全文获取类型
收费全文 | 729篇 |
免费 | 23篇 |
国内免费 | 5篇 |
专业分类
化学 | 401篇 |
晶体学 | 28篇 |
力学 | 3篇 |
数学 | 65篇 |
物理学 | 195篇 |
无线电 | 65篇 |
出版年
2023年 | 6篇 |
2022年 | 19篇 |
2021年 | 16篇 |
2020年 | 22篇 |
2019年 | 17篇 |
2018年 | 11篇 |
2017年 | 14篇 |
2016年 | 29篇 |
2015年 | 15篇 |
2014年 | 48篇 |
2013年 | 54篇 |
2012年 | 44篇 |
2011年 | 41篇 |
2010年 | 29篇 |
2009年 | 31篇 |
2008年 | 37篇 |
2007年 | 19篇 |
2006年 | 29篇 |
2005年 | 28篇 |
2004年 | 25篇 |
2003年 | 20篇 |
2002年 | 13篇 |
2001年 | 14篇 |
2000年 | 14篇 |
1999年 | 10篇 |
1998年 | 7篇 |
1997年 | 7篇 |
1996年 | 7篇 |
1995年 | 24篇 |
1994年 | 10篇 |
1993年 | 11篇 |
1992年 | 9篇 |
1991年 | 3篇 |
1989年 | 7篇 |
1988年 | 2篇 |
1987年 | 6篇 |
1986年 | 5篇 |
1985年 | 9篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1981年 | 4篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1977年 | 7篇 |
1976年 | 4篇 |
1974年 | 2篇 |
1973年 | 2篇 |
1972年 | 5篇 |
1936年 | 4篇 |
排序方式: 共有757条查询结果,搜索用时 812 毫秒
11.
Pentacoordinate complex cations of the general formula [(C6F5)2SbL3]3+ stabilized as solid salts in combination with tetraphenylborate (BPh4), tetrafluorobroate (BF4) anions, where L=DMSO, Ph3AsO, PyO, DMF, α-, β- and γ-picoline have been isolated. The newly formed complexes were characterized by elemental analysis, molar conductance measurements, solid-state IR and and NMR. From these results, a five-fold coordination around antimony was required. 相似文献
12.
Majumdar G Goswami M Sarma TK Paul A Chattopadhyay A 《Langmuir : the ACS journal of surfaces and colloids》2005,21(5):1663-1667
In this letter, we report the synthesis of Au nanoparticles (NPs) and polyaniline (PANI) on the same cation-exchange resin beads and demonstrate their use in catalyzing the oxidation of glucose to gluconic acid by Au NPs and simultaneously in detecting the formation of the acid by the color change of PANI. The synthesis was carried out by exchanging the cations of the resins with HAuCl4 and anilinuium chloride and then reducing the metal ions by NaBH4 to produce Au NPs followed by polymerization of aniline using H2O2. The green emeraldine salt form of PANI thus obtained was treated with NaOH to be converted to blue emeraldine base before use. The deposition of Au NPs was confirmed by a change in color of the bead, visible spectroscopy, X-ray diffraction, and scanning electron microscopic measurements. On the other hand, the presence of PANI was confirmed by Fourier transform infrared (FTIR) and ultraviolet-visible (UV-vis) spectroscopy. The formation of gluconic acid from glucose was confirmed by FTIR spectroscopy. We could detect the presence of glucose of a minimum 1.0 mM concentration in water, using the present method. Our experimental observations demonstrate the possibility of the incorporation of multifunctional components on the surfaces of resins for carrying out a chemical reaction as well as detection of the product. 相似文献
13.
The metal ion (M2+) catalysed dissociation of cis-diaquobisoxalatochromate into the tetraaquomonooxalato complex in aqueous perchloric acid medium which follows the rate law — d(complex)/dt = {kH[H+] + kM[M2+]}[complex] has been studied. Based on kM values the order of catalysing effect of the different metal ions studied is Cu2+ > Ni2+ > Co2+ > Mn2+, which is also the order of stabilities (KMOx) of the monooxalato complexes of these metal ions; in fact the plot of log kM vs. log KMOx is linear. This together with the relative values of ΔH≠ and ΔS≠ for the H+ catalysed and M2+ catalysed paths is in agreement with a mechanism involving chelation of the catalysing cation through the free carbonyl oxygens of the oxalate ligand bound to Cr(III), followed by the dissociation of the Cr(III)? O bonds with simultaneous entry of two water molecules into the coordination sphere of Cr(III). 相似文献
14.
Vapor phase catalytic oxidation of toluene over perovskites viz., LaCoO3, LaCrO3 and LaFeO3 has been studied. The maximum activity was observed at 450°C. The oxidation is first order. The order of catalytic reactivity is LaCoO3>LaFeO3>LaCrO3. The activation energy (Ea) values for LaCoO3, LaFeO3 and LaCrO3 are 13.4, 23.93 and 25.31 kJ/mol, respectively. 相似文献
15.
Thioacetals and thioketals of various aldehydes and ketones were obtained directly from carbonyl compounds or by a transthioacetalisation process from cyclic O,O-acetals in the presence of dithiols and a catalytic amount of tetrabutylammonium tribromide (TBATB). Chemoselective thioacetalisation of aromatic aldehydes containing an electron-donating group in the presence of an aldehyde containing an electron-withdrawing group, aldehydes in the presence of ketones, aliphatic cyclic ketones in the presence of aromatic ketones and less hindered ketones in the presence of more hindered ketones have been achieved. A cyclic acetal containing an electron-donating group has been chemoselectively transthioacetalised in the presence of an acetal having an electron-withdrawing substituent. These selectivities are due to the intrinsic reactivity of the substrate themselves and are independent of the catalyst and reaction conditions. Shorter reaction times, mild reaction conditions, stability of acid sensitive protecting groups, high efficiencies, facile isolation of the desired products and the catalytic nature of the reagent are the attractive features of the present method. 相似文献
16.
Anjali Acharya A. Goswami P. K. Pujari S. Sabharwal S. B. Manohar 《Journal of polymer science. Part A, Polymer chemistry》2002,40(8):1028-1036
Positron annihilation lifetime spectroscopy was used to characterize the reentrant volume‐phase‐transition behavior of poly(N‐isopropyl acrylamide) hydrogel in an ethanol/water mixed solvent. The polymer gel was synthesized with γ irradiation. The ortho‐positronium lifetime (τ3) in the gel slowly increased with an increase in the ethanol content in the mixed solvent. τ3 was not influenced by the volume phase transition. The ortho‐positronium intensity decreased with the collapse of the gel in an approximately 10% ethanol/water mixture. When swelled in pure ethanol, τ3 initially increased with the solvent amount in the gel, showing the destruction of intramolecular hydrogen bonding and the relaxation of polymer chains. The lower critical solution temperature of the gel in the 10% ethanol/water mixture was lower than that in pure water, and τ3 for various solvent contents showed behavior similar to that seen in pure solvent. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1028–1036, 2002 相似文献
17.
Rogatsky E Jayatillake H Goswami G Tomuta V Stein D 《Journal of the American Society for Mass Spectrometry》2005,16(11):1805-1811
The development of a sensitive assay for the quantitative analysis of carbohydrates from human plasma using LC/MS/MS is described in this paper. After sample preparation, carbohydrates were cationized by Cs(+) after their separation by normal phase liquid chromatography on an amino based column. Cesium is capable of forming a quasi-molecular ion [M + Cs](+) with neutral carbohydrate molecules in the positive ion mode of electrospray ionization mass spectrometry. The mass spectrometer was operated in multiple reaction monitoring mode, and transitions [M + 133] --> 133 were monitored (M, carbohydrate molecular weight). The new method is robust, highly sensitive, rapid, and does not require postcolumn addition or derivatization. It is useful in clinical research for measurement of carbohydrate molecules by isotope dilution assay. 相似文献
18.
19.
The reaction of 2-arylpyridinecarboxaldimine [RH4C6NC(H)Py, L (1)] with hydrated RuX3 (X = Cl, Br) in boiling C2H5OH affords dark crystals of RuX2L2. Two geometrical isomers of the compound have been isolated and characterized by analytical and spectroscopic data. The trans isomer of RuCl2L2 shows a single sharp band for ν(Ru---Cl), whereas two bands are observed for the corresponding cis isomer. The highresolution 1H NMR spectra of the isolated complexes are reported and completely assigned. All the complexes have multiple t2→π*(L) transitions in the visible region. Each of the complexes display a quasi-reversible oxidative response due to an RuIII/RuII couple in the range 0.25–0.40 V vs S.C.E. at a platinum working electrode. The formal potentials of this couple obey the Hammett relationship. The ligand-based irreversible oxidations are also briefly noted. 相似文献
20.
Jonali Goswami Rajiv L. Bezbaruah Amrit Goswami Naleen Borthakur 《Tetrahedron: Asymmetry》2002,12(24):3343-3348
β-Adrenoreceptor agonists (R)-(−)-denopamine (R)-1 and (R)-(−)-salmeterol (R)-2 have been prepared in good overall yield and high enantioselectivity through a biotransformative pathway. 相似文献