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121.
The formation of cyclic sulfates (4) from syn,syn-3-[N-(benzyloxycarbonyl)amino] 1,2-diols provides a common intermediate to access other diastereomers via two inversion procedures. Thermolysis of the cyclic sulfates in acetonitrile normally leads to inversion of the distal hydroxyl group to form a 1,3-oxazin-2-one (6). Catalytic hydrogenation of the cyclic sulfates under basic conditions (NEt(3)) results in inversion at the proximal hydroxyl group to form a 1,3-oxazolidin-2-one (5). 相似文献
122.
Several criteria are known for determining which connectionsA are determined uniquely by their curvatureF, or byF and its covariant derivatives. On a principal bundle with semi-simple gauge groupG over a 4-manifoldM, a sufficient condition forF to determineA uniquely is that the linear mapB [F B] from Lie algebra-valued 1-forms to 3-forms (pulled back toM via a local gauge) be invertible on an open dense set inM. Recently F. A. Doria has claimed that this condition is also necessary. We present counterexamples to this claim, and also to his assertion thatF determinesA uniquely if the restriction of the bundle to every open subset ofM has holonomy group equal toG andF is not degenerate as a 2-form over spacetime.This research was supported in part by N. S. F. grant MCS80-03419 (first author) and by an NSERCC operating grant (second author) 相似文献
123.
Steven J. P’Pool 《Journal of organometallic chemistry》2005,690(15):3522-3528
Oxidative addition of methyl iodide to Vaska’s complex in the ionic liquids 1-butyl-3-methylimidazolium triflate [C4mim][OTf], [C4mim] bis(trifluormethylsulfonyl)imide [Tf2N], and N-hexylpyridinium [C6pyr][Tf2N] occurred cleanly to give the expected Ir(III) oxidative addition product. Pseudo-first order rate constants were determined for the oxidative addition reaction in each solvent ([Vaska’s] = 0.25 mM, [CH3I] = 37.5 mM). The observed rate constants under these conditions were 5-10 times slower than the rate seen in DMF. At high methyl iodide concentrations (>23 mM), the expected first order dependence on methyl iodide was not observed. In each ionic liquid, there was no change in the reaction rates within experimental error over the methyl iodide concentration range of 23-75 mM. At lower methyl iodide concentration, a decrease in rate was observed in [C4mim][Tf2N] with decreasing concentration of methyl iodide. 相似文献
124.
We report on the coupling of a polymer-based microfluidic chip to a MALDI-TOF MS using a rotating ball interface. The microfluidic chips were fabricated by micromilling a mold insert into a brass plate, which was then used for replicating polymer microparts via hot embossing. Assembly of the chip was accomplished by thermally annealing a cover slip to the embossed substrate to enclose the channels. The linear separation channel was 50 microm wide, 100 microm deep, and possessed an 8 cm effective length separation channel with a double-T injector (V(inj) = 10 nL). The exit of the separation channel was machined to allow direct contact deposition of effluent onto a specially constructed rotating ball inlet to the mass spectrometer. Matrix addition was accomplished in-line on the surface of the ball. The coupling utilized the ball as the cathode transfer electrode to transport sample into the vacuum for desorption with a 355 nm Nd:YAG laser and analyzed on a TOF mass spectrometer. The ball was cleaned online after every rotation. The ability to couple poly(methylmethacrylate) microchip electrophoresis devices for the separation of peptides and peptide fragments produced from a protein digest with subsequent online MALDI MS detection was demonstrated. 相似文献
125.
Dalluge JJ Gort S Hobson R Selifonova O Amore F Gokarn R 《Analytical and bioanalytical chemistry》2002,374(5):835-840
A method has been developed for the direct determination of coenzyme A (CoA) and organic acid-CoA thioesters in mixtures using directly combined liquid chromatography/electrospray ionization-mass spectrometry (LC/ESI-MS). Mixtures of CoA and organic acid-CoA thioesters were analyzed by LC/ESI-MS with detection of protonated molecular ions and characteristic fragment ions for each compound. The identities of the CoA-thioesters were established based on LC retention times and simultaneously recorded mass spectra. Monitoring of the CoA specific fragment ion at m/z 428 throughout the chromatogram provides a unique fingerprint for CoA content in the samples that corroborates the identification of organic acid-CoA thioesters in the mixtures. Furthermore, fragment ions arising from the ester linkage portion of the molecule allow unambiguous identification of the CoA esters in the samples. A second LC elution system was developed that allows the simultaneous separation and identification of 2-hydroxypropionyl-CoA (lactyl-CoA) and 3-hydroxypropionyl CoA (3HP-CoA), which have the same mass and identical MS fragmentation behavior. The utility of LC/ESI-MS employing this elution system is demonstrated by the determination of 3HP-CoA and lactyl-CoA (converted to CoA-thioesters from their corresponding free acids using CoA-transferase) in fermentation broths from Escherichia coli strains engineered for the production of 3-hydroxypropionic acid (3HP). External calibration employing a purified 3HP-CoA standard allowed indirect quantification of 3HP content in the broth with a precision of 1% (RSD). The feasibility of extending the method described above to perform LC/selected reaction monitoring-tandem mass spectrometry for direct determination of organic acid-CoA thioesters in cells was also demonstrated. 相似文献
126.
Prater MR Gogal RM De Fabo EC Longstreth J Holladay SD 《Photochemistry and photobiology》2003,77(4):383-389
Exposure to ultraviolet radiation results in increased levels of intradermal cis-urocanic acid (cUCA) and alters cutaneous immunity by interfering with processing and presentation of antigen by Langerhans cells. Reports on effects of systemic immunotoxicity with 30 day cUCA exposure in laboratory rodents include thymic atrophy, thymic hypocellularity and decreased T-cell-mediated immunity; however, immune effects of single exposure or 5 day cUCA administration, which may better mimic human exposures, are poorly defined. The present study initially evaluated immune effects of single, 5 day, and 4 week cUCA exposure in C57BL/6N mice. Single administration of intradermal cUCA resulted in decreased splenocyte phagocytosis that persisted for 30 days after cUCA exposure. Five day consecutive cUCA exposure decreased numbers of phenotypically mature CD4(+)CD8(-) and CD4(-)CD8(+) (single positive) thymocytes, increased CD4(+)CD8(+) (double positive) immature thymocytes and increased splenocyte proliferation. Prolonged cUCA exposure (4 weeks) caused profound thymic hypocellularity and splenic hypercellularity and increased splenic macrophage chemiluminescence. Because of this apparent sensitivity of C57BL/6N mice to cUCA, thymic hypocellularity was compared between C57BL/6N and C3H/HeN mice dosed with cUCA, and was found to be more pronounced in the C57BL/6N strain. These results are an extension of previous conclusions on immune modulation caused by cUCA in the spleen and thymus. Further, the observed variation in sensitivity between the mouse strains is consistent with known genetic susceptibility of these strains to the immunomodulatory effects of exposure to sunlight. 相似文献
127.
Jason R McCarthyPatricia J Melfi Steven H CapettaChristian Brückner 《Tetrahedron》2003,59(46):9137-9146
The use of AgII as a removable template in synthetic porphyrin chemistry is described. Mild procedures for the insertion of AgII into chlorins and the demetallation of the [chlorinato]AgII complexes are delineated. The UV-vis spectra of the novel [chlorinato]AgII complexes are discussed. The diol cleavage products of [meso-tetraphenyl-2,3-diolchlorinato]silver(II) under a number of conditions are characterized and compared to those resulting from the cleavage of the corresponding free base diol chlorin or its NiII complex, highlighting the unique templating effect of AgII. The scopes and limits of electrospray ionization mass spectrometry (ESI-MS) for the analysis of AgII chlorins is described. The use of AgII as a templating metal is superior over NiII or ZnII for the preparation of free base pyrrole-modified porphyrins along metal templated pathways. 相似文献
128.
For a single peak, mass spectral resolution can be expressed in terms of peak width or ratio of peak position to peak width. Alternatively, for two equally intense peaks of equal width, resolution can be defined as the minimum peak separation such that the height of the valley between the combined peaks is less than a specified ratio (1%, 10%, 50%, 100%) of the individual (or combined) peak maximum. All these definitions depend on peak shape. Conversion formulae between various mass resolution criteria are presented for each of eight spectral peak shapes: Gaussian, triangular, trapezoidal, Lorentzian (absorption-mode, magnitude-mode, and sine-apodized magnitude-mode), and sinc (absorption-mode and magnitude-mode). From these formulae, mass resolutions based upon different criteria are readily compared for the same or different line shapes. 相似文献
129.
David W. Schwenke Steven L. Mielke Donald G. Truhlar 《Theoretical chemistry accounts》1991,79(3-4):241-269
Summary We have developed efficient and accurate techniques for the calculation of quantum mechanical reaction probabilities of atom-diatom exchange reactions in the gas phase, and we have optimized a computer code employing these techniques and applied it sucessfully to several systems. In this paper we consider further strategies for improving the algorithm to allow even more demanding applications. In this context, improvement means that equivalent results can be obtained using fewer computational resources (computer time or storage) or that an equivalent expenditure of resources can yield higher accuracy. The new strategies discussed here lead to improvement in both of these areas. Two areas of special focus in the present paper are (i) the finite difference boundary value method used for calculating distorted wave Green's functions and regular solutions for scattering by the distortion potential and (ii) the choice of the distortion potential itself. Among other results included here is the first application of the outgoing wave or scattered wave variational principle to reactive scattering. 相似文献
130.
Pablo A. Molina R. Steven Sikorski Jan H. Jensen 《Theoretical chemistry accounts》2003,109(3):100-107
The relationship between hydrogen bonding and NMR chemical shifts in the catalytic triad of low-pH α-chymotrypsin is investigated
by combined use of the effective fragment potential [(2001) J Phys Chem A 105:293] and ONIOM–NMR [(2000) Chem Phys Lett 317:589] methods. Our study shows that while the His57 Nδ1−H bond is stretched by a relatively modest amount (to about 1.060 ?) this lengthening, combined with the polarization due
to the molecular environment, is sufficient to explain the experimentally observed chemical shifts of 18.2 ppm. Furthermore,
the unusual down-field shift of Hɛ1 (9.2 ppm) observed experimentally is reproduced and shown to be induced by interactions with the C=O group of Ser214 as previously
postulated. The free-energy cost of moving Hδ1 from His57 to Asp102 is predicted to be 5.5 kcal/mol.
Received: 26 September 2001 / Accepted: 6 September 2002 / Published online: 21 January 2003
Contribution to the Proceedings of the Symposium on Combined QM/MM Methods at the 222nd National Meeting of the American
Chemical Society, 2001
Correspondence to: J. H. Jensen e-mail: jan-jensen@uiowa.edu
Acknowledgements. This work was supported by a Research Innovation Award from the Research Corporation and a type G starter grant from the
Petroleum Research Fund. The calculations were performed on IBM RS/6000 workstations obtained through a CRIF grant from the
NSF (CHE-9974502) and on supercomputers at the National Center for Supercomputer Applications at Urbana-Champaign. The authors
are indebted to Visvaldas Kairys for help with the CHARMM program, and to Daniel Quinn for many helpful discussions. 相似文献