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71.
Twenty-six derivatives of [SalenMn(III)](+) (1) bearing halogen, nitro, amino, ether, alkyl, or aryl substituents on the aromatic rings and/or at the imine positions or containing 1,3-propylene-, 1,2-phenylene-, 1,2-cyclohexane-, or 1,2-diphenylethylenediamine in place of ethylenediamine as the bridging moiety have been synthesized. The DNA binding/cleaving properties of these complexes in the presence of terminal oxidants have been examined using DNA affinity cleaving techniques. Active derivatives produced DNA cleavage from the minor groove at sites containing multiple contiguous A:T base pairs. For aryl-substituted derivatives, DNA cleavage efficiency was found to vary with both the identity and position of attachment of substituents. The precise patterns of cleavage at A:T target sites varied with the position of attachment of substituents, but not with the identity of the substituents. The results suggest that substituents alter specificity through both steric and electronic effects. The 3,3'-difluoro and -dichloro derivatives produced cleavage patterns that match those of the parent complex, suggesting that the activated form of 1 produces cleavage from an orientation in which the concave edge of the complex faces away from the floor of the DNA minor groove. Bridge modifications yield complexes with reduced DNA cleaving activity relative to 1. DNA cleaving efficiency was found to vary with both the structure and stereochemistry of the bridge. Cleavage efficiency for the complex derived from (R,R)-cyclohexanediamine was 5 times greater than that for the (S,S) enantiomer. Cleavage patterns produced by the enantiomeric complexes at A:T rich target sites were different, demonstrating enantiospecific recognition and cleavage of right-handed double-helical DNA. 相似文献
72.
Igarashi RY Laryukhin M Dos Santos PC Lee HI Dean DR Seefeldt LC Hoffman BM 《Journal of the American Chemical Society》2005,127(17):6231-6241
We here show that the iron-molybdenum (FeMo)-cofactor of the nitrogenase alpha-70(Ile) molybdenum-iron (MoFe) protein variant accumulates a novel S = (1)/(2) state that can be trapped during the reduction of protons to H(2). (1,2)H-ENDOR measurements disclose the presence of two protons/hydrides (H(+/)(-)) whose hyperfine tensors have been determined from two-dimensional field-frequency (1)H ENDOR plots. The two H(+/)(-) have large isotropic hyperfine couplings, A(iso)( )() approximately 23 MHz, which shows they are bound to the cofactor. The favored analysis for these plots indicates that the two H(+/)(-) have the same principal values, which indicates that they are chemically equivalent. The tensors are further related to each other by a permutation of the tensor components, which indicates an underlying symmetry of binding relative to the cofactor. At present, no model for the structure of the iron-molybdenum (FeMo)-cofactor in the S = (1)/(2) state trapped during the reduction of H(+) can be shown unequivocally to satisfy all of the constraints generated by the ENDOR analysis. The data disfavors any model that involves protonation of sulfides, and thus suggests that the intermediate instead contains two chemically equivalent bound hydrides; it appears unlikely that these are terminal monohydrides. 相似文献
73.
James P. Kutney Eugene Dimitriadis Gary M. Hewitt Philip J. Salisbury Mahatam Singh James A. Servizi Dennis W. Martens Robert W. Gordon 《Helvetica chimica acta》1982,65(5):1343-1350
One of the fish-toxic chlorinated resin acids, 14-chlorodehydroabietic acid ( 5 ), found in kraft mill effluent is examined. When exposed to the fungus Mortierella isabellina, 5 is converted into a number of hydroxylated derivatives which show low levels of toxicity to fish. These biotransformation products were isolated and characterized. 相似文献
74.
The reduction of 1,10-phenanthroline-5,6-quinone(I), 5,8-quinolinequinone(II) and 6,7-dichloro-5,8-quinolinequinone(III) was investigated using cyclic voltammetry and coulometry at mercury electrodes and 50% dimethylsulfoxide+water solvent. Each compound is reduced to the corresponding hydroquinone in a diffusion-controlled, reversible two-electron process. The pH-dependence of the reversible potential indicated that the quinone forms were unprotonated, but the hydroquinones could be protonated at the heterocyclic nitrogen atom with pKa = 5.3 for I and 3.5 for III. Careful analysis of the cyclic voltammetric peak shape revealed that the difference between the standard potentials for the introduction of successive electrons, E20 ? E10, was 70 ±20, >100 and 80 ± 20 mV for I–III. Investigation of the pH-dependence of E10 and E20 showed that the pKa of the semiquinone of I was about 8. 相似文献
75.
H. Kurreck S. Aguirre H. Dieks J. Gtschmann J. v. Gersdorff H. Newman H. Schubert M. Speck T. Stabingis J. Sobek P. Tian A. Wiehe 《Radiation Physics and Chemistry》1995,45(6):853-865
Porphyrin quinones (P-Qs), covalently linked via different aliphatic bridges, have been synthesized and studies in their (porphyrin) cationic and (semiquinone) anionic radical states by EPR, ENDOR and TRIPLE resonance techniques. Electron transfer (ET) from the porphyrin donor to the quinone acceptor could be observed by time-resolved picosecond fluorescence spectroscopy (singlet ET) and by time-resolved EPR spectroscopy (triplet ET) in isotropic fluid solution and in anisotropic media (liquid crystals and reversed micelles). Steady-state in situ photoexcitation of P-Qs in CTAB cationic reversed micelles yielded the corresponding semiquinone radical anions. In TRITON X-100 reversed micelles both the radical cation of the porphyrin and the radical anion of the semiquinone could be detected, which occured in complete emission. In covalently linked porphyrin flavins ET from the photoexcited porphyrin fragment to the flavin and, in addition, energy transfer from the photoexcited flavin to the porphyrin could be observed. 相似文献
76.
Kinsel GR Knochenmuss R Setz P Land CM Goh SK Archibong EF Hardesty JH Marynick DS 《Journal of mass spectrometry : JMS》2002,37(11):1131-1140
The photoionization of (pro)(n)DHB (pro = proline, DHB = 2,5-dihydroxybenzoic acid, n = 0, 1, 2 or 4) clusters was studied both experimentally and computationally. Experimentally the (pro)(n)DHB clusters are generated in the gas phase by laser desorption and supersonic jet entrainment. The photoionization thresholds are then determined by the mass-selective measurement of both one- and two-color photoionization efficiency curves. These experiments demonstrate that the ionization energies (IEs) of the (pro)(n)DHB clusters are substantially reduced in comparison with the IE of free DHB. Computational studies of the (pro)(n)DHB clusters provide insights into the mechanism of IE reduction. For the (pro)DHB system the IE reduction results from spin delocalization in the ion state of the cluster. In contrast, for the (pro)(2)DHB and (pro)(4)DHB clusters the IE reduction results from an inductive delocalization of electron density from pro to DHB in the ground state of the cluster. This latter effect, which is a result of the specific hydrogen-bonding interactions occurring in the mixed clusters, leads to IE reductions of >1 eV. Finally, determination of the energetics of the (pro)(2)DHB radical cation demonstrate that the DHB-to-proline proton transfer reaction is a barrierless, exoergic process in the ion state and that energetic demands for cluster dissociation to protonated (pro)(2) plus a deprotonated DHB radical are substantially lower than those for cluster dissociation to (pro)(2) plus DHB(+*). Cumulatively, these studies provide new energetic and mechanistic insights into both primary and secondary MALDI ionization processes. 相似文献
77.
We apply an operator splitting method to develop a simulation algorithm that has complete analytical solutions for the Gaussian thermostated SLLOD equations of motion [D. J. Evans and G. P. Morriss, Phys. Rev. A 30, 1528 (1984)] for a system under shear. This leads to a homogeneous algorithm for performing both equilibrium and nonequilibrium isokinetic molecular dynamics simulation. The resulting algorithm is computationally efficient. In particular, larger integration time steps can be used compared to simulations with regular Gaussian thermostated SLLOD equations of motion. The utility and accuracy of the algorithm are demonstrated through application to the Weeks-Chandler-Anderson fluid. Although strict conservation of the kinetic energy suppresses thermal fluctuations in the system, this algorithm does not allow simulations at lower shear rates than those normally afforded by older nonequilibrium molecular dynamics simulations. 相似文献
78.
[reaction: see text] A regioselective method for the preparation of 1,5-trisubstituted 1H-1,2,3-triazoles via a 1,3-dipolar cycloaddition of 1-trimethylsilylacetylenes with organoazides is described. Immobilization of the azide on REM resin and subsequent cycloaddition afforded a 2 x 2 x 4 x 3 membered 1,5-disubstituted 1H-1,2,3-triazole library with an average purified yield of 68%. 相似文献
79.
80.
Colorimetric peroxidase-coupled procedures for the determination of several cerebrospinal fluid (CSF) lipid classes are described. These methods were modified to increase the effectiveness of each cerebrospinal fluid lipid assay by using the sample as the primary diluent for a highly concentrated reagent in an inverse concentration technique. Direct enzymic assays for the determination of CSF cholesterol (free and total), choline phospholipids, and triglycerides were adapted from existing assays to require less than 0.5 ml of sample per assay. This made determinations of the several lipid analytes possible even when samples were from pediatric specimens. In a study model, 51 pediatric CSF samples were analyzed for these lipid constituents. Mean values and standard deviations were determined. Within and between-run studies were performed by sampling from a pool of cerebrospinal fluid specimens. Within-run coefficients of variation for the several proposed procedures were less than 3% while the between-run findings for all of the procedures were less than 5%. 相似文献