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721.
Bromo Complexes of Molybdenum(IV) [MoBr6]2? and [Mo2Br10]2?. Crystal Structure of (PPh3Me)2[MoBr6] · 2 CH2Br2 The bromomolybdates(IV) (PPh3Me)2[MoBr6] · 2 CH2Br2 and (PPh4)2[Mo2Br10] are obtained by reactions of molybdenum tetrabromide with PPh3MeBr and PPh4Br, respectively. They form black-brown, hydrolysis sensitive crystal powders. The crystal structure of (PPh3Me)2[MoBr6] · 2 CH2Br2 was determined by X-ray diffraction (2376 independent observed reflexions, R = 0.082). Crystal data: a = 1024, b = 1131, c = 1179 pm, α = 108.2°, β = 106.8°, γ = 99.0°, space group P1 , Z = 1. The compound consists of PPh3Me+ ions, CH2Br2 molecules and nearly octahedral [MoBr6]2? ions with MoBr bond lengths between 252.7 and 254.0 pm. 相似文献
722.
Scheffer U Strick A Ludwig V Peter S Kalden E Göbel MW 《Journal of the American Chemical Society》2005,127(7):2211-2217
2-aminopyridine and 2-aminobenzimidazole were chosen as structural analogues to substitute guanidinium groups in receptor molecules designed as phosphoryl transfer catalysts. Shifting the pKa of the guanidinium analogues toward 7 was expected to raise catalytic activities in aqueous buffer. Although the pKa's of both heterocycles are similar (6.2 and 7.0), only 2-aminobenzimidazole led to active RNA cleavers. All cleavage assays were run with fluorescently labeled substrates and a DNA sequencer. RNase contaminations would degrade RNA enantioselectively. In contrast, achiral catalysts such as 9b and 10b necessarily induce identical cleavage patterns in RNA and its mirror image. This principle allowed us to safely rule out contamination effects in this study. The most active catalysts, tris(2-aminobenzimidazoles) 9b and 10b, were shown by fluorescence correlation spectroscopy (FCS) to aggregate with oligonucleotides. However, at very low concentrations the compounds are still active in the nonaggregated state. Conjugates of 10b with antisense oligonucleotides or RNA binding peptides, therefore, will be promising candidates as site specific artificial ribonucleases. 相似文献
723.
Andreja Lesac H. Loc Nguyen Sanja Naran
i Ute Baumeister Siegmar Diele Duncan W. Bruce 《Liquid crystals》2006,33(2):167-174
New, bent-core mesogens are described in which the core of the molecule is a semiflexible, di(4-aminocyclohexyl)methane spacer. The compounds show nematic, columnar nematic and columnar phases as shown by a combination of X-ray diffraction and optical microscopy. The potential of these new mesogens as biaxial nematic candidates is considered. 相似文献
724.
Distinct bacterial species belonging to the family of enterobacteriaceae produce cellulose as an exopolysaccharide. Cellulose produced by Salmonella enterica serovar Typhimurium was isolated by various methods. Subsequently, transmission electron microscopy (TEM) studies of the isolated material revealed novel structural details of enterobacterial cellulose. 相似文献
725.
Roman Zaremba Ute Ch. Rodewald Rolf-Dieter Hoffmann Rainer P?ttgen 《Monatshefte für Chemie / Chemical Monthly》2007,17(1):523-528
The rare earth-transition metal-indides RE
4RhIn (RE = Gd–Tm, Lu) were prepared by arc-melting of the elements and subsequent annealing. Single crystals were grown via slowly cooling of the samples. The indides were investigated via X-ray powder diffraction and several structures were refined from X-ray single crystal diffractometer data:
F[`4]3mF{\bar 4}3m
, a = 1370.7(9) pm, wR2 = 0.049, 428 F
2 values for Gd4RhIn, a = 1360.3(6) pm, wR2 = 0.028, 420 F
2 values for Tb4RhIn, a = 1354.5(2) pm, wR2 = 0.041, 380 F
2 values for Dy4RhIn, a = 1349.2(3) pm, wR2 = 0.029, 410 F
2 values for Ho4RhIn, a = 1342.5(5) pm, wR2 = 0.037, 403 F
2 values for Er4RhIn, a = 1337.8(3) pm, wR2 = 0.038, 394 F
2 values for Tm4RhIn with 14 variable parameters per refinement, and a = 1329.7(3) pm for Lu4RhIn. In this new structure type, the rhodium atoms have a trigonal prismatic rare earth coordination. Condensation of the
RhRE
6 prisms leads to a three-dimensional network which leaves voids that are filled by regular In4 tetrahedra (317 pm In–In distance) in Gd4RhIn. The indium atoms have twelve nearest neighbors (3 In + 9 RE) in icosahedral coordination. The gadolinium atoms build up a three-dimensional, adamantane-like network of condensed, face-sharing
empty octahedra. 相似文献
726.
Vasyl I. Zaremba Ihor R. Muts Ute Ch. Rodewald Viktor Hlukhyy Rainer Pttgen 《无机化学与普通化学杂志》2004,630(12):1903-1907
The ternary indides RE10Ni9+xIn20 (RE = Tb, Dy) were synthesized from the elements by arc‐melting under argon and subsequent annealing. YbNiIn2 was prepared in a sealed tantalum tube in a water‐cooled sample chamber of a high‐frequency furnace. X‐ray powder and single crystal data revealed isotypism with the tetragonal Ho10Ni9In20 type structure, space group P4/nmm for the RE10Ni9+xIn20 compounds: a = 1337.0(2), c = 909.5(2) pm, wR2 = 0.0527, 1795 F2 values, 65 variables for RE = Tb, and a = 1333.63(7), c = 907.2(1) pm, wR2 = 0.0590, 1346 F2 values, 65 variables for RE = Dy. Both indides show an additional nickel site (Ni4) with partial nickel occupancy leading to the refined compositions Tb10Ni9.34(2)In20 and Dy10Ni9.32(2)In20. YbNiIn2 adopts the orthorhombic MgCuAl2‐type structure: Cmcm, a = 430.67(9), b = 1033.0(2), c = 758.1(1) pm, wR2 = 0.0262, 424 F2 values and 16 variable parameters. The crystal chemistry of the RE10Ni9+xIn20 and RENiIn2 compounds is briefly discussed. 相似文献
727.
Ba2(CN2)(CN)2 and Sr2(CN2)(CN)2 – the First mixed Cyanamide Cyanides The mixed cyanamide-cyanides M2(CN2)(CN)2 (M = Ba, Sr) were synthesized by the reaction of Ba2N and SrCO3, respectively, with HCN at 630°C. The crystal structure of Ba2(CN2)(CN)2 was determined from single-crystal X-ray investigations at room temperature and ?100°C; the isostructural Sr2(CN2)(CN)2 was refined using powder methods (P63/mmc; Ba2(CN2)(CN)2: a = 1 066.52(5) pm, c=696.82(3) pm; Sr2(CN2)(CN)2: a = 1 035.91(1) pm, c = 664.23(1) pm; Z = 4). The crystal structure is a partially filled defect variant of the anti-NiAs structure type with a distorted hexagonal close packed arrangement of M2+-ions. All CN22? and one quarter of the CN? ions occupy 3/4 of the octahedrally coordinated interstices, the remaining cyanide anions are located at 3/8 of the tetrahedral sites. In the crystal structure the CN? are coordinated to the cations both end-on and side-on. All anions can be distinguished by vibrational spectroscopy. 相似文献
728.
Kohlmeyer U Kuballa J Jantzen E 《Rapid communications in mass spectrometry : RCM》2002,16(10):965-974
A method using high-performance liquid chromatography/inductively coupled plasma mass spectrometry (HPLC/ICP-MS) has been developed to determine inorganic arsenic (arsenite, arsenate) along with organic arsenic compounds (monomethylarsonic acid, dimethylarsinic acid, arsenobetaine, arsenocholine, trimethylarsine oxide, tetramethylarsonium ion and several arsenosugars) in fish, mussel, oyster and marine algae samples. The species were extracted by means of a methanol/water mixture and a dispersion unit in 2 min, with extraction efficiencies ranging from 83 to 107% in the different organisms. Up to 17 different species were determined within 15 min on an anion-exchange column, using a nitric acid gradient and an ion-pairing reagent. As all species are shown in one chromatogram, a clear overview of arsenic distribution patterns in different marine organisms is given. Arsenobetaine is the major compound in marine animals whereas arsenosugars and arsenate are dominant in marine algae. The method was validated with CRM DORM-2 (dogfish muscle). Concentrations were within the certified limits and low detection limits of 8 ng g(-1) (arsenite) to 50 ng g(-1) (arsenate) were obtained. 相似文献
729.
Ute Bahr 《Annalen der Physik》1977,489(4):267-285
Effective Elastic Properties of Finite Heterogeneous Media - Application to Rayleigh-waves Rayleigh waves in a heterogeneous material (multiphase mixtures, composite materials, polycrystals) are governed by integrodifferential equations derived by the aid of known methods for infinite heterogeneous media. According to this wave equation the velocity depends on the frequency, and the waves are damped. After some simplifications (isotropy, nonrandom elastic constants) the following is obtained: if the fluctuations of the mass density are restricted to the vicinity of the boundary, the frequency dependent part of the velocity behaves like \documentclass{article}\pagestyle{empty}\begin{document}$ \frac{{l^3 \omega ^3}}{{{\mathop c\limits^\circ} _t^3}} $\end{document} and the damping is proportional to \documentclass{article}\pagestyle{empty}\begin{document}$ \frac{{l^4 \omega ^5}}{{{\mathop c\limits^\circ} _t^5}} $\end{document}, whereas \documentclass{article}\pagestyle{empty}\begin{document}$ \frac{{l^2 \omega ^2}}{{{\mathop c\limits^\circ} _t^2}} $\end{document} respectively \documentclass{article}\pagestyle{empty}\begin{document}$ \frac{{l^3 \omega ^4}}{{{\mathop c\limits^\circ} _t^4}} $\end{document} is found if the fluctuations are present in the whole half-space. From this it is seen, what assumptions are necessary to describe the waves by differential equations with frequenc y-dependent mass density. 相似文献
730.
Palladium-mediated cross-coupling reactions are attractive organometallic transformations for the generation of C--C, C--N, C--O, and C--S bonds. Despite being widely employed in small-scale syntheses, cross-coupling reactions have not found important industrial applications because until recently, only reactive aryl bromides and iodides could be used as substrates. These substrates are generally more expensive and less widely available than their chloride counterparts. Over the past few years, new catalytic systems with the ability to activate unreactive and sterically hindered aryl chlorides have been developed. The new catalysts are based on palladium complexes that contain electron-rich and bulky phosphine or carbene ligands. The enhanced reactivity observed with these new systems has been attributed to the formation of unsaturated and reactive [PdL] species which can readily undergo oxidative addition reactions with ArX to yield [Pd(Ar)X(L)]. 相似文献