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991.
Butane-1,2-diacetals are selective protecting groups for trans-diequatorial-1,2-diols and have been widely used in carbohydrate chemistry. The scope of diacetal protection has been extended more recently to include other important hydroxylated chiral templates containing trans-1,2-diols, such as quinic and shikimic acids, the protection of which as diacetals leads to a strong conformational rigidity that induces excellent diastereoselectivity control. In addition, the chiral information stored in the diacetal backbone has also been exploited in the synthesis of important building blocks, such as glycerate, glycolate and tartrate diacetal derivatives. In this critical review, the synthetic power of the conformational rigidity and the chirality stored in the diacetal backbone is described. This phenomenon will be illustrated with recent examples of applications in the synthesis of natural products or biologically interesting compounds (80 references). 相似文献
992.
The (13)C and (15)N CPMAS NMR spectra of 18 pyrazoles substituted at position 1 by dinitrophenyl and trinitrophenyl (picryl) groups have been recorded. To help in the assignments, some of these compounds were studied in DMSO-d(6) solution. Phenomena such as the conformation of the N-aryl groups and broadening of splittings due to quadrupolar nuclei are discussed. 相似文献
993.
Barberá J Giorgini L Paris F Salatelli E Tejedor RM Angiolini L 《Chemistry (Weinheim an der Bergstrasse, Germany)》2008,14(35):11209-11221
The synthesis of chiral liquid-crystalline polymers of well-controlled structure (linear and three-armed star-shaped) with distinct average chain lengths and low polydispersity was achieved by atom transfer radical polymerisation (ATRP) of a new optically active monomer (S)-4-[6-(2-methacryloyloxypropanoyloxy)hexyloxy)]-4'-ethoxyazobenzene [(S)-ML6A], containing the L-lactic residue of one absolute configuration in the side-chain. All the obtained polymeric samples, characterised by differential scanning calorimetry (DSC), X-ray diffraction (XRD) and polarised optical microscopy (POM), exhibit a smectic A(1/2) (fully interdigitated) liquid-crystalline phase and high cleaning points, with transition temperatures dependent on the average polymerisation degree and the macromolecular structure. The chirality originated at the molecular level by the asymmetric functionality of the L-lactic acid residue provides the polymers, in the smectic phase, of highly homogeneous conformations with a prevailing chirality related to the presence of H-aggregates having conformational dissymmetry of one prevailing screw-sense. By irradiating with circularly polarised light (CPL), it is possible to photomodulate the chiroptical properties of these intrinsically chiral polymeric thin films. Upon irradiation with left-handed CPL (l-CPL), the circular dichroism (CD) spectra of the films show enhancement of ellipticity and a net inversion of sign. The effect is reversible and the mirror image of the CD spectrum can be restored by pumping with right-handed CPL radiation (r-CPL). The results show the ability of l-CPL to invert the supramolecular chirality of the materials and demonstrate the essential role of azoaromatic aggregates. 相似文献
994.
Ros-Lis JV Casasús R Comes M Coll C Marcos MD Martínez-Máñez R Sancenón F Soto J Amorós P El Haskouri J Garró N Rurack K 《Chemistry (Weinheim an der Bergstrasse, Germany)》2008,14(27):8267-8278
Dual-function hybrid material U1 was designed for simultaneous chromofluorogenic detection and removal of Hg(2+) in an aqueous environment. Mesoporous material UVM-7 (MCM41 type) with homogeneously distributed pores of about 2-3 nm in size, a large specific surface area exceeding 1000 m(2) g(-1), and nanoscale particles was used as an inorganic support. The mesoporous solid is decorated with thiol groups that were treated with squaraine dye III to give a 2,4-bis(4-dialkylaminophenyl)-3-hydroxy-4-alkylsulfanylcyclobut-2-enone (APC) derivative that is covalently anchored to the inorganic silica matrix. The solid was characterised by various techniques including X-ray diffraction, transmission electron microscopy, Raman spectroscopy, and nitrogen adsorption. This hybrid solid is the chemodosimeter for Hg(2+) detection. Hg(2+) reacts with the APC fragment in U1 with release of the squaraine dye into the solution, which turns deep blue and fluoresces strongly. Naked-eye Hg(2+) detection is thus accomplished in an easy-to-use procedure. In contrast, U1 remains silent in the presence of other thiophilic transition metal ions, alkali and alkaline earth metal ions, or anions ubiquitously present in water such as chloride, carbonate, sulfate, and phosphate. Material U1 acts not only as chemodosimeter that signals the presence of Hg(2+) down to parts-per-billion concentrations, but at the same time is also an excellent adsorbent for the removal of mercury cations from aqueous solutions. The amount of adsorbed mercury ranges from 0.7 to 1.7 mmol g(-1), depending on the degree of functionalisation. In addition, hybrid material U1 can be regenerated for both sensing and removal purposes. As far as we know, U1 is the first example of a promising new class of polyfunctional hybrid supports that can be used as both remediation and alarm systems by selective signalling and removal of target species of environmental importance. Model compounds based on silica gel (G1), fumed silica (F1), and micrometre-sized MCM-41 scaffolds (M1) were also prepared and studied for comparative purposes. 相似文献
995.
García-Fandiño R Gulías M Castedo L Granja JR Mascareñas JL Cárdenas DJ 《Chemistry (Weinheim an der Bergstrasse, Germany)》2008,14(1):272-281
The mechanism of the palladium-catalysed [3+2] intramolecular cycloaddition of alkylidenecyclopropanes to alkynes has been computationally explored at DFT level. The energies of the reaction intermediates and transition states for different possible pathways have been calculated in a model system that involves the use of PH3 as a ligand. The results obtained suggest that the most favourable reaction pathway involves the initial C--C oxidative addition of the cyclopropane to a Pd0 complex to give an alkylidenepalladacyclobutane, which isomerises to a methylenepalladacyclobutane intermediate. Subsequent cyclisation by alkyne carbometallation, followed by reductive elimination affords the final product. An alternative mechanism consisting of a palladaene-type rearrangement is less probable in terms of Gibbs energy, but cannot be fully discarded because it is competitive if one considers electronic energies. For substrates that present an ester group at the terminal position of the triple bond we have found an alternative, more favourable mechanistic route that explains why the [3+2] cycloaddition of these types of systems does not lead to the expected cycloadducts. 相似文献
996.
Gholipour Y Nonami H Erra-Balsells R 《Journal of the American Society for Mass Spectrometry》2008,19(12):1841-1848
Single-cell cytoplasm sap (1-10 pL) was extracted by using a pressure probe glass microcapillary tip from tulip leaf and bulb and analyzed by UV-MALDI-TOF MS for free underivatized carbohydrate content. Three matrices including 2,5-dihydroxybenzoic acid (DHB), 2,4,6-trihydroxyacetophenone (THAP), and carbon nanotubes (CNTs) in positive ion mode were selected for analysis because of acceptable carbohydrate-related signal reproducibility. Disaccharide and oligosaccharide (up to 15 Hex when THAP was used, 11 Hex with DHB, and 7 Hex with CNTs) were detected in tulip bulb cell cytoplasm sample. When DHB was used as matrix, neutral carbohydrates were more abundantly detected as sodiated cations; the sugar-related signals, however, appeared as dominant potassiated cations when THAP and CNTs were used. Small amount of monosaccharide was also detected in bulb cell cytoplasm with CNTs as matrix. UV-MALDI-TOF MS of leaf cell extract resulted in high-resolution detection of hexose and disaccharide with DHB, THAP, and CNTs. 相似文献
997.
Time-resolved kinetic studies of the reaction of dideutero-silylene, SiD 2, generated by laser flash photolysis of phenylsilane-d 3, have been carried out to obtain rate constants for its bimolecular reaction with C 2H 2. The reaction was studied in the gas phase over the pressure range 1-100 Torr in SF 6 bath gas, at five temperatures in the range 297-600 K. The second-order rate constants obtained by extrapolation to the high-pressure limits at each temperature fitted the Arrhenius equation log( k (infinity)/cm (3) molecule (-1) s (-1)) = (-10.05 +/- 0.05) + (3.43 +/- 0.36 kJ mol (-1))/ RT ln 10. The rate constants were used to obtain a comprehensive set of isotope effects by comparison with earlier obtained rate constants for the reactions of SiH 2 with C 2H 2 and C 2D 2. Additionally, pressure-dependent rate constants for the reaction of SiH 2 with C 2H 2 in the presence of He (1-100 Torr) were obtained at 300, 399, and 613 K. Quantum chemical (ab initio) calculations of the SiC 2H 4 reaction system at the G3 level support the initial formation of silirene, which rapidly isomerizes to ethynylsilane as the major pathway. Reversible formation of vinylsilylene is also an important process. The calculations also indicate the involvement of several other intermediates, not previously suggested in the mechanism. RRKM calculations are in semiquantitative agreement with the pressure dependences and isotope effects suggested by the ab initio calculations, but residual discrepancies suggest the possible involvement of the minor reaction channel, SiH 2 + C 2H 2 --> Si( (3)P 1) + C 2H 4. The results are compared and contrasted with previous studies of this reaction system. 相似文献
998.
M. Pappalardo M. F. M. Sciacca D. Milardi D. M. Grasso C. La Rosa 《Journal of Thermal Analysis and Calorimetry》2008,93(2):575-581
The role played by the metal ion in thermodynamics of azurin folding was addressed by studying the thermal denaturation of
the apo-form by differential scanning calorimetry (DSC), and by comparing the results with data concerning the holo protein.
The thermal unfolding experiments showed that at 25°C the presence of metal ion increases the thermodynamic stability of azurin
by 24 kJ mol−1. A comparison between the unfolding and the copper binding free energies allow us to assert that the unfolded polypeptide
chain binds copper and subsequently folds into native holo azurin, being this the thermodynamically most favourable process
in driving azurin folding. 相似文献
999.
Redondo R Machado VC Baeza M Lafuente J Gabriel D 《Analytical and bioanalytical chemistry》2008,391(3):789-798
Biogas is produced by biological processes under anaerobic conditions and may contain up to 20,000 ppmv hydrogen sulfide (H2S), a corrosive substance that attacks power engines and can affect the health of the industrial staff. H2S must be removed from the biogas, especially in co-generation facilities where the biogas is burnt for energy production.
Nowadays, biofiltration is being studied and considered as an interesting alternative for removing H2S from the biogas besides classical chemical processes. The novelty of this work is the design and construction of an automated
H2S on-line analyser to assess the composition of the liquid and gas phases of gas-phase bioreactors. The analyser is made of
two parallel flow configurations which share the same detection device. The first configuration is a single-channel flow injection
analyser (FIA) to detect S2− in the liquid phase. The second configuration is a continuous flow analyser (CFA) with a gaseous diffusion step (GD–CFA)
for detecting H2S in the gas phase. The diffusion step enables separation of the H2S(g) from the sample and its conversion into a detectable chemical species (S2−). S2− detection was performed with an Ag2S ion-selective electrode (ISE) selective to . The main response parameters of the FIA system are a linear range between 3 × 10−5 and 1 × 10−1 mol L−1 S2− (0.61–3,200 mg L−1), with a sensitivity of 27.9 mV decade−1 and a detection limit of 1.93 × 10−5 mol L−1 S2−. The GD–CFA configuration presents a linear range between 400 and 10,000 ppmv H2S(g) with a sensitivity of 26.1 mV decade−1 and a detection limit of 245 ppmv H2S. The proposed analyser was used by analysing real gas and liquid samples with optimal results at a full-scale biotrickling
filter for biogas treatment at a municipal wastewater treatment plant.
Figure The novelty of this work is the design and construction of an automated H2S on-line analyzer to assess the composition of the liquid and gas phases of gas-phase bioreactors. The analyser is made of
two parallel flow configurations which share the same detection device. The first configuration is a single-channel flow injection
analyser (FIA) to detect S2- in the liquid phase. The second configuration is a continuous flow analyser (CFA) with a gaseous diffusion step (GD-CFA)
for detecting H2S in the gas phase. 相似文献
1000.
Morais S Tamarit-López J Carrascosa J Puchades R Maquieira A 《Analytical and bioanalytical chemistry》2008,391(8):2837-2844
A sensitive and versatile methodology involving recordable compact disks as molecular screening surfaces and a standard optical
CD/DVD drive as detector, is reported. Quantitative immunoanalysis, in microarray format, of a cancer marker (alpha-fetoprotein,
AFP) and a selective herbicide (atrazine) on four types of audio-video disc was conducted. Enzyme or gold nanoparticle-labeled
antibodies were used as tracers, forming a precipitate on the sensing disk surface. The principle of disk reading is based
on capture of analog signals with the disk drive that were proportional to the darkness of the immunoreaction product. Detection
limits for AFP (8.0 μg L−1) and for atrazine (0.04 μg L−1) were under the threshold needed to detect nonseminomatous testicular cancer, and below the maximum E.U. residue limit for
drinking water, respectively. The described methodology improves the previous developments using CDs and highlights the enormous
potential of immunoassay methods using standard audio–video disk surfaces in combination with the CD/DVD drive for clinical
analysis, drug discovery, or high-throughput multiresidue screening applications.
Figure
Eye-catching image The analytical potential of commercial audio–video discs as molecular screening surfaces in combination with use of a standard
CD/DVD drive as detector for quantitative immunoanalysis of a cancer marker and agrochemical residues is demonstrated. 相似文献