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111.
The temperature behaviour of the torque amplification factor η in the E48-D4 dye-host/liquid crystal-guest dyed system illuminated by a laser beam has been investigated using a single beam Z-Scan experimental technique. A plot of η versus temperature showed a monotonic aspect, different from other anthraquinone dyed liquid crystals. This behaviour, in our case, is consistent with the monotonic dependence on temperature of the linear αo and non-linear β optical absorption coefficients. In particular, the non-linear optical absorption coefficient related to the reorienting process is negative and of the order of-0.25 mm W-1. The temperature dependence of η seems to be directly correlated to β and not to αo. Since drastic conformational modifications on dye molecules are not expected on passing from the ground to the excited state, the role of the electronic structure seems to be dominant in the reorienting process in dyed nematics. 相似文献
112.
Amelia P. Rauter José A. Figueiredo Isabel Ismael Maria S. Pais António G. Gonzalez Jesus Diaz 《Journal of carbohydrate chemistry》2013,32(2):259-272
Abstract Synthesis of stereoisomeric α-methylene-γ-lactones in furanose-and furanuronoamide derivatives was easily accomplished by Reformatsky Reaction with ethyl bromcmethylacrylic ester and zinc. Pyridinium chlorochromate/3Å molecular sieve powder showed to be an excellent reagent for the oxidation of secondary hydroxyl groups of a furanose system and of α-hydroxy amides. 相似文献
113.
Ana Fragoso Dr. Pedro Lamosa Prof. Rita Delgado Dr. Olga Iranzo 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(6):2076-2088
Designing small peptides that are capable of binding Cu2+ ions mainly through the side‐chain functionalities is a hard task because the amide nitrogen atoms strongly compete for Cu2+ ion coordination. However, the design of such peptides is important for obtaining biomimetic small systems of metalloenyzmes as well as for the development of artificial systems. With this in mind, a cyclic decapeptide, C‐Asp, which contained three His residues and one Asp residue, and its linear derivative, O‐Asp, were synthesized. The C‐Asp peptide has two Pro? Gly β‐turn‐inducer units and, as a result of cyclization, and as shown by CD spectroscopy, its backbone is constrained into a more defined conformation than O‐Asp, which is linear and contains a single Pro? Gly unit. A detailed potentiometric, mass spectrometric, and spectroscopic study (UV/Vis, CD, and EPR spectroscopy) showed that at a 1:1 Cu2+/peptide ratio, both peptides formed a major [CuHL]2+ species in the pH range 5.0–7.5 (C‐Asp) and 5.5–7.0 (O‐Asp). The corrected stability constants of the protonated species (log K*CuH(O?Asp)=9.28 and log K*CuH(C?Asp)=10.79) indicate that the cyclic peptide binds Cu2+ ions with higher affinity. In addition, the calculated value of Keff shows that this higher affinity for Cu2+ ions prevails at all pH values, not only for a 1:1 ratio but even for a 2:1 ratio. The spectroscopic data of both [CuHL]2+ species are consistent with the exclusive coordination of Cu2+ ions by the side‐chain functionalities of the three His residues and the Asp residue in a square‐planar or square‐pyramidal geometry. Nonetheless, although these data show that, upon metal coordination, both peptides adopt a similar fold, the larger conformational constraints that are present in the cyclic scaffold results in different behaviour for both [CuHL]2+ species. CD and NMR analysis revealed the formation of a more rigid structure and a slower Cu2+‐exchange rate for [CuH(C‐Asp)]2+ compared to [CuH(O‐Asp]2+. This detailed comparative study shows that cyclization has a remarkable effect on the Cu2+‐coordination properties of the C‐Asp peptide, which binds Cu2+ ions with higher affinity at all pH values, stabilizes the [CuHL]2+ species in a wider pH range, and has a slower Cu2+‐exchange rate compared to O‐Asp. 相似文献
114.
Prof. Dr. Andrei V. Malkov Dr. Sigitas Stončius Dr. Mark Bell Dr. Fabiomassimo Castelluzzo Dr. Pedro Ramírez‐López Dr. Lada Biedermannová Prof. Dr. Vratislav Langer Dr. Lubomír Rulíšek Prof. Dr. Pavel Kočovský 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(28):9167-9185
Detailed kinetic and computational investigation of the enantio‐ and diastereoselective allylation of aldehydes 1 with allyltrichlorosilanes 5 , employing the pyridine N‐oxides METHOX ( 9 ) and QUINOX ( 10 ) as chiral organocatalysts, indicate that the reaction can proceed through a dissociative (cationic) or associative (neutral) mechanism: METHOX apparently favors a pentacoordinate cationic transition state, while the less sterically demanding QUINOX is likely to operate via a hexacoordinate neutral complex. In both pathways, only one molecule of the catalyst is involved in the rate‐ and selectivity‐determining step, which is supported by both experimental and computational data. 相似文献
115.
Holger Wondraczek Katrin Petzold-Welcke Pedro Fardim Thomas Heinze 《Cellulose (London, England)》2013,20(2):751-760
Nano-scaled particles were obtained from two different cellulose acetates, cellulose acetate propionate, and cellulose acetate butyrate using the emulsification solvent evaporation procedure and the low energy methods of solvent displacement (dialysis and controlled precipitation). The relationship between the formulation parameters and the particle properties were evaluated in case of the emulsification-evaporation technique. For the solvent displacement procedures, the influence of the formulation parameters, and the intrinsic polymer properties like the hydrophilic-hydrophobic balance was evaluated. Comparing the methods, it could be shown that large amounts of small and uniform nanoparticles can be obtained by the emulsification solvent evaporation procedure. The solvent displacement techniques turned out to be very easy to use and to yield narrowly distributed particles as well. 相似文献
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Lóren C.C. Gonçalves Francine N. Victória David B. Lima Pedro M.Y. Borba Gelson Perin Lucielli Savegnago Eder J. Lenardão 《Tetrahedron letters》2014
(E)-1,2-Bis-chalcogen alkenes were stereoselectively prepared in good yields by the addition of diorganyl dichalcogenides to terminal alkynes using CuI/Zn/glycerol as a recyclable catalytic system. The antioxidant activity in vitro of four (E)-1,2-bis-chalcogen alkenes synthesized was evaluated and (E)-1,2-bis-(4-methoxyphenylselanyl)styrene 3b presented excellent activity. The catalytic system used in the synthesis was recovered and used directly up to 5 cycles without loss of activity. 相似文献