全文获取类型
收费全文 | 666篇 |
免费 | 2篇 |
国内免费 | 2篇 |
专业分类
化学 | 399篇 |
晶体学 | 3篇 |
力学 | 25篇 |
数学 | 119篇 |
物理学 | 124篇 |
出版年
2022年 | 2篇 |
2020年 | 4篇 |
2019年 | 4篇 |
2018年 | 8篇 |
2017年 | 6篇 |
2016年 | 16篇 |
2015年 | 6篇 |
2014年 | 7篇 |
2013年 | 24篇 |
2012年 | 45篇 |
2011年 | 58篇 |
2010年 | 25篇 |
2009年 | 22篇 |
2008年 | 47篇 |
2007年 | 40篇 |
2006年 | 47篇 |
2005年 | 43篇 |
2004年 | 42篇 |
2003年 | 30篇 |
2002年 | 30篇 |
2001年 | 8篇 |
2000年 | 5篇 |
1999年 | 17篇 |
1998年 | 3篇 |
1997年 | 10篇 |
1996年 | 11篇 |
1995年 | 4篇 |
1994年 | 6篇 |
1993年 | 6篇 |
1992年 | 7篇 |
1991年 | 6篇 |
1990年 | 5篇 |
1989年 | 3篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1986年 | 6篇 |
1985年 | 10篇 |
1984年 | 7篇 |
1983年 | 8篇 |
1982年 | 5篇 |
1981年 | 5篇 |
1980年 | 3篇 |
1979年 | 6篇 |
1978年 | 1篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1974年 | 4篇 |
1973年 | 3篇 |
1972年 | 1篇 |
排序方式: 共有670条查询结果,搜索用时 15 毫秒
51.
Lameiras P Boudesocque L Mouloungui Z Renault JH Wieruszeski JM Lippens G Nuzillard JM 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2011,212(1):161-168
NMR of weakly polar analytes in an apolar ultraviscous solvent has recently been proposed for mixture analysis as a pertinent alternative to the DOSY experiment. The present article reports the first use of glycerol and glycerol carbonate as polar solvents for the NMR analysis of a model mixture of dipeptides. This work demonstrates the high potentiality of these solvents for the analysis of mixtures made of polar and potentially bioactive compounds. Medium-sized molecules slowly reorient in glycerol and glycerol carbonate under particular temperature conditions, so that solute resonances may show spin diffusion in NOESY spectra, thus opening the way to mixture analysis. Glycerol and glycerol carbonate have turned out to be ultraviscous solvents of choice for the individualization of four structurally close mixed dipeptides: Leu-Val, Leu-Tyr, Gly-Tyr and Ala-Tyr by means of 1D and 2D NOESY experiments. Selective sample excitation and signal detection were implemented to eliminate the intense proton signals of the non-deuterated solvents. Moreover, the recording of a multiplet selective 2D NOESY-TOCSY has shown that the analytical power of NMR in highly viscous solvents is not limited to the extraction of mixture component 1D subspectra but may also yield some supplementary information about atom connectivity within components. 相似文献
52.
Our efforts in construction the 1,2-dioxane ring of plakortolides through two approaches are described. The first one involved as a key step an acid catalyzed 6-endo ring closure of β-hydroperoxy trans-epoxides directed by a vinyl group adjacent to the epoxide function. By this route, an advanced intermediate of plakortolides was obtained in six steps and 35% overall yield. The second approach featured a 1,2-dioxane ring forming by a double opening of bis-epoxides by ethereal hydrogen peroxide. This reaction did not proceed in the expected sense and exclusive formation of hydroperoxy tetrahydropyran derivatives was observed via a tandem oxacyclization–hydroperoxidation sequence. 相似文献
53.
Sylvain Dulaurent Christian Moesch Pierre Marquet Jean-Michel Gaulier Gérard Lachâtre 《Analytical and bioanalytical chemistry》2010,396(6):2235-2249
In clinical or forensic toxicology, general unknown screening procedures are used to identify as many xenobiotics as possible,
belonging to numerous chemical classes. We present here a general unknown screening procedure based on liquid chromatography
coupled with use of a single linear ion trap mass spectrometer, and focus on the identification of pesticides and/or metabolites
in whole blood. After solid-phase extraction (SPE), the compounds of interest were separated using a reversed-phase column
and identified by the mass spectrometer operated first in the full-scan mass spectrometry (MS) mode, in the positive and negative
polarities, followed by MS2 and MS3 scanning of ions selected in data-dependent acquisition. The total scan time was 2.45 s. Two mass spectral libraries (MS2 and MS3), each of 450 spectra, were created for the 320 pesticides and metabolites detected after injection of pure solutions. Robustness
of the spectra and matrix effects were studied and were satisfactory for the present application. Detection limits for the
320 compounds were studied by extracting 1 mL spiked blood at concentrations between 10 μg/L and 10 mg/L. If necessary, it
was possible to decrease the detection limits of some compounds by 10–100-fold by scanning MS2 in only one polarity, owing to a shorter total scan time. However, at the same time, the detection specificity decreased
as no confirmation could be recorded in the following MS3 scan and no information could be registered in the other polarity. So, in these rare cases, confirmation by another method
was required. 相似文献
54.
Jean-Michel Henlin Hans Rink Erich Spieser Gerhard Baschang 《Helvetica chimica acta》1992,75(2):589-603
Starting from 3-(benzyloxy)cyclobutane-1,1-dimethanol, 3-thyminyl- and 3-adenylcyclobutane-1,1-dimethanol were synthesized by direct introduction of the heterocycles (Scheme 1). The mono-O-substituted methoxytrityl derivatives were separately converted to octameric phosphodiesters on an aminomethyl-polystyrene carrier by the phosphotriester method. These oligomers of carba-nucleosides were prepared in order to study their annealing behaviour towards ribo- and deoxyribo-nucleic acids as well as their potential for homologous hybridization. 相似文献
55.
Tunable distributed feedback lasing output based on reflection grating configuration instead of the traditional transmission one was realized from rhodamine 6G (R6G)-doped ethanol and 4-dicyanomethylene-2-methyl-6- (p-dimethylaminostyryl)-4H-pyran-(DCM)-doped methanol. Pure gain coupling and additional index coupling were obtained in R6G-doped ethanol and DCM-doped methanol, respectively. The tuning, which was found to be independent of the refractive index of the lasing media, went through the entire stimulated emission band for the two cases. Dual-peak lasing emission indicative of the existence of an index grating was observed in DCM-doped methanol. The interval between the two peaks increased with pump energy. 相似文献
56.
In this paper we deal with the flow of natural convection between two vertical planes with horizontal temperature gradient. We show that periodic steady flow occur when Rayleigh number increases. Critical values are obtained numerically and a nonlinear analysis is presented according to Center manifold method. 相似文献
57.
The authors study the large time asymptotics of a solution of the Fisher-KPP reaction-diffusion equation,with an initial condition that is a compact perturbation of a step function.A well-known result of Bramson states that,in the reference frame moving as 2t-(3/2) log t+x∞,the solution of the equation converges as t-→ +o∞ to a translate of the traveling wave corresponding to the minimal speed c* =2.The constant x∞ depends on the initial condition u(0,x).The proof is elaborate,and based on probabilistic arguments.The purpose of this paper is to provide a simple proof based on PDE arguments. 相似文献
58.
Valentin Goujard Jean-Michel Tatibouët Catherine Batiot-Dupeyrat 《Plasma Chemistry and Plasma Processing》2011,31(2):315-325
The carbon dioxide reforming of methane to synthesis gas was investigated in a dielectric barrier discharge reactor at room
temperature. The influence of dilution of reactants by helium was studied. We showed that, at a fixed contact time, the conversions
of CH4 and CO2 increase when the amount of helium in the gas mixture increases. This result is attributed to the “penning ionization” phenomenon,
which corresponds to an energy transfer from excited He to molecules in ground state (CH4, CO2). The selectivity to products is affected by the dilution factor. As soon as helium is present in a large amount the formation
of products resulting from recombination of methyl radicals (such as C2, C3 and C4) is less favourable due to the lowest probability of collisions to proceed. A kinetic model is proposed based on the assumption
that the reactant molecules CH4 or CO2 are attacked by active species produced by the plasma discharges, and the production of this active species are function
of the plasma power. This model which takes into account the dilution by helium fits particularly well the experimental data
we obtained. 相似文献
59.
Bisson J Poupard P Pawlus AD Pons A Darriet P Mérillon JM Waffo-Téguo P 《Journal of chromatography. A》2011,1218(36):6079-6084
The phytochemical study of the root extract of the stilbenoid-rich Vitis riparia × Vitis berlandieri grapevine was carried out by centrifugal partition chromatography (CPC). For this reason, we developed a new elution mode we named back-step, which allowed us to obtain cleaner fractions and a more efficient separation process when used in conjunction with a classical elution approach. Three hydroxystilbenes: (E)-resveratrol, (E)-?-viniferin and (E)-vitisin C, with greater than 90% purity were thus obtained through such process, with minimal sample handling and purification steps. Online coupling of CPC to ESI mass spectrometry was used for optimization of the separation parameters and to facilitate the characterization of the stilbenoids. This study details the first phytochemical investigation of stilbenoids from the hybrid species together with a new elution mode able to widen the range of ARIZONA biphasic systems. 相似文献
60.
Taking pyridine as a prototypal example of biologically important nitrogen bases involved in proton-coupled electron transfers, it is shown with the example of the photochemically triggered oxidation of phenol by Ru(III)(bpy)(3) that this proton acceptor partakes in a concerted pathway whose kinetic characteristics can be extracted from the overall kinetic response. The treatment of these data, implemented by the results of a parallel study carried out in heavy water, allowed the determination of the intrinsic kinetic characteristics of this proton acceptor. Comparison of the reorganization energies and of the pre-exponential factors previously derived for hydrogen phosphate and water (in water) as proton acceptors suggests that, in the case of pyridine, the proton charge is delocalized over a primary shell of water molecules firmly bound to the pyridinium cation. 相似文献