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931.
Paul C. Blainey Philip J. Reid 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2001,57(14):1083-2774
The effect of halogen substitution on intermolecular hydrogen-bonding in ethanol is studied. Specifically, Fourier-transform infrared (FTIR) spectra of ethanol, 2,2,2-trifluoroethanol (TFE), and 2,2,2-trichloroethanol dissolved in carbon tetrachloride are reported as a function of temperature and concentration. The spectral intensities corresponding to monomer, dimer, and multimer formation are used to determine the effect of halogen substitution on intermolecular hydrogen-bonding. The enthalpy for dimerization was found to evolve from -4.2+/-0.3 kcal/mol in ethanol to -6.8+/-1.0 kcal/mol in TFE. An opposite trend was observed for multimer formation with enthalpies of -3.7+/-0.5 in ethanol and -2.1+/-1.4 kcal/mol in TFE. The majority of this evolution is assigned to the ability of ethanols to form intramolecular hydrogen bonds involving the hydoxyl proton and the halogen substituents. 相似文献
932.
Henrik Birkedal Dieter Schwarzenbach Philip Pattison 《Acta Crystallographica. Section C, Structural Chemistry》2001,57(8):975-977
The crystal structure of the neuroactive artificial dipeptide N‐benzyloxycarbonylprolyl‐d ‐leucine, C19H26N2O5, was solved using synchrotron radiation data collected on a very small crystal (20 × 20 × 380 μm). The molecules form hydrogen‐bonded 21 helices. The acid carbonyl group does not participate in strong hydrogen bonds. This is interpreted as a consequence of close‐packing requirements. 相似文献
933.
David E. Mery Amanda J. Compadre Paola E. Ordez Edward J. Selvik Vladimir Morocho Jorge Contreras Omar Malagn Darin E. Jones Philip J. Breen Michael J. Balick Flavio G. Gaudio Monica L. Guzman Cesar M. Compadre 《Molecules (Basel, Switzerland)》2022,27(9)
A method to identify anticancer compounds in plants was proposed based on the hypothesis that these compounds are primarily present in plants to provide them with an ecological advantage over neighboring plants and other competitors. According to this view, identifying plants that contain compounds that inhibit or interfere with the development of other plant species may facilitate the discovery of novel anticancer agents. The method was developed and tested using Magnolia grandiflora, Gynoxys verrucosa, Picradeniopsis oppositifolia, and Hedyosmum racemosum, which are plant species known to possess compounds with cytotoxic activities. Plant extracts were screened for growth inhibitory activity, and then a thin-layer chromatography bioautography assay was conducted. This located the major antileukemic compounds 1, 2, 4, and 5 in the extracts. Once the active compounds were located, they were extracted and purified, and their structures were determined. The growth inhibitory activity of the purified compounds showed a significant correlation with their antileukemic activity. The proposed approach is rapid, inexpensive, and can easily be implemented in areas of the world with high biodiversity but with less access to advanced facilities and biological assays. 相似文献
934.
935.
Dr. Maria Concistré Dr. Subhradip Paul Marina Carravetta Ilya Kuprov Dr. Philip T. F. Williamson 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(68):15852-15854
Combining dynamic nuclear polarization with proton detection significantly enhances the sensitivity of magic-angle spinning NMR spectroscopy. Herein, the feasibility of proton-detected experiments with slow (10 kHz) magic angle spinning was demonstrated. The improvement in sensitivity permits the acquisition of indirectly detected 14N NMR spectra allowing biomolecular structures to be characterized without recourse to isotope labelling. This provides a new tool for the structural characterization of environmental and medical samples, in which isotope labelling is frequently intractable. 相似文献
936.
Dr. C. Frank Lee Dr. Carla E. Brown Dr. Alexander J. Nielsen Changmo Kim Dr. Izhar Livne-Bar Prof. Dr. Philip J. Parsons Dr. Christophe Boldron Dr. François Autelitano Prof. Dr. Donald F. Weaver Prof. Dr. Jeremy M. Sivak Prof. Dr. Mark A. Reed 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(35):e202200360
Two stereocontrolled, efficient, and modular syntheses of eicosanoid lipoxin B4 (LXB4) are reported. One features a stereoselective reduction followed by an asymmetric epoxidation sequence to set the vicinal diol stereocentres. The dienyne was installed via a one-pot Wittig olefination and base-mediated epoxide ring opening cascade. The other approach installed the diol through an asymmetric dihydroxylation reaction followed by a Horner-Wadsworth-Emmons olefination to afford the common dienyne intermediate. Finally, a Sonogashira coupling and an alkyne hydrosilylation/proto-desilylation protocol furnished LXB4 in 25 % overall yield in just 10 steps. For the first time, LXB4 has been fully characterized spectroscopically with its structure confirmed as previously reported. We have demonstrated that the synthesized LXB4 showed similar biological activity to commercial sources in a cellular neuroprotection model. This synthetic route can be employed to synthesize large quantities of LXB4, enable synthesis of new analogs, and chemical probes for receptor and pathway characterization. 相似文献
937.
Philipp Mostosi Prof. Dr. Hermann Schindelin Prof. Dr. Philip Kollmannsberger Dr. Andrea Thorn 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(35):14898-14905
In recent years, three-dimensional density maps reconstructed from single particle images obtained by electron cryo-microscopy (cryo-EM) have reached unprecedented resolution. However, map interpretation can be challenging, in particular if the constituting structures require de-novo model building or are very mobile. Herein, we demonstrate the potential of convolutional neural networks for the annotation of cryo-EM maps: our network Haruspex has been trained on a carefully curated set of 293 experimentally derived reconstruction maps to automatically annotate RNA/DNA as well as protein secondary structure elements. It can be straightforwardly applied to newly reconstructed maps in order to support domain placement or as a starting point for main-chain placement. Due to its high recall and precision rates of 95.1 % and 80.3 %, respectively, on an independent test set of 122 maps, it can also be used for validation during model building. The trained network will be available as part of the CCP-EM suite. 相似文献
938.
Philip Eckert Prof. Michael G. Organ 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(21):4861-4865
The role that LiBr plays in the lifetime of Pd-NHC complexes has been investigated. A bromide ion is proposed to coordinate to Pd thereby preventing beta hydride elimination (BHE) (to form NHC-H+) of the reductive elimination (RE) intermediate that normally completes with the desired cross-coupling catalytic cycle. Coordinating groups, such as anilines, are able to bind suitably well to Pd to prevent this pathway from occurring, thus reducing the need for the added salt. The metal hydride formed from BHE is very unstable and RE of the hydride to the NHC ligand occurs very rapidly giving rise to the corresponding hydrido-NHC (i.e., NHC-H+). The use of the per deuterated dibutylzinc shows a significant deuterium isotope effect, shutting down catalyst death almost completely. The use of bis-neopentylzinc, now possessing no hydrides, eliminates catalyst death all together leading to a very long-lived catalytic cycle and confirming the untoward role of BHE. 相似文献
939.
Philip Taylor 《Accreditation and quality assurance》2004,9(1-2):24-32
An operational interlaboratory comparison programme is described which disseminates SI-traceable reference values to laboratories worldwide. These reference values have an uncertainty and traceability that is demonstrated at the highest metrological level. Participating laboratories can use these values to establish the degree of equivalence of their measurement results and can use this to support their measurement capability claims, e.g. towards third parties. The programme has been run by the Institute for Reference Materials and Measurements (IRMM) since 1988, in the first phase as an awareness programme. Currently, IRMM is focusing its efforts on educational aspects of metrology via a collaboration with the European Co-operation for Accreditation, national metrological institutes (NMIs) and interested academic networks. The viewgraphs used are presented in the “Electronic Supplementary Material” of this ACQUAL issue. 相似文献
940.
Philip G. McQueen 《Journal of statistical physics》1997,86(5-6):1397-1398