全文获取类型
收费全文 | 721篇 |
免费 | 21篇 |
国内免费 | 3篇 |
专业分类
化学 | 539篇 |
晶体学 | 17篇 |
力学 | 10篇 |
数学 | 106篇 |
物理学 | 73篇 |
出版年
2022年 | 5篇 |
2021年 | 12篇 |
2020年 | 5篇 |
2019年 | 8篇 |
2018年 | 5篇 |
2017年 | 10篇 |
2016年 | 14篇 |
2015年 | 14篇 |
2014年 | 15篇 |
2013年 | 33篇 |
2012年 | 27篇 |
2011年 | 48篇 |
2010年 | 14篇 |
2009年 | 21篇 |
2008年 | 29篇 |
2007年 | 44篇 |
2006年 | 35篇 |
2005年 | 35篇 |
2004年 | 28篇 |
2003年 | 35篇 |
2002年 | 40篇 |
2001年 | 20篇 |
2000年 | 6篇 |
1999年 | 11篇 |
1998年 | 8篇 |
1997年 | 9篇 |
1996年 | 14篇 |
1995年 | 11篇 |
1994年 | 15篇 |
1993年 | 7篇 |
1992年 | 9篇 |
1991年 | 6篇 |
1990年 | 9篇 |
1989年 | 11篇 |
1988年 | 7篇 |
1987年 | 7篇 |
1986年 | 8篇 |
1985年 | 11篇 |
1984年 | 9篇 |
1983年 | 11篇 |
1982年 | 9篇 |
1981年 | 14篇 |
1980年 | 8篇 |
1979年 | 11篇 |
1978年 | 6篇 |
1977年 | 6篇 |
1976年 | 5篇 |
1974年 | 5篇 |
1973年 | 4篇 |
1926年 | 2篇 |
排序方式: 共有745条查询结果,搜索用时 15 毫秒
111.
Andrews KW Schweitzer A Zhao C Holden JM Roseland JM Brandt M Dwyer JT Picciano MF Saldanha LG Fisher KD Yetley E Betz JM Douglass L 《Analytical and bioanalytical chemistry》2007,389(1):231-239
As part of a study initiating the development of an analytically validated Dietary Supplement Ingredient Database (DSID) in
the United States (US), a selection of dietary supplement products were analyzed for their caffeine content. Products sold
as tablets, caplets, or capsules and listing at least one caffeine-containing ingredient (including botanicals such as guarana,
yerba mate, kola nut, and green tea extract) on the label were selected for analysis based on market share information. Two
or three lots of each product were purchased and analyzed using high-pressure liquid chromatography (HPLC). Each analytical
run included one or two National Institute of Standards and Technology (NIST) Standard Reference Materials (SRMs) and two
products in duplicate. Caffeine intake per serving and per day was calculated using the maximum recommendations on each product
label. Laboratory analysis for 53 products showed product means ranging from 1 to 829 mg caffeine/day. For products with a
label amount for comparison (n = 28), 89% (n = 25) of the products had analytically based caffeine levels/day of between −16% and +16% of the claimed levels. Lot-to-lot
variability (n = 2 or 3) for caffeine in most products (72%) was less than 10%. 相似文献
112.
Dwyer JT Holden J Andrews K Roseland J Zhao C Schweitzer A Perry CR Harnly J Wolf WR Picciano MF Fisher KD Saldanha LG Yetley EA Betz JM Coates PM Milner JA Whitted J Burt V Radimer K Wilger J Sharpless KE Hardy CJ 《Analytical and bioanalytical chemistry》2007,389(1):37-46
This article illustrates the importance of having analytical data on the vitamin and mineral contents of dietary supplements
in nutrition studies, and describes efforts to develop an analytically validated dietary supplement ingredient database (DSID)
by a consortium of federal agencies in the USA. Preliminary studies of multivitamin mineral supplements marketed in the USA
that were analyzed as candidates for the DSID are summarized. Challenges are summarized, possible future directions are outlined,
and some related programs at the Office of Dietary Supplements, National Institutes of Health are described. The DSID should
be helpful to researchers in assessing relationships between intakes of vitamins and minerals and health outcomes. 相似文献
113.
Del Bene JE Elguero J Alkorta I Yañez M Mó O 《The journal of physical chemistry. A》2007,111(3):419-421
This paper reports computed NMR spectral data for the diazaborole anion (C2H4B1N2-) and the corresponding neutral five-membered rings with B-H (C2H5B1N2, diazaborole) and B-Li (C2H4B1Li1N2, Li-diazaborole) bonds, which are the central moieties of newly synthesized nucleophilic organoboryl five-membered rings, recently reported by Segawa et al. (Science 2006, 314, 113). Both spin-spin coupling constants and chemical shifts were obtained using high-level ab initio calculations. These data are a necessary complement to the very scarce experimental information available. 相似文献
114.
Christopher Glidewell John N. Low Janet M. S. Skakle James L. Wardell 《Acta Crystallographica. Section C, Structural Chemistry》2003,59(3):o95-o97
Molecules of the title compound, C12H9IN2O2S, are linked by one N—H⃛O hydrogen bond [H⃛O = 2.16 Å, N⃛O = 2.935 (4) Å and N—H⃛O 147°] and two C—H⃛O hydrogen bonds [H⃛O both 2.49 Å, C⃛O = 3.231 (5) and 3.220 (5) Å, and C—H⃛O = 135 and 134°] into sheets which themselves are weakly linked by an aromatic π–π‐stacking interaction between iodinated rings in centrosymmetrically related molecules. There are no iodo–nitro interactions. 相似文献
115.
Matthew G. Skellern R. Alan Howie Eric E. Lachowski Janet M. S. Skakle 《Acta Crystallographica. Section C, Structural Chemistry》2003,59(2):i11-i14
Barium‐deficient forms of celsian (barium aluminium silicate) with the formula Ba1−xAl2−2xSi2+2xO8 (x = 0.20 and 0.06) have been identified. In contrast with the celsian–orthoclase solid solutions which have been reported previously, these forms, refined in the space group C2/m, with Ba and one O atom in the 4i sites with m site symmetry, and a further O atom in a 4g site with twofold axial symmetry, suggest a slight solid solution with silica. The serendipitous preparation of the compounds represents a possible hazard associated with solid‐state synthesis. 相似文献
116.
Christopher Glidewell John N. Low Janet M. S. Skakle James L. Wardell 《Acta Crystallographica. Section C, Structural Chemistry》2003,59(11):o636-o637
Molecules of the title compound, C13H8I2N2O3, are linked into C(4) chains by a single N—H⋯O=C hydrogen bond [H⋯O = 2.10 Å, N⋯O = 2.832 (5) Å and N—H⋯O = 140°]. Two independent two‐centre iodo–nitro interactions, both involving the same O atom but different I atoms [I⋯O = 3.205 (3) and 3.400 (3) Å, and C—I⋯O = 160.4 (2) and 155.7 (2)°], link the hydrogen‐bonded chains into bilayers. 相似文献
117.
118.
Sugam Kharel Kyle J. Cluff Nattamai Bhuvanesh John A. Gladysz Janet Blümel 《化学:亚洲杂志》2019,14(15):2704-2711
The three secondary phosphine oxides [CH2=CH(CH2)4]2HPO ( 1 ), [CH2=CH(CH2)5]2HPO ( 2 ), and [CH2=CH(CH2)6]2HPO ( 3 ), and two diphosphine dioxides, {[CH2=CH(CH2)6]2PO(CH2)7}2 ( 4 ) and {[CH2=CH(CH2)6]2PO(CH2)4}2 ( 5 ), incorporating long methylene chains, are described. The single crystal X‐ray structures of 1 , 2 , and 5 have been determined. The phosphine oxides 3 , 4 , and 5 have been adsorbed on silica in submonolayer quantities to give 3 a – 5 a . The 1H, 13C, and 31P solid‐state NMR spectra of polycrystalline 3 – 5 have been analyzed and compared with those of 3 a – 5 a . The changes of the solid‐state NMR characteristics upon adsorption and the surface mobilities of the phosphine oxides are discussed. 相似文献
119.
Background
Interstitial cystitis is a chronic condition associated with bladder inflammation and, like a number of other chronic pain states, symptoms associated with interstitial cystitis are more common in females and fluctuate during the menstrual cycle. The aim of this study was to determine if estrogens could directly modulate signalling pathways within bladder sensory neurons, such as extracellular signal-related kinase (ERK) and p38 mitogen-activated protein (MAP) kinases. These signalling pathways have been implicated in neuronal plasticity underlying development of inflammatory somatic pain but have not been as extensively investigated in visceral nociceptors. We have focused on lumbosacral dorsal root ganglion (DRG) neurons projecting to pelvic viscera (L1, L2, L6, S1) of adult female Sprague-Dawley rats and performed both in vitro and in vivo manipulations to compare the effects of short- and long-term changes in estrogen levels on MAPK expression and activation. We have also investigated if prolonged estrogen deprivation influences the effects of lower urinary tract inflammation on MAPK signalling. 相似文献120.
Carlos E. M. Carvalho Solange M. S. V. Wardell James L. Wardell Janet M. S. Skakle John N. Low Christopher Glidewell 《Acta Crystallographica. Section C, Structural Chemistry》2004,60(1):o28-o32
Molecules of the title compounds N2‐(benzoyloxy)benzamidine, C14H12N2O2, (I), N2‐(2‐hydroxybenzoyloxy)benzamidine, C14H12N2O3, (II), and N2‐benzoyloxy‐2‐hydroxybenzamidine, C14H12N2O3, (III), all have extended chain conformations, with the aryl groups remote from one another. In (I), the molecules are linked into chains by a single N—H⋯N hydrogen bond [H⋯N = 2.15 Å, N⋯N = 3.029 (2) Å and N—H⋯N = 153°] and these chains are linked into sheets by means of aromatic π–π stacking interactions. There is one intramolecular O—H⋯O hydrogen bond in (II), and a combination of one three‐centre N—H⋯(N,O) hydrogen bond [H⋯N = 2.46 Å, H⋯O = 2.31 Å, N⋯N = 3.190 (2) Å, N⋯O = 3.146 (2) Å, N—H⋯N = 138° and N—H⋯O = 154°] and one two‐centre C—H⋯O hydrogen bond [H⋯O = 2.46 Å, C⋯O = 3.405 (2) Å and C—H⋯O = 173°] links the molecules into sheets. In (III), an intramolecular O—H⋯N hydrogen bond and two N—H⋯O hydrogen bonds [H⋯O = 2.26 and 2.10 Å, N⋯O = 2.975 (2) and 2.954 (2) Å, and N—H⋯O = 138 and 163°] link the molecules into sheets. 相似文献