全文获取类型
收费全文 | 586篇 |
免费 | 28篇 |
国内免费 | 3篇 |
专业分类
化学 | 469篇 |
晶体学 | 5篇 |
力学 | 8篇 |
数学 | 59篇 |
物理学 | 76篇 |
出版年
2023年 | 4篇 |
2022年 | 10篇 |
2021年 | 6篇 |
2020年 | 15篇 |
2019年 | 13篇 |
2018年 | 8篇 |
2017年 | 2篇 |
2016年 | 17篇 |
2015年 | 14篇 |
2014年 | 11篇 |
2013年 | 33篇 |
2012年 | 33篇 |
2011年 | 44篇 |
2010年 | 29篇 |
2009年 | 30篇 |
2008年 | 35篇 |
2007年 | 23篇 |
2006年 | 41篇 |
2005年 | 33篇 |
2004年 | 30篇 |
2003年 | 32篇 |
2002年 | 35篇 |
2001年 | 7篇 |
2000年 | 11篇 |
1999年 | 3篇 |
1998年 | 6篇 |
1997年 | 8篇 |
1996年 | 7篇 |
1995年 | 5篇 |
1994年 | 5篇 |
1993年 | 3篇 |
1992年 | 3篇 |
1991年 | 3篇 |
1990年 | 4篇 |
1988年 | 3篇 |
1985年 | 5篇 |
1984年 | 7篇 |
1982年 | 4篇 |
1981年 | 3篇 |
1980年 | 2篇 |
1979年 | 4篇 |
1976年 | 2篇 |
1973年 | 4篇 |
1971年 | 2篇 |
1970年 | 3篇 |
1969年 | 3篇 |
1968年 | 3篇 |
1938年 | 1篇 |
1921年 | 1篇 |
1909年 | 1篇 |
排序方式: 共有617条查询结果,搜索用时 31 毫秒
101.
It is important to understand the mechanism to integrate visual and other information for the perception of gravitational
vertical under different conditions of gravity, although most of the experiments to investigate the topic reported so far
has been carried out under normal-gravity (1 G) and under micro-gravity produced in short duration of parabolic flight. The
purpose of this research is to investigate human perception of gravitational vertical under hyper-gravity (1.5G and 2G) to
compare that under normal-gravity (1 G). We measured perceived gravitational vertical using a visual probe presented at the
center while manipulating the orientation of visual stimuli (scenery picture) presented in the surround and the magnitude
of gravitational acceleration. The environment of hyper-gravity was produced by centrifugal force of a rotating flight simulator.
The results show that the accuracy of perceived gravitational vertical increased and the precision of it decreased under hyper-gravity
condition relative to those under the normal-gravity condition when no visual stimulus was presented. However, the effect
of the tilt of visual stimuli did not change under different gravity conditions. 相似文献
102.
103.
104.
Three new monoterpene glucosides, lamiuamplexosides A–C ( 1 – 3 ), along with thirteen known glucosides, were isolated from the whole plant of Lamium amplexicaule L. The structures of 1 – 3 were elucidated on the basis of spectroscopic, chemical, and physicochemical evidence. 相似文献
105.
Rie Fredrickson Takagi Daisy Torino Hjelmqvist Mats Johnsson Sven Lidin 《Solid State Sciences》2009,11(1):13-17
The compounds CaCo2Te3O8Cl2, SrCo2Te3O8Cl2 and SrNi2Te3O8Cl2 were synthesized via solid–gas reactions and investigated using single-crystal X-ray diffraction. While the compound CaCo2Te3O8Cl2 formed large enough single crystals to allow for a detailed structural analysis, crystals of the Sr-containing compounds yielded evidence that they are isostructural. CaCo2Te3O8Cl2 crystallizes in the monoclinic system, space group P21/c, a = 6.537(2) Å, b = 9.088(2) Å, c = 19.500(9) Å, β = 113.36(4)°, Z = 4. It exhibits [CoO5Cl] helical chains along the [010] direction, connected by [CaO8] polyhedra, [TeO3E] tetrahedra and [TeO4E] trigonal bipyramids (the lone pair of electrons on TeIV is designated as E) to form a layer. The layers are held together only by weak van der Waals forces; the shortest interlayer distance is a Te?Cl contact of 3.432(4) Å. 相似文献
106.
Jin Y Hada N Oka J Kanie O Daikoku S Kanie Y Yamada H Takeda T 《Chemical & pharmaceutical bulletin》2006,54(4):485-492
Stereocontrolled syntheses of model compounds related to a major antigenic epitope against antibupleurum 2IIc/PG-1-IgG from antiulcer pectic polysaccharide are described. A trisaccharide derivative (13) was prepared as a precursor and a novel and simple approach for the rational design of a glycocluster and glycodendrimer was developed, through the syntheses of the fluorescence-labeled glycocluster (2) and glycodendrimer (3). 相似文献
107.
Kawaguchi M Ito R Sakui N Okanouchi N Saito K Nakazawa H 《Journal of chromatography. A》2006,1105(1-2):140-147
A novel method for the trace analysis of 17beta-estradiol (E2) in river water sample was developed, which involved stir bar sorptive extraction (SBSE) with in situ acylation (first derivatization) and thermal desorption (TD) with quartz wool assisted (QWA) in tube silylation (second derivatization), followed by gas chromatography-mass spectrometry (GC-MS), and is called the "dual derivatization method." The optimum conditions for SBSE with in situ acylation, such as the volume of acetic acid anhydride and the extraction time, were investigated. In addition, the optimum conditions for TD with QWA in tube silylation, such as the volume of N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) and the TD temperature and hold time, were investigated as well. The detection limit (S/N = 3) and the quantitation limit (S/N>10) of E2 in the river water sample were 0.5 and 2 pg ml(-1) (ppt), respectively, by the dual derivatization method. In addition, the detection limit was 0.1 pg ml(-1) by using dual derivatization method with multi-shot mode. The calibration curve for E2 was linear in the range of 0.002-10 ng ml(-1) with correlation coefficients >0.999. The average recoveries of E2 (n = 6) at the concentrations of 0.05 and 1.0 ng ml(-1) from the river water sample were 93.1 (RSD: 1.4%) and 98.4% (RSD: 0.8%), respectively, with correction using the added surrogate standard, 17beta-estradiol-(13)C(4). This simple, accurate, sensitive and selective analytical method may be applicable to the determination of trace amounts of E2 in water samples. 相似文献
108.
Ooya T Choi HS Yamashita A Yui N Sugaya Y Kano A Maruyama A Akita H Ito R Kogure K Harashima H 《Journal of the American Chemical Society》2006,128(12):3852-3853
A biocleavable polyrotaxane, having a necklace-like structure consisting of many cationic alpha-cyclodextrins (alpha-CDs) and a disulfide-introduced poly(ethylene glycol) (PEG), was synthesized and examined as a nonviral gene carrier. The polyrotaxane formed a stable polyplex having positively charged surface even at low charge ratio. This is likely to be due to structural factors of the polyrotaxane, such as the mobile motion of alpha-CDs in the necklace-like structure. Rapid endosomal escape was observed 90 min after transfection. The positively charged surface and the good buffering capacity are advantageous to show the proton sponge effect. The pDNA decondensation occurred through disulfide cleavage of the polyrotaxane and subsequent supramolecular dissociation of the noncovalent linkages between alpha-CDs and PEG. Transfection of the DMAE-SS-PRX polyplex is independent of the amount of free polycation. Those properties played a key role for delivery of pDNA clusters to the nucleus. Therefore, the polyplex nature and the supramolecular dissociation of the polyrotaxane contributed to the enhanced gene delivery. 相似文献
109.
Nakayama T Kamachi T Jitsumori K Omi R Hirotsu K Esaki N Kurihara T Yoshizawa K 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(27):8392-8402
The high substrate specificity of fluoroacetate dehalogenase was explored by using crystallographic analysis, fluorescence spectroscopy, and theoretical computations. A crystal structure for the Asp104Ala mutant of the enzyme from Burkholderia sp. FA1 complexed with fluoroacetate was determined at 1.2 ? resolution. The orientation and conformation of bound fluoroacetate is different from those in the crystal structure of the corresponding Asp110Asn mutant of the enzyme from Rhodopseudomonas palustris CGA009 reported recently (J. Am. Chem. Soc. 2011, 133, 7461). The fluorescence of the tryptophan residues of the wild-type and Trp150Phe mutant enzymes from Burkholderia sp. FA1 incubated with fluoroacetate and chloroacetate was measured to gain information on the environment of the tryptophan residues. The environments of the tryptophan residues were found to be different between the fluoroacetate- and chloroacetate-bound enzymes; this would come from different binding modes of these two substrates in the active site. Docking simulations and QM/MM optimizations were performed to predict favorable conformations and orientations of the substrates. The F atom of the substrate is oriented toward Arg108 in the most stable enzyme-fluoroacetate complex. This is a stable but unreactive conformation, in which the small O-C-F angle is not suitable for the S(N)2 displacement of the F(-) ion. The cleavage of the C-F bond is initiated by the conformational change of the substrate to a near attack conformation (NAC) in the active site. The second lowest energy conformation is appropriate for NAC; the C-O distance and the O-C-F angle are reasonable for the S(N) 2 reaction. The activation energy is greatly reduced in this conformation because of three hydrogen bonds between the leaving F atom and surrounding amino acid residues. Chloroacetate cannot reach the reactive conformation, due to the longer C-Cl bond; this results in an increase of the activation energy despite the weaker C-Cl bond. 相似文献
110.
Efficient monocyclic 1,2-diazepine formation via a tandem electrocyclization reaction of cyclobutenones with lithiodiazoacetate is demonstrated. The reaction proceeds through an oxy anion-accelerated 4π-ring opening of cyclobutene followed by an 8π-ring closure of the resultant oxy anion-substituted diazo-diene under mild conditions to furnish a 1,2-diazepine via formal diazomethylene insertion into the C-C bond of cyclobutenone. 相似文献