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281.
Milan Randic Wayne L. Woodworth Ante Graovac 《International journal of quantum chemistry》1983,24(5):435-452
For most structures (molecules, graphs, lattices) a count of random walks for nonequivalent sites will give different numbers, particularly for walks of many steps. Occasionally one finds the same count of walks for nonequivalent sites. These have been termed “unusual walks” and have been closely examined in the case of trivalent graphs. While it remains to be understood what structural factors are critical, some regularities have been observed and are discussed. Unusual walks within a single structure signal “isospectural” points in a graph. A number of structures possessing unusual walks have been displayed, and a few constructive steps which do not alter the “unusual” characteristics of selected vertices have been indicated. 相似文献
282.
Photochemistry and phototoxicity of aloe emodin 总被引:2,自引:0,他引:2
Photochemical pathways leading to the phototoxicity of the aloe vera constituent aloe emodin were studied. The results indicate a photochemical mechanism involving singlet oxygen to be the most likely pathway responsible for the observed phototoxicity. Aloe emodin was found to efficiently generate singlet oxygen when irradiated with UV light (phidelta = 0.56 in acetonitrile). The survival of human skin fibroblast cells in the presence of aloe emodin was found to decrease upon irradiation with UV light. A further decrease in cell survival was observed in D2O compared with H2O, suggesting the involvement of singlet oxygen as the primary pathway. Laser flash photolysis experiments were also carried out on aloe emodin alone and in the presence of various biological substrates. Aloe emodin proved to be relatively photostable (phi = 1 x 10(-4)) and a poor photo-oxidant (E*red = +1.02 V). Only absorption bands caused by the triplet state of aloe emodin (lambdamax = 480 nm) and the aloe emodin conjugate base (lambdamax = 520 nm) were observed in the transient spectra. 相似文献
283.
Wayne W. Y. Lau C. S. Lee S. Y. Lee S. H. Goh 《Journal of polymer science. Part A, Polymer chemistry》1993,31(6):1499-1503
Copolymers of 1,2,2,6,6-pentamethyl-4-piperidinyl m-isopropenyl-α,α-dimethylbenzyl carbamate (CB) with styrene (S) and with methyl methacrylate (MMA) were synthesized using AIBN as initiator. S–CB copolymers made from feed ranging from 0.45–0.94 mole fractions S and MMA-CB copolymers made from feed of 0.34–0.88 mole fractions MMA were used to determine the monomer reactivity ratios r1 and r2. The structure of S–CB copolymers was inferred to be mainly of a random nature and in the MMA–CB copolymerization system there is a stronger tendency to form alternating copolymers. © 1993 John Wiley & Sons, Inc. 相似文献
284.
285.
Beck KM Joly AG Dupuis NF Perozzo P Hess WP Sushko PV Shluger AL 《The Journal of chemical physics》2004,120(5):2456-2463
We demonstrate laser control of the electronic product state distribution of photodesorbed halogen atoms from alkali halide crystals. Our general model of surface exciton desorption dynamics is developed into a simple method for laser control of the relative halogen atom spin-orbit laser desorption yield. By tuning the excitation laser photon energy in a narrow region of the absorption threshold, the yield of excited state chorine atoms, Cl(2P(1/2)), can be made to vary from near 0 to 80% for KCl and from near 0 to 50% for NaCl relative to the total yield of Cl atoms. We describe the physical properties necessary to obtain a high degree of product state control and the limitation induced when these requirements are not met. These results demonstrate that laser control can be applied to solid state surface reactions and provide strong support for surface exciton-based desorption models. 相似文献
286.
Martin K Steinberg TH Goodman T Schulenberg B Kilgore JA Gee KR Beechem JM Patton WF 《Combinatorial chemistry & high throughput screening》2003,6(4):331-339
Protein kinases represent one of the largest families of regulatory enzymes, with more than 2,000 of them being encoded for by the human genome. Many cellular processes are regulated by the reversible phosphorylation of proteins and upwards of 30% of the proteins comprising the eukaryotic proteome are likely to be phosphorylated at some point during their existence. In the past, analysis of global protein phosphorylation has been accomplished through radiolabelling of samples with inorganic (32P or [gamma-32)P] ATP. The approach is limited to specimens amenable to radiolabelling and poses certain safety and disposal problems. Alternatively, immunodetection with antibodies to the common phosphoamino acids may be employed, but the antibodies are relatively expensive and exhibit limited specificity and a certain degree of cross-reactivity. Pro-Q Diamond dye is a new fluorescent phosphosensor technology suitable for the detection of phosphoserine-, phosphothreonine- and phosphotyrosine-containing proteins directly in isoelectric focusing gels, SDS-polyacrylamide gels and two-dimensional gels. Additionally, the technology is appropriate for the detection of phosphoproteins or phosphopeptides arrayed on protein chips or affixed to beads. Dye-stained proteins and peptides can be excited with a laser-based light source of 532 or 543 nm or with a xenon-arc lamp-based system equipped with appropriate band pass filters. Alternatively, ultraviolet light of about 302 nm may be employed, providing that sufficiently long exposure times are used to collect the fluorescence signal. Pro-Q Diamond dye emits maximally at approximately 580 nm. The fluorescence-based detection technology is easy to conduct, cost effective and allows rapid large-scale screening of protein and peptide phosphorylation in a variety of solid-phase assay formats. 相似文献
287.
Yip VL Varrot A Davies GJ Rajan SS Yang X Thompson J Anderson WF Withers SG 《Journal of the American Chemical Society》2004,126(27):8354-8355
Among the numerous well-characterized families of glycosidases, family 4 appears to be the anomaly, requiring both catalytic NAD+ and a divalent metal for activity. The unusual cofactor requirement prompted the proposal of a mechanism involving key NAD+-mediated redox steps as well as elimination of the glycosidic oxygen. Primary kinetic isotope effects for the 2- and 3-deutero substrate analogues, isotopic exchange with solvent, and structural analysis of a 6-phospho-beta-glucosidase, BglT (E.C. 3.2.1.6), provided evidence in support of the proposed mechanism, which has striking resemblances to that of the sugar dehydratases. Furthermore, analysis of the stereochemical outcome indicated that family 4 enzymes are retaining glycosidases. 相似文献
288.
The diffusion coefficient of O*2(1Δg) in O2(3Σ?g) has been measured as a function of pressure, D* = 0.201 ± 0.005 cm2 s?1 at 1 atmosphere and 298 K. 相似文献
289.
Frank J. Millero Wayne C. Duer Elizabeth Shepard Peter V. Chetirkin 《Journal of solution chemistry》1978,7(12):877-889
The enthalpies of dilution of aqueous solutions of HCl, H3PO4, NaOH, NaH2PO4, Na2HPO4 and Na3PO4 in the molality range 0.1 to 1.0 mole-kg–1 have been determined at 30°C. The relative apparent molal enthalpies L of HCl, NaOH, NaH2PO4 and Na2HPO4 have been determined with the aid of an extended form of the Debye-Hückel limiting law. The relative apparent molal enthalpies for Na3PO4 solutions have been corrected for hydrolysis. A value of H
H
o
=9525±150 cal-mole–1 was determined for the heat of hydrolysis of PO
4
–3
. This value gives H
3
o
=3815±150 cal-mole–1 for the ionization of H2PO
4
–
, which is in good agreement with the value of H
3
o
=3500±500 cal-mole–1 determined directly by Pitzer at 25°C. The relative apparent molal enthalpies for H3PO4 solutions have been corrected for ionization. A value of H
1
o
=–1900±150 cal-mole–1 was obtained for the heat of ionization of H3PO4 to H++H2PO
4
–
. This value is in good agreement with the value of H
1
o
=–2031 cal-mole–1 at 30°C determined by Harned and Owen from the temperature coefficient of the equilibrium constant and H
1
o
=–1950±80 cal-mole–1 at 25°C determined from calorimetry by Pitzer. 相似文献
290.
The transformation matrix method and the concept of virtual bonds has been applied to the study of the persistence lengths of two recently developed polyimide fibers, PMDA-PFMB, synthesized from pyromellitic dianhydride (PMDA) and 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (PFMB), and BPDA-PFMB, synthesized from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and PFMB. The elements of the averaged transformation matrices are obtained from a molecular dynamics trajectory of 1 ns and from a consideration of the symmetry of the torsion potential energy function for selected bonds. The persistence length is found to be ∼ 496 AÅ for PMDA-PFMB and ∼ 66 AÅ for BPDA-PFMB. The large persistence length of PMDA-PFMB, comparable to that of the rodlike polymer poly(benzobisoxazole) (PBO), is caused by the colinearity of the virtual bonds in the chain and the stiffness of the bond angles. The inclusion of the electrostatic interactions does not seem to affect the result significantly, although a very slight decrease (∼ 5%) of the persistence length was detected. The effect of substitution of methyl groups for the trifluoromethyl groups in these polyimide fibers is also studied, with an increase of ∼ 14% in the persistence length of PMDA-PFMB and ∼ 1.4% in the persistence length of BPDA-PFMB. Since a good solvent that would be appropriate for use in a light scattering measurement has not been found for either of these polyimide fibers, the theoretical estimates of the persistence length cannot be compared with experiment. © 1996 John Wiley & Sons, Inc. 相似文献