全文获取类型
收费全文 | 3481篇 |
免费 | 98篇 |
国内免费 | 8篇 |
专业分类
化学 | 2455篇 |
晶体学 | 37篇 |
力学 | 36篇 |
综合类 | 1篇 |
数学 | 139篇 |
物理学 | 919篇 |
出版年
2023年 | 16篇 |
2022年 | 25篇 |
2021年 | 31篇 |
2020年 | 44篇 |
2019年 | 57篇 |
2018年 | 35篇 |
2017年 | 33篇 |
2016年 | 78篇 |
2015年 | 65篇 |
2014年 | 100篇 |
2013年 | 157篇 |
2012年 | 183篇 |
2011年 | 215篇 |
2010年 | 100篇 |
2009年 | 104篇 |
2008年 | 234篇 |
2007年 | 242篇 |
2006年 | 218篇 |
2005年 | 209篇 |
2004年 | 189篇 |
2003年 | 156篇 |
2002年 | 140篇 |
2001年 | 68篇 |
2000年 | 60篇 |
1999年 | 49篇 |
1998年 | 43篇 |
1997年 | 46篇 |
1996年 | 47篇 |
1995年 | 49篇 |
1994年 | 42篇 |
1993年 | 40篇 |
1992年 | 57篇 |
1991年 | 28篇 |
1990年 | 28篇 |
1989年 | 36篇 |
1988年 | 21篇 |
1987年 | 33篇 |
1986年 | 23篇 |
1985年 | 29篇 |
1984年 | 29篇 |
1983年 | 17篇 |
1982年 | 29篇 |
1981年 | 24篇 |
1980年 | 23篇 |
1979年 | 28篇 |
1978年 | 22篇 |
1977年 | 14篇 |
1976年 | 16篇 |
1975年 | 14篇 |
1973年 | 16篇 |
排序方式: 共有3587条查询结果,搜索用时 15 毫秒
61.
Five Ci-Se...O...Se-Ci atoms in anthraquinone and 9-(methoxy)anthracene bearing phenylselanyl groups at 1,8-positions align linearly, the origin of which is shown to be a nonbonded 5c-6e interaction of the five atoms. 相似文献
62.
Ishii Y Iwahama T Sakaguchi S Nakayama K Nishiyama Y 《The Journal of organic chemistry》1996,61(14):4520-4526
A novel class of catalysts for alkane oxidation with molecular oxygen was examined. N-Hydroxyphthalimide (NHPI) combined with Co(acac)(n)() (n = 2 or 3) was found to be an efficient catalytic system for the aerobic oxidation of cycloalkanes and alkylbenzenes under mild conditions. Cycloalkanes were successfully oxidized with molecular oxygen in the presence of a catalytic amount of NHPI and Co(acac)(2) in acetic acid at 100 degrees C to give the corresponding cycloalkanones and dicarboxylic acids. Alkylbenzenes were also oxidized with dioxygen using this catalytic system. For example, toluene was converted into benzoic acid in excellent yield under these conditions. Ethyl- and butylbenzenes were selectively oxidized at their alpha-positions to form the corresponding ketones, acetophenone, and 1-phenyl-1-butanone, respectively, in good yields. A key intermediate in this oxidation is believed to be the phthalimide N-oxyl radical generated from NHPI and molecular oxygen using a Co(II) species. The isotope effect (k(H)/k(D)) in the oxidation of ethylbenzene and ethylbenzene-d(10) with dioxygen using NHPI/Co(acac)(2) was 3.8. 相似文献
63.
Yoshiaki?YuguchiEmail author Takahiro?Hirotsu Jun?Hosokawa 《Cellulose (London, England)》2005,12(5):469-477
Xyloglucan is a type of hemicellulose with a cellulose backbone containing (1→6)-α-xylose or (1→2)-β-galactoxylose as a side chain. It is soluble in water. Its aqueous solution forms a gel or gel-like precipitate by addition
of Congo red. Xyloglucan gel structures with various concentrations of Congo red were observed by small angle X-ray scattering
(SAXS) at the nano-level. SAXS results indicated that the xyloglucan chains interacted with Congo red, and that an increase
of concentration of Congo red induced a characteristic cross-linking domain, which consisted of a flat structure containing
stacked xyloglucan chain assemblies. The Congo red molecules are inserted between the xyloglucan chains. 相似文献
64.
Takahiro Hosoya 《Tetrahedron》2005,61(29):7037-7044
From the fresh leaves of Wasabia japonica Matsum., five novel flavonoids 1-5, isovitexin derivatives having a trans-sinapoyl group at C-7, were isolated together with five known flavonoids, and their structures were elucidated on the basis of their spectroscopic data (NMR, MS, UV, and IR) and chemical evidence. 相似文献
65.
Miyuki Narita Junya Itoh Tohru Kikuchi Fumio Hamada 《Journal of inclusion phenomena and macrocyclic chemistry》2002,42(1-2):107-114
-Cyclodextrin dimer linked with ethylenediamine at the upper rim of the cyclodextrin has been synthesized and then modified with two dansyl moieties inthe presence of N,N'-dicyclohexylcarbodiimide. The sensing ability and bindingproperty of the title compound were investigated for steroids and terpenoids. Thefluorescence intensity of this dimer was decreased when a host–guest complex was formed. The value I/I0, where I0 and I are fluorescence intensitiesin the absence and presence of a guest and I is I0- I, was used as a parameter of sensitivity. This host exhibited a much higher sensitivity and selective molecular recognition ability for bile acids such as ursodeoxycholic acid andchenodeoxycholic acid and terpenoids such as (-)-borneol than the dansyl-modifiedcyclodextrins reported previously including -cyclodextrin dimer. The behaviors of the appended moieties of the host during the formation of host–guest complexes were studied using induced circular dichroism (ICD) and fluorescence spectra. The ICD intensityof this dimer was decreased on accommodation of a guest and this spectral pattern of the title dimer was opposite to that of bis dansyl-modified -cyclodextrin monomer. Theguest-induced variations in the fluorescence and ICD intensities suggest that this dimer formed a 1 : 1 host–guest complex and the appended moieties act as a hydrophobic cap. 相似文献
66.
Large-scale separation of metallic and semiconducting single-walled carbon nanotubes 总被引:6,自引:0,他引:6
Maeda Y Kimura S Kanda M Hirashima Y Hasegawa T Wakahara T Lian Y Nakahodo T Tsuchiya T Akasaka T Lu J Zhang X Gao Z Yu Y Nagase S Kazaoui S Minami N Shimizu T Tokumoto H Saito R 《Journal of the American Chemical Society》2005,127(29):10287-10290
In the applications of single-walled carbon nanotubes (SWNTs), it is extremely important to separate semiconducting and metallic SWNTs. Although several methods have been reported for the separation, only low yields have been achieved at great expense. We show a separation method involving a dispersion-centrifugation process in a tetrahydrofuran solution of amine, which makes metallic SWNTs highly concentrated to 87% in a simple way. 相似文献
67.
[reaction: see text] Highly basic lithium enolates are shown to be applicable to radical trifluoromethylation. The reaction is extremely fast, and the minimum reaction time is approximately 1 s. 相似文献
68.
Masaki Kashiwara Takahiro Kawai Toshio Oshima 《Communications in Mathematical Physics》1978,60(2):97-130
We use the holonomic character of Feynman integrals to describe their singularity structure explicitly in some simple cases. The results in §1 show that under moderate conditions Feynman amplitudes can be locally expressed essentially in terms of Legendre functions near the points where two positive- Landau-Nakanishi surfaces meet. Related topics such as hierarchical principle in perturbation theory are also discussed in terms of holonomic systems involved. In §4 we use the concrete expressions for Feynman amplitudes obtained in §1 to discuss the validity of Sato's conjecture.Supported in part by NSF MCS 75-2333Supported in part by NSF GP 36269 相似文献
69.
The spectrum of magnetic resonance is typically affected by a modulated structure in an incommensurate phase, resulting in the broadened spectrum characterized by ‘edge singularities’. We demonstrate that the modulated structure in the crystal where molecular motion exists is responsible for the narrowing of the spectrum in the fast motional region by means of the simulation of 2H NMR spectrum of [Mg(H2O)6][SiF6]. The program developed by combining the theories of phase solitons in the incommensurate phase and motional change of 2H NMR spectrum was used for the simulation. We obtained a good agreement between the theoretical 2H NMR spectrum and experimental one for [Mg(H2O)6][SiF6]. 相似文献
70.
A series of iminodiacetic acid (IDA)-linked polyamides (DpPyPyPy-IDA-PyPyPyDp) were prepared and constitute polyamides joined head-to-head by a functionalizable five-atom linker. It was found that the IDA linker exerts a unique influence over the DNA binding conformation differing from both the beta-alanine (extended) or gamma-aminobutyric acid (hairpin) linkers, resulting in cooperative parallel side-by-side 2:1 binding in an extended conformation most likely with a staggered versus stacked alignment. A generalized variant of a fluorescent intercalator displacement (FID) assay conducted on a series of hairpin deoxyoligonucleotides containing a systematically varied A/T-rich binding-site size was used to distinguish between the binding modes of the IDA-linked polyamides. 相似文献