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排序方式: 共有618条查询结果,搜索用时 263 毫秒
91.
Silva MA Dantelle G Mortier M Monteil A Ribeiro SJ Messaddeq Y Briois V Poulain M 《The Journal of chemical physics》2008,128(24):244516
Erbium L(3)-edge extended x-ray absorption fine structure (EXAFS) measurements were performed on rare earth doped fluorosilicate and fluoroborate glasses and glass ceramics. The well known nucleating effects of erbium ions for the crystallization of cubic lead fluoride (based on x-ray diffraction measurements) and the fact that the rare earth ions are present in the crystalline phase (as indicated by Er(3+) emission spectra) seem in contradiction with the present EXAFS analysis, which indicates a lack of medium range structural ordering around the Er(3+) ions and suggests that the lead fluoride crystallization does not occur in the nearest neighbor distance of the rare earth ion. Molecular dynamics simulations of the devitrification process of a lead fluoride glass doped with Er(3+) ions were performed, and results indicate that Er(3+) ions lower the devitrification temperature of PbF(2), in good agreement with the experimental results. The genuine role of Er(3+) ions in the devitrification process of PbF(2) has been investigated. Although Er(3+) ions could indeed act as seeds for crystallization, as experiments suggest, molecular dynamics simulation results corroborate the experimental EXAFS observation that the devitrification does not occur at its nearest neighbor distance. 相似文献
92.
93.
Advances in Iodine(III)‐Mediated Halogenations: A Versatile Tool to Explore New Reactivities and Selectivities 下载免费PDF全文
Andreas M. Arnold Anna Ulmer Dr. Tanja Gulder 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(26):8728-8739
Within the repertoire of organic chemical transformations, the halogenation of substrates is among the most versatile, reliable, and broadly applicable reactions. Although a multitude of different methods are known today, there is still a huge demand for novel and, in particular, catalytic halogenation methods that exhibit new reactivities and selectivities. The class of hypervalent iodanes meets exactly these needs and thus offers a great opportunity to fuel this highly desirable direction within the field of halogenation chemistry. This Concept gives a short overview of recent examples focusing on selective and/or mechanistically unusual halogenations. 相似文献
94.
Site‐Selective Disulfide Modification of Proteins: Expanding Diversity beyond the Proteome 下载免费PDF全文
Dr. Seah Ling Kuan Dr. Tao Wang Prof. Dr. Tanja Weil 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(48):17112-17129
The synthetic transformation of polypeptides with molecular accuracy holds great promise for providing functional and structural diversity beyond the proteome. Consequently, the last decade has seen an exponential growth of site‐directed chemistry to install additional features into peptides and proteins even inside living cells. The disulfide rebridging strategy has emerged as a powerful tool for site‐selective modifications since most proteins contain disulfide bonds. In this Review, we present the chemical design, advantages and limitations of the disulfide rebridging reagents, while summarizing their relevance for synthetic customization of functional protein bioconjugates, as well as the resultant impact and advancement for biomedical applications. 相似文献
95.
96.
Bringmann G Haagen Y Gulder TA Gulder T Heide L 《The Journal of organic chemistry》2007,72(11):4198-4204
Streptomyces cinnamonensis DSM 1042 produces the polyketide-isoprenoid compound furanonaphthoquinone I (FNQ I) and isoprenylated phenazines, predominantly endophenazine A. However, the recently identified biosynthetic gene cluster for these compounds only contains a single gene for a mevalonate pathway enzyme, that is, a putative mevalonate kinase gene. This is in strong contrast to all Streptomyces strains examined so far, where all six genes encoding the mevalonate pathway enzymes are clustered in a single operon of 6.8 kb and, thus, raised the question about the biosynthetic origin of the isoprenoid moieties of FNQ I and endophenazine A. In this study, we investigated the incorporation of [13C2]acetate and [2-13C]glycerol into FNQ I and endophenazine A. The results unequivocally prove that the isoprenoid building blocks of both compounds are predominantly formed via the mevalonate pathway (approximately 80%) but that the MEP pathway (approximately 20%) contributes to the biosynthesis of these molecules, too. In actinomycetes, this is the first experimentally proven example of the utilization of both biosynthetic routes for the formation of one single secondary metabolite. The incorporation pattern of [2-13C]glycerol was consistent with a "reverse" prenyl transfer, that is, with the formation of a C-C bond from C-3 of GPP to the polyketide nucleus of FNQ I. 相似文献
97.
F. Calvo P. Poulain 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2009,51(1):15-23
Monte Carlo simulations of gas-phase polyalanine peptides have been carried out with the Amber ff96 force field. A low-temperature structural transition takes place between the α-helix stable conformation and β-sheet structures,
followed by the unfolding phase change. The transition state ensembles connecting the helix and sheet conformations are investigated
by sampling the energy landscape along specific geometric order parameters as putative reaction coordinates, namely the electric
dipole μ, the end-to-end distance d, and the gyration radius Rg. By performing series of shooting trajectories, the committor probabilities and their distributions are obtained, revealing
that only the electric dipole provides a satisfactory transition coordinate for the α↔β interconversion. The nucleus at the
transition is found to have a high helical content. 相似文献
98.
Tobat P.I. Saragi Christoph Schmidt Tanja Weis Arno Ehresmann 《Journal of magnetism and magnetic materials》2009,321(14):2204-2209
The surface and interface morphology and magnetization characteristics of Co70Fe30 thin films deposited on bare glass and p-Si/SiO2 substrates and on conjugated polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) thin films on such substrates have been studied by atomic force microscopy and magneto-optic Kerr effect. It was found that the average absolute magnitude of the coercive field of Co70Fe30 correlates with the roughness of the underlayer prior to Co70Fe30 deposition. P3HT deposited on p-Si/SiO2 substrates possesses an increased surface roughness as compared to the p-Si/SiO2 surface, but displays a decreased surface roughness as compared to the one of a bare glass substrate. 相似文献
99.
Tanja Soldatović Predrag Čanović Radivoje Nikolić Ratomir Jelić Živadin D. Bugarčić 《Journal of solution chemistry》2009,38(1):57-71
The formation equilibria of the [Pt(SMC)(H2O)2]+ complex with some biologically relevant ligands such as L-methionine (L-met) and glutathione (GSH) were studied. The stoichiometry
and stability constants of the formed complexes are reported, and the concentration distribution of the various complex species
has been evaluated as a function of pH. The reaction between [PtCl2(SMC)] and guanosine-5′-monophosphate (5′-GMP) was studied by 1H NMR spectroscopy. The NMR spectra indicated that first step is the hydrolysis of the [PtCl2(SMC)] complex and second step is the substitution of an aqua ligand, either in the cis or trans position with guanosine-5′-monophosphate in molar ratio 1:1. The values of rate constant showed faster substitution of coordinated
H2O in the trans position to the S donor atom of S-methyl-L-cysteine, whereas the slower reaction was assigned to the displacement of the
cis coordinated aqua molecule. This is due to the strong trans labilization effect of coordinated sulfur.
Electronic Supplementary Material The online version of this article () contains supplementary material, which is available to authorized users. 相似文献
100.
Tanja Grkovic 《Tetrahedron》2009,65(32):6335-207
A survey of the secondary metabolite chemistry profiles of New Zealand sponges of the genus Latrunculia has yielded new members of the discorhabdin A- and B-type families. The structure elucidation of (+)-(6R,8S)-1-thiomethyldiscorhabdin G∗/I (5) and both enantiomers of 16a,17a-dehydrodiscorhabdin W (6) are reported. Absolute configurations were assigned by comparison with a dataset of recently reported electronic circular dichroism spectra of discorhabdin alkaloids. A stereochemical correction of the recently reported natural product (+)-3-dihydrodiscorhabdin A from (3S,5R,6S,8S)-(7) to the C3-epimeric (+)-(3R,5R,6S,8S)-(8) and assignment of absolute configuration is also presented. Semi-synthesis of (+)-(3S,5R,6S,8S)-(7) from (+)-discorhabdin A provided further evidence for this structure revision. Cytotoxicity data is reported for 5-8. 相似文献