全文获取类型
收费全文 | 6801篇 |
免费 | 330篇 |
国内免费 | 73篇 |
专业分类
化学 | 4938篇 |
晶体学 | 31篇 |
力学 | 140篇 |
综合类 | 1篇 |
数学 | 1099篇 |
物理学 | 995篇 |
出版年
2023年 | 58篇 |
2022年 | 105篇 |
2021年 | 120篇 |
2020年 | 162篇 |
2019年 | 130篇 |
2018年 | 111篇 |
2017年 | 95篇 |
2016年 | 203篇 |
2015年 | 171篇 |
2014年 | 216篇 |
2013年 | 341篇 |
2012年 | 482篇 |
2011年 | 540篇 |
2010年 | 274篇 |
2009年 | 199篇 |
2008年 | 442篇 |
2007年 | 434篇 |
2006年 | 423篇 |
2005年 | 392篇 |
2004年 | 358篇 |
2003年 | 270篇 |
2002年 | 250篇 |
2001年 | 113篇 |
2000年 | 74篇 |
1999年 | 74篇 |
1998年 | 49篇 |
1997年 | 66篇 |
1996年 | 75篇 |
1995年 | 51篇 |
1994年 | 53篇 |
1993年 | 63篇 |
1992年 | 43篇 |
1991年 | 54篇 |
1990年 | 47篇 |
1989年 | 35篇 |
1988年 | 36篇 |
1987年 | 44篇 |
1986年 | 40篇 |
1985年 | 49篇 |
1984年 | 43篇 |
1983年 | 34篇 |
1982年 | 31篇 |
1981年 | 37篇 |
1980年 | 37篇 |
1979年 | 35篇 |
1978年 | 24篇 |
1977年 | 20篇 |
1976年 | 24篇 |
1975年 | 25篇 |
1974年 | 21篇 |
排序方式: 共有7204条查询结果,搜索用时 15 毫秒
41.
Yu JQ Wu HC Ramarao C Spencer JB Ley SV 《Chemical communications (Cambridge, England)》2003,(6):678-679
A robust and recyclable palladium catalyst [Pd0EnCat] has been prepared by ligand exchange of polyurea-encapsulated palladium(II) acetate with formic acid, resulting in deposition of Pd(0) in the support material; Pd0EnCat is shown to be a highly efficient transfer hydrogenation catalyst for chemoselective reduction of a wide range of aryl ketones to benzyl alcohols. 相似文献
42.
A general method is described for asymmetric synthesis of both syn- and anti-1,3-amino alcohols. The first application of metalloenamines derived from N-sulfinyl imines is reported for the highly diastereoselective addition to aldehydes. The reduction of the product beta-hydroxy N-sulfinyl imines 2 with catecholborane and LiBHEt(3) provides syn- and anti-1,3-amino alcohols with very high diastereomeric ratios. This method was found to be effective for a variety of substrates incorporating either aromatic or various aliphatic substituents. The convergent and efficient asymmetric syntheses of the two natural products, (-)-8-epihalosaline and (-)-halosaline, were also accomplished. 相似文献
43.
Ana A. C. Pacheco Arnaldo F. da Silva Filho Kristoffer Kortsen Magnus W. D. Hanson-Heine Vincenzo Taresco Jonathan D. Hirst Muriel Lansalot Franck D'Agosto Steven M. Howdle 《Chemical science》2021,12(3):1016
Reversible addition–fragmentation chain transfer (RAFT) dispersion polymerisation of methyl methacrylate (MMA) is performed in supercritical carbon dioxide (scCO2) with 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (DDMAT) present as chain transfer agent (CTA) and surprisingly shows good control over PMMA molecular weight. Kinetic studies of the polymerisation in scCO2 also confirm these data. By contrast, only poor control of MMA polymerisation is obtained in toluene solution, as would be expected for this CTA which is better suited for acrylates. In this regard, we select a range of CTAs and use them to determine the parameters that must be considered for good control in dispersion polymerisation in scCO2. A thorough investigation of the nucleation stage during the dispersion polymerisation reveals an unexpected “in situ two-stage” mechanism that strongly determines how the CTA works. Finally, using a novel computational solvation model, we identify a correlation between polymerisation control and degree of solubility of the CTAs. All of this ultimately gives rise to a simple, elegant and counterintuitive guideline to select the best CTA for RAFT dispersion polymerisation in scCO2.RAFT dispersion polymerisation of methyl methacrylate is performed in scCO2 with 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (DDMAT) present as chain transfer agent (CTA) and surprisingly shows good control over PMMA molecular weight. 相似文献
44.
Controlled drug delivery remains a research focus for public health to enhance patient compliance, drug efficiency and reduce
the side effects of drugs. Pectin, an edible plant polysaccharide, has been shown to be useful for the construction of drug
delivery systems for specific drug delivery. Several pectin derived formulations have been developed in our laboratory and
tested in vitro, ex vivo, and in vivo for the ability to deliver bioactive substances for therapeutic purposes in the context of interactions with living tissues.
Pectin derivatives carrying primary amine groups were more mucoadhesive and have shown potential in nasal drug delivery and
other mucosal drug delivery. Pectin derivatives with highly esterified galacturonic acid residues are more hydrophobic and
able to sustain the release of incorporated fragrances for a prolonged duration. Less esterified pectin derivatives are able
to penetrate deeper into the skin and may be useful in aromatherapy formulations. Pectin, in combination with zein, a corn
protein, forms hydrogel beads. The bound zein restricts bead swelling and retains the porosity of the beads; the pectin networks
shield the zein from protease attack. The complex beads are ideal vehicles for colon-specific drug delivery. Studies presented
in this paper indicate the flexibility and possibility to tailor pectin macromolecules into a variety of drug delivery systems
to meet different clinical requirements.
Mention of trade names or commercial products in this article is solely for the purpose of providing specific information
and does not imply recommendation or endorsement by the US Department of Agriculture. 相似文献
45.
Oxyallyl cations generated in situ from dihaloketones have been found to undergo the Favorskii rearrangement in preference to an intramolecular Type-II 4 + 3 cycloaddition reaction in trifluoroethanol and hexafluoropropan-2-ol solvents, generating acrylate esters with high cis selectivity. A synthetic alternative to the intramolecular Type-II 4 + 3 cycloaddition has been developed on the basis of intramolecular enolate alkylation to close a 7-membered ring with an exocyclic double bond. 相似文献
46.
Direct-current cyclic voltammetry is used to investigate the suitability of a ferrocene derivative as a mediator with galactose, glycolate and l-amino acid oxidases. The three enzymes coupled catalytically to ferrocene monocarboxylic acid exhibiting homogeneous second-order rate constants in the range 0.4 × 105 to 8.5 × 105 l mol?1 s?1. Enzyme electrodes which responded to d-galactose, glycolate or l-amino acids were constructed. The appropriate oxidase was retained behind a dialysis membrane at a carbon paste electrode containing the poorly soluble derivative 1,1′-dimethylferrocene. All the electrodes responded rapidly to millimolar concentrations of their respective substrates producing 95% of the steady-state current response in <2 min. This general method of biosensor construction should be widely applicable to oxidases and other oxidoreductase enzymes. 相似文献
47.
Jonathan A. Yoffe Gerald M. Maggiora A.Terry Amos 《Theoretical chemistry accounts》1981,58(2):137-144
Stieltjes conditions and the ratio test provide necessary but not sufficient conditions onS(-2k) dipole sums. If the dipole sums are accurate the associated [n, n –1] Padé approximant provides a better representation of (), the frequency-dependent dipole polarizability, than a truncated series expression and, in addition, should bound () below. It is shown how constraints on the dipole sums effect the form of the [2,1] Padé approximant and an additional constraint is derived that ensures the analyticity of the approximant on 0 < 1. There then follows a discussion of the reliability of available literature dipole sum values for small molecules containing H, C, N and O. 相似文献
48.
The crystal and molecular structure of [2.2](2,5)furano(1,4)naphthalenophane (1) was determined by X-ray crystallography. The molecule exists in the anti-conformation and the study represents the first instance in which the structural features of a naphthalenoid ring within a cyclophane were determined. Crystals of cyclophane 1 are orthorhombic, space group Pbca, with a = 7.859(2). b = 11.482(3) and c = 28.818(8) Å. While the nonbridged portion of the naphthalenoid ring is planar, the portion which is bridged to the furanoid ring through its 1 and 4 C atoms is puckered and boat-shaped. These C atoms are positioned 14° out of the plane of the other four C atoms of this ring. The furanoid ring is essentially planar but is not parallel to the naphthalenoid ring. It is inclined 22° to the least squares plane of the bridged portion of the naphthalenoid ring. This angle of inclination staggers the atoms of the furanoid and bridged naphthalenoid ring and positions the 3 and 4 C atoms, the 2 and 5 C atoms and the 0 atom of the furanoid ring 3.4. 2.9 and 2.6 Å. respectively, from the least squares plane of the bridged portion of the naphthalenoid ring. While the internal angles around the bridging C atoms α- to the naphthalenoid ring are 109°, those α- to the furanoid ring are 113°. In addition unusually large bond angles (137°) at the 2 and 5 C atoms of the furanoid ring, external to the ring, are also observed. The distortions are considered with respect to the strain within the cyclophane macrocycle and are compared with other similar systems. 相似文献
49.
Coldham I Crapnell KM Fernàndez JC Moseley JD Rabot R 《The Journal of organic chemistry》2002,67(17):6181-6187
A synthesis of the core ABC ring system of the manzamine alkaloids is described, starting from arecoline. The key steps involve a Claisen rearrangement to set up a 4-substituted-3-methylenepiperidine and a stereoselective azomethine ylide dipolar cycloaddition reaction. Condensation of the aldehyde 6 and sarcosine ethyl ester hydrochloride salt gives an intermediate azomethine ylide, which undergoes an intramolecular cycloaddition reaction to set up two new rings and three new chiral centers stereoselectively. The aldehyde 6 was not a suitable substrate for related azomethine ylide cycloaddition reactions with other amines. However, the related dimethyl acetal 26 could be condensed with a variety of amines to give the desired tricyclic products. The cycloaddition reaction with N-methyl or N-allyl glycine ethyl ester gave almost exclusively the exo adduct, whereas cycloaddition with glycine ethyl ester gave the endo adduct. 相似文献
50.
G. L. Marshall 《Journal of mass spectrometry : JMS》1983,18(4):168-172
The electron impact mass spectra of the acetates of zinc, magnesium, cobalt and manganese have been investigated using a direct inlet probe. Volatile tetrahedral complexes are produced on heating, and ions of the form [M4(OCOCH3)6O]+˙ (M = Co and Mn) and [N4(OCOCH3)5O]+ (N = Zn or Mg) are observed. A mixture of magnesium and cobalt acetates produces ions of the form [MgnCo4–n(OCOCH3)6O]+˙. 相似文献