全文获取类型
收费全文 | 5621篇 |
免费 | 182篇 |
国内免费 | 14篇 |
专业分类
化学 | 4022篇 |
晶体学 | 45篇 |
力学 | 119篇 |
综合类 | 1篇 |
数学 | 511篇 |
物理学 | 1119篇 |
出版年
2023年 | 48篇 |
2022年 | 197篇 |
2021年 | 194篇 |
2020年 | 142篇 |
2019年 | 153篇 |
2018年 | 133篇 |
2017年 | 97篇 |
2016年 | 219篇 |
2015年 | 182篇 |
2014年 | 235篇 |
2013年 | 336篇 |
2012年 | 376篇 |
2011年 | 430篇 |
2010年 | 282篇 |
2009年 | 286篇 |
2008年 | 365篇 |
2007年 | 307篇 |
2006年 | 273篇 |
2005年 | 222篇 |
2004年 | 191篇 |
2003年 | 146篇 |
2002年 | 119篇 |
2001年 | 96篇 |
2000年 | 86篇 |
1999年 | 57篇 |
1998年 | 50篇 |
1997年 | 46篇 |
1996年 | 40篇 |
1995年 | 39篇 |
1994年 | 43篇 |
1993年 | 45篇 |
1992年 | 47篇 |
1991年 | 25篇 |
1990年 | 28篇 |
1989年 | 18篇 |
1988年 | 21篇 |
1987年 | 15篇 |
1986年 | 15篇 |
1985年 | 16篇 |
1984年 | 18篇 |
1983年 | 21篇 |
1982年 | 12篇 |
1981年 | 22篇 |
1980年 | 11篇 |
1979年 | 13篇 |
1978年 | 16篇 |
1977年 | 9篇 |
1976年 | 9篇 |
1975年 | 9篇 |
1973年 | 8篇 |
排序方式: 共有5817条查询结果,搜索用时 15 毫秒
71.
Maria Fernanda Silva Estela Soledad Cerutti Luis D. Martinez 《Mikrochimica acta》2006,155(3-4):349-364
The purpose of this article is to review and evaluate cloud point extraction of metals and its coupling to different contemporary
instrumental methods of analysis. This review covers a selection of the literature published on this topic over the period
mainly between 1997 and September 2005 (consisting of 50 publications). The current state of the art for CPE concerning metals
and metal chelates is presented with special emphasis on the hyphenation of this interesting extraction/preconcentration approach
mediated by surfactants to spectrophotometry, atomic spectroscopy and separation techniques. We present contemporary CPE developments
concerning metal speciation and determination and their application to different environmental, clinical, geological and food
samples. Strategies for method development as well as future perspectives are also covered. 相似文献
72.
Pinotti Laura M. Silva Rosineide G. Giordano Roberto C. Giordano Raquel L. C. 《Applied biochemistry and biotechnology》2002,98(1-9):679-686
This article reports studies concerning the production of penicillin G acylase (PGA) by Bacillus megaterium. This enzyme has industrial use in the hydrolysis of penicillin G to obtain 6-aminopenicillanic acid, an essential intermediate
for the production of semisynthetic β-lactam antibiotics. Although most microorganisms produce the enzyme intracellularly,
B. megaterium provides extracellular PGA. The enzyme production by microorganisms involves several steps, resulting in a many operational
variables to be studied. The study of the inoculum is an important step to be accomplished, before addressing other issues
such as culture optimization and downstream processing. In this study, using a standard inoculum as reference, several runs
were performed aiming at the definition of operational conditions in the PGA production. Cell concentration and PGA activity
in the production medium were measured after 24, 48, and 72 h of the beginning of the production phase. This study encompasses
the duration of the inoculum germination phase and the concentration of cells used to startup the germination. Based on these
results, PGA productivity during the production phase was maximized. The selected values for these variables were 1.5 × 107 spores/mL of germination medium, germination during 24 h, and 72 h for the production phase. 相似文献
73.
E. A. Guimares Costa F. De Brito Mota A. Ferreira Da Silva 《International journal of quantum chemistry》1992,41(3):517-524
The density of states is calculated for a random distribution of donor-pairs of hydrogenlike impurities in three- and two-dimensional systems. Recent investigations of the hydrogen molecule in the alternant–molecular–orbital approximation are here extended. We found that the lowest excited state 1Σu (i.e., H+H?), which is optically connected to the ground state, plays a relevant role in the absorption spectra of semiconductor systems. 相似文献
74.
Dehydroluciferyl-coenzyme A (L-CoA) was chemically synthesized and characterized by MS, UV-vis spectrometry and RP-HPLC. The identity of the chemically synthesized compound with the one that was produced by firefly luciferase was confirmed. Moreover, the reversibility of the enzymatic conversion of dehydroluciferin ? dehydroluciferyl-adenylate ? L-CoA was also confirmed. The chemical synthesis of L-CoA, described here, may help the clarification of the activator effect of CoA on luciferase bioluminescent assays, in which the enzyme catalyzed formation of L-CoA and the consequent destruction of L-AMP is one of the possible explanations for that effect. 相似文献
75.
Ribeiro da Silva MD Gomes JR Gonçalves JM Sousa EA Pandey S Acree WE 《The Journal of organic chemistry》2004,69(8):2785-2792
The mean (N-O) bond dissociation enthalpies were derived for three 2-methyl-3-(R)-quinoxaline 1,4-dioxide (1) derivatives, with R = methyl (1a), ethoxycarbonyl (1b), and benzyl (1c). The standard molar enthalpies of formation in the gaseous state at T = 298.15 K for the three 1 derivatives were determined from the enthalpies of combustion of the crystalline solids and their enthalpies of sublimation. In parallel, accurate density functional theory-based calculations were carried out in order to estimate the gas-phase enthalpies of formation for the corresponding quinoxaline derivatives. Also, theoretical calculations were used to obtain the first and second N-O dissociation enthalpies. These dissociation enthalpies are in excellent agreement with the experimental results herewith reported. 相似文献
76.
Detailed spectral simulations based on ab initio density functional theory computations of the amide I and II infrared (IR) and vibrational circular dichroism (VCD) spectra for Ac-(Ala)(4)-NH(2), Ac-(Aib-Ala)(2)-NH(2), and Ac-(Aib)(4)-NH(2) constrained to 3(10)- and alpha-helical conformations are presented. Parameters from these ab initio calculations are transferred onto corresponding larger oligopeptides to simulate the spectra for dodecamers. The differences between conformations and for different Aib substitution patterns within a conformation are reflected in observable spectral patterns where data are available. Simulated IR spectra show small frequency shifts in the amide I maxima between 3(10)- and alpha-helices, but the same magnitude shifts occur within one conformation upon Aib substitution. Thus, from a computational basis, the frequency of the amide I maximum does not discriminate between the 3(10)- or alpha-helical conformations. Calculated VCD band shapes for 3(10)-helices showed more significant changes in amplitude, with change in the fraction of Aib, than those for alpha-helices. Generally, with increasing Aib content, the overall amide I VCD intensity becomes weaker and the amide I couplet becomes more conservative, while the amide II VCD is less affected. Although the detailed band shape is shown to be sensitive to alpha-Me substitution, the basic pattern of amide I and II relative VCD intensities still differs between alpha- and 3(10)-helices and, as a consequence, successfully discriminates between them. These predictions are all borne out in experimental spectra of Aib, mixed Aib-Ala, and Ala-based helical peptides, where available. 相似文献
77.
Study of the behaviour of amino acids in aqueous solution by time-domain NMR and high-resolution NMR
Khallouk M Rutledge DN Silva AM Delgadillo I 《Magnetic resonance in chemistry : MRC》2005,43(4):309-315
The study of protein hydration by time-domain NMR is complicated by the great number of interactions involved, resulting from the presence of several amino acids and the possible modifications produced by the various structures. Moreover, a good comprehension of the molecular interactions of the simple amino acids in solution is essential to elucidate the mechanism of the biological functions of proteins. Measurements of transverse relaxation rates of the protons of water (R(2) = 1/T(2)) in aqueous solutions of amino acids such as L-glycine, L-asparagine, L-arginine and L-tryptophan were carried out in order to study the effects of chemical exchange and molecular diffusion on the amplitude of R(2). The values of R(2) measured by the Carr-Purcell-Meiboom-Gill (CPMG) sequence were studied while varying the solution pH and the parameters of the CPMG sequence. The dependence of R(2) on pH and tau (inter-pulse delay between the first and the second pulses of the CPMG sequence) is interpreted in terms of chemical exchange between the protons of water and those of the labile amino acid groups. This interpretation was confirmed by the analysis of the proton spectra acquired using a 300 MHz NMR spectrometer. 相似文献
78.
Menezes PH Gonçalves SM Hallwass F Silva RO Bieber LW Simas AM 《Organic letters》2003,5(10):1601-1604
[reaction: see text] Several (77)Se NMR experiments were performed by titrating a sample of selenides with the chiral shift reagent methylbenzylamine (MBA), followed by acquisition of (77)Se NMR spectra. Eventually, we observed the appearance of two anisochronous resonances, with a relatively large separation, from 37 to 56 Hz, corresponding to the formation of the diastereomeric complexes. This methodology avoids derivatization processes, and the studied compound can be easily recovered from the NMR tube. 相似文献
79.
A systematic investigation of optimal conditions for determining three of the most common phenoxyl-type N-methylcarbamate pesticides (carbaryl, carbofuran and propoxur) in fruit juices by HPLC with peroxyoxalate-chemiluminescence detection is described. The required pre-column hydrolysis of pesticides and derivatization of their hydrolytic metabolites with dansyl chloride was simultaneously carried out in a short time thanks to the micellar catalytic effect provided by cetyltrimethyl ammonium bromide micelles on the hydrolysis step. The liquid chromatographic separation of the dansylated phenols was performed on a reversed-phase C18 column with isocratic elution. The analytes were detected by using an integrated derivatization chemiluminescence detection unit based on the bis(2,4,6-trichlorophenyl)oxalate-hydrogen peroxide system. Fruit juice samples containing 4.0-1500 microg/l pesticides were analysed with a precision of ca. 6.5%. After contamination of the fruit juice samples, average recovery > 93% at fortification levels of 10-100 microg/l was obtained. 相似文献
80.
A kinetic method is described for the determination of trace amounts of manganese(II), based on its catalytic effect on the oxidation of 2-hydroxybenzaldehyde thiosemicarbazone by hydrogen peroxide. The reaction is followed by measuring the rate of change of fluorescence (lambda(ex) 365 and lambda(em) 440 nm). The calibration is linear over the manganese range 2-9 ng ml with a precision of +/-1%. The proposed method suffers from few interferences. 相似文献