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71.
72.
Attila Makó Zsolt Rapi György Keglevich Áron Szöllősy László Drahos László Hegedűs Péter Bakó 《Tetrahedron: Asymmetry》2010,21(8):919-925
The chiral monoaza-15-crown-5 lariat ethers annellated to methyl-4,6-O-benzylidene-α-d-glucopyranoside-1 or mannopyranoside 2 have been applied as phase-transfer catalysts in the epoxidation of substituted chalcones and chalcone analogues with tert-butylhydroperoxide resulting in significant asymmetric induction. It was found that the position of the substituents in the aromatic ring of the chalcone had an influence both on the chemical yields and enantiomeric excesses. The lowest enantioselectivities (62–83% ee) were found in the case of ortho-substituted model compounds. The highest ee values (ee of 83–97%) were obtained in the case of para-substituted models. From among the chalcone analogues, the maximum ee (90–92%) was detected for the model compound having α-tert-butyl- and β-aryl groups. Using glucose-based crown ether 1, formation of the (?)-enantiomer was preferred, while applying mannose-based 2 as the catalyst, the (+)-enantiomer was in excess. 相似文献
73.
Stiedl Bernadett Kovcs-Kiss Dorottya Ludnyi Krisztina Bdis Attila Klebovich Imre Antal Istvn 《Chromatographia》2010,71(1):109-113
Tolperisone HCl is a central muscle relaxant, which was incorporated in a matrix system formulated with poly(ethylene oxide)–PEO, in order to achieve adequate gastric residence time. This tablet presents considerable analytical difficulties in the quantitative determination of the drug, because the PEO matrix causes significant increase of viscosity in the samples. Our purpose was to develop a reproducible sample preparation method, which is adapted from parameters of the in vitro dissolution test and validate an LC-UV analytical method, which allows good recovery of the drug (99.97%). The developed analytical method was suitable for quantitative analysis of tolperisone HCl in matrix tablets. 相似文献
74.
The structure and energetics of hyaluronan oligomers from di- to decasaccharides have been studied by density functional theory
calculations at the B3LYP/6-31G** level. The study covered selected conformers of the sodium salt, anionic disaccharides,
and neutral acids in the isolated state and in aqueous solution using the PCM model approach. We investigated the structural
changes of the hyaluronan chain when the Na+ ion is removed or replaced by proton. These processes result in some characteristic changes in the glycosidic torsional angles
and hydrogen bonding interactions. We evaluated the folding for the hyaluronan chains and obtained values between 2.2 and
3.2, somewhat smaller than reported for the crystals. We found a contraction of the hyaluronan chains upon enlargement in
most derivatives attributed partly to the helical character of hyaluronan. The energy consequences of the enlargement have
been modeled by isodesmic reactions. The enlargement processes proved to be exothermal and the energies consistent within
the gradual enlargement. 相似文献
75.
The link of a vertex v of a graph G is the subgraph induced by all vertices adjacent to v. If all the links of G are isomorphic to a finite graph L, then G is called a realization of L, and L is called a link graph. At the Smolenice symposium of 1963, Zykov posed the problem of recognizing link graphs. There are two versions of that problem, namely the finite (the existence of a finite realization is required) and the infinite one. Bulitko (see “On Graphs with Prescribed Links of Vertices” [in Russian], Trudy mat. inst. im. Steklova, Vol. 133, 1973, pp. 78-94) proved that the infinite version is algorithmically unsolvable. The solution of both versions is known only for special classes of graphs as paths, cycles, and graphs homeomorphic to a star (see M. Brown and R. Connelly, “On Graphs with a Constant Link I,” New Directions in the Theory of Graphs, Academic Press, New York, 1973, pp. 19-51; On Graphs with a Constant Link II, Discrete Mathematics, Vol. 11, 1975, pp. 199-232). The finite version for trees with less than 10 vertices has been solved by Blass, Harary, and Miller (see “Which Trees Are Link Graphs?” Journal of Combinatorics Theory Series B, Vol. 29, 1980, pp. 277-292). Trees that are link graphs are called link trees. Using some previous results of Bulitko (see “On a Recursive Property of Block-Complete Graphs” [in Russian], Proceedings of Czechoslovak Conference on Graphs, Zemplínska ?irava, 1978, p. 20-30), we present a polynomial time algorithm recognizing link trees. The applied methods have some remarkable consequences concerning the study of link graphs. © 1995 John Wiley & Sons, Inc. 相似文献
76.
77.
Attila Sali 《Combinatorica》1992,12(3):351-361
LetL(A) be the set of submatrices of anm×n matrixA. ThenL(A) is a ranked poset with respect to the inclusion, and the poset rank of a submatrix is the sum of the number of rows and columns minus 1, the rank of the empty matrix is zero. We attack the question: What is the maximum number of submatrices such that any two of them have intersection of rank at leastt? We have a solution fort=1,2 using the followoing theorem of independent interest. Letm(n,i,j,k) = max(|F|;|G|), where maximum is taken for all possible pairs of families of subsets of ann-element set such thatF isi-intersecting,G isj-intersecting andF ansd,G are cross-k-intersecting. Then fori≤j≤k, m(n,i,j,k) is attained ifF is a maximali-intersecting family containing subsets of size at leastn/2, andG is a maximal2k?i-intersecting family. Furthermore, we discuss and Erd?s-Ko-Rado-type question forL(A), as well. 相似文献
78.
Lewis Acid Catalyzed Condensation–Cyclization Cascade: Direct Synthesis of Di/Trifluoromethyl‐1,2,3,4‐tetrahydroquinazolines 下载免费PDF全文
Prof. Dr. G. K. Surya Prakash Dr. Attila Papp Socrates B. Munoz Nathan May John‐Paul Jones Prof. Dr. Ralf Haiges Prof. Dr. Pierre Mothé Esteves Dr. Thomas Mathew 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(28):10170-10178
Condensed heterocycles such as quinazolines constitute the framework of many promising drugs. The great impact of the dramatic fluorine effect in pharmaceuticals prompted a great surge in the quest for fluorinated drug design resulting in over 20 % fluorine‐containing drugs in the market today. Therefore, finding an efficient and cost‐effective method for the direct synthesis of fluorine‐tagged quinazoline systems is of great significance in the pharmaceutical arena. For the first time, a one‐pot sequential condensation–cyclization reaction to form selectively the difluoro/trifluoromethylated tetrahydroquinazolines from simple components difluoro/trifluoroacetaldehyde hemiacetal and aromatic amines is reported. Our recent studies using difluoro/trifluoroacetaldehyde hemiacetal as simple and elegant difluoro/trifluoromethyl synthons and metal triflates such as gallium triflate as safe and stable Lewis acid catalysts led us to this direct synthesis protocol for the expedient and convenient synthesis of fluorinated quinazolines. DFT calculations at PCM/B3LYP/6‐31++G** were carried out for evaluating a possible reaction mechanism for this cyclization. According to the DFT calculations, product stereochemistry is thermodynamically driven, favoring the cis isomer as the major product, which is also confirmed experimentally. 相似文献
79.
We describe general methods for enumerating subsemigroups of finite semigroups and techniques to improve the algorithmic efficiency of the calculations. As a particular application we use our algorithms to enumerate all transformation semigroups up to degree 4. Classification of these semigroups up to conjugacy, isomorphism and anti-isomorphism, by size and rank, provides a solid base for further investigations of transformation semigroups. 相似文献
80.
Several crude oil accumulations in the Pannonian Basin are trapped in uncommonly hot (>170°C) reservoirs. Their maturities range from mature to very mature on the basis of cracking parameters of their biological marker homologous series (ratio of products to reactants). A stable carbon isotopic study of these oils, poor in biological markers commonly used for correlation purposes, did not provide a reliable oil-to-oil correlation. As an alternative tool, the hydrogen isotope compositions of oil fractions separated on the basis of different polarities were measured, and hydrogen isotope type-curves were generated for a set of mature to very mature crude oil samples. This method of presenting hydrogen isotope composition of fractions as type-curves is novel. Nineteen samples (17 crude oils from SE-Hungary, 1 oil condensate and 1 artificial oil) were chosen for the present study. The aim was to examine the applicability of hydrogen isotope type-curves in oil-to-oil correlation and to test the simultaneous application of carbon and hydrogen isotope type-curves in the field of petroleum geochemistry. We have shown that, although the conventionally used co-variation plots proved to be inadequate for the correlation of these hot and mature oils, the simultaneous use of carbon and the newly introduced hydrogen isotope type-curves allows us to group and distinguish oils of different origins. 相似文献