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761.
762.
Neil M. Glagovich Thomas H. Webb Hongsuk Suh Steven Geib Craig S. Wilcox 《Journal of Chemical Sciences》1994,106(5):955-970
Three new synthetic receptors based on Tr?ger’s base are prepared and evaluated. These three receptors are structural isomers
of a successful receptor reported earlier from this laboratory, and they are all far inferior to that original receptor. Two
of these receptors fail almost entirely and do so for an interesting reason. In water (not in other solvents) they collapse,
forming a deflated structure. Collapsed or “sicklied” receptors do not bind to alicyclic or hydrophobic substrates in water
because an important component of the driving force, the high energy waters solvating the interior of the macrocyclic structure,
is lost when the receptor collapses. The results clearly illustrate how important it is to use computational methods that
include good aqueous solvation models when designing new cyclophane receptors. 相似文献
763.
764.
A number of 3-mono- and 3,3-disubstituted derivatives of 1-methyl 2,1-benzisothiazoline 2,2-dioxide (IIa), a unique benzosultam, were synthesized. Starting materials for the condensation and aminoalkylation reactions utilized were IIa and the 3-methyl compound IIb, which were prepared by cyclization of the 2′-chloro-N-methylalkylsulfonanilides, Ia and b, respectively. 相似文献
765.
Magnetic and porous nanospheres from ultrasonic spray pyrolysis 总被引:1,自引:0,他引:1
We have used an inexpensive high-frequency ultrasound generator from a household humidifier to create a useful source for ultrasonic spray pyrolysis and produced submicrometer silica particles that are porous on the nanometer scale. By using two heated zones, we first initiate polymerization of organic monomers in the presence of silica colloid, which creates in situ a composite of silica with an organic polymer, followed by a second heating to pyrolyze and remove the polymer. The morphology and surface area of the final porous silica are controlled by varying the silica-to-organic monomer ratio. In a single flow process, ferromagnetic cobalt nanoparticles can be easily encapsulated in the porous silica, and the resulting nanospheres are extremely resistant to air oxidation. Products were characterized by SEM, (S)TEM, EDS, XPS, and SQUID. 相似文献
766.
Kojima N Sitthithaworn W Viroonchatapan E Suh DY Iwanami N Hayashi T Sankaw U 《Chemical & pharmaceutical bulletin》2000,48(7):1101-1103
cDNAs encoding geranylgeranyl diphosphate synthase (GGPPS) of two diterpene producing plants, Scoparia dulcis and Croton sublyratus, were isolated using the homology-based polymerase chain reaction method. Both cloned genes showed high amino acid sequence homology (60-70%) to other plant GGPPSs and contained highly conserved aspartate-rich motifs. The obtained clones were functionally expressed in Escherichia coli and showed sufficient GGPPS activity to catalyze the condensation of farnesyl diphosphate (FPP) and isopentenyl diphosphate to form geranylgeranyl diphosphate. To investigate the factor determining the product chain length of plant GGPPSs, S. dulcis GGPPS mutants in which either the small amino acids at the fourth and fifth positions before the first aspartate-rich motif (FARM) were replaced with aromatic amino acids or in which two additional amino acids in FARM were deleted were constructed. Both mutants behaved like FPPS-like enzymes and almost exclusively produced FPP when dimethylallyl diphosphate was used as a primer substrate, and failed to accept FPP as a primer substrate. These results indicate that both small amino acids at the fourth and fifth positions before FARM and the amino acid insertion in FARM play essential roles in product length determination in plant GGPPSs. 相似文献
767.
The ability to control the deposition and location of adherent and non-adherent cells within microfluidic devices is beneficial for the development of micro-scale bioanalytical tools and high-throughput screening systems. Here, we introduce a simple technique to fabricate poly(ethylene glycol)(PEG) microstructures within microfluidic channels that can be used to dock cells within pre-defined locations. Microstructures of various shapes were used to capture and shear-protect cells despite medium flow in the channel. Using this approach, PEG microwells were fabricated either with exposed or non-exposed substrates. Proteins and cells adhered within microwells with exposed substrates, while non-exposed substrates prevented protein and cell adhesion (although the cells were captured inside the features). Furthermore, immobilized cells remained viable and were stained for cell surface receptors by sequential flow of antibodies and secondary fluorescent probes. With its unique strengths in utility and control, this approach is potentially beneficial for the development of cell-based analytical devices and microreactors that enable the capture and real-time analysis of cells within microchannels, irrespective of cell anchorage properties. 相似文献
768.
Experimental difficulties in studying nanostructures stem from their small size, which limits the use of traditional techniques for measuring their physical properties. We have developed a nanostructure manipulation device to apply tension to chain aggregates mounted in a transmission electron microscope. A 1-mm-long slit was cut in the center of a lead-tin alloy disc, measuring 3 mm in diameter and 200 microm in thickness. The disc was heated to about 140 degrees C before it was pressed between two quartz slides. The disc was then thinned by mechanical dimpling and ion milling until holes developed around the slit. The edges of the slit were 0.2 to 3 microm in thickness while the gap between them was up to a few microns. This disc was bonded to the two plates of a cartridge. The slit could be widened or narrowed at controlled speeds of 0.5 to 300 nm/s. The system was tested using titania (TiO2) nanoparticle chain aggregates (NCA) deposited across the slit. The ends of the NCA remained attached to the edges of the slit, which was widened at about 0.7 nm/s. In this way, the NCA was stretched up to 176% of its initial length before breaking. 相似文献
769.
Tantalum(IV) amido complexes have been synthesized from Ta(V) precursors. Ta(N(SiMe(3))(2))(2)Cl(3) reacts with Na/Hg to give Ta(N(SiMe(3))(2))(2)Cl(2), and Ta(NEt(2))(2)Cl(3) reacts with LiNPh(2) and Na/Hg to yield Ta(NPh(2))(2)(NEt(2))(2). Ta(N(SiMe(3))(2))(2)Ph(2) is prepared by reacting Ta(N(SiMe(3))(2))(2)Cl(2) with LiPh. Attempts to prepare other organometallic derivatives failed to yield clean products. X-ray crystallographic studies show that Ta(N(SiMe(3))(2))(2)Cl(2), Ta(N(SiMe(3))(2))(2)Ph(2), and Ta(NPh(2))(2)(NEt(2))(2) have distorted tetrahedral geometries. 相似文献
770.
Richard D. Bowen Dennis Suh Johan K. Terlouw 《Journal of mass spectrometry : JMS》1994,29(12):791-805
The reactions of ionized di-n-butyl ether are reported and compared with those of ionized n-butyl sec-butyl and di-sec-butyl ether. The main fragmentation of metastable (CH3CH2CH2CH2)2O+. is C2H5? loss (?85%), but minor amounts (2–4%) of CH3?, C4H7?, C4H9?, C4H10 and C4H10O are also eliminated. In contrast, C2H5? elimination is of much lower abundance (20 and 4%, respectively) from metastable CH3CH2CH2CH2OCH(CH3)CH2CH3+. and [CH3CH2(CH3)CH]2O+., which expel mainly C2H6 and CH3? (35–55%). Studies on collisional activation spectra of the C6H13O+ oxonium ions reveal that C2H5? loss from (CH3CH2CH2CH2)2O+. gives the same product, (CH3CH2CH2CH2 +O?CHCH3) as that formed by direct cleavage of CH3CH2CH2CH2OCH(CH3)CH2CH3+.. Elimination of C2H5? from (CH3CH2CH2CH2)2O+. is interpreted by means of a mechanism in which a 1,4-H shift to the oxygen atom initiates a unidirectional skeletal rearrangement to CH3CH2CH2CH2OCH(CH3)CH2CH3+., which then undergoes cleavage to CH3CH2CH2CH2+O?CHCH3 and C2H5?. Further support for this mechanism is obtained from considering the collisional activation and neutralization-reionization mass spectra of the (C4H9)2O+. species and the behaviour of labelled analogues of (CH3CH2CH2CH2)2O+.. The rate of ethyl radical loss is suppressed relative to those of alternative dissociations by deuteriation at the γ-position of either or both butyl substituents. Moreover, C2H5? loss via skeletal rearrangement and fragmentation of the unlabelled butyl group in CH3CH2CH2CH2OCH2CH2CD2CH3+. occurs approximately five times more rapidly than C2H4D? expulsion via isomerization and fission of the labelled butyl substituent. These findings indicate that the initial 1,4-hydrogen shift is influenced by a significant isotope effect, as would be expected if this step is rate limiting in ethyl radical loss. 相似文献