全文获取类型
收费全文 | 3868篇 |
免费 | 183篇 |
国内免费 | 12篇 |
专业分类
化学 | 2830篇 |
晶体学 | 8篇 |
力学 | 142篇 |
数学 | 480篇 |
物理学 | 603篇 |
出版年
2023年 | 38篇 |
2022年 | 65篇 |
2021年 | 81篇 |
2020年 | 87篇 |
2019年 | 85篇 |
2018年 | 72篇 |
2017年 | 58篇 |
2016年 | 126篇 |
2015年 | 111篇 |
2014年 | 144篇 |
2013年 | 178篇 |
2012年 | 301篇 |
2011年 | 342篇 |
2010年 | 152篇 |
2009年 | 147篇 |
2008年 | 283篇 |
2007年 | 246篇 |
2006年 | 243篇 |
2005年 | 238篇 |
2004年 | 200篇 |
2003年 | 118篇 |
2002年 | 135篇 |
2001年 | 39篇 |
2000年 | 31篇 |
1999年 | 33篇 |
1998年 | 16篇 |
1997年 | 30篇 |
1996年 | 32篇 |
1995年 | 21篇 |
1994年 | 26篇 |
1993年 | 23篇 |
1992年 | 26篇 |
1990年 | 18篇 |
1989年 | 13篇 |
1988年 | 12篇 |
1987年 | 8篇 |
1986年 | 12篇 |
1985年 | 23篇 |
1984年 | 22篇 |
1983年 | 15篇 |
1982年 | 18篇 |
1981年 | 22篇 |
1980年 | 14篇 |
1979年 | 18篇 |
1978年 | 18篇 |
1977年 | 19篇 |
1976年 | 14篇 |
1975年 | 16篇 |
1974年 | 16篇 |
1973年 | 8篇 |
排序方式: 共有4063条查询结果,搜索用时 15 毫秒
41.
Michielin O Zoete V Gierasch TM Eckstein J Napper A Verdine G Karplus M 《Journal of the American Chemical Society》2002,124(37):11131-11141
A conformational analysis of a stereochemically complete set of peptide analogues based on a cis-enediol unit is presented. The cis-enediol unit, which can replace a two or a three amino acid segment of a peptide, contains two "side chains", four asymmetrical carbon atoms, and six free dihedral angles. To determine the accessible conformational space, the molecules are divided into three fragments, each containing two free dihedral angles. The energy surfaces are computed for all dihedral angle values, and the possible conformations of the cis-enediol unit analogues are built using all combinations of the surface minima. Such a "build-up" procedure, which is very fast, is able to reproduce 75% of the minima obtained from a full dihedral angle exploration of the conformational space. The cis-enediol unit minima are compared with the corresponding di- and tripeptide minima; all peptide minima can be closely matched by a cis-enediol unit minimum of low energy (less than 2.2 kcal/mol above the lowest energy conformer). However, there are low energy minima of the cis-enediol unit that have no corresponding minima in peptides. The results are shown to depend strongly on the chirality of the analogues. The ability of each of the stereoisomers to mimic natural peptides, evaluated by the present approach, is correlated with its experimental activity in a renin inhibition assay. 相似文献
42.
Folmer-Andersen JF Lynch VM Anslyn EV 《Journal of the American Chemical Society》2005,127(22):7986-7987
A new method is described for the determination of optical purity of alpha-amino acid samples in protic media. No derivatization of the analyte or multistep synthesis is required, and high accuracy is obtained from the colorimetric output. Chiral discrimination is achieved through the use of an optically pure trans-1,2-diaminocyclohexane-derived Cu(II)-containing host that differentiates amino acid enantiomers by a factor of about 2. Enantioselective signaling arises from the implementation of an indicator displacement assay based on competitive Cu(II) coordination involving the chiral Cu(II)-containing host, the amino acid guest, and a metal ion indicator. The molecular structure of the host/guest complex was determined by X-ray analysis and exhibits chelation of the Cu(II) center by the amino acid to provide substrate organization. 相似文献
43.
E. Vincent L. Verdonck L. Naessens G.P. Van Der K.Elen 《Journal of organometallic chemistry》1984,277(2):235-244
The infrared, Raman and (1H, 13C) NMR spectra of trivinyltin chloride, bromide and iodide have been analyzed and discussed. The vibrational assignment has been confirmed by an approximate normal coordinate analysis. Evidence has been found for a marked influence of the X substituent on the tin-carbon bond due to isovalent rehybridization. Variations in the π electron system of the vinyl group are hardly significant. 相似文献
44.
Fillion E Carson RJ Trépanier VE Goll JM Remorova AA 《Journal of the American Chemical Society》2004,126(47):15354-15355
A one-pot, two-step process that transforms terminal alkynes into ethyl methyl-substituted benzylic quaternary carbon centers is described. (E)-2,2-Disubstituted-1-alkenyldimethylalanes have been shown to participate in 1,2-alkyl migration from aluminum to carbon with concomitant arylation at the 2-position to furnish ethyl methyl-substituted benzylic quaternary carbon centers, when reacted intramolecularly with aryl halides and triflates in the presence of a Pd(0) catalyst. The protocol is initiated with Cp2ZrCl2-catalyzed methylalumination of terminal alkynes followed by palladium-catalyzed intramolecular arylation of the resulting (E)-2,2-disubstituted-1-alkenyldimethylalanes, leading to 1,2-methyl shift from aluminum to carbon. In that sequence, a total of three new C-C single bonds are made, and two of the three alkyl groups on Me3Al transferred to the substrate on vicinal carbons. This method was applied to a variety of substrates, and the mechanism was investigated by deuterium-labeling experiments, which revealed that protodealumination of the final dialkylaluminum triflate or halide intermediates by CH3CN results in the formation of the fourth bond in the course of the transformation. 相似文献
45.
Baeten V von Holst C Garrido A Vancutsem J Michotte Renier A Dardenne P 《Analytical and bioanalytical chemistry》2005,382(1):149-157
In this paper we present an alternative method for detection of meat and bone meal (MBM) in feedstuffs by near-infrared microscopic (NIRM) analysis of the particles in the sediment fraction (dense fraction (d >1.62) from dichloroethylene) of compound feeds. To apply this method the particles of the sediment fraction are spread on a sample holder and presented to the NIR microscope. By using the pointer of the microscope the infrared beam is focussed on each particle and the NIR spectrum (1112–2500 nm) is collected. This method can be used to detect the presence of MBM at concentrations as low as 0.05% mass fraction. When results from the NIRM method were compared with the classical microscopic method, a coefficient of determination (R2) of 0.87 was obtained. The results of this study demonstrated that this method could be proposed as a complementary tool for the detection of banned MBM in feedstuffs by reinforcement of the monitoring of feeds. 相似文献
46.
Fingering instabilities in films moving along wetted surfaces, dimpling in horizontal liquid films, and the drainage of vertical soap films by marginal regeneration are caused by surface tension gradients along the perimeter of the thin film. These gradients lead to a mechanical instability which involves Marangoni type liquid flow. It is possible to describe the conditions for the onset of marginal regeneration with a critical number of the ratio between the driving force for the Marangoni flow and the friction of film elements that move relative to their surroundings. This ratio is called the Mysels number. A linear stability analysis leads to a scaling relation lambda approximately h(Ca)(-1/3) between the wavelength lambda of the instability and the capillary number Ca (Ca=/etaV(s)/gamma. In experiments with several Marangoni-driven instabilities this scaling relation has been found; it illustrates the general applicability in the understanding of flow phenomena of this type. Copyright 2001 Academic Press. 相似文献
47.
Spehar AM Koster S Linder V Kulmala S de Rooij NF Verpoorte E Sigrist H Thormann W 《Electrophoresis》2003,24(21):3674-3678
This paper characterizes the basic electrokinetic phenomena occurring within native poly(dimethylsiloxane) (PDMS) microchannels. Using simple buffers and current measurements, current density and electroosmosis data were determined in trapezoidal, reversibly sealed PDMS/PDMS and hybrid PDMS/glass channels with a cross-sectional area of 1035.5 microm(2) and about 6 cm length. This data was then compared to that obtained in an air-thermostated 50 microm inner diameter (1963.5 microm(2) cross-sectional area) fused-silica (FS) capillary of 70 cm length. Having a pH 7.8 buffer with an ionic strength (I) of 90 mM, Ohms's law was observed in the microchannels with electric field strengths of up to about 420 V/cm, which is about twice as high as for the FS capillary. The electroosmotic mobility (micro(EO)) in PDMS and FS is shown to exhibit the same general dependences on I and pH. For all configurations tested, the experimentally determined micro(EO) values were found to correlate well with the relationship micro(EO) = a + b log(I), where a and b are coefficients that are determined via nonlinear regression analysis. Electroosmotic fluid pumping in native PDMS also follows a pH dependence that can be estimated with a model based upon the ionization of silanol. Compared to FS, however, the magnitude of the electroosmotic flow in native PDMS is 50-70% smaller over the entire pH range and is difficult to maintain at acidic pH values. Thus, the origin of the negative charge at the inner wall of PDMS, glass, and FS appears to be similar but the density is lower for PDMS than for glass and FS. 相似文献
48.
Robert Faure Jean-Pierre Galy Emile-Jean Vincent Jos Elguero 《Journal of heterocyclic chemistry》1977,14(8):1299-1304
The substituent effects on azido/tetrazole equilibrium on a series of 2-substituted thiazoles, benzothiazoles, thiadiazoles, benzoxazoles and isoxazoles has been studied by proton magnetic resonance in two solvents (DMSO-d6 and deuteriochloroform). For thiazole an excellent Hammett relationship was found, both for the 4 and 5 positions. 相似文献
49.
Wiskur SL Lavigne JJ Metzger A Tobey SL Lynch V Anslyn EV 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(15):3792-3804
The thermodynamics of guanidinium and boronic acid interactions with carboxylates, alpha-hydroxycarboxylates, and diols were studied by determination of the binding constants of a variety of different guests to four different hosts (7-10). Each host contains a different combination of guanidinium groups and boronic acids. The guests included molecules with carboxylate and/or diol moieties, such as citrate, tartrate, and fructose, among others. The Gibbs free energies of binding were determined by UV/Vis absorption spectroscopy, by use of indicator displacement assays. The receptor based on three guanidinium groups (7) was selective for the tricarboxylate guest. The receptors that incorporated boronic acids (8-10) had higher affinities for guests that included alpha-hydroxycarboxylate and catechol moieties over guests containing only carboxylates or alkanediols. Isothermal titration calorimetry revealed the enthalpic and entropic contributions to the Gibbs free energies of binding. The binding of citrate and tartrate was investigated with hosts 7-10, for which all the binding events were exothermic, with positive entropy. Because of the selectivity of hosts 8-10, a simple boronic acid (14) was also investigated and determined to be selective for alpha-hydroxycarboxylates and catechols over amino acids and alkanediols. Further, the cooperativity of 8 and 9 in binding tartrate was also investigated, revealing little or no cooperativity with 8, but negative cooperativity with 9. A linear entropy/enthalpy compensation relationship for all the hosts 7-10, 14, and the carboxylate-/diol-containing guests was also obtained. This relationship indicates that increasing enthalpy of binding is offset by similar losses in entropy for molecular recognition involving guanidinium and boronic acid groups. 相似文献
50.
Oscillating Chemical Reactions. III. Effects of the Temperature and Chemical Composition on the ‘Induction Period’ of the BrO /Ce4+/Cyclohexanon and BrO /Ce4+/Cyclopentanon Systems A study of the influence of the temperature and composition of the BrO/Ce4+/cyclohexanon (S1) and BrO/Ce4+/cyclopentanon (S2) systems has shown a very particular behaviour for τind. for a given ratio α of concentrations: Moreover, for 0.27 ? α ? 0.32, log1/τind. is no longer a linear function of the inverse of the temperature: a break in the line log1/τind. = f(1/T) occurs. 相似文献