |
1988年 | 12篇 |
1987年 | 12篇 |
1986年 | 11篇 |
1985年 | 14篇 |
1984年 | 11篇 |
1983年 | 11篇 |
1982年 | 14篇 |
1981年 | 13篇 |
1980年 | 14篇 |
1979年 | 13篇 |
1978年 | 15篇 |
1977年 | 17篇 |
1976年 | 7篇 |
1975年 | 7篇 |
1974年 | 17篇 |
1973年 | 21篇 |
1972年 | 7篇 |
1954年 | 9篇 |
排序方式: 共有928条查询结果,搜索用时 0 毫秒
11.
Reed M. Izatt Gypzy C. Lindh Peter Huszthy Glen A. Clark R. L. Bruening Jerald S. Bradshaw James J. Christensen 《Journal of inclusion phenomena and macrocyclic chemistry》1989,7(5):501-509
The macrocycle-mediated fluxes of alkali, alkaline earth, and several transition metal cations have been determined and compared in a H2O-CH2Cl2-H2O liquid membrane system using four water-insoluble macrocycles containing a dialkylhydrogenphosphate moiety. Transport of alkali metal cations by these ligands was greatest from a source phase pH = 12 or above into an acid receiving phase (pH = 1.5). Very low fluxes were observed for the transport of the alkaline earth cations and all transition metal ions studied except Ag+ and Pb2+ which were transported reasonably well by these new macrocycles.Deceased: September 5, 1987. 相似文献
12.
The rate of the reaction between D2O and the calcium aluminum oxides Ca3Al2O6, Ca5Al6O14, CaAl2O4, and CaAl4O7 was investigated by on-line neutron diffraction powder methods at temperatures from room temperature to 100°C. The rate of the reaction increases with increasing calcium content of the compounds and with increasing temperature for each of the compounds. The crystallographic stable hydrate Ca3Al2(OD)12 is obtained from CaAl4O7 and CaAl2O4 at temperatures above 63°C, from Ca5Al6O14 at temperatures above 49°C, and from Ca3Al2O6 at temperatures as low as 7°C. 相似文献
13.
Pittelkow M Nielsen CB Broeren MA van Dongen JL van Genderen MH Meijer EW Christensen JB 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(17):5126-5135
Host-guest interactions between the periphery of adamantylurea-functionalized dendrimers (host) and ureido acetic acid derivatives (guest) were shown to be specific, strong and spatially well-defined. The binding becomes stronger when using phosphonic or sulfonic acid derivatives. In the present work we have quantified the binding constants for the host-guest interactions between two different host motifs and six different guest molecules. The host molecules, which resemble the periphery of a poly(propylene imine) dendrimer, have been fitted with an anthracene-based fluorescent probe. The two host motifs differ in terms of the length of the spacer between a tertiary amine and two ureido functionalities. The guest molecules all contain an acidic moiety (either a carboxylic acid, a phosphonic acid, or a sulfonic acid) and three of them also contain an ureido moiety capable of forming multiple hydrogen bonds to the hosts. The binding constants for all 12 host-guest complexes have been determined by using fluorescence titrations by monitoring the increase in fluorescence of the host upon protonation by the addition of the guest. The binding constants could be tuned by changing the design of the acidic part of the guest. The formation of hydrogen bonds gives, in all cases, higher association constants, demonstrating that the host is more than a proton sensor. The host with the longer spacer (propyl) shows higher association constants than the host with the shorter spacer (ethyl). The gain in association constants are higher when the urea function is added to the guests for the host with the longer spacer, indicating a better fit. Collision-induced dissociation mass spectrometry (CID-MS) is used to study the stability of the six motifs using the corresponding third generation dendrimer. A similar trend is found when the six different guests are compared. 相似文献
14.
A new synthesis of the carbapenem ring system, as found in thienamycin and related natural products, has been developed. The key step involves a highly efficient carbene insertion reaction which produces the bicyclic ring system by forming the N3 bond. 相似文献
15.
Hau Sun Sam Chan Amber L. Thompson Kirsten E. Christensen Jonathan W. Burton 《Chemical science》2020,11(42):11592
Laurefurenynes C–F are four natural products isolated from Laurencia species whose structures were originally determined on the basis of extensive nuclear magnetic resonance experiments. On the basis of a proposed biogenesis, involving a tricyclic oxonium ion as a key intermediate, we have reassigned the structures of these four natural products and synthesized the four reassigned structures using a biomimetic approach demonstrating that they are the actual structures of the natural products. In addition, we have developed a synthesis of the enantiomers of the natural products laurencin and deacetyllaurencin from the enantiomer of (E)-laurefucin using an unusual retrobiomimetic strategy. All of these syntheses have been enabled by the use of tricyclic oxonium ions as pivotal synthetic intermediates.The synthesis and structural reassignment of laurefurenynes C–F has been achieved, with the new structures fitting with a proposed biosynthesis. Also reported is the synthesis of ent-laurencin and ent-deacetyllaurencin via a retrobiomimetic approach. 相似文献
16.
Harry A. Frank Roya Farhoosh Mila L. Aldema Beverly DeCoster Ronald L. Christensen Ronald Gebhard Johan Lugtenburg 《Photochemistry and photobiology》1993,57(1):49-55
Four carotenoids, 3,4,7,8-tetrahydrospheroidene, 3,4,5,6-tetrahydrospheroidene, 3,4-dihydrospheroidene and spheroidene, have been incorporated into the B850 light-harvesting complex of the carotenoidless mutant, photosynthetic bacterium, Rhodobacter sphaeroides R-26.1. The extent of π-electron conjugation in these molecules increases from 7 to 10 carbon-carbon double bonds. Carotenoid-to-bacteriochlorophyll singlet state energy transfer efficiencies were measured using steady-state fluorescence excitation spectroscopy to be 54 ± 2%, 66 ± 4%, 71 ± 6% and 56 ± 3% for the carotenoid series. These results are discussed with respect to the position of the energy levels and the magnitude of spectral overlap between the S, (2′AJ state emission from the isolated carotenoids and the bacteriochlorophyll absorption of the native complex. These studies provide a systematic approach to exploring the effect of excited state energies, spectral overlap and excited state lifetimes on the efficiencies of carotenoid-to-bacteriochlorophyll singlet energy transfer in photosynthetic systems. 相似文献
17.
Nicolai Stuhr-Hansen Jakob Kryger Srensen Kasper Moth-Poulsen Jrn Bolstad Christensen Thomas Bjrnholm Mogens Brndsted Nielsen 《Tetrahedron》2005,61(52):12288-12295
Simple and readily accessible aryl bromides are useful building blocks for thiol end-capped molecular wires. Thus, 4-bromophenyl tert-butyl sulfide and 1-bromo-4-(methoxymethyl)benzene serve as precursors for a variety of oligo(phenylenevinylene) and oligo(phenyleneethynylene) wires via efficient synthetic transformations as presented in this paper. 相似文献
18.
19.
20.