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61.
Correia I Costa Pessoa J Duarte MT Henriques RT Piedade MF Veiros LF Jakusch T Kiss T Dörnyei A Castro MM Geraldes CF Avecilla F 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(9):2301-2317
The Schiff base N,N'-ethylenebis(pyridoxylideneiminato) (H(2)pyr(2)en, 1) was synthesized by reaction of pyridoxal with ethylenediamine; reduction of H(2)pyr(2)en with NaBH(4) yielded the reduced Schiff base N,N'-ethylenebis(pyridoxylaminato) (H(2)Rpyr(2)en, 2); their crystal structures were determined by X-ray diffraction. The totally protonated forms of 1 and 2 correspond to H(6)L(4+), and all protonation constants were determined by pH-potentiometric and (1)H NMR titrations. Several vanadium(IV) and vanadium(V) complexes of these and other related ligands were prepared and characterized in solution and in the solid state. The X-ray crystal structure of [V(V)O(2)(HRpyr(2)en)] shows the metal in a distorted octahedral geometry, with the ligand coordinated through the N-amine and O-phenolato moieties, with one of the pyridine-N atoms protonated. Crystals of [(V(V)O(2))(2)(pyren)(2)].2 H(2)O were obtained from solutions containing H(2)pyr(2)en and oxovanadium(IV), where Hpyren is the "half" Schiff base of pyridoxal and ethylenediamine. The complexation of V(IV)O(2+) and V(V)O(2) (+) with H(2)pyr(2)en, H(2)Rpyr(2)en and pyridoxamine in aqueous solution were studied by pH-potentiometry, UV/Vis absorption spectrophotometry, as well as by EPR spectroscopy for the V(IV)O systems and (1)H and (51)V NMR spectroscopy for the V(V)O(2) systems. Very significant differences in the metal-binding abilities of the ligands were found. Both 1 and 2 act as tetradentate ligands. H(2)Rpyr(2)en is stable to hydrolysis and several isomers form in solution, namely cis-trans type complexes with V(IV)O, and alpha-cis- and beta-cis-type complexes with V(V)O(2). The pyridinium-N atoms of the pyridoxal rings do not take part in the coordination but are involved in acid-base reactions that affect the number, type, and relative amount of the isomers of the V(IV)O-H(2)Rpyr(2)en and V(V)O(2)-H(2)Rpyr(2)en complexes present in solution. DFT calculations were carried out and support the formation and identification of the isomers detected by EPR or NMR spectroscopy, and the strong equatorial and axial binding of the O-phenolato in V(IV)O and V(V)O(2) complexes. Moreover, the DFT calculations done for the [V(IV)O(H(2)Rpyr(2)en)] system indicate that for almost all complexes the presence of a sixth equatorial or axial H(2)O ligand leads to much more stable compounds. 相似文献
62.
A highly stereoselective and efficient asymmetric total synthesis of (+)-laurencin (1) has been accomplished from the known oxazolidinone 5 in 15 steps. The route features an efficient internal alkylation to form oxocene 3 from 4 and a novel use of acetonitrile anion as a two-carbon acetaldehyde equivalent for direct synthesis of ketone 2 from alpha-alkoxy amide 3. 相似文献
63.
Joó F Kovács J Bényei AC Nádasdi L Laurenczy G 《Chemistry (Weinheim an der Bergstrasse, Germany)》2001,7(1):193-199
Hydrolysis and hydrogenation of [RhCl(tppms)3] (1) and trans-[RhCl(CO)(tppms)2] (2) was studied in aqueous solutions in a wide pH range (2 < pH < 11) in the presence of excess TPPMS (3-diphenylphosphinyl-benzenesulfonic acid sodium salt). In acidic solutions hydrogenation of 1 yields a mixture of cis-mer- and cis-fac-[RhClH2(tppms)3] (3a,b) while in strongly basic solutions [RhH(H2O)(tppms)3] (4) is obtained, the midpoint of the equilibrium between these hydride species being at pH 8.2. The paper gives the first successful 1H and 31P NMR spectroscopic characterization of a water soluble rhodium(I)-monohydride (4) bearing only monodentate phosphine ligands. Hydrolysis of 2 is negligible below pH 9 and its hydrogenation results in formation of [Rh(CO)H(tppms)3] (5), which is an analogue to the well known and industrially used hydroformylation catalyst [Rh(CO)H(tppts)3] (6) (TPPTS = 3,3',3'-phosphinetriyltris(benzenesulfonic acid) trisodium salt). It was shown by pH-potentiometric measurements that formation of 5 is strongly pH dependent in the pH 5-9 range, this gives an explanation for the observed but previously unexplained pH dependence of several hydroformylation reactions. Conversely, the effect of pH on the rate of hydrogenation of maleic and fumaric acid catalyzed by 1 in the 2 < pH < 7 range can be adequately described by considering solely the changes in the ionization state of these substrates. All these results warrant the use of buffered (pH-controlled) solutions for aqueous organometallic catalysis. 相似文献
64.
Purification of plasmid DNA vectors by aqueous two-phase extraction and hydrophobic interaction chromatography 总被引:4,自引:0,他引:4
The current study explores the possibility of using a polyethyleneglycol(PEG)-ammonium sulphate aqueous two-phase system (ATPS) as an early step in a process for the purification of a model 6.1 kbp plasmid DNA (pDNA) vector. Neutralised alkaline lysates were fed directly to ATPS. Conditions were selected to direct pDNA towards the salt-rich bottom phase, so that this stream could be subsequently processed by hydrophobic interaction chromatography (HIC). Screening of the best conditions for ATPS extraction was performed using three PEG molecular weights (300, 400 and 600) and varying the tie-line length, phase volume ratio and lysate load. For a 20% (w/w) lysate load, the best results were obtained with PEG 600 using the shortest tie-line (38.16%, w/w). By further manipulating the system composition along this tie-line in order to obtain a top/bottom phase volume ratio of 9.3 (35%, w/w PEG 600, 6%, w/w NH4)2 SO4), it was possible to recover 100% of pDNA in the bottom phase with a three-fold increase in concentration. Further increase in the lysate load up to 40% (w/w) with this system resulted in a eight-fold increase in pDNA concentration, but with a yield loss of 15%. The ATPS extraction was integrated with HIC and the overall process compared with a previously defined process that uses sequential precipitations with iso-propanol and ammonium sulphate prior to HIC. Although the final yield is lower in the ATPS-based process the purity grade of the final pDNA product is higher. This shows that it is possible to substitute the time-consuming two-step precipitation procedure by a simple ATPS extraction. 相似文献
65.
Hydroxypropyl chitosan-graft-carboxymethyl beta-cyclodextrin (HPCH-g-CM beta-CD) was synthesized by grafting CM beta-CD onto HPCH using water soluble 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) as the condensing agent. Due to the presence of hydrophobic beta-CD rings onto the HPCH backbone, this polymer can be used as a matrix for controlled drug release. The adsorption of a hydrophobic model drug, ketoprofen, by HPCH-g-CM beta-CD microparticles (using tripolyphosphate as an ionic crosslinking agent) fitted well in the Langmuir isotherm equation. The drug dissolution profile showed that HPCH-g-CM beta-CD microparticles provided a slower release of the entrapped ketoprofen than chitosan, and the release behavior was influenced by the pH value of the medium. These results suggest that beta-CD grafted with chitosan derivatives may become a potential biodegradable delivery system to control the release of hydrophobic drugs with pH-responsive capability. 相似文献
66.
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68.
Alternative substrates for DNA and RNA polymerases offer an important set of biochemical tools. Many of the standard methods for nucleoside triphosphate synthesis fail in the cases of nonpurine and nonpyrimidine nucleosides. An efficient preparation of the 5'-O-tosylates for both the deoxy- and ribonucleosides enabled preparation of the diphosphate esters by displacement with tris(tetra-n-butylammonium) pyrophosphate. Enzymatic synthesis of the azole carboxamide deoxyribonucleoside triphosphate was based on ATP as the phosphate donor, nucleoside diphosphate kinase as the catalyst, coupled with phosphoenol pyruvate (PEP) and pyruvate kinase as an ATP regeneration system. Ribonucleoside triphosphate synthesis required PEP as the phosphate donor and pyruvate kinase as the catalyst. An optimized purification procedure based upon boronate affinity gel was developed to yield highly purified nucleoside triphosphates. The strategy outlined here provides a new and efficient method for preparation of nucleoside 5'-triphosphate and is likely applicable to a broad variety of base and sugar modified nucleoside analogues. 相似文献
69.
Gonçalves SA De Pauli SH Tedesco AC Quina FH Okano LT Bonilha JB 《Journal of colloid and interface science》2003,267(2):494-499
The counterion binding at a water/Aerosol-OT (AOT)/heptane microemulsion interface was treated in the context of the pseudo-phase ion exchange formalism. Two approaches were used to determine the selectivity coefficient for copper/sodium counterion binding at the AOT microemulsion interface: measurements of the Cu(II) concentration taken up by the reverse micelle in a Winsor II microemulsion system and steady-state emission quenching measurements of an anionic water-solubilized probe, the tris-(4,4'-dicarboxy-2,2'-bipyridine) ruthenium (II) ion. In addition, the selectivity coefficient for methyl viologen/sodium at the microemulsion surface was determined by the same photophysical technique. The value for copper (II)/sodium exchange (K(Cu/Na)) is 1.1+/-0.3 and that for methyl viologen/sodium (K(MV/Na)) is 0.9+/-0.3. The results show that the pseudo-phase ion exchange model can be used to obtain the selectivity coefficient in a microemulsion system. 相似文献
70.
Translational to vibrational (T-->V) energy conversion in the course of inelastic collisions of n-butylbenzene molecular ions with thiolate self-assembled monolayer (SAM) gold surfaces is studied to better understand internal energy uptake by the hyperthermal projectile ions. The projectile ion is selected by a mass spectrometer of BE configuration and product ions are analyzed using a quadrupole mass analyzer after kinetic energy selection with an electric sector. The branching ratio for formation of the fragment ions m/z 91 and m/z 92, measured over a range of collision energies, is used to estimate the average internal energy with the aid of calculations based on unimolecular dissociation kinetics [Rice-Ramsperger-Kassel-Marcus (RRKM) theory]. The measured T-->V conversion efficiencies (the fraction of the laboratory kinetic energy converted into internal energy) are 11 approximately 12% for dodecanethiolate SAM (H-SAM) and 19 approximately 20% for 2-perfluorooctylethanethiolate SAM (F-SAM), respectively, over ranges of a few 10s of eV. The values are similar to those reported earlier for other thermometer molecules undergoing surface collisions. Chemical sputtering leading to ionization of the surface is a prominent feature of the surface-induced dissociation (SID) spectra of n-butylbenzene acquired using the H-SAM surface but not the F-SAM surface because of the lower ionization energy of the former. 相似文献