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1.
G. Richard Geier III 《Tetrahedron》2004,60(50):11435-11444
To better understand the effects of diverse substituents on reactions leading to porphyrins, pyrrole+aldehyde condensations and related reactions of dipyrromethanes were examined. The course of pyrrole+aldehyde condensations was investigated by monitoring the yield of porphyrin (by UV-Vis spectroscopy), reaction of aldehyde (by TLC), and changes in the composition of oligomers (by laser desorption mass spectrometry). Reaction reversibility was examined via exchange experiments. Reversibility of reactions leading to porphyrin was further probed with studies of dipyrromethanes. The reaction course was found to depend on the nature of the substituent and the acid catalyst. Alkyl or electron-donating substituents displayed levels of reversibility (exchange/scrambling) on par or greater than that of the phenyl substituent, whereas electron-withdrawing or sterically bulky substituents exhibited little to no reversibility. The results obtained provide insight into the electronic and steric effects of different substituents and should facilitate the design of synthetic plans for preparing porphyrinic macrocycles. 相似文献
2.
Danny E. Akin W. Herbert Morrison III Luanne L. Rigsby Franklin E. Barton II David S. Himmelsbach Kevin B. Hicks 《Applied biochemistry and biotechnology》2006,129(1-3):104-116
Information is presented on structure, composition, and response to enzymes of corn stover related to barriers for bioconversion
to ethanol. Aromatic compounds occurred in most tissue cell walls. Ferulic acid esterase treatment before cellulase treatment
significantly improved dry weight loss and release of phenolic acids and sugars in most fractions over cellulase alone. Leaf
fractions were considerably higher in dry weight loss and released sugars with esterase treatment, but stem pith cells gave
up the most phenolic acids. Results help identify plant fractions more appropriate for coproducts and bioconversion and those
more suitable as residues for soil erosion control. 相似文献
3.
P. Dawson J. W. Haas III K. B. Alexander J. Thompson T. L. Ferrell 《Surface science》1991,250(1-3):L383-L388
The intensity of surface enhanced Raman scattering from benzoic acid derivatives on mildly roughened, thermally evaporated Ag films shows a remarkably strong dependence on metal grain size. Large grained (slowly deposited) films give a superior response, by up to a factor of 10, to small grained (quickly deposited) films, with films of intermediate grain size yielding intermediate results. The optical field amplification underlying the enhancement mechanism is due to the excitation of surface plasmon polaritons (SPPs). Since surface roughness characteristics, as determined by STM, remain relatively constant as a function of deposition rate, it is argued that the contrast in Raman scattering is due to differences in elastic grain boundary scattering of SPPs (leading to different degrees of internal SPP damping), rather than differences in the interaction of SPPs with surface inhomogeneities. 相似文献
4.
5.
Martin R. Bridson Charles F. Miller III 《Proceedings of the American Mathematical Society》2004,132(1):59-65
We give examples of direct products of three hyperbolic groups in which there cannot exist an algorithm to decide which finitely presented subgroups are isomorphic.
6.
C. E. Aalseth F. T. Avignone III R. L. Brodzinski H. S. Miley J. H. Reeves 《Journal of Radioanalytical and Nuclear Chemistry》1998,233(1-2):119-125
A fast digital oscilloscope based pulse shape discrimination (PSD) system has been tested with intrinsic germanium detectors
large enough to allow ionizing events which generate localized electron-hole pairs at a single site to be segregated from
those depositing energy at several different sites in the crystal. Drift velocities of the electrons and holes result in pulses
several hundred nanoseconds long. Since the electric field varies by almost a factor of 10 between the outer and inner surfaces,
collection of electrons and holes can frequently be dinstinguished, and pulses due to multi-site events can be distinguished
from single site events. 相似文献
7.
Oscar H. Will III Natalie A. Newland Kaylan L. Blelville 《Photochemistry and photobiology》1985,41(3):283-287
Abstract— Exposure of variously pigmented strains of Ustilago violacea to high intensity incandescent radiation resulted in the generation of three types of survival curves. High carotene, low cytochrome c containing strains of U. violacea were generally characterized by linear type I survival curves with slopes approximately equal to zero. Strains which lacked carotenes were characterized by exponential decay type II survival curves. A third survival curve, type III was observed with carotene accumulating strains which also contained large amounts of cytochrome c. The type III curves are characterized by an initial loss of viability, similar to the type II curves, followed by a recovery period, with eventual stability in survival. The survival curve type appears to be dependent on the relative mg quantities of cytochrome c and carotenes in the cells. Strains with carotene/cytochromec ratios of0–1 × 10-1 ,3–15× 10-1 and l6 × 10-1 and above had type II, type III and type I survival curves, respectively. 相似文献
8.
Abstract— The effect of high intensity incandescent radiation on survival and mitotic recombination in the phlph + diploid strain of Ustilago violacea was studied with and without atmospheric O2 . In the presence of atmospheric O2 , strain phlph+ was characterized by photokilling to approximately 50% survival, and induction of mitotic recombination to about 60% by 90 min of light exposure. No photokilling and little induction of mitotic recombination were observed when light exposure was carried out in an 02 depleted environment. Photokilling and photo-induced mitotic recombination in U. violacea may be due to DNA damage or repair in response to a photosensitized reaction, involving an endogeneous photosensitizer. 相似文献
9.
On the basis of ab initio calculations (configuration interaction and generalized valence bond), we find eight excited states of ozone with vertical excitation energies less than 7 eV. Using these results the various experimental transitions are assigned. One state has an equilateral triangle as the minimum energy geometry (1.5 eV above the ground state). 相似文献
10.