全文获取类型
收费全文 | 14501篇 |
免费 | 299篇 |
国内免费 | 72篇 |
专业分类
化学 | 8235篇 |
晶体学 | 104篇 |
力学 | 503篇 |
数学 | 2001篇 |
物理学 | 4029篇 |
出版年
2022年 | 86篇 |
2021年 | 114篇 |
2020年 | 166篇 |
2019年 | 112篇 |
2018年 | 121篇 |
2017年 | 92篇 |
2016年 | 199篇 |
2015年 | 214篇 |
2014年 | 250篇 |
2013年 | 530篇 |
2012年 | 572篇 |
2011年 | 729篇 |
2010年 | 408篇 |
2009年 | 359篇 |
2008年 | 617篇 |
2007年 | 676篇 |
2006年 | 674篇 |
2005年 | 702篇 |
2004年 | 567篇 |
2003年 | 451篇 |
2002年 | 416篇 |
2001年 | 409篇 |
2000年 | 379篇 |
1999年 | 224篇 |
1998年 | 198篇 |
1997年 | 192篇 |
1996年 | 249篇 |
1995年 | 231篇 |
1994年 | 211篇 |
1993年 | 256篇 |
1992年 | 238篇 |
1991年 | 217篇 |
1990年 | 174篇 |
1989年 | 179篇 |
1988年 | 205篇 |
1987年 | 188篇 |
1986年 | 144篇 |
1985年 | 192篇 |
1984年 | 185篇 |
1983年 | 138篇 |
1982年 | 173篇 |
1981年 | 153篇 |
1980年 | 153篇 |
1979年 | 152篇 |
1978年 | 157篇 |
1977年 | 120篇 |
1976年 | 136篇 |
1975年 | 102篇 |
1974年 | 119篇 |
1973年 | 97篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
231.
Weigelt J Wikström M Schultz J van Dongen MJ 《Combinatorial chemistry & high throughput screening》2002,5(8):623-630
NMR based screening has become an important tool in the pharmaceutical industry. Methods that provide information on the location of small molecule binding sites on the surface of a drug target (e. g. SAR-by-NMR and related techniques) are of particular interest. In order to extend the applicability of such techniques to drug targets of higher molecular weight, selective labeling strategies may be employed. Dual-amino acid selective labeling and site directed non-native amino acid replacement (SNAAR) allow for the selective detection of NMR resonances of a specific amino acid residue. This results in significantly reduced spectral complexity, which not only enables application to higher molecular weight systems, but also eliminates the need for sequential resonance assignment in order to identify the binding site. Regio-selective (or segmental) labeling of an entire protein domain of a multi domain protein may also be achieved. Labeling only a selected part of a multi domain protein (e. g. a catalytic or ligand binding domain) is an attractive way to simplify the spectral interpretation without disturbing the system under study. 相似文献
232.
K. V. Narayana Andreas Martin B. Lücke M. Belmans F. Boers D. van Deynse 《无机化学与普通化学杂志》2005,631(1):25-30
The syntheses of transition metal promoted (M = Co, Cr, Fe, Mo) supported vanadium phosphate (VPO) catalysts (TiO2 (anatase), γ‐Al2O3) and their characterization by N2‐adsorption, X‐ray diffractometry (XRD), FTIR‐spectroscopy and determination of V‐valence state is reported. The catalytic properties were checked in the heterogeneous catalytic ammoxidation of 2, 6‐dichlorotoluene to the corresponding nitrile. The catalyst samples were prepared by synthesis of the precursor compound VOHPO4 · 0.5 H2O, impregnation using various metal salt solutions and mixing with the support materials. The characterization revealed increased surface areas for all the promoted samples in comparison to the basic materials. XRD showed the formation of (VO)2P2O7 after calcinations as well as patterns of support materials (anatase, γ‐Al2O3). The formation of crystalline proportions of mixed oxides were not observed. The catalytic ammoxidation runs revealed a significant effect of the promoter metals on the catalytic properties by an increase of yield by ca. 20 % compared to bulk VPO. Almost complete conversion of 2, 6‐dichlorotoluene and 81 % yield of nitrile were observed using a 25 %VPCoO/γ‐Al2O3 catalyst. 相似文献
233.
The design of a knowledge-based system for the interpretation of combined spectral data for structure elucidation (EXSPEC) is described. Some basic design features are discussed and the functioning of the knowledge base, inference mechanism and user-interface is outlined. Attention is focussed on the development of a spectrum interpreter for infrared and mass spectral data. Interpretation of spectra for 120 liquid alcohols used for rule generation was successful. The system can be run on a Macintosh II or, more slowly, on a Macintosh Plus. 相似文献
234.
The phase β-K0.25Pb0.75F1.75 previously found in the KF-PbF2 system appears to be metastable at low temperatures relative to a mixture of orthorhombic PbF2 and a new phase suspected to be KPbF3 II. KPbF3 II transforms to KPbF3 I at 298.5°C at atmospheric pressure. The KPbF3 II/I transition line rises with pressure, but the substance appears to reversibly disproportionate above ~360°C, 5 kbar, possibly to a mixture of PbF2 and K4PbF6. Instead of β-K0.25Pb0.75F1.75, a mixture with this composition yielded, in addition to weak heat events due to the KPbF3 II/I transition, strong heat events at 254.5°C and atmospheric pressure (thermal hysteresis ~13°C) which were ascribed to the PbF2 orthorhombic/cubic transition. This transition rises with pressure to 673°C at 37.8 kbar. 相似文献
235.
CIDNP has been studied during thermal decomposition, photolysis, and sensitized photolysis of benzoyl chloroacetyl peroxide. The ratio of the CIDNP intensities for the recombination products benzyl chloride and chloromethyl benzoate is dependent on the mode of decomposition, reflecting the extent of rapid decarboxylation of the primary formed benzoyloxy radicals. 相似文献
236.
J. G. C. M. van Duijneveldt-van de Rijdt Dr. F. B. van Duijneveldt 《Theoretical chemistry accounts》1970,19(1):83-91
Using perturbation theory of intermolecular forces (including exchange effects) hydrogen-bond energies are calculated for a simple model representing the water dimer. Several orientations of the proton acceptor with respect to the O...O axis are considered and it is found that the optimum orientation depends sensitively on the oxygen lone-pair hybridization assumed. The main orientation-dependent energy term is found to be the classical electrostatic energy between the unperturbed molecules. The relation of the present results to the structures of the ices I and Ic, and to the results of recent SCF studies are discussed briefly.
For some recent CNDO/2 calculations on (H2O)2 which concentrate mainly on rotations of the donor see Ref. [5]. 相似文献
Zusammenfassung Die Energien von Wasserstoffbindungen werden für ein einfaches Modell des dimeren Wassers mit Hilfe der Störungstheorie für intermolekulare Kräfte (mit Austauscheffekten) berechnet. Verschiedene Orientierungen des Protonen-Akzeptors bezüglich der O ... O-Achse werden untersucht; man findet, daß die optimale Orientierung empfindlich von der angenommen Hybridisierung des einsamen Elektronenpaars des Sauerstoffs abhängt. Der hauptsächliche, von der Orientierung abhängige Energieterm ist die klassische elektrostatische Energie zwischen den beiden ungestörten Molekülen. Die Beziehung der vorliegenden Ergebnisse zu den Strukturen von Eis I und Ic sowie zu den Ergebnissen kürzlicher SCF-Rechnungen wird kurz diskutiert.
For some recent CNDO/2 calculations on (H2O)2 which concentrate mainly on rotations of the donor see Ref. [5]. 相似文献
237.
P. J. G. Cornelissen G. M. J. Beijersbergen † van Henegouwen† G. R. Mohn † 《Photochemistry and photobiology》1980,32(5):653-659
The photobiological activity of chlordiazepoxide, an active ingredient of the drug Librium, which is known to induce phototoxic effects, and two of its metabolites, desmethylchlordiazepoxide and demoxepam, was investigated. Upon irradiation of these biologically active compounds with longwave UV light, the main decomposition product formed is an oxaziridine. Using a strain of Salmonella typhimurium as a test organism for cytotoxicity, it could be demonstrated that not only the drug itself, but also the major mammalian metabolites are phototoxic and, furthermore, that the respective oxaziridines are responsible for the toxic effects found upon irradiation. A close relationship appears to exist between the phototoxicity of the nitrones and the toxicity in the dark of their respective oxaziridines. Investigations of the photobiological activity of a few closely structurally related benzodiazepines could establish that a 4-oxide moiety in the benzodiazepine nucleus is the structural characteristic responsible for the appearance of phototoxicity; in those compounds which contain a 4-oxide in the benzodiazepine nucleus, photo-decomposition to a toxic oxaziridine is observed, while the analogues lacking the 4-oxide moiety do not show this characteristic and, therefore, no phototoxic effects can be observed. Finally, mutagenicity tests performed with the same bacterial indicator as used for phototoxic studies, and including chlorpromazine as a positive reference compound, indicate that under the present experimental conditions photoproducts formed upon irradiation of chlordiazepoxide and its metabolites with longwave UV light do not exert a mutagenic effect. 相似文献
238.
Z. I. Kolar W. Martens P. A. J. van Hooft 《Journal of Radioanalytical and Nuclear Chemistry》1990,146(6):391-400
A method for the preparation of a radioactive tracer for sand,56Mn-labelled sand, using NH4MnF3 as the label carrier was developed. The labelling process is based on the adsorption of the label carrier onto the sand particles. 相似文献
239.
J. L. M. de Boer Walter van Leeuwen U. Kohlmeyer Peter M. Breugem 《Analytical and bioanalytical chemistry》1998,360(2):213-218
The performance of a proportional correction matrix effect reduction procedure was investigated for an axially viewed inductively coupled plasma. It was shown that the proportional correction factor (ratio of analyte matrix effect and internal standard matrix effect) was sufficiently stable over the investigated matrix element concentration ranges (0–2000 mg/L of Na and 0–400 mg/L of Ca) for the procedure to be successful. Proportional correction results in the best correction for matrix effects compared to the classical 1?:?1 intensity ratio correction procedure or the approach without any correction, as was shown in recovery experiments using analyte spiked groundwater samples. Matrix effects as high as 18% without correction were reduced to less than 4% applying proportional correction. 相似文献
240.
van Bokhoven JA Roelofs JC de Jong KP Koningsberger DC 《Chemistry (Weinheim an der Bergstrasse, Germany)》2001,7(6):1258-1265
The changes in the layered structure of Mg-Al hydrotalcite (Mg/ Al = 2) during heat treatment have been investigated by using in situ XAFS simultaneously at the Mg and Al K-edges. The development of unique in situ instrumentation allowed the coordination environments at both the Mg and Al centers to be monitored as a function of the temperature and heat treatment. The results of this study show that the hydrotalcite structure is highly flexible, and should lead to the further development of hydrotalcites as new solid basic catalysts. Moreover, the Mg and Al cations in the cation layers show different behavior as a function of temperature. The coordination of some octahedral Al ions decreases already at a temperature of 425 K, whereas the coordination about Mg does not show any modification at this temperature. However, hydrotalcite treated at 425 K, followed by cooling down to room temperature resulted in a complete reversal to the original octahedral Al coordination. It is proposed that Al-OH bond breakage occurs at 425 K, without the evolution of H2O. This bond is restored after cooling to room temperature. The actual dehydroxylation of hydrotalcite commences between 425 and 475 K, as indicated by a change in coordination of both the Mg and Al centers. This is accompanied by the evolution of H2O molecules and the changes are hence irreversible without the presence of excess water. Heat treatment at 725 K leads to the development of an MgO-like phase (octahedral Mg) and a mixed octahedral/tetrahedral Al phase. A subsequent rehydration at room temperature entirely restores the original coordination about the Al and Mg centers of hydrotalcite to a distance of 15 A, to which XAFS spectroscopy is sensitive. 相似文献