全文获取类型
收费全文 | 1313篇 |
免费 | 34篇 |
国内免费 | 10篇 |
专业分类
化学 | 836篇 |
晶体学 | 7篇 |
力学 | 90篇 |
数学 | 153篇 |
物理学 | 271篇 |
出版年
2022年 | 26篇 |
2021年 | 19篇 |
2020年 | 7篇 |
2019年 | 16篇 |
2018年 | 7篇 |
2017年 | 7篇 |
2016年 | 26篇 |
2015年 | 15篇 |
2014年 | 36篇 |
2013年 | 57篇 |
2012年 | 75篇 |
2011年 | 96篇 |
2010年 | 42篇 |
2009年 | 63篇 |
2008年 | 80篇 |
2007年 | 62篇 |
2006年 | 72篇 |
2005年 | 69篇 |
2004年 | 37篇 |
2003年 | 46篇 |
2002年 | 39篇 |
2001年 | 31篇 |
2000年 | 31篇 |
1999年 | 12篇 |
1998年 | 11篇 |
1997年 | 15篇 |
1996年 | 30篇 |
1995年 | 17篇 |
1994年 | 14篇 |
1993年 | 23篇 |
1992年 | 21篇 |
1991年 | 23篇 |
1990年 | 15篇 |
1989年 | 14篇 |
1988年 | 15篇 |
1987年 | 8篇 |
1986年 | 13篇 |
1985年 | 21篇 |
1984年 | 15篇 |
1983年 | 10篇 |
1982年 | 15篇 |
1981年 | 9篇 |
1980年 | 9篇 |
1978年 | 7篇 |
1977年 | 24篇 |
1976年 | 11篇 |
1975年 | 5篇 |
1974年 | 7篇 |
1973年 | 6篇 |
1972年 | 5篇 |
排序方式: 共有1357条查询结果,搜索用时 15 毫秒
991.
Pritam Mondal Izumi Ishigami Emilie F. Grard Chaeeun Lim Syun-Ru Yeh Sam P. de Visser Gayan B. Wijeratne 《Chemical science》2021,12(25):8872
Heme superoxides are one of the most versatile metallo-intermediates in biology, and they mediate a vast variety of oxidation and oxygenation reactions involving O2(g). Overall proton-coupled electron transfer (PCET) processes they facilitate may proceed via several different mechanistic pathways, attributes of which are not yet fully understood. Herein we present a detailed investigation into concerted PCET events of a series of geometrically similar, but electronically disparate synthetic heme superoxide mimics, where unprecedented, PCET feasibility-determining electronic effects of the heme center have been identified. These electronic factors firmly modulate both thermodynamic and kinetic parameters that are central to PCET, as supported by our experimental and theoretical observations. Consistently, the most electron-deficient superoxide adduct shows the strongest driving force for PCET, whereas the most electron-rich system remains unreactive. The pivotal role of these findings in understanding significant heme systems in biology, as well as in alternative energy applications is also discussed.Electronic characteristics of heme significantly influence the feasibility of hydrogen atom abstraction by synthetic heme superoxide moieties, shedding new light on analogous scenarios implicated in both biological and alternate energy applications. 相似文献
992.
Trace amount of mercury in human hair has been determined by destructive neutron activation analysis. It was found that the contents of mercury in the hair of normal people in Taiwan area is the order of 6 ppm. It is of interest to point out that very little fluctuation is observed in the contents of mercury in human hair regardless of sex and age of the normal people under investigation. On the other hand for those people who are working in the caustic soda manufacturing plant in which mercury is used as cathode in the electrolysis process of saline water, the mercury contents in their hair are definitely much higher (ca. 30 times) than the average values of normal people. 相似文献
993.
The solution conformation of the homodetic decapeptide antibiotic Gramicidin S has been determined by two-dimensional 1H NMR spectroscopy and distance geometry followed by energy minimization. All structures corresponding to energy minima have four intramolecular hydrogen bonds between Leu and Val residues. 相似文献
994.
Meso substitution opposite to the spacer provides a convenient approach for tuning the pocket sizes of pillared cofacial bisporphyrins. The synthesis and coordination chemistry of xanthene and dibenzofuran anchored platforms structurally modified with 2,6-dimethoxyaryl groups are described. Comparative structural analysis of xanthene derivatives confirms the ability of the trans-aryl groups to adjust the vertical dimension of the cofacial cleft: 7 (C(97)H(106)Cl(4)N(8)O(5)), monoclinic, space group P2(1)/c, a = 28.8353(12) A, b = 17.1139(7) A, c = 17.5978(7) A, beta = 98.826(1) degrees, Z = 4; 8 (C(101)H(123)Cl(2)N(8)O(11.5)Zn(2)), monoclinic, space group P2(1)/n, a = 14.5517(6) A, b = 22.9226(10) A, c = 28.5155(13) A, beta = 90.312(14) degrees, Z = 4; 12 (C(99)H(102)Cl(14)N(8)O(5)Mn(2)), monoclinic, space group P2/c, a = 19.5891(3) A, b = 15.0741(2) A, c = 33.2019(6) A, beta = 91.947(10) degrees, Z = 4. The convenience and versatility of this synthetic method offers intriguing opportunities to specifically tailor the binding pockets of cofacial bisporphyrins for the study of small-molecule activation within a proton-coupled electron transfer framework. 相似文献
995.
Small-angle x-ray scattering (SAXS) was used to determine the structural changes in polyethylene induced by radiation. The changes in densities of the crystalline and amorphous phases,
c
and
a
, were calculated after direct determination of the mean square density fluctuation <2>.
a
increases with increasing radiation dose for both linear and branched polyethylene. This accounts for the serious discrepancy between crystallinities determined from wide-angle x-ray scattering and density measurements. This study confirms our previous proposal that crosslinks occur primarily in the noncrystalline phase, most likely at the defects in the lateral grain boundary regions.Dedicated to Prof. Dr. W. Pechhold on the occasion of his 60th birthday. 相似文献
996.
A low-noise wideband optical fiber receiver has been successfully designed using super-beta bipolar transistors (BJTs) at the front end. Even with commercially available super-beta devices, which are not optimized for our application, the obtainable input sensitivity for medium- and high-bandwidth optical receivers is comparable or superior to the best FET design. To demonstrate this concept, a 10-MHz analog receiver was built with a super-beta BJT at the input stage. This receiver achieved an expected average input noise current density of less than 0.4 PA/√Hz over the full bandwidth for a transresistance of 500 kΩ. Detailed design procedures are given in this paper. The noise characteristics of a 50-MHz receiver using super-beta BJTs are also obtained. 相似文献
997.
The natural abundance carbon-13 nuclear magnetic resonance spectra of 3-arylsydnone, 3-aryl-4-formylsydnone, 4-acetyl-3-arylsydnone, 3-aryl-4-cyanosydnone as well as some derivatives of the above compounds have been studied. The data indicate that the electron withdrawing effect on the C4 atom of the sydnone ring decreases in the sequence: C≡N>COCH3~CHO. The solvent effect on the chemical shift has also been investigated. 相似文献
998.
999.
1000.