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181.
A phtalimido group can be readily introduced at the 5″‐position of N‐Boc ribostamycin 6 under Mitsunobu reaction conditions. However, in the case of N‐Boc neomycin B 4 a competitive bicyclization process occurred within the ring IV of the antibiotic, which led to compound 8, whose structure was fully elucidated by NMR spectroscopic methods. Accordingly, particular care must be exercised to achieve this chemical modification of the four‐ring antibiotic. 相似文献
182.
A. Farran E. Figuerola J. de Pablo S. Hernandez 《International journal of environmental analytical chemistry》2013,93(3-4):245-256
Abstract The treatment of waste water containing three organophosphorous pesticides: diazinon, azinphos-methyl and fenthion by using continuous flow methodologies coupled on line with HPLC-UV detection is presented. Two continuous flow techniques: completely continuous flow and flow injection, both combined with two separation methods: liquid-liquid extraction and adsorption resins are discussed, as well as the influence of physical and chemical parameters on the analysis. Adsorption resins seem to be more suitalbe for the treatment of waste water at low pesticide concentraions while liquid-liquid extraction is more selective. 相似文献
183.
Romain Lafay Dr. German Montes‐Hernandez Dr. Emilie Janots Dr. Rodica Chiriac Nathaniel Findling François Toche 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(17):5417-5424
Herein, we report new insights into the nucleation and growth processes of chrysotile nanotubes by using batch and semi‐continuous experiments. For the synthesis of this highly carcinogenic material, the influences of temperature (90, 200, and 300 °C), Si/Mg molar ratio, and reaction time were investigated. From the semi‐continuous experiments (i.e., sampling of the reacting suspension over time) and solid‐state characterization of the collected samples by XRPD, TGA, FTIR spectroscopy, and FESEM, three main reaction steps were identified for chrysotile nucleation and growth at 300 °C: 1) formation of the proto‐serpentine precursor within the first 2 h of the reaction, accompanied by the formation of brucite and residual silica gel; 2) spontaneous nucleation and growth of chrysotile between about 3 and 8 h reaction time, through a progressive dissolution of the proto‐serpentine, brucite, and residual silica gel; and 3) Ostwald ripening growth of chrysotile from 8 to 30 h reaction time, as attested to by BET and FESEM measurements. Complementary results from batch experiments confirmed a significant influence of the reaction temperature on the kinetics of chrysotile formation. However, FESEM observations revealed some formation of chrysotile nanotubes at low temperatures (90 °C) after 14 days of reaction. Finally, doubling the Si/Mg molar ratio promoted the precipitation of pure smectite (stevensite‐type) under the same P (8.2 MPa)/T (300 °C)/pH (13.5) conditions. 相似文献
184.
Hernandez David Alejandro Rodriguez-Zavala Jaime Gustavo Tenorio Francisco J. 《Structural chemistry》2020,31(1):359-369
Structural Chemistry - Quantum chemistry calculations were performed to compare the reactivity indexes obtained within the conceptual density functional theory of phenolic and allyl-phenolic... 相似文献
185.
Antonia Kagkoura Javier Hernandez‐Ferrer Ana M. Benito Wolfgang K. Maser Nikos Tagmatarchis 《化学:亚洲杂志》2020,15(15):2350-2356
A facile strategy for the controllable growth of CdS nanoparticles at the periphery of MoS2 en route the preparation of electron donor‐acceptor nanoensembles is developed. Precisely, the carboxylic group of α‐lipoic acid, as addend of the modified MoS2 obtained upon 1,2‐dithiolane functionalization, was employed as anchor site for the in situ preparation and immobilization of the CdS nanoparticles in an one‐pot two‐step process. The newly prepared MoS2/CdS hybrid material was characterized by complementary spectroscopic, thermal and microscopy imaging means. Absorption spectroscopy was employed to register the formation of MoS2/CdS, by observing a broad shoulder centered at 420 nm due to CdS nanoparticles, while the excitonic bands of MoS2 were also evident. Moreover, based on the efficient quenching of the characteristic fluorescence emission of CdS at 725 nm by the presence of MoS2, strong electronic interactions at the excited state between the two species within the ensemble were identified. Photoelectrochemical assays of MoS2/CdS thin‐film electrodes revealed a prompt, steady and reproducible anodic photoresponse during repeated on‐off cycles of illumination. A significant zero‐current photopotential of ?540 mV and an anodic photocurrent of 1 μA were observed, underlining improved charge‐separation and electron transport from CdS to MoS2. The superior performance of the charge‐transfer processes in MoS2/CdS is of direct interest for the fabrication of photoelectrochemical and optoelectronic devices. 相似文献
186.
A. Emin Atik Oscar Hernandez Philippe Maître Talat Yalcin 《Journal of mass spectrometry : JMS》2014,49(12):1290-1297
Characterization of ε‐N‐acetylated lysine containing peptides, one of the most prominent post‐translational modifications of proteins, is an important goal for tandem mass spectrometry experiments. A systematic study for the fragmentation reactions of b ions derived from ε‐N‐acetyllysine containing model octapeptides (KAcYAGFLVG and YAKAcGFLVG) has been examined in detail. Collision‐induced dissociation (CID) mass spectra of bn (n = 4–7) fragments of ε‐N‐acetylated lysine containing peptides are compared with those of N‐terminal acetylated and doubly acetylated (both ε‐N and N‐terminal) peptides, as well as acetyl‐free peptides. Both direct and nondirect fragments are observed for acetyl‐free and singly acetylated (ε‐N or N‐terminal) peptides. In the case of ε‐N‐acetylated lysine containing peptides, however, specific fragment ions (m/z 309, 456, 569 and 668) are observed in CID mass spectra of bn (n = 4–7) ions. The CID mass spectra of these four ions are shown to be identical to those of selected protonated C‐terminal amidated peptides. On this basis, a new type of rearrangement chemistry is proposed to account for the formation of these fragment ions, which are specific for ε‐N‐acetylated lysine containing peptides. Consistent with the observation of nondirect fragments, it is proposed that the b ions undergo head‐to‐tail macrocyclization followed by ring opening. The proposed reaction pathway assumes that bn (n = 4–7) of ε‐N‐acetylated lysine containing peptides has a tendency to place the KAc residue at the C‐terminal position after macrocyclization/reopening mechanism. Then, following the loss of CO, it is proposed that the marker ions are the result of the loss of an acetyllysine imine as a neutral fragment. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
187.
The use of the inclusion complex of 1-amino-4-hydroxyanthraquinone (AHA) in the internal cavity of β-cyclodextrin is compared with AHA alone as a ligand for the spectrofluorimetric determination of beryllium. The organizing ability of the cyclodextrin medium and the protection of the ligand from the micro-environment confers increased sensitivity, selectivity and detection limit, and allows the determination of 10–70 ng ml?1 beryllium, compared to 60–500 ng ml?1 in the absence of the cyclodextrin. 相似文献
188.
The reaction of [AuCl(PPh3)] with Pb(SR)2(R = C2H5, C6H5, CH2C6H5, C6F5, C6H2Me3-2,4,6, Pri and But) provides a clean method to obtain complexes of the type [Au(SR)(PPh3)] in good yields. The new compounds have been characterized by IR, 1H, 31P, 19F and 31C NMR. A study by FAB mass spectrometry indicates that an ion-molecule aggregation process takes place. 相似文献
189.
190.
Through the distinction between “real” and “virtual” links between the nodes of a graph, we develop a set of simple rules leading to scale-free networks with a tunable degree distribution exponent. Albeit sharing some similarities with preferential attachment, our procedure is both faster than a naïve implementation of the Barabási and Albert model and exhibits different clustering properties. The model is thoroughly studied numerically and suggests that reducing the set of partners a node can connect to is important in seizing the diversity of scale-free structures. 相似文献