全文获取类型
收费全文 | 4882篇 |
免费 | 145篇 |
国内免费 | 15篇 |
专业分类
化学 | 3230篇 |
晶体学 | 32篇 |
力学 | 74篇 |
数学 | 541篇 |
物理学 | 1165篇 |
出版年
2022年 | 92篇 |
2021年 | 104篇 |
2020年 | 97篇 |
2019年 | 75篇 |
2018年 | 54篇 |
2017年 | 78篇 |
2016年 | 134篇 |
2015年 | 114篇 |
2014年 | 126篇 |
2013年 | 225篇 |
2012年 | 214篇 |
2011年 | 264篇 |
2010年 | 157篇 |
2009年 | 154篇 |
2008年 | 201篇 |
2007年 | 201篇 |
2006年 | 155篇 |
2005年 | 180篇 |
2004年 | 141篇 |
2003年 | 116篇 |
2002年 | 106篇 |
2001年 | 70篇 |
2000年 | 85篇 |
1999年 | 69篇 |
1998年 | 49篇 |
1997年 | 47篇 |
1996年 | 59篇 |
1995年 | 68篇 |
1994年 | 67篇 |
1993年 | 72篇 |
1992年 | 83篇 |
1991年 | 66篇 |
1990年 | 54篇 |
1989年 | 53篇 |
1988年 | 47篇 |
1987年 | 52篇 |
1986年 | 42篇 |
1985年 | 68篇 |
1984年 | 53篇 |
1983年 | 60篇 |
1982年 | 57篇 |
1981年 | 51篇 |
1980年 | 58篇 |
1979年 | 60篇 |
1978年 | 49篇 |
1977年 | 53篇 |
1976年 | 42篇 |
1975年 | 33篇 |
1974年 | 48篇 |
1973年 | 54篇 |
排序方式: 共有5042条查询结果,搜索用时 687 毫秒
71.
Th. Weber E. Riedle H. J. Neusser 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1991,20(1):43-46
Sub-Doppler electronic spectra with hundreds of resolved rotational lines are now available for benzene-Ar dimers and trimers. From their analysis the structure of these clusters is precisely determined. The analysis of two bands, 6 0 1 and 16 0 2 , of C6H6 · Ar is presented in detail. It leads to accurate values of the van der Waals bond length in the electronic ground and excited state. The change in frequency upon clustering is found to be a factor of 17 larger for the overtone of the out-of-plane modev 16 than for the in-plane vibrationv 1. This can be tentatively explained by an interaction of the low frequency out-of-plane motion of the ring with the van-der-Waals motion of the Ar atom. 相似文献
72.
O. Gabriel Rosa Ganser Monika Geyer C. Jungwirth A. Orleanski G. Stehlik O. Hoffmann-Ostenhof 《Monatshefte für Chemie / Chemical Monthly》1956,87(6):695-700
Scheinbare Divergenzen zwischen den seinerzeit aufgestellten Phosphatbilanzen in Säureextrakten aus phosphatangereicherter und-verarmter Hefe und den in der letzten Mitteilung dieser Reihe erhobenen Befunden über den Gehalt an freien Nucleotiden in demselben Material, veranlaßten eine genauere Überprüfung der Frage, inwieweit die Extraktionsmethoden die Ergebnisse der Nucleotidbestimmung und der Phosphatbilanzen beeinflussen. Es zeigte sich, daß bei Bestimmung der freien Nucleotide, trotz Verwendung verschiedener Extraktionsmittel, sowohl qualitativ als auch quantitativ weitgehend entsprechende Ergebnisse erhalten wurden.Die Resultate können wieder in dem Sinne erklärt werden, daß während der Phosphatanreicherung eine Synthese von Nucleinsäure auf Kosten der freien Nucleotide erfolgt.Gleichzeitig durchgeführte Phosphatbilanzen in den Säureextrakten aus phosphat-verarmter und-angereicherter Hefe ergaben je nach den Extraktionsbedingungen starke, zur Zeit noch schwer deutbare Konzentrationsunterschiede in den einzelnen Phosphatfraktionen.
Mit 2 Abbildungen 相似文献
Mit 2 Abbildungen 相似文献
73.
Kratzsch C Peters FT Kraemer T Weber AA Maurer HH 《Journal of mass spectrometry : JMS》2003,38(3):283-295
A liquid chromatographic/mass spectrometric assay with atmospheric pressure chemical ionization (APCI-LC/MS) is presented for the fast and reliable screening and identification and for the precise and sensitive quantification of 15 neuroleptic (antipsychotic) drugs and three of their relevant metabolites in plasma. It allows confirmation of the diagnosis of a neuroleptic overdose and monitoring of psychiatric patients' compliance. The neuroleptics amisulpride, bromperidol, clozapine, droperidol, flupenthixol, fluphenazine, haloperidol, melperone, olanzapine, perazine, pimozide, risperidone, sulpiride, zotepine and zuclopenthixol and the pharmacologically active metabolites norclozapine, clozapine N-oxide and 9-hydroxyrisperidone were extracted from plasma using solid-phase extraction and were separated on a Merck LiChroCART column with Superspher 60 RP Select B as the stationary phase. Gradient elution was performed using aqueous ammonium formate and acetonitrile. After screening and identification in the scan mode using the authors' new LC/MS library, the neuroleptics were quantified in the selected-ion mode. The quantification assay was fully validated. It was found to be selective and proved to be linear from sub-therapeutic to over therapeutic concentrations for all analytes. The corresponding reference levels are listed. The accuracy and precision data were within the required limits. The analytes were stable in frozen plasma for at least 1 month. The method was successfully applied to several authentic plasma samples from patients treated or intoxicated with various neuroleptics. The validated LC/MS assay has proved to be appropriate for the isolation, separation, screening, identification and quantification of various neuroleptics in plasma for clinical toxicology and therapeutic drug monitoring purposes. 相似文献
74.
N. Thornton Lipscomb Edwin C. Weber 《Journal of polymer science. Part A, Polymer chemistry》1967,5(4):779-784
Studies have been made of the γ-radiation-induced polymerization of methyl methacrylate in bulk, in the solid state at a temperature of ?65°C. and a radiation intensity of 346,000 rad/hr. The reaction was found to have an extremely long induction period (~50 hr.) when pure monomer was used, and to be first-order with respect to polymer concentration. This first-order dependency was confirmed by a series of irradiations in which 0.6% poly(methyl methacrylate) was dissolved in the monomer before irradiation. These irradiations showed no induction period. Nuclear magnetic resonance spectroscopy indicated a much more heterotactic polymer than that obtained in the liquid state at ?49°C. 相似文献
75.
1,3-Oxazolidin-4-ones and 1,3-oxazin-4-ones were synthesized by formal cyclocondensation of imines with α- or β-hydroxy acids. 相似文献
76.
The 13C spectra of 5,6,9,10-tetradehydrocyclodeca[1,2,3,4-def]-benzo [7,8]biphenylene, 1, and 5,6,9,10-tetradehydrocyclodeca [1,2,3,4-def]-naphtho [2,3-7,8]biphenylene, 2, are reported as are those of a number of simpler acetylenic hydrocarbons used as spectral references. Most of the shifts can be assigned unambiguously. The acetylenic shift assignments were verified by ortho-proton, sp-carbon (1H(1)-13Csp(3)) decoupling experiments. A simple additive shift correlation is found for the hydrocarbons containing unstrained acetylenic groups. However, significant discrepencies are found for the 13C shifts for the strained hydrocarbons 1, 2, 1,2-bis(phenylethynyl)-benzene, 12, and 2,3-bis(phenylethynyl)-naphthalene, 13. The discrepencies are particularily large for carbons near the triple bonds and are attributed to a combination of strain, rehybridization, and other proximity effects related to the interaction between the ortho-substituted acetylenic carbons. 相似文献
77.
The principle of an assay of the major thyroid hormones by an electrochemical technique is demonstrated. The separation of 3,3',5-triiodothyronine, 3,3',5'-triiodothyronine, and thyroxine, by reverse-phase high-performance liquid chromatography is followed by their electrochemical oxidation in a thin-layer electrochemical detection cell with a low-temperature isotropic carbon working electrode. The limits of detection found were in the subnanogram range with linear response in the ranges 0–125 ng for T3 and 0–500 ng for T4. The approach makes the simultaneous assay of total serum thyroid hormones feasible. 相似文献
78.
Tomcík P Jencusová P Krajcíková M Bustin D Brescher R 《Analytical and bioanalytical chemistry》2005,383(5):864-868
An interdigitated microelectrode array (IDA) was applied to the determination of formaldehyde released from textiles produced
in industry. The proposed method is based on formaldehyde reaction with hypobromite which is formed in weakly basic media
by control current electrooxidation of bromide on the generator segment of the IDA array. The unreacted hypobromite diffuses
through the gap between individually polarisable IDA segments and it is amperometrically detected on the collector segment
of the IDA. The efficiency of this nonconvective transfer process in the absence of formaldehyde was substantially higher
(78%) in comparison with that when using the rotating ring disc electrode. The influence of the added formaldehyde on the
transfer process can be utilised to develop a simple and sensitive analytical procedure for formaldehyde detection with a
detection limit of 4×10−6 mol dm−3. 相似文献
79.
Winter B Weber R Hertel IV Faubel M Jungwirth P Brown EC Bradforth SE 《Journal of the American Chemical Society》2005,127(19):7203-7214
Photoelectron spectroscopy combined with the liquid microjet technique enables the direct probing of the electronic structure of aqueous solutions. We report measured and calculated lowest vertical electron binding energies of aqueous alkali cations and halide anions. In some cases, ejection from deeper electronic levels of the solute could be observed. Electron binding energies of a given aqueous ion are found to be independent of the counterion and the salt concentration. The experimental results are complemented by ab initio calculations, at the MP2 and CCSD(T) level, of the ionization energies of these prototype ions in the aqueous phase. The solvent effect was accounted for in the electronic structure calculations in two ways. An explicit inclusion of discrete water molecules using a set of snapshots from an equilibrium classical molecular dynamics simulations and a fractional charge representation of solvent molecules give good results for halide ions. The electron binding energies of alkali cations computed with this approach tend to be overestimated. On the other hand, the polarizable continuum model, which strictly provides adiabatic binding energies, performs well for the alkali cations but fails for the halides. Photon energies in the experiment were in the EUV region (typically 100 eV) for which the technique is probing the top layers of the liquid sample. Hence, the reported energies of aqueous ions are closely connected with both structures and chemical reactivity at the liquid interface, for example, in atmospheric aerosol particles, as well as fundamental bulk solvation properties. 相似文献
80.
Andrea Hartwig Leon Mullenders Monika Asmu? Heike Dally M. Hartmann 《Fresenius' Journal of Analytical Chemistry》1998,361(4):377-380
Based on pronounced enhancing effects in combination with other DNA-damaging agents the potentials of Ni(II), Cd(II) and
As(III) to interfere with DNA repair processes in HeLa cells was investigated. With respect to oxidative DNA damage, Ni(II)
and Cd(II) induced DNA strand breaks starting at concentrations of 250 μM and 5 μM, respectively. The induction of oxidative
DNA base modifications like 8-hydroxyguanine was restricted to the cytotoxic concentration of 750 μM Ni(II) and not observed
after treatment with Cd(II). In contrast, the removal of oxidative DNA base modifications was inhibited at concentrations
as low as 50 μM Ni(II) and 0.5 μM Cd(II). Regarding nucleotide excision repair, Ni(II) and Cd(II) disturbed the DNA-protein
interactions involved in the damage recognition step when applying HeLa nuclear protein extracts and a UV-damaged oligonucleotide,
while As(III) inhibited the actual incision event. In the case of Ni(II) and Cd(II), this effect was reversible by the addition
of Mg(II) and Zn(II), respectively. Furthermore, Cd(II) inactivated the isolated bacterial Fpg protein, most likely by the
displacement of Zn(II) from its zinc finger structure. Since DNA is continuously damaged by exogenous and endogenous sources,
an impaired repair capacity might well account for the carcinogenic action of the metal compounds.
Received: 30 July 1997 / Revised: 6 October 1997 / Accepted: 10 October 1997 相似文献