全文获取类型
收费全文 | 2807篇 |
免费 | 128篇 |
国内免费 | 4篇 |
专业分类
化学 | 1833篇 |
晶体学 | 3篇 |
力学 | 78篇 |
数学 | 548篇 |
物理学 | 477篇 |
出版年
2023年 | 19篇 |
2022年 | 28篇 |
2021年 | 50篇 |
2020年 | 47篇 |
2019年 | 49篇 |
2018年 | 48篇 |
2017年 | 35篇 |
2016年 | 124篇 |
2015年 | 87篇 |
2014年 | 106篇 |
2013年 | 199篇 |
2012年 | 193篇 |
2011年 | 192篇 |
2010年 | 134篇 |
2009年 | 92篇 |
2008年 | 150篇 |
2007年 | 143篇 |
2006年 | 131篇 |
2005年 | 118篇 |
2004年 | 103篇 |
2003年 | 90篇 |
2002年 | 105篇 |
2001年 | 47篇 |
2000年 | 42篇 |
1999年 | 21篇 |
1998年 | 30篇 |
1997年 | 33篇 |
1996年 | 26篇 |
1995年 | 25篇 |
1994年 | 18篇 |
1993年 | 21篇 |
1992年 | 25篇 |
1991年 | 20篇 |
1990年 | 26篇 |
1989年 | 18篇 |
1988年 | 15篇 |
1987年 | 12篇 |
1986年 | 19篇 |
1985年 | 17篇 |
1984年 | 17篇 |
1983年 | 14篇 |
1982年 | 17篇 |
1981年 | 18篇 |
1980年 | 28篇 |
1977年 | 13篇 |
1976年 | 11篇 |
1974年 | 11篇 |
1973年 | 10篇 |
1971年 | 10篇 |
1934年 | 11篇 |
排序方式: 共有2939条查询结果,搜索用时 31 毫秒
71.
The fat of mixture of goat and cow milk, extracted by di-ethylic ether, was gas chromatographically analysed in order to propose an analytical procedure suitable to reveal small additions of cow milk to goat milk. The obtained results show evident differences in the percentages of some fatty acids. An attempt to apply the peak area ratios, corresponding to the methyl esters of fatty acids, suggested by the literature for cow milk did not yield good results. Other ratios suitable to reveal even the presence of low percentages (up to 5 %) of cow milk in goat milk were found and proposed in this paper. Such ratios are sensitive and can be defined markers towards cow or goat milk and also towards extraneous fat like lard or colza seeds oil. 相似文献
72.
Zhi-Yong Yang Emilio Jimenez-Vicente Hayden Kallas Dmitriy A. Lukoyanov Hao Yang Julia S. Martin del Campo Dennis R. Dean Brian M. Hoffman Lance C. Seefeldt 《Chemical science》2021,12(20):6913
The electronic structure of the active-site metal cofactor (FeV-cofactor) of resting-state V-dependent nitrogenase has been an open question, with earlier studies indicating that it exhibits a broad S = 3/2 EPR signal (Kramers state) having g values of ∼4.3 and 3.8, along with suggestions that it contains metal-ions with valencies [1V3+, 3Fe3+, 4Fe2+]. In the present work, genetic, biochemical, and spectroscopic approaches were combined to reveal that the EPR signals previously assigned to FeV-cofactor do not correlate with active VFe-protein, and thus cannot arise from the resting-state of catalytically relevant FeV-cofactor. It, instead, appears resting-state FeV-cofactor is either diamagnetic, S = 0, or non-Kramers, integer-spin (S = 1, 2 etc.). When VFe-protein is freeze-trapped during high-flux turnover with its natural electron-donating partner Fe protein, conditions which populate reduced states of the FeV-cofactor, a new rhombic S = 1/2 EPR signal from such a reduced state is observed, with g = [2.18, 2.12, 2.09] and showing well-defined 51V (I = 7/2) hyperfine splitting, aiso = 110 MHz. These findings indicate a different assignment for the electronic structure of the resting state of FeV-cofactor: S = 0 (or integer-spin non-Kramers state) with metal-ion valencies, [1V3+, 4Fe3+, 3Fe2+]. Our findings suggest that the V3+ does not change valency throughout the catalytic cycle.Active site FeV-cofactor of the V-nitrogenase and the EPR spectrum of the reduced cofactor showing 51V-hyperfine coupling. 相似文献
73.
G. Giraudi Claudio Baggiani Antonella Cosmaro Emilio Santià Adriano Vanni 《Fresenius' Journal of Analytical Chemistry》1998,360(2):235-240
A very selective polyclonal antiserum against 1,1,1-trichloro-2,2-bis-(4-chlorophenyl)-ethane (p,p′-DDT) was obtained by a careful choice of the haptenic structure (2,2-bis(4-chlorophenyl)-ethanol hemisuccinate). This hapten
was conjugated to BSA to prepare the immunogen. The effects of different types of solid phases on the equilibrium reaction
between the hapten on solid phase and the polyclonal antiserum were evaluated to obtain a fine tuning of the antiserum performances
in terms of specificity for p,p′-DDT and sensitivity to low levels of this pesticide. The calibration curves obtained show that it is possible to set up
a sensitive assay for p,p′-DDT, employing a p,p′-dichlorodiphenylacetic acid-based solid phase, with a detection limit of 0.12 ng/mL and a working range of about 0.21–40
ng/mL. Selectivity towards several p,p′-DDT-related substances was good (o,p-DDT 17%, p,p′-DDD 1.2% o,p-DDD 6.3%, p,p′-DDE 6.7%).
Received: 4 November 1996 / Revised: 9 June 1997 / Accepted: 16 June 1997 相似文献
74.
Federico Moscardó Miguel Paniagua Emilio San-Fabián 《Theoretical chemistry accounts》1979,53(4):377-381
Anab initio study of the relative stability for the states2
A
1g
and2
E
g of C2H
6
+
has been carried out. The results of the Open Shell Restricted Hartree-Fock calculations lead to assign the2
A
1
g
as the ground state of the molecule in agreement with previous SCF calculations.The correlation energy associated to both states has been calculated within the correlation hole model and the results, contrary to those obtained from Configuration Interaction calculations, do not alter qualitatively the conclusions from SCF. 相似文献
75.
[reaction: see text] The Pauson-Khand reaction is notably accelerated by TEMPO. According to DFT calculations, TEMPO could trigger a radical, low-energy pathway for the reaction by facilitating the decarbonylation of doublet complexes arising either from a CO/nitroxide exchange or from nitroxide addition to a CO ligand. 相似文献
76.
Samuel González-Mancebo Javier Lacadena Yolanda García-Alonso Jesús Hernández-Benito Emilio Calle Julio Casado 《Monatshefte für Chemie / Chemical Monthly》2002,133(2):157-166
Summary. Nitrosation reactions of phenol, o-cresol, 2,6-dimethylphenol, o-tert-butylphenol, 2-hydroxyacetophenone, and 2-allylphenol in water and water/acetonitrile were studied. Kinetic monitoring of
the reactions was accomplished by spectrophotometric analysis of the nitrosated products at 345 nm. The dominant reaction
was C-nitrosation via a mechanism consisting of an attack on the nitrosatable substrate by NO+/NO2H2
+ followed by a slow proton transfer. The values of the rate constants of phenolic C-nitrosation were increased by electron
donating substituents, and a good Hammett correlation was observed with ρ = −6.1. The results also revealed the strong effect of pH and the permitivity of the reaction medium on the rate constant, whose maximum values were observed for pH ≈ 3, decreasing strongly for higher pH values. The study in water/acetonitrile with up to 25% acetonitrile showed that it is possible to inhibit the reaction strongly
by increasing the percentage of the organic component. The conclusions drawn show that (i) it is possible to predict the rate of nitrosation of phenolics as a function of the meta-substituents on the phenol ring and (ii) the nitrosation of phenolics can be strongly inhibited by increasing the pH of the reaction medium as well as by lowering its dielectric constant.
Received July 13, 2001. Accepted (revised) September 18, 2001 相似文献
77.
The ring-closing metathesis (RCM) of two types of unsaturated chiral allylamines III, easily available from enantiomerically enriched epoxy alcohols, has been studied. Fully protected allylamines IIIa [(1)R = CH(2)-(CH(2))(n)()-CH=CH(2); (2)R = Boc; (3)R = PMB] have been prepared from unsaturated epoxy alcohols, whereas bis-allylamines IIIb ((1)R = Ph, (2)R = allyl,(3)R = Boc or PMB) have been prepared from 2,3-epoxy-3-phenylpropanol. Both types have been subjected to RCM to provide either cyclic allylamine I or II. The synthetic potential of these intermediates has been demonstrated by the enantioselective synthesis of (2S,3R,4S)-3,4-dihydroxyproline. 相似文献
78.
Lynch CL Hale JJ Budhu RJ Gentry AL Finke PE Caldwell CG Mills SG MacCoss M Shen DM Chapman KT Malkowitz L Springer MS Gould SL DeMartino JA Siciliano SJ Cascieri MA Carella A Carver G Holmes K Schleif WA Danzeisen R Hazuda D Kessler J Lineberger J Miller M Emini E 《Organic letters》2003,5(14):2473-2475
[reaction: see text] A novel approach to alpha,alpha-disubstituted-beta-amino acids (beta(2,2)-amino acids) was employed in the synthesis of a series of 3-(pyrrolidin-1-yl)propionic acids possessing high affinity for the CCR5 receptor and potent anti-HIV activity. The rat pharmacokinetics for these new analogues featured higher bioavailabilities and lower rates of clearance as compared to cyclopentane 1. 相似文献
79.
In this study, an analytical procedure for the determination in sediment of the most abundant and/or physiological active estrogens (estradiol, estriol, estrone, ethynyl estradiol, and diethylstilbestrol) and progestogens (progesterone, norethindrone. and levonorgestrel) is described. The procedure includes ultrasonic extraction of the lyophilized sediment, clean-up with octadecylsilica cartridges, and analysis by liquid chromatography-diode array detection-mass spectrometry (LC-DAD-MS). MS detection is performed with an electrospray interface in the positive ion mode for determination of the progestogens and in the negative ion mode for determination of the estrogens. The method was applied to the determination of the target compounds in river sediments from the area of Catalonia. Estrogens and progestogens were found at concentrations usually in the low ng g(-1) range. Estriol and norethindrone were the compounds most frequently found whereas maximum concentrations in all sediment samples were obtained for ethynyl estradiol (22.8 ng g(-1)) and estrone (11.9 ng g(-1)). Detection limits were in the range of 0.04-1.00 ng g(-1). Preliminary conjectures with regards to the environmental behavior and impact of estrogens and progestogens in rivers are made. To the authors' knowledge, this is the first work reporting a detailed method for the analysis of estrogens and progestogens in river sediments and data on the environmental occurrence of both groups of compounds. 相似文献
80.
Emilio Toja Giorgio Tarzia Pietro Ferrari Giorgio Tuan 《Journal of heterocyclic chemistry》1986,23(5):1555-1560
A series of 4,7-dihydro-4-oxo-1H-pyrrolo[2,3-b]pyridine-5-carboxylic acids was synthesized from ethyl 5-methyl(or 5H)-2-aminopyrrole-3-carboxylate. The starting pyrroles were obtained by reaction of carbethoxyacetamidine with bromoacetone or chloroacetaldehyde. One compound ( 10 ) showed antibacterial activity in vitro. 相似文献