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741.
We study the erednikDrinfeld p-adic uniformization ofcertain AtkinLehner quotients of Shimura curves overQ. We use it to determine over which local fields they haverational points and divisors of a given degree. Using a criterionof Poonen and Stoll, we show that the ShafarevichTategroup of their jacobians is not of square order for infinitelymany cases. 1991 Mathematics Subject Classification 11G18, 11G20,14G20, 14G35, 14H40. 相似文献
742.
743.
Application of quantitative analytical electron microscopy to the mineral content of insect cuticle.
Ron Rasch Bronwen W Cribb John Barry Christopher M Palmer 《Microscopy and microanalysis》2003,9(2):152-154
Quantification of calcium in the cuticle of the fly larva Exeretonevra angustifrons was undertaken at the micron scale using wavelength dispersive X-ray microanalysis, analytical standards, and a full matrix correction. Calcium and phosphorus were found to be present in the exoskeleton in a ratio that indicates amorphous calcium phosphate. This was confirmed through electron diffraction of the calcium-containing tissue. Due to the pragmatic difficulties of measuring light elements, it is not uncommon in the field of entomology to neglect the use of matrix corrections when performing microanalysis of bulk insect specimens. To determine, firstly, whether such a strategy affects the outcome and secondly, which matrix correction is preferable, phi-rho (z) and ZAF matrix corrections were contrasted with each other and without matrix correction. The best estimate of the mineral phase was found to be given by using the phi-rho (z) correction. When no correction was made, the ratio of Ca to P fell outside the range for amorphous calcium phosphate, possibly leading to flawed interpretation of the mineral form when used on its own. 相似文献
744.
Ron Shepard 《Journal of computational chemistry》1990,11(1):45-57
A data compression method is presented that is generally applicable to first-order convergent iterative procedures that employ subspace expansions or extrapolations based on successive correction vectors. This method is based on the truncation of insignificant information in successive correction vectors. Although the correction vectors themselves may be severely truncated with the proposed approach, the final solution vector may be represented to arbitrary accuracy. A feature of the proposed method is that more slowly convergent iterative procedures allow the correction vectors to be more severely truncated without affecting the overall convergence rate. The method is implemented and applied to the iterative Davidson diagonalization method. If the compressed representation of the expansion vectors can be held in main computer memory, then a significant reduction in the I/O requirements is achieved. 相似文献
745.
Müller M Gras R Appel RD Bienvenut WV Hochstrasser DF 《Journal of the American Society for Mass Spectrometry》2002,13(3):221-231
The molecular scanner combines protein separation using gel electrophoresis with peptide mass fingerprinting (PMF) techniques to identify proteins in a highly automated manner. Proteins separated in a 2-dimensional polyacrylamide gel (2-D PAGE) are digested in parallel and transferred onto a membrane keeping their relative positions. The membrane is then sprayed with a matrix and inserted into a matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometer, which measures a peptide mass fingerprint at each site on the scanned grid. First, visualization of PMF data allows surveying all fingerprints at once and provides very useful information on the presence of chemical noise. Chemical noise is shown to be a potential source for erroneous identifications and is therefore purged from the mass fingerprints. Then, the correlation between neighboring spectra is used to recalibrate the peptide masses. Finally, a method that clusters peptide masses according to the similarity of the spatial distributions of their signal intensities is presented. This method allows discarding many of the false positives that usually go along with PMF identifications and allows identifying many weakly expressed proteins present in the gel. 相似文献
746.
Dendrimer-based transient supramolecular networks 总被引:1,自引:0,他引:1
Versteegen RM van Beek DJ Sijbesma RP Vlassopoulos D Fytas G Meijer EW 《Journal of the American Chemical Society》2005,127(40):13862-13868
Association of a 16-fold excess of a monodisperse telechelic oligo(THF) (Mw = 1270 g/mol) containing two end groups that selectively bind to the 32 binding sites of a fifth generation dendritic host (Mw = 18,511 g/mol and radius R(h) = 2.4 nm) results in the formation of reversible and dynamic supramolecular complexes. The structure of these complexes in solution depends strongly on the concentration. At low concentration, the two end groups of one guest are proposed to complex to the same host, and flowerlike structures are formed with a radius of R(h) = 3.7 nm. At higher concentrations, both end groups of one guest are complexed to different hosts, forming a bridge between them. This gives rise to the formation of larger associates, and eventually to a transient supramolecular network. Dynamic light scattering unequivocally showed that three distinct relaxation processes, associated with the proposed structures, are present in this system. In addition to the dynamics related to the flowerlike (fast) and the transient network structures (slow), an intermediate dynamic process is attributed to the cooperative motion of a few (approximately 6) connected flowerlike structures. Rheological data elucidate the nature of the intermittent network responsible for the slowest process. A monofunctional guest, not capable of forming a network structure, was used as a reference, and starlike supramolecular structures are formed at all concentrations, indeed. 相似文献
747.
Koster S Duursma MC Boon JJ Heeren RM Ingemann S van Benthem RA de Koster CG 《Journal of the American Society for Mass Spectrometry》2003,14(4):332-341
Electron capture dissociation (ECD) of doubly protonated hyperbranched polyesteramide oligomers (1100-1900 Da) was examined and compared with the structural information obtained by low energy collisionally activated dissociation (CAD). Both the ester and amide bonds of the protonated species were cleaved easily upon ECD with the formation of odd electron (OE(.+)) or even electron (EE(+)) fragment ions. Several mechanistic schemes are proposed that describe the complex ECD fragmentation behavior of the multiply charged oligomers. In contrast to studies of biomolecules, the present results indicate that consecutive cleavages induced by intramolecular H-shifts are significant for ECD and of less importance for low energy CAD. The capture of an electron by the ionized species results in fragmentation associated with a redistribution of the excess internal energy over the products and the subsequent bond cleavage. Low energy, multiple collision CAD is found to be a more selective dissociation method than ECD in view of the observation that only amide bonds are cleaved for most of the hyperbranched polymers examined with CAD in this study. ECD appears not to provide complementary structural information compared to CAD in the study of hyperbranched polymers, even though a significantly more complex ECD fragmentation behavior is observed. ECD is shown to be of use for the structural characterization of large oligomers that may not dissociate upon low energy CAD. This is a direct result of the fact that ECD produces ionized hyperbranched oligomers with a relatively high internal energy. 相似文献
748.
Ron Orlando Peter T. M. Kenny Carole Moquin-Pattey Keith A. Lerro Koji Nakanishi 《Journal of mass spectrometry : JMS》1993,28(12):1395-1402
Tandem mass spectrometry (MS/MS) is an attractive technique for sequencing membrane proteins because it can be applied to peptides in mixtures that are difficult to separate chromatographically. To evaluate the suitability of MS/MS sequencing for membrane proteins and to develop protocols for the preparation of the cleaved peptides, we employed the well characterized apoproteins of bacteriorhodopsin and bovine rhodopsin, i.e. bacterioopsin and opsin, respectively. Without separation, nine out of ten peptides resulting from cyanogen bromide cleavage of bacterioopsin were detected by fast atom bombardment MS, the single undetected fragment being a tetrapeptide that was presumably hidden in the low-m/z matrix background. Furthermore, MS/MS was used to confirm the sequence of all the peptides detected with m/z values below 3.5 kDa (40% of the protein). Bovine opsin was analyzed in a similar fashion. Tandem MS/MS has thus allowed the sequencing of substantial portions of two integral membrane proteins by the analysis of unseparated peptide mixtures, demonstrating for the first time that this technique can obviate some of the most serious difficulties associated with sequencing membrane proteins, namely the difficult-to-achieve separation of the ‘sticky’ peptide fragments. 相似文献
749.
Ron S. Dickson Alan P. Oppenheim Geoff N. Pain 《Journal of organometallic chemistry》1982,224(4):377-386
Treatment of the μ(η1)-alkyne complex (η-C5H5)2Rh2(CO)2(CF3C2CF3) with trimethylamine-N-oxide results in mono-decarbonylation to give the μ(η2)- alkyne complex (η-C5H5)2Rh2(μ-CO)(CF3C2CF3). Coordinative addition of a variety of ligands L to the monocarbonyl complex has been achieved at room temperature, and stable adducts (η-C5H5)2Rh2(CO)L(CF3C2CF3) (L CO, CNBut, PPh3, PMePh2, P(OMe)3, AsPh3, PF3 and PF2(NEt2)) have been fully characterized by infrared and NMR spectroscopy. In each complex, there is a μ(η1)-attachment of the hexafluorobut-2-yne and a trans-arrangement of CO and L. The spectroscopic data establish that there is rapid scrambling of CO and L when L CNBut. An unstable adduct is formed when (η-C5H5)2Rh2(μ-CO)(CF3C2CF3) is dissolved in pyridine. 相似文献
750.
Ron Shepard 《International journal of quantum chemistry》1987,31(1):33-44
The theory of MCSCF and CI energy derivatives with respect to geometrical variations is briefly reviewed with special attention given to the MCSCF and MRCI energy gradients. A computational procedure is proposed for MRCI energy gradients that does not require the solution to any “coupled-perturbed MCSCF ” equations, it does not require any expensive direct-CI matrix-vector products involving derivative integrals, and it does not require any derivative integrals to be transformed from the AO basis to the MO basis. An additional feature is that it does not require any changes to existing MCSCF gradient evaluation programs in order to compute MRCI gradients. The only difference in the two cases is the exact nature of the data passed to the gradient evaluation program from the previous steps in the computational procedure. The additional effort required to compute the entire MRCI energy gradient vector is approximately that required for one additional iteration of the MRCI diagonalization procedure and for one additional MCSCF iteration. For large scale MRCI wave functions, the MRCI energy gradient evaluation should only require about 10% of the effort of computing the wave function itself. This computational procedure removes a major computational botleneck of potential energy surface evaluation. 相似文献