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241.
Abstract Cross-polarization, 13C rotating frame spin-lattice relaxation and C laboratory frame spin-lattice relaxation processes have been studied for different cellulose samples by CP/MAS 13C NMR spectroscopy. It was found that the CP process can be described by a simple thermodynamic model and relative intensities of the respective resonance lines are consistent with the atomic ratios for the spectra obtained at a contact time of about 1 ms. The observed rotating frame spin-lattice relaxation times TC 10 were dominantly dependent on the time constant TD CH by which 13C nuclei were coupled to the 1H dipolar spin system. It was, therefore, impossible to obtain information about molecular 相似文献
242.
It was found that linear phosphonitrilic chloride could be used as a catalyst for ring-opening polymerization of tetrahydrofuran. Bulk polymerizations were carried out in a nitrogen atmosphere. After termination of polymerization, the reaction mixture was poured into water, thereby decomposing the catalyst. The product was dissolved in benzene and then subjected to lyophilization. The polymerization of tetrahydrofuran in the presence of linear phosphonitrilic chloride was found to be an equilibrium and a “living” polymerization. The polymerization product includes little phosphorus, and its infrared absorption spectrum agrees well with that of the polymer obtained with PF5 catalyst. The results of the polymerization using epichlorohydrin as a promoter show that the number of active sites in the molecule of linear phosphonitrilic chloride is considerably smaller. Consequently it is conceivable that the catalytic activity of the linear phosphonitrilic chloride is attributed to its terminal ~~P+Cl3PCl?6 structure. Furthermore we presume that the polymerization of tetra-hydrofuran in the presence of this catalyst proceeds through a cationic ring-opening mechanism. 相似文献
243.
244.
Trained singers and nonsingers vocally shadowed sequences of rapidly changing tones. Tone changes within the sequences were unpredictable in terms of direction and extent of frequency change. Subjects' responses to the shadowing task could be evaluated for accuracy of frequency matching, and for time and speed of voice frequency change. In addition, subjects' transitions between tones could be classified as hit, overshoot, undershoot, or oscillate. The two groups were equally accurate in matching the pitches of tones comprising the sequences. Similarly, pitch lowering was faster than pitch raising for both groups of subjects, while speeds for both lowering and raising increased with increases in size of the interval between tones. However, singers required less time than nonsingers in effecting transitions, apparently because they achieved faster peak speeds and took more direct paths between tones. Implications of the data for physiological and mechanical aspects of voice frequency control are discussed. 相似文献
245.
246.
T Maruyama Y Sato T Horii H Shiota K Nitta T Shirasaka H Mitsuya M Honjo 《Chemical & pharmaceutical bulletin》1990,38(10):2719-2725
9-Cyclobutyladenine (4a), cis- and trans-9-[3- (hydroxymethyl)cyclobutyl]adenine (4b) and 9-[3,3-bis(hydroxymethyl)cyclobutyl]adenine(4d) were prepared from the corresponding cyclobutylamine derivatives (1a, 1b and 1d). Guanine congeners (9a, cis- and trans-9b and 9d) and carbocyclic oxetanocin G (1',2'-trans-9f) were also prepared. Carbocyclic oxetanocin A(1',2'-trans-4f), the preparation of which we have already published, and G were found to be active against herpes simplex virus (type 1 and 2) in vitro, while cis-4b and cis-9b showed an in vitro antiretroviral activity against human immunodeficiency virus (type 1). 相似文献
247.
Yuko Chihara Hidetoshi Arima Masayo Arizono Koki Wada Fumitoshi Hirayama Kaneto Uekama 《Journal of inclusion phenomena and macrocyclic chemistry》2006,54(1-2):89-93
Interactions in aqueous dispersion between dipalmitoyl phosphatidyl choline (DPPC) liposomes and dissolved cyclodextrins (CD)
of different chemical compositions, respectively, were studied. Liposomal dispersions with small unilamellar vesicles (SUV)
of monomodal size distribution were prepared and the physical stability of the vesicles both in the presence and absence of
cyclodextrin was investigated. For the characterization of the kinetic stability of the dispersions under various conditions,
size-distribution functions determined by photon-correlation spectroscopy (PCS) were used. The affinity of cyclodextrins to
the liposomes was characterized by ‚binding isothermsȁ9 determined under equilibrium conditions at 25 °C. Based on the quantity
of the cyclodextrins bound to the DPPC bilayers, stability constants for the associates were estimated. The physical stability
of the liposomes and the possible control of stability were also investigated. 相似文献
248.
Yasuaki Takeuchi Koki Ohno Masasuke Yoshida Kei Nagano 《Photochemistry and photobiology》1981,33(4):587-592
Abstract— Light-induced proton release and uptake by acetylated and unmodified bacteriorhodopsin were measured. Bacteriorhodopsin, when illuminated, shows a net proton release at neutral and alkaline pH's, but in acidic pH, it shows an uptake of protons. In the presence of high concentrations of guanidine hydrochloride, light caused only proton release even in acidic pH and the maximum extent of the release was one proton per bacteriorhodopsin molecule around pH 8.
Acetylation of bacteriorhodopsin caused no alteration in the absorption spectrum of purple complex (bR570 ) and M412 -intermediate, but decreased the decay rate of the M412 -intermediate. Light-induced release of protons was not observed even in neutral pH values, and only the proton uptake was noticed by acetylated purple membrane fragments. In high concentrations of guanidine hydrochloride, no proton uptake or release by illumination was observed. Vesicles were reconstituted from acetylated purple membrane. These vesicles had almost no ability for light-induced proton transport. The role of amino group(s) in light-induced proton release and transport through the purple membrane is discussed. 相似文献
Acetylation of bacteriorhodopsin caused no alteration in the absorption spectrum of purple complex (bR
249.
2-Phenylheptamethyltrisilane as a model compound of polysilastyrene was irradiated at a high dose (1440 kGy) of γ-rays without any additive. The trisilane was found to be resistant to γ-rays, while it gave small amounts of Me6Si2, Me3SiSiMe2Ph, Me3SiH, Me3SiSiHMePh and (Me3Si)2 SiMeC6H4SiMe3. These products can be interpreted as being due to a chain contraction of the trisilane with the extrusion of methylphenylsilylene, a methyl migration with the extrusion of dimethylsilylene, a scission of an Si–Si bond due to an attack by hydrogen atoms, and a substitution of a hydrogen atom on the phenyl group by a trimethylsilyl radical. The same reactions, except the chain contraction, are observed in the cases of pentamethylphenyldisilane and 1,2-diphenyltetramethyldisilane. On the basis of the data obtained, the chemical behavior of polysilastyrene during the irradiation is discussed. 相似文献
250.
In order to improve the thermal stability of colored merocyanine formed by the photochromic reaction of spironaphthoxazine, the spiro compound was incorporated in polymeric siloxanes. They are liquid poly(methylphenylsiloxane) and solid methyl‐ or phenyl‐siloxane resins prepared by the hydrolysis and condensation reactions of the corresponding di‐ or tri‐ethoxysilane. To the polymers the spirooxazine moiety isbonded covalently. Higher stability of the colored form was not observed in the liquid polysiloxane, but in the solid resins. The phenyl substituent in the resin is more effective than the methyl one for stabilization, suggesting that the bulky substituent in the vicinity of the spirooxazine moiety is important as well as stiff polymer matrices. The phenylsiloxane resin with the spirooxazine moiety was enclosed in rigid sol–gel silica, but a further improvement of the thermal stability was not observed. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献