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101.
A fluorimetric method for the determination of citrate is described. Through a series of reactions, citrate is converted into citrazinic acid. The ammonium salt of this compound exhibits a fluorescence maximum at 430 nm when an excitation source of 340 nm is used. The method does not suffer from the normal interferences. The limit of detection is 0.01 μg ml-, and the best working range is 0.1–10 μg ml-1. 相似文献
102.
103.
104.
T. K. Kwei Kalle Levon Giuliana Tesoro Frank Liu Abraham Mey-Marom Surya Munukutla 《先进技术聚合物》1993,4(9):537-547
The synthesis of molecular composites where rigid polymer molecules are reinforcing elements in a thermoset bisimide matrix has been investigated. The approach has been designed to avoid phase separation by selecting systems where reaction of amine-terminated rigid and semiflexible oligomers with maleimide unsaturation occurs prior to crosslinking of the thermoset. This objective has been met for some compositions. The concentration and molecular weight of the rigid oligomers have been varied. The structure of the reinforcing polymer, the reactivity of the maleimide and the conditions for composite synthesis are variables of critical importance, and further work must determine the promise and limitations of this approach. 相似文献
105.
The solution conformation of bicyclo [3.1.0] hexan-3-one has been obtained by the use of the shifts induced in its 1H and 13C NMR by Yb(fod)3. Refinement of the angle of pucker, ∝, of the 5-membered ring indicates that the molecule adopts a flattened boat conformation with ∝ = 195°. This prediction is supported by ab initio, STO-3G, calculations on the isolated molecule. Use of a two- or four-site model for lanthanide-substrate complexing adequately reproduces the experimental data whereas a one-site binding model is unsatisfactory. The importance of multi-site binding is further emphasised by results for the C2v-symmetric ketone, adamantanone, where only a four-site model gives satisfactory agreement between observed and calculated lanthanide-induced shifts. 相似文献
106.
Kirmaier C Bautista JA Laible PD Hanson DK Holten D 《The journal of physical chemistry. B》2005,109(50):24160-24172
Subpicosecond transient absorption studies are reported for a set of Rhodobacter (R.) capsulatus bacterial photosynthetic reaction centers (RCs) designed to probe the origins of the unidirectionality of charge separation via one of two electron transport chains in the native pigment-protein complex. All of the RCs have been engineered to contain a heterodimeric primary electron donor (D) consisting of a bacteriochlorophyll (BChl) and a bacteriopheophytin (BPh). The BPh component of the M heterodimer (Mhd) or L heterodimer (Lhd) is introduced by substituting a Leu for His M200 or His L173, respectively. Previous work on primary charge separation in heterodimer mutants has not included the Lhd RC from R. capsulatus, which we report for the first time. The Lhd and Mhd RCs are used as controls against which we assess RCs that combine the heterodimer mutations with a second mutation (His substituted for Leu at M212) that results in replacement of the native L-side BPh acceptor with a BChl (beta). The transient absorption spectra reveal clear evidence for charge separation to the normally inactive M-side BPh acceptor (H(M)) in Lhd-beta RCs to form D+H(M)- with a yield of approximately 6%. This state also forms in Mhd-beta RCs but with about one-quarter the yield. In both RCs, deactivation to the ground state is the predominant pathway of D decay, as it is in the Mhd and Lhd single mutants. Analysis of the results indicates an upper limit ofV2L/V2m < or = 4 for the contribution of the electronic coupling elements to the relative rates of electron transfer to the L versus M sides of the wild-type RC. In comparison to the L/M rate ratio (kL/kM) approximately 30 for wild-type RCs, our findings indicate that electronic factors contribute approximately 35% at most to directionality with the other 65% deriving from energetic considerations, which includes differences in free energies, reorganization energies, and contributions of one- and two-step mechanisms on the two sides of the RC. 相似文献
107.
Huang F Switek KA Zakharov LN Fronczek FR Slebodnick C Lam M Golen JA Bryant WS Mason PE Rheingold AL Ashraf-Khorassani M Gibson HW 《The Journal of organic chemistry》2005,70(8):3231-3241
Four new bis(m-phenylene)-32-crown-10-based cryptands with different third bridges were prepared. Their complexes with paraquat derivatives were studied by proton NMR spectroscopy, mass spectrometry, and X-ray analysis. It was found that these cryptands bind paraquat derivatives very strongly. Specifically, a diester cryptand with a pyridyl nitrogen atom located at a site occupied by either water or a PF(6) anion in analogous complexes exhibited the highest association constant K(a) = 5.0 x 10(6) M(-1) in acetone with paraquat, 9000 times greater than the crown ether system. X-ray structures of this and analogous complexes demonstrate that improved complexation with this host is a consequence of preorganization, adequate ring size for occupation by the guest, and the proper location of the pyridyl N-atom for binding to the beta-pyridinium hydrogens of the paraquat guests. This readily accessible cryptand is one of the most powerful hosts reported for paraquats. 相似文献
108.
Reppert PM Morgan FD Lesmes DP Jouniaux L 《Journal of colloid and interface science》2001,234(1):194-203
An experimental apparatus and data acquisition system was constructed to measure the streaming potential coupling coefficients as a function of frequency. The purpose of the experiments was to measure, for the first time, the real and imaginary portion of streaming potentials. In addition, the measured frequency range was extended beyond any previous measurements. Frequency-dependent streaming potential experiments were conducted on one glass capillary and two porous glass filters. The sample pore diameters ranged from 1 mm to 34 μm. Two frequency-dependent models (Packard and Pride) were compared to the data. Both Pride's and Packard's models have a good fit to the experimental data in the low- and intermediate-frequency regime. In the high-frequency regime, the data fit the theory after being corrected for capacitance effects of the experimental setup. Pride's generalized model appears to have the ability to more accurately estimate pore sizes in the porous medium samples. Packard's model has one unknown model parameter while Pride's model has four unknown model parameters, two of which can be independently determined experimentally. Pride's additional parameters may allow for a determination of permeability. Copyright 2001 Academic Press. 相似文献
109.
Alkyl, aromatic, benzylic and benzoyl halides have been successfully coupled in good yields using lithium wire suspended in tetrahydrofuran and ultrasound. 相似文献
110.
The title compounds may be made from bis(phenylthio) acetals and aldehydes by routes involving PhS migration. 相似文献