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231.
232.
Ukena T Satake M Usami M Oshima Y Fujita T Naoki H Yasumoto T 《Rapid communications in mass spectrometry : RCM》2002,16(24):2387-2393
Negative-ion fast-atom bombardment collision-induced dissociation tandem mass spectrometric (FAB-CID-MS/MS) methodology was successfully applied to verify the highly complex structure of ostreocin-D (MW 2633), a new palytoxin analog isolated from the marine dinoflagellate Ostreopsis siamensis and proposed to be 42-hydroxy-3,26-didemethyl-19,44-dideoxypalytoxin based on NMR data. The charge-remote fragmentations were facilitated by a negative charge introduced to a terminal amino group or to a hydroxyl group at the other terminus by a reaction with 2-sulfobenzoic acid cyclic anhydride. Product ions generated from the [M - H](-) ions provided information on the structural details of ostreocin-D. Comparisons between the spectral data for ostreocin-D and palytoxin also provided a rational basis for the assignments of product ions. 相似文献
233.
Saito K Shiose T Takahashi O Hidaka Y Aiba F Tabayashi K 《The journal of physical chemistry. A》2005,109(24):5352-5357
The thermal decomposition of formic acid was reinvestigated in the gas phase using two types of shock tubes. It was confirmed that the unimolecular decomposition proceeds through a main channel of dehydration (k1) and a minor decarboxylation channel (k2). This result is in good agreement with our previous study (J. Chem. Phys. 1984, 80, 4989). Furthermore, it was confirmed that the dehydration process is in the second-order region and that the decarboxylation is in the falloff region, in the temperature range of 1300-2000 K and over the total density of (0.5-2.5) x 10(-5) mol cm(-3). The experimental ratios between the two channels, k2/k1, are compared with those of theoretical calculations by conventional transition state theory and the Rice-Ramsperger-Kassel-Marcus theory. 相似文献
234.
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236.
Daisuke Domon Yuko Ohtaniuchi Akihiro Takezawa Sayaka Takeda Hidekazu Kawasaki Akio Murai Hidethoshi Kawai Takanori Suzuki 《Tetrahedron letters》2005,46(48):8279-8283
The C42-C52 part of ciguatoxin CTX3C (1) was synthesized from tri-O-acetyl d-glucal. The synthetic segment had a tetrahydropyran ring corresponding to the ‘C49-reduced’ L-ring of 1, designed to avoid side reactions due to acid-labile C49 acetal carbon during acidic reductive conditions planned in further synthesis toward 1. The vicinal dimethyl part at C47-C48 was constructed by a stepwise conjugate addition/methylation procedure. The C50-C52 unit was installed by Grignard addition of the C3 unit followed by spirocyclization and reductive cleavage of the spirocyclic acetal. Stereoselective assembly of the C42-C44 part was achieved by Brown’s asymmetric crotylboration. 相似文献
237.
Kishida T Fujita N Sada K Shinkai S 《Langmuir : the ACS journal of surfaces and colloids》2005,21(21):9432-9439
Porphyrins bearing four urea-linked dodecyl groups (3a) or four urea-linked triethoxysilylpropyl groups (3TEOS) at their peripheral positions were synthesized. 3a tends to assemble into a sheetlike two-dimensional structure due to the predominant hydrogen-bonding interaction among the urea groups and acts as a moderate gelator of organic solvents. On the other hand, its Cu(II) compelx (3a.Cu) tends to assemble into a fibrous one-dimensional structure due to the predominant porphyrin-porphyrin pi-pi stacking interaction and acts as an excellent gelator of many organic solvents. 3TEOS and 3TEOS.Cu, which also act as gelators, afforded similar superstructures as those of 3a and 3a.Cu, respectively, and as evidenced by SEM and TEM observations and XRD measurements, the original superstructures could be precisely immobilized by in situ sol-gel polycondensation of the triethoxysilyl groups. The TEM images of 3a gels and 3TEOS gels after sol-gel polycondensation showed a fine striped structure, the periodical distance of which was either 2 or 4 nm. X-ray crystallographic analysis of a single crystal obtained from a reference porphyrin bearing four urea-linked butyl groups revealed that there are two different porphyrin-stacked columns in the crystal and both the 2 nm distance and the 4 nm distance can appear, depending on the observation tilting angle. The hybrid gel prepared from 3TEOS.Cu by sol-gel polycondensation showed unique physicochemical properties such as a high sol-gel phase-transition temperature (>160 degrees C), sufficient elasticity, high mechanical strength, etc. Thus, the present study has established new concepts for molecular design of porphyrin-based gelators on the basis of cooperative and/or competitive actions of hydrogen-bonding and pi-pi stacking interactions and for immobilization of their superstructures leading to development of new functional organic/inorganic hybrid materials. 相似文献
238.
[reaction: see text] A new total synthesis of macrosphelides A and B using ring-closing metathesis (RCM) as a macrocyclization step is described. The substrate of the RCM could be synthesized from readily available chiral materials, methyl (S)-(+)-3-hydroxybutyrate and methyl (S)-(-)-lactate, with a high efficiency. The RCM proceeded in the presence of Grubbs' Ru-complex, providing a new effective synthetic route to these natural products. 相似文献
239.
Takashi Matsuda Yu Mishima Saeid Azizian Hiroki Matsubara Takanori Takiue Makoto Aratono 《Colloid and polymer science》2007,285(14):1601-1605
We measured the interfacial tension and the density of air/n-hexane, n-decane, 1-perfluorohexane/1-hexyl-3-methyl-imidazolium hexafluorophosphate systems as a function of temperature. From the
air/ionic liquid surface tension values, it was suggested that Coulombic interaction between imidazolium cations and counter
anions are not so much different between the surface and bulk. The density values indicated that the decrease of surface tension
by saturating organics was closely correlated to the mutual solubility between ionic liquid and organics. Interfacial tension
at the oil/ionic liquid interfaces suggested that ionic liquid molecules were more ordered at the oil/ionic liquid interfaces
compared to the air/ionic liquid interfaces, but the decrease of the entropy due to the interfacial orientation of ionic liquid
was compensated by the increase of the entropy due to the contact of different chemical species. The initial spreading coefficients
and the Hamaker constants indicated that all the oil phases spread at the air/ionic liquid interfaces spontaneously, and form
the complete wetting films. 相似文献
240.
Tamura R Fujimoto D Lepp Z Misaki K Miura H Takahashi H Ushio T Nakai T Hirotsu K 《Journal of the American Chemical Society》2002,124(44):13139-13153
The mechanism of Preferential Enrichment, an unusual enantiomeric resolution phenomenon observed upon recrystallization of a series of racemic crystals which are classified as a racemic mixed crystal with fairly ordered arrangement of the two enantiomers, has been studied. On the basis of the existence of polymorphs and the occurrence of the resulting polymorphic transition during crystallization from solution, the mechanism has been accounted for in terms of (1) a preferential homochiral molecular association to form one-dimensional chain structures in the supersaturated solution of the racemate or nonracemic sample with a low ee value, (2) a kinetic formation of a metastable crystalline phase retaining the homochiral chain structures in a process of nucleation, (3) a polymorphic transition from the metastable phase to a stable one followed by enantioselective liberation of the excess R (or S) enantiomers from the transformed crystal into solution at the beginning of crystal growth to result in a slight enrichment (up to 10% ee) of the opposite S (or R) enantiomer in the deposited crystals, together with an enantiomeric enrichment of the R (or S) enantiomer in the mother liquor, and (4) a chiral discrimination by the once formed S (or R)-rich stable crystalline phase in a process of the subsequent crystal growth, leading to a considerable enantiomeric enrichment of the R (or S) enantiomer up to 100% ee in the mother liquor. The processes (3) and (4) are considered to be directly responsible for an enrichment of one enantiomer in the mother liquor. The association mode of the two enantiomers in solution has been investigated by means of (i) the solubility measurement and (ii) the number-averaged molecular weight measurement in solution by vapor pressure osmometry, together with (iii) the molecular dynamics simulation of oligomer models. The polymorphic transition during crystallization has been observed visually and by means of the in situ FTIR technique and DSC measurement. Both metastable and stable crystals have been obtained, and their crystal structures have been elucidated by X-ray crystallographic analysis of their single crystals. 相似文献