全文获取类型
收费全文 | 1045篇 |
免费 | 27篇 |
国内免费 | 15篇 |
专业分类
化学 | 604篇 |
晶体学 | 8篇 |
力学 | 34篇 |
综合类 | 3篇 |
数学 | 195篇 |
物理学 | 243篇 |
出版年
2022年 | 8篇 |
2021年 | 11篇 |
2020年 | 22篇 |
2019年 | 11篇 |
2018年 | 14篇 |
2016年 | 13篇 |
2015年 | 14篇 |
2014年 | 26篇 |
2013年 | 58篇 |
2012年 | 38篇 |
2011年 | 42篇 |
2010年 | 31篇 |
2009年 | 24篇 |
2008年 | 54篇 |
2007年 | 34篇 |
2006年 | 44篇 |
2005年 | 39篇 |
2004年 | 34篇 |
2003年 | 29篇 |
2002年 | 32篇 |
2001年 | 22篇 |
2000年 | 14篇 |
1999年 | 10篇 |
1997年 | 21篇 |
1996年 | 15篇 |
1994年 | 14篇 |
1993年 | 24篇 |
1992年 | 27篇 |
1991年 | 12篇 |
1990年 | 13篇 |
1989年 | 14篇 |
1988年 | 14篇 |
1987年 | 14篇 |
1986年 | 11篇 |
1985年 | 14篇 |
1984年 | 13篇 |
1983年 | 13篇 |
1982年 | 15篇 |
1981年 | 15篇 |
1980年 | 15篇 |
1979年 | 13篇 |
1978年 | 16篇 |
1977年 | 18篇 |
1976年 | 8篇 |
1975年 | 9篇 |
1974年 | 17篇 |
1973年 | 21篇 |
1972年 | 8篇 |
1971年 | 7篇 |
1954年 | 9篇 |
排序方式: 共有1087条查询结果,搜索用时 46 毫秒
21.
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
22.
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
23.
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
24.
BARB FLORIN DAN; IONESCU VLAD; KONING WILLEM DE 《IMA Journal of Mathematical Control and Information》1994,11(4):277-309
In this paper, we deal with the digital output-measurement-feedbackH control problem for Pritchard-Salamon infinite-dimensionalsystems with unbounded input and output operators. A discretePopov-theory-based solution is given in terms of the solvabilityof Kalman-Szegö-Popov-Yakubovitch systems associated withthe equivalent discrete-time time-invariant system obtainedby lifting the T-periodic continuous-time system. 相似文献
25.
26.
27.
28.
29.
A high performance liquid chromatography (HPLC) method with fluorescence detection including an on-line purification was established for determination of catecholamines in human urine. The method was evaluated using samples of pooled urine spiked with catecholamines and validated for measurements of catecholamines in urine of healthy individuals in a field study. The laboratory method evaluation study showed that the recovery of the method was 0.82 (confidence interval (CI): 0.79-0.86) and 0.92 (CI: 0.89-0.95) for noradrenaline and adrenaline, respectively. Thus, correction factors of 0.82(-1) and 0.92(-1) were applied to correct the measurement results for this systematic effect. Furthermore, an uncertainty budget was generated for the analytical method using the BIPM-approach recommended by the International Organization for Standardization. The relative uncertainty of the method was estimated to be 10-12%, which was consistent with the observed relative variability found in the method evaluation. The method was evaluated in accordance with EURACHEM Guidance Document No 1 concerning accreditation for chemical laboratories with respect to accuracy and precision. The field study showed that the standard deviation of the method was sufficiently low to distinguish between persons working with two different processes of garbage collection, i.e. collection using four wheeled containers versus collection using bins. 相似文献
30.
Jerald S. Bradshaw Ralph B. Nielsen Pui-Kwan Tse Giuseppi Arena Bruce E. Wilson N. Kent Dalley John D. Lamb James J. Christensen Reed M. Izatt 《Journal of heterocyclic chemistry》1986,23(2):361-368
A series of macrocyclic polyether (crown) ligands containing the proton-ionizable s-triazole subcyclic unit were prepared by reacting the 1-THP blocked 3,5-bis(chloromethyl)-1H-1,2,4-triazole with various oligoethylene glycols. The starting bis(chloromethyl)triazole is a vessicant and must be used with caution. Triazolo-18-crown-6 ( 5 ) formed stable complexes with barium, strontium, copper and benzylammonium cations but not with potassium or lithium. The crystal structure of 5 showed the triazole proton to be on nitrogen 3 which is outside the macroring cavity. 相似文献